The biogeochemistry of calcium at Hubbard Brook

A synthesis of the biogeochemistry of Ca was done during 1963–1992in reference and human-manipulated forest ecosystems of the Hubbard BrookExperimental Forest (HBEF), NH. Results showed that there has been a markeddecline in concentration and input of Ca in bulk precipitation, an overalldecline in concentration and output of Ca in stream water, and markeddepletion of Ca in soils of the HBEF since 1963. The decline in streamwaterCa was related strongly to a decline in SO +NO in stream water during the period. The soildepletion of Ca was the result of leaching due to inputs of acid rain duringthe past 50 yr or so, to decreasing atmospheric inputs of Ca, and tochanging amounts of net storage of Ca in biomass. As a result of thedepletion of Ca, forest ecosystems at HBEF are much more sensitive tocontinuing inputs of strong acids in atmospheric deposition than expectedbased on long-term patterns of sulfur biogeochemistry. The Ca concentrationand input in bulk precipitation ranged from a low of 1.0 µmol/and 15 mol/ha-yr in 1986–87 to a high of 8.0 µmol/ and 77mol/ha-yr in 1964–65, with a long-term mean of 2.74 µmol/during 1963–92. Average total atmospheric deposition was 61 and 29mol/ha-yr in 1964–69 and 1987–92, respectively. Dry depositionis difficult to measure, but was estimated to be about 20% of totalinput in atmospheric deposition. Streamwater concentration reached a low of21 µmol/ in 1991–92 and a high of 41 µmol/ in1969–70, but outputs of Ca were lowest in 1964–65 (121mol/ha-yr) and peaked in 1973–74 (475 mol/ha-yr). Gross outputs of Cain stream water were positively and significantly related to streamflow, butthe slope of this relation changed with time as Ca was depleted from thesoil, and as the inputs of sulfate declined in both atmospheric depositionand stream water. Gross outputs of Ca in stream water consistently exceededinputs in bulk precipitation. No seasonal pattern was observed for eitherbulk precipitation or streamwater concentrations of Ca. Net soil releasevaried from 390 to 230 mol/ha-yr during 1964–69 and 1987–92,respectively. Of this amount, weathering release of Ca, based on plagioclasecomposition of the soil, was estimated at about 50 mol/ha-yr. Net biomassstorage of Ca decreased from 202 to 54 mol/ha-yr, and throughfall plusstemflow decreased from 220 to 110 mol/ha-yr in 1964–69 and1987–92, respectively. These ecosystem response patterns were relatedto acidification and to decreases in net biomass accretion during the study.Calcium return to soil by fine root turnover was about 270 mol/ha-yr, with190 mol/ha-yr returning to the forest floor and 80 mol/ha-yr to the mineralsoil. A lower content of Ca was observed with increasing elevation for mostof the components of the watershed-ecosystems at HBEF. Possibly as a result,mortality of sugar maple increased significantly during 1982 to 1992 at highelevations of the HBEF. Interactions between biotic and abiotic controlmechanisms were evident through elevational differences in soil cationexchange capacity (the exchangeable Ca concentration in soils wassignificantly and directly related to the organic matter content of thesoils), in soil/till depth, and in soil water and in streamwaterconcentrations at the HBEF, all of which tended to decrease with elevation.The exchangeable pool of Ca in the soil is about 6500 mol/ha, and itsturnover time is quite rapid, about 3 yr. Nevertheless, the exchangeablepools of Ca at HBEF have been depleted markedly during the past 50 years orso, >21,125 mol/ha during 1940–1995. The annual gross uptake oftrees is about 26–30% of the exchangeable pool in the soil.Some 7 to 8 times more Ca is cycled through trees than is lost in streamwater each year, and resorption of Ca by trees is negligible at HBEF. Of thecurrent inputs to the available nutrient compartment of the forestecosystem, some 50% was provided by net soil release, 24% byleaching from the canopy, 20% by root exudates and 6% byatmospheric deposition. Clear cutting released large amounts of Ca tostream water, primarily because increased nitrification in the soilgenerated increased acidity and NO , a mobileanion in drainage water; even larger amounts of Ca can be lost from theecosystem in harvested timber products. The magnitude of Ca loss due towhole-tree harvest and acid rain leaching is comparable for forests similarto the HBEF, but losses from harvest must be superimposed on losses due toacid rain.     

The biogeochemistry of sulfur at Hubbard Brook

A synthesis of the biogeochemistry of S was done during 34 yr(1964–1965 to 1997–1998) in reference and human-manipulated forestecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. There have beensignificant declines in concentration (–0.44µmol/liter-yr) and input (–5.44mol/ha-yr)of SO₄ ^2– in atmospheric bulk wet deposition, and inconcentration(–0.64 µmol/liter-yr) an d output (–3.74mol/ha-yr) of SO₄ ^2– in stream water ofthe HBEF since 1964. These changes arestrongly correlated with concurrent decreases in emissions of SO₂from the source area for the HBEF. The concentration and input ofSO₄ ^2– in bulk deposition ranged from a low of 13.1µmol/liter (1983–1984) and 211 mol/ha-yr(1997–1998) to a high of 34.7 µmol/liter(1965–1966) and 479 mol/ha-yr (1967–1968), with along-term mean of 23.9 µmol/liter and 336mol/ha-yr during 1964–1965 to 1997–1998. Despiterecentdeclines in concentrations, SO₄ ^2– is the dominantanion in both bulk deposition and streamwater at HBEF. Dry deposition is difficult to measure, especially inmountainousterrain, but was estimated at 21% of bulk deposition. Thus, average totalatmospheric deposition was 491 and 323 mol/ha-yr during1964–1969 and 1993–1998, respectively. Based on the long-term³⁴S pattern associated with anthropogenic emissions,SO₄ ^2– deposition at HBEF is influenced by numerousSO₂sources, but biogenic sources appear to be small. Annual throughfall plusstemflow in 1993–1994 was estimated at 346 molSO₄ ^2–/ha. Aboveground litterfall, for thewatershed-ecosystemaveraged about 180 mol S/ha-yr, with highest inputs (190 molS/ha-yr) in the lower elevation, more deciduous forest zone. Weatheringrelease was calculated at a maximum of 50 mol S/ha-yr. Theconcentration and output of SO₄ ^2– in stream waterranged from a low of 42.3µmol/liter (1996–1997) and 309 mol/ha-yr(1964–1965), to a high of 66.1 µmol/liter(1970–1971) and 849 mol/ha-yr (1973–1974), with along-term mean of 55.5 µmol/liter and 496mol/ha-yr during the 34 yrs of study. Gross outputs ofSO₄ ^2– in stream water consistently exceeded inputsin bulkdeposition and were positively and significantly related to annualprecipitationand streamflow. The relation between gross SO₄ ^2–output and annual streamflow changed with time asatmospheric inputs declined. In contrast to the pattern for bulk depositionconcentration, there was no seasonal pattern for streamSO₄ ^2– concentration. Nevertheless, stream outputs ofSO₄ ^2– were highly seasonal, peaking during springsnowmelt, andproducing a monthly cross-over pattern where net hydrologic flux (NHF) ispositive during summer and negative during the remainder of the year. Nosignificant elevational pattern in streamwaterSO₄ ^2– concentration was observed. Mean annual,volume-weightedsoil water SO₄ ^2– concentrations were relativelyuniform by soil horizon andacross landscape position. Based upon isotopic evidence, much of theSO₄ ^2– entering HBEF in atmospheric depositioncycles throughvegetation and microbial biomass before being released to the soil solution andstream water. Gaseous emissions of S from watershed-ecosystems at HBEF areunquantified, but estimated to be very small. Organic S (carbon bonded andestersulfates) represents some 89% of the total S in soil at HBEF. Some 6% exists asphosphate extractable SO₄ ^2– (PSO₄).About 73% of the total S in the soilprofile at HBEF occurs in the Bs2 horizon, and some 9% occurs in the forestfloor. The residence time for S in the soil was calculated to be 9 yr, butonly a small portion of the total organic soil pool turns over relativelyquickly. The S content of above- and belowground biomass is about 2885mol/ha, of which some 3–5% is in standing dead trees. Yellowbirch, American beech and sugar maple accounted for 89% of the S in trees, with31% in branches, 27% in roots and 25% in the lightwood of boles. The pool of Sin living biomass increased from 1965 to 1982 due to biomass accretion, andremained relatively constant thereafter. Of current inputs to the availablenutrient compartment of the forest ecosystem, 50% is from atmospheric bulkdeposition, 24% from net soil release, 11% from dry deposition, 11% from rootexudates and 4% is from canopy leaching. Comparing ecosystem processes for Sfrom 1964–1969 to 1993–1998, atmospheric bulk deposition decreasedby 34%, stream output decreased by 10%, net annual biomass storage decreased by92%, and net soil release increased by 184% compared to the 1964–1969values. These changes are correlated with decreased emissions of SO₂from the source area for the HBEF. Average, annual bulk deposition inputsexceeded streamwater outputs by 160.0 ± 75.3 SD molS/ha-yr,but average annual net ecosystem fluxes (NEF) were much smaller, mostlynegativeand highly variable during the 34 yr period (–54.3 ± 72.9 SDmol S/ha-yr; NEF range, +86.8 to –229.5). While severalmechanisms may explain this small discrepancy, the most likely are netdesorption of S and net mineralization of organic S largely associated with theforest floor. Our best estimates indicate that additional S from dry depositionand weathering release is probably small and that desorption accounts for about37% of the NEF imbalance and net mineralization probably accounts for theremainder (60%). Additional inputs from dry deposition would result fromunmeasured inputs of gaseous and particulate deposition directly to the forestfloor. The source of any unmeasured S input has important implications for therecovery of soils and streams in response to decreases in inputs of acidicdeposition. Sulfate is a dominant contributor to acid deposition at HBEF,seriously degrading aquatic and terrestrial ecosystems. Because of the strongrelation between SO₂ emissions and concentrations ofSO₄ ^2– in both atmospheric deposition and streamwater at HBEF,further reductions in SO₂ emissions will be required to allowsignificant ecosystem recovery from the effects of acidic deposition. Thedestruction or removal of vegetation on experimental watershed-ecosystems atHBEF resulted in increased rates of organic matter decomposition andnitrification, a lowering of soil and streamwater pH, enhancedSO₄ ^2– adsorption on mineral soil and smallerconcentrations andlosses of SO₄ ^2– in stream water. With vegetationregrowth, this adsorbedSO₄ ^2– is released from the soil, increasingconcentrations andfluxes of SO₄ ^2– in drainage water. Streamwaterconcentration ofSO₄ ^2– and gross annual output ofSO₄ ^2–/ha are essentially the same throughout theHubbard BrookValley in watersheds varying in size by about 4 orders of magnitude, from 3 to3000 ha.     

The biogeochemistry of chlorine at Hubbard Brook,New Hampshire,USA

Chlorine is a minor constituent of most rocks and a minor (although essential) element in plants, but it cycles rapidly through the hydrosphere and atmosphere. In forest ecosystem studies, chloride ion (Cl⁻) is often thought to be conservative in the sense that the sources and sinks within the ecosystem are assumed negligible compared to inputs and outputs. As such, Cl⁻ is often used as a conservative tracer to assess sources and transformations of other ions. In this paper we summarize research on chloride over the course of 36 years (1964–2000) at the Hubbard Brook Experimental Forest (HBEF) in central New Hampshire, USA. Evidence presented here suggests that in the 1960s and 1970s the dominant source of atmospheric Cl⁻ deposition was from pollutant sources, probably coal burning. In the 1970s the Cl⁻ inputs in bulk deposition declined, and the lower Cl⁻ deposition in the last two decades is dominated by marine sources. Between 1964 and 2000 there was no significant trend in Cl⁻ export in stream flow, thus the net hydrologic flux (NHF = bulk deposition inputs − streamflow outputs) has changed over this period. Early in the record the NHF was on average positive, indicating net retention of Cl⁻ within the system, but since about 1980 the NHF has been consistently negative, indicating an unmeasured input or source within the ecosystem. Dry deposition can account for at least part of that unmeasured source, and it appears that release of Cl⁻ from mineralization of soil organic matter (SOM) may also play an important role. We believe that accumulation of Cl⁻ in vegetation during the 1960s and 1970s offset the unmeasured source and resulted in net ecosystem retention. Accumulation of vegetative biomass has ceased since about 1982, leading to the apparent net export (negative NHF) since that time. Although we have no direct measurements of Cl⁻ accumulation in vegetation, our estimates suggest that an aggrading forest could sequester about 32 mol Cl ha⁻¹ year⁻¹, or about a third of the annual average bulk deposition flux to this ecosystem. Experimental additions of Cl⁻ to the forest floor cause increases in Cl⁻ concentration in foliage, throughfall, and soil solution. Manipulations of vegetation also affect the Cl⁻ cycle. Harvesting or devegetation of watersheds causes an increase in the Cl⁻ concentration and flux in stream water for several years after the disturbance. This period of release is followed by a period of reaccumulation of Cl⁻ that may last more than 15 years. In this respect, the behavior of Cl⁻ after disturbance parallels that of NO₃⁻, for which export increases after disturbance due to reduced plant nitrogen uptake and mineralization of nitrogen from detritus, rather than SO₄²⁻, for which export decreases after disturbance due to pH-dependent adsorption onto mineral soils. The interannual pattern of Cl⁻ export from the system primarily reflects the atmospheric inputs, but the net retention and cycling of Cl⁻ within the system appears to be largely under biological, rather than geochemical, control.     

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.     

The effects of whole-tree clear-cutting on soil processes at the Hubbard Brook Experimental Forest,New Hampshire,USA

The effects of whole-tree clear-cutting on soil processes and streamwater chemistry were examined in a northern hardwood forest at the Hubbard Brook Experimental Forest, New Hampshire. Soil processes were examined by monitoring soil solution chemistry collected using zero-tension lysimeters from the Oa, Bh and Bs horizons at three sites along an elevational/vegetation gradient. Whole-tree clear-cutting created a severe ecosystem disturbance leading to leaching losses of nutrients from the soil profile, increased acidification, and elevated concentrations of Al-ions in soil solutions and streamwater. The response was driven by the process of nitrification that led to production of nitric acid in both the forest floor and mineral soil horizons. This acidity was largely neutralized by release and leaching of basic cations and inorganic monomeric Al-ions leaching with the NO₃-ions. The major source of nutrient loss was from the forest floor. The chemical response to the clear-cut was most intense during the second year following the treatment and declined to near reference concentrations in 4–5 years. High elevation sites showed the greatest response to disturbance and the slowest recovery of soil solution concentrations to pre-cut concentrations. Shallow soils and a slower recovery of vegetation at the upper elevation sites were the primary factors contributing to the enhanced disturbance and delayed recovery (and enhanced response to disturbance in the upper elevation sites).     

The Biogeochemistry of Carbon at Hubbard Brook

The biogeochemical behavior of carbon in the forested watersheds of the Hubbard Brook Experimental Forest (HBEF) was analyzed in long-term studies. The largest pools of C in the reference watershed (W6) reside in mineral soil organic matter (43% of total ecosystem C) and living biomass (40.5%), with the remainder in surface detritus (14.5%). Repeated sampling indicated that none of these pools was changing significantly in the late-1990s, although high spatial variability precluded the detection of small changes in the soil organic matter pools, which are large; hence, net ecosystem productivity (NEP) in this 2nd growth forest was near zero (± about 20 g C/m²-yr) and probably similar in magnitude to fluvial export of organic C. Aboveground net primary productivity (ANPP) of the forest declined by 24% between the late-1950s (462 g C/m²-yr) and the late-1990s (354 g C/m²-yr), illustrating age-related decline in forest NPP, effects of multiple stresses and unusual tree mortality, or both. Application of the simulation model PnET-II predicted 14% higher ANPP than was observed for 1996–1997, probably reflecting some unknown stresses. Fine litterfall flux (171 g C/m²-yr) has not changed much since the late-1960s. Because of high annual variation, C flux in woody litterfall (including tree mortality) was not tightly constrained but averaged about 90 g C/m²-yr. Carbon flux to soil organic matter in root turnover (128 g C/m²-yr) was only about half as large as aboveground detritus. Balancing the soil C budget requires that large amounts of C (80 g C/m²-yr) were transported from roots to rhizosphere carbon flux. Total soil respiration (TSR) ranged from 540 to 800 g C/m²-yr across eight stands and decreased with increasing elevation within the northern hardwood forest near W6. The watershed-wide TSR was estimated as 660 g C/m²-yr. Empirical measurements indicated that 58% of TSR occurred in the surface organic horizons and that root respiration comprised about 40% of TSR, most of the rest being microbial. Carbon flux directly associated with other heterotrophs in the HBEF was minor; for example, we estimated respiration of soil microarthropods, rodents, birds and moose at about 3, 5, 1 and 0.8 g C/m²-yr, respectively, or in total less than 2% of NPP. Hence, the effects of other heterotrophs on C flux were primarily indirect, with the exception of occasional irruptions of folivorous insects. Hydrologic fluxes of C were significant in the watershed C budget, especially in comparison with NEP. Although atmospheric inputs (1.7 g C/m²-yr) and streamflow outputs (2.7 g C/m²-yr) were small, larger quantities of C were transported within the ecosystem and a more substantial fraction of dissolved C was transported from the soil as inorganic C and evaded from the stream as CO₂ (4.0 g C/m²-yr). Carbon pools and fluxes change rapidly in response to catastrophic disturbances such as forest harvest or major windthrow events. These changes are dominated by living vegetation and dead wood pools, including roots. If biomass removal does not accompany large-scale disturbance, the ecosystem is a large net source of C to the atmosphere (500–1200 g C/m²-yr) for about a decade following disturbance and becomes a net sink about 15–20 years after disturbance; it remains a net sink of about 200–300 g C/m²-yr for about 40 years before rapidly approaching steady state. Shifts in NPP and NEP associated with common small-scale or diffuse forest disturbances (e.g., forest declines, pathogen irruptions, ice storms) are brief and much less dramatic. Spatial and temporal patterns in C pools and fluxes in the mature forest at the HBEF reflect variation in environmental factors. Temperature and growing-season length undoubtedly constrain C fluxes at the HBEF; however, temperature effects on leaf respiration may largely offset the effects of growing season length on photosynthesis. Occasional severe droughts also affect C flux by reducing both photosynthesis and soil respiration. In younger stands nutrient availability strongly limits NPP, but the role of soil nutrient availability in limiting C flux in the mature forest is not known. A portion of the elevational variation of ANPP within the HBEF probably is associated with soil resource limitation; moreover, sites on more fertile soils exhibit 20–25% higher biomass and ANPP than the forest-wide average. Several prominent biotic influences on C pools and fluxes also are clear. Biomass and NPP of both the young and mature forest depend upon tree species composition as well as environment. Similarly, litter decay differs among tree species and forest types, and forest floor C accumulation is twice as great in the spruce–fir–birch forests at higher elevations than in the northern hardwood forests, partly because of inherently slow litter decay and partly because of cold temperatures. This contributes to spatial patterns in soil solution and streamwater dissolved organic carbon across the Hubbard Brook Valley. Wood decay varies markedly both among species and within species because of biochemical differences and probably differences in the decay fungi colonizing wood. Although C biogeochemistry at the HBEF is representative of mountainous terrain in the region, other sites will depart from the patterns described at the HBEF, due to differences in site history, especially agricultural use and fires during earlier logging periods. Our understanding of the C cycle in northern hardwood forests is most limited in the area of soil pool size changes, woody litter deposition and rhizosphere C flux processes.     

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.     

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.     

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.     

The effects of foraging by the superb lyrebird (Menura novae-hollandiae) in Eucalyptus regnans forests at Beenak,Victoria

Abstract In an early spar-stage stand of Eucalyptus regnans at Beenak, Victoria, foraging by lyrebirds in bare floor areas on steep slopes results in a complex microtopography of excavations, accumulations and terracettes. About 200 t ha^?1 of litter and top soil may be displaced an average of 70 cm downhill per year. Magnetic ferruginous pisolite was used as a marker to monitor progressive soil movement over 3 years. Very little disturbance occurred in areas of dense ground fern, but in bare areas the whole forest floor may be turned over every 20 months. In the site studied, foraging activity by lyrebirds varied seasonally and topographically. Disturbance by other biotic agents was minimal. The mean depth of soil cultivation was about 10 cm and litter was frequently buried or mixed intimately with soil. Since buried leaf litter decays more quickly than that on the surface, lyrebird foraging is likely to increase the rate of nutrient cycling. The small, steep clifflets left at the uphill limits of each scratch microsite provide litter-free niches for the establishment of tree fern prothalli and shade-tolerant herbs. All stages in the growth of the rough tree fern, Cyathea australis, were present in bare floor areas, but in dense ground fern patches, young stages were confined to rotten logs and upturned root balls. Since dense tree fern development tends to diminish the cover of dense ground fern, lyrebird foraging activity may maintain an accessible food resource which would otherwise diminish with increased ground fern cover in these forests in the course of secondary succession after fire.     

Calcium, magnesium, and potassium in hair of deer from areas of contrasting soil productivity

Differences in mineral nutrient composition of soils have been considered to affect health and population characteristics of free-ranging animals, particularly herbivores. Contents of Ca, Mg, and K in hair of female fawn white-tailed deer (Odocoileus virginianus) were measured for eight consecutive years to determine if soil and annual effects occurred in two areas of contrasting soil productivity in Illinois. Soil differences may account for some of the autumnal weight difference (7.2 kg for 4 yrs of observation) observed in fawn does from the areas. Ca, Mg, and K were assayed, because these macronutrients were known to differ in soils of the areas and were presumed to differ in forages. In 6 of the 8 yrs, at least one element was significantly different (P ≤ 0.05) between areas. Significant (P ≤ 0.05) differences for K occurred in 5 yr, for Ca in 4 yr, and for Mg in 2 yr. Ca and Mg were lower in hair in 7 yr from deer collected from the area in which extractable Ca and Mg were higher in soils; that is, hair Ca and Mg levels tended to be inversely related to levels of plant-available Ca and Mg in soil. For 7 of the 8 yr, K content was lower in hair from the area of lower soil K content. Within one area, between-year differences occurred for Ca and K and for Ca and Mg in the other area. Between-year differences in diet selection and annual climatic effects on mineral uptake of forages, among other factors, may account for some of the latter differences. Results for hair analyses suggest that macronutrient differences in Ca, Mg, and K occur in the diets of these populations and may account for some of the weight difference observed between the areas.     

The biogeochemistry of basic cations in two forest catchments with contrasting lithology in the Czech Republic

The biogeochemistry of Ca, Mg, K, and Nawere investigated in two forested catchments in theCzech Republic, one underlain by leucogranite, theother by serpentinite. High weathering rates at theserpentinite site at Pluhv Bor resultedin Mg²⁺ as the dominant cation on the soilexchange complex and in drainage water. Other basiccations (Ca²⁺, K⁺, Na⁺) showedrelatively low concentrations and outflow instreamwater. The catchment exhibited high basesaturation in mineral soils (>70%), and nearneutral soil and stream pH, despite elevated inputsof acidic deposition. Slow growth of Norway spruceat Pluhv Bor may be caused by K deficiency, Mgoversupply and/or Ni toxicity. In contrast, thegranitic site at Lysina showed low concentrations ofbasic cations on the soil exchange complex and instreamwater. Soil and drainage water at Lysina werehighly impacted by acidic deposition. Soil pH wasextremely acidic (<4.5) throughout the soilprofile, and the base saturation of the mineral soilwas very low (<5%). Supplies of basic cationsfrom atmospheric deposition and soil processes wereless than inputs of SO^2- ₄ on anequivalence basis, resulting in low pH and highconcentrations of total Al in drainage water. Needle yellowing in Norway spruce was possibly theresult of Mg deficiency at Lysina. Because of theirextremely different lithologies, these catchmentsserve as valuable end-members of ecosystemsensitivity to elevated levels of acidicdeposition.     

Prevalence of Tree Regeneration by Sprouting and Seeding Along a Rainfall Gradient in Hawai'i

Drought stress in tropical dry forests is thought to result in greater asexual regeneration via vegetative sprouting ( e.g ., basal, root, and branch layering) than occurs in moister tropical forests. We tested this hypothesis by examining the prevalence of tree sprouting and seeding in tropical forests located along a rainfall gradient on the island of Hawai'i. Additionally, we examined the potential for novel disturbance, feral pig Sus scrofa rooting and trampling, to alter patterns in tree regeneration mode. We found greater sprouting (in terms of relative density and basal area) in dry forests than in mesic and wet forests, supporting the hypothesis. We also found that feral pig disturbance is negatively correlated with the relative density and basal area of seedlings in wet forests, but is positively correlated with the relative importance of sprouting, and the richness and diversity of sprouting species. Our results suggest rainfall regimes may be an important factor controlling broad-scale patterns in tree regeneration mode, and that exotic ungulates can significantly modify such patterns with potential consequences for the structure and dynamics of tree populations and communities.     

长白山森林生态系统大气氮素湿沉降通量和组成的季节变化特征

2009—2010年期间,利用雨量计收集法在长白山森林生态系统定位站开展定位观测,分析降水中氮素浓度,研究了该区域大气氮素湿沉降通量和组成的季节变化特征。结果表明,各形态氮素月均浓度之间差别较大,具有明显的季节性;其降水中浓度主要受降水量和降水频次的影响。全年氮素湿沉降中TN、TIN和TON的沉降量分别为27.64 kg N hm-2a-1、11.05 kg N hm-2a-1和16.59 kg N hm-2a-1,TON为沉降主体,占60.02%;其大气氮沉降量主要由降水量和降水中氮素浓度共同决定。该地区氮湿沉降量已处于我国中等水平,考虑到氮素的干湿沉降比例,本区域的年氮沉降量已接近或超过本区域的营养氮沉降临界负荷,存在一定的环境风险。该地区生长季(5—10月)的氮沉降量(16.59 kg N hm-2a-1)占全年氮沉降量的比例达到73.20%。生长季的氮沉降对于促进植物生长直接生态意义重大,而非生长季的氮沉降对于大量补充次年植物生长初期所需养分的间接生态意义明显。     

Element Fluxes and Landscape Position in a Northern Hardwood Forest Watershed Ecosystem

Chemical changes along headwater streams at the Hubbard Brook Experimental Forest in New Hampshire suggest that important differences exist in biogeochemical cycles along an altitudinal gradient within small watershed ecosystems. Using data collected during the period 1982–92, we have constructed element budgets [Ca, Mg, K, Na, Si, Al, dissolved organic carbon (DOC), S, and N] for three subcatchments within watershed 6, a forested watershed last logged around 1917–20. The biogeochemistry of the high-elevation spruce-fir–white birch subcatchment was dominated by processes involving naturally occuring organic compounds. Stream water and soil solutions in this zone had elevated concentrations of organic acidity, DOC, and organically bound monomeric aluminum (Al_o), relative to lower-elevation sites. The middle-elevation subcatchment, dominated by hardwood vegetation, had the greatest net production of inorganic-monomeric aluminum (Al_i), and exhibited net immobilization of DOC and Al_o. The low-elevation subcatchment, also characterized by deciduous vegetation, had the highest rates of net production of base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) among the subcatchments. Living biomass of trees declined slightly in the spruce-fir–white birch subcatchment during the study period, remained constant in the middle-elevation zone, and increased by 5% in the low-elevation subcatchment. Coupling the corresponding changes in biomass nutrient pools with the geochemical patterns, we observed up to 15-fold differences in the net production of Ca, Mg, K, Na, and Si in soils of the three subcatchments within this 13.2-ha watershed. Release of Ca, Na, and dissolved Si in the highest-elevation subcatchment could be explained by the congruent dissolution of 185 mol ha⁻¹ y⁻¹ of plagioclase feldspar. The rate of plagioclase weathering, based on the net output of Na, increased downslope to 189 and 435 mol ha⁻¹ y⁻¹ in the middle-elevation and low-elevation subcatchments, respectively. However, the dissolution of feldspar in the hardwood subcatchments could account for only 26%–37% of the observed net Ca output. The loss of Ca from soil exchange sites and organic matter is the most likely source of the unexplained net export. Furthermore, this depletion appears to be occurring most rapidly in the lower half of watershed 6. The small watersheds at the Hubbard Brook Experimental Forest occupy a soil catena in which soil depth and soil-water contact time increase downslope. By influencing hydrologic flowpaths and acid neutralization processes, these factors exert an important influence on biogeochemical fluxes within small watersheds, but their influence on forest vigor is less clear. Our results illustrate the sensitivity of watershed-level studies to spatial scale. However, it appears that much of the variation in element fluxes occurs in the first 10–20 ha of drainage area. Received 13 August 1998; accepted 7 September 1999.     

不同森林恢复方式对我国南方红壤区土壤质量的影响

2008-2009年在我国南方红壤区,研究了3种典型森林恢复方式(自然恢复的天然次生林、人工恢复的本地种马尾松人工林和引进种湿地松人工林)对林地土壤质量的影响.结果表明： 天然次生林的土壤含水量、土壤容重、土壤粒径构成、土壤全碳、全氮、全磷、有机碳、速效氮、速效磷、速效钾含量均优于两种人工林.综合土壤物理性状、化学性状和微生物性状得到土壤质量综合指数.天然次生林土壤的综合质量指数(1.20±0.10)显著高于马尾松人工林(0.59±0.03)和湿地松人工林(0.59±0.06),而两种人工林之间差异不显著.在我国南方红壤区,自然恢复的天然次生林土壤质量优于人工恢复的马尾松林和湿地松林.     

A Review of the Genera Croconema Cobb, 1920 and Pseudochromadora Daday, 1899 (Nematoda, Desmodoroidea): New Species from the Coasts of Kenya and Australia

This article is a review of the subfamily Desmodorinae (Nematoda, Desmodoroidea) and two related genera within this subfamily, Croconema Cobb, 1920 and Pseudochromadora Daday, 1899 with keys to genus or species level, genus diagnoses and lists of valid species. An emended diagnosis of, and discussion on, Sibayinema Swart & Heyns, 1991, is presented. Three new species are described: Croconema floriani sp.n. from the coast of Kenya, Pseudochromadora galeata sp.n. and P. securis sp.n. from the coast of Australia.     

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.     

Disentangling the long‐term effects of disturbance on soil biogeochemistry in a wet tropical forest ecosystem

Climate change is increasing the intensity of severe tropical storms and cyclones (also referred to as hurricanes or typhoons), with major implications for tropical forest structure and function. These changes in disturbance regime are likely to play an important role in regulating ecosystem carbon (C) and nutrient dynamics in tropical and subtropical forests. Canopy opening and debris deposition resulting from severe storms have complex and interacting effects on ecosystem biogeochemistry. Disentangling these complex effects will be critical to better understand the long‐term implications of climate change on ecosystem C and nutrient dynamics. In this study, we used a well‐replicated, long‐term (10 years) canopy and debris manipulation experiment in a wet tropical forest to determine the separate and combined effects of canopy opening and debris deposition on soil C and nutrients throughout the soil profile (1 m). Debris deposition alone resulted in higher soil C and N concentrations, both at the surface (0–10 cm) and at depth (50–80 cm). Concentrations of NaOH‐organic P also increased significantly in the debris deposition only treatment (20–90 cm depth), as did NaOH‐total P (20–50 cm depth). Canopy opening, both with and without debris deposition, significantly increased NaOH‐inorganic P concentrations from 70 to 90 cm depth. Soil iron concentrations were a strong predictor of both C and P patterns throughout the soil profile. Our results demonstrate that both surface‐ and subsoils have the potential to significantly increase C and nutrient storage a decade after the sudden deposition of disturbance‐related organic debris. Our results also show that these effects may be partially offset by rapid decomposition and decreases in litterfall associated with canopy opening. The significant effects of debris deposition on soil C and nutrient concentrations at depth (>50 cm), suggest that deep soils are more dynamic than previously believed, and can serve as sinks of C and nutrients derived from disturbance‐induced pulses of organic matter inputs.     

重庆市近郊大气无机氮、硫沉降特征及其来源分析

以重庆市近郊中梁山槽谷为研究区,利用气象站和沉降仪获取2017年5月-2018年4月的大气无机氮、硫沉降数据和降水δ¹⁵N-NO₃^-、δ¹⁸O-NO₃^-和δ³⁴S-SO₄^2-、δ¹⁸O-SO₄^2-数据,通过离子浓度比值、同位素值和气团后向轨迹探讨了研究区大气中氮、硫沉降变化特征及其来源。结果表明：（1）大气DIN总沉降量为19.99 kg/hm²,干、湿沉降量分别占11%和89%;大气S总沉降量为32.62 kg/hm²,干、湿沉降量分别占13%和87%。大气氮、硫湿沉降量与降水量均呈正相关（n=12,P < 0.01）,氮、硫干湿沉降量具有明显的季节差异。（2）降水NH₄⁺-N/NO₃^--N比值介于0.45-2.2之间,雨季（5-10月）NH₄⁺-N/NO₃^--N>1,旱季（11-次年4月）NH₄⁺-N/NO₃^--N<1,表明雨季氮主要来源于农业源,旱季来源于工业和交通源;降水NO₃^-/SO₄^2-比值介于0.1-1.25之间,平均值为0.63,表明硫来源以固定污染源（燃煤）为主。（3）大气降水δ¹⁵N-NO₃^-、δ¹⁸O-NO₃^-值分别为-3.8‰-3.9‰（平均值为0.4‰±2.6‰）和58.7‰-98.7‰（平均值为76.1‰±14.3‰）,夏季偏负,冬季偏正;降水δ³⁴S-SO₄^2-和δ¹⁸O-SO₄^2-变化范围分别为1.3‰-3.2‰（平均值为2.3‰±1‰）和5.3‰-8.5‰（平均值为7.1‰±1.6‰）,大气降水中NO₃^-和SO₄^2-主要来源于当地的化石燃料燃烧,同时受到周边污染物的远距离传输影响。（4）气团后向轨迹表明影响研究区氮、硫干湿沉降来源的主要因素是东亚季风,北东-南西走向的川东平行岭谷大地貌格局加剧了季风的影响。