Labile Organic Matter And Terminal Electron Acceptors Affect Greenhouse Gas Emissions,Iron Reduction And Phosporous Mobilization From Soils

Greenhouse gas（GHG）emissions occurred widely from rice paddies and forest soils and pose a significant threat to the atmosphere,consequently leading to global warming（GW）.So far,plentiful efforts have been put forward to mitigate the emission of GHG from flooded rice fields and forest ecosystems.However,flooded rice fields emit both methane（CH₄）and nitrous oxide（N₂O）simultaneously because the geochemistry of submerged soil supports the production of both gases.Under anoxic conditions,anaerobic microbes sequentially reduce terminal electron acceptors（TEAs）such as O2,NO^3-,Mn4+,Fe³⁺ and SO4^2-according to their energy status and availability.TEAs are considered to potentially reduce CH₄ emission from rice paddies.However,labile carbon（C）compounds and nitrogen（N）fertilization are the main drivers responsible for CH₄,carbon dioxide（CO₂）and N₂O emissions.Several laboratory studies were conducted to evaluate the effects of different labile C compounds and TEAs on GHG emissions from rice paddies and forest soils.In addition,the impacts of different labile C compounds on iron（Fe）reduction and phosphorous（P）mobilization in paddy and forest soils under fluctuating redox（Eh）conditions were also investigated.Gas chromatography（GC）,high performance liquid chromatography（HPLC）,elemental analyzer and spectroscopy were used to assess the influence of the amendments on greenhouse gas emission and geochemistry of C,Fe and P.Our results would provide scientific basis for exploring the important scientific and technological issues relevant to GHG emissions and coupled cycling of C,Fe and P in rice paddies and forest ecosystems.The main results showed as follows:（1）Addition of acetate increased CH₄ emissions and microbial biomass carbon（MBC）,suggesting that parts of acetate were mineralized rapidly and other were incorporated into microbial biomass.Addition of different TEAs(i.e.SO4^2-and NO^3-)concentrations with acetate levels reduced soil acetate content,p H values,and CH₄ emissions.This indicated that SO4^2-and NO^3-quantity might have reduced the turnover of acetate through competitive effects between sulfate and nitrate reducers with methanogens,resulting in low CH₄ emissions.On the contrary,addition of TEAs solely and combined with acetate increased soil NO^3-content,and their subsequent consumption facilitated N₂ O emission.The faster consumption of NO^3-than NH4+ under submerged conditions revealed that denitrification was the dominant processes contributing N₂ O emissions in rice paddies.In conclusion,the relative reduction in CH₄ and N₂ O emission was a function of SO4^2-and NO^3-in paddy soil.（2）We investigated the role of acetate,formate,oxalate,and propionate on Fe（Ⅲ）reduction,P mobilization,and CO₂ efflux in two paddy and forest soils（i.e.oxisol and ultisol）of varying soil organic C（OC）and Fe contents under fluctuating Eh conditions.Microbial mineralization of added labile compounds decreased Eh and increased the rates of Fe（Ⅲ）reduction followed by P mobilization in both oxisol and ultisol.This indicated that microbially-mediated Fe（Ⅲ）reduction was intensified by labile organic compounds,which acted as energy sources and electron donors in both paddy and forest soils.The release of available P via Fe reduction was accompanied by peaks of Fe（Ⅱ）content,dissolved OC content,and p H and was followed by a decrease in iron-bound P（Fe-P）.This implied that Fe-P was the main source of available P in paddy and forest soils.The significant correlation between available P and dissolved OC suggested that labile C inputs and soil dissolved OC acted as the main electron donors releasing substantial P amounts in both oxisol and ultisol.However,the faster release of available P in the oxisol than the ultisol indicated that the higher OC and Fe content in the oxisol allowed a fast Eh decrease,leading to rapid microbial oxygen（O2）consumption,and consequently faster and more intensive Fe（Ⅲ）to Fe（Ⅱ）reduction.This conclusion was supported by a fast Eh decrease corresponding to early P mobilization after input of labile C in the oxisol,and it suggested that Eh-driven Fe transformations and P mobilization were strongly modulated by labile C mineralization.Consequently,O2 consumption by labile organic matter（OM）mineralization decreased Eh and thereby facilitated Fe reduction coupled to P mobilization in paddy soils.Moreover,the p H increased mainly because of Fe（Ⅲ）reduction coupled with P mobilization in both forest and paddy soils.However,Eh decreased owing to labile C mineralization released occluded and adsorbed P primarily through Fe reduction and provided a transient supply of P in both sub-tropical paddy and forest soils.Overall,the results revealed that the turnover of labile C compounds significantly increased dissolved OC contents of the soils,which further facilitated CH₄ and CO₂ emissions from rice paddies and forest soils.In contrast,SO4^2-and NO^3-application offerred a probable means to mitigate CH₄ emission from rice paddies.Moreover,increased SO4^2-and NO^3-availability promoted higher NO^3-concentrations,and their further consumption facilitated both nitrification and denitrification and increased N₂ O emissions from rice paddies.We suggested that the use of SO4^2-and NO^3-containing amendments to reduce CH₄ emission from rice paddies is a better option to mitigate CH₄ emission,but it could increase N₂ O emission through NH4+ oxidation and faster NO^3-consumption（denitrification）.Moreover,the turnover of labile C compounds seemmed to be an important source of P for biota through Fe cycling and could provide a transient supply of P to P-limited subtropical paddy and forest soils.Three main mechanisms seemmed to be responsible for increased P availability in rice paddies（1）labile C compounds serve as electron donors and energy source to fuel Fe（Ⅲ）reduction and associated P mobilization（2）labile C compounds as well as inherent soil OC determines the intensity of Fe（Ⅲ）reduction and P mobilization（3）the stabilization of p H at a new level after an initial decline due to labile C compounds appears to be one of the prerequisite for the solubalization,and release of occluded and adsorbed P.Finally,P release associated with Fe reduction appears to provide a transient supply of P to Plimited subtropical rice paddies and presumably,P liberated via Fe reduction would likely to be available to rice roots under field conditions.