Chapter Two - 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, Ypot, the yield at no applied N, YN0, the marginal agronomic efficiency (AEN0) 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 Ypot. YN0 increased with Fines20 (proportion of soil minerals < 20 μm). AEN0 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 Ypot. 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. [ABSTRACT FROM AUTHOR]