Soil organic nitrogen fractions mediate the response of ammonia volatilization to water-nitrogen regulation in Populus tomentosa plantations

Ammonia volatilization (AV) stands out as a major cause of nitrogen loss in plantation ecosystems, significantly undermining the effectiveness of nitrogen fertilizers. While soil organic nitrogen is known to serve as a crucial internal reservoir for nitrogen transformation, the specific pathways through which its various fractions influence AV—particularly under different water-nitrogen regulations—remain poorly understood. We conducted a split-plot field experiment in Populus tomentosa plantations and applied partial least squares path modeling (PLS-PM) to explore the dynamics of gaseous nitrogen loss under combined irrigation and fertilization treatments. Our results reveal that moderate irrigation, when coupled with higher nitrogen inputs, markedly increases AV, with the two factors exhibiting a strong interactive effect. A comprehensive evaluation determined that maintaining 45% field capacity (FC) combined with a nitrogen application rate of 304.8 kg·ha⁻¹ (W1N3) constitutes the optimal management strategy. Among the acid-hydrolyzable nitrogen (AHN) fractions, acid-hydrolyzable ammonium nitrogen (AN) and amino acid nitrogen (AAN) were especially responsive to water-nitrogen interactions, showing a clear and significant positive correlation with cumulative AV. Water and nitrogen inputs drive volatilization primarily by modifying soil physical and chemical conditions and accelerating mineralization processes. This process enriches the pool of mineral nitrogen substrates, indirectly intensifying AV. These findings underscore the central role of organic nitrogen turnover in driving ammonia emissions and offer valuable insights for optimizing nitrogen use efficiency. Through targeted irrigation and fertilization strategies, it is possible to reduce nitrogen losses and enhance sustainability in plantation forest management.