Accumulation Characterisitcs And Stabilization Mechanisms Of Organic Carbon In Soils Developed From Different Parent Materials In Subtropical Regions

Soil organic carbon?SOC?contains about three times more carbon than the atmosphere.Understanding the SOC sequestration potential of soils and clarifying the mechanisms involved in the building up of SOC in soils are fundamental for the comprehension of carbon dynamics.Soil parent material,as one of the five state factors of soil formation,have a major impact on soil physical and chemical properties.Limestone,quaternary red earth,granite,basalt and tertiary red sandstone are the five most common parent materials and exhibited different soil properties in subtropical China.However,the mechanisms controlling formation and stabilization of SOC in these soils have not been well understood.This study,therefore,investigated major influencing factors for SOC formation and stabilization in soils developed from limestone,quaternary red earth,granite,basalt and tertiary red sandstone from the perspective of formation of chemical bonds between SOC and mineral surfaces as well as formation of stable aggregates to protect SOC against microbial attack.Main results are as follows:?1?The effect of parent material on SOC stocks and various organic C fractions of different stability were investigated by sampling 140 soils?0-20 cm?under two land uses?forest and cultivated?and five parent materials but with similar topography.Factors influencing SOC were explored using a combination of stepwise multiple regression analysis and redundancy analysis.The results showed that total SOC stocks varied obviously amongst the five parent materials,with the greatest value observed in soils developed from limestone and the lowest in soils developed from tertiary red sandstone.While particulate OC?POC?,Na OCl-removable OC?AOC?,mineral-associated OC?MOC?and biochemically recalcitrant OC?BROC?content were both significantly?p<0.05?influenced by parent material and land use,the water soluble OC?WSOC?,microbial biomass C?MBC?were only significantly?p<0.05?influenced by land use,and the relative proportion of AOC,MOC and BROC to total SOC were only significantly?p<0.05?influenced by parent material.Our results have also shown the SOC stocks in soils developed from limestone,quaternary red earth and basalt were significantly higher in forest than in arable soils,while no significant land-use effect was found for soils developed from granite and tertiary red sandstone.Soil aggregates,texture,content of Fe oxides and multivalent cations accounted for 67.8%of the variation in absolute quantity and relative proportion of various organic C fractions,and the content of total SOC and various organic C fractions were more closely related to aggregation and soil texture.The step-wise regression analysis further demonstrated that the major SOC retention pattern was related to parent material,with p H,Ca_ex and Mg_ex being the best predictors for total SOC content for soils developed from limestone,while p H was the best predictor in soils derived from granite and tertiary red sandstone.For soils developed from quaternary red earth and basalt,total SOC content was best predicted by aggregates and Fe-oxides,with aggregates best explaining the observed variance.Aggregates were most closely linked with SOC stocks.?2?The thermal stability and chemical composition of SOC in soils derived from different parent materials as well as their implications for temperature induced changes in SOC decomposition were investigated.The SOC in soils developed from limestone and granite were more susceptible to biodegradation and exhibited higher temperature sensitivity(Q₁₀)than soils developed from quaternary red earth and basalt.Meanwhile,SOC of deep soils?10-20 cm?was more biologically stable than surface soils?0-10 cm?.The results from solid-state ¹³C-NMR spectroscopy showed that the main difference in the SOM chemical composition among soils came from a larger contribution of alkyl C?0-45 ppm?in soils developed from limestone and granite than that in soils developed from quaternary red earth and basalt,and the percentage of aromatic C?110-160 ppm?showed a reverse variation trend to that observed in alkyl C.Besides,the results from simultaneous thermogravimetry and differential scanning calorimetry?TG-DSC?showed that the soils developed from limestone and granite were characterized with higher ED?energy density?and Exo₁/Exo₂?the ratio of weight loss associated with the two exothermic peaks?as well as lower TG-50 value?temperature at which half of the SOC was pyrolyzed?than soils developed from quaternary red earth and basalt.Redundancy analysis?RDA?demonstrated that variables assessed by ¹³C-NMR and TG-DSC explained 92.87%of the variations in C mineralization and Q₁₀ values,indicating strong linkage between the thermal stability,chemical composition and biological stability of SOC.Besides,results from RDA also revealed that SOC decomposition at 5^oC and 20^oC was mainly driven by SOC quality,while it was more related to substrate availability at high temperature?35^oC?,implying that factors controlling the temperature sensitivity of SOC decomposition varied with temperature.We hence demonstrated that there might be an obvious fluctuation in the observed temperature sensitivity of SOC decomposition across soils developed from these parent materials when temperature rising becomes more pronounced.?3?Dissolved organic carbon?DOC?adsorption capacity and their influence factors on soils developed from five different parent materials were investigated by equilibrium methods.The stability of DOC bounded to soil minerals was subsequently assessed by a combination of batch desorption experiments,chemical extraction and laboratory incubation.The results showed that under the same p H,DOC adsorption capacity?as percentage of added DOC?for soils followed the order of basalt>quaternary red earth>limestone>granite>tertiary red sandstone.The adsorption capacity of DOC of the soil under conditions of p H 3.5 and 4.0 was higher than that under p H of 7.0.The removal of allophane and ferrihydrite using oxalate,and a sequential extraction of oxalate and DCB solution,decreased the Langmuir adsorption maximum(Q_max)of all soils by a percentage of 19.13%-27.47%and 54.60%-69.78%,respectively.The sorption capacity of soils developed from quaternary red earth and basalt was mostly influenced by removal of oxides.The removal of exchangeable cations also reduced adsorption capacity by 11.74%-43.48%compared to the untreated soil sample.The effect was stronger in the soils developed from limestone.Meanwhile,increasing concentration of Ca²⁺facilitated adsorption processes for all soils.Desorption experiment also showed that desorption of DOC?as percentage of sorbed DOC?for soils followed the order of tertiary red sandstone>granite>limestone>quaternary red earth,basalt.Removal of total?hydr?oxides significantly?p<0.05?increased the desorption of DOC,and the increase was most significant in soils developed from quaternary red earth and basalt.A lower percentage of Na OCl-removabe OC and cumulative mineralized OC during 125 days of incubation to sorbed DOC were also observed in soils developed from basalt,quaternary red earth and limestone,indicating the sorbed DOC in soils developed from basalt,quaternary red earth and limestone were of greater chemical stability and biological stability.Both the single regression analysis and adsorption experiment showed that the high stability of adsorbed DOC in soils developed from basalt and quaternary red earth was contributed by high level of oxides,while it was more related to the presence of multivalent cations in soils derived from limestone.?4?Dynamics of soil aggregates and distribution of soil organic carbon in relation to the addition of organic amendments after 7 days and 184 days of laboratory incubation,as well as their linkages to SOC mineralization in soils derived from five different parent materials were investigated.The results showed that 1-0.5,0.5-0.25 and<0.25 mm aggregates accounted for the largest proportion of aggregates in soils developed from limestone,quaternary red earth and basalt in the absence of organic amendments,while the soils developed from granite and tertiary red sandstone were maily contributed by<0.25 mm microaggregates.Addition of organic amendments increased the proportion of>2 mm and 2-1 mm aggregates across all soils,and a more prominent effect was detected in soils developed from quaternary red earth and basalt.The stability of macroaggregates was also higher in soils developed from limestone,quaternary red earth and basalt than those developed from granite and tertiary red sandstone.Besides,the ratios of accumulative mineralized CO₂-C to total soil organic carbon in soils developed from limestone,quaternary red earth and basalt were significantly?p<0.05?lower than those developed from granite and tertiary red sandstone in amended soils after 184 days of incubation.The correlation analysis further suggested that ratios of cumulative respired CO₂-C to total soil organic carbon was significantly and positively correlated?p<0.01?with the proportion of free light organic carbon?f LOC?,but inversely correlated?p<0.01?with the proportion of>0.25mm macroaggregate-associated SOC.By applying ¹³C-NMR to characterize inherent chemical composition of soil organic carbon fractions,we noted that the the alkyl C/O-alkyl C ratio and aromaticity value of f LOC and intra-aggregate light organic carbon?intra-aggregate LOC?were higher in siols developed from granite and tertiary red sandstone.This result indicated that these two SOC fractions were more advanced decomposed in soils developed from granite and tertiary red sandstone.We demonstrated that enhanced physical protection of SOC by forming more stable macroaggregates contribute to carbon accumulation in soils developed from limestone,quaternary red earth and basalt treated with organic amendments.Above all,our results suggested that the soils developed from quaternary red earth,basalt and limestone showed greater ability for SOC accumulation and stabilization.The possible mechanisms involved in protecting SOC in soils developed from quaternary red earth and basalt included higher retention of dissolved organic carbon by oxides and oxides-mediated processes to form stable macroaggregates.Multivalent cations(i.e.,Ca²⁺/Mg²⁺)acting as efficient binding agents for interaction between organic matter and minerals as well as for macroaggregates formation may play a crucial role for accumulation of stable organic carbon fractions in soils developed from limestone.However,the soils developed from granite and tertiary red sandstone lacked the properties for SOC preservation,due to the low oxides content,coarse texture and limited capacity for macroaggregate formation.