Stephen K. Hamilton

Stream denitrification across biomes and its response to anthropogenic nitrate loading

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.

Have we overemphasized the role of denitrification in aquatic ecosystems? A review of nitrate removal pathways

The removal of nitrogen (N) in aquatic ecosystems is of great interest because excessive nitrate in groundwater and surface water is a growing problem. High nitrate loading degrades water quality and is linked to eutrophication and harmful algal blooms, especially in coastal marine waters. Past research on nitrate removal processes has emphasized plant or microbial uptake (assimilation) or respiratory denitrification by bacteria. The increasing application of stable isotopes and other tracer techniques to the study of nitrate removal has yielded a growing body of evidence for alternative, microbially mediated processes of nitrate transformation. These include dissimilatory (the reduction of nitrogen into other inorganic compounds, coupled to energy producing processes) reduction of nitrate to ammonium (DNRA), chemoautotrophic denitrification via sulfur or iron oxidation, and anaerobic ammonium oxidation (anammox), as well as abiotic nitrate removal processes. Here, we review evidence for the importance of...

Nitrous oxide emission from denitrification in stream and river networks

Nitrous oxide (N2O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N2O via microbial denitrification that converts N to N2O and dinitrogen (N2). The fraction of denitrified N that escapes as N2O rather than N2 (i.e., the N2O yield) is an important determinant of how much N2O is produced by river networks, but little is known about the N2O yield in flowing waters. Here, we present the results of whole-stream 15N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N2O at rates that increase with stream water nitrate (NO3−) concentrations, but that <1% of denitrified N is converted to N2O. Unlike some previous studies, we found no relationship between the N2O yield and stream water NO3−. We suggest that increased stream NO3− loading stimulates denitrification and concomitant N2O production, but does not increase the N2O yield. In our study, most streams were sources of N2O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y−1 of anthropogenic N inputs to N2O in river networks, equivalent to 10% of the global anthropogenic N2O emission rate. This estimate of stream and river N2O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.

Foodweb analysis of the Orinoco floodplain based on production estimates and stable isotope data

Stable isotope data and indirect methods for estimating consumer production were used in a foodweb analysis for the Orinoco River floodplain, Venezuela. The ratio of annual production to mean annual biomass (Pa/B̄) was estimated from mass at maturity for all major categories of consumers. Field data on B̄ then were used to estimate Pa for each major category. Carbon sources for all categories of consumers were identified through δ13C analysis, and trophic shifts in δ15N were used in assigning trophic levels to consumers. The ultimate C source for both invertebrates and fish was algae (phytoplankton and periphyton), even though macrophytes and litterfall from the floodplain forest composed 98% of potentially available C. Production of invertebrate consumers (benthos, rhizofauna, and zooplankton), which occurred almost entirely through 1st-level consumption, was estimated as 14 g C m−2 y−1. Fish, which produced 11 g C m−2 y−1, showed a mean δ15N trophic shift of 1.8 units above the algal C source, suggesting that ∼20% of fish production was directly accountable to consumption of algal C (1st-level consumption), and that most of the remainder was attributable to 1st-level carnivory. Data on production and trophic shifts implied trophic efficiency of 5% for invertebrates and 20% for fish. Although the food web is focused on a very small fraction of potentially available primary C (algae), this C source can account quantitatively for the observed production of both invertebrates and fish. The food web showed marked trophic compression (large consumers supported by trophic levels 1 and 2), which is the only means by which high production of large consumers can be sustained on a small fraction of the potentially available C.

Ecological Determinism on the Orinoco Floodplain

ll oo dplains of l a r ge rivers are among the most produ ctive of eco s ys tem s ; they are also among the f i rst to be altered by econ omic devel opm ent and population growth.Floodplains are vulnerable not only to direct agents of change, such as agriculture and settlement, but also to changes in the hydrology or quality of water in the rivers to wh i ch they are con n ected . Thu s , even floodplains not yet clear ed or diked may show drastic ecological change caused by shifts in flo oding regime, sediment transport, or n utrient budgets as a result of human activities upstream. Because the floodplains of large rivers are vulnerable to a diverse array of anthropogenic influences, their natural functions cannot be obse rved or documented in an y region of dense population or where natural resources and a gri c u l tu ral po ten tial have been ex p l oi ted ex ten s ively. Nearly all of the major floodplain rivers in the Northern Hemisphere,and many in the Southern Hemisphere,have been le veed or impounded f or na vigation, agriculture, power g eneration, or flo od c ontrol (P ower et al. 1995, Vitousek et al.1997).Only large rivers of the far north and in some parts of the tropics now offer a view of large floodplains as they existed b efore human intervention (Cushing and Cummings 1995). Information on the ecological characteristics o f these few floodplains is the only practical means of understanding the natural functioning of floodplains, and is p robably the most useful basis for planning the restoration or protection of floodplains. All large floodplains give the impression of great ecological complexity. The combination of constant physical change, which is driven by the rise and fall of the river, spatial heterogeneity of vegetation reflecting varied depth and duration of inundation, and longitudinal gradients of f l oodplains proj ects intract a ble mu l ti d i m en s i on a l i ty. Closer inspection shows, however, a sur prising degree o f order and repetition in the physical and chemical environment, which in turn suggests the possibility of order and predictability in the biotic components of the system. The most feasible candidate for a test of this idea is a large floodplain whose physical characteristics remain essentially natural.

Passive microwave observations of inundation area and the area/stage relation in the Amazon River floodplain

Inundation patterns in Amazon River floodplains are revealed by analysis of the 37 GHz polarization difference observed by the Scanning Multichannel Microwave Radiometer on the Nimbus-7 satellite. Flooded area is estimated at monthly intervals for January 1979 through August 1987 using mixing models that account for the major landscape units with distinctive microwave emission characteristics. Results are presented separately for 12 longitudinal reaches along the Amazon River main stem in Brazil as well as for three major tributaries (the Jurua, Purus and Madeira rivers). The total area along the Amazon River main stem that was flooded (including both floodplain and open water) varied between 19000 and 91 000km2. The correlation between flooded area and river stage is used to develop a predictive relationship and reconstruct regional inundation patterns in the floodplain of the Amazon River main stem over the past 94 years of stage records (1903-1996). The mean flooded area along the Amazon River during t...

Carbon debt of Conservation Reserve Program (CRP) grasslands converted to bioenergy production

Over 13 million ha of former cropland are enrolled in the US Conservation Reserve Program (CRP), providing well-recognized biodiversity, water quality, and carbon (C) sequestration benefits that could be lost on conversion back to agricultural production. Here we provide measurements of the greenhouse gas consequences of converting CRP land to continuous corn, corn–soybean, or perennial grass for biofuel production. No-till soybeans preceded the annual crops and created an initial carbon debt of 10.6 Mg CO2 equivalents (CO2e)·ha−1 that included agronomic inputs, changes in C stocks, altered N2O and CH4 fluxes, and foregone C sequestration less a fossil fuel offset credit. Total debt, which includes future debt created by additional changes in soil C stocks and the loss of substantial future soil C sequestration, can be constrained to 68 Mg CO2e·ha−1 if subsequent crops are under permanent no-till management. If tilled, however, total debt triples to 222 Mg CO2e·ha−1 on account of further soil C loss. Projected C debt repayment periods under no-till management range from 29 to 40 y for corn–soybean and continuous corn, respectively. Under conventional tillage repayment periods are three times longer, from 89 to 123 y, respectively. Alternatively, the direct use of existing CRP grasslands for cellulosic feedstock production would avoid C debt entirely and provide modest climate change mitigation immediately. Incentives for permanent no till and especially permission to harvest CRP biomass for cellulosic biofuel would help to blunt the climate impact of future CRP conversion.

The biogeochemistry of bioenergy landscapes: carbon, nitrogen, and water considerations.

The biogeochemical liabilities of grain-based crop production for bioenergy are no different from those of grain-based food production: excessive nitrate leakage, soil carbon and phosphorus loss, nitrous oxide production, and attenuated methane uptake. Contingent problems are well known, increasingly well documented, and recalcitrant: freshwater and coastal marine eutrophication, groundwater pollution, soil organic matter loss, and a warming atmosphere. The conversion of marginal lands not now farmed to annual grain production, including the repatriation of Conservation Reserve Program (CRP) and other conservation set-aside lands, will further exacerbate the biogeochemical imbalance of these landscapes, as could pressure to further simplify crop rotations. The expected emergence of biorefinery and combustion facilities that accept cellulosic materials offers an alternative outcome: agricultural landscapes that accumulate soil carbon, that conserve nitrogen and phosphorus, and that emit relatively small amounts of nitrous oxide to the atmosphere. Fields in these landscapes are planted to perennial crops that require less fertilizer, that retain sediments and nutrients that could otherwise be transported to groundwater and streams, and that accumulate carbon in both soil organic matter and roots. If mixed-species assemblages, they additionally provide biodiversity services. Biogeochemical responses of these systems fall chiefly into two areas: carbon neutrality and water and nutrient conservation. Fluxes must be measured and understood in proposed cropping systems sufficient to inform models that will predict biogeochemical behavior at field, landscape, and regional scales. Because tradeoffs are inherent to these systems, a systems approach is imperative, and because potential biofuel cropping systems and their environmental contexts are complex and cannot be exhaustively tested, modeling will be instructive. Modeling alternative biofuel cropping systems converted from different starting points, for example, suggests that converting CRP to corn ethanol production under conventional tillage results in substantially increased net greenhouse gas (GHG) emissions that can be only partly mitigated with no-till management. Alternatively, conversion of existing cropland or prairie to switchgrass production results in a net GHG sink. Outcomes and policy must be informed by science that adequately quantifies the true biogeochemical costs and advantages of alternative systems.

Carbon and nitrogen stoichiometry and nitrogen cycling rates in streams

Stoichiometric analyses can be used to investigate the linkages between N and C cycles and how these linkages influence biogeochemistry at many scales, from components of individual ecosystems up to the biosphere. N-specific NH4+ uptake rates were measured in eight streams using short-term 15N tracer additions, and C to N ratios (C:N) were determined from living and non-living organic matter collected from ten streams. These data were also compared to previously published data compiled from studies of lakes, ponds, wetlands, forests, and tundra. There was a significant negative relationship between C:N and N-specific uptake rate; C:N could account for 41% of the variance in N-specific uptake rate across all streams, and the relationship held in five of eight streams. Most of the variation in N-specific uptake rate was contributed by detrital and primary producer compartments with large values of C:N and small values for N-specific uptake rate. In streams, particulate materials are not as likely to move downstream as dissolved N, so if N is cycling in a particulate compartment, N retention is likely to be greater. Together, these data suggest that N retention may depend in part on C:N of living and non-living organic matter in streams. Factors that alter C:N of stream ecosystem compartments, such as removal of riparian vegetation or N fertilization, may influence the amount of retention attributed to these ecosystem compartments by causing shifts in stoichiometry. Our analysis suggests that C:N of ecosystem compartments can be used to link N-cycling models across streams.

Beyond carbon and nitrogen: how the microbial energy economy couples elemental cycles in diverse ecosystems

Microbial metabolism couples elemental reactions, driving biogeochemical cycles. Assimilatory coupling of elemental cycles, such as the carbon (C), nitrogen (N), and phosphorus cycles, occurs when these elements are incorporated into biomass or released through its decomposition. In addition, many microbes are capable of dissimilatory coupling, catalyzing energy-releasing reactions linked to transformations in the oxidation state of elements, and releasing the transformed elements to the environment. Different inorganic elements provide varying amounts of energy yield, and the interaction of these processes creates a microbial energy economy. Dissimilatory reactions involving C, N, iron, and sulfur provide particularly important examples where microbially mediated oxidation–reduction (redox) transformations affect nutrient availability for net primary production, greenhouse-gas emissions, levels of contaminants and natural toxic factors, and other ecosystem dynamics. Recent discoveries of previously unrec...