Post-Harvest Organic Carbon Amendments to Minimize Mineral Nitrogen Losses in Cole Crop Production.

Cole crops, compared to many other crops, may pose the greatest risk of inorganic N losses during the post-harvest period due to substantial amounts of readily mineralizable N in crop residues. A management practice of amending soil with readily decomposable organic carbon has the potential to immobilize excess N in agricultural systems. In a broccoli (Brassica oleracea var. Italica cv ‘Ironman’) -spring wheat (Triticum aestivum L.) rotation, a two-year replicated field study (2009-2010 and 2010-2011) was conducted to determine the effects of organic carbon amendment on soil nitrate content (0-30 cm depth) after broccoli harvest. The experiment was a randomized complete block design with four replications, in both Early and Late broccoli production. Post-harvest treatments included a positive control (incorporated broccoli crop residue), a negative control (no broccoli crop residue) a reference (oat (Avena sativa L.) cover crop), and three different broccoli crop residue + organic carbon amendments (at 5 Mg ha^-1): (i) crop residue + wheat straw (ii) crop residue + yard waste, and (iii) crop residue + used cooking oil. After broccoli harvest, crop residue was mulched, organic carbon amendments were hand applied, and plots disked twice for incorporation. In the subsequent growing season spring wheat was grown on same plots without addition of N fertilizer, and a control treatment of spring applied N fertilizer (103 kg N ha^-1) was also included. In 2009 after Early harvest, wheat straw and used cooking oil significantly reduced soil nitrate concentrations relative to broccoli residue control by 52 and 64 kg N ha^-1, respectively, while after Late harvest used cooking oil reduced control nitrate concentrations by 65 kg N ha^-1. In 2010 after Early harvest, wheat straw, yard waste, and used cooking oil significantly reduced nitrate concentrations relative to broccoli residue control by 61, 72, and 77 kg N ha^-1, respectively, while after Late harvest used cooking oil reduced positive control nitrate concentrations by 59 kg N ha^-1. There was no impact of organic carbon amendments on spring wheat yield or grain N content in the subsequent growing season in 2010. The study provides evidence that organic carbon can reduce nitrate levels after broccoli harvest, without impacting subsequent yield.