Effect of biochars on water retention properties of northeast region black soils

Abstract: The water retention capacities and organic matter content of valuable black soil is decreasing year by year with unreasonable farming and excessive dependence on chemical fertilizers and pesticides continuously. Black soil erosion is serious and black soil layer is less and less. It is urgent to protect black soil resource. Biochar is recognized gradually in recent years for improving soil structure, soil water retention capacity and crop yield, and for reducing surface runoff and soil erosion as a soil conditioner. However, most researches focus on the improvement in the acidic soil and sandy soil with less organic matter because of the high pH of biochar and the relative good fertility of black soil. The application of biochar in black soil in northeast region is relatively rare. The water retention capacity improvement is one of the important indexes to characterize the physical and chemical properties of black soil. In this paper, the effects of biochars including poplar wood carbon and bamboo carbon on the water retention properties of loamy soil, sandy loam and sandy soil were studied systematically with three different addition ratios (2%, 5%, 10%) and three different particle sizes (0.25, 0.5, 1 mm) of biochars. The water characteristic curves of different northeast black soils added with biochars were studied by the centrifugation method, and were fitted by the models of Van-Genuchten and Broods-Corey. The results showed that the field capacity and the saturated moisture content of loamy soil, sandy loam and sandy soil all increased significantly because of biochars addition. The field capacity of black soils increased significantly with the increase of addition ratios of biochars, but decreased with the increase of particle sizes of biochars. However, an insignificant difference in 0.5 and 1 mm particle sizes was achieved. The effects of poplar wood carbon on the water retention properties of black soils were superior to those of bamboo carbon, significantly. The particle size of 0.25 mm and addition ratio of 10% was optimum for poplar wood carbon to achieve high water retention capacities of northeast black soils. Under optimum condition, the field capacity and the saturated moisture content increased by 64.97% and 47.60% for loamy soil, 66.42% and 38.93% for sandy loam, and 69.39% and 31.18% for sandy soil, respectively. Furthermore, the effect of grain size distribution of black soil itself on its water retention properties was significant. A negative relationship was between soil field capacity and sand particle content of soil. However, the soil field capacity increased significantly with the increase of the silt and clay particle content of soil. Loamy soil has the largest water holding capacity, followed by sandy loam.The water holding capacity of sandy soil was the smallest. Moreover, the water characteristic curves of three black soils with biochar addition decreased gradually with thethe increase of centrifugal time and suction. Van-Genuchten model fitted the water characteristic curves of three black soils, more precisely. All the fit goodness of cubic curves simulating the relationship of soil volume moisture content and suction were more than 0.99. Centrifugation time of 100 min was optimum. The results in this study provided good insight into the effects of biochars on the water retention properties of different black soils in the northeast region.