The inhibitory efficacy of procyanidin on soil denitrification varies with N fertilizer type applied

Denitrification is a major process of the nitrogen (N) cycle by converting nitrate (NO₃^-) back to gaseous nitrogen (N₂), which leads to massive losses of N, including fertilizer N, from agricultural systems. One mitigation strategy for these N losses involves denitrification inhibition by plant-derived biological denitrification inhibitors (BDIs). Procyanidin was recently identified as a new class of BDI in root extracts from Fallopia spp. However, the efficacy of this compound on soil denitrification under different N fertilizer sources is not well understood. Here, a 14-day microcosm experiment was conducted using three nitrate-based fertilizers (NH₄NO₃, KNO₃, and Ca(NO₃)₂) to investigate the impact of procyanidin on soil denitrification and associated microbial pathways. Results showed that procyanidin inhibited denitrification activity regardless of the source of N fertilizer applied, but the inhibitory efficacy of procyanidin varied with N fertilizer types. Addition of procyanidin had greater denitrification inhibition in the soils applied with NH₄NO₃ than with other types of N fertilizer. Moreover, nitrate reductase activity was significantly suppressed by procyanidin addition across all three N fertilizers tested. Quantification of denitrifying functional genes (nirS, nirK, and nosZ) demonstrated that procyanidin inhibited the activity and growth of nirS- and nirK-type denitrifiers, but stimulated the growth of nosZI-containing denitrifiers. These findings indicate that the inhibition of soil denitrification by procyanidin was mainly a result of the suppression of nitrate reductase activity and nirS- and nirK-type denitrifiers abundance. The use of procyanidin together with N fertilizers, especially NH₄NO₃, can be an effective way to reduce the N losses by denitrification.