Influencing factors and partitioning methods of carbonate contribution to CO_(2)emissions from calcareous soils

In calcareous soils,recent studies have shown that soil-derived CO_(2)originates from both soil organic carbon(SOC)decomposition and soil inorganic carbon(SIC)dissolution,a fact often ignored in earlier studies.This may lead to overestimation of the CO_(2)emissions from SOC decomposition.In calcareous soils,there is a chemical balance between precipitation and dissolution of CaCO_(3)-CO_(2)-HCO_(3),which is affected by soil environmental factors(moisture,temperature,pH and depth),root growth(rhizosphere effect)and agricultural measures(organic materials input,nitrogen fertilization and straw removal).In this paper,we first introduced the contribution of SIC dissolution to CO_(2)emissions from calcareous soils and their driving factors.Second,we reviewed the methods to distinguish two CO_(2)sources released from calcareous soils and quantify the 13C fractionation coefficient between SIC and SIC-derived CO_(2)and between SOC and SOC-derived CO_(2),and to partition three CO_(2)sources released from soils with plants and organic materials input.Finally,we proposed methods for accurately distinguishing three CO_(2)sources released from calcareous soils.This review helps to improve the accuracy of soil C balance assessment in calcareous soils,and also proposes the direction of further investigations on SIC-derived CO_(2)emissions responses to abiotic factors and agricultural measures.

CO_(2)emission and source partitioning from carbonate and non-carbonate soils during incubation

The accurate quantification and source partitioning of CO_(2)emitted from carbonate(i.e.,Haplustalf)and non-carbonate(i.e.,Hapludult)soils are critically important for understanding terrestrial carbon(C)cycling.The two main methods to capture CO_(2)released from soils are the alkali trap method and the direct gas sampling method.A 25-d laboratory incubation experiment was conducted to compare the efficacies of these two methods to analyze CO_(2)emissions from the non-carbonate and carbonate-rich soils.An isotopic fraction was introduced into the calculations to determine the impacts on partitioning of the sources of CO_(2)into soil organic carbon(SOC)and soil inorganic carbon(SIC)and into C3 and/or C4 plant-derived SOC.The results indicated that CO_(2)emissions from the non-carbonate soil measured using the alkali trap and gas sampling methods were not significantly different.For the carbonate-rich soil,the CO_(2)emission measured using the alkali trap method was significantly higher than that measured using the gas sampling method from the 14 th day of incubation onwards.Although SOC and SIC each accounted for about 50%of total soil C in the carbonate-rich soil,SOC decomposition contributed 57%–72%of the total CO_(2)emitted.For both non-carbonate and carbonate-rich soils,the SOC derived from C4 plants decomposed faster than that originated from C3 plants.We propose that for carbonate soil,CO_(2)emission may be overestimated using the alkali trap method because of decreasing CO_(2)pressure within the incubation jar,but underestimated using the direct gas sampling method.The gas sampling interval and ambient air may be important sources of error,and steps should be taken to mitigate errors related to these factors in soil incubation and CO_(2)quantification studies.

Effects of nitrogen addition on carbonate-derived CO_(2) emission after biochar addition

●We studied the effect of nitrogen and biochar on CO_(2) emission from SOC and SIC.●Nitrogen increased SIC-derived CO_(2) by 41%but decreased SOC-derived CO_(2) by 20%.●Biochar reduced total soil-derived CO_(2) by neutralizing nitrogen-induced acidity.●We proposed a method for 3-or 4-source partitioning CO_(2) emission from calcareous soils.Biochar addition generally increases the alkalinity regeneration to resist soil acidification driven by nitrogen(N)fertilization.Calcareous soils contain soil organic carbon(SOC)and inorganic C(SIC).Owing to technical limitations in three-source partitioning CO_(2),how biochar addition affects SOC-and SIC-derived CO_(2) emission has not been clarified yet.Therefore,we conducted a 70-day incubation experiment of ammonium-N and maize-straw-derived biochar additions to investigate the N plus biochar impacts on SOC-and SIC-derived CO_(2) emission.Over the 70-day incubation,we found that the N-only addition increased the SIC-derived CO_(2) emission by approximately 41%compared with the control,but decreased the SOC-derived CO_(2) emission by approximately 20%.This suggests that the distinct responses of SIC-and SOC-derived CO_(2) emission to N-only addition come from N-induced acidification and preferential substrate(N)utilization of soil microorganisms,respectively.Compared with N-only addition,N plus biochar addition decreased the SIC-derived CO_(2) emission by 17%−20%during the first 20 days of incubation,but increased it by 54%during the next 50 days.This result suggested that biochar addition reduced the SIC-derived CO_(2) emission likely due to the alkalization capacity of biochar exceeding the acidification capacity of ammonium-N in the short term,but it may increase the SIC-derived CO_(2) emission induced by the weak acidity produced from biochar mineralization in the long term.This study is helpful to improve the quantification of CO_(2) emission from calcareous soils.