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 ( R s ) 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 R s and corresponding temperature and moisture of soil. Lastly, the daily R s 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 R s 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 R s by 96 %–152 % in contrast to bare soil. (iii) The variations of R s 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. • Biocrusts increase soil organic carbon (C) content at 0–5 cm depth by up to 159 %. • Biocrusts lead to 45 %–113 % increase of soil C stock at 0–5 cm compared to bare soil. • Measured respiration rate of biocrusts is 96 %–152 % higher than bare soil. • Estimated annual soil C efflux of biocrusts is 66 %–292 % higher than bare soil. • The rise of C stocks and efflux caused by biocrusts is dependent on biocrust type.