LAUSR

Abstract Nitrogen (N) deposition has been critical to the increase in the terrestrial carbon (C) sink in recent decades. Clarifying the C-N cycling responses in rangelands to N enrichment is essential for quantitatively understanding and predicting how N enrichment influences terrestrial C sinks. However, the N enrichment effects on rangeland C-N cycling under livestock grazing, rather than grazing exclusion, have rarely been investigated. Here, we conducted a 4-year in situ field experiment to examine the effects of multilevel N addition (NH4NO3: 0, 10, 20, 40, 80 and 160 kg N ha−1 yr−1) on soil N characteristics and ecosystem C fluxes in an alpine pasture on the Tibetan Plateau. We found that there were no significant differences in the soil organic C, soil total N, or soil C:N ratio across the six N addition levels. Over the four years, N addition generally increased the average soil inorganic N, with an increase ranging from 6% for the addition of 10 kg N ha−1 yr−1 to 292% for the addition of 160 kg N ha−1 yr−1. Such an increase in soil inorganic N was dominated by soil NO3−-N enrichment rather than NH4+-N variations, resulting in a decreased soil NH4+/NO3− ratio. Unlike the significant response of soil inorganic N to N addition, the net ecosystem CO2 exchange, gross ecosystem productivity and ecosystem respiration averaged over the growing season were not significantly changed by N addition in the third and fourth years, possibly due to the regulation by livestock grazing. In the last experimental year, N addition significantly enhanced belowground biomass but had no significant effect on aboveground biomass, leading to an increase in the root:shoot ratio. Our study highlights that N addition altered C-N cycling in alpine pastoral ecosystems and provides experimental evidence for quantitatively parameterizing the C process responses to N variations in terrestrial ecosystem models.