Phosphorus Loss Characteristics And Mechanism Of Protected Soil Under Different Irrigation Patterns

P loss is the main reason of agricultural non-point source pollution.Soil phosphorus accumulation is becoming serious in greenhouse soil because of continuous P fertilizer application.Irrigation and rainfall lead to not only loss of phosphorus but also pollution of water source.Here we focused on the soil for tomato growing,which individually was under forrow irrigation（FI）,surface drip irrigation（SUR）,subsurface drip irrigation（SDI）for 13 consecutive years to determine the different phosphorus forms through soil profiles under different irrigation patterns.We also calculated soil phosphorus Change-Point and phosphorus sorption parameters to evaluate phosphorus leaching risk.Based on the relationship between soil properties and phosphorus forms,analysis of impact of different irrigation patterns on phosphorus distribution,forms,absorbtion and release were performed then.We also investigated the impact of different irrigation patterns on phosphorus loss and proposed better irrigation methods.In this project,FI and SUR were the same with local protection.The porous pipe was transpiration semi hose of 2 cm diameter and the depth was 30 cm under SDI.Soil wetting proportion were 1.0,0.5 and 0.5 in FI,SUR,and SDI respectively.The irrigation control upper and lower limits were set to soil water suction for 6.3 KPa and 40 KPa.Soil samples for analysis were collected at different depths within 0-60 cm from three different irrigation patterns in the same greenhouse.The main research results were shown as follows:1.The forms and contents of phosphorus through soil profilesDifferent irrigation methods significantly affected the forms and contents of phosphorus through soil profiles.（1）The contents of total P（TP）contents were 0.83-3.18 g kg^-1 through soil profiles.The contents of TP in soils decreased with the increasing soil depth in all three irrigation patterns.The contents of TP of the soils of 0-30cm depth ordered FI>SUR>SDI,while that of 30-60cm depth ordered SUR>FI>SDI.（2）The contents of sequence of extraction for soil P fractionation（H₂O-P,NaHCO₃-P,NaOH-P and HCl-P）based on modified from that of Hedley decreased with the increasing soil depth.NaOH-P was the main phosphorus form in the soils of 0-20 cm,and HCl-P was the main phosphorus form in the soils of 30-60 cm under FI and SUR.HCl-P was the main phosphorus form in the soils of 0-60 cm in SDI.2.Influencing factors of phosphorus forms in soilThe forms and contents of phosphorus through soil profiles were affected by pH value,organic matter,available phosphorus,active iron and active aluminum in different irrigation patterns.（1）The pH values increased with the increasing soil depth,while the contents of organic matter,available phosphorus,active iron and active aluminum decreased with the increasing soil depth in three irrigation patterns.The pH values were extremely negatively correlated with organic matter,available phosphorus,active iron and active aluminum,and each of organic matter,available phosphorus,active iron and active aluminum was extremely positively correlated with each other in FI.The p H values were extremely negatively correlated with organic matter and available phosphorus respectively.Organic matter and available phosphorus were extremely positively correlated with each other.Active iron was extremely positively correlated with active aluminum at 1%in SUR and SDI.（2）The pH value of soil,organic matter and available phosphorus as well as other forms of phosphorus were closely related with each other,and their relationship was dependent on irrigation pattern.The NaOH-P content was negatively correlated with pH value,and positively correlated with organic matter and available phosphorus at 1%,and the HCl-P content was extremely positively correlated with pH value,and negatively correlated with organic matter and available phosphorus in all three irrigation patterns;The content of active phosphorus（H₂O-P and NaHCO₃-P）was significantly correlated with pH value and organic matter and available phosphorus in SUR and SDI,while such correlation was less than 5%in FI.3.Characteristics of phosphorus absorbance and releaseCharacteristics of phosphorus absorbance and release were different in the soils with different irrigation patterns.（1）Langmuir equations could fit well to the ability of soils to adsorp isotherms of P in three irrigation treatments.Qm and K calculated by Langmuir equations in the soils of same depth ordered SDI>SUR>FI;Qm and K decreased with the increasing soil depth dramatically.The amount and ratio of soil phosphorus desorption increased as phosphorus adsorption increased.The adsorption of same phosphorus decreased with the increasing soil depth;The amount and ratio of soil phosphorus desorption in the soils of same depth ordered FI>SUR>SDI.（2）The accumulation amount of total phosphorus,water-soluble phosphorus and particulate phosphorus in soils of different depth all increased as the extension of release.The accumulation amount of phosphorus in soils of same depth ordered FI>SUR>SDI,and water-soluble phosphorus was the major form of phosphorus loss form.4.Evaluation of the risk of soil phosphorus loss（1）The phosphorus loss risk of the greenhouse soils was evaluated by Change-Point.The Change-Point of the soil of 0-20 cm with FI,SUR,and SDI were 59.4 mg kg^-1,68.6 mg kg^-1and65.3 mg kg^-1 respectively,and 60.61 mg kg^-1,70.58 mg kg^-11 and 66.8 mg kg^-11 in 20-40 cm depth.All three irrigation patterns had risk of phosphorus loss which ordered FI>SUR and SDI.（2）The eutrophication risk index of soil phosphorus under three irrigation patterns was 3.2-7.6%in the soil of 0-60 cm.The eutrophication risk index of soil phosphorus in the soil of same depth ordered FI>SUR>SDI.However,all three irrigations did not induce the eutrophication risk.Soil pH value and organic matter were two major influencing factors of eutrophication risk index of soil phosphorus in SDI.The analysis of the correlation between eutrophication risk index of soil phosphorus and absorption phosphorus index suggested that degree of P adsorption saturation was significantly correlated with eutrophication risk index of soil phosphorus（more than 5%）.In summary,FI should be avoided during protected cultivation,and SUR as well as SDI should be preferred.Moreover,proper parameters of irrigation technology and improved fertilization practice should be performed to reduce phosphorus loss and decrease environmental risk with the prerequisite of the crop high quality and yield.