| Aluminum is the most common elements distributed widely in the nature earth's crust, take up 7.45% of the total weight of the earth's crust, second only to oxygen and silicon, which is the most abundant content metallic elements and inorganic mineral elements. In the nature aluminum usually undissolved of silicate or aluminum oxide forms exist in a series aluminum particles,the other aluminum with various forms exist. The existences of aluminum form in soil have effect on soil structure and properties which affect human and animal and plant ecology and environment [1-6]. Since the German silviculturist Ulrich [7] put forward aluminum toxicity theory of forest recession, the each form of aluminum and their toxicity and formation mechanism became hot research in foreign countries earlier than domestic research, domestic research on aluminum toxicity after 1980s become a research hot spot.In recent years, degrationg was result from successive planting of forest, soil recession has become forestry workers research hot spot, many scholars thinked soil degradation and toxic aluminum forms accumulation in soil had a great relationship [8-14]. With the deepening of the research on the aluminum, the effect of soil aluminum on forest ecosystems take more and more attention by people. Judging from the current research results, there were many research on plants aluminum stress [15-18] aluminum toxicity resistance mechanism of plants [19-20], simulated experiments studied the formation mechanism of soil aluminum toxicity [21-23] and the measures of alleviating soil aluminum toxicity [24-27], but the study on soil aluminum toxicity and it's development trend under the status of natural condition is less. Meanwhile, the related researches on aluminum forms in Chinese fir plantation ecosystem soil were relatively weak and the material was less. Whether aluminum toxicity exists in multiple generation Chinese fir plantation soil, effective evidence is still not fully, still under debate.So, choosing 5 forest ecosystem—naturally regenerated evergreen broad-leaved forest , the old-growth Chinese fir model, abandoned natural regeneration Chinese fir, the second-generation Chinese fir planted in1991 and 1993 year—built in Xiqin Teaching Forest Farm in Nanping City as the series of quadrates, and using located research in the field, this paper studied on the dynamic change law of soil total aluminum, aluminum forms and acidity compositions in the process of forest ecosystem conversion. Using simulated acid rain method studied on the variation of the content of each aluminum form in soil extracted by simulated acid rain with different pH and different acidification time of the same pH, and then tried to reveal mechanism of acid rain led to aluminum toxicity. At the same time, using artificial simulated technique, this paper did further research on the variation of the content of each aluminum form in soil which added in forest litter of different tree and exogenous Calcium carbonate cultivation, and their influence mechanism. And then tried to reveal the law of aluminum forms, the mechanism of aluminum toxicity, and lay down the technical measures of alleviate aluminum toxicity, the mechanism of soil degradation in plantation, and provide scientific theory basis and practical guidance for sustainable management in managing plantation. The results were as follows:(1) The total aluminum content differences of each system was very little, the content of second generation Chinese fir plantations (including abandoned natural regeneration Chinese fir model, the second-generation Chinese fir planted in1991 and 1993 year model ) was slightly higher than naturally regenerated evergreen broad-leaved forest model and the old-growth Chinese fir model. The seasonal variation of total aluminum content in the soil of each system was not obvious.(2)The exchangeable aluminum is the strongest biological toxicity of aluminum form in soil .The differences of each system exchangeable aluminum content was very obvious, The content of exchangeable aluminum in soil of second generation Chinese fir plantations was lower than naturally regenerated evergreen broad-leaved forest model and the old-growth Chinese fir model, the content in the abandoned natural regeneration Chinese fir model was lowest. So replanting many generations Chinese fir plantation did not cause exchangeable aluminum accumulation in soil. The seasonal variation of exchangeable aluminum content in each system soil was obvious, January and October was high, and April and July was low.(3)Monomer hydroxyl aluminum as a form of toxicity aluminum in soil is second only to the exchangeable aluminum. The monomer hydroxyl aluminum content difference of each system was very obvious. From the annual average we know that the content of monomer hydroxyl aluminum in the second-generation Chinese fir planted in 1993 year model soil was higher than other systems obviously, showing a certain accumulation of monomer hydroxyl aluminum. The seasonal variation of hydroxyl monomer aluminum content in each system soil was obvious, high in October, low in other months, which consistent with the law proposed by Driscoll et al [79] the content of monomeric aluminum in acidification soil was high in autumn.(4) The differences of Lyso-acid inorganic aluminum and organically complexed aluminum content in each system soil was relatively small, The seasonal variation of Lyso-acid inorganic aluminum content in each system soil was relatively obvious, while the seasonal variation of organically complexed aluminum content in each system soil was obvious, generally higher in July, other months were lower.(5)Slope position had little effect on the content of soil total aluminum and each form of aluminum. Slope aspect had obvious effect on the content of soil total aluminum and exchangeable aluminum. The content of total aluminum and exchangeable aluminum in soil of sunny slope were higher than shady slope, and slope aspect had little effect on the content of soil monomer hydroxyl aluminum, Lyso-acid inorganic aluminum and organically complexed aluminum.(6)The proportion of each form of aluminum in total aluminum in the rule: organically complexed aluminum>Lyso-acid inorganic aluminum>exchangeable aluminum> monomer hydroxyl aluminum. Overall, each form aluminum and active aluminum in soil of the second-generation Chinese fir planted in 1993 year had a higher proportion in total aluminum, while other systems were relatively low; indicating that the second-generation Chinese fir planted in 1993 year had a higher potential risk of aluminum toxicity.(7)The each form of aluminum and soil chemical properties had a certain relationship. From the results of correlation analysis we know that exchangeable aluminum(EX-Al) and total exchangeable acid(EX-AC) was significantly positive correlation, can be fitted with the linear equation: EX-Al=0.0193EX-AC+0.1827(r=0.375*), exchangeable aluminum and soil pH had a very significant negative correlation, can be fitted with the linear equation: EX-Al=-0.2606pH+1.4916 (r=-0.547**); Monomer hydroxyl aluminum (Hy-Al) and organic matter (OM) had a significant positive correlation, can be fitted with the linear equation: Hy-Al=0.001OM+0.024(r=0.394*), monomer hydroxyl aluminum and soil pH had a significant positive correlation, can be fitted with the linear equation: Hy-Al=0.0555pH-0.1915(r=0.352*), monomer hydroxyl aluminum and CEC had a very significant positive correlation, can be fitted with the linear equation: Hy-Al=0.0071CEC-0.0401 (r=0.607**).(8)The differences of exchangeable aluminum (neutralization titration) content in each system soil was very obvious . the content of exchangeable aluminum in soil of second generation Chinese fir plantation was lower than naturally regenerated evergreen broad-leaved forest model and the old-growth Chinese fir model, the content of the abandoned natural regeneration Chinese fir model was lowest. So replanting many generations Chinese fir plantation did not cause exchangeable aluminum accumulation in soil. The differences of Soil pH value between systems was obvious, the value of pH in soil of second generation Chinese fir plantations was higher than naturally regenerated evergreen broad-leaved forest model and the old-growth Chinese fir model, the variation rule of pH value to be contrary to the exchangeable aluminum . The seasonal variation of exchangeable aluminum content in each system soil was not obvious, the seasonal variation of pH value in each system soil was obvious, generally showed in the order: January> April> October> July.(9) The soil of each system extracted by different pH simulated acid rain , the content of aluminum in soil leaching solution increased with simulated acid rain pH decreased, when simulated acid rain pH value higher than 3.5, the content of aluminum in soil leaching solution of each system increase slowly with simulated acid rain pH value decreased, when simulated acid rain pH value lower than 3.5, the content of aluminum in soil leaching solution increased dramatically with simulated acid rain pH value decreased. In five systems, the soil of each system extracted by different pH simulated acid rain, the content of aluminum in soil leaching solution of the abandoned natural regeneration Chinese fir had lower aluminum content. This indicated that the soil of the abandoned natural regeneration Chinese fir model was relatively insensitive to acid rain, in other words , the soil of the abandoned natural regeneration Chinese fir model buffer ability to acid rain was stronger than other system , which is most easily influenced by aluminum toxicity in five systems. The soil in of each system continuous extracted by pH3.5 simulated acid rain 5 times and 15 times, the dynamic change of aluminum content in soil leaching solution decreased with extraction times increased.(10)The soil of each system extracted by different pH simulated acid rain , the value of Al/Ca in soil leaching solution increased with simulated acid rain pH decreased, after extracting by pH4.5 and pH3.5 simulated acid rain, the value of Al/Ca in soil leaching solution of naturally regenerated evergreen broad-leaved forest model, the second-generation Chinese fir planted in1991 and 1993 year model was greater than 3.0, extracted by pH2.5 simulated acid rain, the value of Al/Ca in soil leaching solution of each system was greater than 3.0, each system begun to be under the influence of aluminum toxicity,therefor ,we considered acidification as one of the reasons lead to aluminum toxicity. The content of exchangeable aluminum and monomer hydroxyl aluminum increased with the value of pH decreased, the relation of Lyso-acid inorganic aluminum, organically complexed aluminum and the value of pH was not obvious.(11 ) After the soil of each system extracting by simulated acid rain,the content of exchangeable aluminum and monomer hydroxy aluminum compared with initial soil content had different rate of rising, then the content of exchangeable aluminum and monomer hydroxy aluminum decreased with the extracted times increased.after long-term acidification ,the content of exchangeable aluminum had a decreased trend, the content of monomer hydroxy aluminum had a increased trend, the variation of Lyso-acid inorganic aluminum and organically complexed aluminum content was not obvious.After long-term acidification ,the content of exchangeable aluminum and monomer hydroxy aluminum in soil of the second-generation Chinese fir planted in 1993 year model was the highest than other systems,the content of aluminum in soil leaching solution was the highest than other systems.Can be inferred that the second-generation Chinese fir planted in 1993 year model was most easily influenced by aluminium toxicity in five systems.(12) The soil added in litter had obvious influence on the content of each aluminum form , and the value of pH.After adding in litter cultivation ,the content of exchangeable aluminum and monomer hydroxy aluminum of each treatment compared with the content of contrast soil had different rate of decrease, the content of Lyso-acid inorganic aluminum,organically complexed aluminum and the value of soil pH had different rate of rising.Chinese fir and Nanmu mixed treatment not only had good effect on decreasing the content of exchangeable aluminum and monomer hydroxy aluminum,but also had good effect on rising the value of pH and the content of Lyso-acid inorganic aluminum and organically complexed aluminum. Therefore, in the management of Chinese fir plantation, the Chinese fir withNanmu Mixed plantation can effectively reduce the accumulation of exchangeable aluminum and monomer hydroxy aluminum content in Chinese fir plantations ecsystem, in order to avoid or reduce the aluminum toxicity.(13)The content of exchangeable aluminum and monomer hydroxyl aluminum decreased with litter decomposition time prolonged, the content of Lyso-acid inorganic aluminum, organically complexed aluminum and value of pH increased along with litter decomposition time prolonged.(14) Adding exogenous calcium carbonate in soil had obvious influence on the content of each aluminum form and the value pH. After adding calcium carbonate cultivation in soil, no exchangeable aluminum detected in soil. the content of monomer hydroxy aluminum of each treatment compared with the content of contrast soil had different rate of decrease, in addition, the content of monomer hydroxy aluminum of each treatment decreased with the quantity of calcium carbonate increased, the content of Lyso-acid inorganic aluminum,organically complexed aluminum and the value pH increased along with the quantity of calcium carbonate increased, in the meantime, the content of Lyso-acid inorganic aluminum and organically complexed aluminum increased along with cultivation time prolonged.therefor,using the method of adding exogenous calcium carbonate in soil had obvious effect on controlling the content of each aluminum form and the value pH.(15)The content of exchangeable aluminum and monomer hydroxy aluminum in soil of second generation Chinese fir plantations was lower than naturally regenerated evergreen broad-leaved forest model and the old-growth Chinese fir model. The content of monomer hydroxy aluminum in soil of the second-generation Chinese fir planted in1993 year model was high, the content higher than other system obviously, showing of certain accumulation phenomena, but the content of monomer hydroxyl in each system soil was still in the normal range. Meanwhile,the value of Al/Ca lower than 3.0 in each system,therefor, According to the current evaluation indicators of aluminum toxicity ,we considered that the the possibility of Chinese fir plantation replanting in the study area soil leads to aluminum toxicity was small, according to the simulated acid rain test results , the second-generation Chinese fir planted in 1993 year model was sensitive to acid rain,the potential risk of aluminum toxicity is high when the system appear suffering from acid rain . |
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