Isotopic composition of throughfall nitrates in suburban forests with different vegetations

Tracking the fate of deposited nitrogen and its redistribution in a subtropical watershed in China

“…Therefore, the nitrate transported by H 2 O 18 and HDO 18 or D 2 O 18 from throughfall and soil layer would reach 1,537 kg·yr −1 and 3,391 kg·yr −1 , respectively. Yeon et al () also reported that the NO 3 − concentration in throughfall would always have higher 15 N‐NO 3 and 18 O‐H 2 O values than that in rainfall. This can be explained as vegetation absorbing NO 3 − through leaf apoplast due to throughfall.…”

“…As shown in Figure , δ 15 N‐NO 3 and δ 18 O‐NO 3 in the Xiangxi River watershed were impacted by various factors under the different vegetation cover types due to the distribution of dual isotopic characteristics being the different sources. Rainfall was mainly a source of atmospheric nitrate deposition, so δ 15 N‐NO 3 and δ 18 O‐NO 3 would be affected by atmospheric NO x oxidation reactions and exchanges with atmospheric O 3 (Hastings, Sigman, & Lipschultz, ; Yeon et al, ). The dual isotopic approach also enables us to trace denitrification, because this process generates δ 18 O to δ 15 N between 0.5 and 1 values.…”

“…The δ 15 N‐NO 3 and δ 18 O‐NO 3 showed seasonal changes can be explained by that nitrates would be enriched in 18 O during chemical reactions with tropospheric O 3 . There was a significant difference on solar radiation between winter and summer, so in winter, nitrates are dominantly formed by the N 2 O 5 ‐radical pathway and then increasing δ 18 O‐NO 3 (Hastings et al, ), but in summer, the strong solar radiation would improve the OH‐radical pathway and then decrease δ 18 O‐NO 3 values (Hastings et al, ; Yeon et al, ).…”

“…The dual isotopic approach also enables us to trace denitrification, because this process generates δ 18 O to δ 15 N between 0.5 and 1 values. Certain sampling points in the study area were between the 1:1 and 1:2 ratio, which means that denitrification occurred in certain sampling points in the Xiangxi River watershed (Hao, Gao, Sun, & Wen, 2018 (Hastings et al, 2003;Yeon et al, 2015).…”

“…As shown in Table 3, the different vegetation canopy cover types caused significant dilution effects on N deposition during the rainy season, which may be because available soil N was adequate during this season; thus, vegetation did not need to absorb N via canopy interception (Pallardy, 2008 (Hastings, Sigman, & Lipschultz, 2003;Yeon et al, 2015). The dual isotopic approach also enables us to trace denitrification, because this process generates δ 18 O to δ 15 N between 0.5 and 1 values.…”