Effects Of Biochar On N₂O Emissions And Transformation Of Nitrogen Forms From Soils Of Different Surface Conditions

N₂O is the third largest greenhouse gas after CO₂ and CH₄,and has a strong warming potential,with a potential warming effect of about 298 times than that of CO₂.The main source of N₂O emission is agricultural activity.And,nitrification and denitrification can lead to N₂O production,considered as the main way to N₂O release in farmland soils.As a soil conditioner for biochar,mixed with the soil,can effectively improve the soil organic matter content,soil water retention performance and change the soil pH value.Changes of these environmental factors will inevitably affect the soil biological activity and even the growth of plants,thereby affecting the soil nitrogen cycle.Previous studies have shown that the soil N₂O release under the influence of biochar was not consistent with the increase or decrease,which may be related to factors such as biochar type,biochar consumption,surface vegetation,soil condition etc..Therefore,there is a need for constant multi-perspective research to enrich the understanding of this process.Based on this background,this study conducted a field experiment,aiming at investigating the effect of soil N₂O release,soil total nitrogen and its inorganic nitrogen forms,volatilization of ammonia,soil environment conditions by biochar at varying dosage?biochar applied at 0 t·ha-1yr-1,5 t·ha-1yr-1,45 t·ha-1yr-1?and surface conditions?crop cultivation,bare land?.Field trials were conducted between July 2014 and October2015,followed by maize-wheat-maize seasons.The experiment set up six treatments,referred to as CK+?0 t·ha-1yr-1 biochar+crop cultivation?,CK-?0 t·ha-1yr-1 biochar+bare land?,BC5+?5 t·ha-1yr-1 biochar+crop cultivation?,BC5-?5 t·ha-1yr-1 biochar+bare land?,BC45+?45 t·ha-1yr-1 biochar+crop cultivation?,BC45-?45 t·ha-1yr-1 biochar+bare land?,with 3 repetition of each treatment.The conclusions were as follows:?1?During the wheat growing period,the soil N₂O emissions of CK+,BC5+,BC45+ were 21.70-88.91,21.42-130.09,64.44-179.58 ?g·?m2·h?-1 respectively,and that of BC45+ was significantly higher than that of the other two index?P<0.05?.In comparison with wheat winter period?February 6th?,the soil N₂O emissions of the three treatments in wheat peak stage?March 9 to April 15,2014?were all remarkably reduced?P<0.05?,and the amplification of BC45+ in wheat booting and heading period?April 15th?decreased by 18.43%,14.62%,respectively,with respect to that of CK+ and BC5+.In the earlier stage of maize growth?July 30-August 30,2015?,the soil N₂O emission flux of BC45+ possessed a higher value than that of CK+ and BC5+?P<0.05?,but there were no significant differences among three treatments in maize heading stage?September 17th?and mature stage?September 28th?.However,theenhancing effect of soil N₂O emission that applied larger quantity biochar was effectively weaken with the arrival of the wheat peak stage and the increase of surface mulch.Besides,the result of soil N₂O emission flux in the three treatments had also verified this opinion in bare land.?2?The N₂O warming potential?NWP?and emission intensity?GHGI?in wheat and maize growth season showed as followed: they both enhanced along with the increased dosage of biochar,and the differences between BC45+ and the other two treatments reached the striking level?P<0.05?,but there was no significance difference between BC5+ and CK+.In bare land,there was only a significant difference between NWP of BC5-and CK-,and the results of other treatments were similarities with that of crop cultivation conditions.So,the applied larger quantity biochar could significantly improve the N₂O warming potential and emission intensity,while NWP and GHGI of the applied lower quantity biochar treatment?BC5?were inhibited by crop cultivation?surface vegetation?.?3?Under the conditions of wheat cultivation and homochronous non-cultivation,the soil NO3--N,NH₄+-N and total nitrogen content of BC5 and BC45 treatments were raised with respect to CK,but those dropped significantly,especially for BC45+treatment,declined by 96.44%,69.40% respectively in wheat peak stage,and soil total nitrogen content declined by 6.33% compared to the earlier stage.The soil inorganic nitrogen content of maize growth season had a similar trend in wheat season.Parallel to this result of the apparently falling soil NO3--N and NH₄+-N concentration,the soil N₂O emissions of BC45+ also declined remarkably in peak stage.The decline in respiratory substrate caused by the increase of nitrogen uptake by crop growth,may be one of the reasons for the decrease of N₂O emission.The correlation analysis demonstrated that soil NO3--N,NH₄+-N and total nitrogen all existed distinct correlation,and biochar could impact the mutual transformation among soil NO3--N,NH₄+-N and total nitrogen directly,and further effected the soil N₂O emission.?4?Biochar treatment could effectively reduced the ammonia volatilization of crop cultivation soil.With respect to CK+,the ammonia volatilization of BC5+ and BC45+treatments declined by 7.97%-50.52% and 9.95%-61.80%,respectively.Conversely,that of BC5-,BC45-treatments had a clear increasing trend compared with CK-in bare land.?5?The correlation analysis showed that soil N₂O emission flux of control treatment?CK?and biochar treatments?BC?had a significant positive correlation with soil NO3--N,and the correlation coefficients were 0.71,0.63,in turn.The results of redundant analysis showed that,the contribution rates of each factor to soil N₂O release in the control treatment were: NO3--N >temperature >pH >TN >NH₄+-N >ureaseactivity >humidity >organic matter;and the contribution rates in the biochar treatments were: NO3--N>organic matter>temperature>TN>pH>urease activity>NH₄+-N>humidity.The biochar treatments changed the explanations of the environment factors to the N₂O release,and the factor of the biggest contribution was soil NO3--N.These results indicated that the soil N₂O emission,soil inorganic nitrogen contents and ammonia volatilization in biochar-mediated were more affected by biochar application and crop cultivation?land cover?,and decreased with the arrival of crop growth.Biochar also had a direct effect on other environmental factors?soil organic matter,pH value,urease,etc.?,and the factors formed the influences of soil ammonium nitrogen,nitrate nitrogen etc.,which affected the soil N₂O release.