The Effect of Soil Organic Carbon Amendments on Nitrogen Dynamics after Broccoli (Brassica oleracea L. var. Italica) Production
After broccoli (Brassica oleracea L.) harvest over 400 kg N ha-1 typically remains in the field as readily mineralizable crop residues and soil inorganic N which is susceptible to post-harvest losses. Therefore, the goal of this research was to develop a best management practice (BMP) to minimize potential N losses after broccoli production. Organic C amendments (OCAs) may reduce N losses via immobilization, but for the development of an effective BMP research must evaluate the synchrony of OCA and crop residue decomposition rates, quantity of net N immobilization and fate of crop residue-N, and potential effects of OCA on subsequent crop production. Therefore, N dynamics were evaluated via incubation, field, and tracer studies after broccoli (var. italica) harvest with soil OCAs. In the 56 d laboratory trial, broccoli crop residue or fertilizer-N was incorporated with either wheat straw, yard waste, or used cooking oil. In the field during 2009-2010 and 2010-2011, wheat straw, yard waste, or used cooking oil were applied in a broccoli-spring wheat (Triticum durum L.) rotation. From broccoli to spring wheat in 2011-2012, 15N enrichments were used to trace the fate of residual-N into soil and plant pools, with or without used cooking oil. Net N immobilization was observed with all OCAs in the incubation study, but used cooking oil had a synchronous decomposition rate with broccoli residue and a synergistic effect on N immobilization and microbial activity. In the field studies and compared to the typical practice, wheat straw and yard waste reduced spring wheat yield or had an inconsistent effect in decreasing potential N losses, thus are not recommended as a BMP. In the field and tracer studies relative to no amendment, used cooking oil consistently reduced potential N losses by up to 112 kg N ha-1 and increased recovery of broccoli crop residue-N in soil total, mineral, and organic N fractions by 190-209%, without affecting subsequent spring wheat yield or N content. It is therefore recommended that growers apply used cooking oil as a BMP at broccoli harvest to minimize potential above-ground crop residue-N losses and to increase the soil organic N fraction.