The Chemical Characteristics Of Soil Organic Matter And Its Mineralization Under Different Land Uses

Since the late1950s, land reclamation from lakes as a distinctive way to use land iswidespread along the river or lake basin in China. It has greatly diminished the lake area, andaltered natural ecological succession. However, few studies exist on the effect of this type of landuse changes and little is known about its impact on the carbon cycle of the region. Our experimentaldesign chose four typical land uses within coniferous forest (CF), evergreen broadleaf forest (EBF),bamboo forest (BF) and cropland (CL) in a reclaimed land area from Taihu Lake. In order tounderstand the dynamics of soil organic carbon in the reclaiming land from lake under different landuses, chemical composition and strucuture of DOM and soil C mineralization were analyzed. Theresults showed that:(1)Soil dissolved organic matter (DOM) under the four land uses was significantlydifferent,and its quality and quantity were also affected by extraction method. Both hot-water (DOCHW)and cold-water (DOCCW) extratable carbon from CF soil were larger than other land uses. DOMestimates under the land uses determined by different methods were following in the order:DOCHW> DOCCW> DOCCaCl2(CaCl2extractable C).DOCHWshowed significant correlationwith DOCCWand DOCCaCl2.DOCCWwas related to soil microbial biomass C, while DOCHWandDOCCaCl2was positively correlated with SOC and TN and negatively correlated with pH andbulk density.(2) Spectroscopic characteristics of DOM indicated that chemical structure of DOM fromsoils under the four land uses was different from each other. Under the four land uses, the longestwavelengths of fluorescence emission maxima were recorded for BF. Fourier transform infraredspectroscopy analysis showed that all samples feature C-H, aromatic C=C and carbohydrate C-Ostructures. The ratio of1120/1335(carbohydrate C/C-H3C) in upper soil layer followed the orderof CL> EBF> CF> BF, while the ratio of1630/2920(aromatic C/aliphatic C) showed thehighest value in BF and lowest value in CL. The aromaticity and the humification index(SUVA254, HIXem) revealed that DOM from BF and CF contained more aromatic structure;Humification degree of DOM from forst soil was larger than arable soil and CF was larger thanEBF. Biodegradation of DOM was related to its chemical structure. The larger biodegradabilitywas associated with more microbial source and lower degree of humification and aromaticity,while the lower biodegradability was associated with larger aromatic and pheolic strutures inDOM.(3)The cumulative CO2production under different land uses and sediments ranged from0.88to7.72g CO2-C kg-1soil and there were significant difference between soil depths. The greatestcumulative CO2-C emission and lowest mineralizaton propotion were observed in BF soil. At0-10cm soil depth, the mineralizaton propotion of EBF was larger than CF. The result of doubleexponential model analysis showed that at0-10cm depth both the largest mean residence time(MRT) of active C pool and slow C pool was observed in CL, while at10-25cm depth the largest MRT was observed in EBF and BF, respectively. The soil physico-chemical propertieswere correlated with C mineralization. Cumulative CO2-C emission showed positive correlationswith soil organic matter, total N, C/N、DOC and microbial biomass C, and negative correlationswith soil bulk density. In our study, the cumulative CO2-C, mineralizaton propotion and thepercentage of active C pool in LS were smaller than the terrestrial soisl, and the both MRT ofactive C pool and slow C pool in LS were shorter than the terrestrial soils.(4)At the end of incubation, the increasing percentage of hydrophilic fraction, carbohydrateand phenlos in DOM were observed (except for CL), but the increase in the proportion ofphenlos was greater. The increase of SUVA254, HIXem,and FE under different sites suggestedaromatic structures were accumulating with the decomposition of the labile components of DOMduring incubation. The cumulative CO2-C emission showed a significant positive correlation withproportion of CH and a significant negative relationship with the proportion of phenols at thebeginning of the incubation. C mineralization was inversely related to the SUVA254, whilepositively related to the increasing SUVA254of DOM at the end of incubation. The FE and HIXemfor initial DOM respectively showed negative and positive correlation with C mineralization,while both the increase of FE and ΔHIXemduring incubation were positively correlated with Cmineralization. The correlation of the chemical characteristics of DOM and C mineralizationindicates that the chemical composition and structure of DOM as the intrinsic factor mayinfluence the dynamics of soil carbon.