Paul A. Steudler

Nitrogen saturation in northern forest ecosystems

This article describes ways in which excess nitrogen from fossil fuel combustion may stress the biosphere. Nitrogen emissions can have a direct effect on air quality through both the oxidizing potential of nitrogen oxides and the role these compounds play in the formation of ozone. The complexity of these effects on water quality and on forest nutrition is discussed.

Potential Net Primary Productivity in South America: Application of a Global Model

We use a mechanistically based ecosystem simulation model to describe and analyze the spatial and temporal patterns of terrestrial net primary productivity (NPP) in South America. The Terrestrial Ecosystem Model (TEM) is designed to predict major carbon and nitrogen fluxes and pool sizes in terrestrial ecosystems at continental to global scales. Information from intensively studies field sites is used in combination with continental-scale information on climate, soils, and vegetation to estimate NPP in each of 5888 non-wetland, 0.5° latitude °0.5° longitude grid cells in South America, at monthly time steps. Preliminary analyses are presented for the scenario of natural vegetation throughout the continent, as a prelude to evaluating human impacts on terrestrial NPP. The potential annual NPP of South America is estimated to be 12.5 Pg/yr of carbon (26.3 Pg/yr of organic matter) in a non-wetland area of 17.0 ° 106 km2 . More than 50% of this production occurs in the tropical and subtropical evergreen forest region. Six independent model runs, each based on an independently derived set of model parameters, generated mean annual NPP estimates for the tropical evergreen forest region ranging from 900 to 1510 g°m-2 °yr-1 of carbon, with an overall mean of 1170 g°m-2 °yr-1 . Coefficients of variation in estimated annual NPP averaged 20% for any specific location in the evergreen forests, which is probably within the confidence limits of extant NPP measurements. Predicted rates of mean annual NPP in other types of vegetation ranged from 95 g°m-2 °yr-1 in arid shrublands to 930 g°m@ ?yr-1 in savannas, and were within the ranges measured in empirical studies. The spatial distribution of predicted NPP was directly compared with estimates made using the Miami mode of Lieth (1975). Overall, TEM predictions were °10% lower than those of the Miami model, but the two models agreed closely on the spatial patterns of NPP in south America. Unlike previous models, however, TEM estimates NPP monthly, allowing for the evaluation of seasonal phenomena. This is an important step toward integration of ecosystem models with remotely sensed information, global climate models, and atmospheric transport models, all of which are evaluated at comparable spatial and temporal scales. Seasonal patterns of NPP in South America are correlated with moisture availability in most vegetation types, but are strongly influenced by seasonal differences in cloudiness in the tropical evergreen forests. On an annual basis, moisture availability was the factor that was correlated most strongly with annual NPP in South America, but differences were again observed among vegetation types. These results allow for the investigation and analysis of climatic controls over NPP at continental scales, within and among vegetation types, and within years. Further model validation is needed. Nevertheless, the ability to investigate NPP-environment interactions with a high spatial and temporal resolution at continental scales should prove useful if not essential for rigorous analysis of the potential effects of global climate changes on terrestrial ecosystems.

Factors controlling atmospheric methane consumption by temperate forest soils

Over the past 6 years (1988–1993), we have examined the effects of soil temperature, soil moisture, site fertility, and nitrogen fertilization on the consumption of atmospheric CH4 by temperate forest soils located at the Harvard Forest in Petersham, Massachusetts. We found that soil temperature is an important controller of CH4 consumption at temperatures between −5° and 10°C but had no effect on CH4 consumption at temperatures between 10° and 20°C. Soil moisture exerts strong control on CH4 consumption over a range of 60 to 100% water-filled pore space (% WFPS). As moisture increased from 60 to 100% WFPS, CH4 consumption decreased from 0.1 to 0 mg CH4-C m−2 h−1 because of gas transport limitations. At 20 to 60% WFPS, site fertility was a strong controller of CH4 consumption. High-fertility sites had 2 to 3 times greater CH4 consumption rates than low-fertility sites. Nitrogen-fertilized soils (50 and 150 kg NH4NO3-N ha−1 yr−1 ) had annually averaged CH4 consumption rates that were 15 to 64% lower than annually averaged CH4 consumption by control soils. The decrease in CH4 consumption was related to both the years of application and quantity of nitrogen fertilizer added to these soils.

Long-Term Nitrogen Additions and Nitrogen Saturation in Two Temperate Forests

ABSTRACT This article reports responses of two different forest ecosystems to 9 years (1988–96) of chronic nitrogen (N) additions at the Harvard Forest, Petersham, Massachusetts. Ammonium nitrate (NH4NO3) was applied to a pine plantation and a native deciduous broad-leaved (hardwood) forest in six equal monthly doses (May–September) at four rates: control (no fertilizer addition), low N (5 g N m-2 y-1), high N (15 g N m-2y-1), and low N + sulfur (5 g N m-2 y-1 plus 7.4 g S m-2 y-1). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter production, soil C and N content, and concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In the pine stand, nitrate losses were measured after the first year of additions (1989) in the high N plot and increased again in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further increases in 1996. Overall N retention efficiency (percentage of added N retained) over the 9-year period was 97–100% in the control and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground biomass, fine roots, and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots, suggesting that the one major unmeasured pool, soil organic matter, contains the remaining 68–84%. Short-term redistribution of 15N tracer at natural abundance levels showed similar division between plant and soil pools. Direct measurements of changes in total soil C and N pools were inconclusive due to high variation in both stands. Woody biomass production increased in the hardwood high N plot but was significantly reduced in the pine high N plot, relative to controls. A drought-induced increase in foliar litterfall in the pine stand in 1995 is one possible factor leading to a measured increase in N mineralization, nitrification, and nitrate loss in the pine high N plot in 1996.

Influence of environmental changes on degradation of chiral pollutants in soils

Numerous anthropogenic chemicals of environmental concern—including some phenoxy acid herbicides, organophosphorus insecticides, polychlorinated biphenyls, phthalates, freon substitutes and some DDT derivatives—are chiral. Their potential biological effects, such as toxicity, mutagenicity, carcinogenicity, and endocrine disrupter activity, are generally enantiomer-selective, and different enantiomers are preferentially degraded (transformed) by micro-organisms in various environments. Here we use field and laboratory experiments to demonstrate that environmental changes in soils can alter these preferences, and to suggest that the preferences shift owing to different groups of related microbial genotypes being activated by different environmental changes. In Brazilian soils, almost all pasture samples preferentially transformed the non-herbicidal enantiomer of dichlorprop ((RS)-2-(2,4-dichlorophenoxy)propionic acid), while most forest samples either transformed the herbicidal enantiomer more readily or as rapidly as the non-herbicidal enantiomer. Organic nutrient enrichments shifted enantioselectivity for methyl dichlorprop ((RS)-methyl 2-(2,4-dichlorophenoxy)propionic acid) strongly towards preferentially removing the non-herbicidal enantiomer in soils from Brazil and North America, potentially increasing phytotoxicity of its residues relative to that of the racemate. Assessments of the risks chemical pollutants pose to public health and the environment need to take into account the chiral selectivity of microbial transformation processes and their alteration by environmental changes, especially for pesticides as up to 25 per cent are chiral.

Methane fluxes between terrestrial ecosystems and the atmosphere at northern high latitudes during the past century : a retrospective analysis with a process-based biogeochemistry model

minus consumption) from these soils have increased by an average 0.08 Tg CH4 yr � 1 during the twentieth century. Our estimate of the annual net emission rate at the end of the century for the region is 51 Tg CH4 yr � 1 . Russia, Canada, and Alaska are the major CH4 regional sources to the atmosphere, responsible for 64%, 11%, and 7% of these net emissions, respectively. Our simulations indicate that large interannual variability in net CH4 emissions occurred over the last century. Our analyses of the responses of net CH4 emissions to the past climate change suggest that future global warming will increase net CH4 emissions from the Pan-Arctic region. The higher net CH4 emissions may increase atmospheric CH4 concentrations to provide a major positive feedback to the climate system. INDEX TERMS: 1610 Global Change: Atmosphere (0315, 0325); 1615 Global Change: Biogeochemical processes (4805); 1620 Global Change: Climate dynamics (3309); 1890 Hydrology: Wetlands; KEYWORDS: methane emissions, methane oxidation, permafrost

Plant and Soil Responses to Chronic Nitrogen Additions at the Harvard Forest, Massachusetts

Data are presented on changes in plant and soil processes in two forest types (red pine plantation and oak-maple forest) at the Harvard Forest, Petersham, Massachusetts, in response to 3 yr of chronic N fertilization. The hardwood stand exhibited greater N limitation on biological function than the pine stand prior to fertilization as evidenced by a lower net N mineralization rate, nearly undetectable rates of net nitrification, and very low foliar N content. N additions were made in six equal applications throughout the growing season, and consisted of 5 and 15 g°m-2 °yr-1 of N as ammonium nitrate. The pine stand showed larger changes than the hardwood stand for extractable N, foliar N, nitrification, and N leaching loss. Retention of added N was essentially 100% for all but the high application pine plot from which significant N leaching occurred in the 3rd yr of application. From 75 to 92% of N added to fertilized plots was retained in the soil, with larger fractions retained in the hardwood stand than the pine stand for all treatments. As hypothesized, the stands are exhibiting highly nonlinear patterns of nitrogen output in response to continuous nitrogen inputs. The implications of this nonlinearity for regional eutrophication of surface waters and atmospheric deposition control policy are discussed.

Soil warming, carbon–nitrogen interactions, and forest carbon budgets

Soil warming has the potential to alter both soil and plant processes that affect carbon storage in forest ecosystems. We have quantified these effects in a large, long-term (7-y) soil-warming study in a deciduous forest in New England. Soil warming has resulted in carbon losses from the soil and stimulated carbon gains in the woody tissue of trees. The warming-enhanced decay of soil organic matter also released enough additional inorganic nitrogen into the soil solution to support the observed increases in plant carbon storage. Although soil warming has resulted in a cumulative net loss of carbon from a New England forest relative to a control area over the 7-y study, the annual net losses generally decreased over time as plant carbon storage increased. In the seventh year, warming-induced soil carbon losses were almost totally compensated for by plant carbon gains in response to warming. We attribute the plant gains primarily to warming-induced increases in nitrogen availability. This study underscores the importance of incorporating carbon–nitrogen interactions in atmosphere–ocean–land earth system models to accurately simulate land feedbacks to the climate system.

SOIL CARBON AND NITROGEN STOCKS FOLLOWING FOREST CLEARING FOR PASTURE IN THE SOUTHWESTERN BRAZILIAN AMAZON

Tropical soils contain large stocks of carbon and nitrogen that can be altered by clearing for agriculture. In the Brazilian Amazon, cattle pasture is the predominant use for cleared forest lands. We examined changes to soil bulk density and C and N stocks in seven chronosequences, each consisting of an intact forest and pastures of different ages created directly from cleared forest (7 forests, 18 pastures), along a 700-km transect in Rondonia in the southwestern Amazon Basin. The transect included sites with a similar climate but a range of soil types. We used soil δ13C distributions to determine the origin of soil C and to infer changes to soil C cycling patterns after forest clearing. Soil bulk density increased under pasture; these increases were significant in 6 of 18 pastures examined. Changes in C stocks to a depth of 30 cm under pasture ranged from a loss of 0.72 kg/m2 to an increase of 1.77 kg/m2. Soil C stocks increased in 14 of 18 pastures, but these increases were significant in only 4 pastures. Changes in soil N stocks to a depth of 30 cm ranged from a loss of 0.25 kg/m2 to a gain of 0.23 kg/m2 and showed a similar pattern to C, except in one site where we measured significant N loss. Five of 18 pastures accumulated significant amounts of N, and one pasture lost a significant amount of N. Soil δ13C values were greater in pastures than in the original forests, and δ13C values increased with a longer time under C4 pasture vegetation. Bulk density increases were greater on soils with higher clay contents. Carbon accumulation increased with pasture age but was independent of soil texture. Soil C increases to a depth of 30 cm of up to 1.77 kg/m2 amounted to an increase of >50% of the original soil C stock and represented up to 12% of the C in the biomass of forest vegetation. In contrast, changes to soil N stocks in the range of 0.25 kg/m2 approximately equaled the N stock in the original forest vegetation. Our results indicated that when site history was controlled by considering only pastures formed directly from cleared forest, C and N accumulation was the dominant trend in pasture soils. Absence of a correlation between C and N accumulation and soil texture suggested that site history and management may be more important than soil type as determinants of the direction and magnitude of changes in soil C and N stocks.

Nitrogen dynamics in soils of forests and active pastures in the western Brazilian Amazon Basin

To investigate the influence of forest conversion to pasture on soil N transformations, we compared soil inorganic-N pools and net mineralization and nitrification rates along two chronosequences of upland (terra firme) forest and pastures ranging in age from 4 to 82 years in the state of Rondonia in the western Brazilian Amazon Basin. Forest and pasture soils had similar total extractable inorganic-N pools at 0–5 and 5–10 cm depths. Ammonium-N and NO3−N pools were of similar magnitude in forest soils (2–10 μg N g−1 dry soil), while NH4+N dominated pasture soil inorganic-N pools. Annual average net N mineralization rates for the two chronosequences at 0–5 cm depth in the forests were 1.31–1.88 μg N g−1 d.s. d−1 and exceeded the annual average net N mineralization rates measured in pastures of −0.11-0.02 μg N g−1 d.s. d−1. Annual average net nitrification rates at 0–5 cm depth in forest (1.09–1.46 μg N g−1 d.s. d−1) were also higher than in pastures (0.24–0.25 μg N g−1 d.s. d−1). Pasture soils had lower net N mineralization and net nitrification rates than forest soils even though they had approximately equal or higher total C and total N content. Pasture age did not affect NH4+N pools or net nitrification rates, but decreased NO3−N pools and net N mineralization rates. Net N mineralization rate was unaffected by soil moisture, but net nitrification rate decreased at higher soil moisture. Higher net mineralization and nitrification rates in forest soils suggest a higher potential for NO3−N losses either through leaching or gaseous emissions from intact forests compared with established pastures.