Long Term Effects Of Combined Application Of Biochar And Nitrogen Fertilizer On Crucial Factors Of Soil Fertility And Crops Yield In Upland Red Soil

Upland red soil,a total area of about 2.04×106 km2,is an important land resource in south China because of the abundant hydrothermal resources.However,due to the higher soil buk density,acidity and lower porosity combined with the unreasonable farming methods,the upland red soil has lower soil nutrient content,aggregate stability,soil microbes and enzyme activity and poorer soil hydraulic properties.The application of high dose of fertilizer led to nitrate leaching,affecting the quality of groundwater seriously;additionally,it also caused the increase of agricultural production cost and restricted the local agriculture sustainable development.At present,various researches have shown the beneficial effects of biochar as an amendment to improve soil fertility with the co-benefits of waste management,pollutant immobilization,crop productivity increasement,and soil and groundwater pollution reductions.However,the crucial factors of soil fertility improvement after the biochar application was not clear,and few studies have investigated the optimal biochar and nitrogen fertilizer application rates in worldwide.A six-year field trial was conducted to study the effects of biochar(0,2.5,5,10,20,30 and 40 t·ha-1)combined with nitrogen fertilizer(0,60,90,120 kg·ha-1)on soil pH,soil nutrient content,soil hydraulic properties,soil aggregate,soil microbial and enzyme activity,soil organic carbon mineralization,soil nitrate leaching and crops yield in upland red soil in Jinxian county,Jiangxi province.This research tried to find the the crucial factors of soil fertility improvement after the biochar application and the optimal biochar and nitrogen fertilizer application rates.The mian results were as follows:(1)Biochar combined with nitrogen can significantly reduce soil bulk density,improve soil saturated hydraulic conductivity,saturated water content,field capacity and available water content.At the same time,the large dry and water-stable aggregates proportion also increased with the increasing of biochar application.The most significant improvement of soil hydraulic properties was observed in the first year after biochar used.In addition,the application of biochar can not reduce the wilting coefficient of upland red soil,and the application of nitrogen fertilizer had no significant effect on soil bulk density.Four years after biochar application,C6N2 treatment significantly improved the proportion of>2mm dry aggregates and 2-1mm and 1-0.5mm water-stable aggregates in the 0-15cm soil layer,meanwhile,the proportion of<0.25mm microaggregates decreased with the increasing of biochar application.Biochar application can also increase the proportion of>2mm dry aggregates in the 15-30cm soil layer,but the increasement is less than that in the 0-15cm soil layer.During the six-year experiment period,biochar can continuously improve soil hydraulic properties,however,the influence of biochar on soil hydraulic properties weakened over time with the extension of biochar application.(2)The combined application of biochar and nitrogen fertilizer can significantly reduce soil acidity and increase the nutrient content,such as soil available phosphorus,organic carbon,total nitrogen,ammonium nitrogen and nitrate nitrogen.The soil nutrient content increased with the increasing of biochar and nitrogen fertilizer application.The soil pH and available phosphorus reached the highest value in C6N3 treatment in 2012 and increased by 11.9%and 93.3%,respectively,over CON0.The soil total nitrogen content reached the peak value in C6N3 treatment after biochar was applied for six years because of the nitrogen accumulation in the soil,as C6N3 treatment significantly increase 92.3%of the total nitrogen compared with CON0.During the six-year experiment period,biochar combined with nitrogen fertilizer can improve the soil nutrient content continuly,but the effectiveness weakend over time.In 2016,only high dose of biochar(C5 and C6 level)can significantly improve the soil nutrient content.(3)The combined application of biochar and nitrogen fertilizer can significantly increase the soil microbial biomass carbon and nitrogen.The soil microbial biomass carbon and nitrogen content in C6N3 treatment peaked after three years of biochar application(May,2014).However,the increasing trend was noted until in May,2012,after which the soil microbial biomass carbon and nitrogen content in all biochar treatments declined throughout the experimental period..Application of biochar can also improve soil urease,catalase and invertase activity,and the influence of biochar on these three oxidoreductase in all treatments paralleled the pattern of soil microbial biomass carbon and nitrogen.The C6N2 treatments significantly increased soil urease,catalase and invertase activity by 65.6%,66.1%and 35.1%,respectively,compared with C0N2 treatment.The application of biohar can significantly reduce the soil organic carbon mineralization and the decomposition rate of organic carbon in 2012.In the early stage of soil organic carbon mineralization incubation,biochar can significantly increase the rate of organic carbon mineralization,and the higher the biochar application amount,the faster the rate of soil organic carbon mineralization.However,with the extension of incubation time,biochar showed the negative effect on soil organic carbon mineralization rate.In addition,the ratio of cumulative mineralization of soil organic carbon(Cm)and SOC decreased with the increasing of biochar application,the ratio of cumulative mineralization of soil organic carbon(Cm)and SOC reduced by 95.3%in C6N2 treatment,as compared with C0N2.The results showed that biochar can reduce the proportion of soil easy mineralization organic carbon,reduce the mineralization of soil organic carbon,and reduce the decomposition of soil organic carbon.(4)The application of biochar can significantly reduce the nitrate leaching in upland red soil.The soil column experiment showed that biochar can significantly improve the nitrate flux in horizontal and vertical transport,and improve the adsorption of nitrate in soil.The measured values and simulated values obtained by CXTFIT 2.0 were highly significantly related,as shown by high correlation coefficients(r>0.85).The highly significant positive correlations were found between the initial time,total time,water flux and the application rate of biochar,while cumulative mass of leached NO3--N and peak concentration showed significant negative correlations with the increasing of biochar application rates.The results showed that the biochar added to soil could be an effective way to reduce nutrient leaching in soil column experiment in laboratory.(5)High dose of biochar combined with nitrogen fertilizer can increase rapeseed and sweet potato yields,as well as SYI,resulting in increased production.The increasement of rapeseed yield by biochar was not durable,however,the increasemrnt of sweet potato yield by biochar was more lasting.By the end of this six-year experiment,the yield of sweet potato with biochar addition was still significantly higher than that of CK.Principal component analysis and path analysis showed that the most susceptible parameters in the upland red soil were soil hydraulic and acidity properties following biochar addition,soil pH and hydraulic properties have more significant effects on soil fertility and rape yield than soil nutrients,microbial biomass and enzymatic activities do.High dose of biochar combined with nitrogen fertilizer can increase rapeseed and sweet potato yields,as well as SYI,resulting in increased production,but this does not necessarily increase net income.This research suggests that local farmers can apply biochar and nitrogen fertilizer with 11.4 t·ha-1 and 102 kg·ha-1,respectively,to earn more profits,based on the current prices of biochar and nitrogen fertilizer.The results showed that the biochar combined with nitrogen fertilizer can inprove soil fertility and crop yield,reduce the nitrate leaching and the amount of soil organic carbon mineralization,however,the effectiveness was not durable.It is importment to find the optimum biochar application rate to obtain the highest economic benefits.