Society is in need of new agricultural systems that increase ecosystem services while enhancing food and energy production. Current rangeland management practices evenly distribute fire and grazing across pastures. Patch-burning-grazing (PBG) which consists of burning small sectors of a pasture restores heterogeneity via grazing-fire interactions, promoting diversity and nutrient cycling. How innovative PBG management impacts biogeochemistry of grazed pastures is still uncertain, particularly of subtropical and tropical grasslands, which occupy over a fourth of the global land area. Using the eddy covariance technique we investigated how PBG affects CO2 fluxes from subtropical pastures, and determined the mechanisms governing changes in CO2 fluxes in response to fire-grazing manipulation through impacts on Gross Primary Productivity (GPP) and ecosystem respiration (Reco). Plots consisted of two burning treatments of subtropical pastures with the same grazing intensity: the business-as-usual full burn grazing (FBG), in which prescribed fire was implemented on whole pastures during the first year of the experiment; and, the innovative PBG, in which only one-third of the pasture was burned annually. Treatments were implemented on both intensively-managed and less-intensively-managed semi-natural pastures that dominate the grazing landscape in subtropical South Eastern USA. Pre-treatment CO2 fluxes were similar in both PBG and FBG pasture-types.
Results/Conclusions
In the first year after burns, FBG intensively-managed pasture was a stronger sink of CO2 than PBG pasture because the stimulation of GPP was larger than the increase of Reco. Increased GPP in FBG was caused by higher aboveground net primary productivity (ANPP) and greenness along with higher nitrogen (N) in plant tissue in FBG than in PBG pastures. Within PBG intensively-managed pasture, burning did not affect GPP compared to the unburned sector because although burning stimulated ANPP, grazers removed large amounts of biomass as they spent more time on burned areas. PBG semi-natural pasture was a stronger sink of CO2 than FBG pasture as fire stimulated GPP in burned areas of PBG pasture and it did not increase GPP in FBG pasture. This was caused by the preferential presence of grazers in burned sectors of PBG pasture compared to the FBG treatment which increased ANPP and greenness likely by enhancing N input and alleviating N limitation despite of large amounts of biomass removed by grazers. Our results showed that the impact of PBG on C fluxes of subtropical pasture depends on pasture type and the differential impact is discussed.