| Mollisols region as one of the most important bread basket of the China, the soil fertility is highly related to the food security of the country. Soil phosphorus (P) as the main nutrition for crop growth was focused on by many researchers in years. As well as, one of the main objects was to clarify the spatial heterogeneity of P to guide farmers fertilization, increase crop yield and reduce non-point source pollution. Generally, a watershed can be recognized as the relatively complete unit, and which include of the complete landform condition and hydrological processes. In the stuty, the spatial variation of Total phosphorus (TP) and Available phosphorus (AP, Olsen-P and CaCl2-P) within a watershed scale was carried, and random sample method was adopted. Both of the geostatistical and traditional analysis were used to describe the spatial distribution of P at layers (0-20,20-30,30-40,40-50and50-60cm, a total of610soil samples), also cultivated field and secondary forest areas were compared in the watershed. Furthermore, topographical factors and hydrologic factors were combined to systematically analyze the soil P spatial distribution, and its main drive mechanism. The main results were as below:(1) The horizontal distribution of P was mainly influenced by structural factors (58-95%).The spatial autocorrelation was strong for AP at all20-60cm soil layers, and for TP at all soil layers over the study depth of0-60cm (Co/Co+C<25%). The autocorrelation distance of AP (ranging from42to255m) and TP (ranging from70to225m) differed in various layers, but did not change systematically with soil depth.(2) The horizontal distribution of TP was higher at the summit and the bottom slope due to soil erosion and deposition, especially got higher along the hydrological reach direction, and was highest at the outlet of the watershed. The highest value of TP was2.14g·kg-1. While, TP was lower at the back slope due to soil erosion, and the lowest value was0.03g·kg-1However, AP was typically higher in south facing slope in light of solar radiation, erosion and penetration, in0-40cm depths, and the highest was50.01mg·kg-1. Furthermore, AP decreased along the down slope direction, and the lowest value got0.43mg·kg-1(3) The vertical distribution of TP typically decreased with increasing depths in farmland, gradually decreased from0.65g·kg-1to0.49g·kg-1.The variation is not systematical in secondary forest, in0.54g·kg-1to0.64g·kg-1. TP didn’t changed systematically with soil depth in forestland. In farmland, AP decreased at first and then increased with soil depth increasing, and the highest value is15.04mg·kg-1. Also, the AP dynamics in soil profiles were not performing systematically change. (4) Our results indicate that P in the secondary forest soil profile was still influenced by the previous cultivation, although the farmland had been converted to forest over ten years earlier.(5) AP over the sufficient levels only occurred on85%of the total area at the0-20cm soil depth, and less than12%at20-60cm depths (12%、5%、5%、9%in0-20,20-30,30-40,40-50and50-60cm, respectively).(6) In this study area, only less than2.4%area could have the P loss risk in0-20cm layer, and P loss by water dissolved is not main channel. However, high fertility farmland was easy influenced by the severe soil erosion.Generally, the horizontal distribution of P was mainly influenced by soil loss, deposition and topographical factors, while the vertical distribution of P, especially AP was more easily disturbed by fertilization, moisture, temperature, organic matter and crop absorption in the watershed. In order to better improve the bio-availability of phosphorus, P fertilization combined with conservation tillage could be used, especailly on the back slope. These were beneficial to (1) avoid the excessive P applicaion resulting agricultural non-point source pollution,(2) promote the food production, despite the most of study area has a higher P ferlility level at0-20cm layer. |
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