|Other Abstract||Biological soil crusts (BSCs) are composed of various components of cyanobacteria, algae, fungi, lichens and mosses and other organisms that may constitute as much as 40% of the living cover in dry land ecosystems, even reach to 70% in some extreme environment regions where interference is less. BSCs are the main builder of desert ecosystem and ecological system engineer, the light efficiency ratio of biocrust lichen is 0.5-2%, which is in the same order with vascular plant, its photosynthetic rate ranges from 0.1 to 11.5 μmol m-2 s-1. Although instantaneous photosynthetic rates of BSCs are less than those of vascular plants, coverage of BSCs in semiarid and arid areas is huge, thus making them potential contributors of soil organic carbon (SOC) in arid and semiarid regions. In addition, as the main types of ground cover in arid areas, unique biological and physical characteristic of BSCs significantly influence soil structure and soil hydrological processes, then indirectly affect the release process of soil carbon. Therefore, understanding the carbon exchange of BSCs and biologically crusted soil is an important part of soundly revealing carbon cycle in desert ecosystem.
In the current study, we chose the Gurbantunggut Desert as a study site to survey the spatial distribution of BSCs in the desert, to examine the way in which CO2 flux rates of biologically crusted soil respond to changes in soil moisture and temperature and how removal of BSCs impacts on soil CO2 flux, and measure carbon fixation of BSCs using field observation and laboratory simulation methods. We specifically aimed to: (1) obtain new data and insights into the effect of BSCs on carbon flux and SOC storage; (2) determinate the extent to which BSCs enhance carbon sequestration and minimize carbon loss; (3) provide valuable data in optimizing land management practices to protect desert lands. The main results as follows:
Distribution and development characteristic of BSCs: In this study, we tried to get the distribution information of BSCs and calculated the coverage of each component using maximum likelihood supervised classification. Results indicated that the coverage of BSCs reached to 41.34% in the whole desert, and the coverage of lichen/cyanobacteria crusts accounted for more than 90%. Despite their low biomass relative to other primary producer, crust organisms have a disproportionally large influence on biogeochemistry in arid regions. In addition, we found that BSCs coverage significantly increased from 25% to 94% with the increase of annual precipitation, and moss crust coverage increased from 0% to 18%. Similarly, the content of clay and soil organic carbon (SOC) increased with the increase of rainfall. These results indicated that precipitation was the key factor driving the distribution and development of BSCs and SOC content.
The influence of BSCs on soil organic content: BSCs not only fix carbon during photosynthesis, but also be able to catch carbon from deposition. The spatial distribution of SOC in different succession of biologically crusted soil was investigated. Results showed that SOC significantly increased with the increase of BSCs development, and the occurrence of BSCs could significantly enhance the accumulation of SOC in the surface 0-5 cm soil layer compared with areas devoid of crust cover. Moreover, we found that once BSCs had been removed, in the absence of autotrophs there was no primary production and SOC gradually decreased. However, SOC began to slowly increase 6 months later, accompanied by a continuous increase in chlorophyll a and this appeared to be as a result of re-colonization of the bare soils. Thus, the dynamic change in SOC driven by BSCs and the significant difference in SOC between biologically crusted soils and bare soils clearly demonstrated the effect of BSCs on SOC.
The influence of BSCs on soil moisture and temperature: In the desert, BSCs reduce rainfall infiltration, and make soil moisture shallow stratification. Also, BSCs can increase soil temperature and promote soil moisture evaporation. Soil moisture and temperature control soil respiration, thus BSCs can indirectly affect soil respiration by regulating soil water content and temperature.
The influence of BSCs on soil respiration: BSCs not only directly involved in the process of soil respiration, but indirectly affect soil respiration by changing soil moisture and temperature. Results indicated that soil respiration significantly changed when BSCs were removed. Soil respiration increased when BSCs were removed in dry season and decreased in wet season. BSCs don’t participate in soil respiration when organisms are disccated and inactive, but have a barrier effect on subsoil respiration and reduced 5% to 27% of soil respiration. In wet season, precipitation primarily drives crust-derived respiration, which significantly influences the magnitude of soil CO2 efflux. During a rainfall event (from wet to dry), crust’s respiration contributed 65% to 75% to the whole soil respiration (crust respiration and subsoil respiration), and the ratio of crust respiration increased with increase of rainfall intensity.
Photosynthetic carbon sequestration of BSCs: Biocrust microphytes are poikilohydric organisms in which the water content of cells is in equilibrium with the surrounding environment. Rainfall and snowmelt water is the main moisture source of BSCs photosynthesis in the desert. Laboratory experiment showed that maximum rates of gross and net carbon fixation were typically reached at 60% relative mass water content. Optimal temperatures for photosynthesis occured between 10 and 20oC. And, higher temperature reduced wet duration, the effective photosynthetic time decreased. Interaction of moisture and temperature should be taken into consideration for estimating total carbon sequestration of BSCs. Field observation and laboratory simulation experiment demonstrated that relative lower temperature (5-15oC) and higher water content (40%-80%) resulted in lager carbon sequestration, and suitable environment conditions occur in spring in the desert. The annual carbon fixation was 21.77 ± 2.14 gCm-2 for lichen-cyanobacteria dominated crusts and 26.68 ± 4.09 gCm-2 for moss dominated crusts, and about 47%-49% of annual carbon fixation was got in spring.
The influence of BSCs on soil-atmosphere CO2 exchange: Temporal variation of NCE varied with soil moisture and temperature. Soil moisture alone could explain 71-74% of variation in NCE. Soil type (moss crusted soil or bareland) also had a significant effect on NCE (P < 0.01), but this was dependent on soil moisture which was directly linked to precipitation pulse. Without precipitation, organisms in BSCs were disccated and inactive, BSCs reduced soil carbon efflux. After a 2 mm precipitation pulse, the crust gross photosynthetic rate (GPc) was lower than the crust respiration rate (Rc), resulting in C efflux. When precipitation intensity was equal to or greater than 5 mm, GPc fully offset total respiration, resulting in an increase in C uptake. C gain was positively correlated with intensity of precipitation pulse. Regardless of different precipitation intensities, Rc was significantly higher than that of subsoil respiration. Thus, the regulation of atmospheric-soil C balance by BSCs depends on the intensity of precipitation. Overall, net carbon fixation of BSCs was 4.21-6.27 gCm-2, which can offset 11%～13% carbon release from soil respiration.
Lastly, given the large areas covered by BSCs in arid and semiarid lands, the unequivocal consideration of crust-derived CO2 exchange in future empirical and modeling studies may greatly contribute to the understanding of the global C cycle and enable better prediction of the effects of global environmental change on soil C balance.|