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 (NO) back to gaseous nitrogen (N), 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 (NHNO, KNO, and Ca(NO)) 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 NHNO 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 NHNO, can be an effective way to reduce the N losses by denitrification.