Nitrogen Cycling In Forests Research Articles

Nitrogen cycling in forested catchments: A Chapman Conference

The American Geophysical Union sponsored a Chapman Conference on “Nitrogen Cycling in Forested Catchments” on September 16–20, 1996. This conference brought together scientists from many disciplines to share recent observations and to discuss advances in the study of nitrogen (N) cycling in forests. Conference presentations and discussions focused on mechanisms controlling the retention and losses of N in forests and on the effects of atmospheric deposition, land use, and climate on watershed N loss. The importance of dissolved organic nitrogen in the biogeochemistry of N was a subject of considerable discussion. Several critical issues were identified for future research on N cycling in forested catchments.

Nitrogen deposition and the forest nitrogen cycle: role of denitrification

The fate and effects of anthropogenic N deposition in forest ecosystems are discussed, and a conceptual hypothesis for the ecosystem response to elevated N deposition is presented. The concept of ‘nitrogen saturation’ is discussed and defined. The interactions of effects of excess N (nutrient deficiencies, loss of mycorrhiza, soil acidification, etc.) are described; these may destabilize the forest ecosystem and contribute to forest decline. Estimates of long-term critical N loads for forest ecosystems of 2–20 kg N ha −1 year −1, depending on biomass utilization are derived from simple mass balance considerations. Denitrification is supposed to be an insignificant process in the N-cycling of most forests. It may not alleviate the impact of deposition inputs, but it may be an important process in relation to atmospheric stability.

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.

Foliar retention of 15N-nitrate and 15N-ammonium by red maple ( Acer rubrum) and white oak ( Quercus alba) leaves from simulated rain

Studies of nitrogen cycling in forests indicate that trees assimilate atmospheric nitrate and ammonium and that differences between atmospheric deposition to the forest canopy and deposition measured in forest throughfall can be attributed to the removal of these ions from rain by tree leaves. Red maple and white oak leaves were exposed to artificial rain solutions (pH 4.1) containing 15N-labeled nitrate (3.5 μg N/ml) or ammonium (2.2 μg N/ml). At two time intervals after exposure (2 hr and 2 days) an exposed leaf and a control (non-exposed) leaf were removed from replicate seedlings. Based on results from 15N analysis, most of the nitrate applied to tree leaves was removed by washing with water; the mean per cent removal (± standard error, N = 4) was 87 ± 1 and 73 ± 4% of the 15NO-N Applied to red maple and white oak leaves, respectively. Relative retention of 15NH 4-N by the leaves was greater than that observed for 15NO 3-N. In red maple and white oak leaves, 58 ± 9 and 84 ± 7% (mean ± standard error, N = 4), respectively, of the applied ammonium was not removed by washing treatments. Our results show that the foliar uptake of 15NH 4 + from simulated rain by deciduous tree leaves is greater than that for 15NO 3 −. Greater retention of NH 4 + than NO 3 − ions by red maple and white oak leaves from simulated rainfall is consistent with field observations showing a preferential retention of ammonium from rainfall by forest canopies. As nitrogen chemistry and the relative importance of nitrogen compounds in the atmosphere change in response to proposed emission reductions (and possibly climate change), an improved understanding of the fate of airborne nitrogen compounds in forest biogeochemical cycles will be necessary.

Land use of wastewater and sludge

A review is presented of current technologies, treatment performance and research needs for the land disposal of municipal waste. Sludge use issues continue to center on the public health concerns over cadmium, lead, toxic organics and pathogens. Other issues discussed include design criteria for forest applications, nitrogen cycling in forests, application rates and plant selection for drastically disturbed lands and reassessment of the phytotoxicity standards for metals. 49 references.