Acidification of Forest Soils: A Model for Analyzing Impacts of Acidic Deposition in Europe - Version II

YO? FOR QTJOTATIOY WITBOLT PERgISSION OF THE AUTHORS

ACIDIFTCATION OF FOREST SOILS:

A MODEL FOR

ANALYZING

IMPACTS OF ACIDIC DEPOSITION IN EUROPE VERSION 11

P e k k a Kauppi Juha Kamari Maximilian Posch Lea Kauppi E g b e r t Matzner

June

1 9 8 5 CP-85-27

CoLLaborative P a p e r s r e p o r t work which h a s not been performed solely

at

t h e International Institute f o r Applied Systems Analysis and which h a s r e c e i v e d only limited review. Views o r opinions e x p r e s s e d h e r e i n d o not necessarily r e p r e s e n t t h o s e of t h e Insti- t u t e , i t s National Member Organizations, o r o t h e r organizations supporting t h e work.

INTERNATIONAL INSTITUTE FOR APPLIED SYSTEMS ANALYSIS 2361 Laxenburg, Austria

AUTHORS

Pekka Kauppi is a former r e s e a r c h scholar of t h e International Insti- t u t e f o r Applied Systems Analysis, Laxenburg, Austria. He is now wfth t h e Ministry of t h e Environment, P.O.Box 306, SF-00531 Helsinki, Finland.

Juha Kamari and Lea Kauppi are from t h e National Board of Waters, Water Research Institute, P.O. Box 250, SF-00101 Helsinki 10, Finland (form- e r l y they w e r e wfth t h e International Institute f o r Applied Systems, Laxen- burg, Austria).

Egbert Matzner is from t h e Research Center f o r Forest Ecosystem/Forest Decline, University of Gottingen, Biisgenweg 2, D-3400 Gottingen, FRG

Maximilian Posch is with t h e International Institute f o r Applied Systems Analysis, Laxenburg, Austria.

PREFACE

The IIASA "Acid Rain" P r o j e c t s t a r t e d in 1983 in o r d e r t o provide t h e European decision makers with a too! which can b e used t o evaluate policies f o r controlling acid rain. This modelling e f f o r t is p a r t of t h e official cooperation between IIASA and t h e UN Economic Commission of Europe (ECE).

The IIASA model c u r r e n t l y contains t h r e e linked compartments: Pollu- tion Generation, Atmospheric Processes and Environmental Impact. Each of these compartments c a n b e filled by different substitutable submodels. The submodels currently available a r e Sulfur Emissionst t h e EMEP Long Range Transport Model, Forest Soil pH and Lake Acidity. In addition, two submodels are under development: t h e NO, Emissions submodel and t h e Direct Forest Impacts submodel. The f i r s t version of t h e Forest Soil pH submodel was presented in May 1984. Since then several changes have been implemented following t h e advice of e x p e r t s . This p a p e r describes t h e Forest Soil pH model as i t stands in March 1985.

Leen Hordijk

Acid Rain P r o j e c t Leader

P r o f e s s o r B. Ulrich from t h e University of Gottingen encouraged t h e development of this study, and contributed significantly t o t h e successful collaboration between IIASA and t h e University of Gottingen. W e gratefully acknowledge his s u p p o r t .

W e would also like t o thank especially Prof.C.0. Tamm, Dr.N. van Bree- men and Dr. I. Nilsson f o r t h e i r valuable advice during t h e development of this study.

ABSTRACT

Acidification is an unfavorable p r o c e s s in f o r e s t soils. Timber logging, n a t u r a l accumulation of biomass in t h e ecosystem, and acidic deposition a r e s o u r c e s of acidification. Acidification c a u s e s a r i s k of damage t o plant roots and a subsequent r i s k of a decline in ecosystem productivity.

A dynamic model i s introduced f o r describing t h e acidification of forest soils.

In one-year time s t e p s t h e model calculates t h e soil pH as function of acid stress and t h e b u f f e r mechanisms of t h e soil. Acid stress is defined as t h e hydrogen ion input into t h e t o p soi!. The b u f f e r mechanisms c o u n t e r a c t acidification by provid- ing a sink f o r hydrogen ions. The concepts b u f l e r rate and b u m r capacity a r e used t o quantify t h e b u f f e r mechanisms. The model compares

(t)

t h e rate of t h e a c i d stress (annual amount) t o t h e b u f f e r rate, and (ii) t h e accumulated acid stress ( o v e r s e v e r a l y e a r s ) t o t h e b u f f e r capacity. The comparisons p r o d u c e a n estimate of t h e soil acidity as t h e output.

Since t h e f i r s t version in May 1984 s e v e r a l changes have been implemented following t h e advice of t h e e x p e r t s . For aluminum and i r o n b u f f e r r a n g e s a n equili- brium a p p r o a c h h a s b e e n introduced. The pH of t h e silicate, cation exchange and u p p e r aluminum b u f f e r r a n g e s i s now a function of b a s e s a t u r a t i o n . In t h e c u r r e n t version of t h e model f o r e s t s are assumed t o a b s o r b sulfur compounds more effec- tively than a g r i c u l t u r a l lands and, moreover, forests are assumed

to

grow on p o o r soil t y p e s r a t h e r t h a n on t h e a v e r a g e soil t y p e of a grid.

The model system as a whole i s now available for analyzing t h e impact of dif- f e r e n t emission scenarios. The soil acidification model assumes s u l f u r deposition estimates from t h e o t h e r submodels as input, and as output i t computes t h e t o t a l a r e a of f o r e s t s in E u r o p e with t h e estimated soil pH lower t h a n any s e l e c t e d t h r e s - hold value. Additionally i t p r o d u c e s estimates of t h e acidity of European f o r e s t soils in a map format.

TABLE OF CONTENTS

1. Introduction

2. Soil Acidification 2.1 Acid Stress

2.2 Buffering Processes 3. Model Development

3.1 Basic Assumptions 3.2 Model Structure 3.3 Model Demonstration 4. Model Application

4.1 Specific As sump tions

4.2 Initialization of Buffering Variables 4.3 Results of Model Runs

5. Discussion References Appendix

ACIDIFICATION OF FOREST SOILS:

A MODEL FOR IMPACTS

OF ACIDIC DEPOSITION IN EUROPE VERSION

II

Pekka Kauppi, Juha Kamari? Maximilian Posch , Lea Kauppi and E g b e r t Matzner

1. Introduction

F o r e s t damage h a s been observed in r u r a l areas in Central Europe t o

a

l a r g e e x t e n t since t h e 1970's. I t was f i r s t r e p o r t e d on s i l v e r f i r (Schiitt, 1977) and l a t e r on Norway s p r u c e , Scots pine, b e e c h , and o t h e r t r e e species

as

well (Schiitt et al., 1983). In 1984, in t h e Federal Republic of Germany damage w a s r e p o r t e d f o r a f o r e s t a r e a of 2,549,000 h a (Lammel, 1984). F o r e s t damage i s a r e s u l t of many f a c t o r s such as d i r e c t impact of a i r pollutants on t r e e foliage, soil acidification, and climate. In t h i s study we a d d r e s s one of them, soil acidification, which h a s been demonstrated as a n important link between a i r pollution and f o r e s t damage. I t i s intended t h a t o t h e r f a c t o r s contributing t o f o r e s t damage will b e i n c o r p o r a t e d into t h e model at a l a t e r s t a g e .

The f i r s t version of t h e soil pH model w a s p r e s e n t e d in May 1984 (IIASA CP-84-16). S e v e r a l soil scientists were t h e n asked t o review t h e model.

According t o t h e i r suggestions t h e model s t r u c t u r e w a s substantially changed in t h e description of aluminum and iron b u f f e r ranges. Instead of assuming

a

c e r t a i n buffer rate, a n equilibrium a p p r o a c h was introduced. In addition, t h e pH of t h e soil in t h e silicate, cation exchange and u p p e r alumi- num r a n g e i s now calculated as a function of b a s e saturation. Besides t h e s e s t r u c t u r a l changes, suggestions concerning t h e l a r g e s c a l e application t o Europe have been i n c o r p o r a t e d into t h e model: f o r e s t s a r e known t o grow on poor soils r a t h e r t h a n on t h e a v e r a g e soil type. T h e r e i s a l s o s t r o n g evi- dence on t h e filtering e f f e c t s of f o r e s t s , i.e. t h e deposition velocity o v e r f o r e s t s i s l a r g e r than t h a t o v e r open land. Some o t h e r suggestions, although considered c o r r e c t , could not b e t a k e n into account, because of t h e p r o j e c t ' s focus on a l a r g e s p a t i a l scale.

2. Soil Acidification

Soil a c i d i f i c a t i o n h a s been defined as being a d e c r e a s e in t h e acid neutralization capacity of t h e soil (Van Breemen

et

a l . , 1984). Such a d e c r e a s e may coincide with a d e c r e a s e in soil pH. I t may a l s o t a k e place in conditions of a relatively constant pH assuming efficient buffering processes. In such a c a s e t h e buffering of t h e soil c o u n t e r a c t s t h e f a c t o r s tending t o d e c r e a s e t h e soil pH s o t h a t o v e r long periods of time t h e soil pH stabilizes

at

a constant level.

Y e t

t h e neutralization capacity is being con- sumed and t h e soil is s u b j e c t t o acidification.

2.1. Acid Stress

Acid s t r e s s i s defined as t h e input of hydrogen ions (protons) into t h e top soil. Acid

stress

c a n r e s u l t from acidic deposition of a i r pollutants, from biomass utilization, and from t h e n a t u r a l biological activity of ecosys-

t e m s

(Ulrich, 1983a; Van Breemen

et

al., 1984). Any one of t h e s e s o u r c e s can dominate t h e flux of protons entering t h e soil. The acid

stress

due t o a i r pollution can r e s u l t from t h e d i r e c t deposition of hydrogen ions o r from t h e indirect e f f e c t of acid producing substances such as t h e d r y deposition of sulfur compounds.

Acid

stress

h a s two important aspects. One i s t h e accumulating amount of

stress

and t h e o t h e r i s t h e instantaneous r a t e of t h e stress. The v a r i a b l e a m o u n t of s t r e s s r e f e r s t o t h e load, and involves accumulation o v e r s e v e r a l y e a r s . The unit f o r t h e amount of

stress

is kilomoles of acidity p e r h e c t a r e (kmol h a -I). The v a r i a b l e s t r e s s r a t e r e f e r s , in principle, t o t h e time derivative of t h e amount of stress although in p r a c t i c e i t is given a s annual hydrogen ion input. The unit f o r t h e stress rate i s kilomoles of aci- dity p e r h e c t a r e and y e a r (kmol h a y r -I).

2.2. Buffering Processes

Soil r e a c t s t o t h e acid

stress

depending on t h e soil p r o p e r t i e s . Acid stress implies t h e flux of hydrogen ions into t h e soil, and in t h e correspond- ing way t h e b u m r i n g p r o p e r t i e s of the s o i l imply t h e consumption of hydrogen ions within t h e soil profile. Buffering i s described using two vari- ables, one f o r t h e g r o s s potential and t h e o t h e r f o r t h e rate of t h e r e a c - tion. Both v a r i a b l e s r e f e r t o t h e intrinsic p r o p e r t i e s of t h e soil. They can b e quantified f o r any volume of t h e reacting soil.

& n e r c a p a c i t y , t h e g r o s s potential, i s t h e t o t a l r e s e r v o i r of t h e buffering compounds in t h e soil. The unit f o r t h e buffer capacity i s t h e same as t h a t f o r t h e amount of acid

stress

(kmol ha-').

& n e r r a t e , t h e rate variable, i s defined

as

t h e maximum potential r a t e of t h e r e a c t i o n between t h e buffering compounds and t h e hydrogen ions. This v a r i a b l e i s needed because t h e r e a c t i o n kinetics i s sometimes of importance. Although t h e buffer capacity i s high, t h e r a t e sometimes limits hydrogen ion consumption. The buffer rate i s e x p r e s s e d in units which

are

comparable t o those of t h e

stress

rate (kmol h a y r -I).

The proton consumption r e a c t i o n s in soils h a v e been systematically described by Ulrich (1981, 1983b). A consecutive s e r i e s of chemical r e a c - tions h a s been documented in soils in which t h e acidification proceeds.

Information r e g a r d i n g t h e dominant r e a c t i o n s h a s been used f o r defining

c a t e g o r i e s , called b u f f e r r a n g e s . They are briefly d e s c r i b e d in t h e follow- ing p a r a g r a p h s and summarized in Table 1. The name of e a c h b u f f e r r a n g e r e f e r s t o t h e dominant b u f f e r r e a c t i o n and t h e t y p i c a l pH r a n g e s given refer t o t h e pH of a soil/water suspension (pH(H20)).

Table 1 : Classification of t h e acid buffering r e a c t i o n s in f o r e s t soils (Ulrich, 1981,1983b)

I I

I

PH Base

1

1 I

r a n g e

)

s a t u r a t i o n

,

Buffer r e a c t i o n

I ! I I

1 Carbonate

/

^{8.0-6.2 1.00}

¹

^CaC03

⁺

^HzC03

^{-> ca2+ +}

^2HC03

¹

/

^Cation

/

^{5.0-4.2 0.05-0.70}

ⁱ

clay mineral=Ca

+

2~ +

-> i

I

exchange I

I

I I

1 ^I

H-clay mineral-H

+

c a 2 + I

I

1

Silicate

1

^6.2-5.0

i !

1 I 1

I

I i

1

^{Aluminum 4.2-3.0 1 0.00-0.05}

¹

^AlOOH

⁺

^{3 ~ '}

^->

^{~ 1}

⁺

^{~2H20 '}

¹

I

I

^{- !}

/

^Iron

1

^{<3.8 i 0.00} F e O O H + 3 H t - > F e 3 + + 2 H $

1

0.70-1.00

C a r b o n a t e b u m r r a n g e

CaAl2si2O8

+

2H2C03

+

H20

->

Soils containing CaC03 in t h e i r fine e a r t h f r a c t i o n ( c a l c a r e o u s soils) are classified into t h e c a r b o n a t e b u f f e r r a n g e (pH r 6.2).

caZ+

is t h e dom- inant cation in t h e soil solution and in t h e exchange s u r f a c e s of t h e soil p a r t i c l e s . The b u f f e r c a p a c i t y of soils in t h i s r a n g e is p r o p o r t i o n a l t o t h e amount of CaC03 in t h e soil. In case CaC03 i s evenly distributed in t h e soil, t h e b u f f e r rate, i.e. t h e dissolution

rate

of CaC03, i s high enough t o b u f f e r any o c c u r r i n g rate of acid

stress.

S i t i c a t e b u . r r a n g e

If t h e r e i s no CaC03 in t h e fine e a r t h f r a c t i o n and t h e c a r b o n i c acid is t h e only acid being produced in t h e soil, t h e soil i s classified into t h e sili- c a t e b u f f e r r a n g e (6.2

>

^pH r 5.0). In t h i s r a n g e t h e only buffer p r o c e s s acting in t h e soils i s t h e weathering of silicates and t h e associated release of b a s e cations, s i n c e t h e dissolution of aluminum compounds d o e s not

start

in significant amounts until

at

pH less t h a n 5.0. The b u f f e r rate i s often quite low. The b u f f e r c a p a c i t y , in t u r n , i s high as i t is formed by t h e mas- sive s t o r a g e of t h e s i l i c a t e material. The weathering of s i l i c a t e s o c c u r s throughout all b u f f e r r a n g e s . The switch t o lower b u f f e r r a n g e s implies t h a t

t h e weathering r a t e of silicates i s not sufficient t o buffer t h e acid stress completely.

a t i o n exchange range

The soils are classified into t h e cation exchange buffer r a n g e when t h e cation exchange r e a c t i o n s play t h e major r o l e in t h e acid buffering. This implies t h a t t h e silicate buffer r a n g e is not capable of buffering t h e acid stress completely. The e x c e s s s t r e s s , not buffered by t h e r e a c t i o n s of t h e silicate buffer r a n g e , i s adsorbed in form of

H+-

o r Al-ions

at

t h e exchange s i t e s , t h u s displacing t h e b a s e cations. The cation exchange r e a c t i o n s a r e f a s t and, t h e r e f o r e , t h e buffer rate of soils in t h i s r a n g e effectively coun- t e r a c t s any occurring

rates

of t h e acid

stress.

The t o t a l b u f f e r capacity (=

cation exchange capacity, CECht) i s generally r a t h e r low depending mainly on t h e soil t e x t u r e . The remaining buffer capacity

at

any given time is quantified by base saturation, t h e p e r c e n t a g e of b a s e cations of t h e t o t a l CEC. A s long as t h e b a s e saturation s t a y s above 5-10 p e r c e n t , t h e e x c e s s stress i s buffered by t h e cation exchange r e a c t i o n s and t h e soil pH t a k e s a value between 5.0 and 4.2, t h e actual value depending on t h e b a s e s a t u r a - tion.

ALuminum and iron bumr ranges

Below t h e c r i t i c a l value of t h e b a s e saturation soils a r e classified into t h e aluminum buffer r a n g e . Hydrogen ions a r e consumed in releasing alumi- num mainly from clay minerals. These r e a c t i o n s merely change t h e form of acidity from hydrogen ions t o ~ 1 ~ ' . The l e a c h a t e t h u s h a s a potential of aci- difying t h e a d j a c e n t ecosystems. High aluminum ion concentrations c h a r a c - t e r i z e t h e soil solution and may cause toxic e f f e c t s t o b a c t e r i a and plant roots.

Aluminum compounds a r e abundant in soils, s o t h a t t h e buffer capacity hardly e v e r r e s t r i c t s t h e reaction. The soil pH is determined by t h e equili- brium with solid p h a s e s of aluminum compounds. A s long as t h e soil pH s t a y s within t h e r a n g e 4.2-3.8, t h e soil i s classified into t h e aluminum buffer r a n g e .

A t t h e extreme s t a g e of acidification (pH

<

3.8) soil may b e classified into t h e iron buffer r a n g e . Increasing solubility of iron oxides i s observed.

This leads t o visible (colour) symptoms in t h e soil profile, which is not t h e c a s e f o r aluminum, although in quantitative

terms

aluminum may still a c t as a dominant buffer compound. The pH values

as

low as 3.8 indicate toxicity and nutrient deficiency t o living organisms.

3. Model Development

3.1. Basic Assumptions

The requirement of a l a r g e spatial s c a l e necessitates s e v e r a l simplifi- cations in t h e model. The assumptions affecting t h e model s t r u c t u r e itself are briefly described h e r e , whereas t h e additional assumptions included in t h e model application

at

i t s p r e s e n t s t a g e a r e discussed in a subsequent c h a p t e r .

The soil i s considered as a homogeneous box. I t is, however, possible t o divide t h e soil into s e v e r a l l a y e r s if i t i s considered important when estimating t h e e f f e c t s of soil acidification. In f a c t , t h i s h a s a l r e a d y been done in connection with t h e IIASA s u r f a c e

water

acidification model (Kamari

et

al., 1984). In t h a t case two l a y e r s were introduced.

The ion exchange and buffering p r o p e r t i e s of o r g a n i c m a t t e r are not taken into account s e p a r a t e l y from t h e inorganic b u f f e r systems. The infor- mation about t h e humus content of t h e soil o r t h e thickness of t h e moor l a y e r i s not commonly available from different p a r t s of Europe. A t least in Northern Europe, where t h e accumulation of o r g a n i c m a t t e r i s significant, i t would b e important t o t a k e t h e buffering p r o p e r t i e s of o r g a n i c m a t t e r into account.

The model w a s designed t o focus on t h e year-to-year changes of soil acidity. Seasonal, monthly o r even daily p a t t e r n s of soil acidity are poten- tially v e r y important as t h e y may effectively a c t

as

key situations t r i g g e r - ing biological effects. Our model d e s c r i b e s t h e annual baseline level instead of t h e s h o r t

t e r m

p e a k s of low o r high acidity. In t h i s way i t does not d i r e c t l y focus on t h e potentially c r u c i a l events but i t estimates t r e n d s of increasing probabilities of such events. This r e s t r i c t i o n of focus made i t possible t o omit r e d o x p r o c e s s e s and sulphate adsorption p r o c e s s e s from t h e model. I t w a s assumed t h a t t h e s e p r o c e s s e s g e n e r a t e seasonal variabil- ity in soil acidity which levels out in the long r u n without affecting t h e year-to-year t r e n d .

The weathering

rate

of silicates and t h e connected release of b a s e cations i s assumed independent from t h e pH of t h e soil. In some l a b o r a t o r y experiments i t h a s been shown t h a t t h e release of silicates i n c r e a s e s with decreasing pH (e.g. Wollast, 1967; Busenberg and Clemency, 1975; Stumm

et

al., 1983). However, t h e r e l e a s e of silica does not necessarily imply t h a t b a s e cations a r e released

at

t h e same r a t e . They may p r e c i p i t a t e with aluminum compounds t o form clay minerals. Increased b a s e cation leaching i s usually due t o cation exchange r e a c t i o n s , not necessarily t o increased weathering

rate.

In Solling (FRG), no deviation in t h e weathering

rate

of sil- i c a t e s from t h e long term a v e r a g e has been observed, although t h e pH of t h e soil h a s d e c r e a s e d (Matzner, unpublished).

9.2. Model structure

The model d e s c r i b e s soil acidification in terms of t h e sequence of t h e buffer ranges. The model compares (i) t h e amount of

stress

(cumulative value o v e r t h e time period of i n t e r e s t ) t o t h e buffer capacity, and (ii) t h e s t r e s s r a t e (year-to-year basis) t o t h e buffer r a t e . The comparisons are made s e p a r a t e l y f o r t h e c a r b o n a t e , silicate and cation exchange buffer ranges. The model t h u s assumes, t h a t values f o r t h e buffering v a r i a b l e s

-

buffer capacity and b u f f e r r a t e

-

a r e determined s e p a r a t e l y f o r e a c h of t h e s e buffer ranges.

A l l t h e buffering v a r i a b l e s do not have t o b e considered in t h e model.

The buffer

rates

of t h e c a r b o n a t e r a n g e and t h e cation exchange r a n g e a r e s o high t h a t in p r a c t i c e t h e y c a n not b e exceeded by any o c c u r r i n g

rate

of acid s t r e s s . Moreover, t h e b u f f e r capacities of silicate and aluminum r a n g e s c a n not b e exhausted in t h e time s c a l e of hundreds of y e a r s . For t h e aluminum and iron r a n g e s , a n equilibrium a p p r o a c h w a s chosen. The soil pH

i s assumed t o s t a y in equilibrium with solid phases of aluminum compounds.

Thus, a b u f f e r r a t e i s not needed. The iron r a n g e i s a l s o assumed t o b e quan- titatively i r r e l e v a n t f o r buffering

at

pH-values above 3.0. In t h i s way t h e number of buffering v a r i a b l e s actually included into t h e model r e d u c e s t o t h r e e : buffer capacity of t h e c a r b o n a t e r a n g e (BC&), b u f f e r r a t e of t h e silicate r a n g e ( b r a ) and buffer capacity of t h e cation exchange r a n g e (BCCE )

The model i s used by taking t h e given p a t t e r n of acid s t r e s s as t h e input variable. The program compares t h e (annual) acid s t r e s s t o t h e buffer r a t e determined f o r t h e prevailing buffer range. I t a l s o compares t h e accumulated amount of acid s t r e s s t o t h e buffer capacity. With t h e s e comparisons t h e program calculates which buffer r a n g e prevails e a c h y e a r , and then computes t h e approximation of t h e prevailing soil pH.

Acid stress t o t h e t o p soil i s p a r t l y o r as a whole neutralized by t h e weathering of c a r b o n a t e o r silicate minerals. I t i s assumed t h a t soils con- taining f r e e c a r b o n a t e s (calcareous soils) always have a b u f f e r

rate

high enough t o neutralize any

rate

of acid stress. In t h i s c a s e t h e soil pH i s assumed t o s t a y

at

6.2

as

long as t h e buffer capacity of t h i s r a n g e i s not exhausted. In non-calcareous soils, neutralization depends on t h e intensity of silicate weathering (silicate buffer r a t e ) . A s long as this b u f f e r r a t e i s l a r g e r t h a n t h e acid stress no d e c r e a s e in soil pH i s assumed t o o c c u r .

If t h e acid

stress

e x c e e d s t h e a c t u a l b u f f e r r a t e of t h e silicates, t h e soil s h i f t s into t h e cation exchange buffer range. Then t h e hydrogen ions gradually r e p l a c e t h e b a s e cations on t h e exchange s i t e s of t h e soil parti- c l e s thus decreasing t h e b a s e saturation of t h e soil. The capacity of t h e cation exchange b u f f e r system i s depleted with a rate t h a t equals t h e differ- e n c e between t h e acid s t r e s s rate and t h e b u f f e r r a t e of silicates. This has t o d o with t h e equilibrium between-the ions a t t a c h e d t o t h e soil p a r t i c l e s and those dissolved in t h e soil solution. The g r a d u a l c h a r a c t e r

was

intro- duced also f o r t h e r e c o v e r y . The soil pH i s t h e n estimated o n t h e basis of t h e prevailing b a s e saturation within t h e cation exchange r a n g e and t h e u p p e r aluminum r a n g e

at

pH from 5.6 t o 4.0. If t h e cation exchange capa- city i s totally exhausted t h e hydrogen ion concentration i s assumed t o b e determined by equilibrium with solid phases aluminum which implies dissolu- tion o r precipitation of aluminum until a n equilibrium s t a t e i s r e a c h e d . The specific equations i n c o r p o r a t e d into this model s t r u c t u r e are p r e s e n t e d in Appendix. The main c h a r a c t e r i s t i c s of t h e model are summarized in t h e flow c h a r t (Figure 1).

3.3. Model Demonstration

The dynamic f e a t u r e s of t h e mode!

are

demonstrated in this section by producing two input-output p a t t e r n s . These f i g u r e s d e s c r i b e t h e r e a c t i o n s of only one soil type, Dystric Cambisol (Bd). Table 2 indicates t h e c h a r a c - t e r i s t i c s of t h i s soil t y p e assumed t o p r e v a i l in t h e beginning of t h e 100 y e a r study period. When fixing t h e s e values t h e r e a c t i n g soil l a y e r w a s assumed t o b e 50 cm. BC& being z e r o indicates t h a t Dystric Cambisol i s f r e e of lime. The input f o r this model demonstration consists of two hypothetical time p a t t e r n s of t h e acid

stress

f o r t h e period of 100 y e a r s . The output is t h e time p a t t e r n of t h e soil pH, corresponding t o t h e mean

hydrogen ion concentration of t h e soil l a y e r of 50 c m .

Table 2: Initial conditions and p a r a m e t e r values f o r model demonstra- tion (Soil type: Dystric Cambisol, Bd). Soil thickness of 50 c m is assumed.

Carbonate b u f f e r capacity Bc& 0.0 kmol ha-I

I

!

Silicate b u f f e r

rate bra

^{1.0 kmol} ha -'yr

) I

Total cation exchange capacity CECt,, 1105.0 kmol ha-' 1

1

Base saturation

I fi 0.15

!

!

1

Volumetric water content

at Of

0.27

(

field capacity

I

i

j Precipitation; Central E u r o p e P 0.90 m yr-I

I I

Evapotranspiration; Central E u r o p e E 0.50 m yr-I

i

Figure 2 indicates t h a t f o r this soil t h e soil pH is gradually declining from 4.6 down t o 4.0 in 100 y e a r s when t h e soil is s u b j e c t t o a growing stress from 1 t o 8 kmol ha - l y r - I . The silicate b u f f e r r a n g e accounts f o r t h e buffering of 1 kmol ha yr of t h e acid s t r e s s . The e x c e s s stress is buffered by t h e p r o c e s s e s of t h e cation exchange range. The buffering within t h e silicate b u f f e r r a n g e , essentially due t o t h e weathering of t h e sil- i c a t e mineral, i s acting through a l l t h e buffer ranges. After 60 y e a r s t h e b u f f e r capacity of t h e cation exchange r a n g e is d e c r e a s e d t o a b a s e s a t u r a - tion level of 5%. A t t h i s point, none of t h e h i g h e r b u f f e r r a n g e s is capable of buffering t h e s t r e s s , and t h e soil pH declines t o t h e level which c o r r e s p o n d s t o t h e pH r a n g e of t h e aluminum b u f f e r system. The acid s t r e s s , p a r t l y buffered within t h e silicate b u f f e r r a n g e , finally determines t h e new equilibrium pH in t h e soil solution according t o t h e aluminum solu- bility assumed. This p r o c e s s r e s u l t s in a slowly decreasing soil pH due t o t h e growing

stress

r a t e .

A dramatic p a t t e r n of t h e acid stress w a s selected t o summarize t h e dynamic behavior of t h e model (Figure 3). The p a t t e r n includes a constant s t r e s s of 8 kmol ha -l yr f o r 30 y e a r s , a l i n e a r decline t o z e r o in t h e sub- sequent 40 y e a r s , and a constant z e r o stress o v e r t h e remaining 30 y e a r s .

Figure 1 . Flow diagram of t h e soil acidification model

The soil with initial conditions as in Table 2 r e a c t s in t h e following way:

First, t h e r e is a gradual but accelerating decline in pH from 4.6

to

4.2.

Next, t h e r e is a r a p i d decline of pH n e a r t o t h e pH value 3.7. The buffer capacity of t h e cation exchange r a n g e is exhausted and t h e buffer r a t e of t h e aluminum r a n g e cannot keep t h e pace with t h e acid

stress rate.

Next, t h e r e is a n increase of t h e soil pH t o 4.0. A t t h a t point t h e acid

stress

has declined s o t h a t t h e joint buffering of t h e silicate and t h e aluminum range i s capable of increasing t h e pH. Finally, a r e c o v e r y

starts

from pH 4.0 upwards. This is possible because t h e acid

stress

declines t o levels where t h e silicate buffer

rate

i s sufficient f o r buffering t h e

stress

alone. During t h e gradual r e c o v e r y in t h e soil, weathering slowly r e p l a c e s hydrogen ions from t h e cation exchange sites. The cation exchange capacity i s refilled, s t a r t i n g

at

pH 4.0, with a

rate

equal t o t h e difference of t h e buffer

rate

of silicate buffer r a n g e and t h e

rate

of acid stress. A b a s e saturation level of 4% will b e r e a c h e d by t h e end of t h e 100 y e a r period.

Figure 2. Input-output relationship: r e s p o n s e of t h e soil t o an increasing stress

4. Model Application

This application i s p a r t of t h e IIASA Acid Rain P r o j e c t which h a s t h e general objective of analyzing a l t e r n a t i v e c o n t r o l s t r a t e g i e s of t h e Euro- pean sulfur emissions. The focus of t h e application i s h e n c e r e s t r i c t e d t o t h e s t r e s s due t o a i r pollution. The IIASA framework

sets

t h e p r e r e q u i s i t e of a l a r g e spatial scale. The p r o j e c t h a s provided a n energy-emission model f o r generating s c e n a r i o s of f u t u r e sulfur emissions in Europe assuming optional programs f o r e n e r g y development and sulfur control (Alcamo

et

al.

1984). The computed emissions a r e converted into sulfur deposition s c e n a r i o s by using t h e long-range t r a n s p o r t model f o r air pollutants developed within t h e EMEP-program (see Eliassen and Saltbones, 1983). This mode! h a s been applied

at

IIASA by reducing i t t o a s o u r c e r e c e p t o r matrix (Alcamo e t al. 1984). Sulfur deposition i s then transformed into a n approxi- mation of t h e acid stress, and t h i s information i s used as t h e driving vari- a b l e of t h e soil acidification model (Figure 4).

4.1. Specific Assumptions

For t h e time being, t h e acid s t r e s s w a s estimated on t h e basis of sulfur deposition only, simply by assuming acid s t r e s s t o b e proportional t o sulfate ion equivalents in t h e

water

e n t e r i n g t h e soil. The a c t u a l acid stress associ- a t e d with sulfur deposition depends on t h e neutralization intensity of, e.g.

atmospheric dust and canopy. The s p a t i a l variation of t h e s e p r o c e s s e s w a s

Figure 3. Input-output relationship: r e s p o n s e of t h e s o i l t o a declining stress

n o t t a k e n i n t o account. A single r e l a t i o n s h i p w a s assumed as t h e f i r s t s t e p f o r t h e whole of E u r o p e . ~ n t e r n a l p r o t o n production, i.e. p r o t o n production r e s u l t i n g from t h e e x c e s s accumulation of c a t i o n s t o t h e biomass and humus w a s n o t included in t h e e s t i m a t e s of a c i d stress.

The EMEP model assumes c o n s t a n t deposition velocity o v e r all land s u r - f a c e s (Eliassen & S a l t b o n e s , 1983). This assumption i s n e c e s s a r y as t h e model c o v e r s t h e whole of E u r o p e ; i t would b e a n enormous t a s k t o d e s c r i b e t h e s p a t i a l v a r i a b i l i t y of t h e deposition velocity in detail. Model validation s u g g e s t s t h a t , in g e n e r a l , t h e assumption of c o n s t a n t deposition velocity c a n b e s u p p o r t e d when aiming

at

modeling t h e c o n c e n t r a t i o n s of s u l f u r

com-

pounds on a l a r g e s p a t i a l scale. From local e x p e r i m e n t s i t a p p e a r s , how- e v e r , t h a t f o r e s t s h a v e a r a t h e r s t r o n g f i l t e r i n g e f f e c t o n air pollutants, s o t h a t t h e deposition velocity o v e r f o r e s t s i s l a r g e r t h a n t h a t of o p e n land by a f a c t o r of two t o t h r e e , depending on t h e tree s p e c i e s . We believe t h a t t h e a p p a r e n t c o n t r o v e r s y between model validations and local e x p e r i m e n t s c a n b e explained by assuming t h a t within a n y of t h e l a r g e g r i d s q u a r e s t h e a v e r - a g e deposition velocity i s t h e same as t h a t s e l e c t e d f o r t h e EMEP model. In t h i s way t h e EMEP model p r o d u c e s q u i t e s a t i s f a c t o r y r e s u l t s as f a r as t h e variability between t h e g r i d s q u a r e s i s c o n c e r n e d . In l o c a l scale within t h e g r i d s q u a r e , however, i t u n d e r e s t i m a t e s t h e deposition o n f o r e s t land. A s f o r e s t s were t h e main t a r g e t ecosystems f o r o u r model i t w a s considered

t Energy

-

Emissions

I

Suttur emissions

4

Atmospheric Rocsow

Environmntil omput

Form roil pH

Figure 4. The IIASA acid r a i n framework and p r o c e d u r e f o r using t h e model

necessary t o include t h e filtering effect into t h e model.

Based on t h e validation experiments of t h e

EMEP

model t h e a v e r a g e total deposition of a grid s q u a r e , d t o t , was assumed c o r r e c t . The deposition on t h e f o r e s t within t h i s g r i d , d f , w a s then assumed t o b e q times l a r g e r than t h e deposition on open land, d o

Since

P d f + (1

-P

)do

=

titot

where

p

is t h e f r a c t i o n of f o r e s t within t h e grid, w e g e t f o r d f d f

=

dtot (1 + (cp-1)P)

from which acid s t r e s s , as, w a s derived.

QS

=

0 - d f

The f a c t o r a implies t h a t p a r t of t h e sulfur deposition i s neutralized b e f o r e i t e n t e r s t h e soil. This holds especially f o r d r y deposition, which may b e neutralized by dust, canopy, e t c . The above calculation p r o c e d u r e t a k e s into account (i) t h e estimated g r o s s deposition on e a c h g r i d s q u a r e , (ii) t h e filtering f a c t o r q , (iii) t h e f r a c t i o n of f o r e s t s in e a c h g r i d s q u a r e ,

2 ,

digi- talized from t h e World F o r e s t r y Atlas (Weltforstatlas, 1975), and (iv) t h e acid

stress

f a c t o r , a. A s a n output i t produces t h e allocation of deposition between f o r e s t s and t h e a g r i c u l t u r a l land within e a c h g r i d s q u a r e . This specific f e a t u r e of t h e IIASA model gives t h e f i r s t p r i o r i t y t o t h e long r a n g e t r a n s p o r t model as f a r

as

l a r g e s c a l e variability of deposition i s concerned and y e t d e s c r i b e s t h e filtering e f f e c t of f o r e s t s by including small s c a l e information on t h e distribution of f o r e s t s vs. open land within t h e g r i d square. A f a c t o r q

=

2 i s used as long as detailed information on t h e s p a t i a l distribution of q i s not available. The acid

stress

coefficient, a, seems t o have values between 0.5 and 0.75 in some European f o r e s t s (e.g. Matzner, 1983; Wright and Johannessen, 1980). The value a

=

^{2 / 3} w a s chosen

as

a tentative approximation.

I t is conceivable t h a t f o r e s t s , as they r e p r e s e n t a r e a s neglected by a g r i c u l t u r e , grow on p a r t i c u l a r l y susceptible soils. Soils which have low specific weathering r a t e s and low levels of b a s e s a t u r a t i o n a r e more sus- ceptible t o acidification t h a n soils on t h e a v e r a g e . The concentration of f o r e s t s on p o o r soils, although hypothetical, w a s considered s o obvious t h a t i t w a s included as p a r t of t h e model. R a t h e r t h a n assuming t h e f r a c t i o n of f o r e s t s constant on all soil t y p e s we used t h e following calculation pro- cedure: F o r e s t s of

a

given g r i d s q u a r e were allocated s t a r t i n g from soil t y p e s with t h e lowest weathering r a t e s and cation exchange capacity values and continuing until all f o r e s t s were distributed. In t h i s way a g r i c u l t u r e w a s located on t h e

most

f e r t i l e soils whereas p o o r soils of

a

g r i d were assumed f o r f o r e s t s .

In t h e presentation of r e s u l t s a n important indicator i s t h e "critical acidity". A t p r e s e n t t h e switch t o aluminium buffer r a n g e (base saturation 0.05, pH 4.2) i s assumed t o imply a n increased r i s k f o r f o r e s t damage. T h e r e a r e s e v e r a l r e a s o n s why t h i s d e g r e e of acidity w a s assumed t o b e critical:

soil chemistry changes quite drastically; Al-concentration in t h e soil solu- tion i n c r e a s e s and Ca/Al-ratio r e a c h e s t h e level t h a t implies t h e r i s k of soil borne toxicity t o t r e e r o o t s (Matzner and Ulrich, 1984; Ulrich

et

al. 1984).

More r e s e a r c h , however, would b e needed f o r relating t h e r i s k of

forest

damage t o t h e soil acidity. The final decision about t h e ' c r i t i c a l pH' is l e f t t o t h e model u s e r .

4.2. Initialization of Buffering Variables

Initialization of t h e soil v a r i a b l e s

w a s

based on t h e chemistry informa- tion available on European soils, and on t h e soil thickness selected

to

approximate t h e rooting zone. The b u f f e r capacity of t h e c a r b o n a t e r a n g e i s proportional t o t h e lime content of t h e soil; t h e b u f f e r

rate

of t h e silicate r a n g e is r e l a t e d t o t h e chemical weathering r a t e of t h e silicate minerals;

t h e b u f f e r capacity of t h e cation exchange rate depends on t h e clay content and on t h e b a s e s a t u r a t i o n of t h e soil; and t h e b u f f e r rate of t h e aluminum r a n g e depends on t h e accessability of aluminum compounds. Although such relationships, especially t h o s e regarding t h e aluminum accessability a r e

only partially understood, they can be used as a guideline in quantifying t h e susceptibility of t h e soils t o acidification. The values f o r t h e buffer capaci- t i e s and buffer

rates

were initialized accordingly based on t h e International Geological Map of Europe and t h e Mediterranean Region (1972) and t h e FXO-UNESCOSoil Map of t h e World (1974). The depth of t h e reacting soil w a s assumed 50 cm throughout t h e study area. The y e a r 1960 w a s selected as being t h e baseline y e a r .

All information regarding soils w a s stored into a computerized grid- based format. Each grid s q u a r e had t h e extension of 1 d e g r e e longitude times 0.5 d e g r e e s latitude. In this way t h e size of a grid w a s fixed

at

56 km in t h e south-north direction, but in t h e

east-west

direction i t varied from 9 1 km t o 38 km depending on t h e latitude. The number of t h e grid squares w a s 2304.

Detailed soil chemistry information regarding t h e o t h e r soil variables w a s available from t h e Soil Map. The fraction of each soil type within t h e grid s q u a r e w a s computerized with

an

accuracy of 5 p e r c e n t units. The resolution of t h e map is such t h a t t h e standard grid s q u a r e w a s composed of 1-7 soil types. The number of different soil types w a s 80. The soil d a t a base consists of 5212 soil units, t h e mean number of soil types p e r grid square being 2.2. One 70 y e a r simulation f o r t h e whole of Europe r e q u i r e s then about 365,000 mode! runs.

Initial values f o r t h e soil variables were given f o r every soil type (Table 3). The Soil Map, however, could not provide t h e information regard- ing t h e buffer

rate

of t h e silicate buffer range which is equal t o t h e weath- ering r a t e of t h e p a r e n t material. The approximation of this variable w a s based on o t h e r sources. Ulrich (1983b) r e p o r t s a r a n g e of variation in European soils from 0.2 t o 2.0 kmol ha yr m ^-I. Four classes f o r t h e reacting 50 cm soil l a y e r were introduced with t h e following buffer r a t e s (in kmol ha yr -I) :

The Geological Map was used t o determine p a r e n t materials of soils in each grid square. Depending on t h e dominant parent material t h e soil of each grid square w a s classified into one of t h e above categories.

Based on this information t h e model is applicable f o r producing acidifi- cation scenarios f o r f o r e s t soils. The model is r u n separately f o r each soil type within each grid square. An estimate of t h e soil pH is produced

as

t h e output.

class

I

, 1 / 2

buffer r a t e 0.25

1

^0.50

4.3. Results of Model R u n s

Two example scenarios were introduced using t h e IIASA energy- emission model, and t h e long range t r a n s p o r t model supplied by t h e EMEP programme. From 1960 until 1980 t h e scenarios were identical. From t h a t on t h e scenarios departed so t h a t t h e 'high' deposition scenario assumed high r a t e s of energy development throughout Europe,

as

defined by t h e ECE 'trends continued' scenario (ECE, 1983) linearly extrapolated t o 2030. The 'low' deposition scenario w a s constructed according t o t h e ECE

3 0.75

4 1.00

Table 3. Buffer c a p a c i t i e s of t h e c a r b o n a t e a n d c a t i o n e x c h a n g e b u f f e r r a n g e s estimated f o r t h e y e a r 1960 f o r soil t y p e s of t h e FAO-UNESCO Soil Map of t h e World (1974). Soil thick- n e s s of 50 cm is assumed.

I I s o i l

1

^BC A0

I

Bd

1

Be

!

Bk

i Cd

I C e

I

Ch

I

^Cm

I

Hc

!

Hg

!

^{J c}

/

^{J e}

Kh

I

kmoL ha

200.0

1

^910.0

;

^{s o i l} 1 _{I BCca}

;

^{B ~ C E I i}

j t y p e

I

¹

1 I kmoL ha

I

1

i

^{Kk 8000.0 1170.0} 1

1 ;

^{3000.0 I 170.7}

ⁱ

0.

1

^138.8

!

Lg 0. , 146.3

I

^Lo

^I

^0.

^I

^107.3

¹

L v

1

^{3000.0 1225.0}

'

Mo

i

^{0. 1495.0 i} j

Od

I

I

^o.

1

^72.0

Oe 0. 1 168.8

1

P g 180.0

I

1 Ph

1 ^{:: 1}

^49.0

¹

1 P1 I 0.

1

^68.3

1

1 i

^{0. 1 78.0 1}

1 Pp

1

^0. j ^239.2

i

1 Qc

1

^100.0 ( 227.5 1

/

Q1 0. 1 117.0

I

Rca 0.

1

^47.3

i

Rcb

I

^0.

1

^136.5 1

1

^{RCC 500.}

Re

1

^0.

::::: ¹

I

^{SO 500.0 . 1183.0 1}

I ^Sm

1

^0.

1

^236.3 !

1

Th 1

i

^0.

I

^127.5 j

! Tm , 0.

1

^136.5 1

!

T o I 0. 183.8 1

I TV i 0.

!

^{120.0 i}

I

u ^I 0. 136.5

I I

:

^{s o i l BCca}

^I

^BCm

,

t y p e I I

1

^Vc

1

VP

I Wd

1 we

'

^{500.0 1410.5}

1

Xk 1 43000.0 1170.0

1

I X y

1

^40000.0

1

^{1225.0 1}

I

z g 15000.0 1225.0

I Bc-LC

I

^{3000.0 685.6}

1

I I-Bc

I

^200.0

1

^1050.0

I-Bc-LC

1

^{1500.0 1 469.1} 1 ¹

,

^I-Bd

_I

^{0. / 151.2 1}

I-Be 0. 1 ^765.6 1

I I-Be-LC

1

^1500.0

ⁱ

^{533.9 1}

I I-Bh-C' I 0. I 136.5 1 i I-C 1 500.0 ( 910.0 I I I-E 1 10000.0 1 1750.0 1

I :-L I 0. 1 149.3 1

I I-LC 1 1500.0 / 153.6 1 1 I-LO-BC I 0. 1 408.5 1 I I-LC-E ¹ 10000.0 1 1500.0 1

1

I-Po 1 0. 1 126.8 1

1 I-Po-Od 1 0. 1 108.5 1

1 I-Rc-ZEk I 20000.0

1

^1500.0

I I-Re-Rx 0. 106.8 1

I !-c

1

^{0. 136.5 1}

* Lo-LC 1 1500.0

)

^139.1

I

kmoL ha I

32000.0 1170.0 9000.0 3640.0 0.

1

^47.3

1

'conservation' s c e n a r i o , assuming lower

rates

of e n e r g y

use

and, in addition

to

t h a t , effective measures t a k e n f o r t h e c o n t r o l of t h e sulfur emissions (Figure 5). The specific method of generating d i f f e r e n t s c e n a r i o s i s p r e s e n t e d by Alcamo

et

al. (1984).

The model c a n b e used f o r producing estimates of t h e time p a t t e r n s of t h e t o t a l f o r e s t

area

with soils below

a

selected c r i t i c a l pH f o r any s c e n a r i o (Figure 6). The

area

of t h e f o r e s t in e a c h g r i d s q u a r e i s calculated and t h e

t i m e

evolution of t h e

area

of European f o r e s t s with soil pH below

a

selected c r i t i c a l value i s t h e n displayed. Another option is

to

display t h e

areas

^with soils below

a

c r i t i c a l pH f o r

a

selected y e a r in

a

map format. Different shadings indicate t h e p e r c e n t a g e of t h e total f o r e s t

area

with soil pH below t h e s e l e c t e d value ( s e e Figures 7 a n d 8a, b).

As p a r t of t h e IIASA study t h i s application of t h e soil acidification m o d e l i s designed f o r quick comparisons of sulfur emission scenarios. I t is up

to

t h e model u s e r t o decide what kind of s c e n a r i o s should b e compared.

The two examples

were

s e l e c t e d

to

demonstrate t h e model behaviour. There- f o r e , t h e examples

are

relatively

useless as

f a r as selecting feasible policy options i s concerned. The following p a r a g r a p h s discuss t h e e f f e c t s of t h e 'low' vs. t h e 'high' s c e n a r i o but t h i s discussion i s intended merely t o demon-

strate

t h e p r o p e r t i e s of t h e model.

By t h e y e a r 1980, t h a t is, assuming t h e historical deposition p a t t e r n , t h e m o d e l p r e d i c t s a decline in t h e f o r e s t soil pH in relatively l a r g e regions of Central E u r o p e (Figure 7). Continuing with t h e 'high' deposition s c e n a r i o t h e

area

of low pH substantially e n l a r g e s by t h e y e a r 2010 a n d much of t h e soils in Central Europe and S o u t h e r n Scandinavia r e a c h t h e aluminum buffer r a n g e (Figure 8a). When t h e 'low' s c e n a r i o is used as t h e input, t h e r e s u l t s indicate much less r i s k of f o r e s t damage by t h e y e a r 2010. As indicated by Figure 8 b t h e f o r e s t

area

with more acidic soils than t h e threshold is estimated two times l a r g e r with t h e 'high' s c e n a r i o t h a n with t h e 'low' scenario.

5. Discussion

The model developed in t h i s study c a n b e used f o r quantifying some

aspects

of t h e acidification problem of f o r e s t soils which have e a r l i e r been discussed using qualitative

terms.

The soil acidification model and t h e appli- cation

to

t h e European overview

are

simplifications. which necessarily include uncertainties. Many solutions,

as

t h e y stand now,

are

c r u d e approx- imations which need clarification in f u t u r e r e s e a r c h . I t is t h e hope of t h e a u t h o r s , however, t h a t t h e model s t r u c t u r e would

act as a tool

f o r organiz- ing t h e data a n d f o r identifying r e s e a r c h needs. Even in i t s p r e s e n t s t a g e

REFERENCE YERRS

Figure 5. Total sulfur emitted in Europe according t o the 'high' and 'lowe emission scenario from coal and oil s e c t o r s

T I M E [ Y E A R S 1

Figure 6 . Time evolution of the total forest area with soils in aluminum and iron buffer ranges (pH less than 4.2) in Europe assuming the two emission scenarios

Figure 7. Model estimates of f o r e s t soils below pH 4.2 in 1980. The shading determines the fraction of f o r e s t soils below the threshold pH in each grid.

m

^{5 % -}

B 3 0 % -

1

7 80 OF FOREST UITH Pn

-

-8 0 8 16 2 A 32 A 0

78 5 % - 3 0 %

m 3 0 %

-

^{80 %}

I

~ 8 0 % OF FOREST ARER

65 UITH PH

-

^4.2

60

55

(b) 50

A5

48

35

Figure 8 . A comparison of the area of risk in 2010, (pH

<

4.2), result- ing from the high emission scenario (a) and from the low emission scenario (b)

.

t h e model might a p p e a r useful in evaluating policies t o combat t h e acidifica- tion of f o r e s t soils.

The model makes a distinction between reversible and i r r e v e r s i b l e changes in t h e soil chemistry. Exhaustion of t h e buffer capacity is more o r less irreversible. The case of an insufficient buffer r a t e , in turn, may b e reversible: The buffer r a t e is again sufficient when t h e s t r e s s rate (annual load) is reduced below a threshold; this threshold i s t h e value of t h e buffer r a t e variable. This f e a t u r e of t h e model should b e useful a s i t indicates whether a decrease in t h e acid

stress

would result in a recovery of t h e soil, o r whether i t would merely cause a delay in t h e acidification process.

The model, designed f o r studies on f o r e s t soils, a p p e a r s too complex f o r studies on agricultural soils. Intensive agriculture maintains high pH values in soils by means of liming and o t h e r practices. In theory, t h e model could b e used f o r calculating t h e amount of lime needed t o counteract, f o r example, t h e acidic deposition. This calculation, however, can b e done using more straightforward methods.

. The application of t h e model t o t h e problem of acidic deposition in Europe indicates t h a t soil buffering fails in maintaining adequate pH levels in l a r g e p a r t s of Central Europe. In Northern Europe, although t h e buffer- ing is generally less efficient, t h e acidic deposition would cause less trouble in this respect. This does not prove t h a t t h e problem of soil acidification is r e s t r i c t e d t o Central Europe. Acidification due t o biomass accumulation, i.e. t h e so-called internal proton production, has a special r o l e in Northern Europe where low temperatures r e t a r d biomass decomposition. High inter- nal proton production increases t h e susceptibility of t h e environment t o t h e acidification due t o a i r pollutants. This additional stress needs t o b e addressed in future r e s e a r c h .

The soil variables were initialized f o r 1960. This does not imply t h a t no acid

stress

was assumed before t h a t time. The initialization should b e viewed a s fixing a r e f e r e n c e point r a t h e r than a manifestation of t h e s t a t e of virgin forests. The initialization should b e based on field measurements;

in t h e present application this goal was only partially fulfilled.

The reacting volume was fixed a t t h e top 50 cm of t h e soil. No horizon- tal gradients were explicitly assumed. Including d e e p e r l a y e r s into t h e reactive p a r t of t h e soil would add t o t h e reacting volume and i t would thus postpone t h e possible problem. Including t h e gradients would involve f a s t e r acidification in t h e very top of t h e soil and slower acidification in t h e d e e p e r layers. The above results correspond t o t h e a v e r a g e situation in t h e volume. This a v e r a g e value may b e inaccurate in some cases due t o t h e nonlinearities of t h e model. Moreover, t h e model assumes t h a t all deposi- tion actually r e a c t s within t h e top soil. This may not always b e t h e case. If p a r t of t h e deposition flows unchanged through t h e top soil, t h e soil response will b e delayed and t h e acidification problem is t r a n s f e r r e d into t h e adjacent ecosystems o r t o t h e groundwater. Within t h e IIASA Acid Rain P r o j e c t a regional lake acidification model has been developed, where t h e soil pH model is used f o r describing t h e soil chemistry in t h e catchment.

Soil acidification poses a t h r e a t t o f o r e s t ecosystems and generates predisposing

stress

in ecosystems a s defined by Manion (1981). Forest dam- age, however, is a multicausal phenomenon. Many f a c t o r s a r e involved such a s ozone pollution, heavy metals, exceptional climatic conditions, and

cultivation of

tree

s p e c i e s outside of t h e i r n a t u r a l sites. The interactions of soil acidification and t h e o t h e r f a c t o r s d e s e r v e c o n c e r t e d r e s e a r c h e f f o r t . I t does not

seem

possible today t o d e s c r i b e t h e f o r e s t damage in s a t i s f a c t o r y detail with any specific model. But emphasizing t h e complexity of t h e f o r e s t damage as a n argument against s e r i o u s modeling e f f o r t s may w e l l cause a delay in obtaining a b e t t e r understanding of t h e phenomenon.

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