Subsoil amelioration in agriculture: Deep loosening and compost incorporation in a Retisol

Subsoils can store significant amounts of water, soil organic carbon and nutrients. In consequence, agricultural subsoil management is being increasingly tested as an option to sustain crop productivity under unfavourable conditions. The Soil3 project funded by the Federal Ministry of Education and Research of Germany aims at investigating the potential of subsoil management for agriculture. In the frame of this project, we collected samples from a field experiment taking place in Thyrow (Brandenburg, Germany), at a location with low precipitations and the soil was classified as a Retisol. The experiment was designed to investigate the potential benefits of deep ploughing together with deep placement of organic fertilizers on agricultural productivity and soil organic matter stocks. We focus on three treatments, namely the control plots, the plots after deep loosening, and the plots after deep loosening and compost incorporation. We quantified the changes in C and N stocks and in two size fractions obtained by wet sieving (<20µm and >20µm). We also recorded hyperspectral images of 1-metre soil cores in the Vis-NIR range (400-990 nm) and modelled the C distribution at a high spatial resolution (pixel size = 53×53 μm²). The spatial distribution of soil organic matter resulting from the incorporation of organic fertilizer in the subsoil is modelled at the sub-millimetric scale. The organic matter stocks and C:N stoichiometry are both impacted by the agricultural management and the imaging technique allows us to distinguish between increased amount of organic matter in hotspots or in soil mineral matrix, and to discuss the mechanisms controlling the observed changes.