The Role of Soil Organic Matter for Maintaining Crop Yields: Evidence for a Renewed Conceptual Basis

Soil organic carbon (SOC) is believed to play a crucial role for many soil functions and ecosystem services. Despite much research, a lower threshold of SOC for sustainable crop production has not been identified across soil types. We addressed a comprehensive dataset with yields of winter wheat for nearly a 1000 field nitrogen (N) response experiments performed over 3 decades in Denmark. The soils ranged from coarse sand to clay loam (0.016-0.406 kg clay kg(-1) minerals) with SOC ranging from 0.005 to 0.039 kg kg(-1) minerals. All experiments were located on arable soil that was fully fertilized with all other plant nutrients. Based on the yield response at 4-6 levels of applied mineral N, we estimated the potential yield not restricted by any nutrient, Y-pot, the yield at no applied N, Y-N0, the marginal agronomic efficiency (AE(N0)) in terms of the increase in yield per unit of applied N at zero N, and the level of mineral N, Nopt, needed to obtain Y-pot. Y-N0 increased with Fines20 (proportion of soil minerals < 20 mu m). AE(N0) decreased with SOC independent of soil type. Nopt decreased with increase in SOC. Ypot increased significantly with a squared expression of Fines20 and decreased significantly with SOC. Ypot was better explained by RH = Fines20/SOC than SOC per se. Our results indicate a positive effect of SOC in terms of a reduction of mineral N needed to obtain the potential crop yield, and a negative effect on Y-pot. We suggest a multigate regulation concept for future studies addressing the non-N effect of SOC. We further draw attention to the importance of the quality of the SOC for better understanding effects on crop yields.