ShengliGuo, Zhiqi Wang, Rui Wang, Yaxian Hu

　　Institute of Soil and Water Conservation, Northwest A&F University

　　Globally, more than 60% of the land area are slopes with gradients > 8^o. When compared with the flatlands, the spatial distribution of water, heat and sediment changes dramatically on sloping lands, resulting in great variation of soil properties and plant growth. Therefore, it is of great importance to improve our understanding of soil CO₂ flux on sloping lands, so as to evaluate its potential role in terrestrial ecosystem and future climate change.

　　The Loess Plateau, due to its complex terrains and various slopes, is an ideal region to investigate the characteristics of soil CO₂ flux on sloping lands. Field investigations were conducted at a typically small watershed, at the Changwu State Key Agro-ecological Experimental Stationfrom 2013 to 2015. Soil CO₂ flux, soil temperature, soil moisture, runoff, sediment, organic carbon in soil (SOC) and sediment, fine root biomass and winter wheat yield were measured.

　　The results show that: 1) Soil CO₂ flux significantly decreased with the increase of slope gradient. However, the extent of such variations varied with terrain types (gentle slopes; steep slopes). Specifically, on gentle slopes, slight changes in slope gradient from S_0.5 to S₁ or S₃ could reduce the annual cumulative CO₂ flux by 13.4% and 25.5%. This means, by every 1^o increase of gradient on gentle slopes, soil CO₂ flux decreased about 19.5%. However, on the steep slopes, the annual cumulative CO₂ flux was only reduced by 13.3% and 22.1%, when slope gradients evidently changed from S₅ to S₁₀ and S₂₀. This equals to merely 2.4% soil CO₂ decrease when the slope gradient increased by 1o on steep slopes. This was probably because that the decreasing extent of soil moisture and SOC varied with slope gradients, i.e., whenthe slope was < 5^o, the soil moisture and SOC decreased greatly by 2.87 % and 0.73 g kg^–1 by every 1° increase of slope gradient; whereas, when the slope was > 5^o, the soil moisture and SOC decreased slightly by 0.34 % and 0.03 g kg^–1 by every 1° increase of slope gradient.

　　2) The soil CO2 flux was significantly influenced by the slope position, and followed the order of the bottom slope > the middle slope > the upper slope, resulted from the difference of soil moisture, fine root biomass and SOC among slope positions. Furthermore, the relative increase (%) of soil CO₂ flux at the bottom slope compared to the upper slope was more pronounced on slopes of greater gradients (P<0.01), i.e., the relative increase (%) of soil CO₂ flux at the bottom slope compared to the upper slope was amplified by 1.3 times with every 1° increase of slope gradient.

　　3) The soil CO₂ flux was significantly negatively related with slope gradient (P<0.01) following the exponential pattern. Accordingly, the concept of slope coefficient (β) was developed from the response coefficient of soil CO₂ flux with slope gradients:

　　Where F is the soil CO₂ flux, x is the slope gradient, F ₀ is the constant, α is related to soil inherent properties, and β reflects the response of soil CO₂ flux to slope gradient changes for a certain soil.

　　4) In addition, Q_{1 0} was significantly reduced with the slope gradient. Meanwhile, Q ₁₀ values showed significant differences among slope positions, with the trend of the bottom slope> the middle slope> the upper slope. Spatial variations in soil moisture, fine root biomass and SOC were the main factors for the changes of Q ₁₀ among slope gradients and slope positions.