| Swine manure biochar is a kind of biochar formed by pyrolysis of swine manure under high temperature and anaerobic conditions,and conversation of swine manure into biochar will be an important way to completely eliminate manure pollution in current large-scale pig farms.The fact of high content of organic matter,available phosphorus and potassium,and large specific surface area and high porosity in swine manure biochar,presents that swine manure biochar is rich in nutrients and strong in stabilizing heavy metal elements,therefore,it is of great significance to strengthen its application in agriculture.Meanwhile,over reliance on chemical fertilizer is widespread in agricultural production based on reddish yellow paddy soils in southern China,leading to the decline of soil comprehensive fertility and the accumulation of heavy metals,which seriously affect the sustainable development of rice farming.In this paper,swine manure biochar was used as soil conditioner to explore it’s effects on the environmental safety of nitrogen,phosphorus and heavy metals in reddish yellow paddy soil.In this study,swine manure biochar was applied to the reddish yellow paddy soil at the proportion of 2.5%,5.0%,and 7.5%,respectively.Rice and packchoi were planted by pot experiment in rice-vegetable-rice rotation system,and the rice was medium rice.Soil pore water samples were taken at the main growth stages of rice in each cropping season,and plant samples and soil samples were taken after harvest in each cropping season.The contents of nitrogen,phosphorus and heavy metals in the samples of soil pore water,soil,and plants were tested,respectively,and the micro physical and chemical background of soil was analyzed also,as well the corresponding indexes were evaluated.The research results are as follows:(1)After applying swine manure biochar,the soil microstructure were changed from lamellar structure to mixed structure of lamellar clay and biochar-clay complex;the pH value of the soil in each treatment were increased by 0.29～0.67,and the redox potential were decreased by 10～66mv;the total nitrogen content in the soil was increased by 28.8%～78.8%,and the nitrogen activity entropy were decreased gradually with the application rate increasing;the soil total phosphorus content increases by 1.28～4.39 g/kg,and the soil phosphorus activity entropy were increased with the application rate increasing.The Cu and Zn content in the soil were increased by 1.94-5.14 times and 0.37-3.40 times,respectively;the content of weak acid extraction of Pb and Cd decreased,and the content of reducible As and Hg increased,which shows obvious passivation.(2)After the application of swine manure biochar,the average total nitrogen content of the soil pore water during the first rice-cropping season increased by-6.5%一 63.9%,and increased by 10.8%to 44.6%during the second rice-cropping season;the total phosphorus content of the soil pore water were increased by 2.1～9.3 times during the first rice-cropping season,increased by 8.2-14.1 times during the second rice-cropping season;the content of water-soluble Cu,Zn and Pb in the soil pore water were increased during the first rice-cropping season,among which the Pb content exceeded the environmental quality standard of class III surface water(GB 3838-2002),which reached moderate to severe pollution;during the second rice-cropping season,the content of heavy metals in the soil pore water treated by swine manure biochar was lower than that of the control,and the pollution level were light or safe,which reflected the passivation effect of swine manure biochar on soil heavy metals.(3)After application of swine manure biochar,the accumulation of Cu and Zn in the pakchoi was increased significantly with the application ratio increasing,and slightly increased in the brown rice compared with the control.In the reddish yellow paddy soil treated with 2.5%to 7.5%swine manure biochar,the Cu content was suitable for the growth of Chinese cabbage and rice,while the Zn content in the soil of 7.5%treatment was so high that the packchoi and rice show zinc toxicity.The accumulation of Pb,Cd,As and Hg in brown rice in the first cropping season were decreased with the increase of swine manure biochar except for Pb in the treatment 2.5%.In the second cropping season,the accumulation of Pb,Cd,As and Hg in brown rice were decreased with the increase of swine manure biochar application as well,and the accumulation amount was significantly lower than that of the control.The accumulation of Pb in pakchoi in treatments of application of swine manure biochar was higher than that in the control,while the accumulation of Cd,As,and Hg were lower than that in the control.The accumulated amount of heavy metals in each treatment did not exceed the standard.(4)According to the comprehensive evaluation of heavy metal pollution of crops,the comprehensive pollution index of heavy metals of Pakchoi in each treatment was lower than 0.7,which belonged to the safety level,and the comprehensive pollution index of each treatment were ranked as follows:CK>T2.5>T5.0>T7.5.In the first rice-cropping season,the comprehensive pollution index of brow rice in each treatment were ranked as:T2.5>CK>1>T5.0>0.7>T7.5,and the main pollutant was Pb;in the second rice-cropping season,the comprehensive pollution index of brown rice were ranked as:1>CK>T2.5>0.7>T5.0>T7.5,and the main pollutants were Pb and As.(5)The results of the food safety evaluation of crops showed that the higher the application rate of pig manure carbon,the lower the risk coefficient of cabbage and brown rice.In summary,Appropriate application of swine manure biochar(T2.5 treatment)can improve the environmental quality of reddish yellow paddy soil,change the microscopic morphology and chemical properties of the soil,supplement the nutrients of N,P,Cu,and Zn,and inhibit the migration of heavy metals through soil pore water.While excessive application of swine manure biochar(T5.0 and T7.5 treatments)can pollute the soil with Cu and Zn,increase the content of nitrogen and phosphorus in soil pore water,and cause the risk of eutrophication of surrounding water.However,increasing the application amount of swine manure biochar can reduce the comprehensive score of heavy metal pollution in pakchoi and brown rice,reduce the food safety risk value of children and adults,and improve the quality of crops. |
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