Spatial distribution of soil nutrient at depth in black soil of Northeast China: a case study of soil available potassium

Spatial mapping of potassium is very urgently needed to develop economically and environmentally sound soil management plans, and deeper-layer soil available potassium (AK) resources on the farm are important for evaluating the long-term need for purchased K fertilizer. The objectives of this study are to analyze AK spatial distribution in deep soil depths from a small watershed, and to provide reference for farmers better managing fertilizer application and protecting environment in the farmland. In order to describe AK spatial distribution in deeper depths, six hundred and ten soil samples were collected from 122 soil profiles (0-60 cm) in a representative random sample method. Kriging procedures, correlation analysis and regression analysis were used to describe the spatial variability of soil AK in a small watershed from typical black soil of northeast China, and AK in cultivated field and forest (field returned to forest over 10 years) areas was compared. Gaussian models were recognized as the best for predicting AK in the soil at different layers. Spatial autocorrelations as influenced by human activities were weak for AK at 0-30, and 40-60 cm layers and was moderate at 30-40 cm, and autocorrelation distance (A(0) = 3,110 m) is uniform at all layers. AK heterogeneity was different among soil layers, and was influenced by soil parent material, landscape, slope position, land use, soil management and so on in the watershed. The content of AK was 21.1-428.4 mg kg(-1) in the cultivated areas and 117.2-401.0 mg kg(-1) in the forest (field returned to forest over 10 years) in the entire profile (0-60 cm). AK in the forest areas were larger 18, 3, 22, 16 and 5 % in 0-20, 20-30, 30-40, 40-50 and 50-60 cm, respectively than in the farmland. AK variance in space decreased from the 0-20 to the 50-60 cm depth. The spatial pattern of AK was similar in all layers, and typically decreased from forest to surrounding farmland. AK also deceased along the water flow direction. Furthermore, AK was influenced by steepness, aspect, elevation, leading to the big difference between the two banks of the hydrographic reaches. In the field, AK was negatively correlated to gross soil loss and steepness at all depths, and became significant at the 0-20 cm depth, while it was significantly positively correlated to elevation except at the 40-50 cm depth. Generally, AK is sufficient for crop production for at least 160 year with a little potassium fertilizer application needed during the growing season in the study area. However, crop residue management and fertilizer application, especially manure and nitrogen application in special conditions (dry seasons, intensive cropping without residue return and so on) and sensitive areas (such as Low elevation, closer forest and so on) is still necessary in order to increase potassium storage and promote the efficiency of AK for plants.