Unraveling the drivers for interannual variabilities of NO fluxes from forests soils across climatic zones.

Forest soils are an important source of nitrous oxide (NO), however, field observations of NO emission have often exhibited large variabilities when compared with managed agricultural lands. In the last decade, the number of forest NO 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 NO monitoring studies (≥ 2 years) based on a global database, and extracted variabilities (VAR) as well as relative variabilities (VAR, in proportions) of annual NO 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 NO emission on a long-term scale. Our results show that mean VAR is 0.43 kg N ha yr and VAR is 28.68%. Across climatic zones, the subtropical forests have the largest annual NO fluxes, as well as the largest fluctuations among annual budgets, while the tropics were the smallest. We found that the regulating factors for VAR and VAR are fundamentally different, i.e., MAT and N input determine the annual fluxes as well as VAR while MAP and other limiting soil parameters determine VAR. The relative contributions of different seasons to flux variabilities were also explored, indicating that NO 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 NO emission from different climatic zones forest soils; by unraveling the divergent drivers for VAR and VAR, we have provided the possibility of improving NO simulation models for constraining the heterogeneity of NO emission processes from climatic zones forest soils.