Presence of spring-thaw N2O emissions are not linked to functional gene abundance in a drip-fertigated cropped soil in arid northwestern China

Abstract

Spring-thaw represents a significant source for nitrous oxide (N2O) emissions from fertilized croplands in temperate regions. In this study, we present surface N2O fluxes, soil-profile N2O concentrations at 5, 15, 30 and 60 cm depths along with the abundance of nitrifiers and denitrifiers over the winter and spring-thaw periods in an arid, drip- fertigated cotton field, which had received spring application of 240 kg N ha−1 as urea alone (Urea), polymer-coated urea (ESN), and urea plus urease and nitrification inhibitors. Nitrous oxide emissions from December to April were generally unaffected by fertilizer treatments with a cumulative average of 186 g N ha−1, accounting for 39% of the annual N2O emissions. Emission peaks occurred at spring-thaw and coincided with increasing soil-profile N2O concentrations at all depths, suggesting the burst in N2O fluxes was due to new N2O production, rather than a physical release of N2O trapped in the soil profiles over winter. The abundance of nitrifier and denitrifier genes changed over the winter and spring-thaw periods but was not affected by fertilizer treatments from the previous spring, suggesting the abundance of N2O-producing microorganism was more controlled by environmental conditions than N sources applied in the previous spring. The daily N2O flux rate from December to April was positively correlated with soil temperature, water-filled pore space, and denitrifying enzyme activity, but not with the gene copy number of AOA, AOB, narG, nirS, nirK and nosZ, indicating that variation in the abundance of these genes was not contributing to the N2O emissions. These results suggest that N2O emissions in spring-thaw are substantial for drip-fertigated croplands in the arid regions and should be considered in the annual budgets. The environmental factors such as soil temperature and moisture are likely more important than the copy-numbers of N2O-producing functional genes in driving the variability in spring-thaw emissions.