Grazing by livestock modifies the response of soil microbes to elevated atmospheric carbon dioxide

Grasslands, vital for pastoral agriculture, face global change impacts from elevated CO2(eCO2) and livestock grazing, yet their combined effects on soil ecosystems remain unclear. In a 23-year experiment, uniquely incorporating livestock grazing with and without excrement return, we assessed soil microbiome and biogeochemical responses.We found support for altered carbon and nitrogen cycling in soils under eCO2 and grazing, with effects largely dependent on animal excrement return to the soil. 16S and ITS sequencing revealed that both eCO2 and grazing increased bacterial diversity at the expense of fungal diversity. Community shifts were primarily driven by grazing, particularly in the absence of excrement return, and were associated with an increased abundance of oligotrophic taxa. This was accompanied by significant declines in cation exchange capacity (-1.83cmol·kg−1), pH(-0.52), and OlsenP (-11.5mg·L−1). Also, grazing with excrement return elicited a large significant decrease in C:N(-0.21) and near 3-fold increase in ammonium (+0.6 μg N/g dry soil), while eCO2 significantly decreased OlsenP (-9.13mg·L−1) and C:N(0.26).Microbial phospho- and neutral-lipid quantification indicated significant interactive effects of eCO2 and the grazing with excrement return treatments, increasing total microbial biomass by 12.9% driven by large increases in Actinobacteria (35.6%), Gram-positive (24.5%) and Gram-negative bacteria (10.8%), and total bacterial biomass (13.8%), in response to the increased substrate supply from excreta.Our novel experiment revealed significant interactive effects of eCO2 and grazing on the soil microbiome, suggesting that full understanding of the stability and sustainability of pastoral grasslands to eCO2 can only be achieved through the inclusion of grazing animals.