Critical deposition levels for nitrogen and sulphur on dutch forest ecosystems

Critical loads for N and S on Dutch forest ecosystems have been derived in relation to effects induced by eutrophication and acidification, such as changes in forest vegetation, nutrient imbalances, increased susceptibility to diseases, nitrate leaching, and Al toxicity. The criteria that have been used are N contents in needles, nitrate concentrations in groundwater (drinking water), and NH₄/K ratios, Ca/Al ratios, and Al concentrations in the soil solution. Assuming an equal contribution of N and S, all effects seem to be prevented at a total deposition level below 600 mol_c ha^?1 yr^?1 due to N uptake by stemwood and acid neutralization by base cation weathering. The most serious effects will probably be prevented at total deposition levels between 1500 and 2000 mol_c ha^?1 yr^?1. The current average deposition in the Netherlands is 4900 mol_c ha^?1 yr^?1.     

Environmental pollution and impacts on soils and forests nutrition in North America

The effects of acid deposition, excess N deposition, and elevated CO₂ on forest soils and nutrition in North America are reviewed. While there remains the possibility that acid deposition and excess N deposition are contributing to declines in red spruce, sugar maple, and southern pines, clearcut cause and effects are still not evident. Climate is clearly a major factor in red spruce decline in the northeastern U.S., but air pollution may contribute. There is some evidence that soil solution Al may be approaching deleterious levels in southeastern red spruce forests. Lack of proper management may be a major factor in the sugar maple and southern pine declines, but once again, air pollution as a potential contributor cannot be ignored. Nutrient budget analyses and discoveries of soils base cation depletion in certain sites suggest that base cation status is declining in forests of the southeastern U.S., but thus far, base cation deficiencies are uncommon. Recent research has revealed that there are more cases of N-saturated forests in North America than was previously suspected. These systems are characterized by high rates of soil N mineralization, high atmospheric N inputs, low uptakes, or some combination of these factors. Soil leaching and Al mobilization in such systems is often dominated by nitrate. However, the geographical extent of these types of systems is limited, and the traditional view that most forest ecosystems are N limited remains valid, especially where forest management is intensive. The limited information available on tree response to CO₂. suggests N-deficient plants often grow faster with elevated CO₂, whereas P-deficient plants often do not. Research is needed to 1) determine if the differences in response between N- and P-deficient plants is common, 2) the responses of plants deficient in other nutrients to elevated CO₂, and 3) the interactions of CO₂ increase, nutrient deficiencies, climate change.     

Successional changes in microbial biomass,respiration and nutrient status during litter decomposition in an aspen and pine forest

Microbial biomass, microbial respiration, metabolic quotient (qCO₂), C_mic/C_org ratio and nutrient status of the microflora was investigated in different layers of an aspen (Populus tremuloides Michx.) and pine forest (Pinus contorta Loud.) in southwest Alberta, Canada. Changes in these parameters with soil depth were assumed to reflect successional changes in aging litter materials. The microbial nutrient status was investigated by analysing the respiratory response of glucose and nutrient (N and P) supplemented microorganisms. A strong decline in qCO₂ with soil depth indicated a more efficient C use by microorganisms in later stages of decay in both forests. C_mic/C_org ratio also declined in the aspen forest with soil depth but in the pine forest it was at a maximum in the mineral soil layer. Microbial nutrient status in aspen leaf litter and pine needle litter indicated N limitation or high N demand, but changes in microbial nutrient status with soil depth differed strongly between both forests. In the aspen forest N deficiency appeared to decline in later stages of decay whereas P deficiency increased. In contrast, in the pine forest microbial growth was restricted mainly by N availability in each of the layers. Analysis of the respiratory response of CNP-supplemented microorganisms indicated that growth ability of microorganisms is related to the fungal-bacterial ratio.     

Effect of nitrogen source on aluminum toxicity in nonmycorrhizal and ectomycorrhizal pitch pine seedling

The effect of elevated nitrate [(NO₃‐nitrogen (N)] or ammonium (NH₄)‐N on the response of nonmycorrhizal (NM) and ectomycorrhizal (ECM) pitch pine (Pintis rigida Mill.) seedlings to aluminum (Al) was determined in experiments in which N was increased three times above ambient levels. Seedlings with and without the mycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch were grown in sand irrigated with nutrient solution (pH 3.8) containing 0, 10, or 20 mg Al L^‐1 (0, 370, or 740 μM Al). The nutrient solution simulated that for the sandy, nutrient‐poor soil of the New Jersey Pine Barrens. Elevated NO₃‐N had no significant effect on Al toxicity in NM seedlings, but Al toxicity at ambient NH₄‐N was ameliorated by elevated NH₄‐N. Symptoms of Al toxicity in roots (thick and stunted) of ECM seedlings at ambient N levels were reduced by elevated NH₄‐N and absent at elevated NO₃‐N. When N was elevated by an increase in NO₃‐N or NH₄‐N, uptake of N and relative increases in total biomass were greater in ECM than in NM seedlings.     

The wood-decaying fungus Hygrophoropsis aurantiaca increases P availability in acid forest humus soil, while N addition hampers this effect

We evaluated the influence of the brown rot fungus Hygrophoropsis aurantiaca on P solubility in the humus layer of a podzolic forest soil. This fungus is known to exude large amounts of oxalic acid that may stimulate weathering of minerals and increase dissolution of humus, which in turn may increase P availability in the soil surrounding the fungus. Humus was inoculated using small wooden pieces colonised by the fungus. The presence of the fungus resulted in elevated concentration of PO₄⁻ in the humus solution. In a second experiment birch seedlings grown in the same humus were able to utilise the PO₄⁻ mobilised by the fungus to increase their internal P content. The factor determining this increased P uptake and the increased available P might be oxalate produced by fungus. The acid may directly dissolve P or change organic forms of P making it more susceptible to reaction with phosphatases. This fungal effect on P solubility diminished when N was added to the soil in the form of a slow release N fertilizer (methyl urea), or when a soil with a higher soil N concentration was used. We found a strong correlation between NH₄⁺ concentration and total organic carbon in the soil solution at high NH₄⁺ concentrations, suggesting the dissolution of humus as a result of the high NH₄⁺ content in the solution. This study demonstrates that the wood-decaying fungus H. aurantiaca influences nutrient turnover in forest soil, and thereby nutrient uptake by forest trees. An intensified harvest of forest products such as whole tree harvesting may decrease the active biomass of the wood decomposers and may thereby change the availability of P and the leaching of N.     

Effects of reduced atmospheric deposition on soil solution chemistry and elemental contents of spruce needles in NE—Bavaria,Germany

The decrease in anthropogenic deposition, namely SO₄^2— and SO₂, in European forest ecosystems during the last 20 years has raised questions concerning the recovery of forest ecosystems. The aim of this study was to evaluate if the long term data of element concentrations at the Fichtelgebirge (NE‐Bavaria, Germany) monitoring site indicates a relationship between the nutrient content of needles and the state of soil solution acidity. The soil at the site is very acidic and has relatively small pools of exchangeable Ca and Mg. The trees show medium to severe nutrient deficiency symptoms such as needle loss and needle yellowing. The Ca and Mg concentrations in throughfall decreased significantly during the last 12 years parallel to the significant decline in the throughfall of H⁺ and SO₄^2— concentrations. Soil solution concentrations of SO₄^2—, Ca and Mg generally decreased while the pH value remained stable. Aluminum concentrations decreased slightly, but only at a depth of 90 cm. Simultaneously a decrease in the molar Ca/Al and Mg/Al ratios in the soil solution was observed. Ca and Mg contents in the spruce needles decreased, emphasizing the relevance of soil solution changes for tree nutrition. The reasons for the delay in ecosystem recovery are due to a combination of the following two factors: (1) the continued high concentrations of NO₃^— and SO₄^2— in the soil solution leading to high Al concentrations and low pH values and, (2) the decreased rates of Ca and Mg deposition cause a correlated decrease in the concentration of Ca and Mg in the soil solution, since little Ca and Mg is present in the soil's exchangeable cation pools. It is our conclusion that detrimental soil conditions with respect to Mg and Ca nutrition as well as to Al stress are not easily reversed by the decreasing deposition of H⁺ and SO₄^2—. Thus, forest management is still confronted with the necessity of frequent liming to counteract the nutrient depletion in soils and subsequent nutrient deficiencies in trees.     

Charcoal production at kiln sites affects C and N dynamics and associated soil microorganisms in Quercus spp. temperate forests of central Mexico

Temperate forests dominated by Quercus spp. cover large parts of Central Mexico and rural communities depend on these forests for wood and charcoal. The impacts of charcoal production on selected chemical properties including C and N dynamics, and populations of ammonifiers, nitrifiers and denitrifiers were investigated on surface soils (0–15 cm) collected during the dry and rainy season of these forests. Organic C was halved in soil at the kiln sites compared to undisturbed forest soil. Concentrations of exchangeable Ca²⁺, K⁺ and Mg²⁺ increased >1.6 times at kiln sites and pH increased from 4.5 in undisturbed soil to 7.0 at kiln sites. The kiln sites had 1.3 times and 2.4 times lower microbial biomass C and N, respectively, than undisturbed forest sites during the rainy season. Although the effect of charcoal production on NH₄⁺, NO₂^? and NO₃^? concentrations was small, the ammonifying, nitrifying and denitrifiers were 16 times lower at the kiln sites than in the undisturbed forest soil. This research found that the charcoal production had a negative effect on the cultivable microorganisms involved in N cycling and the soil microbial biomass C and N compared to undisturbed forest soil. Differences in inorganic N dynamics were more affected by seasonality, i.e. precipitation, than by charcoal production.     

Effects of fertilization with MgSO4 and (NH4)2SO4 on soil solution chemistry,mycorrhiza and nutrient content of fine roots in a norway spruce stand

This paper focuses on the short-term reaction of fine root and mycorrhiza on changes in soil solution chemistry following application of MgSO₄ (Kieserite) and (NH₄)₂SO₄ (ammonium sulfate). The experiments were conducted within the ARINUS Experimental Watershed Area near Schluchsee in the Black Forest (SW Germany). Yellowing of the older needles as related to Mg deficiency was the typical symptom observed within this 45 yr old Norway spruce stand. On the N treated plot the relative mycorrhiza frequency declined and the percentage of nonmycorrhizal root tips increased, whereas in the Mg fertilized plot these parameters did not differ from the control. The observed changes cannot be caused by Al, because elevated concentrations of potentially toxic Al species and extremely low Ca/A1 molar ratios appeared in the soil solution of both treatments and did not result in reduced growth of long roots as reported from solution culture experiments. Moreover, the Al content of fine roots did not increase. Therefore, it is concluded that the thresholds for Al toxicity derived from solution culture experiments with nonmycorrhizal seedlings cannot be transferred to forest stands. A direct toxic effect of elevated NH₄ ⁺ concentrations on mycorrhiza is unlikely, but cannot be excluded. Enhanced root growth due to a higher uptake of NH₄ ⁺ from soil solution may provide a more plausible explanation for the observed increase in the percentage of nonmycorrhizal root tips after N application. Even though the N content of fine roots did not increase, the diminished K content gives some indirect indication for NH₄ ⁺ uptake by the roots. This is also consistent with reduced Mg content due to NH ₄ ⁺ /Mg²⁺ antagonism. On the MgSO₄ treated plot, Mg contents of the fine roots increased thus reflecting Mg uptake by the deficient stand.     

Carbon,nitrogen and phosphorus dynamics of ant mounds (Formica rufa group) in managed boreal forests of different successional stages

Wood ants (Formica rufa group) are ubiquitous in European boreal forests and their large long-lived mound nests, which mainly consist of forest litter and resin, accumulate carbon (C) and nutrients. The C and nutrient dynamics of wood ant mounds in response to forest succession have received minor attention in boreal forests. We aimed to study whether the C, nitrogen (N) and phosphorus (P) concentrations and the bulk density of ant mounds differ from those of the surrounding forest soil, to estimate the C, N and P pools in ant mounds, and to test whether the concentrations and pools change with forest age. Norway spruce (Picea abies (L.) Karst.) stands on medium-fertile sites in 5-, 30-, 60- and 100-year stand age classes were studied in eastern Finland. Carbon and P concentrations in the above-ground mound material were higher than those in the surrounding organic layer. The C, N and extractable P concentrations were higher in the soil under the ant mounds than in the surrounding mineral soil (0–21 cm). The low bulk densities in the ant mounds and the soil below them could be a result of the porous structure of ant mounds and the soil-mixing activities of the ants. The C/N ratios were higher in the mounds than in the organic layer. Carbon concentrations in the ant mounds increased slightly with stand age. Carbon, N and P pools in the ant mounds increased considerably with stand age. Carbon, N and P pools in ant mounds were <1% of those in the surrounding forest soil. Nevertheless, the above- and belowground parts of the ant mounds contained more C, N and P per sampled area than the surrounding forest soil. Wood ants therefore increase the spatial heterogeneity in C and nutrient distribution at the ecosystem level.     

Direct contribution by soil arthropods to nutrient availability through body and faecal nutrient content

Summary The direct contribution made by soil arthropods to nutrient dynamics was investigated in pine forests that differed in soil nutrient status. Nutrient concentrations (K⁺, Ca²⁺, Mg²⁺, PO ₄ ^3– , N, C) in the most abundant species and groups of arthropods in two Pinus nigra forests were compared, and distinct differences were found among taxonomic groups. In the rank order: collembolans, oribatides, isopods, diplopods, Ca²⁺ and Mg²⁺ concentrations increased, while N and C concentrations decreased. The nutrient concentrations in individuals of the same species but originating from the different forests were similar, except for the isopod Philoscia muscorum. The total and available nutrient concentrations in food and faeces of the collembolan Tomocerus minor and the isopod Philoscia muscorum were compared. The isopod faeces contained relatively less K⁺ and Mg²⁺, and more Ca²⁺, PO ₄ ^3– , and greater N availability, compared with the food material. The collembolan faeces showed a higher availability of all nutrients measured. The N species appeared to be changed by collembolans; their faeces contained high NO ₃ ^– concentrations, while their food contained relatively high concentrations of NH ₄ ⁺ . These findings were examined in relation to their significance for ecosystem functioning. It was concluded that about 12% of the total K⁺, PO ₄ ^3– , N and 2% of the Ca²⁺ in the organic layer was found in the mesofauna. It was calculated that faeces production by the collembolans resulted in a 2.4 times higher NO ₃ ^– availability in the forest floor.     

Potential hazards of lime application in a damaged pine forest ecosystem in Berlin,Germany

Ninety percent of the pines (P. Sylvestris) in the forests of Berlin (West) are classified as damaged. Needle and leaf analyses do not indicate nutrient deficiencies. In site of high S-inputs (55 kg ha^?1 yr^?1 with throughfall) total acid inputs are moderate (2.4 kmol ha^?1 yr^?1) due to their neutralization by carbonatic dusts. Heavy metal depositions have led to accumulations in the forest floor (e.g. Pb 150 mg kg^?1, Cd 0.5 mg kg^?1). The dominating soil type, a cambic arenosol (Ustipsamment) is strongly acidified (pH 3.2 – 4.0) and poor in available nutrients. On an experimental plot, the application of dolomitic lime (6.1 tons ha^?1) and fertilizer (145 kg ha^?1 K₂SO₄) led to a significant increase m pH and base saturation in the top 10 cm of the mineral soil after 2 yr. The data on element fluxes give evidence for increased mineralization rates, enhanced heavy metal accumulation in the forest floor and increased soil solution concentrations of potentially hazardous substances (Al, Cd, NO₃). The lime application is discussed in terms of site specific effects on ecosystem stability and groundwater quality.     

Intensive management of a bamboo forest significantly enhanced soil nutrient concentrations but decreased soil microbial biomass and enzyme activity: a long-term chronosequence study

Purpose

Intensive long-term management practices in forest ecosystems can markedly influence soils’ physicochemical and microbial properties. However, their effects on the magnitude of nutrient pools and activities of enzymes regarding nutrient cycling in subtropical forest soils remain unclear. This study aimed to examine effects of long-term intensive management (organic mulching and chemical fertilization) on concentrations of different C, N, and P fractions and activities of enzymes involved with nutrient cycling in a subtropical Lei bamboo (Phyllostachys violascens) forest soil.

Materials and methods

Soil samples were taken from a chronosequence of Lei bamboo forests with intensive management spanning 0, 5, 10, and 15 years. Concentrations of various forms of C, N, and P, as well as activities of β-glucosidase, cellobiohydrolase, urease, protease, and acid phosphatase were measured.

Results and discussion

The results revealed that the concentrations of different classes of C (water-soluble organic C, hot-water-soluble organic C, and readily oxidizable C), N (NH₄⁺-N, NO₃^?-N, and water-soluble organic N), and P [resin-inorganic P (P_i), NaHCO₃-P_i, NaHCO₃-organic P (P_o), NaOH-P_i, NaOH-P_o, HCl-P_i, and residual-P] were enhanced markedly with prolonged duration of intensive management. Furthermore, activities of β-glucosidase, cellobiohydrolase, urease, protease, and acid phosphatase were increased following a 5-year treatment, while they were markedly reduced from 5- to the 15-year treatments. The 15 years of intensive management significantly reduced microbial biomass C and N concentrations by 8.2% and 31.9%, respectively, compared to the control.

Conclusions

We concluded that long-term intensive management led to the accumulation of C, N, and P, while it negatively impacted microbial biomass and activities of enzymes involved in nutrient cycling in subtropical Lei bamboo forest soils. Consequently, a reduction in chemical fertilizers should be considered toward the long-term sustainable development of subtropical Lei bamboo forests.

     

Soil–plant relationships and tree distribution in old growth Nothofagus betuloides and Nothofagus pumilio forests of Tierra del Fuego

The evergreen Nothofagus betuloides and deciduous Nothofagus pumilio live close together in Tierra del Fuego. The fact that these two species rarely form mixed forests suggests that the soil factor can contribute to the distribution of two species on a local scale. Most of these forests are undisturbed; therefore, soil characteristics may reflect both the influence of the species and of the dominant pedogenetic processes. In this paper, we aim to study how soil characteristics can affect nutrient cycling, the strategies of tree nutrition and the distribution of Nothofagus forests in Tierra del Fuego. Twenty-two soils in these two types of forests in contrasted topographic positions were sampled and analysed. The dominant soil processes were waterlogging in the evergreen forest and podzolisation in the deciduous one. Within each type of forest we observed large variability in soils. Nutrient-poor litter slowed down nitrogen cycling. However, phosphorus mobilisation was high in nutrient-poor environments because of low pH and waterlogging. Chemical and biological characteristics of the Oa horizon were able to discriminate between forest types while mineral soil characteristics were not. Biological activity of the organic horizons was limited by low pH and, in N. betuloides forests, also by waterlogging conditions. According to this pattern, N. betuloides grows in less fertile soils and thus has lower nutrient concentrations in leaves. In contrast, N. pumilio grows in soils with higher fertility and consequently has high nutrient content in leaves. Despite growing in low fertility sites, N. betuloides grows at a rate similar to that of N. pumilio. This suggests that the evergreen species is more efficient in using nutrients and better adapted to waterlogging conditions.     

Consequences of N Deposition to Forest Ecosystems - Recent Results and Future Research Needs

Wet and dry deposition of atmospheric nitrogen (N) compounds into forest ecosystems and their effect on physical, chemical and microbial processes in the soil has attracted considerable attention for many years. Still the consequences of atmospheric N deposition on N metabolism of trees and its interaction with soil microbial processes has only recently been studied. Atmospheric N deposited to the leaves is thought to enter the general N metabolism of the leaves, but the processes involved, the interaction with different metabolic pathways, and the connection between injury by atmospheric N and its metabolic conversion are largely unknown. Laboratory and field experiments have shown that N of atmospheric NO₂ and NH₃, deposited to the leaves of trees, is subject to long-distance transport in the phloem to the roots. This allocation can result in considerable decline of N uptake by the roots. Apparently, the flux of N from the soil into the roots can be down-regulated to an extent that equals N influx into the leaves. This down-regulation is not mediated by generally enhanced amino-N contents, but by elevated levels of particular amino acids. Field experiments confirm these results from laboratory studies: Nitrate (NO₃) uptake by the roots of trees at a field sites exposed to high loads of atmospheric N is negligible, provided concentrations of Gln in the roots are high. At the ecosystem level, consequences of reduced N uptake by the roots of trees exposed to high loads of atmospheric N are (1) an increased availability of N for soil microbial processes, (2) enhanced emission of gaseous N-oxides from the soil, and (3) elevated leaching of NO₃ into the ground water. How recent forest management practices aimed at transforming uniform monocultures to more structured species-rich forests will interact with these processes remains to be seen. Possible implications of these forest management practices on N metabolism in trees and N conversion in the soil are discussed particularly in relation to atmospheric N deposition.     

Effects of nitrogen deposition on soil organic carbon fractions in the subtropical forest ecosystems of S China

Experiments were conducted between 2003 and 2008 to examine how N additions influence soil organic C (SOC) and its fractions in forests at different succession stages in the subtropical China. The succession stages included pine forest, pine and broadleaf mixed forest, and old‐growth monsoon evergreen broadleaf forest. Three levels of N (NH₄NO₃)‐addition treatments comprising control, low‐N (50 kg N ha^–1 y^–1), and medium‐N (100 kg N ha^–1 y^–1) were established. An additional treatment of high‐N (150 kg N ha^–1 y^–1) was established in the broadleaf mixed forest. Soil samples were obtained in July 2008 for analysis. Total organic C (TOC), particulate organic C (POC, > 53 μm), readily oxidizable organic C (ROC), nonreadily oxidizable organic C (NROC), microbial biomass C (MBC), and soil properties were analyzed. Nitrogen addition affected the TOC and its fractions significantly. Labile organic‐C fractions (POC and ROC) in the topsoil (0–10 cm) increased in all the three forests in response to the N‐addition treatments. NROC within the topsoil was higher in the medium‐N and high‐N treatments than in the controls. In the topsoil profiles of the broadleaf forest, N addition decreased MBC and increased TOC, while no significant effect on MBC and TOC occurred in the pine and mixed forests. Overall, elevated N deposition increased the availability of labile organic C (POC and ROC) and the accumulation of NROC within the topsoil irrespective of the forest succession stage, and might enhance the C‐storage capacity of the forest soils.     

Carbon mineralization is promoted by phosphorus and reduced by nitrogen addition in the organic horizon of northern hardwood forests

Limitations to the respiratory activity of heterotrophic soil microorganisms exert important controls of CO₂ efflux from soils. In the northeastern US, ecosystem nutrient status varies across the landscape and changes with forest succession following disturbance, likely impacting soil microbial processes regulating the transformation and emission of carbon (C). We tested whether nitrogen (N) or phosphorus (P) limit the mineralization of soil organic C (SOC) or that of added C sources in the Oe horizon of successional and mature northern hardwood forests in three locations in central New Hampshire, USA. Added N reduced mineralization of C from SOC and from added leaf litter and cellulose. Added P did not affect mineralization from SOC; however, it did enhance mineralization of litter- and cellulose- C in organic horizons from all forest locations. Added N increased microbial biomass N and K₂SO₄-extractable DON pools, but added P had no effect. Microbial biomass C increased with litter addition but did not respond to either nutrient. The direction of responses to added nutrients was consistent among sites and between forest ages. We conclude that in these organic horizons limitation by N promotes mineralization of C from SOC, whereas limitation by P constrains mineralization of C from new organic inputs. We also suggest that N suppresses respiration in these organic horizons either by relieving the N limitation of microbial biomass synthesis, or by slowing turnover of C through the microbial pool; concurrent measures of microbial growth and turnover are needed to resolve this question.     

Influence of vegetation types and soil properties on microbial biomass carbon and metabolic quotients in temperate volcanic and tropical forest soils

Abstract

There is limited knowledge about the differences in carbon availability and metabolic quotients in temperate volcanic and tropical forest soils, and associated key influencing factors. Forest soils at various depths were sampled under a tropical rainforest and adjacent tea garden after clear-cutting, and under three temperate forests developed on a volcanic soil (e.g. Betula ermanii and Picea jezoensis, and Pinus koraiensis mainly mixed with Tilia amurensis, Fraxinus mandshurica and Quercus mongolica), to study soil microbial biomass carbon (MBC) concentration and metabolic quotients (qCO₂, CO₂-C/biomass-C). Soil MBC concentration and CO₂ evolution were measured over 7-day and 21-day incubation periods, respectively, along with the main properties of the soils. On the basis of soil total C, both CO₂ evolution and MBC concentrations appeared to decrease with increasing soil depth. There was a maximal qCO₂ in the 0–2.5 cm soil under each forest stand. Neither incubation period affected the CO₂ evolution rates, but incubation period did induce a significant difference in MBC concentration and qCO₂ in tea soil and Picea jezoensis forest soil. The conversion of a tropical rainforest to a tea garden reduced the CO₂ evolution and increased the qCO₂ in soil. Comparing temperate and tropical forests, the results show that both Pinus koraiensis mixed with hardwoods and rainforest soil at less than 20 cm depth had a larger MBC concentration relative to soil total C and a lower qCO₂ during both incubation periods, suggesting that microbial communities in both soils were more efficient in carbon use than communities in the other soils. Factor and regression analysis indicated that the 85% variation of the qCO₂ in forest soils could be explained by soil properties such as the C:N ratio and the concentration of water soluble organic C and exchangeable Al (P < 0.001). The qCO₂ values in forest soils, particularly in temperate volcanic forest soils, decreased with an increasing Al/C ratio in water-soluble organic matter. Soil properties, such as exchangeable Ca, Mg and Al and water-soluble organic C:N ratio, were associated with the variation of MBC. Thus, MBC concentrations and qCO₂ of the soils are useful soil parameters for studying soil C availability and microbial utilization efficiency under temperate and tropical forests.     

Are Indicators for Critical Load Exceedance Related to Forest Condition?

The aim of this study was to evaluate the suitability of the (Ca?+?Mg?+?K)/Al and the Ca/Al ratios in soil solution as chemical criteria for forest condition in critical load calculations for forest ecosystems. The tree species Norway spruce, Sitka spruce and beech were studied in an area with high deposition of sea salt and nitrogen in the south-western part of Jutland, Denmark. Throughfall and soil water were collected monthly and analysed for pH, NO₃-N, NH₄-N, K, Ca, Mg, DOC and Al_tot. Organic Al was estimated using DOC concentrations. Increment and defoliation were determined annually, and foliar element concentrations were determined every other year. The throughfall deposition was highest in the Sitka spruce stand (maximum of 40 kg N ha^?1yr^?1) and lowest in the beech stand (maximum of 11 kg N ha^?1yr^?1). The Sitka spruce stand leached on average 12 kg N ha^?1yr^?1 during the period 1988–1997 and leaching increased throughout the period. Only small amounts of N were leached from the Norway spruce stand whereas almost no N was leached from the beech stand. For all tree species, both (Ca?+?Mg?+?K)/Al and Ca/Al ratios decreased in soil solution at 90 cm depth between 1989 and 1999, which was mainly caused by a decrease in concentrations of base cations. The toxic inorganic Al species were by far the most abundant Al species at 90 cm depth. At the end of the measurement period, the (Ca?+?Mg?+?K)/Al ratio was approximately 1 for all species while the Ca/Al ratio was approximately 0.2. The lack of a trend in the increment rates, a decrease in defoliation as well as sufficient levels of Mg and Ca in foliage suggested an unchanged or even slightly improved health condition, despite the decreasing and very low (Ca?+?Mg?+?K)/Al and Ca/Al ratios. The suitability of these soil solution element ratios is questioned as the chemical criteria for soil acidification under field conditions in areas with elevated deposition rates of sea salts, in particular Mg.     

Soil changes induced by air pollutant deposition and their implication for forests in central Europe

A survey of leaf and needle losses of European forests in 1993 revealed that 23% of the total forested area had defoliation of more than 25%. The focus of this defoliation is in Central Europe, namely in Poland, Slowakia, Czech Republic, and Germany. The annual surveys of leaf losses and discoloration indicated only small changes during the last years for the coniferous forests in Germany. However, the increasing leaf losses of oak and beech during the last years were alarming. Evaluating the potential relation between air pollutant deposition, soil changes and forest damage, we focus here on the recent changes in deposition and soil conditions, and their implication on tree root development and drought susceptability of trees. While deposition of SO₄ ^2?, H⁺ and Ca²⁺ in many Central European forests decreased in the last decade, input of NH₄ ⁺ and NO₃ ^? remained high or even increased. The H⁺ load of many forest soils today is thus still high compared to weathering rates, but the proportion of the H⁺ load resulting from turnover of deposited N has increased. Recent effects of changing depositions on acid forest soils were: depletion of soil Al-pools, release of formerly stored soil SO₄ ^2?, accumulation of N in soil organic matter, increasing N availability to trees and decreasing concentration of Ca²⁺ in the soil solution. We hypothesise that soil acidification and increased N availability will decrease the fine root biomass of trees and shift the rooting zone to upper soil layers. Increased above ground growth, observed in many areas of Europe, will furthermore decrease the root/shoot ratio. This development will finally cause increased drought susceptability of trees and is thus of destabilizing nature. The proposed chain of events might be overlapped by other effects of air pollutants on forest ecosystems, namely direct effects of gases on leaves, nutritional inbalances, and interactions with pests.