Altered precipitation patterns raise the risk of reduction in deposited nitrogen induced carbon sinks in an alpine grassland

Abstract

Even if the total amount of precipitation remains constant, the intra-annual precipitation distribution could profoundly affect the nitrogen (N) cycling and carbon (C) sequestration. However, previous studies have concentrated on N retention and C sinks in response to N deposition, with limited evidence on scenarios involving different precipitation regimes. We conducted a ¹⁵NO₃⁻ and ¹⁵NH₄⁺ tracer experiment with alterations in the precipitation pattern to investigate the retention of deposited N and associated C sink responses in alpine grasslands via the stoichiometric scaling method. We manipulated precipitation patterns by redistributing 40 % of the precipitation from the early to late growing season. The present study revealed that the total retention rates of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were 1.59 and 1.27 kg N ha⁻¹ yr⁻¹, respectively, under natural precipitation conditions in alpine grasslands. However, changes in precipitation distribution reduced total ¹⁵NO₃⁻ retention by 14.21 % to 19.04 % and total ¹⁵NH₄⁺ retention by 12.71 % to 14.06 %, indicating a strong sensitivity of N cycling to intra-annual precipitation variability. The C sink induced by ¹⁵N deposition was 83.57 kg C ha⁻¹ yr⁻¹ under natural precipitation conditions. The alternative precipitation distribution reduced this C sink, which was induced by ¹⁵N deposition by 11.39 % to 15.05 %. The reduction in the C sink induced by N deposition under changing precipitation patterns is attributed mainly to decreased N retention resulting from a temporal decoupling between plant N demand and soil N availability of N in alpine grasslands.