Responses Of Cycling Of Soil Water And Carbon To Plant Coverage In The Transitional Belt Of Wind And Water Erosion Of The Loess Plateau

The transitional belt of wind and water erosion is the center of the intensive soil erosion. Although increasing plant coverage can effectively control soil erosion, limited soil water resources restrict increasing plant coverage. At the same time, the fragile eco-environment of the transitional belt of wind and water erosion indicates that the cycling process and mechanism of soil water and carbon in the region are different from other region of the Loess Plateau. Therefore, it is very important for sustainable development of eco-environment to study the effect of plant coverage on cycling of soil water and carbon to determine the optimal plant coverage. In this study, we analyzed the responses of soil CO₂ efflux, leaf photosynthesis and soil water to plant coverage by field observation. The Simultaneous Heat and Water Transfer (SHAW) model was used to simulate soil water content variations with plant coverage for a representative climatic year to determine the optimal plant coverage for two dominant shrubs (Caragana korshinkii Kom and Salix psammophila) in this area, based on soil water balance. The main results are as follows:1. Plant coverage had effect on soil CO₂ efflux. Generally, plant coverage was higher, soil CO₂ efflux was the larger. For the same plant coverage treatment, monthly variation of soil CO₂ efflux was observed during the growth period: maximum soil CO₂ efflux was achieved in August. In addition, vegetation type had pronounced impact on soil CO₂ efflux, the soil CO₂ efflux of C. korshinkii were significantly greater than those of S. psammophila, growing in silt loam soil.2. The regulation of biotic factors and abiotic factors to the response of soil CO₂ efflux to plant coverage. During the experiment, biotic factors (root biomass and aboveground biomass) were important driving factors mediating the response of soil CO₂ efflux to plant coverage for two shrubs. Among the abiotic factors, the soil water content (0-6 cm), soil temperature (0-5 cm) and their interaction could partly regulate the response of soil CO₂ efflux to plant coverage. Soil water content (0-6 cm) was more closely correlated with soil CO₂ efflux than soil temperature (0-5 cm).3. The significant effects of plant coverage on leaf net photosynthesis rate and plant growth. Leaf photosynthesis rate was significantly larger in low plant coverage than in high plant coverage for two shrubs, on the contrary, the canopy photosynthetic carbon fixation was significantly greater in high plant coverage than in low plant coverage. The interaction of light availability and plant water availability could mediate the response of leaf photosynthetic capacity to plant coverage. In C. korshinkii plots, high plant coverage plants had lower plant height and stem diameter compared to low plant coverage plants, which were in contrast to S. psammophila. The response of plant growth to plant coverage may be mainly caused by belowground competition for water (symmetric competition) in C.korshinkii plots, whereas by aboveground competition for light (asymmetric competition) in S. psammophila plots.4. Soil texture and rain pulse significantly affected soil CO₂ efflux and leaf net photosynthetic rate. Soil CO₂ efflux of C. korshinkii was larger growing in silt loam soil than in sandy soil. On the contrary, soil CO₂ efflux of S. psammophila was lower growing in silt loam soil than in sandy soil. This showed that heterotrophic respiration of C. korshinkii was more sensitive to soil texture, but autotrophic respiration of S. psammophila. Soil texture had significant effects on leaf net photosynthesis rate: leaf net photosynthesis rate was significantly larger in silt loam soil than in sandy soil for two shrubs, except that the late growth stage of S. psammophila in 2010. Soil water condition was important driving factors mediating the response of leaf net photosynthesis rate to plant coverage for two shrubs. Significant monthly variation of soil CO₂ efflux and leaf photosynthesis showed that the maximum leaf photosynthesis rates lagged behind the maximum soil CO₂ efflux. When soil became wet, large amount of rainfall could have pronounced impact on soil CO₂ efflux. Soil water distribution from the depth of rainfall infiltration was the main factors mediating the leaf photosynthesis response to the interaction of rainfall event and soil texture.5. The effects of plant coverage on temporal and spatial variation of soil water. The average soil water content, soil water storage in root layer and soil evaporation (0-15 cm) decreased with the increase of plant coverage. There were significant differences in soil water content of different plant coverage. The soil water content in the upper 0-100 cm had larger change range, which was sensitive to rainfall, root water uptake, and soil evaporation. During young-age period, the degree of soil desiccation increased for the shrubs with increasing plant coverages and stand age.6. The transitional belt of wind and water erosion suffered intensive soil erosion. Although increasing plant coverage can effectively control soil erosion, limited soil water resources restrict increasing plant coverage. Therefore, the determination of optimal plant coverage, based on soil water balance, is the key for sustainable development of eco-environment. We used field observation data to calibrate and validate SHAW model. The representative dry year (i.e. 10% probability of a drier year) was determined, according to past climate data. Based on soil water balance, the optimal coverage for the C. korshinkii and S. psammophila shrub species corresponded to a maximum LAI of 1.27 and 0.70, respectively.The results showed that the differences in soil CO₂ efflux among plant coverage could be attributed to the different root respiration (autotrophic respiration) in the transitional belt of wind and water erosion. The responses of leaf photosynthesis and plant growth to plant coverage were caused by interplaying of competition for light, water and space. The effects of soil texture on soil CO₂ efflux and leaf photosynthesis suggests that considering soil texture heterogeneous is important in carbon cycling of semiarid ecosystems. The effects of plant coverage on temporal and spatial variation of soil water were closely related with plant growth and seasonal distribution of rainfall. During young-age period, the degree of soil desiccation increased for the shrubs with increasing plant coverages and stand age. Therefore, based on soil water balance, the optimal coverage is the key to maintain the ecosystem sustainability in semiarid region. The conclusions of this study could help to reveal cycling processes of water and carbon in the transitional belt of wind and water erosion and to guide for vegetation restoration and revegetation.