The influence of reduced tillage on soil fertility indicators in two long-term organic field experiments

Summary

Conventional agriculture calls for global change towards more sustainable agrosystems. Reduced tillage (RT) presents an interesting alternative to conventional tillage (CT) in order to reduce soil degradation while keeping control of weed pressure. However, research gaps on soil quality assessment in long-term organic farming under RT need to be filled, especially for different soil textures. . The aim of this work was to study the effects of RT on soil fertility indicators in two organic long-term field experiments (LTEs). The study compared the effects of RT (chisel plow to 10 cm) and CT (traditional moldboard ploughing 15-20 cm) on two LTEs from FiBL in Northern Switzerland differentiated by high clay (Frick, AG) and silt-loam (Aesch, BL) textures, including two associated untilled grasslands as a reference. To assess soil fertility, physical (bulk density), chemical (pH, SOC, TN, Cmic and Nmic, micronutrients) and biological (bacteria, fungi, earthworms) soil properties were measured at three depths (0-10 cm, 10-20 cm, 20-30 cm). Additional effort was brought to the in-depth analysis of microbial communities using next generation sequencing technique. Statistical analysis included ANOVA, PCA, RDA and Mantel tests to evaluate the significant effect of tillage, depth and sites on soil parameters and microbial populations. The results showed that RT enhanced most of the soil fertility indicators in both sites compared to CT. The positive effect of RT was more pronounced in the clayey soil of Frick for chemical parameters (SOC stocks, TN, micronutrients), with a visible stratification of nutrients with depth, thus resembling to the natural grassland (G) plots, while CT lead to more homogenous layers. Fungi copies were also enhanced in the Frick soil under RT. The biological indicators (total soil DNA, bacteria, and earthworms) surprisingly seemed to be more enhanced by RT in the silty soil of Aesch. RT could significantly decrease bulk density in the top layer of Frick site, and only minorly in Aesch site, while it increased compared to CT in the second layers of both sites. Biomolecular samples were composed of 60'000 sequences each, and the data obtained by sequencing showed an excellent correlation with all environmental variables (r=0.5853, p=0.001***). PCA on microbial community showed a clear distinction between both sites and between the natural G plots and the cultivated RT and CT plots, especially in Frick. In Aesch, microbial community showed significantly different structures linked with depth and without tillage differentiation, while in Frick the microbial community was very similar regardless of tillage treatment, and minorly with of depth. Nitrifiers included few Archaea (Gen. Nitrosopumilus), and Bacteria (Phylum Nitrospirae with Gen. Nitrospira, Class α- and β-Proteobacteria with Gen. Nitrobacter, Nitrosomonas, Nitrosovibrio and Nitrospira). Microbial diversity indices showed that the diversity was greater in Aesch than in Frick, greatest in the top layers, and lower in the G plots than in the RT and CT plots. This work enabled to confirm enhanced soil fertility indicators with RT practices under two organic LTEs, and especially in the high SOC-sequestering clay soil, while the biomolecular analysis surprisingly showed enhanced biological parameters in the silty soil. The underlying processes regarding site specific changes still need to be untangled through deeper analyses, including specific target on soil physical parameters under different tillage conditions and for different soil textures, and extended knowledge on the microbial community behavior at a functional level.