Biological soil properties in a long-term tillage trial in Germany
Sebastian Ulrich1, Sabine Tischer2, Bodo Hofmann1*,andOlaf Christen1
1Agronomy and Organic Farming, Institute for Agricultural and Nutritional Sciences, Martin-Luther-University Halle-Wittenberg, Betty-Heimann-Str. 5, 06120 Halle/Saale, Germany
2Soil Biology and Soil Ecology, Institute for Agricultural and Nutritional Sciences, Martin-Luther-University Halle-Wittenberg, von-Senckendorff-Platz 3, 06120 Halle/Saale, Germany
Abstract
After 37 years of different soil-tillage treatments in a long-term field experiment in Germany, a number of biological soil characteristics was measured. The field trial comprised six major treat- ments with different implements and various depths. In this paper, results from a comparison of long-term use of a plow (to 25 cm depth), a chisel plow (to 15 cm depth), and no-tillage are pre- sented. The biological soil characteristics measured include the soil-organic-carbon (SOC) con- tent, microbial biomass, enzyme activities, and the abundance and biomass of earthworms.
Long-term use of a chisel plow and no-tillage increased the organic-C content in the uppermost soil layer (0–10 cm) compared with the plow treatment. The microbial biomass and the enzyme activities arginine-ammonification,b-glucosidase, and catalase decreased with depth in all treat- ments. Arginine-ammonification and catalase were higher in the plow treatment in soil layers 10 to 30 cm. Additionally, the chisel plow caused an increase in number and biomass of earthworms compared to both other tillage treatments. Differences in earthworm numbers and biomass be- tween plowing and no-tillage were not statistically significant.
Key words:no-tillage / organic-carbon content / microbial activities / earthworms Accepted June 12, 2009
1 Introduction
The long-term effects of husbandry treatments on soil char- acteristics are an important component of sustainable agricul- tural systems. This applies especially to differences in soil-til- lage treatments with its fundamental effect on the interaction of soil biological activity, soil physical properties, and follow- ing long-term changes in soil C content (Beck, 1991;Larink, 1998;Kandeleret al., 1999;Frahm, 2000). Reduced-tillage systems and no-tillage are practices that could maintain and improve soil quality. Land-use change can alter land-cover biomass and cause associated adjustment in C stocks with consequences for the mitigation of greenhouse-gas emis- sions.
Carbon sequestration by no-tillage management or afforesta- tion is associated with aboveground biomass but little is known on the effects of land-use change on the composition of soil C pools (Martenset al., 2003). The influence of soil-til- lage systems on soil organic matter (SOM) in temperate soils is experimentally detectable only over extended time periods, whereas changes in the microbial biomass and microbial pro- cesses may become manifest over a shorter time scale.
Therefore, the microbial biomass and enzyme activities can be used as early indications of changes in biological soil properties induced by changed tillage (Kandeleret al., 1999).
Tillage modifies the distribution of SOM in the soil Ap horizon.
In conventionally cultivated soils, SOM is more uniformly dis- tributed than in reduced-tillage or no-tillage soils where crop residues are concentrated in the uppermost soil layer
(Arshadet al., 1990). As a consequence, the microbial activ- ities in topsoils under no-tillage conditions were significantly greater than in plowed soils, while the reverse trend in deeper layers was observed (KandelerandBöhm, 1996).
Many studies have examined the long-term effects of differ- ent tillage management on the soil microbial biomass and enzyme activities, but less information is available under low- precipitation conditions in different soil layers up to 40 cm soil depth and the combination of biological indicators like micro- bial activities and the earthworm abundance. Those interac- tions were studied in a long-term field experiment with differ- ent tillage treatments under dry conditions of Central Ger- many. The soil parameters measured in the experiment comprise organic C, microbial biomass, enzyme activities, and earthworm number and biomass.
2 Materials and methods
The experimental site Seehausen (51°24′ N, 12°26′ E) is located close to the city of Leipzig (Saxony, NE Germany). At Seehausen, the precipitation averages 552 mm, and the long-term mean air temperature is 9.1°C (1963–1999). The soil is a sandy loam. According toFAO(2006), it is classified as a Stagnic Luvisol. It is susceptible to surface sealing and crust formation. Depending on weather conditions, the growth of crops may adversely be affected by water logging and
* Correspondence: Dr. B. Hofmann;
e-mail: bodo.hofmann@landw.uni-halle.de
the field trial in 2002. With the change in crop rotation in 1990/91, some tillage treatments were also changed:
(1) plow 25 cm (unchanged since 1965),
(2) chisel plow 25 cm (plow 25–35 cm until 1990), (3) plow 15 cm (unchanged since 1965),
(4) chisel plow 15 cm (unchanged since 1965),
(5) chisel harrow 10 cm (plow 25 cm to sugar beet or potatoes and chisel plow to 10 cm) for small grains,
(6) no-tillage (until 1990 plow to 25 cm and subsoiling to 45 cm either before sugar beet or potatoes).
Here, the main focus is put on the three treatments “plow 25 cm”, “chisel plow 15 cm”, and “no tillage”. However, results from the other treatments are included in the regression anal- ysis. Data for single or mean values for the respective treat- ments are not shown.
Soil samples for the soil-biological measurements were taken in April 2002 in the six soil layers: 0–5, 5–10, 10–15, 15–20, 20–30, and 30–40 cm, respectively. The methods applied are as follows: SOC and total N were estimated by dry combus- tion (according toISO 10694, 1995, and ISO 13878, 1998, respectively). The measurements of basal respiration and microbial biomass were done by the substrate-induced respiration (SIR) according toAndersonandDomsch(1978) as well asHeinemeyeret al. (1989),ISO 16072(2002), and ISO 14240–1(2001). The determination of dry bulk density (dB) was based on the use of 250 cm3 cores (according to ISO 11272, 2001). Accumulation of SOC, total N, and micro- bial biomass were calculated with restriction of dry bulk den- sity. Three enzyme activities of several metabolic cycles were chosen. The activity of the aerobically living organisms in the soil was mainly determined by the catalase activity (EC 1.11.-) according toBeck(1971). The specific reaction of the cata- lase is based upon the cleavage of the H2O2 molecule into water and oxygen. Low amounts of hydrogen peroxide are formed during the respiratory metabolism.
The b-glucosidase activity (EC 3.2.1.21) was determined because it is involved in the last limiting step of cellulose degradation (Hoffmann and Dedeken, 1965, modified by Tischer and Altermann, 1992). The method of arginine- ammonification (EC 3.4.-) is based upon the measurement of the released ammonia from an N-compound (L-arginine)
The statistical analysis was done using the SAS program packet (General Linear Model) (SAS, 1999). The normal dis- tribution of the data was tested by the Kolmogorov-Smirnov test. The correlation was carried out by SAS procedure “reg”.
3 Results
3.1 Organic-C and total N contents
The different tillage treatments caused changes in the C and N contents in the uppermost soil layer (Fig. 1 and 2). Espe- cially the vertical distribution of the organic-C and total N con- tents was affected by the differences in the long-term tillage treatments. In the “plow” treatment, the organic-C and N dis- tributions were fairly similar down to 30 cm. This effect was caused by the typical mixing of manure and plant residues in the uppermost 30 cm of the profile. In contrast, the long-term use of conservation tillage with a chisel plow and the no-til- lage system lead to higher organic-C and N contents in the soil layers 0–5 and 5–15 cm. Due to the limited input in the lower soil layers in the conservation and no-tillage systems, lower organic-C contents occurred.
For given bulk densities (Tab. 1) and a soil depth of upper- most 15 cm, the pools of C and total N result in the following pattern: chisel plow (29.7 Mg SOC; 2.7 Mg N ha–1), no tillage (26.9 Mg SOC; 2.4 t N ha–1), and plow (21.8 Mg SOC; 2.1 Mg N ha–1). In the subsoil (15–40 cm), the following pattern occurs: chisel plow (25.6 Mg SOC; 2.5 Mg N ha–1), plow
Figure 1:Influence of different soil tillage on organic-C contents.
(29.2 Mg SOC; 2.7 Mg N ha–1), and no tillage (31.2 Mg SOC;
2.8 Mg N ha–1).
3.2 Microbial biomass
The microbial biomass was higher under reduced tillage and no-tillage than under conventional tillage, especially in the uppermost soil layers 0–10 cm (Tab. 2). We could assume a Cmic: SOC ratio of 2.18% in the plowed, 3.14% in the chisel- plow treatment, and 3.18% in the no-tillage variant. The microbial biomass and organic matter (OM) correlates with r2
= 0.82*** (p
<
0.001,n= 36, all treatments together).The metabolic quotient (qCO2) is a measure of the effective- ness of the microbial metabolism. The lowest metabolic quo- tient could be estimated in the soil layers 0–15 cm. In the sub- soil (30–40 cm), the highest qCO2were below the plow layer in the plow plots or in the chisel-plow treatment from 20–30 up to 30–40 cm.
3.3 Enzyme activities
The tested enzyme activitiesb-glucosidase, arginine-ammo- nification, and catalase showed a correlation with SOC (r2= 0.74***, r2= 0.58**, r2= 0.76***; ***p
<
0.001, **p<
0.01,n= 36). In general, enzyme activities of tested soils decline with soil depth. The highest enzyme activities were measured in the uppermost 10 cm layer in the chisel-plow and no-tillage treatment (Tab. 2), whereas in the plow treatment the micro-bial activity was fairly similar in the different soil layers.
Almost no differences were recorded in the soil layer 30–40 cm.
Due to the high aeration in the plow treatment, the arginine- ammonification showed high activities down to a depth of 30 cm. If C mineralization is put in relation to N mineralization, like basal respiration in relation to arginine-ammonification, high numbers are an indication of an increased C mineraliza- tion or a low N mineralization (Dilly and Munch, 1995). At 30–40 cm soil depth, the basal respiration showed no relation to the tillage treatment and averaged only 0.5lg C g–1h–1. The arginine-ammonification was very low. In contrast, the highest ratios of basal respiration to arginine-ammonification were measured in the chisel-plow and in the plow treatments at a depth of 30–40 cm. In the plow treatment, a balanced ratio between C and N mineralization was detected down to a depth of 30 cm, whereas in the chisel-plow and the no-tillage treatment a greater C mineralization and a smaller N minera- lization were measured at a soil depth below 15 cm. In total of the Ap horizons, the highest N and C mineralization were measured in the chisel-plow treatment (Fig. 3).
Figure 2:Influence of different soil tillage on total N contents.
Table 1:Influence of different tillage systems on bulk density in differ- ent soil-depth intervals (different letters indicate a significant differ- ence ata<0.05 for a soil depth).
Variant / depth interval
0–6 cm 6–12 cm 18–24 cm 24–30 cm 32–38 cm
Plow 1.41 a 1.49 a 1.46 b 1.63 a 1.64 a
Chisel plow 1.34 a 1.53 a 1.54 ab 1.61 a 1.64 a No-tillage 1.40 a 1.55 a 1.61 a 1.59 a 1.70 a
Figure 3:Quotient from basal respiration [lg C-CO2(g dw soil)–1h–1] and arginine-ammonification [lg N (g dw soil)–1h–1] for three soil- tillage variants.
Figure 4:Relation of total N to basal respiration (n= 36, all tillage variants together).
A strong correlation (r2= 0.74***,p
<
0.001) between basal respiration and total N (Fig. 4) could be found and showed the close relation between N and C cycles.3.4 Earthworms
The earthworm population was strongly affected by the differ- ent soil-tillage treatments (Fig. 5). The highest abundance and biomass of earthworms was measured in the treatment with the chisel plow with 221 animals m–2and 64.7 g m–2, re- spectively. Differences in earthworm numbers and biomass between plowing and no-tillage were not statistically signifi- cant. The highest number of adult individuals was found in the no-tillage system (27%), although the total numbers and biomass were on a comparable level with the figures in the plow treatment. The highest number of earthworm species (Lumbricus terrestris, Aporrectodea caliginosa, Aporrectodea rosea, Allolobophora chlorotica, Octolasion cyaneum) was observed in the plow and in the chisel-plow treatment. In the no-tillage system, only three species [A. caliginosa (26%), A. rosea (9%), and A. chlorotica (65%)] were found. The highest species diversity occurred in the plow variant (1.26) followed by chisel plow (0.91) and no-tillage (0.85). In our
experiments,L. terrestris, an anoecic species, was observed after plowing or using the chisel plow with a dominance of 5%
and 10%, respectively. A comparison of the juvenile earth- worms (Lumbricus sp.) based on the eco-morphological traits revealed a high proportion of 33% of juveniles under no-til- lage conditions. In contrast, after plowing, no juveniles of the respective species were observed. In the treatment with the chisel plow, 5% juveniles occurred.
4 Discussion
Long-term comparisons of soil-tillage systems may reveal dif- ferences in the distribution of SOC and N in the soil profile. A reduction in the soil-tillage intensity and/or depth leads to an increase in SOC and soil N in the uppermost 10 cm of the profile. With increasing depth, the respective concentrations decrease. According to Teebrüggeund Düring (1999), the reason is the decreased input of organic manure and crop residues in deeper layers under such conditions. In contrast, the use of the plow causes a much more homogenized and even distribution in the Ap horizon. The high SOC concentra- tions in the soil layer 30–40 cm in the no-tillage treatments are due to a subsoiling in the treatment in 1979, 1982, 1985,
Plow 5.47 b 6.66 b 5.90 b 6.48 a 6.52 a 15.04 b
Chisel plow 3.40 a 3.47 a 5.31 b 7.92 b 13.36 b 16.64 b
No-tillage 3.58 a 3.69 a 4.08 a 6.05 a 7.95 a 9.24 a
Cmic: SOC ratio/ %
Plow 2.18 a 1.95 a 1.79 b 2.08 c 1.39 c 0.88 a
Chisel plow 3.14 b 2.31 b 1.52 a 0.84 a 0.57 a 0.86 a
No-tillage 3.18 b 2.51 b 1.61 ab 1.21 b 1.03 b 0.82 a
Arginine-ammonification/lg N (g dw soil)–1
Plow 1.04 a 0.88 a 1.12 b 1.09 b 1.17 b 0.02 a
Chisel plow 1.25 a 0.92 a 0.59 a 0.09 a 0.02 a 0.02 a
No-tillage 1.61 b 0.79 a 0.49 a 0.21 a 0.11 a 0.04 a
b-glucosidase/lg saligenin (g dw soil)–1(3 h)–1
Plow 23.7 a 24.9 a 28.3 a 23.6 a 23.1 b 10.8 a
Chisel plow 92.0 c 45.9 b 28.0 a 20.6 a 10.0 a 9.0 a
No-tillage 54.8 b 42.3 b 29.2 a 23.6 a 18.6 b 16.2 a
Catalase number
Plow 9.2 a 7.9 a 8.4 a 9.9 b 6.9 b 2.0 a
Chisel plow 20.9 b 11.5 b 7.3 a 4.7 a 2.8 a 1.7 a
No-tillage 21.7 b 12.7 b 7.1 a 4.6 a 3.0 a 2.5 a
and 1988. The SOC pools in the no-tillage and chisel-plow plots are at the soil depth of 15 cm (with respect to dry bulk density) 1.2 times (5.2 Mg SOC ha–1) and 1.4 times (7.9 Mg SOC ha–1) higher on average than in the plowed variants (21.8 Mg SOC ha–1). More CO2 would be sequestered, and therefore the chisel-plow and the no-tillage plots act as car- bon sinks in relation to the conventional-tillage treatment. In the subsoil (15–40 cm), the highest C storage could be assumed in the no-tillage variants.
The C : N ratios ranged from 10.5 (0–15 cm) in the plow treat- ment to 11.3 in no-tillage and 10.9 in chisel-plow treatment.
These results mean that more N is immobilized by microbes in the reduced-tillage treatments than by continuous plowing.
As a consequence, the microbial biomass and various micro- bial processes tended to increase in surface soils after til- lage-management change. The SOC and total N in the uppermost soil layer of reduced- and no-tillage systems rep- resent a sink or a source of nutrients. These findings con- firmed the observations of other authors (Kandeler et al., 1999;Frahm, 2000;Acosta-Martinezet al., 2003) that types of reduced tillage enrich topsoils with SOC.
The microbial biomass is defined as the living component of SOM (JenkinsonandLadd, 1981). As a very broad generali- zation, the amount of microbial biomass in a soil reflects the total OM content, with the living microbial component forming a low proportion of the total. The proportion present as micro- bial biomass comprises 1%–5% (w/w) of SOC (Sparling, 1997). In the temperate zone, the Cmic: SOC ratio is≈2.5%
(Anderson and Weigel, 2003). In our studies, we could assume a ratio of 2.18% in the plow, 3.14 in conservation-til- lage, and 3.18% in no-tillage treatment. These results have shown that changes in the OM contents will be reflected in this ratio. The microbial biomass and OM correlate strongly.
These findings confirmed the observation of other authors (Friedelet al., 1996;DengandTabatabai, 1997;Kandeleret al., 1999) that reduced tillage enriches topsoils with microbial biomass. According to Paoletti (2001), the composition of bacterial population has a great influence on the degradation rate of OM and the nutritional cycle. Under conventional til- lage is colonized another bacterial population with a specific degradation rate of OM than in the variants with reduced til- lage. The greater bacterial diversity by conventional tillage
has more possibilities for degradation of OM and for stress resistance. A humus accumulation cannot take place under these conditions. In conclusion, more CO2 from the atmo- sphere is sequestered in the reduced-tillage soils and these amounts can be released again upon changing the tillage management. In several studies (Anderson and Domsch, 1978, 1993;InsamandDomsch, 1988), the microbial respira- tion per unit microbial biomass was measured and found use- ful as an indicator of the overall metabolic status of a given microbial community. High amounts of microbial biomass in the soils indicate favorable habitat conditions for the growth and the energy metabolism of many microorganisms. Since microorganisms live predominantly heterotrophically in the soil, the microbial biomass is affected by the amount and quality of the organic substrate. The efficiency in using organic energy sources is decisive for the formation of the microbial biomass. Therefore, the metabolic quotient (qCO2) is a measure of the effectiveness of the microbial metabo- lism. At poor site conditions like in the subsoil of investigated soils, the microorganisms need more energy for maintaining their biomass; C is lost to the microflora. The lowest meta- bolic quotient could be estimated in the topsoil, and the high- est qCO2 were below the loosening layers by conventional (30–40 cm) and conservation tillage (20–40 cm). In conclu- sion, one can assume that the decreased soil density in the no-tillage treatment has a positive influence on the effective- ness of the microbial metabolism.
Mineralization of OM involves a wide range of metabolic pro- cesses with the active participation of soil enzymes (de la Paz Jimenezet al., 2002). Therefore, soil enzymes are ideal for the assessment of environmental alteration leading to changes in the metabolism (Kandeler et al., 1999; Tischer, 2005a). Especially, theb-glucosidase activity has been found to be sensitive to soil management and has been proposed as a soil-quality indicator (Dicket al., 1997;KnightandDick, 2004). Theb-glucosidase can detect changes in soil manage- ment within relatively short time periods (1–3 y) and it is rela- tively stable with seasons (BandickandDick, 1999). Gener- ally a decrease in SOC content causes decreased enzyme activities as a result of reduction in microbial biomass and changes in the composition of soil microflora and root devel- opment (de la Paz Jimenezet al., 2002). The tested enzyme activities b-glucosidase, arginine-ammonification, and cata- Figure 5: Effect of three different soil-tillage variants on abundance and biomass of earthworms (different letters mark significant differences ata<0.05).
cation is an indicator for the C and N mineralization. Follow- ing a long-term use of a chisel plow and a plow, the highest ratios were detected at the soil depth of 30–40 cm, which can be interpreted as a high C and a low N mineralization. The overall highest numbers were measured in the chisel-plow treatment.
A strong correlation between basal respiration and total N could be found and showed the relation between N and C cycles (cf., Haag2004).
Earthworms are an essential part of many agroecosystems and may be useful indicators for sustainability (Hubbardet al., 1999; Frahm, 2000; Tischer, 2005b). Earthworms are probably the most important soil animals in terms of plant pro- ductivity because of their significant influence on soil physi- cal, chemical, and biological properties related to plant yields.
However, agricultural management practices such as tillage, crop rotation, and use of agrochemicals greatly affect earth- worm population. Numerous studies have shown that greater earthworm abundance was found in no-tillage than in conven- tionally tilled agroecosystems (Hubbardet al., 1999;Frahm, 2000;Emmerling, 2001). In our investigations, the earthworm population was strongly affected by the different soil-tillage treatments. The highest abundance and biomass of earth- worms was measured in the treatment with the chisel plow.
The differences in earthworm numbers and biomass between plow and no-tillage were not statistically significant. These results are in agreement with investigations ofFrahm(2000).
The high dominance of both species A. caliginosa and A. chlorotica under conventional management could be explained with their tolerance towards intensive tillage (Paoletti, 2001;Tischer, 2005b). Directly after plowing, a high number of injured and dead earthworms was observed, which concurs with results of Bostum (1995), Emmerling (2001), and Chan (2001). The authors consistently describe the harmful effects of intensive soil tillage on earthworm popula- tions.
5 Conclusions
Long-term differences in soil-tillage treatments have conse- quences for soil biological characteristics. The differences in the distribution of crop residues within the Ap horizon follow- ing a less intensive soil tillage causes an increase in organic substances (SOC, N) near the soil surface in the topsoil, whereas an intensive tillage has the opposite effect.
Similar effects of the tillage treatments were also observed for other soil biological parameters like microbial biomass
In our experiment—with exception of the long-term conserva- tion treatment—the earthworms showed little response to the different tillage treatments. Only the chisel-plow treatment had a positive effect on the earthworm population. The num- ber of adult individuals in the no-tillage variant was higher compared with the plowed treatments.
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