Effect Of Biogas Slurry Application Of Large-scale Pig Farms On Soil Quality In The Fujiang River Basin

Biogas project is an effective way to realize resource recovery and innocuity of pig manure from large-scale farms,while the excessive or improper application of biogas slurry will cause secondary soil environment pollution.In this study,20 large-scale pig farms in Mianyang and Suining around the Fujiang River basin were selected as the research objects.The changes of main nutrients and heavy metals in pig manure water and biogas slurry before and after biogas project treatment was analyzed.Combining with field sampling and pot experiment,the effects of biogas slurry on soil fertility quality and heavy metals accumulation after long-term and short-term application was researched.The main research results and conclusions are as following:(1)Components analysis of pig manure water and biogas slurry of large-scale farms.The nutrients concentration was relatively high in pig manure water and biogas slurry of 20 large-scale pig farms selected in Mianyang and Suining.Among them,the average concentration of COD,TN,TP and NH4+-N in biogas slurry were 3875.1 mg/L,1932.7 mg/L,38.8 mg/L and 804.2 mg/L,respectively.The main heavy metal pollutants had relatively high concentration,such as Zn,Cu,Mn,As and Cr in pig manure water and biogas slurry with an average electrical conductivity of 8.38 mS/cm.Directly agricultural application of pig manure water and biogas slurry may cause heavy metal pollution risks and soil salinization problems.(2)Effect of biogas slurry long-term application on soil fertility quality.Sample plot in farmland surrounding the large-scale pig farm through continuous application of biogas slurry compared with controlled plot shows that:long-term application of biogas slurry increased the content of>0.25 mm mechanical macro-aggregate in soil.The proportion of>2 mm mechanical macro-aggregate was around 73.99%～88.03%.The content of>2 mm and>0.25 mm water-stable aggregate of vegetable land were higher than paddy field and wheat field.The content of<0.25 mm organic matter in aggregate increased significantly.With the continuous application of biogas slurry,the content of organic matter,available nitrogen and available phosphorus in the topsoil increased.(3)Effect of biogas slurry on soil fertility and vegetable biomass after three seasons application.The pot experiment shows that:>2 mm soil mechanical and water-stable aggregate presented an increasing trend after biogas slurry application for three seasons.The maximum leaf length,fresh weight and root weight of vegetables were increased.The content of organic matter in soil,available nitrogen and phosphorus in vegetable were positively correlated with the concentration and dosage of biogas slurry.Suitable biogas slurry application for leafy-vegetables need to be diluted about 2～5 times,root vegetables about 1～2 times,and the appropriate biogas slurry once application was 500 mL/pot.(4)Effect of biogas slurry application on accumulation of heavy metals in soil.The comparative study of greenhouse pot experiment and field trials show that:heavy metals in soil(0～10 cm),such as Zn,Cu,Cr,As had relative high concentration after biogas slurry long-term application in field,and obvious cumulative phenomenon.The accumulation of Zn,Cu,Pb and Cr was not obvious in vegetable soil after three season biogas application for indoor pot.The content of heavy metals like As and Cd was relative high,and heavy metals like As and Cd were easier accumulate in root vegetable land compared with leafy vegetable land.The greenhouse pot experiment results show that:available state content of Zn and Cu in the third vegetable soil account for 1.08%～2.12%and 2.31%～5.65%of the total amount of heavy metals,and As and Cd account for 0.67%～5.99%and 1.24%～60.00%,respectively.This indicates that the accumulation of As and Cd in soil should be highly concerned.Heavy metals,such as As,Cd,Cr and Pb in the edible part of vegetable,which content were relatively high,especially the Pb content in root vegetable conspicuously higher than leafy vegetable.Biogas slurry was allowed to be absorbed about 134.44～191.05 m3/hm2 per season.