Soil organic matter dynamics under irrigated perennial forage–annual crop rotations

Abstract Reduced soil disturbance and increased organic matter inputs in forage production systems may improve soil quality and crop production through their effects on soil organic matter ( SOM ) dynamics and nutrient cycling. We evaluated the effects of conventional ( CV ), organic ( OR ) and reduced‐tillage ( RT ) management systems on SOM pools in perennial forage–annual crop rotations established on soils previously under long‐term monocropped corn ( Zea mays L.) field. The SOM pools evaluated include potentially mineralizable carbon ( PMC ) and nitrogen ( PMN ), dissolved organic carbon ( DOC ) and nitrogen ( DON ), microbial biomass carbon ( MBC ) and nitrogen ( MBN ), inorganic nitrogen (inorganic‐N), soil organic carbon ( SOC ) and soil total nitrogen ( STN ). The crop rotation was 3 years of alfalfa ( Medicago sativa L.)–grass mixture (2009‐2011) followed by a fourth year of corn (2012) across all management systems. While soils under the RT system had the highest rate of labile carbon (C) and nitrogen (N) accrual, all management systems accumulated SOC and STN during 2009‐2011. The response of different C and N pools was not consistent in 2012 when the fields were transitioned from alfalfa‐grass mixture to corn production. Soil PMN and inorganic‐N contents were significantly greater under CV than other management systems, whereas PMC and MBC remained greater under RT than other management systems. This suggests CV may provide immediate benefits to crop production by increasing N availability, but RT has potential to increase SOC accumulation and improve soil quality in the long‐term under perennial forage–annual crop rotations.