Considerations in modeling change in temperate forest nitrogen cycles

The general features of nitrogen (N) cycles in temperate forests and the important processes to consider when modeling change in these cycles include atmospheric inputs, N fixation, litter (root and aboveground) transfers and decomposition, soil processes, N uptake and effects on productivity and litter quality, and N outputs. Nitrogen cycling is closely linked with the carbon (C) and water cycles. Thus models of N cycling must include aspects of these other cycles. Although much is known about individual processes, development of a generic model of forest N cycling is not possible at present because the links and interactions among the individual processes are not well understood. The weakest links with respect to the N cycle are: quantification of atmospheric (especially dry) deposition rates in polluted environments, controls on C and N allocation in vegetation, controls on N turnover in fine roots, controls on decomposition of the older components of soil organic matter, and feedbacks among N availability, litter "quality" and subsequent N mineralization rates. To examine possible effects of long-term change in climate or atmospheric chemistry on the storage of C and other elements in forest ecosystems, we need to model in detail the effects of these factors on complex soil processes such as organic matter decomposition. Some promising models have been developed, but they need to be validated across a range of forest types before they can be used with confidence for long-term prediction. Mean annual leaf litter N concentration offers potential as a simple index of annual N uptake in forests.