Effects Of Biological Soil Crusts On Soil CO₂ Efflux And Their Influence Factors In A Typical Grassland

Biological soil crusts are a key biotic component of dryland ecosystems worldwide. Soil CO2 efflux in the arid and semi-arid ecosystems mainly include plant root respiration, soil microbial respiration and CO2 efflux of biological soil crusts. However, most studies carried out to date on carbon (C) fluxes in these ecosystems have neglected them. Four years’experiment had been conducted in the fenced and grazing grassland of loess plateau. Using the Li-8150 automated soil CO2 flux system, we investigated the seasonal patterns of soil CO2 efflux as well as the annual cumulative CO2 efflux in the bareland and crustal plots, and quantified the contribution of biological soil crusts in the soil CO2 efflux. The influences of stimulated precipitation on soil respiration, microbial respiration, root respiration and net CO2 efflux of biological soil crusts were detected too. By a three-year study, we compared soil respiration and microbial respiration in the fenced and grazing grasslands, and differences among temperature sensitivities of soil respiration and microbial respiration in the both grassland were also test. By stimulating trampling, the responses of soil CO2 efflux and water infiltration rates of biological soil crusts to grazing were estimated. The objectives of this study are to quantify the contributions of biological soil crusts in the C cycling of grassland, and to identify how precipition and grazing affect it. This results will be helpful for precisely estimating the C cycling in this ecosystem, and for the sustainable development of this grassland.The main results are as follows:1. Biological soil crusts play a critical role in modulating soil CO2 efflux. When compared to the bareland, biological soil crusts significantly lowered the soil CO2 efflux, the cyanobacteria and mosses declined 18.2% and 12.4% of soil surface CO2 efflux, respectively. The gross photosyntheic rates of cyanobacteria and mosses were similar, there were 61.6 g C m-2 y-1 and 60.0 g Cm-2 y-1, respectively; and the carbon sequestration of cyanobacteria and mosses were 36.5 g C m-2 y-1 and 24.8 g Cm-2 y-1, respectively, which indicated that cyanobacteria had the higher carbon fixation capacity. The contribution of biological soil crusts to systemic carbon balance was 22.4%-32.2%. The impacts of biological soil crusts on soil CO2 efflux were mainly caused by the increments of SOC, TN, TP, SMBC and SMBN, and by the alterations of fungi/bacteria, etc.2. In the crustal microsites, soil CO2 efflux were more sensitivity to the changes of temperature (higher Q10), when compared to the bareland. In addition, the carbon fixation of biological soil crusts always happened in the growing season which had higher soil moisture. The existence of biological soil crusts had a trend to lower the soil moisture threshold of soil CO2 efflux.3. The precipitation-manipulated experiment indicated that the rainfall amount was the most dominated factor which controlled net carbon efflux of biological soil crusts. The carbon fixation of biological soil crusts had precipitation threshold, when the rainfall amount was lower than this value, biological soil crusts released carbon. The precipitation threshold for cyanobacteria was lower than 2 mm, and for mosses was among 2 mm and 5 mm. After exceeded this lowest precipitation, different species and different cover of biological soil crusts did not change the soil CO2 efflux.4. Similar with CO2 efflux of biological soil crusts, precipitaion stimulated microbial respiration and root respiration immediately. In addition, soil antecedent water status played a key role in modulating this responses:soil CO2 efflux response to rainfall addition amplified at drier antecedent soil conditions and dampened under wetter antecedent soil conditions. Like the results of biological soil crusts, the response magnitude and duration of soil CO2 efflux to water addition depended on the precipitation pulse sizes. During the rainfall time, soil moisture was the major factor that determined soil respiraion and its components, while soil temperature had a negetive relationship with soil respiration and its components.5. Based on our data, we developed a model:IR=(aX2+b) X1+cX2+d, where IR were increased ratios of soil respriation and its components; X1 was the rainfall amount; X2 was soil antecedent water status; a, b, c and d were constants, to predict the increase ratios of SR and its components by using precipitation pulse size and antecedent soil moisture. This will be helpful for the accurate calculation of carbon balance of this grassland.6. Simulated trampling declined soil CO2 efflux and water infiltration rates of biological soil crusts microsites significantly. As increasing of trampling intensity, the water infiltration rates of crustal microsites were declining. Meanwhile, biological soil crusts highered the water infiltration rates. Trampling tremendously decreased soil CO2 efflux and net carbon efflux of biological soil crusts, however, the different trampling intensity did not cause significant differences. It took 10 days for the net CO2 efflux of biological soil crusts returned to original status after trampling. Trampling did not alter the relationship between soil CO2 efflux and soil temperature, while biological soil crusts increased the temperature sensitivity (Q10) of soil CO2 efflux.7. Graing significantly declined plant root biomass and soil organic carbon content, thereby decreasing soil respiration and microbial respriation. Meanwhile, grazing significantly lower the temperature sensitivities (long-term Q10) of soil respiration and microbial respiration. However, grazing did not differ the they short-term Q10 (which was consistent with that trampling did not change Q10 of soil CO2 efflux). There was also no difference between short-term Qio of soil respiration and that of microbial respiration. In both grasslands, the short-term Q10 had a similar seasonal pattern with higher values in the beginning and ending of growing season. The short-term Qio was significantly and negatively related to soil temperature at 2-cm depth, but no relationship was found between soil moisture (0-10 cm soil layer) and short-term Q10. Due to the plant phenological process and other climate factors, a discrepancy existed between long-term Q10 and short-term Q10. Additionally, in both grasslands, long-term Q10 of microbial respiration was lower than that of soil respiration, suggesting that autotrophic respiration was more sensitive than heterotrophic respiration to temperature.