Export of chloride after clear-cutting in the Hubbard Brook sandbox experiment

The objective of this study was to discern the source of higher than usual concentrations of chloride in drainage water collected from experimental forest plots after clear-cutting. When the sandbox experiments were initiated at the Hubbard Brook Experimental Forest Station three vegetation types were established: red pine, grass, and minimally vegetated (scattered lichens and bryophytes) as the bare control plot. After 15 years of growth the trees were cut down and above-ground biomass removed from the red pine sandbox. For several years prior to the cut, high concentrations ( 75 M) of dissolved Cl^– in drainage waters occurred in November/December. This is attributed to the buildup of rainfall-derived Cl^– due to evapotranspiration that depletes soil moisture to low levels resulting in a lack of drainage during this period. The excess Cl^– is quickly flushed out by subsequent drainage over a few weeks and Cl^– concentrations return to values characteristic of rainfall and throughfall. After the trees were removed in May, 1998, Cl^– continued to be leached from the system. The concentration of Cl^– peaked (175 M) in Sept. 1998 and did not return back to base level concentration until Dec. 1999. The Cl^– release pattern is distinctly different from that of dissolved NO₃ ^–, which peaked about one year later than Cl^–. An excess (over that of the control sandbox) of 78 g Cl^– was released in the 1.5 year period after clear-cut, showing that a large amount of leachable chloride is stored in the bulk soil/root/organic matter fraction. Lack of uptake by trees may be part of the reason for this chloride pulse. But an analysis of chloride content in roots and litter indicates that as much as 50% of the chloride leached from the sandbox may have come from the decaying roots and litter. Additional chloride may have been released from the soil organic matter by decomposition. The biochemical behavior of Cl^– in systems such as this should be evaluated before assuming Cl^– to be conservative for purposes of hydrological transport or soil weathering studies.     

Deforestation causes increased dissolved silicate losses in the Hubbard Brook Experimental Forest

Globally significant increases in the riverine delivery of nutrients and suspended particulate matter have occurred with deforestation. We report here significant increases in streamwater transport of dissolved silicate (DSi) following experimental forest harvesting at the Hubbard Brook Experimental Forest, NH, USA. The magnitude of the streamwater response varied with the type of disturbance with the highest DSi export fluxes occurring in the manipulations that left the most plant materials on the soil surface and disturbed the soil surface least. No measurable loss of amorphous silica (ASi) was detected from the soil profile; however, ASi was redistributed within the soil profile after forest disturbance. Mass‐balance calculations demonstrate that some fraction of the DSi exported must come from dissolution of ASi and export as DSi. Land clearance and the development of agriculture may result in an enhanced flux of DSi coupled with enhanced erosion losses of ASi contained in phytoliths.     

High Nitrate Retention during Winter in Soils of the Hubbard Brook Experimental Forest

Stream export of nitrogen (N) as nitrate (NO₃⁻; the most mobile form of N) from forest ecosystems is thought to be controlled largely by plant uptake of inorganic N, such that reduced demand for plant N during the non-growing season and following disturbances results in increased stream NO₃⁻ export. The roles of microbes and soils in ecosystem N retention are less clear, but are the dominant controls on N export when plant uptake is low. We used a mass balance approach to investigate soil N retention during winter (December through March) at the Hubbard Brook Experimental Forest by comparing NO₃⁻ inputs (atmospheric deposition), internal production (soil microbial nitrification), and stream output. We focused on months when plant N uptake is nearly zero and the potential for N export is high. Although winter months accounted for only 10–15% of annual net nitrification, soil NO₃⁻ production (0.8–1.0 g N m⁻² winter⁻¹) was much greater than stream export (0.03–0.19 N m⁻² winter⁻¹). Soil NO₃⁻ retention in two consecutive winters was high (96% of combined NO₃⁻ deposition and soil production; year 1) even following severe plant disturbance caused by an ice-storm (84%; year 2) We show that soil NO₃⁻ retention is surprisingly high even when N demand by plants is low. Our study highlights the need to better understand mechanisms of N retention during the non-growing season to predict how ecosystems will respond to high inputs of atmospheric N, disturbance, and climate change.     

Biogeochemical responses of two forest streams to a 2-month calcium addition

1. Calcium (Ca) has been lost from forest soils at the Hubbard Brook Experimental Forest (HBEF) because of decreased atmospheric input of Ca and high input of acid anions. Through time, this Ca loss has led to low streamwater Ca concentration and this change may affect stream ecosystem processes.
2. To test both the biogeochemical response of streams to increased calcium concentration and the role of streams in retaining calcium lost from soils, we added c. 120 μeq Ca L^?1 as CaCl₂ to two second‐order streams at HBEF for 2 months. One stream (buffered) also received an equivalent amount of NaHCO₃ to simulate the increase in pH and alkalinity if Ca were added with associated HCO₃^? ion. The other stream (unbuffered) received only CaCl₂. We collected water samples along a transect above and below the addition site at 11 dates: two before, seven during, and two after the addition.
3. Increase in pH in the buffered stream ranged from 5.6 to about 7.0 in the treated section. There was a net uptake of Ca on all sampling dates during the addition and these uptake rates were positively related to pH. Between 10 and 50% of the added Ca was taken up during the release in the 80‐m study reach. In the unbuffered stream, there was net uptake of Ca on only two dates, suggesting lower Ca uptake.
4. Water samples collected after the addition was stopped showed that a small fraction of the added Ca desorbed from sediments; the remainder was apparently in longer‐term storage in the sediments. No Ca desorbed from the stream sediments in the unbuffered stream, showing that sorption/desorption may be controlled by a pH‐induced increase in the number of exchange sites.
5. These streams appeared to be a significant sink for Ca over a 2‐month time scale, and thus, change in streamwater Ca during a year may be due to processing of Ca within the stream channel, as well as to changes in inputs from the catchment.     

Soil processes and sulfate loss at the Hubbard Brook Experimental Forest

The Hubbard Brook Ecosystem Study was designed to evaluate element flux and cycling in a northern hardwood forest and the effects of disturbance on these processes. In the original experiment, an entire watershed was deforested and regrowth was inhibited for three years using herbicides. Initial effects of the treatment included: elevated stream discharge, large increases in streamwater solute concentrations and elevated losses of those ions from the watershed. In contrast, streamwater concentrations and net ecosystem output of sulfate decreased in response to the treatment. During the post treatment period, the concentrations of most dissolved ions declined relative to a reference watershed while, again in contrast, sulfate concentrations increased relative to the reference. In this paper we develop a hypothesis which links acidification and sulfate adsorption processes in the soil to explain the observed trends in sulfate losses from the Hubbard Brook Experimental Forest.     

Nitrogen Dynamics in Ice Storm-Damaged Forest Ecosystems: Implications for Nitrogen Limitation Theory

Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO₃ ^–) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO₃ ^– concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO₃ ^- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO₃ ^-. At the regional scale, we observed high variability in the magnitude of NO₃ ^- losses: while six of the surveyed watersheds showed accelerated rates of NO₃ ^– loss, three did not. Moreover, in contrast to the strong linear relationship between NO₃ ^– loss and crown damage within HBEF watersheds [r ²: (W1 = 0.91, W6 = 0.85)], stream water NO₃ ^– concentrations were weakly related to crown damage (r ² = 0.17) across our regional sites. The efflux of NO₃ ^– associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO₃ ^– fluxes associated with forest harvesting. Because over one half of the entire years worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems. Present address: Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA.     

Stable sulfur isotopic biogeochemistry of the Hubbard Brook Experimental Forest, New Hampshire

In natural ecosystems, differences often exist in the relative abundanceof stable S isotopes (°³⁴S) that can provide clues as tothe source, nature, and cycling of S. Values of °³⁴S inprecipitation, throughfall, soils, soil solution, and stream waters weremeasured at the Hubbard Brook Experimental Forest (HBEF), New Hampshire.Values of °³⁴S in precipitation and throughfall weresimilar to each other but differed seasonally. Precipitation°³⁴S values were higher in the dormant season[°³⁴S = 5.9±0.6 (17)][Mean + SE(N)]than in the growing season [°³⁴S = 5.0±0.6(40)] but throughfall growing-season values were higher[°³⁴S = 5.6±0.6(68)] than for the dormantseason [°³⁴S = 4.9±0.7 (9)]. Different treespecies did not affect throughfall °³⁴S values. In soilsolution, °³⁴S values were higher in the growing season(°³⁴S = 8.9±2.8; 8.8±1.7;and 4.0±0.6 for Oa, Bh, and Bs horizons, respectively) thanin the dormant season (°³⁴S = 5.6±1.5;3.7±2.4; and 3.4±1.2 for Oa, Bh, and Bshorizons, respectively). These seasonal differences in°³⁴S were probably caused by biological isotopicfractionation. The °³⁴S values in streams were generally2 lower and more variable than those in precipitation andthroughfall, suggesting fractionation and/or different isotopic sources inthe soil.     

Nitrate accumulation in leaves of vegetation of a forested ecosystem receiving high amounts of atmospheric ammonium sulfate

Vegetation of an acid woodland, receiving an atmospheric ammonium input of about 3 kmol (40 kg N) per hectare per year, was analyzed on the content of organic nitrogen, ammonium and nitrate. A high nitrate content (50–320 μmol g⁻¹ dry weight) was found in bird-cherry, black-berry and bracken, whereas only low amounts (up to 2 μmol g⁻¹ dry weight) of nitrate were present in mountain-ash, hazel and the two dominant tree species oak and birch. The impact of this nitrate uptake and nitrate accumulation on soil pH and autotrophic nitrification is discussed.     

An Unexpected Nitrate Decline in New Hampshire Streams

Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition and as forest nutrient requirements decline with age. The latter theory was supported initially by measurements of stream NO₃ ⁻ concentration in old-growth and successional stands on Mount Moosilauke, New Hampshire (Vitousek and Reiners 1975; Bioscience 25:376–381). We resampled 28 of these and related streams to evaluate their response to 23 years of forest aggradation and chronic N deposition. Between 1973–74 and 1996–97, mean NO₃ ⁻ concentration in quarterly samples from Mount Moosilauke decreased by 71% (25 μmol/L), Ca²⁺ decreased by 24% (8 μmol/L), and Mg²⁺ decreased by 22% (5 μmol/L). Nitrate concentrations decreased in every stream in every season, but spatial patterns among streams persisted: Streams draining old-growth stands maintained higher NO₃ ⁻ concentrations than those draining successional stands. The cause of the NO₃ ⁻ decline is not evident. Nitrogen deposition has changed little, and although mechanisms such as insect defoliation and soil frost may contribute to the temporal patterns of nitrate loss, they do not appear to fully explain the NO₃ ⁻ decline across the region. Although the role of climate remains uncertain, interannual climate variation and its effects on biotic N retention may be responsible for the synchronous decrease in NO₃ ⁻ across all streams, overriding expected increases due to chronic N deposition and forest aging. Received 4 December 2001; accepted 30 May 2002.     

Spatial and temporal patterns of nitrogen concentrations in pristine and agriculturally-influenced prairie streams

Long-term data on nitrogen chemistry of streams draining Konza Prairie Biological Station (Konza), Kansas were analyzed to assess spatial and temporal patterns and examine the influence of agricultural activity on these patterns. Upland watersheds of Konza are predominantly tallgrass prairies, but agricultural fields and riparian forests border the lower reaches of the streams. We have up to 11 years of data in the relatively pristine upland reaches and 4 years of data on wells and downstream reaches influenced by fertilized croplands. Seasonal and spatial patterns in total nitrogen (TN) concentrations were driven largely by changes in the nitrate (NO₃ ^–) concentrations. A gradient of increasing NO₃ ^– concentrations occurred from pristine upland stream reaches to the more agriculturally-influenced lowland reaches. Nitrate concentrations varied seasonally and were negatively correlated with discharge in areas influenced by row-crop agriculture (p = 0.007). The NO₃ ^– concentrations of stream water in lowland reaches were lowest during times of high precipitation, when the relative influence of groundwater drainage is minimal and water in the channel is primarily derived from upland prairie reaches. The groundwater from cropland increased stream NO₃ ^– concentrations about four-fold during low-discharge periods, even though significant riparian forest corridors existed along most of the lower stream channel. The minimum NO₃ ^– concentrations in the agriculturally influenced reaches were greater than at any time in prairie reaches. Analysis of data before and after introduction of bison to four prairie watersheds revealed a 35% increase of TN concentrations (p < 0.05) in the stream water channels after the introduction of bison. These data suggest that natural processes such as bison grazing, variable discharge, and localized input of groundwater lead to variation in NO₃ ^– concentrations less than 100-fold in prairie streams. Row-crop agriculture can increase NO₃ ^– concentrations well over 100-fold relative to pristine systems, and the influence of this land use process over space and time overrides natural processes.     

Measuring Nitrogen and Phosphorus Uptake by Intact Roots of Mature Acer saccharum Marsh., Pinus resinosa Ait., and Picea abies (L.) Karst

A common method for measuring uptake by intact roots in situ is the depletion method, wherein intact fine roots are separated from soil and placed in nutrient solution. The difference between initial and final amounts of nutrient in solution is attributed to root uptake. Variations on this method include applying pretreatment solutions, training roots to grow into bags or trays, and varying concentrations of nutrient solution. We tested whether variations in methods affected measured net uptake rates of NH ₄ ⁺ , NO ₃ ⁻ , and PO ₄ ³⁻ . Intact roots of 60 year-old sugar maple (Acer saccharum Marsh.), red pine (Pinus resinosa Ait.), and Norway spruce (Picea abies (L.) Karst.) were given one of four treatments prior to measuring net uptake. “Trained” roots were grown in a sand-soil mixture. “Recovered” roots were excavated and allowed to recover in nutrient solution for two or four days (“two-day recovery” and “four-day recovery”, respectively). “No recovery” roots were excavated and used immediately in experiments. We exposed roots to three concentrations of nutrient solutions to observe the effects of initial nutrient solution concentration. Initial nutrient solution concentration was an important source of variation in measured uptake rates, and N uptake was stimulated by low antecedent concentrations. We found no significant differences in net uptake rates between pretreatments for any of the species studied, indicating that our pretreatments were not effective in improving measurement of uptake. Such pretreatments may not be necessary for measuring net uptake and may not hinder the comparison of rates measured using variations of the depletion method.     

Mini-Review: Nutrient Cycling in Lakes and Streams: Insights from a Comparative Analysis

Understanding of general ecosystem principles may be improved by comparing disparate ecosystems. We compared nutrient cycling in lakes and streams to evaluate whether contrasts in hydrologic properties lead to different controls and different rates of internal nutrient cycling. Our primary focus was nutrient cycling that results in increased productivity, so we quantified nutrient cycling by defining the recycling ratio (ρ) as the number of times a nutrient molecule is sequestered by producers before export. An analytic model of nutrient cycling predicted that in lakes ρ is governed by the processes that promote the mineralization and retard the sedimentation of particulate-bound nutrients, whereas in streams, ρ is governed by processes that promote the uptake and retard the export of dissolved nutrients. These differences were the consequence of contrast between lakes and streams in the mass-specific export rates (mass exported · standing stock^-1· time^-1) of dissolved and particulate nutrients. Although ρ is calculated from readily measured ecosystem variables, we found very few published data sets that provided the necessary data for a given ecosystem. We calculated and compared ρ in two well-studied P-limited ecosystems, Peter Lake and West Fork Walker Branch (WFWB). When ecosystems were scaled so that water residence time was equal between these two ecosystems, ρ was three orders of magnitude greater in WFWB. However, when we scaled by P residence time, ρ was nearly equal between these two ecosystems. This suggests broad similarities in ρ across ecosystem types when ecosystem boundaries are defined so that turnover times of limiting nutrients are the same. Received 19 November 1998; accepted 6 October 1999.     

Positive effect of shredders on microbial biomass and decomposition in stream microcosms

1. Animals play a major role in nutrient cycling via excretory processes. Although the positive indirect effects of grazers on periphytic algae are well understood, little is known about top‐down effects on decomposers of shredders living on leaf litter. 2. Nutrient cycling by shredders in oligotrophic forest streams may be important for the microbial‐detritus compartment at very small spatial scales (i.e. within the leaf packs in which shredders feed). We hypothesised that insect excretion may cause local nutrient enrichment, so that microorganism growth on leaves is stimulated. 3. We first tested the effect of increasing concentration of ammonium (+10, +20 and +40 μg NH₄⁺ L^?1) on fungal and bacterial biomass on leaf litter in a laboratory experiment. Then we performed two experiments to test the effect of the presence and feeding activity of shredder larvae. We used two species belonging to the trichopteran family Sericostomatidae: the Palaearctic Sericostoma vittatum and the Neotropical Myothrichia murina, to test the effect of these shredders on fungal and bacterial biomass and decomposition on leaves of Quercus robur and Nothofagus pumilio, respectively. All experiments were run in water with low ammonium concentrations (2.4 ± 0.34 to 14.47 ± 0.95 μg NH₄⁺ L^?1). 4. After 5 days of incubation, NH₄ concentrations were reduced to near‐ambient streamwater concentrations in all treatments with leaves. Fungal biomass was positively affected by increased ammonium concentration. On the other hand, bacteria abundance was similar in all treatments, both in terms of abundance (bacteria cells mg^?1 leaf DW) and biomass. However, there was a tendency towards larger mean cell size in treatments with 20 μg NH₄ L^?1. 5. In the experiment with S. vittatum, fungal biomass in the treatment with insects was more than twice that in the control after 15 days. Bacteria were not detected in treatments with insects, where hyphae were abundant, but they were abundant in treatments without larvae. In the decomposition experiment run with M. murina, leaf‐mass loss was significantly higher in treatments with larvae than in controls. 6. Our hypothesis of a positive effect of shredders on fungal biomass and decomposition was demonstrated. Insect excretion caused ammonium concentration to increase in the microcosms, contributing to microbial N uptake in leaf substrata, which resulted in structural and functional changes in community attributes. The positive effect of detritivores on microbes has been mostly neglected in stream nutrient‐cycling models; our findings suggest that this phenomenon may be of greater importance than expected in stream nutrient budgets.     

Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem

Reductions in snow cover undera warmer climate may cause soil freezing eventsto become more common in northern temperateecosystems. In this experiment, snow cover wasmanipulated to simulate the late development ofsnowpack and to induce soil freezing. Thismanipulation was used to examine the effects ofsoil freezing disturbance on soil solutionnitrogen (N), phosphorus (P), and carbon (C)chemistry in four experimental stands (twosugar maple and two yellow birch) at theHubbard Brook Experimental Forest (HBEF) in theWhite Mountains of New Hampshire. Soilfreezing enhanced soil solution Nconcentrations and transport from the forestfloor. Nitrate (NO₃ ^–) was thedominant N species mobilized in the forestfloor of sugar maple stands after soilfreezing, while ammonium (NH₄ ⁺) anddissolved organic nitrogen (DON) were thedominant forms of N leaching from the forestfloor of treated yellow birch stands. Rates ofN leaching at stands subjected to soil freezingranged from 490 to 4,600 mol ha^–1yr^–1, significant in comparison to wet Ndeposition (530 mol ha^–1 yr^–1) andstream NO₃ ^– export (25 mol ha^–1yr^–1) in this northern forest ecosystem. Soil solution fluxes of P_i from the forestfloor of sugar maple stands after soil freezingranged from 15 to 32 mol ha^–1 yr^–1;this elevated mobilization of P_i coincidedwith heightened NO₃ ^– leaching. Elevated leaching of P_i from the forestfloor was coupled with enhanced retention ofP_i in the mineral soil Bs horizon. Thequantities of P_i mobilized from the forestfloor were significant relative to theavailable P pool (22 mol ha^–1) as well asnet P mineralization rates in the forest floor(180 mol ha^–1 yr^–1). Increased fineroot mortality was likely an important sourceof mobile N and P_i from the forest floor,but other factors (decreased N and P uptake byroots and increased physical disruption of soilaggregates) may also have contributed to theenhanced leaching of nutrients. Microbialmortality did not contribute to the acceleratedN and P leaching after soil freezing. Resultssuggest that soil freezing events may increaserates of N and P loss, with potential effectson soil N and P availability, ecosystemproductivity, as well as surface wateracidification and eutrophication.     

Effects of chronic elevated nitrate concentrations on the structure and function of river biofilms

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Simulating effects of changing climate and CO2 emissions on soil carbon pools at the Hubbard Brook experimental forest

Carbon (C) sequestration in forest biomass and soils may help decrease regional C footprints and mitigate future climate change. The efficacy of these practices must be verified by monitoring and by approved calculation methods (i.e., models) to be credible in C markets. Two widely used soil organic matter models – CENTURY and RothC – were used to project changes in SOC pools after clear‐cutting disturbance, as well as under a range of future climate and atmospheric carbon dioxide (CO₂) scenarios. Data from the temperate, predominantly deciduous Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA, were used to parameterize and validate the models. Clear‐cutting simulations demonstrated that both models can effectively simulate soil C dynamics in the northern hardwood forest when adequately parameterized. The minimum postharvest SOC predicted by RothC occurred in postharvest year 14 and was within 1.5% of the observed minimum, which occurred in year 8. CENTURY predicted the postharvest minimum SOC to occur in year 45, at a value 6.9% greater than the observed minimum; the slow response of both models to disturbance suggests that they may overestimate the time required to reach new steady‐state conditions. Four climate change scenarios were used to simulate future changes in SOC pools. Climate‐change simulations predicted increases in SOC by as much as 7% at the end of this century, partially offsetting future CO₂ emissions. This sequestration was the product of enhanced forest productivity, and associated litter input to the soil, due to increased temperature, precipitation and CO₂. The simulations also suggested that considerable losses of SOC (8–30%) could occur if forest vegetation at HBEF does not respond to changes in climate and CO₂ levels. Therefore, the source/sink behavior of temperate forest soils likely depends on the degree to which forest growth is stimulated by new climate and CO₂ conditions.     

The role of water exchange between a stream channel and its hyporheic zone in nitrogen cycling at the terrestrial-aquatic interface

The subsurface riparian zone was examined as an ecotone with two interfaces. Inland is a terrestrial boundary, where transport of water and dissolved solutes is toward the channel and controlled by watershed hydrology. Streamside is an aquatic boundary, where exchange of surface water and dissolved solutes is bi-directional and flux is strongly influenced by channel hydraulics. Streamside, bi-directional exchange of water was qualitatively defined using biologically conservative tracers in a third order stream. In several experiments, penetration of surface water extended 18 m inland. Travel time of water from the channel to bankside sediments was highly variable. Subsurface chemical gradients were indirectly related to the travel time. Sites with long travel times tended to be low in nitrate and DO (dissolved oxygen) but high in ammonium and DOC (dissolved organic carbon). Sites with short travel times tended to be high in nitrate and DO but low in ammonium and DOC. Ammonium concentration of interstitial water also was influenced by sorption-desorption processes that involved clay minerals in hyporheic sediments. Denitrification potential in subsurface sediments increased with distance from the channel, and was limited by nitrate at inland sites and by DO in the channel sediments. Conversely, nitrification potential decreased with distance from the channel, and was limited by DO at inland sites and by ammonium at channel locations. Advection of water and dissolved oxygen away from the channel resulted in an oxidized subsurface habitat equivalent to that previously defined as the hyporheic zone. The hyporheic zone is viewed as stream habitat because of its high proportion of surface water and the occurrence of channel organisms. Beyond the channel's hydrologic exchange zone, interstitial water is often chemically reduced. Interstitial water that has not previously entered the channel, groundwater, is viewed as a terrestrial component of the riparian ecotone. Thus, surface water habitats may extend under riparian vegetation, and terrestrial groundwater habitats may be found beneath the stream channel.     

Inhibition of Nitrification Alters Carbon Turnover in the Patagonian Steppe

Human activities are altering biodiversity and the nitrogen (N) cycle, affecting terrestrial carbon (C) cycling globally. Only a few specialized bacteria carry out nitrification—the transformation of ammonium (NH ₄ ⁺ ) to nitrate (NO ₃ ⁻ ), in terrestrial ecosystems, which determines the form and mobility of inorganic N in soils. However, the control of nitrification on C cycling in natural ecosystems is poorly understood. In an ecosystem experiment in the Patagonian steppe, we inhibited autotrophic nitrification and measured its effects on C and N cycling. Decreased net nitrification increased total mineral N and NH ₄ ⁺ and reduced NO ₃ ⁻ in the soil. Plant cover (P < 0.05) and decomposition (P < 0.0001) decreased with inhibition of nitrification, in spite of increases in NH ₄ ⁺ availability. There were significant changes in the natural abundance of δ¹⁵N in the dominant vegetation when nitrification was inhibited suggesting that a switch occurred in the form of N (from NO ₃ ⁻ to NH ₄ ⁺ ) taken up by plants. Results from a controlled-condition experiment supported the field results by showing that the dominant plant species of the Patagonian steppe have a marked preference for nitrate. Our results indicate that nitrifying bacteria exert a major control on ecosystem functioning, and that the inhibition of nitrification results in significant alteration of the C cycle. The interactions between the C and N cycles suggest that rates of C cycling are affected not just by the amount of available N, but also by the relative availability for plant uptake of NH ₄ ⁺ and NO ₃ ⁻ .     

Spatial relationships of aluminum chemistry in the streams of the Hubbard Brook Experimental Forest,New Hampshire

Aluminum chemistry was evaluated in two headwater streams in the White Mountains of New Hampshire. Observed elevational trends in stream aluminum chemistry may be related to spatial variations of vegetation type and mineral soil depth within the watersheds. At the highest elevations maximum densities of spruce and fir vegetation occur and aluminum appears to be mobilized predominantly by transformations involving dissolved organic matter. At the mid-elevations hardwood vegetation predominates and the mechanism of aluminum mobilization shifts to dissolution by strong acids within the mineral soil. At the lowest elevations, relatively thick mineral soil seems to limit aluminum mobility, resulting in low concentrations in streamwater. Comparison of these results with an earlier study of an adjacent watershed, indicates that subtle differences in watershed characteristics such as tree species distribution and topography may cause significant variations in stream aluminum chemistry. Control of aluminum mobility by imogolite minerals was not indicated by the stream chemistry of these watersheds. To determine the relationship between acidic deposition and aluminum mobility, natural variations which occur in the aluminum cycle must be addressed.