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

The spatial variability of soil available phosphorus (AP) and total phosphorus (TP) influences crop yield and the environment. The paper aims to identify the spatial heterogeneity of P (AP and TP) and clarify the main driving mechanisms in a Mollisol watershed of Northeast China. Both geostatistical and traditional analysis were used to describe the spatial distribution of P at different depths. P in cultivated fields on the upper slopes was compared with secondary forest areas on the lower slopes within the same watershed. The horizontal distribution of P was found to be primarily influenced by structural factors (58-95 %). TP was high at both the summit and the bottom of slopes at all depths, being especially high at the watershed outlet due to erosion on the back slope and deposition at the base. AP was higher on south-facing slopes than on north-facing slopes and typically decreased from the summit to the base of south-facing slopes at the 0-40-cm depths, mainly due to solar radiation, soil loss, and water loss. The vertical distribution of TP typically decreased with increasing depths in farmland but did not show systematic variation in the forest profiles. AP was lower in the middle of the 0-60-cm soil profiles in the farmland, reflecting the influence of fertilization, infiltration, and crop absorption. AP in the 30-60-cm and TP in the 20-60-cm layers were lower in farmland than in the secondary forest, and only 2 % of the area showed a risk of P loss through ground flow and infiltration in the 0-20-cm layer. The horizontal distribution of P in the 0-60-cm layers was mainly influenced by soil and water loss, deposition, and hydrothermal dynamics, while the vertical distribution of P, especially AP, was more affected by fertilization, infiltration, organic matter, and crop absorption. Secondary forestland that had been converted from farmland was found to effectively hold P, especially in deep soil layers, as the loss of P dissolved in water is not a primary process.