Legumes and soil organic matter transformations in upper Midwest agroecosystems

Abstract

Cover crops are an agroecological approach that can increase ecosystem service provisioning, reducing erosion, providing spatial and temporal biotic diversification, and increasing soil organic matter (SOM). However, implementation of cover crops in the upper Midwest is limited by short growing seasons and harsh winters with variable temperatures and snowfall. Our objective was to explore the potential of winter annual legume cover crops to stabilize and augment soil C and organic N stocks in upper Midwest agricultural systems in which cover crop biomass production is quite limited, and without major impacts on crop yield. We compared hairy vetch (VET), winter rye (RYE), red clover (CLO), hairy vetch/rye biculture (MIX), and bare-ground control (NOCC), planted in a randomized complete block design at two University of Minnesota Research and Outreach Centers in southwest (LAMB) and north-central (GR) Minnesota. Cover crops were established in fall and terminated in spring prior to sweet corn planting. To measure linked soil C and N dynamics, we determined microbial biomass C (MB-C), permanganate-oxidizable C (POX-C), size-fractionated particulate organic matter C and N (POM-C and POM-N), extractable soil N (EXT-N) and potentially mineralizable N (PMN). We hypothesized that 1) MIX has intermediate biomass and biomass N levels, compared to legumes (high N, low biomass) and RYE (low N, high biomass) monocrops 2) MIX treatment increases both soil C and N, legumes primarily increase N and RYE influences labile C parameters, and 3) RYE exhibit sweet corn yield declines due to low N and high residue. Our results indicate winter annual legume cover crops are a viable tool for increasing soil N but had limited impact on labile SOM parameters. Vetch biomass exceeded 2.00 Mg dry matter ha-1, resulting in > 80 kg N ha-1. Extractable N increased after cover crop termination approaching 25 mg N kg soil-1 in VET. Post-termination PMN increased at GR Y1 and LAMB Y2, to 30-65 mg N kg soil-1, from baseline values of ~ 20 mg N kg soil -1 pre-termination. Yet soil N results displayed major site differences, with PMN declining in LAMB Y1 and GR Y2 pre-termination to post-termination. Soil C results were variable, indicating significant effect of sampling time and environment yet no treatment differences for MB-C or POX-C. However, MB-C was correlated with PMN (R2= 0.24, p = 0.002), indicating labile C may be an effective indicator of N dynamics. Corn yields were highest in LAMB 2015, VET and MIX exceeded 13 Mg ha-1 and out-yielded RYE (~ 8 Mg ha-1). Our project suggests winter annual legume cover crops may achieve multiple ecosystem services including winter soil coverage, SOM development, and coupled C and N cycling.