| The former stubble of eucalyptus plantation is mainly pure pine and fir forest and mixed coniferous and broad-leaved forest,and the tannin content of crops is low.However,the tannin content of eucalyptus is high,and the input of tannin in woodland soil increases greatly after planting eucalyptus.Moreover,the characteristics and laws of the effects of eucalyptus tannins on soil Fe,Al,Mn and soil aggregates were not clear.In this study,an aqueous solution of eucalyptus leaf decomposed liquid tannin(TFL,eucalyptus tannin),gallic acid(GA,hydrolyzed tannin)and epigallocatechin(EGCG,condensed tannin)was used as the leaching solution.The red soil was leached 18 times in 36 days to study the process and characteristics of the effect of tannin type and concentration for the leaching loss content of Al,Fe and Mn in soil,content of Al,Fe and Mn oxides in various forms and soil aggregate stability.The main findings are as follows:(1)TFL,GA and EGCG could significantly increased the leaching loss of soluble Al,Fe and Mn in soil,and the leaching loss content was positively correlated with the tannin concentration of the leaching solution.Among them,TFL has stronger effect on the dissolution of soil Al,Fe,Mn than GA and EGCG.Distilled water(CK)leaching soil Al,Fe,Mn leaching loss content were 0.66,0.15,0.36 mg/kg;When the tannin concentration was 800 mg/L,the leaching loss content of Al,Fe and Mn in TFL leaching soil were 24.90,62.86 and 65.39mg/kg,respectively.The leaching loss content of Al,Fe and Mn in GA leaching soil were 13.11,26.15 and 30.42 mg/kg,respectively.The leaching loss content of Al,Fe and Mn in EGCG leaching soil were 6.91,6.20 and 7.65 mg/kg,respectively.The leaching loss content of soil Al,Fe and Mn was TFL>GA>EGCG.(2)The leaching of tannins mainly promotes the transformation of soil amorphous Al,Fe,Mn oxides to other soluble oxides.Amorphous Al,Fe,Mn oxides decreased significantly.Complex Al,Fe,Mn oxide content increased significantly.Leaching of soluble Al,Fe,Mn also promotes free Al,Fe,Mn oxides reduce.The leaching loss content of soluble Al,Fe,Mn is significantly negatively correlated with amorphous Al,Fe,Mn oxides,significantly positively correlated with complex Al,Fe,Mn oxides,and free Al,Fe,Mn oxides Oxides show a weak negative correlation.The content of amorphous Al,Fe,Mn oxides in CK soil was 2.20,3.74 and 0.244 mg/kg;compared with CK,the content of amorphous Al,Fe,Mn oxides in TFL,GA,EGCG leaching soil at 800 mg/L decreased by 10%,5%,26%,and 4%,7%,13%,and 2%,8%,4%,respectively.The content of complex Al,Fe,Mn oxides in CK soil was 1.41,4.92 and 0.042mg/kg;compared with CK,the content of complexed Al,Fe,Mn oxides in TFL,GA,EGCG leaching soil at 800 mg/L increased by-1%,16%,149%,and 7%,20%,117%,and 3%,13%,142%,respectively.The free Al,Fe and Mn oxide contents in CK soil were 2.85,20.93 and 0.301 mg/kg;compared with CK,the free Al,Fe and Mn oxide contents in TFL,GA and EGCG leaching soil of 800mg/L decreased by 8%,11%,22%,and 0%,-4%,9%,and 7%,13%,6%,respectively.(3)The leaching of TFL,GA and EGCG promoted the aggregation of small particle size aggregates(<3 mm)to large particle size aggregates(10-3mm)in the soil and enhanced the water stability of soil aggregate.The change range of the mass percentage of large particle size aggregates(10-3 mm)in the leaching soil of each treatment was 22% in the CK group,20%-60% in the TFL group,21%-27% in the GA group,and 18%-38% in the EGCG group.The mass percentage of other soil particle fractions(<3 mm)decreased to varying degrees.The mean mass diameter(MWD)and geometric mean diameter(GMD)of each tannin leached soil were significantly positively correlated with tannin concentration,total tannin retention and complex Fe and Mn oxides,while negatively correlated with amorphous Al,Fe and Mn oxides.The results of correlation analysis indicated that the retention of tannins by soil and the increase of the content of complex Al,Fe,Mn oxides contributed to the enhancement of the water stability of soil aggregates.In conclusion,tannin leaching promotes the transformation of soil amorphous Al,Fe,Mn oxides and free Al,Fe,Mn oxides to soluble Al,Fe,Mn.A small part of soluble Al,Fe and Mn is leached away by water flow.Most of the soluble Al,Fe and Mn were re-complexed under the strong chelation of tannin,and were adsorbed by soil minerals to form complex Al,Fe and Mn oxides,which promoted the aggregation of small and medium particle size aggregates(<3 mm)to large particle size aggregates(10-3 mm).Thus enhancing the water stability of soil aggregates.However,the long-term leaching of eucalyptus tannins in the woodland soil of the eucalyptus plantation leads to the gradual acidification of the soil and the leaching of a large amount of soil soluble Al,Fe,and Mn,which will reduce the formation of complex Al,Fe,and Mn oxides,and the excessive leaching of other forms of Al,Fe,Mn oxides,reducing the cohesion of Al,Fe,Mn oxides on soil aggregates,and reducing the stability of soil aggregates.The results of this study are helpful for people to predict the changes of soil characteristics in areas such as eucalyptus plantations that have suffered from high concentrations of tannin leaching for a long time,and provide a scientific basis for maintaining the safety and sustainable development of eucalyptus plantation ecosystems. |
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