Do corn-soybean rotations enhance decomposition of soil organic matter?

Abstract

Corn and soybean crops are often grown in rotation, requiring lower nitrogen (N) inputs than continuous corn. However, soil organic carbon (C) may be declining in corn-soybean systems despite sustained residue inputs. We asked whether corn-soybean rotations increase decomposition of litter and soil C as compared with continuous corn. We incubated soils from a long-term field experiment including continuous corn and both phases of the corn-soybean rotation. Soils were amended with corn litter, soybean litter, or no litter. We measured natural abundance C stable isotopes (δ13C values) in respiration and microbial biomass to partition C sources. Addition of soybean litter increased microbial biomass while corn did not. However, corn litter addition consistently increased (i.e., primed) soil C decomposition while soybean litter did not. Soils most recently planted to corn following soybeans had the greatest soil C decomposition and N mineralization irrespective of litter addition, and they decomposed corn litter faster and had a faster priming response than the other treatments. Our data support the hypothesis that alternating inputs of N-rich soybean litter and relatively N-poor corn litter could enhance litter and SOC decomposition by driving microbial growth following the soybean phase and stimulating priming following the corn phase. Increased decomposition and N mineralization from litter and SOC in corn-soybean rotations may contribute to the soybean N credit but could also contribute to longer-term soil C and N declines, consistent with field data.