Enhancing the Mineralizable Nitrogen Pool Through Substrate Diversity in Long Term Cropping Systems

The development of sustainable N management systems requires a better understanding of the contribution of on‐farm resources to the active N pool size and its mineralization. This study explores the effect of substrate diversity on improving N supply through mineralization. A “diverse system”, consisting of a corn ( Zea mays L.)‐corn–soybean ( Glycine max L.)‐wheat ( Triticum aestivum L.) rotation with cover crops and fertilized with composted manure was compared with a corn monoculture with conventional fertilizers. Nitrogen mineralization was measured in situ and in laboratory incubations as was the ability of each soil to mineralize added compost and red clover ( Trifolium pratense ) residue in the 6th and 7th yr of rotation. Net mineralized N in the diverse system was 90 and 40% higher than that in the monoculture at 70 and 150 d of laboratory incubations respectively. Comparable response was found in situ where a 70% higher net mineralization was observed in the diverse system at 70 d. The 70‐ and 150‐d mineralizable N pools increased over time, but the ability of soil organisms to break down additional substrate did not change as a result of system diversity. At 150 d of laboratory incubation, a synergistic effect was observed when 5 Mg ha −1 of compost plus 5 Mgha −1 of clover was added to either soil. The combination of the two organic materials mineralized more N than the sum of their individual mineralization results. A more diverse cropping system may increase the soil's capacity to supply N to a growing crop while maintaining desirable levels of soil organic matter. This is essential for the overall long‐term productivity and sustainability of agricultural systems.