Effects of biochar application on tilth soil hydraulic properties of slope cropland of purple soil

Abstract: Biochar is a kind of solid residual produced by thermal decomposition of orgnic material under limited or absent supply of oxygen, and relatively low temperatures, biochar has the properties of high internal surface area and microporosity, furthmore, non-biological and biological stability. It used as a soil amendment could greatly improve soil physical and chemical properties, reduce the biological effectiveness of soil pollutant and greatly reduce the emission of carbon dioxide and other greenhouse gases and sequestrated soil carbon in recent years. In this study, a one-year field trail of biochar application in the hilly area of central Sichuan Basin, was carried out in sloping farmland plots, which was located at Yanting Agro-ecological Experimental Station of Purple Soil (105°27′E，31°16′N), Sichuan, Southwest China, to investigate the effects on hydraulic properties of cultivated purple soil (an entisol). Two treatments were set up: control (NPK) and biochar amended (NPK-BC), with each being replicated three times. Comparison between biochar amended and control plots was made by determining soil hydraulic parameters, soil pore size distribution and the contribution of each pore size to flow at two depths (2-7 and >7-12 cm) of the plough layer. Results showed that: 1) due to biochar application, the soil contact angle was increased by 6.7°and 0.5°at the 2-7 and >7-12 cm depth, respectively. This implies that soil water absorption ability was increased and nutrients will be more easily dissolved in the soil.2) After one year of biochar application, the residual water content (θr)，which is unavailable to plants and water content in structure pores (θstr), which is easy to be drained out，was decreased, respectively. But the water content in soil matrix pores (θtxt), which is available to plants, increased significantly (P<0.05) from 0.058?0.003 cm3.cm-3 to 0.085?0.002 cm3/cm3. This implies its stronger ability to retain plant-available water after biochar amended, ; 3) due to biochar application, The effective porosity of r>125 μm pores increased by 54% and 8% at the 2-7 and >7-12 cm depth, respectively.Particularly, the effective porosity of r>500 μm pores increased most markedly, reaching 110% and 355% for the two depths, respectively. This shows that biochar application reduces the 250< r < 500 μm pores volume in the soil, but increased the volume of smaller pore (125< r <250 μm) and larger pore (r>500 μm); 4) the saturated hydraulic conductivity at the two depths (2-7 and >7-12 cm) increased by 45% and 35%, respectively, after a year of biochar application. Tension infiltration data show that soil macropores (r>125 μm) were the main contributing (accounting for 92-94%) pores to the fast drainage at the 2-7 and >7-12 cm depth, under control and biochar amended r, in spite of their very low percentage (3-4%) of total porosity. 5)Therefore, it can be inferred that, on one hand, the application of biochar could increase the soil's capacity to hold plant-available water and thus enhance resistance to drought; on the other hand, it can also enhance water permeability of soil, which can reduce surface runoff and potential soil erosion.