Effects Of Dolomite Application On Organic Carbon Mineralization In Acidic Paddy Soils

The application of lime materials is a widely used agricultural practice for soil acidity amelioration.The application of lime materials also affects other soil processes,such as the cycling of carbon,nitrogen,and other elements.Soil organic carbon?SOC?mineralization not only strongly affects the size of soil carbon pool,but also has close relationships with atmospheric CO₂ concentration and global climate change.Therefore,the regularities and controlling factors of carbon storage,emission,and transformation of farmland ecosystem are important issues in carbon cycle researches.However,little is known on the combined effects of the application of lime materials and other agronomic practices on SOC mineralization in acidic paddy soils.In particular,it is still lack of understanding the effects of simultaneous application of exogenous organic matters and lime materials on SOC mineralization and the microbial mechanisms involved.Therefore,it is of great scientific significance to study the regularities and controlling factors of SOC mineralization under the combined effects of the application of lime materials and other agronomic practices.In the present study,two kinds of acidic paddy soils collected from Qichun county?S1?and Chibi city?S2?in Hubei Province were selected.A series of incubation experiments were performed to investigate the effects of different sized dolomite application and combined effects of dolomite application and other agronomic practices?i.e.,soil water variation,nitrogen fertilizer application,and straw returning?on SOC mineralization.Soil p H,mineral nitrogen?ammonia nitrogen and nitrate nitrogen?,dissolved organic carbon?DOC?,microbial biomass carbon?MBC?,soil enzyme activities and carbon decomposition functional genes abundances related to cellulose and lignin decomposition,greenhouse gases emissions and isotope characteristics of CO₂-¹³C were determined during incubation.The main results of the present study are as follows:1. The ameliorations of dolomite on the two acidic paddy soils were affected by dolomite particle size,soil moisture condition and nitrogen fertilizer application.Soil acidities were best ameliorated by fine-sized?<0.15mm?dolomite addition.Low soil moisture condition,i.e.,50%water holding capacity?WHC?,resulted in the suitable soil p H considerably or nearly matching to the target value of 6.50.The ameliorations of dolomite on the two soils were weakened by nitrogen fertilizer application.The combined effect of dolomite application and straw addition on soil amelioration was significant only in soil S2.2. The organic carbon content of soil S1 was significantly higher than that of soil S2.In addition,the DOC content and CO₂ emission amount of soil S1 were also higher than those of soil S2 during incubation,indicating that the native SOC content affects the quantity of its mineralization.Compared with the soil with low organic carbon content,the soil with high organic carbon content may provide more carbon substrate for microbial utilization,thus producing higher CO₂ emissions than the former.3. The cumulative CO₂ emission and SOC turnover rate were significantly affected by dolomite particle size.Both CO₂ emission and SOC mineralization rate were significantly affected by treatment with the fine-sized dolomite addition.Compared with the control,the cumulative CO₂emissions increased by 3%,11%,and 32%in S1and by 7%,21%,and 49%in S2 in treatments with the coarse-?0.25-0.5mm?,medium-?0.15-0.25mm?,and fine-sized?<0.15mm?dolomite addition,respectively.Moreover,SOC turnover rate was dependent on the dolomite particle size.In the two soils,the decomposition rate constant?k?significantly increased in treatment with the fine-sized dolomite addition rather than other two sizes.4. CO₂ emissions of the two soils significantly increased with dolomite addition under three moisture levels,as the following order:130%WHC(0.73 g C kg^-1)>90%WHC(0.66 g C kg^-1)>50%WHC(0.29 g C kg^-1)in S1 and 90%WHC(0.51 g C kg^-1)>130%WHC(0.45 g C kg^-1)>50%WHC(0.19 g C kg^-1)in S2.The amounts of SOC mineralization in both soils increased when soil moisture content increased from50%WHC to 90%WHC.Compared with 90%WHC,flooding condition?130%WHC?promoted the amount of SOC mineralization in S1 but had inhibitory effect in S2.5. In general,N₂O emission increased in S1 but decreased in S2 with dolomite addition.The global warming potential?GWP?of greenhouse gases?GHGs?emissions of the two soils was the lowest when dolomite was applied under the soil moisture level of 50%WHC.According to the combined effects of moisture and dolomite addition on soil acidity amelioration and GWP of GHGs,50%WHC was recommended as the optimal water management strategy for acidic paddy soils amelioration by dolomite in this study.6. SOC mineralization was not changed with nitrogen fertilizer addition,whereas increased by 39%and 51%with the incorporated application of dolomite and nitrogen fertilizer in S1 and S2,respectively.Compared with nitrogen fertilizer addition,dolomite application was still the main factor affecting SOC mineralization.Soil nitrification in response to dolomite or nitrogen fertilizer application was soil acidity dependent in the two soils;the lower the soil acidity,the faster and stronger the nitrification response.When we evaluate the effect of liming on the amelioration and organic carbon mineralization of acidic soils,it is necessary to consider the inputs of nitrogen and the properties of soils,so as to provide guidance and suggestion for the rational application of liming in agricultural production practice.7. SOC mineralization significantly increased by 15%-29%with dolomite addition,in which 80.75%-86.33%of carbon was released as CO₂ in two soils in the dolomite and straw addition experiment.SOC mineralization significantly increased by 15%-20%but decreased by 18%-32%with straw addition and the incorporated addition of dolomite and straw,respectively,in two soils.8. The activities of endo-?-1,4-glucanase?Egase?,cellobiohydrolase?CBH?,and laccase,as well as the abundances of GH48,cbh?,and LMCO functional genes were significantly affected by dolomite and/or straw addition.The results of redundancy analysis indicated that soil p H,DOC,mineral nitrogen,MBC,activities of Egase,CBH,and laccase,abundances of GH48,cbh?,and LMCO genes were the main factors affecting SOC mineralization.The structural equation model further showed that soil DOC and mineral nitrogen were the main direct effects while p H was the main indirect effect on SOC mineralization.These results implied that the main agronomic practices,such as liming and straw returning can not only change soil p H,but also alter nutrient?i.e.,mineral nitrogen?and microbial available substrate?i.e.,DOC?content,thereby affect the abundances of carbon decomposition function genes,and then the activity of cellulase and laccase,and finally regulate SOC mineralization.Overall,the process and rate of SOC mineralization will be affected with dolomite application alone or combined with other agronomic practices?e.g.,use of nitrogen fertilizer,straw returning?in acidic paddy fields.In the process of soil acidity amelioration,much attention should be paid to the combined influence of liming and other agronomic practices on SOC mineralization.Furthermore,much attention should be paid not only to amelioration result but also to SOC loss and GHG mitigation.It is of important scientific significance of this study to provide guidance for agricultural production practices,accurate estimation of carbon release in the process of soil acidity amelioration,prediction of the impact of paddy soil carbon cycle on global climate change,and sustainable development of agriculture.