Abstract
Grazing and cultivation are two important management practices worldwide that can cause significant soil organic carbon (SOC) losses. However, it remains elusive how soil microbes have responded to soil carbon changes under these two practices. Based on a four-decade long field experiment, this study investigated the effects of grazing and cultivation on SOC stocks and microbial properties in the semi-arid grasslands of China. We hypothesize that grazing and cultivation would deplete SOC and depress microbial activities under both practices. However, our hypotheses were only partially supported. As compared with the adjacent indigenous grasslands, SOC and microbial biomass carbon (MBC) were decreased by 20% or more under grazing and cultivation, which is consistent with the reduction of fungi abundance by 40% and 71%, respectively. The abundance of bacteria and actinomycetes was decreased under grazing but increased under cultivation, which likely enhanced microbial diversity in cultivation. Invertase activity decreased under the two treatments, while urease activity increased under grazing. These results suggest that nitrogen fertilizer input during cultivation may preferentially favor bacterial growth, in spite of SOC loss, due to rapid decomposition, while overgrazing may deteriorate the nitrogen supply to belowground microbes, thus stimulating the microbial production of nitrogen acquisition enzymes. This decade-long study demonstrated differential soil microbial responses under grazing and cultivation and has important applications for better management practices in the grassland ecosystem.
Highlights
Grassland covers approximately 20% of the earth’s land surface area [1] and contains 12% of global soil organic carbon (SOC) [2]
The SOC contents in the grazing plot were 24% and 20% lower than those in the ungrazing plot at the two soil layers, respectively, while the SOC stocks were decreased by 4.57 Mg·ha−1 and 4.90 Mg·ha−1 at both soil depths after 30 years of grazing
Cultivation significantly (p < 0.01) decreased the SOC content by 19% at 0–10 cm and 29% at 10–20 cm, the SOC stocks were reduced by 6.23 Mg·ha−1 and 7.42 Mg·ha−1 at the two soil depths after the native grassland had been cultivated for 38 years
Summary
Grassland covers approximately 20% of the earth’s land surface area [1] and contains 12% of global soil organic carbon (SOC) [2] This implies that modest changes in C storage in grassland ecosystems have the potential to modify the global C cycle. About 313 million ha (~78%) of the grasslands are located in the northern arid or semi-arid temperate zone This region was subject to an increasing demand for natural resources and animal products due to a human population increase, which has resulted in a 34% overload in livestock carrying capacity, and approximately 5% of the total area of the existing grassland has been cultivated for cropland [4,5]. It is imperative to monitor SOC change and develop appropriate management and conservation polices in order to improve soil productivity and ecosystem sustainability
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