Ecological and site historical aspects of N dynamics and current N status in temperate forests

Ecological developments during Holocene age and high atmospheric depositions since industrialization have changed the N dynamics of temperate forest ecosystems. A number of different parameters are used to indicate whether the forests are N‐saturated or not, most common among them is the occurrence of nitrates in the seepage water below the rooting zone. The use of different definitions to describe N saturation implies that the N status of ecosystems is not always appropriately assessed. Data on N dynamics from 53 different German forests were used to classify various development states of forest ecosystems according to the forest ecosystem theory proposed by Ulrich for which N balances of input – (output plus plant N increment) were used. Those systems where N output equals N input minus plant N increment are described as (quasi‐) Steady State Type. Those forests where N output does not equal N input minus plant N increment as in a ‘transient state.’ Forests of the transient state may lose nitrogen from the soil (Degradation Type) or gain nitrogen [e.g., from atmospheric depositions (Accumulation Type)]. Forest ecosystems may occur in four different N states: (a) (quasi‐) Steady State Type with mull type humus, (b) Degradation Type with mull type humus, (c) Accumulation Type with moder type humus, and (d) (quasi‐) Steady State Type with moder type humus. Forests with the (quasi‐) steady state with mull type humus in the forest floor (n= 8) have high‐soil pH values, high N retention by plant increment, high N contents in the mineral soils, and have not undergone large changes in the N status. Forests of the Degradation Type lose nitrogen from the mineral soil (currently degradation is occurring on one site). Most forests that have moder or mor type humus and low‐soil pH values, and low N contents in the mineral soil have gone through the transient state of organic matter loss in the mineral soils. They accumulate organic matter in the forest floor (accumulation phase, currently 21 sites are accumulating 6–21 kg N ha^?1 yr^?1) or have reached a new (quasi‐) steady state with moder/mor type humus (n= 15). N retention in the accumulation phase has significantly increased in soil with N deposition (r²= 0.38), soil acidity (considering thickness of the forest floor as indices of soil acidity, r²= 0.43) and acid deposition (sulfate deposition, r²= 0.39). Retention of N (4–20 kg N ha^?1 yr^?1) by trees decreased and of soils increased with a decrease in the availability of base cations indicating the important role of trees for N retention in less acid soils and those of soils in more acid soils. Ecosystem theory could be successfully applied on the current data to understand the dynamics of N in temperate forest ecosystems.     

Dynamics of the internal soil nitrogen cycles under moder and mull forest floor types on a slope in a <Emphasis Type="Italic">Cryptomeria japonica</Emphasis> D. Don plantation

To examine the influence of microbial carbon (C) availability on the internal soil nitrogen (N) cycles under moder and mull forest floor types within the same slope sequence, surface mineral soils (0–5cm depth) taken at upper (moder-type forest floor) and lower (mull-type forest floor) positions on a slope in a Cryptomeria japonica D. Don plantation were incubated for 300days. During the incubation, changes in net and gross N transformations, the organic C and N pools, and microbial respiration were monitored. Despite relatively small differences in net N mineralization in both soils, very rapid rates of gross N transformations were found in mull soil during the initial 15days of the experiment. A rapid net nitrification occurred after days 150 and 100 in moder and mull soils, respectively, presumably because of decreased microbial C availability. However, a rapid net nitrification also occurred in the mull soil during the initial 15days when microbial C availability was high, and gross nitrification was detected in both soils, except at day 0 in the moder soil. Changes in gross N transformations and in organic C and N pools over the experiment suggested that the start of rapid net nitrification might be influenced not only by microbial C availability, but also by the microbial availability of N relative to C.     

Consequences of liming and gypsum top-dressing on nitrogen and carbon dynamics in acid forest soils with different humus forms

The aim of this study was to understand the effects of lime and gypsum on nitrogen and carbon turnover of the soil. A pot experiment was conducted in parallel with a field experiment which was set up in 1989 in a declining forest of the French Ardennes. A dystric cambisol, associated with a moder and mull humus separately, was used to study changes in the soil chemistry as a result of added lime and gypsum top-dressing.The lime was applied to the surface of an acid mull humus of an oak (Quercus petraea) stand and of a moder humus of a spruce (Picea abies) stand. A quantity of 2.8 t ha^-1 equivalent CaO was supplied as CaCO₃, CaCO₃+MgO and CaSO₄.2H₂O. The experiment was installed in an open-air nursery for 20 months, during which the organic carbon and nitrogen in the solution were analysed monthly. They were analysed in the solid phase after 20 months. At the end of this period the changes in the soil and leachate depended mainly on the type of the material added.The leachate was enriched with nitrogen from the third month of the experiment under lime treatments and in the control. The same pattern was found under the two humus types but the magnitude was higher in soil with a mull humus. The nitrogen was mostly leached as NO₃ ^--N in the carbonate treatments and in the control, whereas it was predominantly NH₄ ⁺-N under gypsum. The NO₃ ^--N was 50% higher than NH₄ ⁺-N in the control and CaCO₃, CaCO₃+MgO treatments. In the CaSO₄ treatment this phenomenon was reversed. The leaching of organic carbon was greater under gypsum than under the other treatments whatever the humus.In the solid phase of the soil (organic layers) the organic carbon and nitrogen concentration decreased significantly after liming, especially in the mull humus. Consequently it induced a decrease in C:N ratio of about 18% with respect to the control.     

Carbon flow in an upland grassland: effect of liming on the flux of recently photosynthesized carbon to rhizosphere soil

The effect of liming on the flow of recently photosynthesized carbon to rhizosphere soil was studied using ¹³CO₂ pulse labelling, in an upland grassland ecosystem in Scotland. The use of ¹³C enabled detection, in the field, of the effect of a 4‐year liming period of selected soil plots on C allocation from plant biomass to soil, in comparison with unlimed plots. Photosynthetic rates and carbon turnover were higher in plants grown in limed soils than in those from unlimed plots. Higher δ¹³C‰ values were detected in shoots from limed plants than in those from unlimed plants in samples clipped within 15 days of the end of pulse labelling. Analysis of the aboveground plant production corresponding to the 4‐year period of liming indicated that the standing biomass was higher in plots that received lime. Lower δ¹³C‰ values in limed roots compared with unlimed roots were found, whereas no significant difference was detected between soil samples. Extrapolation of our results indicated that more C has been lost through the soil than has been gained via photosynthetic assimilation because of pasture liming in Scotland during the period 1990–1998. However, the uncertainty associated with such extrapolation based on this single study is high and these estimates are provided only to set our findings in the broader context of national soil carbon emissions.     

Quantitative traits associated with adaptation of three barley (Hordeum vulgare L) cultivars to suboptimal iron supply

A laboratory method was developed that allows determination of in situ net nitrification with high sensitivity and at high temporal resolution. Nitrate in soils is quantitatively converted into nitrous oxide under strictly anaerobic conditions in the presence of 10 kPa acetylene by the soil endogenous denitrifier population, with the N₂O detected by a gas chromatograph equipped with a ⁶³Ni electron capture detector. Thus, even low net nitrification rates, i.e. small net increases in soil nitrate concentrations can easily be detected. Comparison of results using this method with results obtained using the classical in situ incubation method (buried bag soil incubation) revealed excellent agreement. Application of the new method allowed both determination of the seasonal pattern of net nitrification as well as correlation analysis between in situ NO and N₂O flux rates and in situ net nitrification rates of the forest soils studied. Regardless of the forest site studied (spruce, spruce limed, beech), and during each year of a 3 years period (1995–1997), net nitrification varied strongly with season and was least during winter and greatest during summer. The long-term annual, mean rate of net nitrification for the untreated spruce site, the limed spruce site and the beech site were 1.54 ± 0.27 mg N kg^–1 sdw d^–1, 1.92 ± 0.23 mg N kg^–1 sdw d^–1 and 1.31 ± 0.23 mg N kg^–1 sdw d^–1, respectively. In situ rates of nitrification and NO and N₂O emission were strongly correlated for all sites suggesting that nitrification was the dominate source of NO as well as N₂O.     

Buried germinative seeds in mature beech forests with different herbaceous vegetation and soil types

The seed banks in the soils of seven mature beech forests in southern Sweden were examined using the seedling-emergence method. Seedling emergence in the field was also studied. In the studied forests, covering a wide range of vegetation and soil types, between 1020 and 4500 germinating seeds m⁻² were found in the seed bank. Seed abundance showed no apparent relationship with the type of soil or vegetation at the sites, but the highest number of species in the seed bank was found on the mull sites. The species diversity of germinative seeds increased strongly with increasing soil fertility.
Only a minor part (10–35%) of the species in the germinable part of the seed bank were represented in the vegetation. Soil samples from the oligotrophic sites mainly contained graminoids, notably Carex spp., while herbs dominated the seed banks of the rich mull sites. Seeds of e.g. Stellaria nemorum, Oxalis acetosella, Moehringia trinervia, Viola rivinianalreichenbachiana, Melica uniflora , and Milium effusum were especially abundant in the mull soils. Woody species were infrequent on all sites, and no representatives of the early spring flora were noted in the germination tests.
In the mor and moder soils most seeds were recovered from the mineral soil, and they were believed to originate from early stages in the succession of the forest. Several sites had comparatively large seed banks of species that typically occur in disturbed forests, e.g. Juncus effusus and Rubus idaeus , but very few germinating seeds belonged to species restricted to non-forest habitats. Emerging seedlings in the field were only observed on the mull and moder sites. With few exceptions they belonged to species, which were common both in the seed banks and in the vegetation.     

Increases in soil organic carbon sequestration can reduce the global warming potential of long‐term liming to permanent grassland

The application of calcium‐ and magnesium‐rich materials to soil, known as liming, has long been a foundation of many agro‐ecosystems worldwide because of its role in counteracting soil acidity. Although liming contributes to increased rates of respiration from soil thereby potentially reducing soils ability to act as a CO₂ sink, the long‐term effects of liming on soil organic carbon (C_org) sequestration are largely unknown. Here, using data spanning 129 years of the Park Grass Experiment at Rothamsted (UK), we show net C_org sequestration measured in the 0–23 cm layer at different time intervals since 1876 was 2–20 times greater in limed than in unlimed soils. The main cause of this large C_org accrual was greater biological activity in limed soils, which despite increasing soil respiration rates, led to plant C inputs being processed and incorporated into resistant soil organo‐mineral pools. Limed organo‐mineral soils showed: (1) greater C_org content for similar plant productivity levels (i.e. hay yields); (2) higher ¹⁴C incorporation after 1950s atomic bomb testing and (3) lower C : N ratios than unlimed organo‐mineral soils, which also indicate higher microbial processing of plant C. Our results show that greater C_org sequestration in limed soils strongly reduced the global warming potential of long‐term liming to permanent grassland suggesting the net contribution of agricultural liming to global warming could be lower than previously estimated. Our study demonstrates that liming might prove to be an effective mitigation strategy, especially because liming applications can be associated with a reduced use of nitrogen fertilizer which is a key cause for increased greenhouse gas emissions from agro‐ecosystems.     

Nitrification and occurrence of Nitrobacter by MPN-PCR in low and high nitrifying coniferous forest soils

The nitrification of three coniferous forest soils was investigated: a podzol (Fontainebleau, France) with low N deposition and no nitrate accumulation, an acid mull (Haldon, UK) with low N deposition and moderate nitrate accumulation, and a podzol (Wekerom, The Netherlands) with high N deposition and high nitrate accumulation. Twenty-one months in situ lysimeter experiments and short-term and long-term in vitro incubations were performed to respectively establish the status of NO ₃ ^- accumulation and potential nitrification of these soils. These complementary approaches allowed to conclude that the absence of NO ₃ ^- accumulation in Fontainebleau soil was effectively due to a lack of nitrifing activity in the whole profile while the high NO ₃ ^- accumulation observed in the Wekerom soil essentially resulted from an active nitrifying activity in the Oh horizon of this soil. For the Haldon soil, the inadequacy between the lysimeter and the short-term in vitro experiment was discussed.Surprisingly, relatively high densities of the nitrite-oxidiser Nitrobacter (enumerated by PCR-MPN technique) were found in all the studied horizons of the three soils. Moreover, a long-term incubation experiment showed that inhibition of nitrification in the Oh Fontainebleau soil could be removed after submitting the soil samples to constant conditions during 82 days. This demonstrates that in these soils, ammonium-oxidisers (as well as nitrite-oxidisers) are present and that extrinsic factors were involved in the inhibition of nitrification.     

Investigations into the exploitation of heterogeneous soils by Lupinus albus L. and L. pilosus Murr. and the effect upon plant growth

In calcareous soils, genotypes of Lupinus albus L. generally grow poorly, resulting in stunted plants that often develop lime-induced chlorosis. In contrast, some genotypes of L. pilosus Murr. occur naturally in calcareous soils without developing any visible symptoms of stress. Some genotypic variation for tolerance to calcareous soil does exist in L. albus and the tolerance mechanisms need to be determined. The adaptation through root system morphological plasticity of L. albus and L. pilosus, to heterogeneous limed soil profiles (pH 7.8) containing either patches of acid (non-limed) soil, or vertically split between acid and limed soil, was investigated. When grown in the presence of patches of acid soil, L. albus had a 52% greater shoot dry weight and visibly greener leaves compared with plants grown in the homogeneous limed soil. Total root dry matter in the acid-soil patches was greater than in the control limed-soil patches. This was due to a four-fold increase in the cluster root mass, accounting for 95% of the root dry matter in the acid-soil patch. Although these cluster roots secreted no more citric acid per unit mass than those in the limed soil did, their greater mass resulted in a higher citrate concentration in the surrounding soil. L. pilosus responded to the patches of acid soil in a manner comparable with L. albus. When grown in the homogeneous limed soil, L. pilosus had a greater maximum net CO₂ assimilation rate (P_max) than L. albus, however, the P_max of both species increased after they had accessed a patch of acid soil. Differences were apparent between the L. albus genotypes grown in soil profiles split vertically into limed and acid soil. A genotype by soil interaction occurred in the partitioning between soils of the cluster roots. The genotype La 674 was comparable with L. pilosus and produced over 11% of its cluster roots in the limed soil, whereas the other genotypes produced only 1–3% of their cluster roots in the limed soil. These results indicate L. pilosus is better adapted to the limed soil than L. albus, but that both species respond to a heterogeneous soil by producing mainly cluster roots in an acid-soil patch. This revised version was published online in June 2006 with corrections to the Cover Date.     

Site and temporal variation of soil respiration in European beech, Norway spruce, and Scots pine forests

Global warming and changes in rainfall amount and distribution may affect soil respiration as a major carbon flux between the biosphere and the atmosphere. The objectives of this study were to investigate the site to site and interannual variation in soil respiration of six temperate forest sites. Soil respiration was measured using closed chambers over 2 years under mature beech, spruce and pine stands at both Solling and Unterlüß, Germany, which have distinct climates and soils. Cumulative annual CO₂ fluxes varied from 4.9 to 5.4 Mg C ha^?1 yr^?1 at Solling with silty soils and from 4.0 to 5.9 Mg C ha^?1 yr^?1 at Unterlüß with sandy soils. With one exception soil respiration rates were not significantly different among the six forest sites (site to site variation) and between the years within the same forest site (interannual variation). Only the respiration rate in the spruce stand at Unterlüß was significant lower than the beech stand at Unterlüß in both years. Soil respiration rates of the sandy sites at Unterlüß were limited by soil moisture during the rather dry and warm summer 1999 while soil respiration at the silty Solling site tended to increase. We found a threshold of ?80 kPa at 10 cm depth below which soil respiration decreased with increasing drought. Subsequent wetting of sandy soils revealed high CO₂ effluxes in the stands at Unterlüß. However, dry periods were infrequent, and our results suggest that temporal variation in soil moisture generally had little effect on annual soil respiration rates. Soil temperature at 5 cm and 10 cm depth explained 83% of the temporal variation in soil respiration using the Arrhenius function. The correlations were weaker using temperature at 0 cm (r² = 0.63) and 2.5 cm depth (r² = 0.81). Mean Q₁₀ values for the range from 5 to 15 °C increased asymptotically with soil depth from 1.87 at 0 cm to 3.46 at 10 cm depth, indicating a large uncertainty in the prediction of the temperature dependency of soil respiration. Comparing the fitted Arrhenius curves for same tree species from Solling and Unterlüß revealed higher soil respiration rates for the stands at Solling than in the respective stands at Unterlüß at the same temperature. A significant positive correlation across all sites between predicted soil respiration rates at 10 °C and total phosphorus content and C‐to‐N ratio of the upper mineral soil indicate a possible effect of nutrients on soil respiration.     

Spatial variability of NO and NO2 flux rates from soil of spruce and beech forest ecosystems

In order to evaluate differences in the magnitude of NO and NO₂ flux rates between soil areas in direct vicinity to tree stems and areas of increasing distance to tree stems, we followed in 1997 at the Höglwald Forest site with a fully automated measuring system a complete annual cycle of NO and NO₂ fluxes from soils of an untreated spruce stand, a limed spruce strand, and a beech stand using at each stand measuring chambers which were installed onto the soils in such a way that they formed a stem to stem gradient. Flux data obtained since the end of 1993 from measuring chambers placed at the interstem areas of the stands, which had been used for the calculation of the long year annual mean of NO and NO₂ flux rates from soils of the stands, are compared to both (a) those obtained from the interstem chambers in 1997 and (b) those from the stem to stem gradient chambers. Daily mean NO fluxes obtained in 1997 were in a range of 0.3 – 280.1 g NO-N m^–2 h^–1 at the untreated spruce stand, 0.5 – 273.2 g NO-N m^–2 h^–1 at the limed spruce stand and 0.5 - 368.8 g NO-N m^–2 h^–1 at the beech stand, respectively. Highest NO emission rates were observed during summer, lowest during winter. Daily mean NO₂ fluxes were in a range of –83.1 – 7.6 g NO₂-N m^–2 h^–1 at the untreated spruce stand, -85.1 – 2.1 g NO₂-N m^–2 h^–1 at the limed spruce stand and –77.9 to –2.0 g NO₂-N m^–2 h^–1 at the beech site, respectively. As had been observed for the years 1994–1996, also in 1997 NO emission rates were highest at the untreated spruce stand and lowest at the beech stand and liming of a spruce stand resulted in a significant decrease in NO emission rates. For NO₂ no marked differences in the magnitude of flux rates were found between the three different stands. Results obtained from the stem to stem gradient experiments revealed that at all stands studied NO emission rates were significantly higher (between 1.6- and 2.6-fold) from soil areas close to the tree stems and decreased – except at the beech stand - with increasing distance from the stems, while for NO₂ deposition no marked differences were found. Including the contribution of soil areas in direct vicinity to the beech stems in the estimation of the annual mean NO source strength revealed that the source strength has been underestimated by 40% in the past.     

Use of strontium isotopes and foliar K content to estimate weathering of biotite induced by pine seedlings colonised by ectomycorrhizal fungi from two different soils

Pinus sylvestris seedlings, colonised by ectomycorrhizal (EM) fungi from either of two different soils (untreated forest soil and a limed soil from a clear cut area), were grown with or without biotite as a source of K. The biotite was naturally enriched in ⁸⁷Sr and the ratio of ⁸⁷Sr/ ⁸⁶Sr in the plant biomass was estimated and used as a marker for biotite weathering and compared to estimates of weathering based on foliar content of K. Different nutrient regimes were used to expose the seedlings to deficiencies of K with and without an application of nitrogen (NH₄NO₃) in excess of seedling demand. The seedlings were grown for 220 days and the elemental composition of the shoots were analysed at harvest. The EM colonisation was followed by analysing the concentration of ergosterol in the roots and the soils. Bacterial activity of the soil was estimated by the thymidine incorporation technique. The concentration of organic acids in the soil solution was measured in the soil in which seedlings colonised by EM fungi from the untreated forest soil were grown. It was found that seedlings colonised by EM fungi from untreated forest soil had taken up more K in treatments with biotite addition compared to seedlings colonised by EM fungi from the limed forest soil (p<0.05). Seedlings from untreated forest soil had larger shoots and contained more K when grown with biotite compared to KCl as K source, indicating that biotite had a stimulatory effect on the growth of these seedlings which was not related to K uptake. Seedlings from the limed soil, on the other hand, had similar foliar K content when grown with either biotite or KCl as K source. The larger uptake of K in seedlings from untreated forest soil was not an effect of a more developed EM colonisation of the roots since seedlings from the limed soil had a higher ergosterol concentration both in the soil and in the roots. Nutrient regimes had no significant influence on the total uptake of K but the ⁸⁷Sr/ ⁸⁶Sr isotope ratio in the plant biomass indicated that seedlings grown with excess nitrogen supply had taken up proportionally less Sr from the biotite (1.8% of total Sr content) compared to seedlings grown with a moderate nitrogen supply (5.0%). Furthermore, seedlings grown with excess nitrogen supply had a reduced fungal colonisation of roots and soil and bacterial activity was lower in these soils. The ⁸⁷Sr/ ⁸⁶Sr ratio in the plant biomass was positively correlated with fungal colonisation of the roots (r ²=0.98), which may indicate that the fungus was involved in releasing Sr from the biotite. Uptake of K from biotite was not related to the amount of organic acids in the soil solution. Oxalic acid was positively related to the amount of ergosterol in the root, suggesting that oxalic acid in the soil solution originates from the EM symbionts. The accuracy of the estimations of biotite weathering based on K uptake by the seedlings in comparison with the ⁸⁷Sr/⁸⁶Sr isotope ratio measured in the shoots is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

TRAP: a modelling approach to below-ground carbon allocation in temperate forests

Tree root systems, which play a major role in below-ground carbon (C) dynamics, are one of the key research areas for estimating long-term C cycling in forest ecosystems. In addition to regulating major C fluxes in the present conditions, tree root systems potentially hold numerous controls over forest responses to a changing environment. The predominant contribution of tree root systems to below-ground C dynamics has been given little emphasis in forest models. We developed the TRAP model, i.e. Tree Root Allocation of Photosynthates, to predict the partitioning of photosynthates between the fine and coarse root systems of trees among series of soil layers. TRAP simulates root system responses to soil stress factors affecting root growth. Validation data were obtained from two Belgian experimental forests, one mostly composed of beech (Fagus sylvatica L.) and the other of Scots pine (Pinus sylvestris L.). TRAP accurately predicted (R = 0.88) night-time CO₂ fluxes from the beech forest for a 3-year period. Total fine root biomass of beech was predicted within 6% of measured values, and simulation of fine root distribution among soil layers was accurate. Our simulations suggest that increased soil resistance to root penetration due to reduced soil water content during summer droughts is the major mechanism affecting the distribution of root growth among soil layers of temperate Belgian forests. The simulated annual rate of C input to soil litter due to the fine root turnover of the Scots pine was 207 g C m^–2 yr^–1. The TRAP model predicts that fine root turnover is the single most important source of C to the temperate forest soils of Belgium.     

Ecological aspects of an isolate of Steinernema diaprepesi (Rhabditida: Steinernematidae) from Argentina

Ecological aspects of Steinernema diaprepesi isolate SRC were studied to evaluate the species potential as biological control agent of insect pests. Under laboratory conditions, the following aspects were determined: the nematode life cycle, pathogenicity to several arthropods, reproductive capacity, tolerance to desiccation, effect of temperature on survival and infectivity of infective juveniles (IJs), and influence of soil texture and soil water potential on the isolate. The parasitic cycle on last-instar larvae of Galleria mellonella at 25°C was completed 8 days after infection. The nematode showed high virulence to lepidopteran larvae, being limited or nil in the remaining orders of arthropods evaluated. An acceptable offspring production of S. diaprepesi was confirmed in the species G. mellonella and S. frugiperda, suggesting that the isolate would have potential for control of lepidopteran larvae. Optimum temperature for reproduction was 20–25°C. IJs survived exposure to a range of temperatures between 10 and 40°C, with a significant reduction in the number of live IJs at 40°C. The nematodes remained infective at 20–40°C. IJ mortality was 100% on day 6 of exposure to 85% RH. The movement of IJs observed in the soil column experiments revealed that the isolate uses a cruiser-type search strategy. Soil texture and water potential significantly influenced IJ movement, search and penetration of G. mellonella larvae. The efficacy of this isolate was found to be favoured in sandy soils, regardless of the soil water potential.     

Four years continuous record of CH4-exchange between the atmosphere and untreated and limed soil of a N-saturated spruce and beech forest ecosystem in Germany

In order to gain information about seasonal and interannual variations of CH₄-fluxes at a spruce control site, a limed spruce site and a beech site of the Höglwald Forest, Bavaria, Germany, complete annual cycles of CH₄-exchange between the soil and the atmosphere with 2-hourly resolution were followed for 4 consecutive years. The ranges of CH₄ fluxes observed for the different sites were: +12.4 to –69.4 g CH₄ m^–2 h^–1 (spruce control site), +11.7 to –51.4 g CH₄ m^–2 h^–1 (limed spruce site), and –4.4 to –167.3 g CH₄ m^–2 h^–1 (beech site). Lowest rates of atmospheric CH₄-uptake or even a weak net-emission of CH₄ by the soils were observed during winter/spring times, whereas highest rates of CH₄-uptake were always found in summer/spring. Over the entire observation period of 4 years, mean CH₄-uptake rates were –1.82 kg CH₄-C ha^–1 yr^–1 at the spruce control site, –1.31 kg CH₄-C ha^–1 yr^–1 at the limed spruce site, and –4.84 kg CH₄-C ha^–1 yr^–1 at the beech site. The results obtained in this study demonstrate that in view of the huge interannual variations in CH₄-fluxes of approx. 1 kg CH₄-C ha^–1 yr^–1, multiple year measurements of CH₄-fluxes are necessary to accurately characterize the sink strength of temperate forest for atmospheric CH₄. By comparison of CH₄-fluxes measured at the spruce control site and the limed spruce site, a significant negative effect of forest floor liming on CH₄-uptake could be demonstrated. Compared to the spruce stand, the beech stand showed on average approx. 3 times higher rates of atmospheric CH₄-uptake, most likely due to pronounced differences between both sites with regard to the organic layer structure and bulk density of the mineral soil. Regression analysis between CH₄-fluxes and environmental parameters revealed that at all sites the dominating factors regulating temporal variations of CH₄ fluxes were soil moisture and soil temperature. Field measurements of CH₄ concentrations in the soil profile and laboratory measurements of CH₄-oxidation and CH₄-production activity on soil samples taken from different soil depths showed that the CH₄-flux at the Höglwald Forest sites is the net-result of simultaneous occurring production and consumption of CH₄ within the soil. Highest CH₄-oxidation activity was found in the uppermost centimeters of the mineral soil, whereas highest potential CH₄-production activity was found in the organic layer.     

The influence of acid irrigation and liming on the soil microbial biomass in a Norway spruce (Picea abies [L.] K.) stand

Over a period of three years (1990–1992) microbial biomass-C (C_mic), CO₂ evolution, the C_mic:C_org ratio and the metabolic quotient for CO₂ (qCO₂) were determined in a Norway spruce stand (Höglwald) with experimentally acid-irrigated and limed plots since 1984. A clear relationship between soil pH and the level of microbial biomass-(C_mic) was noted, C_mic increasing with increasing soil pH in O_h or A_h horizons. More microbial biomass-C per unit C_{org} (C_mic:C_org ratio) was detected in limed plots with elevated pH of O_h or A_h horizons as compared to unlimed plots with almost 3 times more C_mic per unit C_org in the limed O_h horizon. Differences here are significant at least at the p=0.05 level. The positive effects of liming (higher pH) on the C_mic:C_org ratio was more pronounced in the upper horizon (O_h)). The total CO₂ evolution rate of unlimed plots was only half of that noted for limed plots which corresponded to the low microbial biomass levels of unlimed plots. The specific respiratory activity, qCO₂, was similar and not significantly different between the unlimed control plot and the limed plot.Acid irrigation of plots with already low pH did not significantly affect the level of microbial biomass, the C_mic:C_org ratio or qCO₂. An elevated qCO₂ could be seen, however, for the limed + acid irrigated plot. The biomass seemed extremely stressed, showing with 3.8 g CO₂-C mg_-1 C_mic h_-1 (O_h) the highest qCO₂ value of all treatments. This was interpreted as a reflection of the continuous adaptation processes to the H⁺ ions by the microflora. The negative effect of acid irrigation of limed plots was also manifested in a decreased C_mic:C_org ratio.     

Estimating the relative nutrient uptake from different soil depths in Quercus robur, Fagus sylvatica and Picea abies

The distribution of fine roots and external ectomycorrhizal mycelium of three species of trees was determined down to a soil depth of 55 cm to estimate the relative nutrient uptake capacity of the trees from different soil layers. In addition, a root bioassay was performed to estimate the nutrient uptake capacity of Rb⁺ and NH₄⁺ by these fine roots under standardized conditions in the laboratory. The study was performed in monocultures of oak (Quercus robur L.), European beech (Fagus sylvatica L.) and Norway spruce [Picea abies (L.) Karst.] on sandy soil in a tree species trial in Denmark. The distribution of spruce roots was found to be more concentrated to the top layer (0–11 cm) than that of oak and beech roots, and the amount of external ectomycorrhizal mycelia was correlated to the distribution of the roots. The uptake rate of [⁸⁶Rb⁺] by oak roots declined with soil depth, while that of beech or spruce roots was not influenced by soil depth. In modelling the nutrient sustainability of forest soils, the utilization of nutrient resources in deep soil layers has been found to be a key factor. The present study shows that the more shallow-rooted spruce can have a similar capacity to take up nutrients from deeper soil layers than the more deeply rooted oak. The distribution of roots and mycelia may therefore not be a reliable parameter for describing nutrient uptake capacity by tree roots at different soil depths.     

Soil changes in different age classes of Norway spruce (Picea abies (L.) Karst.) on afforested farmland

The aim of this study was to test the hypothesis that afforestation changes the content and distribution of soil organic carbon, nutrients and pH in the A-horizon of land previously used in agriculture, and that such soil changes depend on stand development. The investigation was evaluated as a completely randomised design with three treatments representing different age classes of trees: 20 years (Y20), 40 years (Y40) and 55 years (Y55). Eighteen trial plots, six per treatment, were established in plantations of Picea abies (L.) Karst. on soils of similar texture and mineralogy. Tree volume was 220 m³ ha^-1 in Y20, 400 in Y40 and 440 m³ ha^-1 in Y55.Concentrations of carbon (C) and nitrogen (N) were significantly higher in the uppermost part of the soil in the older stands Y40 and Y55 than in Y20. The total amount of organic C in the litter layer plus the top 15 cm of the soil differed between age classes, with Y40 and Y55 having the largest amounts. A reference layer (15–20 cm) was used in calculating the amount of soil C that had accumulated in the horizon since afforestation, being about 10 tonnes ha^-1 of C in Y20 and 19 tonnes ha^-1 in Y40 and Y55.Cation exchange capacity (CEC) and base saturation (BS) was higher in the older stands. Carbon contents and CEC were strongly correlated. In Y40 and Y55, pH was significantly lower than in Y20 in the lower part of the soil horizon. There was a general decrease with depth of C, N, CEC, K⁺ and Mg²⁺ in the soil horizon. BS, Ca²⁺, Na⁺ and pH showed a somewhat different pattern of distribution, with deceasing values in the upper part of the soil horizon and increasing values in the lower part of the soil horizon.Abbreviations BD Bulk density - CEC cation exchange capacity - BS base saturation - Ca²⁺ calcium ion - Mg²⁺ magnesium ion - K⁺ potassium ion - Na⁺ sodium ion - C carbon - Ca _a accumulated carbon content - C _t total carbon content - N nitrogen - Y20 age class 20 years - Y40 age class 40 years - Y55 age class 55 years     

Foliage litter turnover and earthworm populations in three beech forests of contrasting soil and vegetation types

Summary Leaf litter decomposition, levels of accumulated litter as well as the abundance and biomass of earthworms were measured in three mature beech forests in southern Sweden: one mor site, one poor mull site, and one rich mull site. The disappearance rate of beech litter, measured with litter bags, increased with increasing soil fertility. On the rich mull site, the disappearance rate was much higher than in the two other forests, due to the combined effects of higher earthworm activity, more favouable soil moisture conditions, and higher litter quality. Incubating the litter in finely meshed bags (1-mm mesh) to exclude macrofauna had a great effect on litter mass loss in the rich mull site, but it had only a minor effect in the other sites. Simultaneous incubations of local and transplanted leaf litter on the three study sites showed that the substrate quality of the litter increased in the order: mor site — poor mull site — rich mull site. Lignin, N, and P concentrations of the leaf litter failed to explain the observed differences in decomposition rates, and acid/base properties are suggested to be more important. Earthworm numbers per m² were 2.5 (1 species) in the mor, 40 (6 species) in the poor mull and 220 (9 species) in the rich mull forest. Soil chemical conditions, notably pH, were suggested as the main factors determining the inter-site differences in abundance and species composition of earthworms. The role of litter decomposition and earthworm activity in the accumulation of organic matter in the forest floor in different types of beech woodlands are discussed.