Occurrence and 15N-quantification of simultaneous nitrification and denitrification in N-fertilised soils incubated under oxygen-limiting conditions

Abstract

Nitrification and denitrification are known to co-occur in soils, but the effect of fertilisation history on NO fluxes and the relative source partitioning of the NO has not been thoroughly investigated. In this study, we therefore combined a high-tech N stable isotope tracing technique with quantitative PCR (qPCR) to explore the relative contributions of nitrification and denitrification to NO production by a sandy-loam Eutric Cambisol soil treated repeatedly with ammonium sulfate [(NH)SO] or potassium nitrate (KNO) for 3 years prior. Both soils (historically (NH)SO and historically KNO treated) were amended separately with (NH)SO and KNO and incubated at 80% water filled pore space for 30 days. Soil NO emissions, NH and NO concentrations and their corresponding N-enrichments were determined. The effect of N addition on N transformation rates was also calculated. The total abundance of nitrifiers was estimated by qPCR of the amoA gene from bacteria and archaea, and that of denitrifiers by using the nirK, nirS, norB and nosZI genes as molecular targets. In the historically (NH)SO-treated soil, 49.0–58.0% of the NO emitted originated from nitrification and 42.0–51.0% from denitrification during incubation. The production of NO was accompanied by a decrease in soil NH concentrations and a parallel increase in the concentration of soil NO . In addition, the abundance of the bacterial and archaeal amoA gene increased during the incubation. Conversely, in the soil historically treated with KNO, the N isotopic analyses showed that denitrification contributed 84.0–99.0% of the total NO produced. Decreases in soil NO concentrations paralleled the increase in N enrichment of NO and the abundance of the nirK, nirS, norB and nosZI genes. The results also showed that values of N enrichment were significantly higher in the KNO-treated soil, which is in line with the higher abundance of the nosZI gene. Calculation of the N transformation rates indicated that autotrophic nitrification and denitrification were responsible for NO production in the historically (NH)SO-treated soil and that denitrification was the most important NO source in the soil treated with KNO. We conclude that N-fertilisation history, and not simply soil oxygen availability, affect the relative contributions of nitrification and denitrification to soil NO emissions. Indeed, here we have shown that nitrification can be an important NO source process even in soils maintained at high moisture contents.