Kinetics and dynamics of dissolved organic nitrogen, and nitrogen cycling genes in the soil profile of an irrigated vineyard with surface-applied organic amendments

Understanding the dynamics of dissolved organic nitrogen (DON) in the vineyard’s soil is crucial for nitrogen (N) management in this agroecosystem. This study investigated the kinetics and dynamics of DON, including underlying microbial mechanisms, in the soil profile of an irrigated Merlot vineyard in the Okanagan Valley, BC, Canada. Biennial treatments from 2011 to 2018 included wood-based mulch with or without agricultural compost (26.7 and 13.7 Mg ha−1 fresh weight, respectively) and a non-amended control. In the Fall of 2018, composite soil samples were collected at 15 cm intervals down to 60 cm depth. Microbial N cycling functional potential was determined, and initial DON concentrations, and potentially releasable DON (N0-DON), and the DON release rate constant (k-DON) were assessed via 100-day aerobic incubation. Pre-incubation DON levels (0.89–44.1 mg N kg−1) were higher in mulch + compost ≥ mulch > control and decreased with depth. DON release followed first-order kinetics, with N0-DON ranging from 12.1 to 188 mg N kg−1 and being unaffected by amendments, but decreased with depth. Cumulative DON averaged 33.8 mg N kg−1 and was also unaffected by amendments, but it decreased with depth. The k-DON (0.016–0.224 wk−1) was influenced by amendments and depth, with higher values in mulch + compost compared to mulch-only and control, and it declined with depth. Organic amendments doubled bacterial proteolytic capacity, positively correlating with N parameters. Bacillota-related proteolytic (npr) and nitrate reduction (nrfA) genes were associated with cumulative DON and nitrate release during the incubation, suggesting that mineral N released by proteolytic activity was re-immobilized into microbial biomass, maintaining plant-available N throughout the soil profile. First-order kinetics further support the key role of soil organisms in DON dynamics. In low-N systems like vineyards, DON supply rivals mineral N and increases with organic amendments. While DON supplementation supports mineral N supply, its potential capacity diminishes over the long term, necessitating periodic applications to maintain effectiveness. The relatively fast release of Nmin and DON from mulch + compost makes it ideal for early spring applications to synchronize N release with grapevine N uptake. These findings are particularly relevant for organic vineyards with limited access to N sources.