Nitrogen deposition linked to increased transpiration in Korean larch (Larix olgensis) depended on precipitation

Abstract

Anthropogenic exogenous nitrogen input is essential for the hydrologic cycle in forest ecosystems and is of critical concern for forest hydrological partitioning. To investigate the impacts of nitrogen deposition on transpiration in Larix olgensis plantations, we manipulated nitrogen deposition using four nitrogen (N) treatments (LN: 20 kg N hm−2 a−1; MN:40 kg N hm−2 a−1;HN:80 kg N hm−2 a−1; and control: CK). In these treatments, we examined the sap flow densities (Js), whole tree transpiration(Er), and canopy transpiration (Ec), and monitored the variation of environmental factors during three successive growing seasons from July 2015 to October 2017 in the northeastern China. 19 months were divided into three groups upon the different hydrothermal conditions. The results indicated that N addition generally increased the daily average Js, while the increment amplitude depended on water conditions. A nonlinear response of Er to N deposition was detected and varied with time; in addition, abundant precipitation may mitigate the Er increases induced by N deposition. Furthermore, N addition also enhanced Ec and promoted the percentage of Ec to precipitation, especially in rainless conditions. The significant linear positive correlation between daily average Js and Rs or VPD was found and the sensitivity of Js to Rs was weak, for the slope was relatively constant regardless of the amount of precipitation or N addition. On the contrary, the sensitivity of Js to VPD increased with N addition and was closely related with the hydrothermal conditions. These results suggested the complex influence of N deposition on plant transpiration depended on precipitation and may aggregate drought pressure.