Isotope Constraints on Nitrate Exchanges Between Precipitation and Forest Canopy

Abstract

AbstractAtmospheric nitrogen (N) deposition is a key process influencing plant‐soil N processes and associated functions of forest ecosystems. However, the N deposition into soils based on open‐field precipitation observations remains inaccurate due to the unconstrained precipitation‐canopy N exchanges, which prevents a better evaluation of N deposition effects on forest N cycles and functions. Nitrate () is a major form of reactive N. Based on a data synthesis of fluxes and isotopes (15N, 17O, 18O) of atmospheric inputs in forests, here we constructed a new method to quantify fractions and fluxes of throughfall () contributors (nitrification () and particulates () in canopies, the original precipitation ()) and then constrain precipitation‐canopy exchanges (i.e., gains from canopy and losses due to canopy retention). Generally, was higher in fluxes but lower in N and O isotopes than , suggesting higher gains than losses and canopy nitrification as a gain contributor. 10%−18% and 40%−47% of were gained from canopy and , respectively, while 43% ± 25% and 20% ± 74% of the original were lost after passing through canopies of broadleaved and coniferous forests, respectively. Importantly, both gain and loss fluxes were found increasing with the fluxes. This work unlocked fractions and fluxes of major precipitation‐canopy exchange processes and revealed a stimulating mechanism of atmospheric pollution on precipitation‐canopy exchanges.