Abstract. Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate ( ) loss to groundwater Ϫ

Abstract. A synthesis of the biogeochemistry of K was conducted during in the reference and human-manipulated watershed-ecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. Results showed that during the first two years of the study , which coincided with a drought period, the reference watershed was a net sink for atmospheric inputs of K. During the remaining years, this watershed has been a net source of K for downstream ecosystems. There have been long-term declines in volume-weighted concentration and flux of K at the HBEF; however, this pattern appears to be controlled by the relatively large inputs during the initial drought years. Net ecosystem loss (atmospheric deposition minus stream outflow) showed an increasing trend of net loss, peaking during the mid-1970s and declining thereafter. This pattern of net K loss coincides with trends in the drainage efflux of SO: and NO;, indicating that concentrations of strong acid anions may be important controls of dissolved K loss from the site. There were no long-term trends in streamwater concentration or flux of K. A distinct pattern in pools and fluxes of K was evident based on biotic controls in the upper ecosystem strata (canopy, boles, forest floor) and abiotic controls in lower strata of the ecosystem (mineral soil, glacial till). This biological control was manifested through higher concentrations and fluxes of K in vegetation, aboveground litter, throughfall and forest floor pools and soil water in the northern hardwood vegetation within the lower reaches of the watershedecosystem, when compared with patterns in the high-elevation spruce-fir zone. Abiotic control mechanisms were evident through longitudinal variations in soil cation exchange capacity (related to soil organic matter) and soil/till depth, and temporal and disturbance-related variations in inputs of strong-acid anions. Marked differences in the K cycle were evident at the HBEF for the periods 1964-69 and 1987-92. These changes included decreases in biomass storage, net mineralization and throughfall fluxes and increased resorption in the latter period. These patterns seem to reflect an ecosystem response to decreasing rates of biomass accretion during the study. Clearcutting disturbance resulted in large losses of K in stream water and from the removal of harvest products. Stream losses occur from release from slash, decomposition of soil organic matter and displacement from cation exchange sites. Elevated concentrations of K persist in stream water for many years after clearcutting. Of the major elements, K shows the slowest recovery from clearcutting disturbance.

Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate (NO3−) loss to groundwater or surface waters is the primary symptom of N excess. Additional symptoms include increasing N concentrations and higher N:nutrient ratios in foliage (i.e., N:Mg, N:P), foliar accumulation of amino acids or NO3−, and low soil C:N ratios. Recent nitrogen‐fertilization studies in New England and Europe provide preliminary evidence that some forests receiving chronic N inputs may decline in productivity and experience greater mortality. Long‐term fertilization at Mount Ascutney, Vermont, suggests that declining and slow N‐cycling coniferous stands may be replaced by fast‐growing and fast N‐cycling deciduous forests. Symptoms of N saturation are particularly severe in high‐elevation, nonaggrading spruce–fir ecosystems in the Appalachian Mountains and in eastern hardwood watersheds at the Fernow Experimental Forest near Parsons, West Virginia. In the Los Angeles Air Basin, mixed conifer forests and chaparral watersheds with high smog exposure are N saturated and exhibit the highest streamwater NO3− concentrations for wildlands in North America. High‐elevation alpine watersheds in the Colorado Front Range and a deciduous forest in Ontario, Canada, are N saturated, although N deposition is moderate (∼8 kg·ha−1·yr−1). In contrast, the Harvard Forest hardwood stand in Massachusetts has absorbed >900 kg N/ha during 8 yr of N amendment studies without significant NO3− leaching, illustrating that ecosystems vary widely in the capacity to retain N inputs. Overly mature forests with high N deposition, high soil N stores, and low soil C:N ratios are prone to N saturation and NO3− leaching. Additional characteristics favoring low N retention capacity include a short growing season (reduced plant N demand) and reduced contact time between drainage water and soil (i.e., porous coarse‐textured soils, exposed bedrock or talus). Temporal patterns of hydrologic fluxes interact with biotic uptake and internal cycling patterns in determining ecosystem N retention. Soils are the largest storage pool for N inputs, although vegetation uptake is also important. Recent studies indicate that nitrification may be widespread in undisturbed ecosystems, and that microbial assimilation of NO3− may be a significant N retention mechanism, contrary to previous assumptions. Further studies are needed to elucidate the sites, forms, and mechanisms of N retention and incorporation into soil organic matter, and to test potential management options for mitigating N losses f...

An intensive soil sampling design was evaluated to determine what resolution could be obtained in N and C pool size estimates in a northern hardwood forest soil. Pits of measured volume were excavated by horizon in the forest floor and in three depth strata in the mineral soil. Future comparisons should be able to detect differences in N and C pool sizes ranging from 8 to 25% of the observed mean values depending upon the element and depth strata. Future sampling should detect changes of 230 and 130 kg N ha−1 in the forest floor (combined O horizons) and 0‐ to 10‐cm stratum in the mineral soil respectively. Similarly, changes of 5.9 and 2.4 Mg C ha−1 should be detectable for forest floor and 0 to 10 cm pools respectively. Soil N content for the forest floor was 1300 kg N ha−1. For the mineral soil depth strata (0–10 cm, 10–20 cm, 20 cm to the bottom of the B horizon), N contents were 1600, 1200 and 3100 kg N ha−1 respectively. Total solum N content was estimated to be 7200 kg N ha−1. Soil C contents for the combined O horizons, 0‐ to 10‐, 10‐ to 20‐ and ≥ 20‐cm strata were 30, 32, 27 and 73 Mg C ha−1 respectively. The total solum C content was estimated to be 160 Mg C ha−1. Concentrations of soil N and C were positively correlated with elevation over the 240 m range studied, but soil pools of N and C were not correlated with elevation or soil mapping unit.