| The covering earth used for reclaiming back-filled with coal gangue contains excess sand but short of clay, So water-retaining capacity of the reclaimed soil becomes poor. In this soil the nutrient such as organic matter, phosphorus and nitrogen gets deficient. So the reclaimed soil is not good for crop growth. The formation of soil humus can improve structure and function of the reclaimed soil, however, the humus may influence the bioavailability and mobility of cadmium in soil, and then increases the potential risk for crops and human health. It is necessary to ameliorate the physical properties and control the cadmium bioavailability of the reclaimed soil by adjusting the quantity, the repartition and the quality of humus, and then improve the fertility and environmental quality of soil. This study investigated the soil under three typical artificial vegetation restoration models (Syriga oblate+ Robinia pseudoacacia+grass, Photinia serrulate grass, Ailanthus altissima+gcass)for seven years in Datong Reclamation District from Huainan city, which had been back-filled with coal gangue. Soil physical variables(bulk density, total porosity, capillary porosity and non-capillary porosity),fractions of humus(fulvic acids, humic acids, iron-linked hum in, clay-combined hum in and insoluble humin residues), fractions of combined humus(loosely-combined humus.stably-combined humus and tightly-combined humus),and fractions of cadmium(Exchangeable fraction, Carbonate-bound fraction, Fe-Mn oxide bound fractions, Organic bound fraction, Residual fraction, and DTPA extracted fraction,) of the topsoil (to a depth of 100 mm) and subsoil (at depths of between 100 and 200 mm) were determined by physical, chemical, and instrumental methods. In addition, the ratio of optical densities or absorbance of dilute, aqueous humic acid solutions at 465 and 665 nm(E4/E6),and the logarithmic difference of absorption coefficient at 400 and 600 nm (ΔlogK)of humic acids were also characterized by visible light colorimetric methods. The relationships between the physical properties, available fractions of cadmium of soil and soil humus were analyzed by multivariate statistical methods, with the purpose of assessing how the quantity, the repartition and the quality of humus and combined humus could ameliorate the physical property and control the cadmium bioavailability of the reclaimed soil.The humus extracted was divided into humic acids and fulvic acids according to modifier methods introduced by Dou Sen. The humin was divided into iron-linked humin, clay-combined humin and insoluble humin residues by modifier methods according to the methods introduced by Pallo. The combined humus was divided into loosely-combined humus,stably-combined humus and tightly-combined humus according to modifier methods introduced by Fu Jiping. Results indicated that the quantity of SOC by vegetation restoration was significantly more than of natural soil, and dropped off with the increase of soil depth. The average soil organic carbon content of the topsoil had three different levels of soil fertility while that of the subsoil had five distinct levels of soil fertility, and the natural soil only got six various levels of soil fertility according to the classification standards of the second national survey in 1980. The soil humus was still in a low humic degree. The repartition and the quality of humus varied with different vegetation collocation. In the topsoil, The mixed plantation of Ailanthus altissima and grass could drive lower degree of humification while the mixed plantation of Syriga oblate, Robinia pseudoacacia and grass could drive higher degree of humification than other vegetation types. The mixed plantation of Photinia serrulat and grass could drive higher aliphatic character than other vegetation types. The ratio of organic carbon to total nitrogen(C/N) was significantly correlated to fractions of humus and combined humus. The C/N was directly correlated with fulvic acids, humic acids, humin,the ratio of humic acids to fulvic acids,ΔlogK of humic acids, the ratio of insoluble humin residues to humin, loosely-combined humus, tightly-combined humus, and the ratio of tightly-combined humus to combined humus, and inversely correlated with E4/E6 of humic acids, the ratio of ron-linked humin to humin, the ratio of clay-combined humin to humin, and the ratio of stably-combined humus to combined humus, respectively.The physical properties were quantified using standard procedures, and analyzed the relationships with the fractions of humus and combined humus. Results indicated that the soil bulk density was lower but total porosity higher significantly by vegetation restoration than of natural soil. The mixed plantation of Ailanthus altissima and grass could drive lower bulk density and higher total porosity than other vegetation types. The porosity of different bore diameters had significantly correlations with fractions of humus and combined humus. Non-capillary was directly correlated with fulvic acids, humic acids, humin, the ratio of insoluble humin residues to humin, loosely-combined humus, tightly-combined humus, and the ratio of tightly-combined humus to combined humus, and inversely correlated with the ratio of iron-linked humin to humin, the ratio of clay-combined humin to humin, and the ratio of stably-combined humus to combined humus, respectively. Capillary had significantly positive correlation with E4/E6 of humic acids, the ratio of iron-linked hum in to humin, the ratio of clay-combined humin to humin, and the ratio of stably-combined humus to combined humus, and significantly negative correlation with fulvic acids, humic acids, humin, the ratio of humic acids to fulvic acids,ΔlogK of humic acids, the ratio of insoluble humin residues to humin, loosely-combined humus, tightly-combined humus, and the ratio of tightly-combined humus to combined humus, respectively.The factions of soil cadmium were divided into exchangeable fraction, carbonate bound fraction, Fe-Mn oxide bound fraction, organic bound fraction, residual fraction according, to sequential extraction introduced by A.tessier. The faction of cadmium extracted by Diethylene Triamine Pentacetate Aci(DTPA) often represented stronger bioavailability and mobility of cadmium. Results indicated that the total cadmium by vegetation restoration do not show significant difference with of natural soil, and the available factions of cadmium varied with different vegetation types. The Exchangeable fraction and DTPA extracted fraction of the topsoil were significantly higher than of the subsoil. In the topsoil, The mixed plantation of Ailanthus altissima and grass could drive higher Exchangeable fraction and DTPA extracted fraction while the mixed plantation of Syriga oblate, Robinia pseudoacacia and grass could drive lower than other vegetation types. The available factions of cadmium (Exchangeable fraction, Carbonate bound fraction, DTPA extracted fraction) were directly correlated with humic acids, humin, ΔlogK of humic acids, the ratio of insoluble humin residues to humin, loosely-combined humus, tightly-combined humus, and the ratio of tightly-combined humus to combined humus, and inversely correlated with E4/E6 of humic acids, the ratio of ron-linked hum in to humin, the ratio of clay-combined humin to humin, and the ratio of stably-combined humus to combined humus, respectively.In total, the improvement of the aliphatic character and the degree of con densation of humic acids, the ratio of iron-linked humin to humin, and the rati o of clay-combined humin to humin may significantly ameliorate the physical properties and control the cadmium bioavailability of the reclaimed soil. |
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