Livestock trampling regulates the soil carbon exchange by mediating surface roughness and biocrust cover

Despite the considerable attention devoted to the potential importance of biocrusts in global carbon cycling, their photosynthetic and respiratory responses to human activity and the underlying mechanisms are not yet fully understood. To evaluate the effect of livestock trampling disturbance on the carbon balance of biocrust-covered sandy soil, the net photosynthetic rate (NP), gross photosynthetic rate (GP), and soil respiration rate (R) of algae-lichen mixed crust and moss crust before and after addition of 2, 5, 10, and 20 mm water under different trampling intensities (CK, no trampling; LD, trampling five times; MD, trampling 20 times; and HD, trampling 45 times on the undisturbed soil with an area of 415 cm2) were determined. The results showed the surface roughness decreased after an initial increase, whereas the biocrust cover decreased exponentially with increasing trampling intensity. There was negligible variation in surface soil NP, GP, and R (approximately 0.07 μmol CO2·m−2·s−1) during dry soil periods. The carbon exchange rate in each trampling treatment changed significantly even after a small amount of rainfall (2 mm), but the surface soil still served as a carbon source. As simulated rainfall increased to ≥ 5 mm, the biocrust NP and GP declined (more negative values represent greater carbon uptake), and R continued to increase. Most notably, the CK and LD surfaces switched from carbon sources to carbon sinks, while the HD surface soil remained a carbon source. In terms of cumulative carbon exchange, LD trampling did not significantly increase carbon loss and even contributed to carbon gains compared with CK, whereas MD and HD caused severe carbon loss. In summary, impact of livestock trampling disturbance on the biocrust-covered soil carbon exchange is driven by rainfall, and its response to rainfall is mediated by surface roughness and biocrust cover. Specifically, surface soil carbon gains did not decrease or even increased slightly under light trampling, probably due to slight decreases in biocrust cover offset by the photosynthetic contribution of increased surface roughness, whereas heavy trampling resulted in severe carbon losses as the biocrust cover sharply decreased. This study emphasizes the important role of livestock trampling-induced changes in surface soil structure on the carbon exchange of sandy ecosystems.