| Hot water extractable organic carbon(HWEOC)is a comprehensive indicator of soils,which can represent the organic carbon activity,fertility and aggregation of soils.Investigation of the sources,distribution and mineralization of HWEOC are very important for the prediction of fertilization and carbon pool in forest soils,which is very limited.This led to the weakness on the theoretical base of the application to use HWEOC as an indicator of soil processes.In this work,soil samples were collected from different forests in Qinling Mountains.Field investigation and lab-incubation experiments were both used.Determination methods such as hot water extraction,physical fractionation,litter removal,and different mathematical statistics methods were used to investigate the sources,the distribution of HWEOC(in surface soils of different forests,in particulate organic matters of different size,and in soil profiles),and the mineralization mechanisms of HWEOC.The objectives are to comprehensively analyze the distribution characteristics and mineralization of HWEOC in mountain forest soils,so as to provide a theoretical basis for the application of HWEOC as an indicator for soil carbon sequestration research and other research.The main results are as follows:(1)Distribution characteristics of HWEOC and its main chemical components in litters from different organs(leaf,branch,cone,bark,moss,fine root and woody debris)of spruce(Picea asperata)and birch(Betula albosinensis)The contents of HWEOC,hot water extracted phenolic-C(Phe-C)and carbohydrate-C(Car-C)in litters form different organs were significantly(P<0.05)different.Within all litters,the litter of spruce cone showed the highest concentration of Phe-C(10.99 g·kg-1)and Car-C(27.16 g·kg-1).The woody debris in H-horizon of birch forest showed the lowest concentration of Phe-C(0.94 g·kg-1)and Car-C(1.22 g·kg-1).For litters from the same organ of the two forests,contents of HWEOC,Phe-C and Car-C were higher in spruce litter than birch litter.Litter removal increased the aromaticity of HWEOC and reduced the contents of Phe-C and Car-C of A-horizon(the surface horizon)soil.Therefore,the source organ of litter had a significant effect on the content and the chemical composition of litter HWEOC.(2)Distribution characteristics of HWEOC in the surface soils of different forests,and in coarse POM(c POM,>0.25 mm)and fine POM(f POM,0.053–0.25 mm)Forest type had a very significant(P<0.01)effect on c POM content and micro-aggregation rate(MIR,percentage of silts and clays in micro-aggregates/percentage of silts and clays in the whole soil)in forest soils.But the effects on contents of HWEOC and f POM were not significant.The c POM in coniferous forest soil(3.68%±2.38%)was significantly higher than that in broad-leaved forest(2.45%±1.48%)and mixed coniferous forest(1.95%±0.51%).The MIR values in coniferous forest(46.5%±4.3%)and broad-leaved(45.6%±4.3%)forest soils were significantly higher than that in mixed(42.3%±5.8%)forest soil,indicating that forest type was an important factor affecting the labile soil organic carbon(SOC)content and soil micro-aggregation.The content of HWEOC(1.48±0.52 g·kg-1),c POM(3.76%±1.84%),f POM(2.86%±0.53%)and MIR(49.12%±1.34%)in the Frigid Udic Cambisols were significantly higher than the content of HWEOC(0.75±0.23 g·kg-1),c POM(1.63%±0.58%),f POM(0.91%±0.39%)and MIR(40.75%±2.87%)in the Hap Udic Cambisols.The forest type and soil type could influence the soil micro-aggregation by their effect on the labile SOC.In the surface soils of spruce,birch,oak(Quercus aliena var.acuteserrata)and pine(Larix gmelinii),the concentrations of HWEOC in c POM and f POM were 9.1–18.7 and 4.8–11.1 times of that in bulk soil,respectively.HWEOC in POM accounted for 0.39%–2.44%and 24.68%–60.57%in total SOC and HWEOC,respectively.The concentration and main components of HWEOC between c POM and f POM were significantly(P<0.05)different.The carbohydrate-C(Car-C)and phenolic-C(Phe-C)accounted for 41.1%–46.9%and 5.5%–8.2%of HWEOC in c POM,respectively,which accounted for 35.3%–68.0%and 2.8%–5.1%of HWEOC in f POM,respectively.Thus,carbohydrates were the main chemical components of HWEOC in POM.(3)Investigation of the distribution characteristics of HWEOC in soil profiles of spruce and birch forests,via dividing the profile into L+F(undecomposed and half-decomposed litter),H(decomposed litter),A-(the surface mineral soil),B-(the subsurface mineral soil)and C-(the bottom mineral soil)horizonsResult of the analyses on the cold water extractable organic carbon(CWEOC)and HWEOC on the entire soil profiles of spruce and birch forests showed that:the percentage of water extractable organic carbon(WEOC,CWEOC+HWEOC)in SOC at mineral horizons(3.36%–3.88%)was significantly higher than that at the H horizon(1.47%–1.79%),and which in A-horizons were 14%and 18%higher those in H horizons of spruce forest and birch forest,respectively.The ratio of CWEOC/HWEOC showed an increasing trend from the H horizon to the B horizon.Phe-C was highly concentrated in the H horizon,while Car-C was relatively enriched in the mineral soil horizons.Distributions of HWEOC,biomass density of fine root and mineral N showed similar trends on soil profiles.After excluding the influence of soil layer and forest type,there are still very significantly positive correlations among the root biomass density,HWEOC content and mineral N content.The increase of fine root biomass density improved the content of HWEOC and the aromaticity of HWEOC,but decreased the ratio of HWEOC/SOC.Thus,the improvement of fine roots led to an increase in the stability of SOC pool.(4)Investigation of the mineralization characteristics of HWEOC and the temperature sensitivity,via lab-incubation experiments using surface soil of birch forestNo obvious mineralization"initail flush"was shown at 5°C and 20°C,but the phenomenon obviously appeared at 35°C.Within the first 14 days during the incubation,there was no significant difference(basically equal)between the loss amount of HWEOC and the total mineralization amount of SOC(Soil organic carbon,SOC)in all treatments.The change of HWEOC could explain the mineralization of SOC in a short term.The increase of temperature improved the mineralizable proportion of HWEOC:The mineralizable HWEOC percentages under 5°C,20°C and 35°C were(6.94±1.63)%(19.22±3.17)%and(24.94±4.56)%of total HWEOC,respectively.Because the temperature-rise greatly increased the mineralization rate of carbohydrates and some other substances,the mineralization of HWEOC was concentrated(79%)at the initial stage of incubation(the first14 days).This intensified the phenomenon of mineralization"initail flush".In conclusion,HWEOC in forest soils of Qinling mountains was a labile SOC,which was affected by tree species,soil type and the source of litter,and enriched in POM.Different chemical components in HWEOC showed different characteristics of distribution in soil profiles and mineralization.HWEOC/SOC and the aromaticity of HWEOC can be used as indicators of SOC stability.The changes of HWEOC and its carbohydrates and phenolic compounds can be used as analytical indicators of SOC mineralization in the short term. |
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