MONITORING FOREST SOILS OVER TIME TO UNDERSTAND ACIDIC DEPOSITION EFFECTS AND RECOVERY

The use of soil monitoring continues to grow due to a recognition that forest soils are more dynamic than previously thought, and a realization that changes can be detected in the chemical properties of soils within a decade or less with proper methods. To develop and promote the use of repeated sampling to monitor soil change, the Northeastern Soil Monitoring Cooperative (NESMC) was formed from a group of American and Canadian forest soil scientists with a shared interest in soil monitoring. In a recent NESMC project, data was pooled from 27 sites across the Northeastern U.S. and eastern Canada to regionally assess the response of soils to decreasing acidic deposition. The sites were exposed to reductions in wet SO₄^2- deposition that ranged from 5.7% to 76%, over soil sampling intervals of 8 to 24 years. Indications of soil recovery from acidic deposition were identified across the region despite different deposition histories, time intervals between sampling, and methods of sampling among the study sites. The most pronounced change was the decrease of exchangeable Al in the O horizon. Increases in pH in the O and B horizons were also seen at most sites. Among all sites, reductions in SO₄^2- deposition were positively correlated with ratios (final sampling/ initial sampling) of base saturation (P < 0.01), and negatively correlated with exchangeable Al ratios (P < 0.05) in the O horizon. These results were highly relevant to policy assessment because they showed that the greater the deposition decrease, the stronger the recovery response. However, at one-third of the sites, base saturation in the B horizon decreased, with no increases. This result was largely due to an increase in exchangeable Al without increases in exchangeable Ca. The increase in B horizon Al may reflect a return to soil processes similar to podzolization. These results are unique in showing that the effects of acidic deposition on North American soils have begun to reverse, and demonstrate the value of soil monitoring in tracking important environmental changes.