Effects Of Long-term Located Fertilization And Paddy Field Continuous Cropping On Phosphorus Speciation In Farmland Soils

Phosphorus(P),one of the indispensable elements for the plants,participates in the whole course of plant growth.Most of P uptaked by plants is derived from soil.Fertilization patterns and continuous paddy field cropping patterns affect the occurrence,transformation and availability of P in red soils.With more and more concerns about environmental quality,the formation and transformation of P also attract considerable attention.It is particularly important to reveal the morphological distribution and temporal and spatial evolution of phosphorus for soil fertility and environmental protection.In this study,the soil samples under long-term fertilization in Qiyang,Hunan and from paddy field under continuous cropping in Changsha Hunan were collected.The Hedley P fractionation method combined with the inorganic P fractionation method was applied to explore the effects of long-term fertilization and cropping paddy field on soil P speciation and transformation.The evolution characteristics of soil clay minerals and iron oxide were also analyzed by powder X-ray diffraction(XRD).The main results are described as follows:Effects of long-term fertilization on red soil P speciation and transformation(1)In the case of long-term non-fertilization control(CK),the Fe activation degree decreased and the ability of soil to fix P enhanced with the increase of tillage period,promoting the conversion of labile phosphorus to stable phosphorus in soil,which led to the decrease of total P and available P in soil by 11.11% and 83.38%,respectively.And the content of inorganic P in the soil increased,inorganic P in the soil mainly existed as Fe-P.These results showed that soil organic phosphorus mineralized rapidly,and tranformed into a more stable form of inorganic phosphorus with lower biovailability.(2)For the long-term application of N fertilizer treatment(N),significant soil acidification was found with the soil pH dropped by 1.61.The transformation of 2:1 type of clay minerals to kaolinite occured significantly,speeding up formation of more stable soil phosphorus.The soil total P and content of available P decreased by 24.44% and 83.88%,respectively.The content of inorganic P in soil decreased greatly,and the proportion of Al-P and Ca-P with moderate plant availability decreased slightly,promoting formation of less available Fe-P and O-P.(3)In the single treatment of organic fertilizer and NPK/organic fertilizer treatments(OM and NPKM),the Fe activation degree increased,and the ability of soil to fix P relatively weakened,promoting the activation of P in soil.This results in increase of soil total P and available P.Meanwhile,the total phosphorus was 3 times and 3.64 times as much as that before the experiment,respectively,and the available phosphorus was 6.55 times and 6.36 times.Both treatments resulted in a significant increase in total P and inorganic P contents in the soil,especially for the Al-P with relativley higher bioavailability.These results showed that the application of organic fertilizer can promote the activation of P in the soil,where inorganic P was transformed into higher bioavailablity P form.(4)Under the balanced treatment of NPK(NPK),the supply of exogenous P source increased the total P and available P in the soil by 91.11% and 97.70%,respectively.As the Fe activation degree increased,the mobility of P,as well as soil active P increased during the season.The absolute content and percentage of Al-P in soil inorganic P pool were obviously increased,but Fe-P and O-P contents were significantly decreased.These results indicated that long-term application of balanced fertilizer promoted the transformation of soil inorganic P components to Al-P with higher bioavailability,the labile phosphorus content increased.Effects of continuous cropping of paddy field on the speciation and transformation of P in soilIn the continuous cropping pattern of paddy field,the content of available P in soil decreased year by year,and the content of unavailable P and recalcitrant P in soil increased,especially the HCl extracted P(HCl-Pt)component which increased the most remarkedly.These results indicated that there was a trend of P tranformation into the steady speciation in the soil.The Al-P component and Ca-P component of the inorganic P fraction in the soil showed a significant decrease trend,while the potential available P,Fe-P component and the stable P(inactive P),O-P component showed a downward trend.The highly active inorganic P in the soil decreased,and transformed to the potential effective or even very stable P forms.Therefore,more inorganic P in the soil was fixed,which can not be uptaked by plants.