Sources, fates, and impacts of nitrogen inputs to terrestrial ecosystems: review and synthesis

Abstract

Abstract. The relative importance of nitrogen inputs from atmospheric deposition and biological fixation is reviewed in a number of diverse, non-agricultural terrestrial ecosystems. Bulk precipitation inputs of N (I-12kgN ha-' yr-') are the same order of magnitude as, or frequently larger than, the usual range of inputs from nonsymbiotic fixation (< 1 -5 kg N ha-' yr-I), especially in areas influenced by industrial activity. Bulk precipitation measurements may underestimate total atmospheric deposition by 3040% because they generally do not include all forms of wet and dry deposition. Symbiotic fixation generally ranges from z lo-160 kg N ha-' yr-' in ecosystems where N-fixing species are present during early successional stages, and may exceed the range under unusual conditions.Rates of both symbiotic and nonsymbiotic fixation appear to be greater during early successional stages of forest development, where they have major impacts on nitrogen dynamics and ecosystem productivity. Fates and impacts of these nitrogen inputs are important considerations that are inadequately understood. These input processes are highly variable in space and time, and few sites have adequate comparative information on both nitrogen deposition and fixation.The existing information on N inputs is deficient in several areas: -more intensive studies of total atmospheric deposition, especially of dry deposition, are needed over a wide range of ecosystems; -additional studies of symbiotic fixation are needed that carefully quantify variation over space and time, examine more factors regulating fixation, and focus upon the availability of N and its effects upon productivity and other nutrient cycling processes; -process-level studies of associative N-fixation should be conducted over a range of ecosystems to determine the universal importance of rhizosphere fixation; -further examination of the role of free-living fixation in wood decomposition and soil organic matter genesis is needed, with attention upon spatial and temporal variation; and -investigations of long-term biogeochemical impacts of these inputs must be integrated with process-level studies using modem modelling techniques.

Coupling Between the Coastal Ocean and Yaquina Bay, Oregon: Importance of Oceanic Inputs Relative to Other Nitrogen Sources

“…Using 1996 vegetation data obtained from the Coastal Landscape Analysis and Modeling Study (http://www.fsl.orst.edu/ clams), we estimate that about 23% of the Yaquina Watershed is vegetated with red alder (assuming that the broadleaf cover is primarily alder). Using published N fixation rates of 50-200 kg N ha −1 year −1 (Boring et al 1988;and Binkley et al 1994) and the coverage of alder in the Yaquina watershed, we estimate that >98% of the annual riverine N loading to Yaquina Bay may be related to the presence of red alder in the watershed. Compton et al (2003) found a relationship between broadleaf and mixed cover and annual N export (N export , kg N ha −1 year −1 ) in the Salmon River basin…”

Nitrogenase activity and N 2 fixation are stimulated by elevated CO 2 in a tropical N 2 -fixing tree

“…However, most terrestrial ecosystems are nitrogen limited (Vitousek and Howarth 1991) and may not respond to elevated CO 2 with increased plant growth unless there is a concomitant increase in nitrogen availability (Kramer 1981). Biological N 2 -fixation currently accounts for 60% of the "new" nitrogen deposited on land annually (Schlesinger 1991) and symbiotic N 2 -fixing trees may be expected to improve the fertility of N-deficient soils in a high CO 2 environment by gradually increasing soil N content (Boring et al 1988;Chapin et al 1994). Thus, N 2 -fixing trees may stimulate photosynthesis and growth under elevated CO 2 conditions, inducing positive feedback on rates of carbon sequestration in forests.…”

Global terrestrial biogeochemistry: Perturbations, interactions, and time scales

“…Allocation may also change over time as the availability of N changes, as may the C:N ratios of the plant tissue (Boring et al 1988, Vitousek et al 1988). These simulations suggest that net C uptake is likely to be more sensitive to any changes in allocation than to C:N ratio changes within each tissue type.…”