Effects Of Combinedapplication Of Straw Andexogenous Substances On Soil Priming Effect And Carbon Sequestration

Soil organic carbon（SOC）is an important indicator of soil fertility securing high and stable crop yields.SOC sequestration not only affects agricultural sustainability,but also plays an important role of the global carbon cycle and climate change.Nowadays in China,with the vigorous promoting of straw returning,crop straw has become one of the most important organic amendment improving SOC sequestration in farmland.At the same time,straw return can maintain the balance of the medium element and microelements.But the majority of straw carbon can be mineralized to CO₂during the crop straw decomposition stage,causing a little straw carbon being sequestered in soil.Therefore,it is of great significance to explore how to reduce soil CO₂emission and sequester more straw carbon in soil.Since the effect of straw returning on the SOC sequestration depends on the balance between the amount of newly formed SOC and the loss of SOC mineralization.The organic materials addition may increase the turnover of SOC in the soil which can induced priming effect,thereby accelerate the native SOC mineralization.Soil aggregate is the basic unit of soil structure,its the important place for physical protection of SOC.They can improve the stability of SOC by making a barrier between the soil microorganisms and SOC,which can prevent decomposition of SOC by soil microorganisms and enzymes.However,most studies have focused on the priming effect of crop straw addition on SOC mineralization in bulk soil,but the knowledge of the priming effect in aggregate size classes is absent.Meanwhile,soil inorganic carbon（SIC）is an important component of the soil carbon pool.It should be combined with reasonable materials while straw returning to consider the effect on SOC and SIC.For these,an incubation experiment was investigated the effect of straw returning on SOC and priming effect（PE）in soil aggregates,and another incubation experiment was conducted to study the effect of straw returning with calcium oxide application on cropland soil carbon sequestration.The main results are as follows:1.In order to investigate the priming effect of straw addition in soil aggregates of long-term different carbon and nitrogen management,this study was used the incubation experiment,and soil were collected from the same type soil that had been continuously subjected to different crop residues and nitrogen management for 16 years(S₀N₀:No straw return+unfertilization;S₁N₁:High straw return+high nitrogen fertilizer:240 kg·hm^-2).Soil samples were separated into three aggregate size classes（large macroaggregates,small macroaggregates and microaggregates）and were incubated with or without maize straw for70 days under 25℃.The results showed that the cumulative CO₂emission from the mineralization of microaggregates in S₁N₁soil without straw addition was 1261.2 mg·kg^-1,which was higher than large macroaggregates and microaggregates by 36.3%～46.9%.The same amount of straw addition can increase the cumulative CO₂emission of each aggregate size classes in both soils.And regardless of soil fertility,the CO₂emission of the small macroaggregates is significantly higher than the other two aggregate size classes.For the apparent priming effect（PE）in aggregate size classes,there is no significant difference in apparent PE in large macroaggregate and microaggregate in S₀N₀soil after straw addition,but the apparent PE in small macroaggregate is significantly higher than large macroaggregate and microaggregate by 5.9%～10.9%.In S₁N₁soil,the apparent PE in aggregate size class in an order of:macroaggregates>microaggregates.Meanwhile,straw addition significantly increased the content of SOC in the aggregate size classes in both soils by 2090 mg·kg^-1,2360 mg·kg^-1and 5570 mg·kg^-1.And the content of SOC was higher in micro-versus macroaggregates,which was the opposite of the apparent PE.Regardless of the addition of straw,the dissolved organic carbon（DOC）in macroaggregates was significantly higher than microaggregates by 41%and 43.2%in S₁N₁soil.However,there was no significant difference in DOC among the aggregate size in S₀N₀soil.There was no significant difference in the microbial biomass carbon（MBC）among the aggregate size without straw addition,but the MBC in microaggregates in S₀N₀soil and S₁N₁soil was significantly higher than macroaggregates after straw addition.2.2.For the effects of lime and straw addition on SOC sequestration in long-term different carbon and nitrogen management soils,two tested soils(S₀N₀:No straw return+unfertilization;S₁N₁:High straw return+high nitrogen fertilizer:240 kg·hm^-2)also used and respectively incubated with or without addition of straw and lime to study the effect of straw return and lime on the carbon sequestration.Our study showed that the soil cumulative CO₂emission was observed 42.9%higher in S₁N₁soil when without straw and lime addition,than in S₀N₀soil.In both soils,the straw addition alone increased the soil cumulative CO₂emission by averages of 81.6%and 70.4%,respectively,compared with straw absence.This shows that straw addition has a greater impact on the native OC mineralization in soil with low initial SOC.Lime addition decreased soil cumulative CO₂emission in both soils whether straw addition or not.Straw addition increased the SOC by 2.95 g·kg^-1and 3.19g·kg^-1,respectively in S₀N₀and S₁N₁soils,while reduced the SOC by 1.36 g·kg^-1in S₁N₁soil and did not affect the SOC in S₀N₀soil when combining with lime addition.Using¹³C stable isotope technology,the newly formed organic carbon（OC）in the two soils both significantly increased after straw addition,therein which increased 25.8%in S₀N₀soil when compared with S₁N₁soil.However,the conjoint addition of lime and straw did not modify the newly formed SOC when compared with the addition of straw alone,which shows that the lime addition have no effect on the decomposition process of straw in the soil.Over all,straw addition alone increased the SOC apparent balance by 841.46 mg?kg^-1and 388.87mg?kg^-1,respectively in S₀N₀and S₁N₁soils.Lime addition reduced the cumulative CO₂emission by 469 mg?kg^-1and 529 mg?kg^-1,respectively in S₀N₀and S₁N₁soils,which approximately equal to the increases in SIC(by 443 mg?kg^-1and 566 mg?kg^-1,respectively).In S₀N₀soil and S₁N₁soil,straw addition significantly increased the MBC by131.71 mg·kg^-1and 145.95 mg·kg^-1,respectively.Regardless of straw addition,lime addition reduced MBC.The addition of strawor lime alone had no significant effect on the Ca-Hs in both soils,but the combination of lime and straw addition significantly increased the Ca-Hs.Straw addition significantly increased the DOC in S₀N₀soil and S₁N₁soil by 47.7%and34.9%,and the combination of straw and lime addition significantly increased the DOC in S₀N₀soil,but had no significant effect on the DOC in S₁N₁soil.Sraw addition significantly reduced the soil p H in both soils.The addition of lime alone increased the soil p H by 3.6%and 3.7%.However,the combination of lime and straw increased the p H by 2.4%and 1.9%in S₀N₀soil and S₁N₁soil,respectively.Based on the above experimental studies,the following conclusions can be made:（1）The PE was higher in macro-versus macroaggregates,but the SOC in microaggregates is higher than macroaggregates.This indicates that microaggregates have a greater protective effect on SOC.（2）Soil with low carbon and nitrogen input in the field can promote the newly formed organic carbon（OC）,and the cumulative soil CO₂emission with low input is always lower than high input.therefore,the potential of soil organic carbon sequestration increases with decreasing soil initial SOC content.（3）The addition of lime significantly reduced the cumulative soil CO₂emission.At the same time,the increment of soil inorganic carbon（SIC）content were approximately equal to the reduced amount of cumulative CO₂emission in both fertility soils after lime addition.This showed that the addition of calcium source can achieve the goal of soil organic carbon sequestration through chemical reaction with soil CO₂.