MICROSCALE HETEROGENEITY OF ACIDITY RELATED STRESS-PARAMETERS IN THE SOIL SOLUTION OF A FORESTED CAMBIC PODZOL

Acid related stress in soils might be caused by high concentrations of H+ and Al3+ in soil solution. Sampling of soil solution so far integr ates over a relatively large soil volume, in the range of dm(3). In or der to study the microscale heterogeneity of acidity related stress-pa rameters 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 s olution 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 als o measured. There was a general trend of increasing pH and decreasing UV absorption with increasing soil depth, however without a clear corr elation of concentration isolines to soil horizon borders. The latter was also true for total Al (Al-tot) and Al3+, with the exception of th e soil horizon border A(he)/B-h, which was very well reflected by Al3 and also by the fraction of bound Al. In the A(he) horizon less than 30%, in deeper mineral soil less than 50% of Al-tot were present as fr ee Al3+. This fact is critical when calculating Ca/Al ratios as a stre ss parameter, because total metal content measured by ICP clearly over estimates the risk of root damage, even in deeper horizons of acid for est soils, where organic complexation of Al is of minor importance. Th e heterogeneity of soil solution chemistry and toxicity parameters on the cm-scale was found to be significant, for example with gradients o f more than 0,5 pH-units within 2 cm. Because plant roots also experie nce soil on a microscale, high resolution investigations of soil solut ion chemistry offer a new approach for looking at the chemical environ ment relevant for root growth and plant nutrition.