Abstract
Cole crops (Brassica vegetables) can pose a significant risk for N losses during the post-harvest period due to substantial amounts of readily mineralizable N in crop residues. Amending the soil with organic C has the potential to immobilize N and thereby reduce the risk for N losses. Four field trials were conducted to determine the effects of organic C amendments (OCA) on N dynamics and spring wheat (Triticum durum L.) harvest parameters proceeding early- and late-broccoli (Brassica olecerea var italica L.) systems in 2009 and 2010. The experimental controls represented the traditional grower practice of incorporated broccoli crop residue (CR-control) and the pre-plant application of N fertilizer (CRN-control) to subsequent spring wheat. Alternative practices were compared to the controls, which included broccoli crop residue removal (CR-removal), an oat (Avena sativa L.) cover crop (CC-oat), and three different OCA of wheat straw (OCA-straw), yard waste (OCA-yard), or used cooking oil (OCA-oil). The treatments, which demonstrated reduced autumn soil mineral N (SMN) concentrations after broccoli harvest, relative to the CR-control, were CR-removal, OCA-straw, and OCA-oil. Although CR-removal and OCA-straw indicated a reduced potential for autumn soil N losses in the early-broccoli system, these practices are not recommended for growers because subsequent spring wheat yield and profit margins were reduced compared to the CR- and CRN-controls. The OCA-oil reduced autumn SMN concentrations by 53 to 112 kg N ha−1 relative to the CR-control after both early- and late-broccoli harvest, suggesting a larger potential for reduced autumn soil N losses, compared to all other treatments. No detrimental effects resulted from the OCA-oil treatment on the subsequent spring yield or grain N. The OCA-oil reduced spring wheat profit margins relative to the CR-control, like the OCA-straw and CR-removal treatments, however profit margins were similar between the OCA-oil and the CRN-control. Therefore, in areas with a high risk of environmental N contamination, growers should consider the OCA-oil practice after cole crop harvest to minimize the risk of N losses.
Highlights
Nitrogen fertilizer applications are frequently used to enhance vegetable crop production, and much research has been done to reduce N losses during the vegetable crop growing season [1,2]
organic C amendments (OCA)-straw indicated a reduced potential for autumn soil N losses in the early-broccoli system, these practices are not recommended for growers because subsequent spring wheat yield and profit margins were reduced compared to the CR- and CRN-controls
The lower yield in the early-broccoli compared to the late-broccoli system agrees with previous research, which found lower total and head plant mass in summer harvested broccoli compared to autumn harvest [39], likely due to different environmental conditions during the broccoli growth periods
Summary
Nitrogen fertilizer applications are frequently used to enhance vegetable crop production, and much research has been done to reduce N losses during the vegetable crop growing season [1,2]. After harvest, a large quantity of N can remain in vegetable crop residues [2,3,4], which readily mineralizes [3] and may be susceptible to post-harvest losses. There is need to minimize N losses during the post-harvest season when the risk of losses is much greater due to the annual water budget in Ontario [5] and elsewhere. Cole crop (Brassica vegetables), in particular, produce optimal yields with high N applications, ranging from 270 to 550 kg N haí for broccoli (Brassica olecerea var italica L.) [4,6,7,8,9].
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