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

Authors: White, Kathryn; Brennan, Eric; Cavigelli, Michel; SMITH, RICHARD - University Of California - Cooperative Extension Service

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2022
Publication Date: 4/18/2022
Citation: White, K.E., Brennan, E.B., Cavigelli, M.A., Smith, R.F. 2022. Winter cover crops increased nitrogen availability and efficient use during eight years of intensive organic vegetable production. PLoS ONE. 17(4). Article e0267757. https://doi.org/10.1371/journal.pone.0267757.
DOI: https://doi.org/10.1371/journal.pone.0267757

Interpretive Summary: For farmers in the Central Coast region of California to meet new regulatory requirements to reduce nitrate leaching to ground water, efficient use of nitrogen is essential. This study provides the first long-term data on nitrogen budgets (i.e., nitrogen applied versus nitrogen removed in harvest crops) for this major region of vegetable production the U.S. It is based the first eight years of on an on-going experiment called the Salinas Organic Cropping System experiment. This important experiment recently provided critical data that helped to increase the nitrogen credit that farmers in the region can receive for best-management practices like cover cropping. The experiment includes systems that differ in cover cropping frequency (annually versus every 4th year) and compost inputs. We tabulated nitrogen inputs from irrigation water, cover crop seed, vegetable transplant plugs, yard waste compost, nitrogen fixation and pelleted and liquid organic fertilizers. Nitrogen removed from the field from eight crops of lettuce and six crops of broccoli were also measured. Across all systems less than a quarter of nitrogen inputs left the field in harvested vegetables and the remaining surplus is at risk of loss. Various scenarios to reduce the risk of nitrogen losses and increase nitrogen cycling in these systems are discussed.

Technical Abstract: Efficient use of nitrogen is essential to protect water quality in highly fertilized intensive organic vegetable production systems, but little is known about the long-term effects of organic management on nitrogen mass balances. We measured soil nitrogen and tabulated nitrogen inputs (organic fertilizers, compost and legume cover crops) and exports in harvested crops (lettuce, broccoli) over eight years to calculate soil surface and soil system nitrogen 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 nitrogen, cover crop nitrogen uptake, and vegetable crop nitrogen uptake and yield, and 2) tabulate nitrogen balances to assess the effects of these treatment factors on nitrogen export in harvested crops, soil nitrogen storage and potential nitrogen loss. Results show that across all systems only 13 to 23% of nitrogen inputs were exported in harvest and the remaining surplus is at risk of loss. Annual compost applications increased soil nitrogen stocks but had little effect on vegetable nitrogen uptake or yield, increasing the cumulative nitrogen balance surplus over eight years by 1017 kg ha-1, relative to the system receiving organic fertilizers alone. Annually planted winter cover crops increased nitrogen availability, crop uptake and export. However, additional nitrogen inputs from 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 nitrogen surplus by 369 kg ha-1 relative to the legume-rye mixture by increasing nitrogen retention and availability without increasing nitrogen inputs. Reduced reliance on external compost inputs and increased use of annually planted winter cover crops can improve efficient nitrogen use and cropping system yield, consequently improving environmental performance.