Mechanisms for aluminum release in Chinese acid forest soils

et al. 2006

Environ Monit Assess

Al i /(Ca + Mg) molar ratio in soil water has been used as an indicator to the effects of acid deposition on terrestrial ecosystems. However, the main factors controlling this ratio have not been well documented in southern and southwestern China. In this study, we presented the variation in inorganic aluminum (Al i ) and Al i /(Ca + Mg) molar ratio in different sites and soil horizons based on two to three years monitoring data, and evaluated the main factors controlling Al i /(Ca + Mg) molar ratio using principle component analysis (PCA) and partial least square (PLS) regression. Monitoring data showed although Al i /(Ca + Mg) molar ratios in most soil water were lower than assumed critical 1.0, higher molar ratios were found in some soil water at TSP and LXH site. Besides acid loading, both soil properties and soil water chemistry affected the value of Al i /(Ca + Mg) molar ratio in soil water. Partial least square (PLS) indicated that they had different relative importance in different soil horizons. In A-horizon, soil aluminum saturation (AlS) had higher influence on Al i /(Ca + Mg) molar ratio than soil water chemistry did; higher soil aluminum saturation (AlS) led to higher Al i /(Ca + Mg) molar ratio in soil water. In the deeper horizons (i.e., B 1 -, B 2 -and BC-horizon), inorganic aluminum (Al i ) in soil water had more and more important role in regulating Al i /(Ca + Mg) molar ratio. On regional scale, soil aluminum saturation (AlS) as well as cation exchange capacity (CEC) was the dominant factor controlling Al i /(Ca + Mg) molar ratio. This should be paid enough attention on when making regional acid rain control policy in China.

Section: Introductionsupporting

confidence: 57%

Section: Site Descriptionmentioning

confidence: 99%

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Evaluating Controlling Factors to Ali/(Ca + Mg) Molar Ratio in Acidic Soil Water, Southern and Southwestern China: Multivariate Approach

et al. 2006

Environ Monit Assess

“…Such an empirical model, described by Reuss et al (1990), gave satisfactory agreement to both experimental and field data (Table 4). Larssen et al (1999b) presented pK values optimized for the Chinese soils they studied (pK = -9.01). However, such empirically fitted constants and equations are known to show large spatial variation and are therefore of limited value in different regions.…”

Section: Possible Processes Controlling Aluminium Activitymentioning

confidence: 99%

Aluminium Mobilization from Acidic Forest Soils in Leigongshan Area, Southwestern China: Laboratory and Field Study

Arch Environ Contam Toxicol

et al. 2006

Abstract. Aluminium mobilization from acidic forest soils in the remote Leigongshan area in southwestern China was investigated in the field and by a laboratory batch experiment using sequential extractions with HCl solutions with pH of approximately 3.4. Experimental data showed that strongly organically bound aluminium (Alp-Alcu) decreased significantly after sequential extraction, whereas exchangeable aluminium (Alex) increased in all three horizons. In the A-horizon with higher base saturation (BS), dissolved aluminium exchanged with base cations (BCs) on soil complex, causing lower aluminium release but significant leaching of BCs. In AB-and B-horizon with lower BS, more dissolved aluminium remained in solution. Al 3+ activities in both experimental soil extracts and field soil waters strongly deviated from those predicted by the gibbsite model (i.e., pAl = pKsp + 3.0 pH) despite a significant increase in Al 3+ with a decrease in pH. The soil organic-bound aluminium model, using Alcu and Alp to account for differences in the size of available Al pools in the soil, gave considerably better fits to both experimental and field data. SOM-Al model, approved by using Alex instead of organically bound aluminium (i.e., Alp and Alcu), could depict the changes in Al activities most successfully. We have concluded that Alp-Alcu is the main source of aqueous aluminium, whereas Alex plays an important role in regulating aluminium solubility during soil acidification.

“…Based on laboratory experiments and limited field monitoring, the significance of some possible influencing factors have been documented by several specific studies (i.e. Larssen et al, 1999;Zhu et al, 2004). However, the main controlling mechanism of Al solubility is far from clearly understood, especially at the regional scale.…”

Section: Introductionmentioning

confidence: 99%

Evaluating main factors controlling aluminum solubility in acid forest soils, southern and southwestern China

et al. 2007

Applied Geochemistry

The mechanism controlling Al solubility in Chinese acidic forest soils is not clearly understood. This is the main limitation to the ability to generate adequate dose-response prediction models of the ecological effect of acid rain. To evaluate the relative significance of possible processes, soils and soil solutions from five forest catchments, located in southern and southwestern China, were collected and analyzed for chemical parameters. Monitoring showed that inorganic Al (Al i ) was the dominant fraction in most soil solutions; organic Al (Al o ) was usually less than 10% of total monomeric Al (Al a ). Aluminum fractions varied significantly between and within the different sites, though appearing to follow a similar pattern. Over the entire pH range of 3.6-5.6, the pAl (i.e. Àlog of the Al 3+ activity) closely correlated with solution pH, following regression slopes of 1.28 and 2.00 for upper and lower soil horizons, respectively. The variations in Al 3+ activity could not be explained satisfactorily using mineral dissolution equilibria. Partial least square (PLS) regression showed that soil acidity (quality) and ionic strength (intensity) of the solution were the main explanatory variables for the variation in the concentration of Al fractions. Aluminum in upper horizons originated from both organic and inorganic solid Al pools, while aqueous Al in lower horizons was dominantly of inorganic origin. Aluminum solubility was strongly influenced by cation exchange, especially in the upper horizon. In the upper horizon, ionic strength (I) had a greater influence on Al solubility due to cation exchange reaction. In the lower horizon, dissolution of inorganic Al pools by the elevated H + concentrations was the main Al release mechanism. So Al activity was more dependent on H + (or pH) in the lower horizon.