Microscale heterogeneity of acidity related stress-parameters in the soil solution of a forested cambic podzol

Abstract

Acid related stress in soils might be caused by high concentrations of H + and Al 3+ in soil solution. Sampling of soil solution so far integrates over a relatively large soil volume, in the range of dm 3 . In order to study the microscale hcterogeneity of acidity related stress-parameters the soil profile of a podzolic cambisol was covered by a 10x6 matrix of micro suction cups with a grid distance of 2 cm. The soil solution collected at 10 sampling events was analyzed for free cations and anions by capillary electrophoresis and for total metal content by a micro injection technique on ICP-OES. pH and UV absorption were also measured. There was a general trend of increasing pH and decreasing UV absorption with increasing soil depth, however without a clear correlation of concentration isolines to soil horizon borders. The latter was also true for total Al (Al tot ) and Al 3+ , with the exception of the soil horizon border A be /B h ,which was very well reflected by Al + and also by the fraction of bound Al. In the A be horizon less than 30%, in deeper mineral soil less than 50% of Al tot were present as free Al 3+ . This fact is critical when calculating Ca/Al ratios as a stress parameter, because total metal content measured by ICP clearly overestimates the risk of root damage, even in deeper horizons of acid forest soils, where organic complexation of Al is of minor importance. The heterogeneity of soil solution chemistry and toxicity parameters on the cm-scale was found to be significant, for example with gradients of more than 0,5 pH-units within 2 cm. Because plant roots also experience soil on a microscale, high resolution investigations of soil solution chemistry offer a new approach for looking at the chemical environment relevant for root growth and plant nutrition.