Winter cover crops increased nitrogen availability and efficient use during eight years of intensive organic vegetable production

Abstract

Efficient use of nitrogen (N) is essential to protect water quality in high-input organic vegetable production systems, but little is known about the long-term effects of organic management on N mass balances. We measured soil N and tabulated N inputs (organic fertilizers, compost, irrigation water, atmospheric deposition, cover crop seed, vegetable transplant plugs and fixation by legume cover crops) and exports in harvested crops (lettuce, broccoli) over eight years to calculate soil surface and soil system N mass balances for the Salinas Organic Cropping Systems study in Salinas, CA. Our objectives were to 1) quantify the long-term effects of compost, cover crop frequency and cover crop type on soil N, cover crop and vegetable crop N uptake, and yield, and 2) tabulate N balances to assess the effects of these factors on N export in harvested crops, soil N storage and potential N loss. Results show that across all systems only 13 to 23% of N inputs were exported in harvest. Annual compost applications increased soil N stocks but had little effect on vegetable N uptake or yield, increasing the cumulative soil system N balance surplus over eight years by 999 kg ha-1, relative to the system receiving organic fertilizers alone. Annually planted winter cover crops increased N availability, crop uptake and export; however, biological N fixation by legumes negated the positive effect of increased harvest exports on the balance surplus in the legume-rye cover cropped system. Over eight years, rye cover crops improved system performance and reduced the cumulative N surplus by 384 kg ha-1 relative to the legume-rye mixture by increasing N retention and availability without increasing N inputs. Reduced reliance on external compost inputs and increased use of annually planted non-legume cover crops can improve efficient N use and cropping system yield, consequently improving environmental performance.

Optimizing the Nitrogen Use Efficiency in Vegetable Crops

“…In an eight-year experiment conducted in Salinas, California, consisting of organic vegetable rotations, a rye-legume cover crop combination also resulted in improved NUE. However, because of the relatively high levels of external inputs, the legume cover crops increased the surplus of N in the system, which may result in environmental losses, while rye was more effective as a cover crop, to minimize N surplus levels from the system [217]. Based on multi-year cover crop evaluations conducted in Japan, including 15 N studies, a combined 2:1 hairy vetch/rye planting ratio was also found to improve the mineralization and synchronization of N release by the individual species, with the N uptake demand of tomato, along with an increase in the overall soil N pool and N cycling [218].…”

Legume Cover Crop Alleviates the Negative Impact of No-Till on Tomato Productivity in a Mediterranean Organic Cropping System

“…), which have a deep and branched taproot and grow relatively quickly to cover the soil and control weeds [18], can also be used as part of other cereal-legume combinations. Mixtures with the right ratio of legume to cereal can guarantee a low residue C: N ratio, which is fundamental for N to be available to the cash crop, especially in situations with slow mineralization rates such as in no-till systems [21,[24][25][26].…”

Reply to Muramoto, J.; Bolda, M. Comments on “Verteramo Chiu, L.J.; Gomez, M.I. A Tale of Two Strawberries: Conventional and Organic Open-Field Production in California. Sustainability 2023, 15, 14363”

“…Compost is environmentally neutral, according to the libraries used in the LCA study. Although some studies report that compost application increases soil carbon sequestration in row crops in California (Tautges et al, 2019; White et al, 2020) [3,4], the amount of soil carbon sequestration varies by crop system, which makes it difficult to generalize to other crop systems. Our LCA study is based on the environmental effects of the inputs and processes reported in the libraries of SimaPro.…”