Mineralizable soil nitrogen and labile soil organic matter in diverse long-term cropping systems

Sustainable soil fertility management depends on long-term integrated strategies that build and maintain soil organic matter and mineralizable soil N levels. These strategies increase the portion of crop N needs met by soil N and reduce dependence on external N inputs required for crop production. To better understand the impact of management on soil N dynamics, we conducted field and laboratory research on five diverse management systems at a long-term study in Maryland, the USDA- Agricultural Research Service Beltsville Farming Systems Project (FSP). The FSP is comprised of a conventional no-till corn (Zea mays L.)–soybean (Glycine max L.)–wheat (Triticum aestivum L.)/double-crop soybean rotation (NT), a conventional chisel-till corn–soybean–wheat/soybean rotation (CT), a 2 year organic corn–soybean rotation (Org2), a 3 year organic corn–soybean–wheat rotation (Org3), and a 6 year organic corn–soybean–wheat–alfalfa (Medicago sativa L.) (3 years) rotation (Org6). We found that total potentially mineralizable N in organic systems (average 315 kg N ha−1) was significantly greater than the conventional systems (average 235 kg N ha−1). Particulate organic matter (POM)–C and –N also tended to be greater in organic than conventional cropping systems. Average corn yield and N uptake from unamended (minus N) field microplots were 40 and 48%, respectively, greater in organic than conventional grain cropping systems. Among the three organic systems, all measures of N availability tended to increase with increasing frequency of manure application and crop rotation length (Org2 < Org3 ≤ Org6) while most measures were similar between NT and CT. Our results demonstrate that organic soil fertility management increases soil N availability by increasing labile soil organic matter. Relatively high levels of mineralizable soil N must be considered when developing soil fertility management plans for organic systems.