Quantifying nitrogen provisioning and release from cover crops in walnut orchards

Walnut orchards in Mediterranean climates are characterized by wet winters that can lead to nitrogen (N) leaching. Cover cropping can help alleviate N losses from leaching, but using cover crops as a N management strategy in orchards requires understanding and quantifying N mineralization and availability throughout the growing season. This study measured total N input, N derived from fixation, and N mineralization rates from cover crop residues across four cool-season cover crop treatments: a clover mixture, a clover-grass mixture, a multiplex mixture, and resident vegetation. We grew cover crops in two consecutive years in a California walnut orchard and measured changes in the N status of orchard soils. By using isotopically labeled cover crop residues, we were able to determine the mineralization rate of cover crop N. We found that the multiplex treatment consistently provided the greatest carbon and N inputs. Moreover, the multiplex treatment also had the most N inputs derived from fixation. Clover and multiplex aboveground residues had the largest N mineralization rates, and the decomposition of these residues on the soil surface appeared to prime N mineralization in the topsoil. In our study, the multiplex treatment offered the greatest overall N benefits, likely due to the overall success of this treatment in establishment and biomass production. Moreover, we found that the non-leguminous species in the multiplex treatment appeared to also utilize and sequester fixed N from the leguminous covers, demonstrating a relatively unexplored benefit of this multiplex cover crop mixture. • We quantified N inputs from different cover crop treatments in walnut orchards. • The multiplex treatment provided the highest C and N inputs and benefits. • Non-N-fixing species utilized fixed N, enhancing N benefits in cover crop mixtures. • Clover and multiplex treatments degraded fastest, releasing the most N.