Evaluating the Influences of Soil Calcium and Aluminum Availabilty on Ecosystem Processes in the Northern Hardwood Forest

Calcium (Ca) depletion and increased bioavailability of aluminum (Al) are potential consequences of soil acidification caused by acidic deposition and other anthropogenic factors. Tree declines are associated with base cation depletion and increased Al toxicity in forest soils in North America, Europe, and Asia. Changes in soil Ca and Al availability may lead to increased oxidative stress and disruptions in carbohydrate relationships in forest trees, as well as to substantial alterations in the capacity for enzymatically controlled processes of decomposition and mineralization in forest soils. Assessments were made to determine if forest systems are prone to disruption associated with altered Ca and Al bioavailability. Foliar elemental concentrations, foliar antioxidant enzyme activities, foliar and woody shoot carbohydrates were measured in sugar maple (Acer saccharum, Marsh.), and soil extracellular enzyme activities (EEA) were assayed at a long-term nutrient perturbation study (NuPert) in the Hubbard Brook Experimental Forest, New Hampshire, USA. Treated plots received Ca to increase soil Ca above ambient depleted levels or Al to further reduce Ca availability. Additions of Ca to soil are associated with greater Ca concentrations in foliage compared to leaves from trees from control and Al-addition plots. Soil Aladditions are associated with lower foliar phosphorus concentrations in comparison with foliage from trees in Ca-addition plots. Additions of Al to soil are associated with higher antioxidant enzyme (glutathione reductase and ascorbate peroxidase) activities in foliage and lower shoot sugar (total sugars, sucrose, glucose and fructose) concentrations relative to trees in Ca-addition and control plots. Al accumulations in distal tissues likely triggered toxicity responses reported for leaves and stems. Soil EEA results highlight treatment-induced alterations to soil processes. Across soil enzyme systems, EEA levels are greatest in Al-addition soils in fall, but are elevated in Caaddition soils in spring compared with ambient conditions. Seasonal differences in EEA levels suggest a differential influence of soil treatments on specific soil communities. Within this native, mature northern hardwood forest, early indications of response in foundation species to Ca and Al manipulation are detected including Al-induced oxidative stress and resulting carbohydrate irregularities in sugar maple trees, and substantial seasonal swings in soil EEA: processes that could foreshadow broader ecosystem alterations as anthropogenic disruptions of soil Ca and Al availability continue