| Climate warming remains a major challenge facing the world today.Current field practices with low carbon or nitrogen inputs to arable lands depleted soil nutrient content,especially with a double-crop system of winter wheat and summer maize rotation.Due to this long intensive cultivation mode,soils in this region possess a very low soil carbon and nitrogen content,and this will surely jeopardize future productivity.Straw returning has became a major agronomic practice to help resolve this issue.However,studies showed that straws can be decomposed within soil in really short time and release greenhouse gases which are chef contributors to global warming.Biochar is an alternative source for soil amendments.Resulting from the highly intense thermal condition of biochar process,their products hold relatively higher carbon content,finest aromatic structure and greatest stability when incorporated into soils.Hence biochar application became a comprehensive solution in resolving world food crisis,environment pollution remediation and global climate change mitigation.Due to its large porous characteristics and high surface area,biochar amendents could increase soil porosity thus reduce soil bulk desity and in turn favoring soil to accumulate moisture thus provide great environment for crop growth.In this thesis,based on the comprehensive literature review about the current progress of biochar as soil amendment,the current research area is broad and results are inconsistent.Further study is needed to target specific soil types and environmental conditions.For instance,little is known about the effect of biochar produced at different temperatures on soil freezing thawing cycles.Therefore,study the effect of biochar,as a potential important bio-resources in sustainable agriculture,incorporation with soil has significant meaning to soil improvement and crop yield managment.Our research used both indoor incubation experiment and in situ plant-soil mesocosms experiment to study different straws and their derived biochars on soil carbon content,and different pyrolysis temperatures on soil nitrogen and plant uptake.The main method and results were as follows:(1)An incubation was carried out with two main factors,including four types of carbon sources(MS as maize straw,WS as wheat straw,MB as maize biochar and WB as wheat biochar)and 3 application rates(5,10,20 g/kg)to investigate CO2 respiration and their decomposition in calcareous soil.The results showed that,the CO2 efflux under wheat and maize biochar treatments are significantly lower than the values under their corresponding straws treatments during the incubation period,and soil respiration rate was decreased by 77% and 76%,respectively.In addition,CO2 cumulative emission on both straw and biochar treatments showed a similar pattern,in which,the total CO2 emission were only 84.6 and 82.4 mg·pot-1 with biochars amendments while they reached 355.8 and 347.0 mg·pot-1 for wheat and maize straw incorporation,respectively.In general,soil mineralization rate fitted with a logarithm model,and the correlationship between the model and tested data achieved significant level.Under the same soil type and environmental set-ups,there was a negative correlationship between different biochar application rates and total carbon loss within hundred years,which in another way,meaning higher application rate holds the highest carbon restoration capability.In conclusion,biochar,as an alternative way of dealing abundant straw resources in Guanzhong Plain,can decrease soil C mineralization significantly,thus its application as a soil amendment could be a productive management for sustainable agriculture.(2)The objective of this study was to evaluate the effect of different straws additions(WS and MS)and biochar applications(WB and MB)on soil C fractions and their responses due to various sensitivity factors.The results showed that,total C loss under biochar treatments were 9.0% and 8.2%,respectively while they reached to 55.6% and 52.6% under wheat and maize straw treatments,approximately 5.3 folds higher than biochar treatments.Both straw and biochar treatments significantly increased soil organic carbon(SOC)content,with WB treatment achieved the highest SOC content(74% increase compared to control treatment).The increase was in order of WB > MB > MS > WS,and the SOC content increased as amendments application rate increases.However,there were no significant effect on soil dissolved organic carbon(DOC)or soil labile organic carbon(LOC)content under different biochar application rates,while there was a negative correlation between decreased soil microbial carbon(MBC)content and increased biochar addition rates.In general,straw treatments had a greater effect on labile soil carbon fractions(LOC,DOC and MBC)than its derived biochar,whereas biochar tended to have a greater effect on the recalcitrant soil fraction(SOC).Our results suggest that the production of biochar and its addition to calcareous soil is an effective management practice for C sequestration,but its advantage over straw addition on the retention of soil carbon should be considered in the context of a longer time scale.(3)An incubation experiment was carried out with soil sterilization as main factor and investigated the interactions between different straw and biochar material and soil microbes on soil physical properties.The temperature was maintained at 25oC and moisture as 75% of field capacity to investigate their decomposition characteristics under the same temperature and moisture conditions.Soil aggregation,particle density,bulk density,TOC,SOC,MBC,MBN,LOC and DOC were measured 70 d after incubation.The results showed that 1)Regarding wet-sieve results,both straw and biochar treatments increased soil macroaggregation(>0.25 mm)by 400% and 50%,respectively,when compared with control.Biochar can also increase soil micro-aggregation(0.25+0.053 mm).No significant effect of soil sterilization and types of crop on soil aggregation.2)Regarding dry-sieve results,soil sterilization treatment significantly decreased soil dried aggregation.Biochar increased all lower level aggregation(<1 mm)while straw only significantly increased 2 mm aggregation.3)Soil amended with biochar lost less water indicating improved water conservation.4)Biochar increased long term carbon fraction than straw,such as SOC,however,straw treatments have better short term effective carbon,such as LOC,DOC,MBC.Additionally,soil sterilization treatment significantly decreased soil MBC.(4)While soil freeze-thaw cycles can decrease soil nutrient retention over winter by increasing leaching losses and greenhouse gas emissions,biochar as a soil amendment could mitigate these effects.Nevertheless,there is often variation in the effectiveness of different biochar formulations with respect to soil nutrient retention.We used soil mesocosms to examine the effects of biochar produced under a series of pyrolysis temperatures(250-600oC)on soil nitrogen retention in response to variation in soil freeze-thaw cycle intensity(-10oC vs.0oC following spring melt).We also examined the subsequent effects on soil nitrous oxide(N2O)emissions and nitrogen leaching losses.1)N2O emission peaked on the 4th day after melting,there was a significant interaction of biochar cooking temperature with soil freezing,that is,under soil freezing condition,higher temperature produced biochar increased N2 O emission.2)Soil freezing increased inorganic nitrogen losses through leaching,but biochar,especially produced under high temperature,can mitigate such effect and reduced nitrate leaching with no significant effect on leachate ammonium.Biochar also decreased the ratio of nitrate and ammonium from 7.1 to 4.1.Overall,our results confirm that biochar application can mitigate soil nitrogen losses over winter,although it may also interact with soil freezing to increase emissions of the greenhouse gas N2 O.(5)Followed the same experiment design as experiment(4),we added 15 N tracer to soil mesocosms to examine the effects of biochar on soil nitrogen retention over winter and its subsequent effects on plant nitrogen uptake.We measured 15 N abundance in soil and plant samples as well as plant N uptake,N concentration and plant yield.Results concluded that 1)Biochar amendment increased both soil 15 N retention over winter and the subsequent plant 15 N uptake,increased from 20% and 1.5%,respectively to 39% and 3.2%,respectively.Biochar generated at the highest temperature exhibiting the strongest effects on plant 15 N uptake.2)Soil freezing significantly reduced crop yield by 30%,biochar addition mitigated the negative soil freezing effect on subsequent plant biomass.Overall,high-temperature produced biochar could effectively increase N retention over winter and help to mitigate the negative effect of soil freezing on crop yield.(6)In agricultural fields,both biochar amendment and the use of cover crops have been proposed as strategies to increase crop N availability,but it is unclear how their interactions may affect soil N retention over winter,particularly when leguminous cover crops are used.We used three factors experiment design,including snow removal(SR),legume cover crop(CL)and biochar temperature(TEMP,250-550 oC),with resin bag installment at bottom to examine the whole winter N(inorganic forms)retention,soil pH,Bulk density(BD),Microbial biomass nitrogen(MBN)following spring melt and plant yield.We found that 1)Biochar could reduce soil nitrate-N leaching over entire winter,especially under legume cover crop treatment(8.7% reduction),the interaction achieved significant level.2)Although MBN was not significantly affected by biochar addition in general,biochar produced under higher pyrolysis temperature,however,caused soil MBN to decrease.3)Regarding the plant yield,biochar and cover crop addition both increased plant yield and significantly mitigated the negative effect of soil freezing,with a better performance of biochars when combined with CL treatments.4)Higher temperature produced biochar can lead to higher soil pH.Overall,our results confirmed that biochar application can mitigate soil N losses over entire winter,and help to increase plant productivity. |
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