Cover crop mixtures enhance maize access to SOM-associated subsoil nutrients in contrasting soil types

The subsoil plays a crucial role in the dynamics of soil organic matter (SOM), influencing soil health. Improving crop access to subsoil nutrients has become increasingly important with frequent droughts. While winter cover crops (CCs) are considered to enhance nutrient use efficiency, understanding of their role in carbon and nutrient cycling, especially when different CC mixtures are used in contrasting soils, is still limited. We investigated how different CC mixtures facilitate maize access to SOM-associated nutrients in subsoils across three contrasting soils in Germany. Using ¹⁵N CC leaf labelling and subsoil tracer application, we quantified nitrogen transfer from CC mixtures to SOM and maize uptake of nutrients originating from both topsoil and subsoil pools.

Our findings reveal that maize nutrient acquisition from subsoils depends heavily on soil type-CC combination. In high-fertility soils with larger SOM stocks, maize appeared to rely predominantly on nutrients originating from SOM. In contrast, in nutrient-poor soils, CC mixtures actively promoted the mobilisation and transfer of both CC-derived and SOM-associated N, evidenced by increased dissolved N pools and higher maize ¹⁵N recovery. Deep-rooting CC mixtures enabled access to nutrients associated with subsoil SOM fractions, likely by enhancing subsoil exploration, despite limited horizon penetrability. These results show that the impact of CC mixtures extends beyond nutrient inputs alone to the accessibility and availability of nutrients bound to SOM at different soil depths. We therefore conclude that optimising CC mixtures is an effective strategy to mobilise nutrients associated with subsoil SOM for maize under a changing climate.