Microbial pathways account for the pH effect on soil N2O production

Clarifying the responses of different N2O production pathways to pH change is helpful to better understand the effect of pH on N2O emission from soils. A(15)N tracing study was carried out on subtropical forest (SF) and cropland (SC) soil in 30-day pH manipulation treatments to quantify the effect of pH on autotrophic nitrification, heterotrophic nitrification and denitrification-derived N2O. Gene abundances of autotrophic nitrification (AOA-amoA, AOB-amoA) and denitrification (nirK, nirS, nosZ)-related N2O production were determined to identify the microbial mechanism behind the pH effect on N2O production. The results showed that the total N2O production rate (N2Ot) of the SF soil was 4.56 and 4.86 mu g N kg(-1) day(-1) at pH 3.5 and 4.5, respectively, which was significantly higher than the other treatments. Similarly, the highest N2Ot of the SC soil was also observed in pH 3.5 treatment (7.15 mu g N kg(-1) day(-1)). Both N2O production rate via denitrification (N2Od) and heterotrophic nitrification (N2Oh) markedly increased, while the N2O production rate via autotrophic nitrification (N2Oa) markedly decreased by decreasing pH in both the SF and SC soil. Thus, denitrification and heterotrophic nitrification were responsible for the high N2O production under strongly acidic conditions. The abundance of nosZ gene was significantly and negatively correlated with N2Od (P < 0.05), and the abundance of AOB-amoA was significantly and positively correlated with N2Oa (P < 0.01). The results of the structural equation modeling (SEM) analysis revealed that soil pH generated a more direct effect on the denitrifying and ammonia-oxidizing gene abundance, in turn, affected N2Od and N2Oa, respectively. In addition, soil pH exerted a significant effect on both the bacterial 16S rRNA and fungal ITS rDNA gene abundance (P < 0.05). However, the role of soil fungi and bacteria on N2Oh could not be determined in this study.