| Mitigations of greenhouse gases(CO2,CH4 and N2O)emissions from agricultural soils play an important role in combating climatic change and ozone depletion around the world.As a significant contributor to the state emission inventory,how to effectively reduce greenhouse gases emissions from agriculture would have great contribution to implement the objective of our government,that is,in 2020 the national CO2 emissions per unit of GDP decreased 40-45%compared to 2005.To investigate the effects of biochar addition on mitigation of greenhouse gases emissions and the increase of soil carbon sequestration,thus,would have beneficial effects on exploring effective practices for attenuating global warming.This study was mainly carried out in rice-wheat rotation system and vegetable fields in eastern China.The static opaque chamber_GC method was used to in situ measure the fluxes of CO2,CH4 and N2O from these systems,and thus,the primary objective was to evaluate the effects of biochar applications on greenhouse gases emissions,crop production and global warming potential(GWP).In this study through collecting the data of biochar decomposition from the studies that 13C or 14C isotope was used,the decomposed amount and rate of biochar and its-induced priming effect on SOM were meta-analyzed.The factors and processes affecting biochar stability in soil are also discussed.In addition,through synthesizing the peer-reviewed results of N2O emissions from vegetable fields,we used an empirical model to estimate the amount and spatial distribution of fertilizer-induced N2O emissions and background emission from Chinese vegetable fields in 2009.In this study,field experiment were accomplished as follows:an outdoor and a field in situ pot experiments were performed to determine the effects of different application levels of rice husk biochar(0,25 and 50 t ha-1)together with or without N fertilizer(0 and 200 kg N ha-1)on CO2,CH4 and N2O emissions and crop yield from a rice-wheat rotation system;one-year field measurement of N2O and CO2 emissions from an intensive vegetable field was conducted to investigate the effects of wheat straw biochar(0 and 30 t ha-1)together with two different N fertilizers(MF and CP)applications on N2O emissions and net ecosystem carbon budget.A 60-day aerobic incubation was also performed to evaluate the effects of rice husk biochar addition on N2O and CO2 emissions from three paddy soils.The main results of this study are presented as follows:1.The decomposed amount of biochar increased logarithmically with experimental duration,and the decomposition rate decreased with time.The biochar decomposition rate varied significantly with experimental duration,feedstock,pyrolysis temperature,and soil clay content.The MRTs of labile and recalcitrant biochar C pools were estimated to be about 108 days and 556 years with pool sizes of 3%and 97%,respectively.The second database(116 observations from 21 studies)was used to evaluate the priming effects after biochar addition.Biochar slightly retarded the mineralization of soil organic matter(SOM;overall mean:-3.8%,95%CI=-8.1-0.8%),compared to the soil without biochar addition.Significant negative priming was common for studies with a duration shorter than half a year(-8.6%),crop-derived biochar(-20.3%),fast pyrolysis(-18.9%),the lowest pyrolysis temperature(-18.5%),and small application amounts(-11.9%).In contrast,biochar addition to sandy soils strongly stimulated SOM mineralization by 20.8%.This indicates that biochar stimulates microbial activities especially in soils with low fertility.Furthermore,abiotic and biotic processes,as well as the characteristics of biochar and soils,affecting biochar decomposition are discussed.2.The seasonal N2O emissions from vegetable fields significantly increased with nitrogen(N)fertilizer application(P<0.001).According to the ordinary least squares(OLS)model,the fertilizer-induced emission factor(EF)and background emissions of N2O were estimated to be 0.55±0.05%and 1.067±0.277 kg N ha-1 yr-1,respectively.The EF was reduced and the background emission of N2O increased when the measurement duration was prolonged from<100 d to>100 and<200 d.Comparable results were obtained by the maximum likelihood(ML)model,with an EF of 0.49±0.06%and background N2O emissions of 1.228±0.189 kg N ha-1 yr-1.Based on the OLS-derived parameters,the fertilizer-induced direct emissions and background emissions of N2O were estimated to be 66.95 Gg N and 19.63 Gg N,respectively,in 2009,and the annual N2O emissions were much higher in the provinces of Shandong,Henan,Hebei and Sichuan.The estimated N2O emissions from vegetable fields accounted for 21.4%of the total direct N2O emissions from Chinese croplands but with large uncertainties.3.Biochar addition significantly decreased N2O emissions during the 60-day period by 73.1%as an average value while the inhibition ranged from 51.4%to 93.5%(P<0.05-0.01)in terms of cumulative emissions.Significant interactions were observed between biochar,N fertilizer,and soil type indicating that the effect of biochar addition on N2O emissions was influenced by soil type.Moreover,biochar addition did not increase CO2 emissions from both paddy soils(P>0.05)in terms of cumulative emissions.4.Biochar addition to the upland soil increased CH4 emissions by 37%during the rice season,while had no effect on CH4 emissions during the wheat season.Biochar amendment decreased N2O emissions up to 54%and 53%during the rice and wheat seasons,respectively,but had no effect on the ecosystem respiration in either crop season,neither on the global warming potentials of CH4 and N2O over the rice-wheat rotation.In the aerobic incubation experiment,biochar addition significantly decreased N2O emissions and increased CO2 emissions from the paddy soil(P<0.01)without urea addition.Biochar addition increased grain yield and biomass if applied with nitrogen fertilizer.Averaged over the two soils,biochar amendment increased the production of rice and wheat by 12%and 17%,respectively,and these increases can be partly attributed to the increased soil nitrate retention.5.Biochar addition did not affect the annual N2O emissions(26-28 kg N ha-1),but reduced seasonal N2O emissions during the cold period.Biochar increased soil organic carbon and CO2 efflux on average by 61%and 19%,respectively.Biochar amendment did not increase yield-scaled N2O emissions after application of mineral fertilizer,and slightly but insignificantly decreased yield-scaled N2O by 15%after nitrapyrin addition.Biochar addition strongly increased carbon gain in the acidic soil,with an average of 11.1 t C ha-1,compared to the treatments without biochar addition(-2.2 t C ha-1)over one year period.In conclusion:1.Following biochar applied in soil,only a small part of biochar C is bioavailable and the remaining 97%contribute directly to long-term C sequestration in soil.Biochar can persist in soils on a centennial scale and that it has a positive effect on SOM dynamics and thus on C sequestration.2.Nitrous oxide emission induced by vegetable cultivation is one of the most sources of N2O emissions from agroecosystem,which is mainly ascribed to large amount of N input and favorable environment conditions.Biochar application stimulated soil heterotrophie respiration.The effect of biochar application on N2O emissions from vegetable field is influenced by both soil temperature and N availability.3.During the rice growing season,biochar application can stimulate CH4 emissions but significantly decrease N2O emissions.The reduction of N2O emissions due to biochar application are attributed to the altered N transformation,as well as its interaction with N fertilizer and soil type.Under N fertilization conditions,biochar application can increase the yields of both rice and wheat. |
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