Isotope Constraints on Nitrate Exchanges Between Precipitation and Forest Canopy

Atmospheric 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 (${{\text{NO}}_3}^{-}$) is a major form of reactive N. Based on a data synthesis of fluxes and isotopes (¹⁵N, ¹⁷O, ¹⁸O) of atmospheric ${{\text{NO}}_3}^{-}$ inputs in forests, here we constructed a new method to quantify fractions and fluxes of throughfall ${{\text{NO}}_3}^{-}$ ($t-{{\text{NO}}_3}^{-}$) contributors (nitrification ($n-{{\text{NO}}_3}^{-}$) and particulates ($p-{{\text{NO}}_3}^{-}$) in canopies, the original precipitation ($b-{{\text{NO}}_3}^{-}$)) and then constrain precipitation-canopy ${{\text{NO}}_3}^{-}$ exchanges (i.e., $t-{{\text{NO}}_3}^{-}$ gains from canopy and $b-{{\text{NO}}_3}^{-}$ losses due to canopy retention). Generally, $t-{{\text{NO}}_3}^{-}$ was higher in fluxes but lower in N and O isotopes than $b-{{\text{NO}}_3}^{-}$, suggesting higher gains than losses and canopy nitrification as a gain contributor. 10%−18% and 40%−47% of $t-{{\text{NO}}_3}^{-}$ were gained from canopy $n-{{\text{NO}}_3}^{-}$ and $p-{{\text{NO}}_3}^{-}$, respectively, while 43% ± 25% and 20% ± 74% of the original $b-{{\text{NO}}_3}^{-}$ were lost after passing through canopies of broadleaved and coniferous forests, respectively. Importantly, both $t-{{\text{NO}}_3}^{-}$ gain and $b-{{\text{NO}}_3}^{-}$ loss fluxes were found increasing with the $b-{{\text{NO}}_3}^{-}$ fluxes. This work unlocked fractions and fluxes of major precipitation-canopy ${{\text{NO}}_3}^{-}$ exchange processes and revealed a stimulating mechanism of atmospheric ${{\text{NO}}_3}^{-}$ pollution on precipitation-canopy ${{\text{NO}}_3}^{-}$ exchanges.