Estimation of annual CO₂ efflux of moss biocrust through measuring and simulating its respiration rate in a semiarid climate

Biocrust is a potentially extensive living cover in drylands, which comprises much of the land surface, but presently its contribution to the soil respiration rate (R_s) and CO₂ efflux is still not clearly understood. In this study, we continuously measured the R_s of moss-dominated biocrust (biocrust and biocrust covered soil) and bare soil, together with soil temperature and moisture, for ~100 days each in a semiarid climate on the Chinese Loess Plateau. We modeled R_s across a two-year period, based on its relationship with soil temperature and moisture. Using the model, the seasonal variation of the R_s was simulated for biocrust and bare soil, and their annual CO₂ efflux were further estimated. We also obtained samples of the biocrust layer and bare soil surface and analyzed their physicochemical properties and enzyme activities to explain the biocrust effects on R_s. Our results showed that the measured R_s of the biocrust and bare soil ranged from 0.05 to 7.62 and 0.02–2.73 μmol m⁻² s⁻¹, respectively, and was closely exponentially related to the dynamics of soil temperature and moisture (in threshold relationships). The variations of R_s in both the biocrust and bare soil were strongly predicted in a regression model by R² ≥ 0.50 and P < 0.001. Through this model, we simulated the seasonal variations of R_s, which averaged 1.03 ± 0.01 and 0.54 ± 0.01 μmol m⁻² s⁻¹ (F = 225.51, P < 0.001) across all seasons for the biocrust and bare soil, respectively. We further estimated that the annual CO₂-C efflux through respiration of the biocrust was 390 g m⁻² yr⁻¹ and that of the bare soil was 203 g m⁻² yr⁻¹, indicating that the presence of the biocrust layer annually contributed 91.8% (187 g m⁻² yr⁻¹) more soil respiration in comparison to the bare soil. Moreover, the main correlates with R_s shifted from largely abiotic, e.g. soil texture and total phosphorus content, in bare soils to biotic, e.g. indicators of carbon and nitrogen content, in biocrust soils. This finding implies that the contribution of the biocrust to R_s is linked to the carbon fixed and organic nutrients stored in the biocrust layer. In conclusion, moss-dominated biocrust highly accelerates soil respiration and increases gross soil CO₂ efflux, possibly through regulating soil temperature and moisture, improving soil physicochemical properties, increasing soil enzyme activities, and accelerating decomposition of biocrust-fixed and -stored carbon.