| Biochar is a carbon-rich refractory solid formed by high-temperature thermal decomposition of straw,wood,organic waste,municipal waste,livestock manure under anaerobic conditions.A large number of studies have confirmed that biochar has a large specific surface area and stable structure,and plays an important role in soil carbon sequestration,improving soil quality,improving soil productivity and reducing greenhouse gas emissions.However,the biochar will gradually age after being applied to the soil.In addition to the change of its own carbon composition,the interaction between its own functional group structure and soil particles will also change,which directly affects soil physicochemical properties,nutrients,nitrogen microorganisms and transformation process,and then affects crop growth and soil N2O emissions.At present,most studies on biochar focus on short-term effects(<2 years),and there is very little research on the effects of longterm application of biochar on soil aggregates,fertility distribution,nutrient use efficiency and N2O emission.Therefore,the paper studied the effects of aging process of biochar on soil aggregation,rice growth and soil N2O emissions through in situ long term rice field experiments,and clarified the microbial mechanism of aged biochar on soil fertility,agricultural productivity and soil N2O emissions.The long term field experiment was established in Nanjing,Jiangsu,where was paddy soil.Six treatments were carried out as following:CKnoP(with urea but no phosphate fertilizer),CK(without urea and biochar),N(urea),NB0y(urea with 0-year aged biochar amended in 2017),NB2y(urea with 2-year aged biochar amended in 2015)and NB5y(urea with 5-year aged biochar amended in 2012).Soil samples were collected after rice harvest and separate biochar particles from soil samples of NB0y,NB2y,and NB5y treatments;the contents of soil aggregate size classes of macro-aggregate(MacroA,250-2000 μm),microaggregate(MicroA,53-250 μm),silt fraction(SiltF,2-53 μm)and clay fraction(ClayF,<2μm)and soil basic physicochemical properties were measured.The rice yields and nitrogen and phosphorus use efficiency(NUE and PUE)were determined.Moreover,annual monitoring of soil N2O emissions in rice and wheat season was conducted,and soil samples were collected after rice and wheat harvest to analyze the changes in the abundance of relevant microbial functional genes during the production process.The results of our study are summeraized as follows:1.Scanning electron microscope,Fourier infrared spectroscopy,X-ray photoelectron spectroscopy and other analytical methods were used to compare the changes in physicochemical properties of fresh and aged biochar.Compared with BFresh,the physicochemical properties of B0y changed little,and compared with B0y,the surface structure of B2y and B5y was seriously damaged;B2y and B5y significantly increased the cation exchange capacity(CEC)by 29.3%and 52.0%,oxygen content(O)by 39.2%and 44.8%,pore volume by 53.6%and 75.0%,H/C by 13.9%and 19.4%,O/C by 57.9%and 68.4%,total specific surface area(SSA)by 156.4%and 208.8%,respectively.The carbon content(C),pH and mean pore diameter were significantly reduced by 9.1%and 10.4%,21.9%and 24.3%,30.1%and 23.7%,respectively.And as the biochar ages,the aliphatic carbon and aromatic carbon reduced due to oxidation,and oxygen-containing functional groups such as carboxyl groups and carbonyl groups increased.2.The NB2y and NB5y treatments significantly increased the ratio of soil aggregates of 250-2000 μm by 69.2%-107.8%,decreased that of 2-53 μm by 13.1%-14.7%and that of<2μm by 6.9%-41.9%,while NB0y did not,compared with N treatment;The NB0y,NB2y and NB5y treatments significantly increased SOC by 13.3%-46.4%,total N by 8.1%-24.2%and total P by 5.7%-17.1%,but did not significantly in SOC and TN content of 2-53 μm fraction,compared to the N treatment.Similarly,both the NB2y and NB5y treatments increased rice yield by 13.7%and 16.3%,but significantly increased NUE by 31.3%and 44.3%,PUE by 35.4%and 45.5%,respectively,while NB0y did not,compared to the P treatment.Based on the structural equation modeling(SEM),the aged biochar(NB2y and NB5y)increased the rice yield,N and P storage by improving soil nutrient status(SOC,TN,TP,etc.)and structure(increased the ratio of soil aggregates of 250-2000 μm).3.Continuous observation of soil N2O emissions during the entire cycle of rice-wheat rotation,aged biochar can significantly reduce the cumulative N2O emissions by 32.4%54.0%,but the reduction capacity of aged biochar for 2 or 5-year was reduced.Compared with N treatment,NB0y,NB2y and NB5y treatment significantly increased soil pH by 9.4%20.7%,NO3--N content by 1.7%-31.3%,and reduced NH4+-N content by 4.2%-25.3%,and ability of improving soil pH was decreased with aging;The NB0y,NB2y and NB5y treatments significantly increased the abundance of nosZ gene by 54.9%-249.4%,but it was decreased with the biochar ages.The soil(nirS+nirK)/nosZ ratio was increased with the biochar ages.Meanwhile,the cumulative N2O emissions showed a significant negative correlation with soil pH,and a positive correlation with NO3--N content and amoA-AOB gene abundance.In summary,biochar’s own properties will change during the aging process,and it will affect on the soil and crop,which improves soil ability to maintain nutrients,may increase agricultural productivity.However,the potential for reducing N2O emissions will decrease as the biochar ages. |
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