Atmospheric concentrations of the greenhouse gas nitrous oxide (N2O) have increased significantly since pre-industrial times owing to anthropogenic perturbation of the global nitrogen cycle1,2, with animal production being one of the main contributors3. Grasslands cover about 20 per cent of the temperate land surface of the Earth and are widely used as pasture. It has been suggested that high animal stocking rates and the resulting elevated nitrogen input increase N2O emissions4,5,6,7. Internationally agreed methods to upscale the effect of increased livestock numbers on N2O emissions are based directly on per capita nitrogen inputs8. However, measurements of grassland N2O fluxes are often performed over short time periods9, with low time resolution and mostly during the growing season. In consequence, our understanding of the daily and seasonal dynamics of grassland N2O fluxes remains limited. Here we report year-round N2O flux measurements with high and low temporal resolution at ten steppe grassland sites in Inner Mongolia, China. We show that short-lived pulses of N2O emission during spring thaw dominate the annual N2O budget at our study sites. The N2O emission pulses are highest in ungrazed steppe and decrease with increasing stocking rate, suggesting that grazing decreases rather than increases N2O emissions. Our results show that the stimulatory effect of higher stocking rates on nitrogen cycling4,7 and, hence, on N2O emission is more than offset by the effects of a parallel reduction in microbial biomass, inorganic nitrogen production and wintertime water retention. By neglecting these freeze–thaw interactions, existing approaches may have systematically overestimated N2O emissions over the last century for semi-arid, cool temperate grasslands by up to 72 per cent.
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This work has been supported by the German Research Foundation (DFG research group 536, ‘Matter fluxes in grasslands of Inner Mongolia as influenced by stocking rate’) and the National Natural Science Foundation of China (grant number 40805061), with co-funding from the NitroEurope Integrated Project of the European Commission. We thank K. K. Goldewijk for providing livestock data for the years before 1961, and Z. Yu, K. Müller, L. Lin, P. Schoenbach, G. Willibald, R. Kiese, C. Werner and C. Liu for support with field measurements.
Author Contributions K.B.-B., N.B., X.Z. and M.D. designed the experiment. B.W., W.C. and Z.Y. carried out the flux measurements. H.W. conducted the microbiological measurements. B.W., W.C., H.W. and M.D. performed data analysis. B.W. carried out the upscaling for temperate grasslands. M.A.S., K.B.-B., N.B., M.D. and B.W. drafted the manuscript.
This file contains Supplementary Figures S1-S9 with legends, Supplementary Table S1, Supplementary Methods and Data, Supplementary Discussions and Supplementary References.
About this article
Journal of Environmental Sciences (2019)