| dc.description.abstracteng | Diversity, abundance, and biomass of soil biota depend on the biotic and abiotic characteristics of the soil environment and are essential for the provision of important ecosystem services. Agricultural systems are shaped by the long-term history of farming practices and the recent management decisions of the farmer. The present thesis reports on the effects of different tillage practices on soil organisms, their functions and performances. Different approaches were applied: (i) first, investigations on microbial and chemical soil parameters were carried out under different climatic and soil conditions across Europe, (ii) second, an experiment on litter breakdown driven by the soil mesofauna was conducted at a long-term experimental field site and (iii) third, detailed insights on the effect of soil inversion and crop residue placement on soil annelids and their functional feedback were gained under laboratory conditions.
Study 1 investigates the effects of reduced tillage applications compared with conventional tillage on microbial and chemical soil parameters at four different long-term experimental field sites at Germany, Romania, Spain and Sweden. Reduced tillage meant either no-tillage (NT) as direct seeding or minimum tillage (MT) that had a working depth between 5 and 12 cm (3 sites) or down to 25-30 cm (1 site) with non-inversive machinery. Conventional tillage (CT) was carried out as mouldboard ploughing at all sites, inverting the soil down to a depth of 23-30 cm. At the German site, MT increased concentrations of organic carbon (C), total nitrogen (N) and microbial biomass carbon (MBC) in the top 10 cm. In contrast, CT increased MBC contents and bulk densities between 20 and 30 cm soil depth. In Sweden, NT and MT on a soil with an acidic pH had positive effects on concentrations of C, N and MBC in the upper 20 cm, compared with CT. At the Romanian site, a clay-rich soil, soil parameters showed no differences between inversion tillage (CT) and non-inversion tillage (MT), both of which had a working depth of 25 to 30 cm. At the Spanish site the use of NT increased the concentrations as well as stocks of C, N and MBC significantly, compared with CT. To sum up, reduced tillage enhanced soil microbial properties in most cases. However, the effectiveness of tillage reduction seems to highly depend on site conditions such as pH level, soil texture and climatic conditions.
Study 2 focuses on the effects of soil mesofauna and reduced tillage on decomposition of crop residues under field conditions. In a long-term field experiment near Göttingen (Germany), a litterbag experiment was carried out to compare decomposition rates at in plots under minimum tillage and ploughing. The litterbags were filled with 5 g of chopped maize leaves (Zea mays L.). Two different mesh sizes were used to quantify the contribution of mesofauna to decomposition processes. The coarse mesh size (2 mm) allowed mesofauna and microorganisms to access the litterbags, while the fine mesh (48 μm) only allowed the colonisation by microorganisms. In October 2017, litterbags were placed at a depth of 5-8 cm in minimum tillage and at a soil depth of 25 cm in the ploughed plots and were removed after 2, 5 and 7 months. In addition to C and N contents of litter material and the overall litter loss, the abundance and species composition of Collembola, as an important representative group of the mesofauna in agricultural systems, were measured. Stable isotopes analysis (δ13C and δ15N) was used to quantify the uptake of the maize-derived C by Collembola. Mesofauna access improved litter quality (indicated by low C/N ratios). This effect is presumably more indirect by stimulating microbial activity and increasing the supply of N. Overall, Collembola abundances were increased in ploughed plots compared with the minimum tilled plots. Collembola species composition was not significantly changed by tillage, although euedaphic species occurred mainly in ploughed plots and epedaphic species under minimum tillage. In deeper soil depths crop residues seem to be important as a habitat and a food source for Collembola, because other carbon sources are difficult to access. The results suggest that soil mesofauna greatly contributes to the transformation of organic matter and the availability of nutrients.
Study 3 is a mesocosm laboratory experiment on the effect of soil inversion with residue displacement on the activity and functions of microorganisms, enchytraeids and earthworms. Undisturbed soil columns (30 cm high, 15 cm Ø) from a long-term experimental field site near Göttingen (Germany) were used. The experiment was shaped to investigate the two factors ‘tillage’ and ‘soil fauna’. Tillage had the factor levels (1) simulated soil inversion (SI): soil columns were ‘ploughed’ manually with residues (5 g leaves of Zea mays L.) placed at a depth of 15 cm and (2) non-inversion (NI): undisturbed soil columns with residues placed on the surface. Soil fauna had four factor levels (addition of different soil organisms to the soil columns): (1) anecic earthworms (Lumbricus terrestris), (2) endogeic earthworms (Octolasion cyaneum), (3) enchytraeids (Enchytraeus crypticus and Enchytraeus christenseni) and (4) control columns without soil fauna. All different combinations of factor levels were replicated four times (n = 4). The experiment ran for 114 days in a dark climate chamber at a temperature of 10 °C. The gas fluxes (CO2, N2O) were measured automatically every 4.5 hours using a gas chromatograph. The concentrations of microbial biomass and ergosterol were quantified as soil biotic properties. The fate of crop residue-derived C was traced using stable isotopes analysis (δ13C and δ15N). The results showed that the organisms reacted differently to the incorporation of the organic material. As expected, L. terrestris consumed more maize than O. cyaneum during the experiment. For endogeic earthworms, the incorporation of residues at a depth of 15 cm was advantageous, whereas tillage had no effect on the uptake of maize-derived C by anecic species. In addition, the number of enchytraeids was not affected by the crop residue placement. Anecic earthworms increased CO2 C emissions by 22% (vs. control). Soil inversion enhanced N2O-N emission by 188%. Overall, there was no interaction between the two factors. Microbial and chemical parameters were strongly affected by soil inversion. Results showed that incorporation of crop residues at deeper soil layers has neutral to even positive effects for soil annelids. Nevertheless, under field-conditions, the incorporation of residues into the soil may impair or endanger annelids mechanically.
In general, reduced tillage systems supported soil biota and thus were able to enhance the ecosystem services they provide. Ploughing can increase the abundance of individual species, however, a broad spectrum of species and thus functional diversity in the soil should be promoted. The incorporation of crop residues appeared to be positive for some functional groups of soil fauna. Even though earthworms increased gas emissions, this effect is considered to be offset by their contribution to carbon sequestration. Soil inversion, on the other hand, appears to make a large contribution to greenhouse gas emissions. The high variability of the results between different locations suggests that recommendations for reduced tillage practices can only be given at a local scale, related to the existing soil and climatic conditions. Overall, this thesis contributes to the development of sustainable management strategies with a focus on soil tillage and highlights the often underestimated importance of soil organisms for agriculture. | de |