Distribution And Release Of Phosphorus Fractions Associated With Soil Aggregate Structure In Restored Wetlands

Wetland is a transitional ecological zone between terrestrial ecosystems and aquatic ecosystems.It is a geographical complex formed by the interaction of soil and organisms,and has the functions of conserving water sources,regulating climate,degrading pollution,protecting biodiversity and maintaining ecosystem stability.However,under the influence of human activities,such as the discharge of industrial and agricultural wastewater,the construction of water conservancy projects,the reclamation and cultivation of transition area between land and water,and the development of tourism landscapes,China's wetlands are facing ecological risks,including the severe degradation of vegetation,the rapid shrinkage of area and the sharp decline in ecosystem productivity.With the improvement of people's understanding of the importance of wetland ecological function,returning farmland to wetland has become an important measure to protect wetland resources.Phosphorus,as an essential nutrient for the growth of phytoplankton and the provision of cellular kinetic energy,participates in metabolic process of organisms in a wide variety of forms,is a participant in important life activities,such as photosynthesis and respiration,and plays an important role in the process of energy storage,migration and transformation.To date,despite considerable advances in our understanding of changes in phosphorus content and morphological in farmland and wetland ecosystems,the accurate essence of phosphorus redistribution and release risk associated with soil aggregate fractions in restored wetlands remains poorly understood.The structure and chemical properties of soil aggregates have an influence on the biogeochemical cycle of nutrient in wetland ecosystems.During wetland restoration duration,how do the structural stability of soil aggregates and the release capacity of phosphorus change? Simultaneously,soil aggregates play a physical-chemical-biological role in the retention-release process of phosphorus,but the specific mechanism needs to be further studied.In addition,the unique geographical location,climatic characteristics and cultivation methods lead to different occurrence patterns and release mechanisms of phosphorus fractions from that in southern China,and the research on seasonal freeze-thaw region has been explored preliminarily.In this regard,restored wetlands in Sanjiang Plain representing different restoration duration of 1,2,3,5,13 and 19 years,respectively,were selected as the research region,and soil aggregates were taken as the basic unit.Scanning electron microscopy energy dispersive spectrometer(SEM-EDS)was used to explore the dynamic changes in soil aggregates and distribution characteristic of surface element during wetland restoration duration.Phosphorus fractionation procedure,and Langmuir and Freundlich adsorption isotherm model were conducted to analyze the redistribution and adsorption-desorption behavior of phosphorus in soil aggregate fractions.By means of redundancy analysis(RDA)and path analysis(PA)multivariate statistical analysis,the primary controlling environmental factors affecting the redistribution of phosphorus in soil aggregate fractions and the retention capacity of soil to phosphorus were identified.The gotten conclusions are expressed as follows:(1)The stability of soil aggregates in restored wetlands was higher than that in farmland,and it increased first and then decreased with the extension of wetland restoration duration.The structure of coarse-aggregates is superior to the fine-aggregates.Fine-aggregates(<0.25 mm)first condensed into coarse-aggregates(>0.25 mm),which was then reversed until the process stabilized after 5 restoration years.(2)During wetland restoration duration,pH,iron(Fe)and manganese(Mn)increased first and then decreased,electrical conductivity(EC)continued to decline,total nitrogen(TN)and organic matter(SOM)showed a bimodal tendency.TN was enriched in soil aggregates with a grain-size of >1 mm.SOM and Mn decreased with the decreasing grain-size of soil aggregates.Soil aggregates with grain-size of 0.25-1 mm and 0.053-0.25 mm carried Fe.(3)For soil aggregate fractions,with the exception of labile inorganic phosphorus(L-Pi),which continuously decreased within wetland restoration duration,total phosphorus(TP),labile organic phosphorus(L-Po),moderately labile organic phosphorus(Ml-Po),iron-aluminum bound phosphorus(Fe.Al-P),calcium-magnesium bound phosphorus(Ca.Mg-P),humic phosphorus(Hu-P)and residual phosphorus(Re-P)presented a unimodal tendency.TP,L-Pi,L-Po and Re-P tended to decrease with the reduction in soil aggregate grain-size,Ml-Po was enriched in soil aggregates with a grain-size of 0.25-1 mm and 0.053-0.25 mm,soil aggregates with a grain-size of 0.25-1 mm carried Fe.Al-P and Ca.Mg-P.(4)Coarse-aggregates showed a considerable capacity to retain phosphorus.The capacity to retain phosphorus of soil aggregates with grain-size of >1 mm and 0.25-1 mm decreased first and then increased within wetland restoration duration,while that of soil aggregates with grain-size of 0.053-0.25 mm and <0.053 mm continued to decline.(5)Environmental factors controlling phosphorus redistribution and the capacity of soil to retain phosphorus were described as the following order: TN > SOM > pH > Fe > Mn > EC?(6)At the initial stage of wetland restoration,the contents of Fe.Al-P and Hu-P were relatively high and were significantly affected by hydrological conditions.Especially in the rainy season of the Sanjiang Plain(June-August),it is necessary to focus on its release risks.