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
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
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
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.
Key Words: soil CO2 efflux; photosynthesis; soil water balance; plant coverage; the
transitional belt of wind and water erosion