Litter accumulation retards the positive effects of nitrogen inputs on plant carbon sequestration in grassland

Purpose A better understanding of the nitrogen (N) saturation threshold for plant carbon (C) sequestration would help project the variations of ecosystem C sequestration under the background of increasing N inputs, but the underlying mechanisms are far from clear. This study aimed to elucidate the role of N-induced litter accumulation in mediating the impacts of N addition on plant C sequestration. Methods Based on a seven-year N addition field experiment in a temperate steppe, we measured soil properties, plant species richness, litter biomass, and aboveground biomass C stocks under mown (low litter accumulation) and unmown conditions (high litter accumulation). Results The impacts of N addition on plant C sequestration in grasslands were dependent on addition rates and mowing conditions. There was a threshold for the positive impacts of increasing N addition rates on plant community aboveground C stocks under unmown conditions but not under mown conditions. Litter accumulation was a key factor driving the divergent effects of N addition under different conditions. Results of structural equation modelling showed that N addition had direct and positive effects on plant C sequestration both in unmown plots with litter mass < 200 g m −2 and mown plots, and that such positive effects disappeared in unmown plots with litter mass > 200 g m −2 . Conclusion Our findings highlight the indirect role of litter in mediating the impacts of N inputs on ecosystem C cycling through changing the capacity of plant C sequestration.