Jerry M. Melillo

Nitrogen and lignin control of hardwood leaf litter decomposition dynamics

The effects of initial nitrogen and lignin contents of six species of hardwood leaves on their decomposition dynamics were studied at the Hubbard Brook Experimental Forest. Rate con- stants (k) for annual leaf mass loss ranged from -0.08 to -0.47. The rate constants (k) had a negative linear correlation (r2 = .89) with the ratio of initial lignin concentration to initial nitrogen concentra- tion. Decomposition dynamics of the litter materials were described by inverse linear relationships between the percentage of original mass remaining and the nitrogen concentration in the residual material. Initial lignin concentration was highly correlated (r2 = .93) with the slope of the inverse linear relationship for each of the litter types.

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.

Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems

Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmospheric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems. Atmospheric carbon dioxide and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s. This recent sink can be largely attributed to northern extratropical areas, and is roughly split between North America and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrowth on abandoned agricultural land and fire prevention, in addition to responses to environmental changes, such as longer growing seasons, and fertilization by carbon dioxide and nitrogen. Nevertheless, there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes.

Changes in the Carbon Content of Terrestrial Biota and Soils between 1860 and 1980: A Net Release of CO"2 to the Atmosphere

Changes in land use over the past two centuries have caused a significant release of CO2 to the atmosphere from the terrestrial biota and soils. An analysis of this release is based on amounts of organic carbon within an ecosystem following changes such as harvest of forests; it is also based on rates of changes, such as conversion of forest to agriculture, deduced from agricultural and forestry statistics. A model is used to calculate the net amount of carbon stored or released each year by the biota and soils of 69 regional ecosystems. Some of the changes, such as afforestation, the growth of harvested forests, and buildup of soil organic matter, result in a storage of carbon; others, such as harvest of forests and increase in pasture and agricultural areas, result in a loss of carbon to the atmosphere. According to this analysis, there has been a net release of carbon from terrestrial ecosystems worldwide since at least 1860. Until 1960, the annual release was greater than release of carbon from fossil fuels. The total net release of carbon from terrestrial ecosystems since 1860 is estimated to have been 180 x 1015 g (a range of estimates is 135-228 x 1015 g). The estimated net release of carbon in 1980 was 1.8-4.7 x 1015 g; for the 22 yr since 1958 the release of C was 38-76 x 1015 g. The ranges reflect the differences among various estimates of forest biomass, soil carbon, and agricultural clear- ing. Improvements in the data on the clearing of tropical forests alone would reduce the range of estimates for 1980 by almost 60%. Estimates of the other major terms in the global carbon budget, the atmospheric increase in C02, the fossil fuel release of C02, and the oceanic uptake of C02, are all subject to uncertainties. The combined errors in these estimates are large enough that the global carbon budget appears balanced if the low estimate for the biotic release of carbon given above is used (1.8 x 1015 g released in 1980) with the higher estimates of oceanic uptake. If higher estimates for biotic release are used, then the carbon budget does not balance, and the estimates of oceanic uptake or of other factors require revision.

Carbon balance of the terrestrial biosphere in the twentieth century: Analyses of CO2, climate and land use effects with four process-based ecosystem models

The concurrent effects of increasing atmospheric CO2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models. Over the long-term (1920-1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO2. During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr(-1), which is within the uncertainty of analysis based on CO2 and O-2 budgets. Three of the four models indicated tin accordance with O-2 evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long-term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the El Nino/Southern Oscillation (ENSO)-scale variability in the atmospheric CO2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO2 suggested that the observed trend may be a consequence of CO2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process-based simulation of historical terrestrial carbon include (1) the transfer of insight gained from stand-level process studies to improve the sensitivity of simulated carbon storage responses to changes in CO2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the Earth system.

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.

Carbon and nitrogen dynamics along the decay continuum: Plant litter to soil organic matter

Decay processes in an ecosystem can be thought of as a continuum beginning with the input of plant litter and leading to the formation of soil organic matter. As an example of this continuum, we review a 77-month study of the decay of red pine (Pinus resinosa Ait.) needle litter. We tracked the changes in C chemistry and the N pool in red pine (Pinus resinosa Ait.) needle litter during the 77-month period using standard chemical techniques and stable isotope, analyses of C and N.Mass loss is best described by a two-phase model: an initial phase of constant mass loss and a phase of very slow loss dominated by degradation of ‘lignocellulose’ (acid soluble sugars plus acid insoluble C compounds). As the decaying litter enters the second phase, the ratio of lignin to lignin and cellulose (the lignocellulose index, LCI) approaches 0.7. Thereafter, the LCI increases only slightly throughout the decay continuum indicating that acid insoluble materials (‘lignin’) dominate decay in the latter part of the continuum.Nitrogen dynamics are also best described by a two-phase model: a phase of N net immobilization followed by a phase of N net mineralization. Small changes in C and N isotopic composition were observed during litter decay. Larger changes were observed with depth in the soil profile.An understanding of factors that control ‘lignin’ degradation is key to predicting the patterns of mass loss and N dynamics late in decay. The hypothesis that labile C is needed for ‘lignin’ degradation must be evaluated and the sources of this C must be identified. Also, the hypothesis that the availability of inorganic N slows ‘lignin’ decay must be evaluated in soil systems.

A Comparative Analysis of Potential Nitrification and Nitrate Mobility in Forest Ecosystems

The controls of potential nitrogen mineralization, nitrate production, and nitrate mo- bilization in a wide range of forest ecosystems were investigated through a combination of field and laboratory experiments. Trenched plot experiments were performed in 17 forests, and laboratory incubation studies of potential ammonium and nitrate production were made on soils from 14 of these sites. The site with the greatest potential for nitrate production in the laboratory was a New Hampshire northern hardwoods forest. Several other sites, including New Hampshire balsam fir, Indiana maple- beech, New Mexico aspen, and Oregon western hemlock forests, also had high potential nitrate production. All of these sites also had rapid nitrate movement to below the rooting zone following trenching in the field. Of nine processes which could be important in preventing or delaying solution losses of nitrate from disturbed forests, two appeared most important among the forests we examined. Low net nitrogen mineralization (caused by either nitrogen immobilization or low gross nitrogen mineralization) and lags in nitrification (probably caused by either low initial populations of nitrifying bacteria or the allelochemic inhibition of nitrification) were identified as important in several sites and in different regions. A direct relationship between the amount of nitrogen in annual litterfall and the proportion of forest floor nitrogen mineralized in laboratory incubations was observed, suggesting that refractory organic nitrogen compounds are produced in nitrogen-poor sites. An inverse relationship was found between the amount of nitrogen in litterfall in these and other sites and the carbon:nitrogen ratio of that litterfall, suggesting that the immobilization capacity of litter is increased in nitrogen-poor sites. The presence and length of lags in nitrification were inversely correlated with the mean concentration of mineral nitrogen in mineral soil. These patterns suggest that nitrogen retention within disturbed forest ecosystems can be caused by low nitrogen availability prior to disturbance.

Reconciling carbon-cycle concepts, terminology, and methods

Recent projections of climatic change have focused a great deal of scientific and public attention on patterns of carbon (C) cycling as well as its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric carbon dioxide (CO2). Net ecosystem production (NEP), a central concept in C-cycling research, has been used by scientists to represent two different concepts. We propose that NEP be restricted to just one of its two original definitions—the imbalance between gross primary production (GPP) and ecosystem respiration (ER). We further propose that a new term—net ecosystem carbon balance (NECB)—be applied to the net rate of C accumulation in (or loss from [negative sign]) ecosystems. Net ecosystem carbon balance differs from NEP when C fluxes other than C fixation and respiration occur, or when inorganic C enters or leaves in dissolved form. These fluxes include the leaching loss or lateral transfer of C from the ecosystem; the emission of volatile organic C, methane, and carbon monoxide; and the release of soot and CO2 from fire. Carbon fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However, even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological advances have led to a diversity of approaches to the measurement of C fluxes at different temporal and spatial scales. These approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully specifying the fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we can provide a less ambiguous framework for understanding and communicating recent changes in the global C cycle.

Global Warming and Terrestrial Ecosystems: A Conceptual Framework for Analysis

raise global mean temperature over the next century by 1.0–3.5 °C (Houghton et al. 1995, 1996). Ecologists from around the world have begun experiments to investigate the effects of global warming on terrestrial ecosystems, the aspect of global climate change that attracts the most public attention (Woodwell and McKenzie 1995, Walker and Steffen 1999). The effort to understand response to warming builds on a history of investigations of the effects of elevated CO 2 on plants and ecosystems (Koch and Mooney 1996, Schulze et al. 1999). There are important differences, however, between increases in atmospheric CO 2 and temperature change, both in the temporal and spatial patterns of change and in how they affect ecosystems. The scientists involved in temperature change research have had to face new technical and conceptual challenges in designing and interpreting their experiments (Schulze et al. 1999). In this paper we describe these challenges and present a conceptual framework for interpreting experimental results and predicting effects of warming on ecosystems.