The Effects Of Nutrients On The Relationship Between Iron And Soil Organic Carbon In Peatlands

Peatlands cover only?3%of the terrestrial surface,yet secure about 30%of global soil carbon.Combustion of fossil fuels and agriculture increase atmosphere nitrogen and phosphorus deposition and fertilization,which promote nutrient levels in peatlands.Iron is also an active element during redox condition in peatlands,it is meaningful to know the effects of nutrient on the relationship between Fe and carbon for carbon stability and peatland management.We conducted our experiments in a temperate fen in Northeastern China,where contained long-term fertilization peatlands and natural peatlands.Both field and incubation experiments were performed to know the enzymatic mechanisms of fertilization on soil nutrient and Fe associated organic carbon,and the effects of temperature and oxygen on Fe-mediated carbon mineralization,finally,we had a synthesis analysis of the effects of nutrients on the relationship between Fe and carbon.We studied the cultivated peatlands?paddy?,their close proximity natural peatlands?peatlands disturbed?and undisturbed peatlands?as reference?to know the effects of nutrient input on soil nutrient status and microbial responses.Cultivation decreased surface soil organic carbon and total nitrogen,and increased soil total phosphorus?p<0.05?.With the impact of agriculture,N and P showed a weak increase at 0-10 cm soil,followed by a higher increase and then showed no differences down the profile.According to the enzymatic vector angle and correlation analysis,we also found that increased P reduced microbial P limitation,and higher P enrichment also inhibited N-degrading enzyme activity in cultivated peatlands.Although soil organic carbon and total nitrogen also affected microbial enzymatic activities,P played a dominant role in the variation of microbial enzymatic activities.We examined Fe associated soil organic carbon?Fe-OC?and the effects of nutrients on Fe-OC at the flooding period in natural peatlands and cultivated peatlands.Our data show that crystalline Fe?300-780%in 20-70cm soil?,reactive Fe?150-300%in 20-50cm soil?and short-ranged Fe?150-200%in 20-40cm soil?were enriched compared with natural peatlands,albeit with an absence of corresponding differences in these Fe oxides associated organic carbon,and ammonium and available phosphorus negatively influenced Fe-OC,suggesting N and P could cause lowering Fe-OC in cultivated peatlands.Incubated experiments also confirmed that microbes from cultivated peatlands could promote Fe reduction with high N and P availability.High N availability has changed iron-reducing bacteria community composition and promoted the abundance.Therefore,we suggested that the long-term N and P fertilization have changed the iron-reducing bacteria community and could promote microbial demand for Fe???,initiating lower Fe associated organic carbon in cultivated peatlands.We used Fe manipulate experiments to know the effects of temperature on the Fe-mediated carbon mineralization.Interestingly,the Fe reduction rate decreased with anoxic incubation time at 25?in Fe treatment,microbial incubation experiments showed that microbial Fe reduction significantly increased from low to high P availability in natural peatlands,but nitrogen addition didn't change the microbial Fe reduction rate.These all suggested that P played an important role in the Fe reduction in natural peatlands.During aerobic treatment,Fe oxides protected carbon decomposition at Fe treatments during 0-5 days,but the carbon mineralization rate at 5-10 and10-15 days was still higher than the control.We found that ferrous could stimulate phenol oxidase activity and release dissolved organic carbon?DOC?from Fe-OC.A further experiment showed phenol oxidase addition could promote carbon mineralization through decomposing phenolics and promoting labile carbon production,which can be called‘Fe cascade effects'.And low temperature?5?and 15??limited and microbial metabolism,as well as Fe reduction rate,which directly decreased Fe medicated‘cascade effects'on carbon mineralization.We used Fe manipulate experiments to know the effects of oxygen on Fe-mediated carbon mineralization.The results showed that the inhibition of Fe oxides on carbon mineralization was stronger than microbial metabolisms under 70%soil oxygen condition,causing the lower carbon mineralization rate.However,CO₂ fluxes in Fe treatment did not show any difference with control under 120%soil oxygen condition.These also suggested that the promotion of microbial metabolisms by high oxygen contents offset the inhibition of Fe oxides on carbon mineralization.In summary,P could promote P accumulation through alleviating microbial P limitation,high P also could decrease N-degrading enzyme and potentially increase N accumulation.Phosphorus loading could reduce soil P limitation in the short-term and long-term fertilization could also change the iron-reducing bacteria community in the temperate fen,which all contributes to a high Fe reduction rate.Nutrients hence act as an‘accelerator'in‘Fe cascade effects'on carbon decomposition.Furthermore,nutrient loading would also change the effects of temperature and oxygen on Fe-mediated carbon mineralization.Peatlands protection and restoration are paid attention worldwide,but it is still at the early development stage.Most studies focused on water level manipulation and ignored the effects of Fe and nutrients on carbon mineralization.Hence we propose that a risk assessment of soil substrate characteristics on GHG emissions is an essential prerequisite to attempt to form systematic strategies to sustainably manage drained peatlands.