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• R s of biocrust was measured and modeled as a function of soil environment factors. • Variation of R s in biocrust & bare soils was strongly predicted in regression models. • R s averaged 1.03 and 0.54 μmol m−2 s−1 for biocrust and bare soil across all seasons. • Annual CO 2 -C efflux of biocrust and bare soil was 390 and 203 g m−2, respectively. • Contribution of biocrust to R s may be linked to the C stored in the biocrust layer. 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 CO 2 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 CO 2 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 R 2 ≥ 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 CO 2 -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 CO 2 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. [ABSTRACT FROM AUTHOR]

Biocrusts are widely distributed throughout the world in dryland ecosystems, but their respiration rate (R s) and responses to soil temperature and mositure have not yet been fully investigated. In a semiarid region of the Loess Plateau in China, the R s of moss-dominated biocrusts was continuously (104 days) measured in summer through an automated soil respiration system, and their relationships with soil temperature and moisture at 2, 5, and 10 cm depths were intensively analyzed. The results showed the biocrusts had 115% higher (t = −3.82, P < 0.001) R s during day and 44% higher (t = −10.22, P < 0.001) R s during night as compared with the underlying soil; they averagely increased R s by 81% (2.52 vs. 1.39 μmol m−2 s−1) across day and night. More specifically, the R s of biocrusts ranged from 0.33 to 6.33 μmol m−2 s−1 in wet conditions (soil moisture ≥ 0.15 cm3 cm−3) and 0.27 to 2.83 μmol m−2 s−1 in dry conditions (soil moisture = 0.05 cm3 cm−3), with a daily mean of 2.11 μmol m−2 s−1. Particularly, in wet conditions the R s of biocrusts had positive responses to increasing temperature, and their relationship was well fitted with an exponential function (R 2 ≥ 0.44; P < 0.001), with a Q 10 value ranging between 1.73 and 2.08. Similarly, the R s of biocrusts was positively correlated with soil moisture, but their relationship was not well fitted with the quadratic function (R 2 ≤ 0.22). A model based on the soil moisture and temperature at 2 cm depth performed much better (R 2 = 0.53; P < 0.001) with R s than that based on the soil moisture and temperature at 5 or 10 cm depth or even air temperature. In conclusion, moss biocrusts made a significant contribution to soil respiration in semiarid ecosystem, firstly (and directly) due to the rich and diversified microbial communities in biocrust layer and secondly (and indirectly) due to the biocrust effects on soil temperature and moisture regimes. The temperature and moisture at 2 cm depth (biocrust layer) should be employed with a higher priority in simulating R s of biocrusted surfaces, rather than the commonly used soil depth (5 cm) in estimating R s of the ordinary soil without biocrusts. Unlabelled Image • Biocrusts increased respiration rate (R s) by 81% as compared with underlying soil. • R s of biocrusts ranged from 0.05 to 9.50 μmol m−2 s−1 with a mean of 2.11 in summer. • R s of biocrusts was highly dependent on moisture and temperature (Q 10 = 1.73–2.08). • Soil temperature and moisture at 2 cm depth are capable of estimating R s of biocrusts. [ABSTRACT FROM AUTHOR]

[Display omitted] • Increased precipitation suppresses respiration rate (R s) of biocrust by 9%–22%. • Moderately reduced precipitation stimulates R s of biocrust by up to 26%. • Relationship between biocrust R s and precipitation amount is nonlinear. • Response of biocrust R s to precipitation gradient is negatively asymmetric. • Precipitation manipulation affects R s via changing biocrusts and soil properties. Global climate change is expected to cause both increased and reduced precipitation in the next decades, especially in drylands with semi-arid climates. Biocrusts are an essential soil surface cover in global drylands, and they strongly influence most soil properties and support fundamental ecosystem functions, especially soil carbon (C) cycling. Nevertheless, the response patterns and mechanisms of biocrust respiration rate (R s) to precipitation changes remain uncertain under changing semi-arid climates. In our study, a field experiment for biocrusts with natural precipitation (CK) and six levels of precipitation manipulation (–50 %, –30 %, –10 %, +10 %, +30 %, and +50 % of CK) were conducted from 2019 to 2021 in a semi-arid climate region of the northern Chinese Loess Plateau. We continuously measured the biocrust R s of all the treatments in the growing seasons, and the fundamental properties of biocrusts and their temperature and water content at 5 cm depth were also measured. Our results showed that the increased precipitation inhibited biocrust R s , while moderately reduced precipitation stimulated biocrust R s. As compared with the CK, the precipitation manipulations of +10 %, +30 %, and +50 % decreased biocrust R s by 8.9 %, 15.3 %, and 22.1 %, respectively, while the treatments of –10 % and –30 % increased biocrust R s by 25.5 % and 8.0 %, respectively. However, the precipitation manipulation of –50 % suppressed biocrust R s by 19.3 %. More importantly, the responses of biocrust R s to precipitation changes were negative and asymmetric, which indicated a higher sensitivity of biocrust R s to reduced precipitation in comparison to increased precipitation. Moreover, the increased precipitation (+10 % to + 50 %) raised soil water content (by 29.7 %–55.0 %), moss biomass (by 12.5 %–50.0 %), and organic matter content (by 11.3 %–12.4 %). In contrast, the reduced precipitation (–10 % to –30 %) decreased soil water content (by 5.2 %–17.6 %) and moss biomass (by 12.5 %–25.8 %). The structural equation modeling analysis showed that the responses of biocrust R s to precipitation manipulation were mainly and directly influenced by soil temperature and biocrust properties. Although soil water content did not have a direct significant effect on biocrust R s , it indirectly influenced biocrust R s by affecting the fundamental properties of soil and biocrusts. Consequently, the changed soil properties and biocrust characteristics caused by precipitation shifts aggravated the negative responses of biocrust R s to precipitation manipulation. Our findings emphasize that biocrusts have a negative and asymmetric response of R s to precipitation changes, implying that the intensified precipitation variation in semi-arid regions caused by global climate change in future may positively affect the stability of soil C stocks contributed by biocrusts and thus reduce soil C efflux in drylands. [ABSTRACT FROM AUTHOR]