Spatio-Temporal Variability Of Soil Phosphorus And Its Fraction Transformation In Farmland

Studying on spatio-temporal variability of soil phosphorus and its fractiontransformation is helpful for rational fertilization and identification of key control spot ofagricultural non-point phosphorus pollution. On the basis of phosphorus sequentialextraction methods combined with modern mathematic means, we can know better aboutcharacteristics of soil phosphorus fraction transformation, which were useful for rationallyfertilization and regulating the banlance of soil phosphorus.With geo-statistics and GIS technique, this paper analyzed the spatio-temporalvariability of soil available P at three times (1982, 1997, and 2002) and total P in two times(1982, 2002) in Rugao County, Jiangsu Province, and analyze the main factors affectingspatio-temporal variation. Then the typical soil series were chosen to study thetransformation of soil phosphorus fractions and distribution in topsoils and profiles.In general, the content of soil available P and total P kept ascendant trend in the past20 years. The results showed that the loam Aquatic Cambosols (Yinfang) in southern partof the area increased in recent years after experienced a decrease at the middle of 1990s,other soils showed different ascendant trend to various extents within each period. Thesandy Aquatic Cambosols (Motou and Taoyuan) derived from river alluvium in themid-western areas increased more slowly than other soils in recent years, meanwhile, thesoils of loam Aquatic Cambosols (Dongchen) also developed from river alluvium innortheastern part and the loam Aquatic Cambosols (Banjing) derived from lacustrinesediment in the northwestern areas increased very fast.Overall, the loamy Stagnic Anthrosols (Baipu) soils in the eastern area, the clayAquatic Cambosols (Changqingsha, Zhanghuanggang) and the sandy Ustic Cambosols(Guoyuan) soils in the southern area had high levels of total P at all times, whereas theMotou and Taoyuan soils in the middle and western areas had low levels. From 1982 to2002, the average of the total P raised from 0.72g kg^-1 to 0.84 g kg^-1. The Dongchen soilshad higher increment whereas the Baipu and Guoyuan soils didn't increase significantly instatistics.The main factors affected spatio-temporal variability of soil available P and total P included fertilization, land utilization and soil property. From the database of soil availableP during 20 years, a fitted equation reflected its trend was obtained. Based on thecalculation in terms of the critical value that produces risk to aquatic environment, soilavailable P will be able to reach the critical value after about 24 years according to presentincreased trend.The amount of Ca-bound P (H₂SO₄-P) was the main fraction of P in soils, and theproportions of various fractions in toral P decreased in order as follows: Ca-bound P(H₂SO₄-P)＞Residual P＞Fe- and Al-bound P (NaOH-P_i)≈organically bound P(NaOH-P_o)＞exchangeable P (NH₄Cl-P). The content of NaOH-P_i and NaOH-P_o and theirproportion of total P increased significantly in the past 20 years, implying that the surplus Padded to soils manily transformed to NaOH-P_i and NaOH-P_o. Meanwhile, NaOH-P_i wasnot only the sources of soil available P but also the sink of surplus P added to soils.Using related coefficient analysis and path coefficient analysis to discuss the relationbetween every P fraction and available P (NaHCO₃-P), it concluded that the contribution ofevery fractions to available P is decreasing in order: NaOH-P_i＞H₂SO₄-P＞Residual P＞NaOH-P_o in 1982, showing that NaOH-P_i and H₂SO₄-P was the effective source ofavailable P and NaOH-P_o and residual P had direct influence upon NaOH-P_i then indirecton available P. In 2002, the contribution of every fractions to available P decreasing inorder: NaOH-P_i＞NaOH-P_o＞H₂SO₄-P＞Residual P, indicating NaOH-P_i was the effectivesource of available P and NaOH-P_o and residual P were indirect sources to available P insoils.In four profiles of Cambosols (Banjing,Motou,changqingsha and Dongchen soils),the content of soil available P and total P is higher in the topsoil than other layers, anddecreased with an increase of depth. In profile of Athrosols (Baipu soils), the content ofavailable P and total P was high in topsoil, decreased in subsoil and then slightly increasedwith the depth.The content of NH₄Cl-P, NaOH-P_i and NaOH-P_o and their proportions of total P wereall higher in the topsoil than in the other layer. In the other layers under the topsoil,H₂SO₄-P and residual P were the main fractions of total P so that the proportion of them intotal P was nearly above 90%, especially, in the bottom layers where the proportion canreached above 99%.According to the results of this study, the amount of application of P fertilizer in riceseason can be properly lowered because the NaOH-P_i accumulated in soils can transform to soil available P under the reduction condition.