Joseph C. von Fischer

Global patterns of terrestrial biological nitrogen (N2) fixation in natural ecosystems

Human activities have clearly caused dramatic alterations of the terrestrial nitrogen cycle, and analyses of the extent and effects of such changes are now common in the scientific literature. However, any attempt to evaluate N cycling processes within ecosystems, as well as anthropogenic influences on the N cycle, requires an understanding of the magnitude of inputs via biological nitrogen fixation (BNF). Although there have been many studies addressing the microbiology, physiology, and magnitude of N fixation at local scales, there are very few estimates of BNF over large scales. We utilized >100 preexisting published estimates of BNF to generate biome- and global-level estimates of biological N fixation. We also used net primary productivity (NPP) and evapotranspiration (ET) estimates from the Century terrestrial ecosystem model to examine global relationships between these variables and BNF as well as to compare observed and Century-modeled BNF. Our data-based estimates showed a strong positive relationship between ecosystem ET and BNF, and our analyses suggest that while the models simple relationships for BNF predict broad scale patterns, they do not capture much of the variability or magnitude of published rates. Patterns of BNF were also similar to patterns of ecosystem NPP. Our “best estimate” of potential nitrogen fixation by natural ecosystems is ∼195 Tg N yr−1, with a range of 100–290 Tg N yr−1. Although these estimates do not account for the decrease in natural N fixation due to cultivation, this would not dramatically alter our estimate, as the greatest reductions in area have occurred in systems characterized by relatively low rates of N fixation (e.g., grasslands). Although our estimate of BNF in natural ecosystems is similar to previously published estimates of terrestrial BNF, we believe that this study provides a more documented, constrained estimate of this important flux.

THERMODYNAMIC CONSTRAINTS ON NITROGEN TRANSFORMATIONS AND OTHER BIOGEOCHEMICAL PROCESSES AT SOIL-STREAM INTERFACES

There is much interest in biogeochemical processes that occur at the interface between soils and streams since, at the scale of landscapes, these habitats may function as control points for fluxes of nitrogen (N) and other nutrients from terrestrial to aquatic ecosystems. Here we examine whether a thermodynamic perspective can enhance our mechanistic and predictive understanding of the biogeochemical function of soil-stream interfaces, by considering how microbial communities interact with variations in supplies of electron donors and acceptors. Over a two-year period we analyzed >1400 individual samples of subsurface waters from networks of sample wells in riparian wetlands along Smith Creek, a first-order stream draining a mixed forested-agricultural landscape in southwestern Michigan, USA. We focused on areas where soil water and ground water emerged into the stream, and where we could characterize subsurface flow paths by measures of hydraulic head and/or by in situ additions of hydrologic tracers. We found strong support for the idea that the biogeochemical function of soil-stream interfaces is a predictable outcome of the interaction between microbial communities and supplies of electron donors and acceptors. Variations in key electron donors and acceptors (NO 3 - , N 2 O, NH 4 + , SO 4 2- , CH 4 , and dissolved organic carbon [DOC]) closely followed predictions from thermodynamic theory. Transformations of N and other elements resulted from the response of microbial communities to two dominant hydrologic flow paths: (1) horizontal flow of shallow subsurface waters with high levels of electron donors (i.e., DOC, CH 4 , and NH 4 + ), and (2) near-stream vertical upwelling of deep subsurface waters with high levels of energetically favorable electron acceptors (i.e., NO 3 - , N 2 O, and SO 4 2- ). Our results support the popular notion that soil-stream interfaces can possess strong potential for removing dissolved N by denitrification. Yet in contrast to prevailing ideas, we found that denitrification did not consume all NO 3 - that reached the soil-stream interface via subsurface flow paths. Analyses of subsurface N chemistry and natural abundances of δ 15 N in NO 3 - and NH 4 + suggested a narrow near-stream region as functionally the most important location for NO 3 - consumption by denitrification. This region was characterized by high throughput of terrestrially derived water, by accumulation of dissolved NO 3 - and N 2 O, and by low levels of DOC. Field experiments supported our hypothesis that the sustained ability for removal of dissolved NO 3 - and N 2 O should be limited by supplies of oxidizable carbon via shallow flowpaths. In situ additions of acetate, succinate, and propionate induced rates of NO 3 - removal (∼1.8 g N.m -2 .d -1 ) that were orders of magnitude greater than typically reported from riparian habitats. We propose that the immediate near-stream region may be especially important for determining the landscape-level function of many riparian wetlands. Management efforts to optimize the removal of NO 3 - by denitrification ought to consider promoting natural inputs of oxidizable carbon to this near-stream region.

THERMODYNAMIC CONSTRAINTS ON NITROGENTRANSFORMATIONS AND OTHER BIOGEOCHEMICALPROCESSES AT SOIL–STREAM INTERFACES

There is much interest in biogeochemical processes that occur at the interface between soils and streams since, at the scale of landscapes, these habitats may function as control points for fluxes of nitrogen (N) and other nutrients from terrestrial to aquatic ecosystems. Here we examine whether a thermodynamic perspective can enhance our mechanistic and predictive understanding of the biogeochemical function of soil–stream interfaces, by considering how microbial communities interact with variations in supplies of electron donors and acceptors. Over a two-year period we analyzed >1400 individual samples of subsurface waters from networks of sample wells in riparian wetlands along Smith Creek, a first-order stream draining a mixed forested–agricultural landscape in southwestern Michigan, USA. We focused on areas where soil water and ground water emerged into the stream, and where we could characterize subsurface flow paths by measures of hydraulic head and/or by in situ additions of hydrologic tracers. We found strong support for the idea that the biogeochemical function of soil–stream interfaces is a predictable outcome of the interaction between microbial communities and supplies of electron donors and acceptors. Variations in key electron donors and acceptors (NO3−, N2O, NH4+, SO42−, CH4, and dissolved organic carbon [DOC]) closely followed predictions from thermodynamic theory. Transformations of N and other elements resulted from the response of microbial communities to two dominant hydrologic flow paths: (1) horizontal flow of shallow subsurface waters with high levels of electron donors (i.e., DOC, CH4, and NH4+), and (2) near-stream vertical upwelling of deep subsurface waters with high levels of energetically favorable electron acceptors (i.e., NO3−, N2O, and SO42−). Our results support the popular notion that soil–stream interfaces can possess strong potential for removing dissolved N by denitrification. Yet in contrast to prevailing ideas, we found that denitrification did not consume all NO3− that reached the soil–stream interface via subsurface flow paths. Analyses of subsurface N chemistry and natural abundances of δ15N in NO3− and NH4+ suggested a narrow near-stream region as functionally the most important location for NO3− consumption by denitrification. This region was characterized by high throughput of terrestrially derived water, by accumulation of dissolved NO3− and N2O, and by low levels of DOC. Field experiments supported our hypothesis that the sustained ability for removal of dissolved NO3− and N2O should be limited by supplies of oxidizable carbon via shallow flowpaths. In situ additions of acetate, succinate, and propionate induced rates of NO3− removal (∼1.8 g N·m−2·d−1) that were orders of magnitude greater than typically reported from riparian habitats. We propose that the immediate near-stream region may be especially important for determining the landscape-level function of many riparian wetlands. Management efforts to optimize the removal of NO3− by denitrification ought to consider promoting natural inputs of oxidizable carbon to this near-stream region.

NITROGEN TRANSFORMATIONS AND NO3− REMOVAL AT A SOIL–STREAM INTERFACE: A STABLE ISOTOPE APPROACH

The natural removal of NO3− by denitrification within riparian zones of streams and rivers is an area of considerable interest owing to its potential to minimize the impacts of excess anthropogenic loadings. In this study we utilize natural variations in stable N isotopic compositions of NO3− and NH4+ within a transect of shallow wells extending 4 m inland from Smith Creek, a southwestern Michigan stream, to provide insight into microbial processes and the extent of NO3− removal within a soil–stream interface. Within this region three water masses with unique biogeochemical characteristics intersect: a shallow flow rich in NH4+ and dissolved organic carbon (DOC), a deep groundwater mass rich in NO3− but depleted in DOC, and stream water low in NO3−, NH4+, and DOC. N isotope values for NO3− within the well transect were highly variable (−7.7–34.1‰) and reflected intense microbial activity within this narrow region. Isotopic variation was primarily controlled by upwelling of deep groundwater near the stream...

In situ measures of methanotroph activity in upland soils: A reaction‐diffusion model and field observation of water stress

[1] Laboratory assays of methanotroph activity in upland (i.e., well-drained, oxic) ecosystems alter soil physical structure and weaken inference about environmental controls of their natural behavior. To overcome these limitations, we developed a chamber-based approach to quantify methanotroph activity in situ on the basis of measures of soil diffusivity (from additions of an inert tracer gas to the chamber headspace), methane concentration change, and analysis of results with a reaction-diffusion model. The analytic solution to this model predicts that methane consumption rates are equally sensitive to changes in methanotroph activity and diffusivity, but that doubling either of these parameters leads to only a p 2 increase in consumption. With a series of simulations, we generate guidelines for field deployments and show that the approach is robust to plausible departures from assumptions. We applied the approach on a dry grassland in north central Colorado. Our model closely fit measured changes in methane concentrations, indicating that we had accurately characterized the biophysical processes underlying methane uptake. Field patterns showed that, over a 7-week period, soil moisture fell from 38% to 15% water-filled pore spaces, and diffusivity doubled as the larger soil pores drained of water. However, methane uptake rates fell by � 40%, following a 90% decrease in methanotroph activity, suggesting that the decline in methanotroph activity resulted from water stress to methanotrophs. We anticipate that future application of this approach over longer timescales and on more diverse field sites has potential to provide important insights into the ecology of methanotrophs in upland soils.

Controls on soil methane fluxes: Tests of biophysical mechanisms using stable isotope tracers

methanogenic mineralization pathways. Application of a new, nondisruptive, 13 CH4 isotope pool dilution technique permitted us to evaluate these mechanisms by distinguishing gross methane fluxes through both productive and consumptive pathways. We quantified each of these pathways in surface soils across broad moisture gradients in tropical montane environments in the Hawaiian Islands and temperate ecosystems in the northeastern United States. We found only limited support for the consumption control hypothesis because consumption was only important in dry soils. We also failed to find support for the carbon supply hypothesis, in that rates of carbon mineralization did not explain the observed variability in net fluxes across landscapes. Rather, dramatic differences in methane production, and thus emission, depended on surprisingly small diversions of soil carbon flow from nonmethanogenic to methanogenic pathways: on average, soils were a net source of methane to the atmosphere if more than 0.04% of total carbon mineralization passed through methanogenic pathways. We infer that fine-scale heterogeneity of soil redox status is critical for regulating soil methane fluxes.

Contrasting above‐ and belowground sensitivity of three Great Plains grasslands to altered rainfall regimes

Intensification of the global hydrological cycle with atmospheric warming is expected to increase interannual variation in precipitation amount and the frequency of extreme precipitation events. Although studies in grasslands have shown sensitivity of aboveground net primary productivity (ANPP) to both precipitation amount and event size, we lack equivalent knowledge for responses of belowground net primary productivity (BNPP) and NPP. We conducted a 2-year experiment in three US Great Plains grasslands--the C4-dominated shortgrass prairie (SGP; low ANPP) and tallgrass prairie (TGP; high ANPP), and the C3-dominated northern mixed grass prairie (NMP; intermediate ANPP)--to test three predictions: (i) both ANPP and BNPP responses to increased precipitation amount would vary inversely with mean annual precipitation (MAP) and site productivity; (ii) increased numbers of extreme rainfall events during high-rainfall years would affect high and low MAP sites differently; and (iii) responses belowground would mirror those aboveground. We increased growing season precipitation by as much as 50% by augmenting natural rainfall via (i) many (11-13) small or (ii) fewer (3-5) large watering events, with the latter coinciding with naturally occurring large storms. Both ANPP and BNPP increased with water addition in the two C4 grasslands, with greater ANPP sensitivity in TGP, but greater BNPP and NPP sensitivity in SGP. ANPP and BNPP did not respond to any rainfall manipulations in the C3 -dominated NMP. Consistent with previous studies, fewer larger (extreme) rainfall events increased ANPP relative to many small events in SGP, but event size had no effect in TGP. Neither system responded consistently above- and belowground to event size; consequently, total NPP was insensitive to event size. The diversity of responses observed in these three grassland types underscores the challenge of predicting responses relevant to C cycling to forecast changes in precipitation regimes even within relatively homogeneous biomes such as grasslands.

Late Quaternary temperature record from buried soils of the North American Great Plains

We present the first comprehensive late Quaternary record of North American Great Plains temperature by assessing the behavior of the stable isotopic composition (δ 13 C) of buried soils. After examining the relationship between the δ 13 C of topsoil organic matter and July temperature from 61 native prairies within a latitudinal range of 46°–38°N, we applied the resulting regression equation to 64 published δ 13 C values from buried soils of the same region to construct a temperature curve for the past 12 k.y. Estimated temperatures from 12 to 10 ka (1 k.y. = 1000 14 C yr B.P.) fluctuated with a periodicity of ∼1 k.y. with two cool excursions between −4.5 and −3.5 °C and two warmer excursions between −1 and 0 °C, relative to modern. Early Holocene temperatures from ca. 10–7.5 ka were −1.0 to −2.0 °C before rising to +1.0 °C in the middle Holocene between 6.0 and 4.5 ka. After a cool interlude from 4.2 to 2.6 ka, when temperatures dropped to slightly below modern, another warm interval ensued from 2.6 to 1 ka as temperatures increased to ∼+0.5 °C. A final decline in temperature to below modern occurred beginning ca. 0.5 ka. Cooler than present temperatures in the Great Plains indicate telecommunications with cool-water episodes in the Gulf of Mexico and North Atlantic potentially governed by a combination of glacial meltwater pulses and low solar irradiance.

Chronology reconstruction for the disturbed bottom section of the GISP2 and the GRIP ice cores: Implications for Termination II in Greenland

and d 18 Oatm in the trapped gases. Our reconstructed ages for basal ice samples are based on comparison of published measurements of CH4 and d 18 Oatm from the disturbed section of the GRIP and GISP2 cores with the same properties in the Vostok ice core. NGRIP d 18 Oice values are also used to constrain the chronology during the end of marine isotope stage 5e. For each sample, we assign an age that represents the unique or most probable time of gas trapping, given its gas composition. Of 157 samples with CH4 and d 18 Oatm data, 10 give unique ages. Twenty-five newly measured values of the triple isotope composition of O2 from the disturbed section of the GISP2 core add a third time-dependent gas property that agrees with our reconstruction. Our reconstruction supports earlier conclusions of Landais et al. (2003) that the disturbed section primarily includes ice from the last interglacial (MIS 5e) and the penultimate glacial period (MIS 6). The oldest ice in the basal layer of GISP2 and GRIP has an age � 237 ka. The climate history we derive suggests that the last interglacial at Summit, Greenland, around 127 ka was slightly warmer than the current interglacial period. Reduction of various ion concentrations in ice and thickening of the ice sheet during Termination II was similar to that in Termination I.

Multigas leakage correction in static environmental chambers using Sulfur hexafluoride and Raman spectroscopy

In static environmental chamber experiments, the precision of gas flux measurements can be significantly improved by a thorough gas leakage correction to avoid under- or overestimation of biological activity such as respiration or photosynthesis. Especially in the case of small biological net gas exchange rates or gas accumulation phases during long environmental monitoring experiments, gas leakage fluxes could distort the analysis of the biogenic gas kinetics. Here we propose and demonstrate a general protocol for online correction of diffusion-driven gas leakage in plant chambers by simultaneous quantification of the inert tracer sulfur hexafluoride (SF6) and the investigated biogenic gases using enhanced Raman spectroscopy. By quantifying the leakage rates of carbon dioxide (CO2), methane (CH4), and hydrogen (H2) simultaneously with SF6 in the test chamber, their effective diffusivity ratios of approximately 1.60, 1.96, and 5.65 were determined, each related to SF6. Because our experiments suggest that the effective diffusivity ratios are reproducible for an individual static environmental chamber, even under varying concentration gradients and slight changes of the chamber sealing, an experimental method to quantify gas leakage fluxes by using effective diffusivity ratios and SF6 leakage fluxes is proposed. The method is demonstrated by quantifying the CO2 net exchange rate of a plant-soil ecosystem (Mirabilis jalapa). By knowing the effective chamber diffusivity ratio CO2/SF6 and the measured SF6 leakage rate during the experiment, the leakage contribution to the total CO2 exchange rate could be calculated and the biological net CO2 concentration change within the chamber atmosphere determined.