Contributions of three types of biocrusts to soil carbon stock and annual efflux in a small watershed of Northern Chinese Loess Plateau

Accurate estimation of soil carbon (C) stock and efflux in drylands is vital for understanding the C cycling processes and predicting global climate change. As a “living skin”, biocrusts strongly change soil C fixation and respiration, but till now it is still challenging to accurately estimate and assess biocrust's contributions to soil C stock and efflux, especially taking different types of biocrusts and annual scale into consideration. In a small representative watershed on the northern Chinese Loess Plateau, cyanobacterial crust (cyano crust), cyanobacteria and moss mixed crust (mixed crust), moss crust, and bare soil were sampled. The organic C content of the samples were determined in laboratory, and subsequently their C stocks were calculated. Moreover, the respiration rate (Rₛ) of biocrusts and bare soil were repetitively measured in field for a four-month period, respectively, and an optimal regression model was established based on the relationship between Rₛ and corresponding temperature and moisture of soil. Lastly, the daily Rₛ was extended to a whole year through model extrapolation, and the seasonal and annual C efflux was obtained through summing the daily respiration together. Our results showed that (i) the organic C content of three types of biocrusts were 69 %–159 % higher than that of bare soil, and the soil C stock at depth of 0–5 cm was 0.35, 0.51, 0.81, and 0.61 kg m⁻² for bare soil, cyano crust, mixed crust, and moss crust, respectively. (ii) During the measurement period, the mean Rₛ of the four habitats were 1.13, 2.21, 2.86, and 2.56 μmol m⁻² s⁻¹, respectively, showing that the biocrusts increased Rₛ by 96 %–152 % in contrast to bare soil. (iii) The variations of Rₛ were best explained by soil temperature and moisture through the Gaussian model, which had the lowest AIC and RMSE. (iv) Based on model extrapolation, the estimated annual C efflux from bare soil, cyano crust, mixed crust, and moss crust were 228.0, 378.0, 894.0, and 679.0 g m⁻², respectively, indicating that in annual scale biocrusts increased C efflux by 66 %–292 % compared with bare soil. The seasonal variations also implied that the C efflux of biocrusts in summer and autumn were significantly higher than that in winter and spring, but they all accounted for a considerable proportion and should not be neglected. In conclusion, different types of biocrusts are the dominant contributors of soil C fixation and respiration in drylands. More importantly, the increased soil C efflux from biocrusts is highly overwhelmed by their elevated photosynthetic capacity, which make biocrusts sequestrate a considerable amount of C. These effects on soil C are highly associated with biocrust's roles in restoring degraded soil and mitigating global climate warming.