Effects of combined application of manure and inorganic fertilizer on N2O emissions and sources in vegetable soils

Abstract: The soils are dominating source of nitrous oxide (N2O), a greenhouse gas that contributes to stratospheric ozone destruction. In China, vegetable soils are amended with the highest level of N fertilizers among agricultural soils, causing large N2O flux. Bacterial nitrification and denitrification are thought to be the primary process for N2O emission from soil. Recently, it has been suggested that the intramolecular distribution of 15N between central (α) and terminal (β) position in the liner N2O molecule (βNαNO, known as site preference or SP), can indicate which processes contribute to N2O fluxes. The goal of this test was to confirm N2O isotopomer signature stability effects on source partitioning of N2O and the contribution of microbial process to N2O production and consumption. Here, a microcosm experiment was performed to partition N2O production pathways and its change pattern emitted from vegetable soils amended with different manure, inorganic fertilizers and their combinations by using abundant isotope technique, which mainly focused on SP. The experiments set up five different manure and inorganic fertilizers treatments: 100%M (manure fertilizers), 100%U (inorganic fertilizers), 80%M+20%U, 50%M+50%U, CK (no fertilizers). N2O concentration, 15Nα and 15Nbulk were measured by an element analyzer isotope ratio mass spectrometer and a trace gas analysis system. SP was calculated by N2O isotope characteristic value, giving the contribution to denitrification and nitrification. The results showed that inorganic fertilizer drove significantly higher N2O emissions than that of manure fertilizer. The higher quantity of inorganic fertilizer was used, the more N2O was released. The cumulative emissions of N2O from manure fertilizer treatment were 6.63 folds higher than that of inorganic fertilizer. The values of SP increased first and then decreased gradually with time, suggesting that production processes of N2O were associated with different microbial pathways. Denitrification was the dominant microbial process within seven days after fertilization in each fertilizer combination, the highest proportion reached 78.89%, SP value of 6.97‰ and then nitrification became the main pathway with the highest proportion of 76.48%, SP value of 25.24‰. Denitrification was likely to occur when applying inorganic fertilizer, the average proportion reached 52.98%, SP value of 15.52‰ and the application of manure fertilizer enhanced the nitrification process with the average proportion of 71.35%and SP value of 23.55‰. Considering both values of N2O emissions and SP, it was suggested that manure fertilizer had a positive effect on reducing N2O emissions in Drab Fluvo-aquic soil of Beijing. Of course, denitrification is most important source of N2O production. The results provide scientific basis for the rational fertilization in vegetable production. At the same time, we need to make further work to explore and characterize isotopic signature of N2O to find out microbial identification of N2O.