Altered precipitation rather than warming and defoliation regulate short-term soil carbon and nitrogen fluxes in a northern temperate grassland

Abstract

Grasslands store large quantities of organic carbon (C) and thus can markedly impact global climate change if they are disturbed. How practices such as grazing intensity and climate change interact to affect soil greenhouse gas (GHG) fluxes within native northern temperate grasslands is unclear. We investigated the responses of three major GHGs (CO2, N2O and CH4) to temperature (ambient vs. warmed), precipitation (ambient vs. reduced or increased) and defoliation (no defoliation vs. low- or high-intensity defoliation) treatments in a factorial experiment over a two-year period in a native northern temperate grassland in Alberta, Canada. Growing season mean soil gas fluxes across all treatments were 93.8 mg CO2 m−2 h−1, −0.025 mg CH4 m−2 h−1, and 0.003 mg N2O m−2 h−1, suggesting that this grassland was a source for CO2 and N2O, but a sink for CH4 over the study period. Structural equation modeling showed that increased precipitation, rather than warming or defoliation, was the primary driver directly regulating fluxes of CO2 and N2O. While increased precipitation also increased microbial biomass C (MBC), the latter had little impact on GHG fluxes, suggesting that MBC was not a critical factor regulating the precipitation-GHG flux relationship. Warming and precipitation effects on CH4 fluxes depended on the month of sampling during the growing season. Warming decreased CH4 uptake but only in mid-July, while increased precipitation reduced CH4 uptake across all sampling dates except early August, at which time increased precipitation increased CH4 uptake. Our study shows that short-term soil GHG fluxes in this native grassland are relatively resilient to moderate warming and defoliation, but sensitive to altered precipitation. Increased precipitation was the main contributor to short-term GHG fluxes in this temperate grassland via changes in CO2 efflux.