Nitrogen Mineralization in Soils Under Organic Farming and Conservation Tillage as Compared to Conventional Management
Cao, B.1, Roelcke, M.2, Dauck, H.P.2, Küsters, A.2, Cai, G.X.1 and Nieder, R.2 1 Nanjing Institute of Soil Science, Chinese Academy of Sciences, Beijing East Road 71,
210008 Nanjing, P.R. China. chencao@sohu.com gxcai@issas.ac.cn
2 Institute of Geoecology, Braunschweig Technical University, Langer Kamp 19c, 38106 Braunschweig, Germany. m.roelcke@tu-bs.de r.nieder@tu-bs.de
Summary
The effect of organic farming and conservation tillage practice as compared to conventional management on nitrogen (N) mineralization was investigated in Lower Saxony, Germany. In a farming systems comparison, Ap horizons (0-30 cm) were sampled from two plots under long-term organic and one plot under conventional farming, in each of two different areas, with Cambisols in Pleistocene sands and Luvisols in loess, respectively. In a tillage investiga-tion, topsoils (0-15 and 15-30 cm; Colluvic Luvisol in loess) were sampled from 3 plots with different tillage practices (conventional ploughing, reduced tillage, zero tillage). A long-term aerobic incubation experiment with intermittent leaching was carried out using the method of Stanford and Smith (1972) in the modification of Nordmeyer and Richter (1985). The field-moist topsoils were incubated at 35 °C for a total of 148 days. Mineralization parameters were estimated from the cumulative amounts of leached NO3--N and NH4+-N via non-linear regres-sion. Results showed that there were no significant differences in the amounts of N mineral-ized between plots under organic and conventional farming of the same soil type. However, significant differences in N mineralization rate were observed between soils of the two differ-ent pardiffer-ent materials. Regardless of farming system, total amounts of N mineralized from the sandy soils were 51.31-58.05 µg g-1 while the corresponding figures for the loess were 80.3-118.8 µg g-1. In the tillage experiment, the amounts of N mineralized were almost identical in the 0-15 cm and 15-30 cm increments of the conventionally ploughed soils. In contrast, they were significantly higher in 0-15 cm compared to 15-30 cm under reduced tillage, and even more so in the zero tillage soils. These differences were consistent with the organic C and total N contents in the corresponding depth increment. Apparently, reduced and zero tillage merely resulted in a redistribution of SOM and total N between the 0-15 and 15-30 cm incre-ments, and consequently in different N mineralization, while mean contents over 0-30 cm were unchanged, compared to the conventional ploughing treatment.
1 Introduction
Global climate concerns have refocused attention on different tillage practices as well as or-ganic vs. conventional farming regimes, which are expected to have long-term effects on the sequestration and availability of carbon (C) and nitrogen (N) in soils. In the past few decades organic farming has become increasingly popular in Europe and other parts of the world. In organic farming systems, animal and green manure are used instead of synthetic fertilizers to improve soil productivity and supply plant nutrients. Many studies showed that there were higher amounts of soil organic matter, enzymes and microbial biomass in the soils under or-ganic farming in comparison with those under conventional farming systems (Liebig and
Doran, 1999; Colla et al., 2002; Pulleman et al., 2003). Compared to conventional tillage, various kinds of conservation tillage proved to markedly affect soil fertility and organic matter dynamics (Logan et al., 1991). Decreasing tillage intensity from the conventional tillage (CT) to zero tillage (ZT) generally leads to crop residues remaining on or near the soil surface, resulting in a stratification of organic matter and nutrients within the plough layer compared to an even distribution in CT (Stockfisch et al, 1999; Terbrügge and During, 1999).
For the past 30 years, the N mineralization potential has been widely used as a means to determine the effect of various agricultural practices such as N fertilization, tillage depth, crop rotation, and organic residue amendment on N availability (EI-Haris et al., 1983; Campbell et al, 1991; Wani et al, 1995; Christenson and Butt, 1997). Numerous authors have used the in-cubation procedure according to Stanford and Smith (1972) to investigate soil N mineraliza-tion. In the following years, different mathematical models have been proposed to fit the cu-mulative mineralized N vs. incubation time.
The aim of this study was to determine the effects of organic farming and conservation tillage practices on soil nitrogen mineralization. Specific objectives were i) to compare effects of long-term organic farming versus conventional farming systems in two different soil types on N mineralization, and ii) to determine the effect of reduced tillage (RT) and zero tillage (ZT) on N mineralization in comparison with conventional tillage (CT).
2 Materialand methods 2.1 Sites and soil sampling
Soil samples for incubation were taken from different field plots in south-eastern Lower Saxony, Germany (Tab. 1). In a farming systems investigation, Ap horizons (0-30 cm) were sampled in two different areas, with Cambisols in Pleistocene sands in the old moraine area (Farm I and II) and Luvisols in loess in the loess area (Farm III and IV), respectively. In each of the two areas, two different plots under long-term organic farming, and a nearby plot under conventional farming were sampled on May 07, 2003. Farm I (128 ha) has practiced organic (bio-dynamic) farming since 1952, with a 9-year cereal-legume rotation and a livestock den-sity (dairy cows) of 0.75 gross weight units (gwu) ha-1. Farm II (650 ha farming cooperative) is conventional, with a 4-year sugar beet-cereal rotation and no livestock. Farm III (47 ha) was converted to organic farming (bio-dynamic) in 1985, with an 8-year cereal-legume rota-tion and a livestock density (beef fattening) of 0.3 gwu ha-1. Farm IV is a conventional farm (38 ha) with a 4-year sugar beet-cereal rotation, without livestock. In a tillage investigation, topsoils (0-15 and 15-30 cm; Colluvic Luvisol in loess) were sampled on three plots of a 10-year field experiment (Farm V, conventional, 75 ha, 3-10-year sugar beet-cereal rotation, no live-stock) with different tillage practices (conventional ploughing (CT), reduced tillage (RT), zero tillage (ZT)) on May 12, 2003. On each plot, ten soil cores were randomly sampled with an auger and subsequently bulked. The field-moist soil samples were stored at +4 °C until the onset of the incubation. On the same occasions in May as well as in July 2003 (after crop har-vest), soil for mineral nitrogen determination (Nmin method) was sampled on the same plots; results will be presented elsewhere.
Table 1: Basic information for the sampling sites. Plot no. (Farm I-V) Location Depth increment [cm] Farming system (Farms I-IV) Tillage system (Farm V) Soil type
(Parent material) Crop
1 (Farm I)
Hämelerwald 0-30 Organic (Pleistocene sands) Stagnic Cambisol Winter rye
2 (Farm I)
Hämelerwald 0-30 Organic Cambisol (Pleistocene sands) Alfalfa-grass
3 (Farm II
Hämelerwald 0-30 Conventional Stagnic Cambisol (Pleistocene sands) Winter barley
4 (Farm III)
Berel 0-30 Organic Luvisol (Loess) Peas
5 (Farm III)
Berel 0-30 Organic Stagnic Cambisol (Loess) Peas
6 (Farm IV)
Berel 0-30 Conventional Luvisol (Loess) Winter barley
7 (Farm V)
Adenstedt 0-15 15-30 Conventional tillage Colluvic Luvisol (Loess) Winter wheat
8 (Farm V)
Adenstedt 0-15 15-30 Reduced tillage Colluvic Luvisol (Loess) Winter wheat
9 (Farm V)
Adenstedt 0-15 15-30 Zero tillage Colluvic Luvisol (Loess) Winter wheat
2.2 Incubation experiments
A long-term aerobic incubation experiment with intermittent leaching was carried out using the method of Stanford and Smith (1972) in the modification of Nordmeyer and Richter (1985). 20 g of field-moist soil was sieved <2 mm and mixed with 15 g of washed and oven-burned quartz sand (1-2 mm). The samples were subsequently incubated in quadruplicate in 60-mL plastic syringes at 35 °C for a total of 148 d. Leaching with 4 x 25 mL of 0.01 M CaCl2 was carried out on days 0, 3, 7, 14, 21, 35, 49, 70, 91, 119 and 148. After adding 25 mL N-free nutrient solution, the excess moisture in the samples was removed under a suction of –80 kPa for 1h (Stanford and Smith, 1972). NH4+-N and NO3--N in the leachates were determined by continuous flow analysis (Skalar, NL). Dissolved organic carbon (DOC) and nitrogen (DON) in the leachates was also determined (not presented here).
2.3 Parameter estimation
Mineralization parameters were estimated from the cumulative amounts of leached N (NO3--N plus NH4+-N) (mean values of 4 samples) via non-linear regression, using the following three mineralization models. The one-component first-order exponential model (referred to as sin-gle model thereafter) (Stanford and Smith, 1972) is expressed as:
)] exp( 1 [ ) (t N0 kt N = − − , (1)
where N(t) is the amount of N mineralized at time t [µg g-1], N0 is the initial amount of poten-tially mineralizable N [µg g-1], and k is the first-order rate constant [day-1]. Because different fractions of organic N in soil may be susceptible to different mineralization, the single model was modified to a two-component first-order exponential model (referred to as double model thereafter) (Molina et al., 1980):
)] exp( 1 [ )] exp( 1 [ ) (t N k t N k t N = a − − a + r − − r , (2)
where N(t) is the amount of nitrogen mineralized at time t [µg g-1], Na and Nr are the amounts of organic N initially present in the available and resistant fractions, respectively [µg g-1], and
ka and kr are the first-order rate constants for the two fractions [day-1]. The right-hand side of Equation (2) can be approximated by a mixed first-order and zero-order reaction (referred to as special model thereafter) (Bonde and Rosswall, 1987):
t C t k N t N( )= a[1−exp(− a )]+ r , (3)
with Cr as the quasi-linear decomposition rate of the resistant fraction [µg g-1 day-1]. For the statistics, means (n=4) of cumulative amounts of mineral N at each leaching event were com-pared for the different soils, using two-sample t-tests assuming different variances.
2.4 Soil analysis
Organic C (Corg) and total N (Ntot) were determined by dry combustion with a Carlo Erba ana-lyzer. The pH was measured in 0.01 M CaCl2 at a soil : solution ratio of 1 : 2.5. Soil texture was analyzed by sieving and sedimentation. The soil properties are listed in Tab. 2.
Table 2: Properties (mean ± stand. dev.) of sampled soils (Ap). (For soils no. 7, 8, 9: pH and
texture of 0-30 cm increments).
Plot
no. Depth increment
[cm] Corg [%, w/w] N[%, w/w] tot C/N pH (CaCl2) Clay (<2µm) Silt (2-63µm) [%, w/w] Sand (63-2000µm) 1 0-30 1.10 ±0.03 0.088 ±0.002 12.5 5.7 6.4 22.6 71.0 2 0-30 0.99 ±0.01 0.096 ±0.001 10.3 6.7 8.1 32.4 59.5 3 0-30 0.89 ±0.01 0.083 ±0.001 10.7 6.6 13.0 34.5 52.5 4 0-30 0.96 ±0.01 0.107 ±0.002 9.0 7.2 11.6 83.2 5.2 5 0-30 1.24 ±0.03 0.134 ±0.004 9.2 6.5 16.5 79.8 3.7 6 0-30 0.95 ±0.01 0.106 ±0.001 9.0 7.2 13.2 82.3 4.5 7 0-15 1.09 ±0.01 0.118 ±0.001 9.2 7 15-30 1.06 ±0.01 0.113 ±0.001 9.3 6.3 15.4 82.2 2.4 8 0-15 1.26 ±0.00 0.132 ±0.001 9.6 8 15-30 0.91 ±0.01 0.101 ±0.001 9.0 6.4 14.4 83.1 2.5 9 0-15 1.18 ±0.01 0.125 ±0.001 9.4 9 15-30 0.96 ±0.01 0.105 ±0.001 9.1 7.3 14.0 82.5 3.5
3 Results and discussion
3.1 Parameter estimation of N mineralization
Tab. 3 gives the N mineralization parameters of the incubated soils optimized with the double model. Both the double and the special model (not shown) produced good predictions for all the soils, except for soil 5. In most cases, the double model resulted in a higher R2 (0.997-0.999) and greater model/error ratio than the special model. In case of soil 5 under organic farming, neither the double nor the special model could be fitted to the measured cumulative mineralized N, resulting in unreasonable parameters. However, N mineralization in soil 5 was well simulated by the single model (Fig. 2; R2=0.997), with the potentially mineralizable ni-trogen N0 (151 µg g-1) being slightly higher than the mean measured value in 148 days (119 µg g-1), and a rate constant k (0.011 day-1) which is in agreement with the literature (Benbi and Richter, 2002). Soil 5 had a higher clay content and stagnic properties, and higher organic carbon and total N contents than soils 4 and 6. Because soil organic matter has heterogeneous components with varying degrees of degradability, models having two components are gener-ally more accurate than a single component model for simulating N mineralization (Deans et al., 1986; Bonde and Rosswall, 1987; Cabrera and Kissel, 1988; Dou et al., 1996; Benbi and Richter, 2002). Occasionally the simulation by the single model can be better than the double model, one possible reason being that more than two fractions of organic N are contributing to
N0 (Richter et al., 1982; Dalal and Mayer, 1987). Relating the estimated Nr values from all the soils (except soil no. 5) to the corresponding Ntot contents (Tab. 2), Nr accounts for 5.5-13.0% of total N. This is in agreement with values found in German (e.g., Nuske, 1983, Nr = 8-15% of Ntot) or Chinese loess soils (9.8%; Roelcke et al., 2002).
Table 3: Optimized mineralization model parameters using non-linear regression (double
exponential model, except soil from plot 5: single exponential model) of the aerobic incuba-tion experiment.
Plot no.
(depth) [µg gNa-1] [dayka-1] [µg gNr-1] [daykr-1] [µg gN0 -1]
(=Na+Nr) Ratio model/error (full regression) 1 (0-30 cm) 10.893 0.217 48.090 0.0124 58.983 14390 2 (0-30 cm) 16.320 0.376 51.058 0.0115 67.378 20431 3 (0-30 cm) 16.264 0.106 87.282 0.0042 103.546 19751 4 (0-30 cm) 9.166 0.209 127.321 0.0056 136.487 7433 5 (0-30 cm) k: 0.011 150.906 4198 6 (0-30 cm) 12.988 0.182 93.426 0.0084 106.414 2254 7 (0-15 cm) 12.966 0.328 86.570 0.0074 99.536 9349 7 (15-30 cm) 10.305 0.292 77.508 0.0098 87.813 9140 8 (0-15 cm) 10.146 0.928 81.020 0.0137 91.166 9121 8 (15-30 cm) 17.704 0.112 76.473 0.0042 94.177 4176 9 (0-15 cm) 21.407 0.336 106.180 0.0091 127.587 20796 9 (15-30 cm) 8.266 0.658 136.636 0.0035 144.902 5844
3.2 Influence of farming system on N mineralization
Figs. 1 and 2 show the pattern of nitrogen mineralization for the sandy soils and loess soils under different farming systems, respectively. There were no significant differences in the amounts of N mineralized between plots under organic and conventional farming of a same soil type. Soils 1 and 2 had higher C and N contents compared to soil 3, yet with no apparent
0 30 60 90 120 150 0 50 100 150
Incubation time [days]
NO 3 - -N + N H4 + -N [µ g g -1 so
il] 1 meas. 1 est.
2 meas. 2 est. 3 meas. 3 est. 0 30 60 90 120 150 0 50 100 150
Incubation time [days]
NO 3 - -N + N H4 + -N [µ g g -1 s o
il] 4 meas. 4 est.
5 meas. 5 est.
6 meas. 6 est.
effect on N mineralization. There were no differences in C and N contents and in N minerali-zation between soils of plots 4 (organic) and 6 (conventional), adjacent to each other. More nitrogen was mineralized in soil 5 under organic farming than in soils 4 and 6 (Fig. 2), proba-bly due to the higher Corg, Ntot and clay contents in soil 5 (Tab. 2). Generally, the inclusion of cover crops and perennial forage crops (as common in organic farming) increases SOM levels (Nieder al., 2003). Several reports have shown that the amount of N mineralized is correlated with total N, total C and microbial N (Dalal and Mayer, 1987; Antonopoulos, 1999), while Li et al. (2003) found N mineralized to be correlated with total N only.
Figure 1: Cumulative N mineralization over 147 days in Ap horizons (0-30 cm) from two
or-ganic plots (1, 2) and one conventional plot (3) (parent material Pleistocene sands). Mean measured values (dots) ± stand. dev., and curves fitted via non-linear regression (Eq. 2).
Figure 2: Cumulative nitrogen mineralization over 147 days in Ap horizons (0-30 cm) from
two organic plots (4, 5) and one conventional plot (6) (parent material loess). Mean meas-ured values (dots) ± stand. dev., and curves fitted via non-linear regression, using Eq. 2 for soils 4 and 6, and Eq. 1 for soil 5.
Significant differences in N mineralization were observed between the two different parent materials Pleistocene sands and loess (Figs. 1 and 2). Differences became more pronounced with increasing incubation time (comparing soils 1, 2, 3 vs. 4 and 6: P<0.05 from day 35 onwards, P<0.01 from day 49 onwards, P<0.001 from day 70 onwards). Regardless of farming system, the amounts of N mineralized after 148 days (N148d) ranged from 51.3 µg g-1 to 58.1 µg g-1 for the sandy soils (nos. 1, 2, 3) and from 80.3 µg g-1 to 118.9 µg g-1 for the loess soils (nos. 4, 5, 6). Accordingly, the N mineralization potentials N0 of the 3 loess soils were greater than those of the 3 sandy soils (Tab. 3). Nordmeyer and Richter (1985) also found that the N mineralization potential in loess was distinctly higher than that in sandy soils of the same region, explaining it with different clay contents, while Richter et al. (1989) and Roelcke et al. (2002) found a close positive correlation between the mineraliza-tion of the resistant N fracmineraliza-tion (Nr · kr, or decomposimineraliza-tion rate Cr) and the Ntot in soil.
3.3 Effect of tillage practice on N mineralization
Tillage practice distinctly affected the distribution of organic C and total N within the 0-30 cm Ap horizon (Tab. 2). Under CT, organic C and total N contents in the 0-15 cm increment were almost identical to those in 15-30 cm, while under RT and ZT, Corg contents in 0-15 cm were 23-39%, and Ntot contents 20-30% higher than those in 15-30 cm. Moreover, Corg contents in the 0-15 cm increment under RT and ZT were 8-16%, and Ntot contents 6-12% higher than under CT. These differences in the upper 0-15 cm between tillage systems cor-responded to 2,000-4,000 kg C and 165-310 kg N. However, opposite results were found in 15-30 cm, with Corg contents 10-14% and Ntot contents 7-11% lower in RT and ZT than un-der CT. There were no differences in mean Corg and Ntot contents over the whole 0-30 cm increment among the different tillage practices, and accordingly, the total mass of C and N remained almost constant. An accumulation of carbon and nitrogen in the upper soil layer under conservation tillage has also been reported by other authors (Logan et al, 1991; Ter-brügge and During, 1999; Feng et al, 2003). The absence of ploughing and lower homog-enization grade result in a stratification of SOM and an enrichment of the topsoil with or-ganic C and N (Frede et al, 1994). In a number of long-term experiments (up to 20 years) in humid regions, ZT increased soil organic C by 3 t ha-1 on average (Paustian et al., 1997).
The effect of tillage practice on nitrogen mineralization is shown in Fig. 3a-c. Generally, differences in N mineralization between the 0-15 and 15-30 cm increments increased with decreasing tillage intensity. No significant differences existed in the CT treatment (soil 7) between soils from the 0-15 cm and the 15-30 cm increments. The amounts of N mineral-ized in 148 days (N148d) in the 0-15 and 15-30 cm increments under conventional ploughing were about equal (70.3 µg g-1 and 69.6 µg g-1, respectively). Significant differences existed between the 0-15 cm and 15-30 cm under RT (soil 8), becoming more pronounced with increasing incubation time (P<0.05 from day 49 onward). N148d were 81.5 and 53.2 µg g-1, for 0-15 and 15-30 cm, respectively. For ZT (soil 9), differences between the 0-15 cm and 15-30 cm increments were significant during the whole course of the experiment (P<0.01,
P<0.001 on several occasions), with N148d of 100.3 (0-15 cm) and 64.0 (15-30 cm), respec-tively. These differences were consistent with the distribution of Corg and Ntot contents in the soil (Tab. 2). Nevertheless, different tillage practices did not lead to significant differences in the total amounts of N mineralized over the whole 0-30 cm Ap horizon, N148d (sums of
0 30 60 90 120 150 0 50 100 150
Incubation time [days]
NO 3 - -N + NH 4 + -N [µ g g -1 s o il] 7 0-15 meas. 7 0-15 est. 7 15-30 meas. 7 15-30 est. 0 30 60 90 120 150 0 50 100 150
Incubation tim e [days]
NO 3 - -N + N H4 + -N [µ g g -1 s o il] 8 0-15 meas. 8 0-15 est. 8 15-30 meas. 8 15-30 est. 0 30 60 90 120 150 0 50 100 150
Incubation time [days]
NO 3 - -N + N H4 + -N [µ g g -1 s o il] 9 0-15 meas. 9 0-15 est. 9 15-30 meas. 9 15-30 est.
both increments) for CT, RT and ZT being 140.0 µg g-1, 134.6 µg g-1 and 164.3 µg g-1, re-spectively. Other studies equally found that N mineralization generally increased in the up-per soil layer as intensity of tillage decreased (Franzluebbers et al., 1994a, 1994b; Torbert et al., 1998; Wienhold and Halvorson, 1999).
a)
b)
c)
Figure 3a-c: Cumulative nitrogen mineralization over 147 days in Ap horizons (0-15 cm
and 15-30 cm increments) under CT (a), RT (b) and ZT (c). Mean measured values (dots) ± stand. dev., and curves fitted via non-linear regression, using Eq. (2).
Conclusions
This study did not find any effect of organic farming on soil N mineralization compared to conventional farming. Significant differences in N mineralization existed between soils with two different parent materials, with N mineralized in the sandy soils much lower than that in the loess soils. In the tillage investigation, reduced and zero tillage seem to have merely resulted in a redistribution of SOM, while the mean contents over 0-30 cm were unchanged, compared to the conventional tillage. Reduced and zero tillage also led to a partitioning of N mineralization in the 0-30 cm soil. N mineralization was significantly higher in 0-15 cm than in 15-30 cm both under reduced and zero tillage, while the amounts of N mineralized were almost identical in the two depth increments under conventional tillage. These differ-ences were in line with the organic C and total N contents in the corresponding soil incre-ments.
Acknowledgements
We thank the farmers of the five farms in Lower Saxony. We also thank Mr. Raphael Thies for his warmhearted support during field sampling. The first author gratefully acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG) and the National Natural Sci-ence Foundation of China (NSFC).
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