Abstract

Winter cover crops (WCCs) are effective at reducing N losses from temperate agroecosystems. Although extensive research on WCCs has demonstrated numerous benefits, overall adoption rates in the Midwest U.S. remain low. We evaluated an alternative to WCC’s ability to reduce nitrate (NO3−)-N leaching; that is, adding an inexpensive, easy-to-apply, form of labile carbon (C) as a soil amendment intended to immobilize N and mitigate leaching. In the autumn in a typical maize–soybean rotation, we added crude glycerol (a C-rich, biodiesel byproduct) and hypothesized that glycerol carbon (Cglyc) would immobilize N and have no effect on crop growth. More specifically, Cglyc was broadcast applied at three rates (0, 216, and 866 kg C ha−1 y−1) and combined factorially with six spring-applied fertilizer N rates (0, 56, 112, 168, 224, and 280 kg N ha−1) at two sites. In response, we measured: soil profile NO3−-N, leached NO3−-N, crop health (via SPAD), yield, and maize agronomic optimum N rate (AONR). Cglyc reduced spring soil profile NO3−-N by 14–24% across site-years, but had highly variable and non-significant effects on NO3−-N leaching. Cglyc had an inconsistent impact on crop SPAD and yield, with Cglyc increasing AONR by ~63 kg N ha−1 (or 31–40%) at one of two sites. Our results show promise for using labile C as a “liquid cover crop” soil amendment. Future studies should explore greater labile C application rates and alternate application timing in order to fine-tune the balance between environmental benefits and crop productivity.

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