Spatial variability and its controlling factors of soil moisture on cropland-forestland mixed hillslope in black soil area of Northeast China

Abstract: Black soil area in Heilongjiang plays an indispensable role in safeguarding national food security as an important commodity grain base. Serious soil erosion and environment degradation restrict the sustainable development of local society and economy. Soil moisture is one of the key factors which control vegetation growth, soil erosion, hydrological and ecological processes at the hillslope, and it also exhibits a great degree of spatial variability. Understanding the spatial variability of soil moisture and its influence mechanism at the slope scale is essential for controlling soil erosion, increasing food production, and improving ecological environment. In this study, spatial variability of soil moisture under the effect of land use and topographic variables was investigated on a cropland-forestland mixed hillslope. The research area was characterized by 4 different land use structures (from top slope to toe slope): cropland-cropland-cropland-cropland-cropland (L1), cropland- cropland-forestland-forestland-cropland (L2), cropland-cropland-forestland-forestland-forestland (L3), forestland-forestland- cropland-forestland-cropland (L4). Soil moisture was collected biweekly during mid-August to mid-September (2016) at 20 sites located in Keshan County of Heilongjiang Province, China. Soil moisture was measured by oven drying method at the depth of 0-20, 20-40, 40-60 cm. The relationship between environmental factors and soil moisture variability was conducted by redundancy analysis method (RDA). The results showed that: 1) Soil moisture content varied from 5.77% to 45.57%; and the soil moisture content of cropland was found markedly higher than that of forestland (P<0.05), with the mean soil moisture content ranging from 20.13% to 22.07% in cropland, and from 13.69% to 15.96% in forestland; the differences of soil moisture content at different soil depths were not significant(P>0.05). 2) Soil moisture content showed a moderate variability in the study area, coefficient of variation (CV) in cropland (35.9%-39.6%) was higher than that in forestland (30.0%-36.5%). 3) Soil moisture variations along hillslope under the impact of 4 land use structures differed from each other. The differences of soil moisture content at middle slope and toe slope in L1 were significant (P<0.05). 4) The spatial variation of soil moisture in the single land use structure (L1, CV was 14.82%) was lower than that in other cropland-forestland mixed land use structures (CV of L2, L3 and L4 was 62.70%, 38.25% and 25.70%, respectively), which indicated that land use mosaic patterns increased the variations of soil moisture. 5) RDA ordination showed that spatial variability of soil moisture was strongly influenced by land use types, and it was also affected by slope gradient, slope position and elevation. In addition, different factors control soil moisture distribution at different soil depths. This study presents that it is necessary to optimize the planting structure and take other proper methods aiming at preventing water loss and soil erosion, in order to improve soil fertility and promote the sustainable development.