IMPLICATIONS OF SODIUM MASS BALANCE FOR INTERPRETING THE CALCIUM CYCLE OF A FORESTED ECOSYSTEM

Abstract

Disturbance of forest ecosystems, such as that caused by harvesting or acid deposition, is thought to alter the ability of the ecosystem to retain nutrients. Although many watershed studies have suggested depletion of available calcium (Ca) pools, interpretation of ecosystem Ca mass balance has been limited by the difficulty in obtaining mineral weathering flux estimates. While many studies have suggested that weathering flux is insufficient to maintain available soil pools, measurements of concomitant changes in available soil pools are rare. Here, we critically examined application of the Ca:Na ratio method in interpreting the long-term Ca budget of six northern hardwood watershed ecosystems at the Hubbard Brook Experimental Forest, Woodstock, New Hampshire, USA. Storage of sodium (Na) in biomass and secondary minerals and on cation exchange sites was low enough so that net ecosystem Na loss was essentially equivalent to mineral weathering flux. Mineral chemistry and mass balance considerations constrained the Ca:Na ratio of weathering products to a sufficiently narrow range that spatial and temporal changes in the ecosystem Ca:Na ratio could be interpreted as changes in contribution of available Ca pools to ecosystem loss. Depletion of available Ca pools was greater in the three experimentally manipulated watersheds with aggrading biomass compared to three reference watersheds with relatively mature forest conditions. Although accelerated loss of Ca in the first few years following disturbance has been documented by prior studies, this study suggests that excess Ca loss continues for at least three decades after treatment, with no evident trend toward the conditions shown in the reference watershed. It is not likely that changes in previously quantified Ca pools account for this sustained loss, suggesting that a previously unstudied Ca pool or release mechanism may be important in ecosystem response to disturbance. Possible sources include Ca oxalate, which is known to accumulate in forest soils, but has not been considered in the context of an ecosystem mass balance.