Relative contributions of different substrates to soil N2O emission and their responses to N addition in a temperate forest

Rapid Responses of Greenhouse Gas Emissions and Microbial Communities to Carbon and Nitrogen Addition in Sediments

Massive labile carbon and nitrogen inputs into lakes change greenhouse gas emissions. However, the rapid driving mechanism from eutrophic and swampy lakes is not fully understood and is usually contradictory. Thus, we launched a short-term and anaerobic incubation experiment to explore the response of greenhouse gas emissions and microbial communities to glucose and nitrate nitrogen (NO3--N) inputs. Glucose addition significantly increased CH4 and CO2 emissions and decreased N2O emissions, but there were no significant differences. NO3--N addition significantly promoted N2O emissions but reduced CH4 accumulative amounts, similar to the results of the Tax4Fun prediction. Bacterial relative abundance changed after glucose addition and coupled with the abundance of denitrification genes (nirS and nirK) decreased while maintaining a negative impact on N2O emissions, considerably increasing methanogenic bacteria (mcrA1) while maintaining a positive impact on CH4 emissions. Structural equation modeling showed that glucose and NO3--N addition directly affected MBC content and greenhouse gas emissions. Further, MBC content was significantly negative with nirS and nirK, and positive with mcrA1. These results significantly deepen the current understanding of the relationships between labial carbon, nitrogen, and greenhouse emissions, further highlighting that labile carbon input is the primary factor driving greenhouse gas emissions from eutrophic shallow lakes.

Unraveling the drivers for interannual variabilities of N2O fluxes from forests soils across climatic zones

Forest soils are an important source of nitrous oxide (N2O), however, field observations of N2O emission have often exhibited large variabilities when compared with managed agricultural lands. In the last decade, the number of forest N2O studies has increased more than tenfold, but only a few of them have looked into the interannual flux variabilities from the regional scale. Here, we have collected 30 long-term N2O monitoring studies (≥ 2 years) based on a global database, and extracted variabilities (VARFlux) as well as relative variabilities (VAR%, in proportions) of annual N2O fluxes. The relationship of mean annual precipitation (MAP), mean annual temperature (MAT), and nitrogen (N) deposition with flux variabilities was examined to explore the underlying mechanisms for N2O emission on a long-term scale. Our results show that mean VARFlux is 0.43 kg N ha-1 yr-1 and VAR% is 28.68%. Across climatic zones, the subtropical forests have the largest annual N2O fluxes, as well as the largest fluctuations among annual budgets, while the tropics were the smallest. We found that the regulating factors for VARFlux and VAR% are fundamentally different, i.e., MAT and N input determine the annual fluxes as well as VARFlux while MAP and other limiting soil parameters determine VAR%. The relative contributions of different seasons to flux variabilities were also explored, indicating that N2O fluxes of warm and cool seasons are more responsible for the fluctuations in annual fluxes of the (sub)tropical and temperate forests, respectively. Overall, despite the limitation in interpretations due to few long-term studies from literature, this work highlights that significant interannual variabilities are common phenomena for N2O emission from different climatic zones forest soils; by unraveling the divergent drivers for VARFlux and VAR%, we have provided the possibility of improving N2O simulation models for constraining the heterogeneity of N2O emission processes from climatic zones forest soils.

Nitrogen addition stimulates N2O emissions via changes in denitrification community composition in a subtropical nitrogen-rich forest

Microbially driven nitrification and denitrification play important roles in regulating soil N availability and N2O emissions. However, how the composition of nitrifying and denitrifying prokaryotic communities respond to long-term N additions and regulate soil N2O emissions in subtropical forests remains unclear. Seven years of field experiment which included three N treatments (+0, +50, +150 kg N ha-1 yr-1; CK, LN, HN) was conducted in a subtropical forest. Soil available nutrients, N2O emissions, net N mineralization, denitrification potential and enzyme activities, and the composition and diversity of nitrifying and denitrifying communities were measured. Soil N2O emissions from the LN and HN treatments increased by 42.37% and 243.32%, respectively, as compared to the CK. Nitrogen addition significantly inhibited nitrification (N mineralization) and significantly increased denitrification potentials and enzymes. Nitrification and denitrification abundances (except nirK) were significantly lower in the HN, than CK treatment and were not significantly correlated with N2O emissions. Nitrogen addition significantly increased nirK abundance while maintaining the positive effects of denitrification and N2O emissions to N deposition, challenging the conventional wisdom that long-term N addition reduces N2O emissions by inhibiting microbial growth. Structural equation modeling showed that the composition, diversity, and abundance of nirS- and nirK-type denitrifying prokaryotic communities had direct effects on N2O emissions. Mechanistic investigations have revealed that denitrifier keystone taxa transitioned from N2O-reducing (complete denitrification) to N2O-producing (incomplete denitrification) with increasing N addition, increasing structural complexity and diversity of the denitrifier co-occurrence network. These results significantly advance current understanding of the relationship between denitrifying community composition and N2O emissions, and highlight the importance of incorporating denitrifying community dynamics and soil environmental factors together in models to accurately predict key ecosystem processes under global change.