Ecosystem processes and nitrogen export in northern U.S. watersheds.

There is much interest in the relationship of atmospheric nitrogen (N) inputs to ecosystem outputs as an indicator of possible "nitrogen saturation" by human activity. Longer-term, ecosystem-level mass balance studies suggest that the relationship is not clear and that other ecosystem processes may dominate variation in N outputs. We have been studying small, forested watershed ecosystems in five northern watersheds for periods up to 35 years. Here I summarize the research on ecosystem processes and the N budget. During the past 2 decades, average wet-precipitation N inputs ranged from about 0.1 to 6 kg N ha(-1) year(-1) among sites. In general, sites with the lowest N inputs had the highest output-to-input ratios. In the Alaska watersheds, streamwater N output exceeded inputs by 70 to 250%. The ratio of mean monthly headwater nitrate (NO3-) concentration to precipitation NO3- concentration declined with increased precipitation concentration. A series of ecosystem processes have been studied and related to N outputs. The most important appear to be seasonal change in hydrologic flowpath, soil freezing, seasonal forest-floor inorganic N pools resulting from over-winter mineralization beneath the snowpack, spatial variation in watershed forest-floor inorganic N pools, the degree to which snowmelt percolates soils, and gross soil N mineralization rates.

Biogeochemistry of a treeline watershed, northwestern Alaska.

Since 1950, mean annual temperatures in northwestern Alaska have increased. Change in forest floor and soil temperature or moisture could alter N mineralization rates, production of dissolved organic carbon (DOC) and organic nitrogen (DON), and their export to the aquatic ecosystem. In 1990, we began study of nutrient cycles in the 800-ha Asik watershed, located at treeline in the Noatak National Preserve, northwestern Alaska. This paper summarizes relationships between topographic aspect, soil temperature and moisture, inorganic and organic N pools, C pools, CO2 efflux, growing season net N mineralization rates, and stream water chemistry. Forest floor (O2) C/N ratios, C pools, temperature, and moisture were greater on south aspects. More rapid melt of the soil active layer (zone of annual freeze-thaw) and permafrost accounted for the higher moisture. The O2 C and N content were correlated with moisture, inorganic N pools, CO2 efflux, and inversely with temperature. Inorganic N pools were correlated with temperature and CO2 efflux. Net N mineralization rates were positive in early summer, and correlated with O2 moisture, temperature, and C and N pools. Net nitrification rates were inversely correlated with moisture, total C and N. The CO2 efflux increased with temperature and moisture, and was greater on south aspects. Stream ion concentrations declined and DOC increased with discharge. Stream inorganic nitrogen (DIN) output exceeded input by 70%. Alpine stream water nitrate (NO3-) and DOC concentrations indicated substantial contributions to the watershed DIN and DOC budgets.

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.

Long-term monitoring for environmental change in U.S. National Parks a watershed approach.

The U.S. National Park Service (NPS) is faced with direct questions about the condition of Park natural resources. The watershed approach to long-term monitoring of natural, and remote areas within the National Parks has provided important data for detecting both spacial and temporal changes in environmental conditions. These data collections allow the partitioning of cause and effect relationships of ecological change within a given watershed. They also serve to meet both reference and early warning objectives. Success in advancing a number of 'acid precipitation' goals has demonstrated the usefulness of these integrated watershed data for inter-ecosystem comparison and for analogy between watersheds. The watershed program owing to the NPS experience is proposed as a model for focusing the National Park Service's inventory and monitoring program inititiative. This approach provides to park researchers and resources managers the needed tools for dealing with today's complex local, regional and global natural resources issues.

Nitrification, soil acidification and streamwater chemistry following deglaciation, glacier bay national park and preserve.

A major tool used in the assessment of anthropic atmospheric effects on aquatic and terrestrial ecosystems is biogeochemical nutrient cycling and budgets. However, to be most effective such study should be done in an ecosystem context. Also some assessment of natural variation in factors affecting nutrient cycling must be in place before trends, often subtle and long-term, attributable to man can be statistically quantified. The input and output balance of chemical species in watershed ecosystems is considerably influenced by ecosystem succession. It is hypothesized that during primary ecosystem succession chemical element output is initially relatively high due to rapid acidification and lack of plant uptake. Outputs decline during the period of high ecosystem productivity and biomass accumulation, and they again rise during late successional stages to approximate inputs from precipitation weathering, and aerosol capture. Glacier Bay provides a unique opportunity to quantify many mechanisms responsible for variation in nutrient cycles without the need for site manipulation. This is especially true for quantifying the rate and magnitude of natural acidification in ecosystems. The park has a spectrum of watersheds differing in stage of primary and secondary succession following deglaciation. These sites are not now subjected to or altered by anthropic atmospheric inputs. The objectives of this research were (1) determine the rate of soil chemical change which occurs following deglaciation, (2) relate soil acidification to presence of organic matter, soil NO inf3 (sup-) , and total N, (3) estimate the downward movement of ionic species within the soil profiles with increasing acidification from advancing plant succession, and (4) determine if such processes and ionic movements might be reflected in watershed stream ionic outputs. We studied five watersheds ranging from 40-350 years since deglaciation. Soil samples were collected and lysimeters installed in seven vegetation successional stages following deglaciation. An anion of ecological importance and a common air contaminant is NO inf3 (sup-) , and its discharge in streamflow from early successional ecosystems was found to be high. The terrestrial biota in such systems was dominated by Alnus sinuata, a major nitrogen fixer. Stream discharge of NO inf3 (sup-) suggested that early successional ecosystem N fixation exceeded biotic uptake. This was confirmed by examining NO inf3 (sup-) in soil extractions and lysimeters. This process was particularly evident beneath >20-year old Alnus (forty years since deglaciation). concurrent with increased NO inf3 (sup-) concentrations below the rooting zone was increased H(+) which increased 100x during 25 years of primary succession. This natural acidification from a mobile NO inf3 (sup-) ion resulted in an pronounced increase in soil base cation leaching and mobilization of aluminium in the soil profile. The magnitude and short time required for such acidification greatly exceeded anything projected or modeled for systems impacted by anthropic inputs. Stream SO inf4 (sup2-) concentrations also were high relative to precipitation inputs suggesting mineralization of sulfur within the ecosystem and/or poor soil adsorption of SO inf4 (sup2-) . This is an important finding in such ecosystems where cation nutrient ion levels are often very low. Should atmospheric inputs of SO inf4 (sup2-) increase additional loss of cations appears imminent. These data suggest that most early successional ecosystems at Glacier Bay would be sensitive to anthropic inputs of both NO inf3 (sup-) and SO inf4 (sup2-) . This is unusual in other ecosystems where many conserve ionic NO inf3 (sup-) inputs, and older systems have considerable SO inf4 (sup2-) adsorption capacity. The effect of any increased atmospheric inputs of these ions would be accelerated cation leaching and ecosystem acidification.

Effects of acid deposition on watershed ecosystems of national parks in the great lakes basin.

Legally protected national parks provide an appropriate substrate for essential long-term study of ecosystem structure and function, and for detecting trends in natural and human-induced stress. The absence of unplanned site manipulation in such areas is especially valuable for such research. Our present research has two major components. The first is the long-term ecosystem-level study of the effects of atmospheric contaminants on ecosystem processes. The overall objective is to evaluate ecosystem aquatic/terrestrial linkages and their role in establishing aquatic ecosystem sensitivity to anthropic atmospheric inputs. Four watershed/lake ecosystems, representative of much of the region's diversity, are under study. Two mature boreal sites on Isle Royale are characterized by first-order perennial surface stream input and lake outflow. Two additional mainland northern hardwood sites, one with shallow soils and one with soils derived from glacial till, are characterized by sensitive aquatic systems. One site is in a private reserve and the other in Pictured Rocks National Lakeshore. Surface outflow is gaged by Parshall flume and stage height recorder. Meteorological stations record variables for estimating evapotranspiration. One-tenth ha plots have been established in all watersheds and three sites have had intensive study of precipitation modification by canopy and forest soil. Five-year mean maximum and minimum lake pH varies from 6.85 to 4.94, Ca(2+) from 1070 to 54 µ eq l(-1), K(+) from 5.42 to 8.35 µ eq l(-1), NH 4 (+) from 10.12 to 3.23 µ eq l(-1), HCO 3 (sup-) from 635 to 24 µ eq l(-1), NO 3 (sup-) from 3.27 to 1.54 µ eq l(-1), and SO 4 (sup2-) from 110 to 52.7 µ eq l(-1). The relatively high NO 3 (sup-) values observed in one lake are the result of stream drainage from a watershed dominated by Alnus rugosa, and another has high seasonal NO 3 (sup-) inputs during spring runoff. However, owing to periodic winter thaws, significant snowpack release of nutrients generally precedes maximum spring stream runoff. Water chemistry in both sensitive and non-sensitive lakes appears to be primarily reflecting how the conterminous terrestrial system is retaining atmospheric inputs more than the quality of direct lake atmospheric input. This is especially evident for H(+), NO 3 (sup-) and SO 4 (sup2-) .The second component is the assessment of watershed acidification, SO 4 (sup2-) output and soil retention across an input gradient. An anthropic deposition gradient provides the opportunity for intersite time-trend analyses as to the effects of inputs. Our study objective was to see if the decreasing west to east input/output values for SO 4 (sup2-) , noted in small first-order watersheds in national parks from Minnesota to Ohio, might be related to present atmospheric inputs, potential and total soil SO 4 (sup2-) adsorption, or soil SO 4 (sup2-) desorption from earlier higher inputs. Precipitation pH ranged from 5.05 at Fernberg, Minnesota to 4.24 at Wooster, Ohio. Minimum and maximum concentrations of NH 4 (+) , NO 3 (sup-) , SO 4 (sup2-) and Cl(-) were also found at these stations. Stream water concentrations of NO 3 (sup-) and SO 4 (sup2-) increase in a similar but sharper gradient. Streams are well buffered. Cation, HCO 3 (sup-) , NO 3 (sup-) and especially SO 4 (sup2-) output increase west to east, but H(+) output decreases. At the eastern site stream SO 4 (sup2-) concentration and output exceed HCO 3 (sup-) . Potential soil SO 4 (sup2-) adsorption capacity increases eastward, but this capacity is filled. Crystalline Fe hydrous oxides appear more effective than amorphous Fe hydrous oxides at adsorbing SO 4 (sup2-) . High anthropic anion inputs, inability of forest soil to adsorb additional inputs and perhaps SO 4 (sup2-) desorption appear responsible for the replacement of HCO 3 (sup-) by SO 4 (sup2-) in stream water. The major cation accompanying SO 4 (sup2-) is Ca(2+).