Labile Soil Organic Matter and Mineralizable Soil Nitrogen Pools In Response to Management History, Inputs, and Crop Rotation Length and Diversity.

Sustainable soil fertility management depends on long-term integrated strategies that build and maintain soil organic matter and mineralizable soil N, thereby increasing the portion of N needs met by soil N cycling and reducing dependence on external N inputs required for crop production. There is a need to improve our understanding of soil organic N cycling to design highly productive and efficient farming systems. We conducted field and laboratory research to characterize soil N dynamics in five diverse management systems of a long-term study in Maryland, the USDA-ARS Beltsville Farming Systems Project (FSP). The five cropping systems of the FSP include a conventional no‑till corn‑soybean‑wheat‑soybean rotation (NT), a conventional chisel-till corn–soybean–wheat/soybean rotation (CT), a 2-yr organic corn–soybean rotation (Org2), a 3-yr organic corn–soybean–wheat rotation (Org3), and a 6-yr organic corn–soybean–wheat–alfalfa hay rotation (Org6). Our specific objectives were: i) compare the influence of management history on labile soil organic matter; ii) quantify mineralizable soil N in each of the cropping systems; and iii) evaluate corn yield response to soil N in each of the five cropping systems. Labile soil organic matter was characterized using physical (particulate organic C and N, POM), chemical (KMnO₄ oxidizable organic matter, CLOM) and biological (phospholipid fatty acid analysis, PLFA) methods.  Long-term (210 d) aerobic incubations were conducted to quantify mineralizable soil N. Corn was grown within un-amended subplots (without cover crop legume, manure and/or fertilizer N) established within FSP wholeplots to determine response to soil N. All measures of labile organic matter, potentially mineralizable soil N, and corn yield response to soil N were greater in organic than conventional cropping systems. Labile fractions of organic matter were generally greater in NT than CT; however, potentially mineralizable N was similar between the two systems. Among the three organic systems, labile organic matter, potentially mineralizalbe N and corn yield response to soil N generally increased with increasing length of crop rotation and complexity (Org2<Org3<Org6). Our results demonstrate that organic management practices increase labile soil organic matter, and mineralizable soil N relative to conventional management and that, among organic management systems, crop rotation length and diversity play an integral role in building and maintaining soil fertility.