Tree Species Effects on Calcium Cycling: The Role of Calcium Uptake in Deep Soils

Soil acidity and calcium (Ca) availability in the surface soil differ substantially beneath sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis) trees in a mixed forest in northwestern Connecticut. We determined the effect of pumping of Ca from deep soil (rooting zone below 20-cm mineral soil) to explain the higher available Ca content in the surface soil beneath sugar maple. We measured the atmospheric input of Ca with bulk deposition collectors and estimated Ca weathering and Ca mineralization in the surface soil (rooting zone above 20-cm mineral soil) from strontium isotope measurements and observed changes in exchangeable Ca in soils during field incubation. Calcium leaching at 20 cm was calculated by combining modeled hydrology with measured Ca soil solution concentrations at 20-cm depth. We measured root length distribution with depth beneath both tree species. Calcium leaching from the surface soil was much higher beneath sugar maple than hemlock and was positively related with the amount of Ca available in the surface soil. Calcium leaching from the surface soil beneath sugar maple was higher than the combined Ca input from atmospheric deposition and soil weathering. Without Ca uptake in the deep soil, surface soils are being depleted in Ca, especially beneath sugar maple. More organically bound Ca was mineralized beneath sugar maple than beneath hemlock. A relatively small part of this Ca release was leached below the surface soil, suggesting that, beneath both tree species, most of the Ca cycling is occurring in the surface soil. Sugar maple had more fine roots in the deep soil than hemlock and a greater potential to absorb Ca in the deep soil. With a simple model, we showed that a relatively small amount of Ca uptake in the deep soil beneath sugar maple is able to sustain high amounts of available Ca in the surface soil. Received 20 June 2001; accepted 6 December 2001.     

Differential effects of sugar maple, red oak, and hemlock tannins on carbon and nitrogen cycling in temperate forest soils

Tannins are abundant secondary chemicals in leaf litter that are hypothesized to slow the rate of soil-N cycling by binding protein into recalcitrant polyphenol–protein complexes (PPCs). We studied the effects of tannins purified from sugar maple, red oak, and eastern hemlock leaf litter on microbial activity and N cycling in soils from northern hardwood–conifer forests of the northeastern US. To create ecologically relevant conditions, we applied tannins to soil at a concentration (up to 2 mg g⁻¹ soil) typical of mineral soil horizons. Sugar maple tannins increased microbial respiration significantly more than red oak or hemlock tannins. The addition of sugar maple tannins also decreased gross N mineralization by 130% and, depending upon the rate of application, decreased net rates of N mineralization by 50–290%. At low concentrations, the decrease in mineralization appeared to be driven by greater microbial-N immobilization, while at higher concentrations the decrease in mineralization was consistent with the formation of recalcitrant PPCs. Low concentrations of red oak and hemlock tannins stimulated microbial respiration only slightly, and did not significantly affect fluxes of inorganic N in the soil. When applied to soils containing elevated levels of protein, red oak and hemlock tannins decreased N mineralization without affecting rates of microbial respiration, suggesting that PPC formation decreased substrate availability for microbial immobilization. Our results indicate that tannins from all three species form recalcitrant PPCs, but that the degree of PPC formation and its attendant effect on soil-N cycling depends on tannin concentration and the pool size of available protein in the soil.     

Calcium weathering in forested soils and the effect of different tree species

Soil weathering can be an important mechanism to neutralize acidity inforest soils. Tree species may differ in their effect on or response to soilweathering. We used soil mineral data and the natural strontium isotope ratio⁸⁷Sr/⁸⁶Sr as a tracer to identify the effect of treespecies on the Ca weathering rate. The tree species studied were sugar maple(Acer saccharum), hemlock (TsugaCanadensis), American beech (Fagusgrandifolia),red maple (Acer rubrum), white ash (FraxinusAmericana) and red oak (Quercus rubra) growingin a forest in northwestern Connecticut, USA. Three replicated sites dominatedby one of the six tree species were selected. At sugar maple and hemlock sitesthe dominant mineral concentrations were determined at three soil depths. Ateach site soil, soil water and stem wood of the dominant tree species weresampled and analyzed for the ⁸⁷Sr/⁸⁶Sr ratio, total SrandCa content. Atmospheric deposition was collected and analyzed for the sameconstituents. Optical analysis showed that biotite and plagioclaseconcentrations were lower in the soil beneath hemlock than beneath sugar mapleand suggested species effects on mineral weathering in the upper 10cm of the mineral soil. These results could not be confirmed withdata obtained by the Sr isotope study. Within the sensitivity of the Sr isotopemethod, we could not detect tree species effects on Ca weathering andcalculatedCa weathering rates were low at all sites (< 60mgm^–2yr^–1). Wefound a positive correlation between Ca weathering and the total Caconcentration in the surface soil. These results indicate that the absolutedifferences in Ca weathering rate between tree species in these acidic surfacesoils are small and are more controlled by the soil parent material(plagioclasecontent) than by tree species.     

Simulated long‐term effects of harvest and biomass residue removal on soil carbon and nitrogen content and productivity for two Upper Great Lakes forest ecosystems

We used the ecosystem process model Biome‐BGC to simulate the effects of harvest and residue removal management scenarios on soil carbon (C), available soil nitrogen (N), net primary production (NPP), and net ecosystem production (NEP) in jack pine (Pinus banksiana Lamb.) and sugar maple (Acer saccharum Marsh) ecosystems in northern Wisconsin, USA. To assess harvest effects, we simulated short (50‐year) and long (100‐year) harvest intervals, high (clear‐cut) and low (selective) harvest intensities, and three levels of residue retention (15%, 25%, and 35%) over a 500‐year period. The model simulation of NPP, soil C accumulation, and NEP agreed reasonably well with biometric and eddy‐covariance measurements of these two ecosystems. The more intensive (50‐year rotation clear‐cuts with low residue retention) harvest scenarios tended to have the greatest NEP (420 and 678 t C ha^?1 for the 500‐year interval for jack pine and sugar maple, respectively). All the harvest scenarios decreased mineral soil C and available mineral soil N content relative to the no‐harvest scenario for jack pine and sugar maple. The rate of change in mineral soil C decreased the greatest in the most intensive biomass removal scenarios (?0.012 and ?0.072 t C ha^?1 yr^?1 relative to no‐harvest for jack pine and sugar maple, respectively) and the smallest decrease was observed in the least intensive biomass removal scenarios (?0.002 and ?0.009 t C ha^?1 yr^?1 relative to no‐harvest for jack pine and sugar maple, respectively). The more intensive biomass removal harvest scenarios in sugar maple significantly decreased peak productivity (NPP) in the simulation period.     

Throughfall,stemflow and interception of rainwater in an evergreen broadleaved forest

In order to analyze the dyanmics of heavy metals in a forest ecosystem, throughfall and stemflow were collected for individual rain showers in an evergreen broad-leaved forest dominated byCastanopsis cuspidata. The relation between throughfall (or stemflow) (Px) and gross rainfall (P) was approximated by a linear regression equationPx=a(P-b). The values of coefficient “a” were 0.32–2.02 for throughfall at each sampling point and the mean values for 1976 and 1977–1978 were 0.682 and 0.767, respectively. The stemflow volume differed widely among individual trees, depending mainly on the tree form of each species. In particular, the tree form ofPasania edulis was found to be especially suited to collecting stemflow. OneP. edulis tree collected 64% of the rainwater that fell onto its crown as stemflow. The ratio of stemflow to gross rainfall decreased in summer resulting from an increase in leaf biomass and an increase in air temperature. In fact, the values of coefficient “a” for mean stemflow per unit area were 0.180 for summer and 0.229 for other seasons in 1976, and 0.145 for summer and 0.155 for other seasons in 1977–1978 for different sampled trees.     

Nitrogen deposition effects on soil organic matter chemistry are linked to variation in enzymes,ecosystems and size fractions

Recent research has dramatically advanced our understanding of soil organic matter chemistry and the role of N in some organic matter transformations, but the effects of N deposition on soil C dynamics remain difficult to anticipate. We examined soil organic matter chemistry and enzyme kinetics in three size fractions (>250 μm, 63–250 μm, and <63 μm) following 6 years of simulated atmospheric N deposition in two ecosystems with contrasting litter biochemistry (sugar maple, Acer saccharum—basswood, Tilia americana and black oak, Quercus velutina—white oak, Q. alba). Ambient and simulated (80-kg NO₃ ⁻–N ha⁻¹ year⁻¹) atmospheric N deposition were studied in three replicate stands in each ecosystem. We found striking, ecosystem-specific effects of N deposition on soil organic matter chemistry using pyrolysis gas chromatography/mass spectrometry. First, furfural, the dominant pyrolysis product of polysaccharides, was significantly decreased by simulated N deposition in the sugar maple–basswood ecosystem (15.9 vs. 5.0%) but was increased by N deposition in the black oak–white oak ecosystem (8.8 vs. 24.0%). Second, simulated atmospheric N deposition increased the ratio of total lignin derivatives to total polysaccharides in the >250 μm fraction of the sugar maple–basswood ecosystem from 0.9 to 3.3 but there were no changes in other size classes or in the black oak–white oak ecosystem. Third, simulated N deposition increased the ratio of lignin derivatives to N-bearing compounds in the 63–250 and >250 μm fractions in both ecosystems but not in the <63 μm fraction. Relationships between enzyme kinetics and organic matter chemistry were strongest in the particulate fractions (>63 μm) where there were multiple correlations between oxidative enzyme activities and concentrations of lignin derivatives and between glycanolytic enzyme activities and concentrations of carbohydrates. Within silt-clay fractions (<63 μm), these enzyme-substrate correlations were attenuated by interactions with particle surfaces. Our results demonstrate that variation in enzyme activity resulting from atmospheric N deposition is directly linked to changes in soil organic matter chemistry, particularly those that occur within coarse soil size fractions. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Use of foliar Ca/Sr discrimination and <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr ratios to determine soil Ca sources to sugar maple foliage in a northern hardwood forest

Calcium/strontium and ⁸⁷Sr/⁸⁶Sr ratios in foliage can be used to determine the relative importance of different soil sources of Ca to vegetation, if the discrimination of Ca/Sr by the plant between nutrient sources and foliage is known. We compared these tracers in the foliage of sugar maple (Acer saccharum) to the exchange fraction and acid leaches of soil horizons at six study sites in the White Mountains of New Hampshire, USA. In a previous study, sugar maple was shown to discriminate for Ca compared to Sr in foliage formation by a factor of 1.14 ± 0.12. After accounting for the predicted 14% shift in Ca/Sr, foliar Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios closely match the values in the Oie horizon at each study site across a 3.6-fold variation in foliar Ca/Sr ratios. Newly weathered cations, for which the Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios are estimated from acid leaches of soils, can be ruled out as a major Ca source to current foliage. Within sites, the ⁸⁷Sr/⁸⁶Sr ratio of the soil exchange pool in the Oa horizon and in the 0–10 cm and 10–20 cm increments of the mineral soil are similar to the Oie horizon and sugar maple foliar values, suggesting a common source of Sr in all of the actively cycling pools, but providing no help in distinguishing among them as sources to foliage. The Ca/Sr ratio in the soil exchange pool, however, decreases significantly with depth, and based on this variation, the exchange pool below the forest floor can be excluded as a major Ca source to the current sugar maple foliage. This study confirms that internal recycling of Ca between litter, organic soil horizons and vegetation dominate annual uptake of Ca in northern hardwood ecosystems. Refinement of our understanding of Ca and Sr uptake and allocation in trees allows improvement in the use of Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios to trace Ca sources to plants.     

Effect of soil K, Ca and Mg saturation and endomycorrhization on growth and nutrient uptake of sugar maple seedlings

Nutrient imbalances of declining sugar maple (Acer saccharum Marsh.) stands in southeastern Quebec have been associated with high exchangeable Mg levels in soils relative to soil K and Ca. A greenhouse experiment was set up to test the hypothesis that the equilibrium between soil exchangeable K, Ca, and Mg ions influences the growth and nutrient status of sugar maple seedlings. Also tested was whether endomycorrhization can alter nutrient acquisition under various soil exchangeable basic cations ratios. Treatments consisted of seven ratios of soil exchangeable K, Ca, and Mg making up a total base saturation of 58%, and a soil inoculation treatment with the endomycorrhizal fungus Glomus versiforme (control and inoculated), in a complete factorial design. Sugar maple seedlings were grown for 3 months in the treated soils. Plant shoot elongation rate, dry biomass and nutrient concentrations in foliage were influenced by the various ratios of soil cations. The predicted plant biomass and foliar K concentration were highest at a soil Ca saturation of 38%, a soil K saturation of 12%, and a soil Mg saturation of 8%. Potassium concentration in foliage was dependent on the level of Ca and Mg saturation in the soil when soil K saturation was close to 12%. Foliar Ca and Mg levels were more dependent on their corresponding levels in soil than foliar K. Colonization by G. versiforme did not influence seedling growth and macronutrient uptake. The results confirm that growth and nutrition of sugar maple are negatively affected by imbalances in exchangeable basic cations in soils.     

Water inputs across a tropical montane landscape in Veracruz,Mexico: synergistic effects of land cover,rain and fog seasonality,and interannual precipitation variability

Land‐cover change can alter the spatiotemporal distribution of water inputs to mountain ecosystems, an important control on land‐surface and land‐atmosphere hydrologic fluxes. In eastern Mexico, we examined the influence of three widespread land‐cover types, montane cloud forest, coffee agroforestry, and cleared areas, on total and net water inputs to soil. Stand structural characteristics, as well as rain, fog, stemflow, and throughfall (water that falls through the canopy) water fluxes were measured across 11 sites during wet and dry seasons from 2005 to 2008. Land‐cover type had a significant effect on annual and seasonal net throughfall (NTF <0=canopy water retention plus canopy evaporation; NTF >0=fog water deposition). Forest canopies retained and/or lost to evaporation (i.e. NTF<0) five‐ to 11‐fold more water than coffee agroforests. Moreover, stemflow was fourfold higher under coffee shade than forest trees. Precipitation seasonality and phenological patterns determined the magnitude of these land‐cover differences, as well as their implications for the hydrologic cycle. Significant negative relationships were found between NTF and tree leaf area index (R²=0.38, P<0.002), NTF and stand basal area (R²=0.664, P<0.002), and stemflow and epiphyte loading (R²=0.414, P<0.001). These findings indicate that leaf and epiphyte surface area reductions associated with forest conversion decrease canopy water retention/evaporation, thereby increasing throughfall and stemflow inputs to soil. Interannual precipitation variability also altered patterns of water redistribution across this landscape. Storms and hurricanes resulted in little difference in forest‐coffee wet season NTF, while El Niño Southern Oscillation was associated with a twofold increase in dry season rain and fog throughfall water deposition. In montane headwater regions, changes in water delivery to canopies and soils may affect infiltration, runoff, and evapotranspiration, with implications for provisioning (e.g. water supply) and regulating (e.g. flood mitigation) ecosystem services.     

Nutrient cycling in Pinus sylvestris stands in eastern Finland

Nutrient cycling within three Pinus sylvestris stands was studied in eastern Finland. The aim of the study was to determine annual fluxes and distribution of N, P, K, Ca, Mg, Zn, Fe, B, and Al in the research stands. Special emphasis was put on determining the importance of different fluxes, especially the internal cycle within the trees in satisfying the tree nutrient requirements for biomass production. The following nutrient fluxes were included, input; free precipitation and throughfall, output; percolation through soil profile, biological cycle; nutrient uptake from soil, retranslocation within trees, return to soil in litterfall, release by litter decomposition. The distribution of nutrients was determined in above- and belowground tree compartments, in ground and field vegetation, and in soil.The nitrogen use efficiencies were 181, 211 and 191 g of tree aboveground dry matter produced per g of N supplied by uptake and retranslocation in the sapling, pole stage and mature stands, respectively. Field vegetation was more efficient in nitrogen use than trees. Stand belowground/aboveground and fine root/coarse root biomass ratios decreased with tree age. With only slightly higher fine root biomass, almost three times more nitrogen had to be taken-up from soil for biomass production in the mature stand than in the sapling stand.The annual input-output balances of most nutrients were positive; throughfall contained more nutrients than was lost in mineral soil leachate. The sulphate flux contributed to the leaching of cations, especially magnesium, from soil in the mature stand.Retranslocation supplied 17–42% of the annual N, P and K requirements for tree aboveground biomass production. Precipitation and throughfall were important in transferring K and Mg, and also N in the sapling stand. Litterfall was an important pathway for N, Ca, Mg and micro nutrients, especially in the oldest stands.     

Decades of atmospheric deposition have not resulted in widespread phosphorus limitation or saturation of tree demand for nitrogen in southern New England

It is commonly assumed that nitrogen (N) is the primary mineral resource limiting the productivity of temperate forests. Sustained inputs of N via atmospheric deposition are altering the N status of temperate forests raising the possibility that nutrients such as phosphorus (P) are increasingly limiting productivity. The objective of this study was to determine whether P availability limits tree growth alone or in combination with N. This study was conducted in two forest types common throughout the New England landscape of the northeastern United States; in sugar maple and white ash dominated stands growing on base rich parent material characterized by rapid rates of N cycling and high N availability, and in red oak–beech–hemlock dominated stands growing on base-poor parent material characterized by slow rates of N cycling and low N availability. Starting in 2004, N and P were added to replicate plots in each forest type in factorial combination at a rate of 150 and 50 kg ha⁻¹ year⁻¹, respectively. Diameter growth rates of all trees >10 cm DBH were measured in 2005 and 2006 using dendrometer bands and converted into units of basal area increment (BAI) and wood production. Following 2 years of fertilization, basal area increment in the sugar maple–white ash forests remained strongly N limited. Fertilization with P did not significantly increase BAI alone, although both N and P fertilization tended (P < 0.10) to increase diameter growth in white ash. Wood production in the N-fertilized plots increased by 100 g C m⁻² year⁻¹, roughly doubling production in the non-fertilized plots. In the red oak–beech–hemlock stands, there was no overall effect of N or P fertilization on BAI or wood production because BAI in some species was stimulated by fertilization with N alone (e.g., black cherry, red oak), while in other species BAI was unaffected (e.g., red maple, beech) or negatively affected by fertilization with N or P (e.g., eastern hemlock). Given that BAI in several tree species responded to fertilization with N alone and that only one species responded to P fertilization once N was added, this study suggests that decades of atmospheric N deposition have not (yet) resulted in widespread P limitation or saturation of tree demand for N.     

Trace elements in the hydrologic cycle of a forest ecosystem

Summary Concentrations of Cu, Fe, Mn, and Zn were measured in bulk atmospheric precipitation, throughfall, stemflow, and soil solutions at 10−, 15−, 25−, and 30-cm depths, in aEucalyptus globulus forest in the Berkeley hills, California, during the 1974–75 wet season after each main storm event. Litter and plant samples were analyzed. There was some similarity in the behavior of Cu, Fe, and Zn, but Mn behaved differently. Mn and Zn are largely deposited on the forest canopy by impaction during dry-deposition episodes, whereas most of the Cu and Fe input occurs in rain. For the hydrologic components measured, concentrations of Cu and Fe increase in the order: precipitation<throughfall<stemflow <soil solutions. For Zn the order is: precipitation<stemflow<throughfall<soil solutions. Concentrations of Cu, Zn, and Fe in the soil solution fluctuate with downward movement of wetting fronts and are negatively correlated with pH. Concentrations of Fe in soil solution are about 10 times greater than those of throughfall and stemflow; the corresponding relative differences for Cu and Zn were much less. Plant uptake of Mn exceeds that of Cu, Zn, and Fe. The increases in Mn concentrations from precipitation to throughfall and stemflow are much greater than those for Cu, Zn, and Fe because precipitation has very low Mn concentrations. The concentration series for Mn is: precipitation<soil solutions<throughfall<stemflow. Concentrations of Mn in the soil solution are negatively correlated with pH. During the dry summer Mn accumulates in the soil, but is quickly flushed by early rains of the wet season.     

The effects of gap disturbance on nitrogen cycling and retention in late-successional northern hardwood–hemlock forests

Late-successional forests in the upper Great Lakes region are susceptible to nitrogen (N) saturation and subsequent nitrate (NO₃⁻) leaching loss. Endemic wind disturbances (i.e., treefall gaps) alter tree uptake and soil N dynamics; and, gaps are particular susceptible to NO₃⁻ leaching loss. Inorganic N was measured throughout two snow-free periods in throughfall, forest floor leachates, and mineral soil leachates in gaps (300–2,000 m², 6–9 years old), gap-edges, and closed forest plots in late-successional northern hardwood, hemlock, and northern hardwood–hemlock stands. Differences in forest water inorganic N among gaps, edges, and closed forest plots were consistent across these cover types: NO₃⁻ inputs in throughfall were significantly greater in undisturbed forest plots compared with gaps and edges; forest floor leachate NO₃⁻ was significantly greater in gaps compared to edges and closed forest plots; and soil leachate NO₃⁻ was significantly greater in gaps compared to the closed forest. Significant differences in forest water ammonium and pH were not detected. Compared to suspected N-saturated forests with high soil NO₃⁻ leaching, undisturbed forest plots in these late-successional forests are not losing NO₃⁻ (net annual gain of 2.8 kg ha⁻¹) and are likely not N-saturated. Net annual NO₃⁻ losses were observed in gaps (1.3 kg ha⁻¹) and gap-edges (0.2 kg ha⁻¹), but we suspect these N leaching losses are a result of decreased plant uptake and increased soil N mineralization associated with disturbance, and not N-saturation.     

Detritus Production and Soil N Transformations in Old-Growth Eastern Hemlock and Sugar Maple Stands

To examine the linkage between forest cover type, litter inputs, and patterns of net N mineralization versus the turnover of N among soil microbes, we measured both the net and gross rates of N mineralization in replicated, adjacent old-growth eastern hemlock [Tsuga canadensis(L.) Carr.] or sugar maple (Acer saccharum Marsh.) stands in upper Michigan. Mean aboveground net primary production and annual litterfall mass were significantly higher (P < 0.01) in the maple forests (870 g·m^-2·y^-1 and 439 g·m^-2·y^-1, respectively) than in the hemlock forests (480 g·m^-2·y^-1 and 344 g·m^-2·y^-1, respectively). Forest floor and coarse woody debris mass, however, were significantly lower (P < 0.05) in the maple forests (2.2 and 0.1 kg·m^-2, respectively) than in the hemlock forests (2.9 and 0.2 kg·m^-2, respectively). Litterfall N concentration was not significantly different (P > 0.10) between the two forest types. In situ gross rates of N mineralization were higher (P < 0.06) in the maple forests than in the hemlock forests (7.5 and 6.1 mg N·kg soil^-1·d^-1 respectively), but in situ net N mineralization varied independently of forest type and stand-level litterfall N concentration. Cover type–dependent differences in detritus production and detritus C quality appear to result in different N turnover rates, but the balance between gross mineralization and immobilization of N is very sensitive to within stand variability and varies at a scale smaller than cover type alone can predict. Received 3 Feburary 1999; accepted 27 August 1999.     

Fine Root Dynamics and Forest Production Across a Calcium Gradient in Northern Hardwood and Conifer Ecosystems

Losses of soil base cations due to acid rain have been implicated in declines of red spruce and sugar maple in the northeastern USA. We studied fine root and aboveground biomass and production in five northern hardwood and three conifer stands differing in soil Ca status at Sleepers River, VT; Hubbard Brook, NH; and Cone Pond, NH. Neither aboveground biomass and production nor belowground biomass were related to soil Ca or Ca:Al ratios across this gradient. Hardwood stands had 37% higher aboveground biomass (P = 0.03) and 44% higher leaf litter production (P < 0.01) than the conifer stands, on average. Fine root biomass (<2 mm in diameter) in the upper 35 cm of the soil, including the forest floor, was very similar in hardwoods and conifers (5.92 and 5.93 Mg ha⁻¹). The turnover coefficient (TC) of fine roots smaller than 1 mm ranged from 0.62 to 1.86 y⁻¹ and increased significantly with soil exchangeable Ca (P = 0.03). As a result, calculated fine root production was clearly higher in sites with higher soil Ca (P = 0.02). Fine root production (biomass times turnover) ranged from 1.2 to 3.7 Mg ha⁻¹ y⁻¹ for hardwood stands and from 0.9 to 2.3 Mg ha⁻¹ y⁻¹ for conifer stands. The relationship we observed between soil Ca availability and root production suggests that cation depletion might lead to reduced carbon allocation to roots in these ecosystems.     

Litterflow chemistry and nutrient uptake from the forest floor in northwest Amazonian forest ecosystems

Samples of the fraction of net rainfall passing through the forest floor collected at monthly intervals in four pristine forests in Colombian Amazonia, during the period between 1995–1997 were analysed for solute concentrations to estimate the element fluxes from the forest floor into the mineral soil and root nutrient uptake from these forest floors. Results were compared with inputs by throughfall, stemflow, litterfall and fine root decay. Element concentrations were tested for their relationship with litterflow amounts, rainfall intensity and length of the antecedent dry period and differences in element fluxes between ecosystems were assessed. Concentrations of elements in litterflow followed a similar pattern as those in throughfall, which indicates that element outputs from the forest floor are strongly related to those inputs in throughfall. In the forests studied, the average concentrations of elements as K, Mg, orthoP and the pH of the litterflow decreased relative to that in throughfall in most events, while the concentration of elements such as dissolved organic carbon, H, SO₄ and Si increased in litterflow from these forests. Element concentrations in litterflow showed a poor correlation with variables such as litterflow amounts, rainfall intensity and antecedent dry period, except for K which showed a significant correlation (p>0.95) with analysed variables in all forests. Outputs were significantly different between forests (p>0.95); these fluxes, which particularly concerned cations, being the largest in the flood plain, while for anions outputs increased from the flood plain to the sedimentary plain. After adding the nutrient contributed by litter decomposition and fine root decay, the net outputs of main elements from the forest floors were still smaller than inputs by net precipitation (throughfall+stemflow) indicating that the litter layers clearly acted as a sink for most nutrients. Accordingly, the element balances confirm that the forest floors acted as a sink for nutrients coming in by throughfall, stemflow, litterfall and fine root decomposition. P, Mg and N appeared to be the most limiting nutrients and the forests studied efficiently recycled these nutrients.     

Spatial heterogeneity of soil chemical properties at fine scales induced by <Emphasis Type="Italic">Haloxylon ammodendron</Emphasis> (Chenopodiaceae) plants in a sandy desert

Spatial heterogeneity is considered a ubiquitous feature in natural ecosystems. Fertile islands represent a typical example for such heterogeneity in desert ecosystems. The soil pH and salinity also show significant heterogeneity in fertile islands. To investigate the distribution of soil salinity and nutrients around individual shrubs and the major factors influencing their distribution, an experiment was conducted at the scale of the rhizosphere, root system, and individual for Haloxylon ammodendron (C. A. Mey.) Bunge (Chenopodiaceae) shrub in the Gurbantünggüt Desert. Specifically, the heterogeneity of the following soil chemical parameters was evaluated: pH, electrical conductivity (EC), soil organic carbon (SOC), total nitrogen (TN), and available phosphorus (AP). The chemical properties of the shrub stemflow were also evaluated to determine its contribution to the formation of fertile islands and the distribution of soil salinity. The results revealed great variance in the soil pH and EC at the rhizosphere and root system scales, indicating that a single root or root system exerts a great effect on soil pH and salinity. At the individual scale, the content of SOC, TN, and AP was significantly enriched in the layers adhering to the taproot, and this enrichment extended 20–40 cm from the taproot. Conversely, the soil pH and EC were significantly lower from the taproot to 10–25 cm away from the root, indicating that the fertile island is also an island of low alkalinity/salinity. Comparison of the chemical properties of stemflow and bulk precipitation revealed a higher content of chemical elements (except pH and CO₃ ²⁻) in the stemflow, indicating that the fertile island and lower pH and EC in this island were likely formed by the effects of stemflow. Specifically, stemflow brings in water and nutrients, while reducing the salt levels. Overall, the high nutrients and low alkalinity/salinity island created around the taproot favor the growth of the plants.     

Nitrogen cycling in a northern hardwood forest: Do species matter?

To investigate the influence of individual tree species on nitrogen (N) cycling in forests, we measured key characteristics of the N cycle in small single-species plots of five dominant tree species in the Catskill Mountains of New York State. The species studied were sugar maple (Acer saccharum), American beech (Fagus grandifolia), yellow birch (Betula alleghaniensis), eastern hemlock (Tsuga canadensis), and red oak (Quercus rubra). The five species varied markedly in N cycling characteristics. For example, hemlock plots consistently showed characteristics associated with "slow" N cycling, including low foliar and litter N, high soil C:N, low extractable N pools, low rates of potential net N mineralization and nitrification and low NO ₃ ^– amounts trapped in ion-exchange resin bags buried in the mineral soil. Sugar maple plots had the lowest soil C:N, and the highest levels of soil characteristics associated with NO ₃ ^– production and loss (nitrification, extractable NO ₃ ^– , and resin bag NO ₃ ^– ). In contrast, red oak plots had near-average net mineralization rates and soil C:N ratios, but very low values of the variables associated with NO ₃ ^– production and loss. Correlations between soil N transformations and litter concentrations of N, lignin, lignin:N ratio, or phenolic constituents were generally weak. The inverse correlation between net nitrification rate and soil C:N that has been reported in the literature was present in this data set only if red oak plots were excluded from the analysis. This study indicates that tree species can exert a strong control on N cycling in forest ecosystems that appears to be mediated through the quality of soil organic matter, but that standard measures of litter quality cannot explain the mechanism of control.     

西藏原始林芝云杉林雨季林冠降水分配特征

利用2006-2007年对西藏米林县南伊沟原始林芝云杉(Picea likiangensis var.linzhiensis)林林外降水、穿透水和树干茎流定位观测数据,对林芝云杉林的林冠降水再分配特征进行研究。结果表明:西藏南伊沟的年降水量为716.4mm,主要集中在4-9月份,占全年降雨量的86.95%。在林芝云杉的生长季节(4-10月份),林冠截留量为338.6mm,占同期林外降水量的51.60%;林内穿透水量为316.3mm,占同期林外降水量的48.21%;树干茎流量仅为1mm,仅占0.19%。林内穿透水(Tp)、树干茎流(Sf)、林冠截留量(Ip)及林冠截留率(PIp)与林外降水量(p)之间的关系分别为:Tp=0.8622p-3.5229,r=0.9964;Sf=0.0004p1.4586,r=0.9458;Ip=1.2222p0.6341,r=0.874;PIp=253.6p-0.7008,r=0.9732。林芝云杉林雨季林冠降水的分配规律与该森林结构复杂、林分年龄高、胸高断面积大密切相关,说明该森林在涵养水源和保持水土等方面发挥着重要的作用。     

Soil organic matter and litter chemistry response to experimental N deposition in northern temperate deciduous forest ecosystems

The effects of atmospheric nitrogen (N) deposition on organic matter decomposition vary with the biochemical characteristics of plant litter. At the ecosystem‐scale, net effects are difficult to predict because various soil organic matter (SOM) fractions may respond differentially. We investigated the relationship between SOM chemistry and microbial activity in three northern deciduous forest ecosystems that have been subjected to experimental N addition for 2 years. Extractable dissolved organic carbon (DOC), DOC aromaticity, C : N ratio, and functional group distribution, measured by Fourier transform infrared spectra (FTIR), were analyzed for litter and SOM. The largest biochemical changes were found in the sugar maple–basswood (SMBW) and black oak–white oak (BOWO) ecosystems. SMBW litter from the N addition treatment had less aromaticity, higher C : N ratios, and lower saturated carbon, lower carbonyl carbon, and higher carboxylates than controls; BOWO litter showed opposite trends, except for carbonyl and carboxylate contents. Litter from the sugar maple–red oak (SMRO) ecosystem had a lower C : N ratio, but no change in DOC aromaticity. For SOM, the C : N ratio increased with N addition in SMBW and SMRO ecosystems, but decreased in BOWO; N addition did not affect the aromaticity of DOC extracted from mineral soil. All ecosystems showed increases in extractable DOC from both litter and soil in response to N treatment. The biochemical changes are consistent with the divergent microbial responses observed in these systems. Extracellular oxidative enzyme activity has declined in the BOWO and SMRO ecosystems while activity in the SMBW ecosystem, particularly in the litter horizon, has increased. In all systems, enzyme activities associated with the hydrolysis and oxidation of polysaccharides have increased. At the ecosystem scale, the biochemical characteristics of the dominant litter appear to modulate the effects of N deposition on organic matter dynamics.