Advances in the positional cloning of nodulation genes in soybean

The effect of liming on the decomposition of Norway spruce needle litter was studied in 40–60-year-old Norway spruce stands. Finely-ground limestone had been spread about 30 years ago at a dose of 2 t ha^–1 and reliming was carried out about 20 yr later at a dose of 4 t ha^–1. Needle litter was collected from both control and limed plots, and it was placed in litter bags in the middle of the humus layer of the plot from which they originated, and similarly to the other plot in May. Litter bags were sampled after 4, 12 and 16 months. The site of origin of the needle litter, whether from control plot or from limed plot, affected mainly the early stages of decomposition. Initially the effect of liming was seen as decreased concentration of water soluble material and then, during decomposition, as decreased mass loss and decreased degradation of lignin, and increased C/N ratio. The incubation site, whether the control or the limed plot, did not affect decomposition significantly.Decomposition of Scots pine needles in a young Scots pine plantation was also studied. The treatments were: 2 t ha^–1 of finely-ground limestone and 2.5 t ha^–1 of bark ash spread 8 months before this study. The treatments did not affect decomposition much, but some stimulation of the treatments on decomposition was observed. Compared to spruce needles, the C/N ratio of pine seedles was lower, they contained less lignin and more water soluble material, and decomposed faster in the first summer.     

Nitrogen mineralization and H+ transfers in a Scots pine (Pinus sylvestris L.) forest soil as affected by liming

H⁺ production due to N uptake in a mature Scots pine stand subjected to high NH₄ ⁺ deposition was previously estimated to amount to approx. 2.2 kmol ha^-1 y^-1. The question whether H⁺ transfers related to N mineralization (ammonification and nitrification) offset or corroborate this proton production is investigated in the present research. To determine N mineralization, soil cores were used of which both ends were closed with layers of ion exchange resin (IER) to prevent influx and efflux of ions. The effect of liming on N mineralization and the resulting H⁺ production was investigated in 7 incubation periods of each ca. 8 wk. Because of its high mobility NO₃ accumulated in both IER layers at the expense of that in the incubated forest floor and mineral soil. Net N mineralization in the soil cores as a whole amounted to 40 and 77 kg N ha^-1 in 384 d in the control and limed plots, respectively. In both treatments ca. 65% of mineralized N was nitrified. H⁺ production due to N mineralization amounted to approx. 1.2 kmol ha^-1 y^-1 in the control and limed plots. Liming reduced the amount of C in the forest floor, but not forest floor mass, because of an increased mixing with mineral particles.     

Insect herbivory, organic matter deposition and effects on belowground organic matter fluxes in a central European oak forest

Apart from the forest floor, the canopy of forested ecosystems functions as the second most important source for dissolved and particulate fractions of organic and inorganic C and N compounds. However, under mass outbreak situations of insect herbivores this flux path of organic matter is considerably intensified clearly exceeding C and N fluxes from the forest floor. In this paper we report on herbivore-altered C and N fluxes from the canopy to the forest floor and effects on forest floor nutrient fluxes during severe defoliating herbivory of the winter moth (Operophtera brumata) and the mottled umber moth (Eranis defoliaria) in an oak forest in Germany. Over the course of 6.5 months we followed the C and N fluxes with bulk deposition, throughfall solution, insect frass deposits (green-fall together with insect faeces) and with forest floor solution in an 117-yr-old oak (Quercus petraea) forest. Compared to the control, herbivore defoliation significantly enhanced throughfall inputs of total and dissolved organic carbon and nitrogen by a factor of 3 and 2.5 (for TOC and DOC), and by 1.4 and 1.3 times (for TNb and DNb), respectively. Frass plus green-fall C and N fluxes peaked in May with 592 kg C?ha^?1 and 33.5 kg N?ha^?1 representing 79.6% (for C) and 78.3% (for N) of the total C and N input over 2.5 months. The quantitative and qualitative C and N input via faeces and litter deposition significantly differ between the insect affected and non-affected site. However, the C and N fluxes with throughfall did not significantly correlate with forest floor leachates. In this context, forest floor fluxes of TOC, DOC and NO₃-N were significantly lower at the infested site compared to the control, whereas fluxes of NH₄-N together with DON were significantly higher. The study demonstrates the importance of linking the population and associated frass dynamics of herbivorous insects with the cycling of nutrients and organic matter in forest ecosystems, highlighting the remarkable alterations in the timing, amounts and nature of organic matter dynamics on the ecosystem level. Consequently, the ecology of phytophagous insects allows partly to explain temporal-spatial alterations in nutrient cycling and thus ecosystem functioning.     

Deposition of ammonia to the forest floor under spruce and beech at the Höglwald site

Laboratory and field measurements of the flux of ammonia to forest floor canopies of spruce and beech stands at the Höglwald site in southern Bavaria are reported. Measurements were performed with an open chamber method. A linearity between ammonia concentration and ammonia flux from the atmosphere to the ground floor canopy was detected. Deposition of ammonia showed no saturation even at air concentrations up to 50 g NH₃ m^–3 air. Temperature, water content and the moss layer of the ground floor canopy had a minor influence on the deposition velocity in laboratory experiments. Deposition velocity of ammonia was higher to the spruce (1.3 cm s^–1), and limed spruce ground floor canopy (1.17 cm s^–1) compared to the beech stand (0.79 cm s^–1). In field studies, a diurnal course of the deposition velocity was detected with highest velocities in midday and minor during night times, but not in the climatic chamber. The flux of ammonia to the ground floor canopy was estimated of app. 10 kg N ha^–1 yr^–1 for the soil under spruce, 9 kg N ha^–1 yr^–1 for the limed spruce and 6 kg N ha^–1yr^–1 for the soil under beech. The fluxes are interpreted as fluxes from the atmosphere to the ground canopies of the stands.     

Influence of Earthworm Invasion on Redistribution and Retention of Soil Carbon and Nitrogen in Northern Temperate Forests

We analyzed soil organic matter distribution and soil solution chemistry in plots with and without earthworms at two sugar maple (Acer saccharum)–dominated forests in New York State, USA, with differing land-use histories to assess the influence of earthworm invasion on the retention or loss of soil carbon (C) and nitrogen (N) in northern temperate forests. Our objectives were to assess the influence of exotic earthworm invasion on (a) the amount and depth distribution of soil C and N, (b) soil ¹³C and ¹⁵N, and (c) soil solution chemistry and leaching of C and N in forests with different land-use histories. At a relatively undisturbed forest site (Arnot Forest), earthworms eliminated the thick forest floor, decreased soil C storage in the upper 12 cm by 28%, and reduced soil C:N ratios from 19.2 to 15.3. At a previously cultivated forest site with little forest floor (Tompkins Farm), earthworms did not influence the storage of soil C or N or soil C:N ratios. Earthworms altered the stable isotopic signature of soil at Arnot Forest but not at Tompkins Farm; the alteration of stable isotopes indicated that earthworms significantly increased the loss of forest floor C but not N from the soil profile at Arnot Forest. Nitrate (NO₃^–) concentrations in tension and zero-tension lysimeters were much greater at Tompkins Farm than Arnot Forest, and earthworms increased NO₃^– leaching at Tompkins Farm. The results suggest that the effect of earthworm invasion on the distribution, retention, and solution chemistry of soil C and N in northern temperate forests may depend on the initial quantity and quality of soil organic matter at invaded sites.     

Soil organic carbon pool changes following land‐use conversions

Carbon (C) can be sequestered in the mineral soil after the conversion of intensively cropped agricultural fields to more extensive land uses such as afforested and natural succession ecosystems. Three land‐use treatments from the long‐term ecological research site at Kellogg biological station in Michigan were compared with a nearby deciduous forest. Treatments included a conventionally tilled cropland, a former cropland afforested with poplar for 10 years and an old field (10 years) succession. We used soil aggregate and soil organic matter fractionation techniques to isolate C pools that (1) have a high potential for C storage and (2) accumulate C at a fast rate during afforestation or succession. These fractions could serve as sensitive indicators for the total change in C content due to land‐use changes. At the mineral soil surface (0–7 cm), afforesting significantly increased soil aggregation to levels similar to native forest. However, surface soil (0–7 cm) C did not follow this trend: soil C of the native forest site (22.9 t C ha^?1) was still significantly greater than the afforested (12.6 t C ha^?1) and succession (15.4 t C ha^?1) treatments. However, when the 0–50 cm soil layer was considered, no differences in total soil C were observed between the cropland and the poplar afforested system, while the successional system increased total soil C (0–50 cm) at a rate of 0.786 t C ha^?1 yr^?1. Afforested soils sequestered C mainly in the fine intra‐aggregate particulate organic matter (POM) (53–250 μm), whereas the successional soils sequestered C preferentially in the mineral‐associated organic matter and fine intra‐aggregate POM C pools.     

Effects of white clover (Trifolium repens L.) on plant and soil nitrogen and soil organic matter in mixtures with perennial ryegrass (Lolium perenne L.)

To increase our insight into the above- and belowground N flows in grass and grass-clover swards relations between crop and soil parameters were studied in a cutting trial with perennial ryegrass (Lolium perenne) monocultures and ryegrass–white clover (Trifolium repens) mixtures. The effects of clover cultivar on herbage yield, the amount of clover-derived nitrogen, apparent N transfer to companion grass, dynamics of N and organic matter in the soil were estimated.The grass monocultures had very low DM yields (<2.1 t ha^-1) and a low N concentration in the harvested herbage. During 1992–1995 the annual herbage DM yield in the mixtures ranged from 7.0 to 14.3 t ha^-1, the white clover DM yield from 2.4 to 11.2 t ha^-1 and the mean annual clover content in the herbage DM harvested from 34 to 78%. Mixtures with the large-leaved clover cv. Alice yielded significantly more herbage and clover DM and had a higher clover content than mixtures with small/medium-leaved cvs. Gwenda and Retor. Grass cultivar did not consistently affect yield, botanical composition or soil characteristics.The apparent N₂ fixation was very high, ranging from 150 to 545 kg N ha^-1 in the different mixtures. For each tonne of clover DM in the harvested herbage 49 to 63 kg N was harvested, while the apparent N transfer from clover to grass varied between 55 and 113 kg N ha^-1 year^-1.The net N mineralization rate was lower under monocultures than under mixtures. The C mineralization and the amounts of C and N in active soil organic matter fractions were similar for monocultures and mixtures, but the C:N ratio of the active soil organic matter fractions were higher under grass than under mixtures. This explains the lower N mineralization under grass.     

Effects of pre-harvest soil acidification,liming and N fertilization on the survival,growth and needle element concentrations of Picea abies L. Karst. seedlings

There are concerns that the anthropogenic acidification of Swedish forest soils may have severe effects on forest yield, and it has been suggested that liming could be used to counter this problem. The aim of this study was to investigate the survival, growth and element concentrations of C+1 needles of Picea abies (L.) Karst. seedlings planted on plots that had been acidified (in 12 annual treatments totalling 600 or 1200 kg S ha^–1 in the form of elemental sulphur), limed (12×500=6000 kg lime ha^–1 in the form of CaCO₃) or N-fertilized (3×200=600 kg N ha^–1 in the form of urea) prior to harvest. Seedlings growing on plots given a combination of the N plus low S treatments were also tested. None of the treatments, given to triplicate plots, significantly influenced seedling survival during the first five growing seasons. Furthermore, none of the treatments significantly affected growth, although the average growth rate was slightly higher for limed plots. The survival and growth of the seedlings are discussed in relation to differences in the cover of field-layer vegetation between the treatments after the final felling. Needles from seedlings in the limed plots showed significantly lower concentrations of Mn and Al, and higher concentrations of Ca and Zn compared with needles from seedlings in the other plots (i.e. control, high S or N-treated). The K concentration in the needles was significantly higher in limed plots than in high-S plots. Changes of element concentrations observed in the soil, associated with the treatments, where in some cases reflected in the needle concentrations.     

Carbon and nitrogen storage in an age-sequence of <Emphasis Type="Italic">Pinus densiflora</Emphasis> stands in Korea

The carbon (C) and nitrogen (N) storage capabilities of Pinus densiflora in six different stand ages (10, 27, 30, 32, 44, and 71 years old) were investigated in Korea. Thirty sample trees were destructively harvested and 12 were excavated. Samples from the above and belowground tree components, coarse woody debris (CWD), forest floor, and mineral soil (0–30 cm) were collected. Tree biomass was highest in the 71-year-old stand (202.8 t ha⁻¹) and lowest in the 10-year-old stand (18.4 t ha⁻¹). C and N storage in the mineral soil was higher in the 71-year-old stand than in the other stands, mainly due to higher soil C and N concentrations. Consequently, the total ecosystem C and N storage (tree+forest floor+CWD+soil) was positively correlated with stand age: increasing from a minimum in the 10 year old stand (18.8 t C ha⁻¹ and 1.3 t N ha⁻¹) to a maximum in the 71-year-old stand (201.4 t C ha⁻¹ and 8.5 t N ha⁻¹). The total ecosystem C storage showed a similar sigmoidal pattern to that of tree C storage as a function of the age-sequence, while N storage in the CWD, forest floor and mineral soil showed no significant temporal trends. Our results provide important insights that will increase our understanding of C and N storage in P. densiflora stands and our ability to predict changes according to stand age in the region.     

The effect of the presence of a forage legume on nitrogen and carbon levels in soils under Brachiaria pastures in the Atlantic forest region of the South of Bahia,Brazil

The impact of forest clearance, and its replacement by Brachiaria pastures, on soil carbon reserves has been studied at many sites in the Brazilian Amazonia, but to date there appear to be no reports of similar studies undertaken in the Atlantic forest region of Brazil. In this study performed in the extreme south of Bahia, the changes in C and N content of the soil were evaluated from the time of establishment of grass-only B. humidicola and mixed B. humidicola/Desmodium ovalifolium pastures through 9 years of grazing in comparison with the C and N contents of the adjacent secondary forest. The decline in the content of soil C derived from the forest (C3) vegetation and the accumulation of that derived from the Brachiaria (C4) were followed by determining the ¹³C natural abundance of the soil organic matter (SOM). The pastures were established in 1987, 10 years after deforestation, and it was estimated that until 1994 there was a loss in forest-derived C in the top 30 cm of soil of approximately 20% (9.1 Mg C ha^–1). After the establishment of the pastures, C derived from Brachiaria accumulated steadily such that at the final sampling (1997) it was estimated 13.9 Mg ha^–1 was derived from this source under the grass-only pasture (0–30 cm). Samples taken from all pastures and the forest in 1997 to a depth of 100 cm showed that below 40 cm depth there was no significant contribution of the Brachiaria-derived C and that total C reserves under the grass/legume and the grass-only pastures were slightly higher than under the forest (not significant at P=0.05). The more detailed sampling under the pastures showed that to a depth of 30 cm there was significantly (P<0.05) more C under the mixed pasture than the grass-only pasture. It was estimated that from the time of establishment the apparent rate of C accumulation (0–100 cm depth) under the grass/legume pastures (1.17 Mg ha^–1 yr^–1) was almost double that under the grass-only pastures (0.66 Mg ha^–1 yr^–1). The data indicated that newly incorporated SOM derived from the Brachiaria had a considerably higher C:N ratio than that present under the forest.     

Forest floor-mineral soil interactions in the internal nitrogen cycle of an old-growth forest

Seasonal patterns and annual rates of N inputs, outputs, and internal cycling were determined for an old-growth mixed-conifer forest floor in the Sierra Nevada Mountains of California. Rates of net N mineralization within the forest floor, and plant N-uptake and leaching of inorganic N from the forest floor were 13, 10, and 9 kg-N ha^-1 yr^-1, respectively. The Mediterranean-type climate appeared to have a significant effect on N cycling within this forest, such that all N-process and flow rates showed distrinct seasonal patterns. We estimated the forest floor supplies less than one-third of the total aboveground plant N-uptake in this forest. The rate of net nitrification within the forest floor was always low (1 kg-NO₃ ^--N ha^-1 30d^-1). Mean residence times for organic matter and N in the forest floor were 13 and 34 years, respectively, suggesting that this forest floor layer is a site of net N immobilization within this ecosystem. We examined the influence of the forest floor on mineral soil N dynamics by injecting small amounts of¹⁵N-enriched (NH₄)₂SO₄ solutions into the surface mineral soil with the forest floor present (+FF) or removed (-FF). K₂SO₄-extractable NO₃ ^--N, total inorganic-N, and total-N pool sizes in the mineral soil were initially increased after forest floor removal (after 4 months), but NO₃ ^--N and total inorganic-N were not significantly different thereafter. Microbial biomass-N and K₂SO₄-extractable total-N pool sizes were also found to be larger in mineral soils without a forest floor after 1 and 1.3 years, respectively. Total¹⁵N-recovery was greater in the +FF treatment compared to the -FF treatment after 1-year (about 50% and 35%, respectively) but did not differ after 1.3 years (both about 35%), suggesting that the forest floor delays but does not prevent the N-loss from the surface mineral soil of this forest. We estimated using our¹⁵N data that fungal translocation from the mineral soil to the forest floor may be as large as 9 kg-N ha^-1 yr^-1 (similar in magnitude to other N flows in this forest), and may account for all of the observed absolute increase of N in litter during the early stages of decomposition at this site. Our results suggest that the forest floor acts both as a source and sink for N in the mineral soil.     

The influence of organic soil amendments on sulfate adsorption and sulfur availability in a Brazilian Oxisol

Soil management practices that involve additions of organic materials may influence plant sulfur availability in highly-weathered, acid soils. This study evaluated the effects of organic additions on sulfate adsorption and sulfur availability in a limed (3,4 t ha^-1) and unlimed Typic Haplustox soil of the Cerrado Region of Brazil. In unlimed soil, the proportion of applied sulfate (600 kg S ha^-1 as gypsum) that was adsorbed temporarily decreased over two cropping seasons by incorporation of 10 t dry matter ha^-1 crop^-1 of guinea grass (Panicum maximum Jacq.) but not when a similar quantity of a tropical legume, feijâo de porco (Canavalia ensiformis L.), was added. Liming reduced sulfate adsorption and resulted in sulfate leaching to a depth of 30 to 45 cm. Both plant materials temporarily reduced sulfate adsorption in laboratory studies when added to an unlimed soil at a rate equivalent to 40 t ha^-1. Analysis of soil properties affected by organic additions and liming showed significant correlations between sulfate adsorption and soil pH, extractable aluminum, calcium and magnesium, and surface charge. Maize dry matter yields increased by 1.3 to 3.5 t ha^-1 with addition of both organic materials. However, only the feijâo de porco treatment resulted in increases in sulfur uptake for the years in which organic materials were applied. Determining the effects of organic material additions on plant sulfur availability is complicated by the combined effects of sulfur mineralization, sulfate adsorption, and the plant's ability to utilize adsorbed subsoil sulfate.Joint contribution of Cornell University and CPAC-EM- BRAPA. This research was supported by USAID through the Title XII CRSP subgrant SM-CRSP-10 from North Carolina State University     

Above- and belowground organic matter storage and production in a tropical pine plantation and a paired broadleaf secondary forest

The distribution of tree biomass and the allocation of organic matter production were measured in an 11-yr-old Pinus caribaea plantation and a paired broadleaf secondary forest growing under the same climatic conditions. The pine plantation had significantly more mass aboveground than the secondary forest (94.9 vs 35.6 t ha^-1 for biomass and 10.5 vs 5.0 t ha^{-1 for litter}), whereas the secondary forest had significantly more fine roots (⩽2 mm diameter) than the pine plantation (10.5 and 1.0 t ha^-1, respectively). Standing stock of dead fine roots was higher than aboveground litter in the secondary forest. In contrast, aboveground litter in pine was more than ten times higher than the dead root fraction. Both pine and secondary forests had similar total organic matter productions (19.2 and 19.4 t ha^-1 yr^-1, respectively) but structural allocation of that production was significantly different between the two forests; 44% of total production was allocated belowground in the secondary forest, whereas 94% was allocated aboveground in pine. The growth strategies represented by fast growth and large structural allocation aboveground, as for pine, and almost half the production allocated belowground, as for the secondary forest, illustrate equally successful, but contrasting growth strategies under the same climate, regardless of soil characteristics. The patterns of accumulation of organic matter in the soil profile indicated contrasting nutrient immobilization and mineralization sites and sources for soil organic matter formation.     

Changes in soil properties after afforestation of former intensively managed soils with oak and Norway spruce

In many European countries, surplus agricultural production and ecological problems due to intensive soil cultivation have increased the interest in afforestation of arable soils. Many environmental consequences which might rise from this alternative land-use are only known from forest establishment on less intensively managed or marginal soils. The present study deals with changes in soil properties following afforestation of nutrient-rich arable soils. A chronosequence study was carried out comprising seven Norway spruce (Picea abies (Karst.) L.) and seven oak (Quercus robur L.) stands established from 1969 to 1997 on former horticultural and agricultural soils in the vicinity of Copenhagen, Denmark. For comparison, a permanent pasture and a ca. 200-year-old mixed deciduous forest were included. This paper reports on changes in pH values, base saturation (BS_eff), exchangeable calcium, soil N pools (N_min contents), and C/N ratios in the Ap-horizon (0–25 cm) and the accumulated forest floor. The results suggest that afforestation slowly modifies soil properties of former arable soils. Land-use history seems to influence soil properties more than the selected tree species. An effect of tree species was only found in the forest floor parameters. Soil acidification was the most apparent change along the chronosequence in terms of a pH decrease from 6 to 4 in the upper 5 cm soil. Forest floor pH varied only slightly around 5. Nitrogen storage in the Ap-horizon remained almost constant at 5.5 Mg N ha^–1. This was less than in the mineral soil of the ca. 200-year-old forest. In the permanent pasture, N storage was somewhat higher in 0–15 cm depth than in afforested stands of comparable age. Nitrogen storage in the forest floor of the 0–30-year-old stands increased in connection with the build-up of forest floor mass. The increase was approximately five times greater under spruce than oak. Mineral soil C/N ratios ranged from 10 to 15 in all stands and tended to increase in older stands only in 0–5 cm depth. Forest floor C/N ratios were higher in spruce stands (26.4) as compared to oak stands (22.7). All stands except the youngest within a single tree species had comparable C/N ratios.     

Gaps and Soil C Dynamics in Old Growth Northern Hardwood–Hemlock Forests

Old growth forest soils are large C reservoirs, but the impacts of tree-fall gaps on soil C in these forests are not well understood. The effects of forest gaps on soil C dynamics in old growth northern hardwood–hemlock forests in the upper Great Lakes region, USA, were assessed from measurements of litter and soil C stocks, surface C efflux, and soil microbial indices over two consecutive growing seasons. Forest floor C was significantly less in gaps (19.0 Mg C ha⁻¹) compared to gap-edges (39.5 Mg C ha⁻¹) and the closed forest (38.0 Mg C ha⁻¹). Labile soil C (coarse particulate organic matter, cPOM) was significantly less in gaps and edges (11.1 and 11.2 Mg C ha⁻¹) compared to forest plots (15.3 Mg C ha⁻¹). In situ surface C efflux was significantly greater in gaps (12.0 Mg C ha⁻¹ y⁻¹) compared to edges and the closed forest (9.2 and 8.9 Mg C ha⁻¹ y⁻¹). Microbial biomass N (MBN) was significantly greater in edges (0.14 Mg N ha⁻¹) than in the contiguous forest (0.09 Mg N ha⁻¹). The metabolic quotient (qCO₂) was significantly greater in the forest (0.0031 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹) relative to gaps or edges (0.0014 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹). A case is made for gaps as alleviators of old growth forest soil C saturation. Relative to the undisturbed closed forest, gaps have significantly less labile C, significantly greater in situ surface C efflux, and significantly lower decreased qCO₂ values.     

Carbon, nitrogen and phosphorus in volcanic soils following afforestation with native birch (Betula pubescens) and introduced larch (Larix sibirica) in Iceland

Afforestation has become an important tool for soil protection and land reclamation in Iceland. Nevertheless, the harsh climate and degraded soils are growth-limiting for trees, and little is know about changes in soil nutrients in maturing forests planted on the volcanic soils. In the present chronosequence study, changes in C, N and total P in soil (0–10 and 10–20 cm depth) and C and N in foliar tissue were investigated in stands of native Downy birch (Betula pubescens Enrh.) and the in Iceland introduced Siberian larch (Larix sibirica Ledeb.). The forest stands were between 14 and 97 years old and were established on heath land that had been treeless for centuries. Soils were Andosols derived from basaltic material and rhyolitic volcanic ash. A significant effect of tree species was only found for the N content in foliar tissue. Foliar N concentrations were significantly higher and foliar C/N ratios significantly lower in larch needles than in birch leaves. There was no effect of stand age. Changes in soil C and the soil nutrient status with time after afforestation were little significant. Soil C concentrations in 0–10 cm depth in forest stands older than 30 years were significantly higher than in heath land and forest stands younger than 30 years. This was attributed to a slow accumulation of organic matter. Soil N concentrations and soil P_tot were not affected by stand age. Nutrient pools in the two soil layers were calculated for an average weight of soil material (400 Mg soil ha⁻¹ in 0–10 cm depth and 600 Mg soil ha⁻¹ in 10–20 cm depth, respectively). Soil nutrient pools did not change significantly with time. Soil C pools were in average 23.6 Mg ha⁻¹ in the upper soil layer and 16.9 Mg ha⁻¹ in the lower soil layer. The highest annual increase in soil C under forest compared to heath land was 0.23 Mg C ha⁻¹ year⁻¹ in 0–10 cm depth calculated for the 53-year-old larch stand. Soil N pools were in average 1.0 Mg N ha⁻¹ in both soil layers and did not decrease with time despite a low N deposition and the uptake and accumulation of N in biomass of the growing trees. Soil P_tot pools were in average 220 and 320 kg P ha⁻¹ in the upper and lower soil layer, respectively. It was assumed that mycorrhizal fungi present in the stands had an influence on the availability of N and P to the trees. Responsible Editor: Hans Lambers.     

Effects of forest clear-cutting on the carbon and nitrogen fluxes through podzolic soil horizons

Effects of clear-cutting on the dissolved fluxes of organic C (DOC), organic N (DON), NO₃ ^– and NH₄ ⁺ through surface soil horizons were studied in a Norway spruce dominated mixed boreal forest in eastern Finland. Bulk deposition, total throughfall and soil water from below the organic (including understorey vegetation and, after clear-cutting, also logging residues), eluvial and illuvial horizons were sampled weekly from 1993 to 1999. Clear-cutting was carried out in September 1996. The removal of the tree canopy decreased the deposition of DOC and DON to the forest floor and increased that of NH₄ ⁺ and NO₃ ^– but did not affect the deposition of total N (DTN, <3 kg ha^–1 a^–1). The leaching of DOC and DON from the organic horizon increased over twofold after clear-cutting (fluxes were on an average 168 kg C and 3.3 kg N ha^–1 a^–1), but the increased outputs were effectively retained in the surface mineral soil horizons. Inorganic N deposition was mainly retained by the logging residues and organic horizon indicating microbial immobilization. Increased NO₃ ^– formation reflected as elevated concentrations in the percolate from below the mineral soil horizons were observed especially in the third year after clear-cutting. However, the changes were small and the increased leaching of DTN from below the illuvial horizon remained small (<0.4 kg ha^–1 a^–1) and mainly DON. Effects of clear-cutting on the transport of C and N to surface waters will probably be negligible.     

Forest-to-pasture conversion influences on soil organic carbon dynamics in a tropical deciduous forest

On a global basis, nearly 42% of tropical land area is classified as tropical deciduous forest (TDF) (Murphy and Lugo 1986). Currently, this ecosystem has very high deforestation rates; and its conversion to cattle pasture may result in losses of soil organic matter, decreases in soil fertility, and increases in CO₂ flux to the atmosphere. The soil organic matter turnover rate in a TDF after pasture conversion was estimated in Mexico by determining natural abundances of¹³C. Changes in these values would be induced by vegetation changes from the C₃ (forest) to the C₄ (pasture) photosynthetic pathway. The rate of loss of remnant forest-soil organic matter (fSOM) was 2.9 t ha^–1 year^–1 in 7-year-old pasture and decreased to 0.66 t ha^–1 year^–1 by year 11. For up to 3 years, net fSOM level increased in pastures; this increment can be attributed to decomposition of remnant forest roots. The sand-associated SOM fraction was the most and the silt-associated fraction the least depleted. TDF conversion to pasture results in extremely high rates of loss of remnant fSOM that are higher than any reported for any tropical forest.     

Biomass,Carbon, and Nitrogen Pools in Mexican Tropical Dry Forest Landscapes

Tropical dry forest is the most widely distributed land-cover type in the tropics. As the rate of land-use/land-cover change from forest to pasture or agriculture accelerates worldwide, it is becoming increasingly important to quantify the ecosystem biomass and carbon (C) and nitrogen (N) pools of both intact forests and converted sites. In the central coastal region of México, we sampled total aboveground biomass (TAGB), and the N and C pools of two floodplain forests, three upland dry forests, and four pastures converted from dry forest. We also sampled belowground biomass and soil C and N pools in two sites of each land-cover type. The TAGB of floodplain forests was as high as 416 Mg ha^–1, whereas the TAGB of the dry forest ranged from 94 to 126 Mg ha^–1. The TAGB of pastures derived from dry forest ranged from 20 to 34 Mg ha^–1. Dead wood (standing and downed combined) comprised 27%–29% of the TABG of dry forest but only about 10% in floodplain forest. Root biomass averaged 32.0 Mg ha^–1 in floodplain forest, 17.1 Mg ha^–1 in dry forest, and 5.8 Mg ha^–1 in pasture. Although total root biomass was similar between sites within land-cover types, root distribution varied by depth and by size class. The highest proportion of root biomass occurred in the top 20 cm of soil in all sites. Total aboveground and root C pools, respectively, were 12 and 2.2 Mg ha^–1 in pasture and reached 180 and 12.9 Mg ha^–1 in floodplain forest. Total aboveground and root pools, respectively, were 149 and 47 kg ha^–1 in pasture and reached 2623 and 264 kg ha^–1 in floodplain forest. Soil organic C pools were greater in pastures than in dry forest, but soil N pools were similar when calculated for the same soil depths. Total ecosystem C pools were 306. The Mg ha^–1 in floodplain forest, 141 Mg ha^–1 in dry forest, and 124 Mg ha^–1 in pasture. Soil C comprised 37%–90% of the total ecosystem C, whereas soil N comprised 85%–98% of the total. The N pools lack of a consistent decrease in soil pools caused by land-use change suggests that C and N losses result from the burning of aboveground biomass. We estimate that in México, dry forest landscapes store approximately 2.3 Pg C, which is about equal to the C stored by the evergreen forests of that country (approximately 2.4 Pg C). Potential C emissions to the atmosphere from the burning of biomass in the dry tropical landscapes of México may amount to 708 Tg C, as compared with 569 Tg C from evergreen forests.     

Exchange of N-gases at the Höglwald Forest – A summary

During 4 years continuous measurements of N-trace gas exchange were carried out at the forest floor-atmosphere interface at the Höglwald Forest that is highly affected by atmospheric N-deposition. The measurements included spruce control, spruce limed and beech sites. Based on these field measurements and on intensive laboratory measurements of N₂-emissions from the soils of the beech and spruce control sites, a total balance of N-gas emissions was calculated. NO₂-deposition was in a range of –1.6 –2.9 kg N ha^–1 yr^–1 and no huge differences between the different sites could be demonstrated. In contrast to NO₂-deposition, NO- and N₂O-emissions showed a huge variability among the different sites. NO emissions were highest at the spruce control site (6.4–9.1 kg N ha^–1 yr^–1), lowest at the beech site (2.3–3.5 kg N ha^–1 yr^–1) and intermediate at the limed spruce site (3.4–5.4 kg N ha^–1 yr^–1). With regard to N₂O-emissions, the following ranking between the sites was found: beech (1.6–6.6 kg N ha^–1 yr^–1) >> spruce limed (0.7–4.0 kg N ha^–1 yr^–1) > spruce control (0.4–3.1 kg N ha^–1 yr^–1). Average N-trace gas emissions (NO, NO₂, N₂O) for the years 1994–1997 were 6.8 kg N ha^–1 yr^–1 at the spruce control site, 3.6 kg N ha^–1 yr^–1 at the limed spruce site and 4.5 kg N ha^–1 yr^–1 at the beech site. Considering N₂-losses, which were significantly higher at the beech (12.4 kg N ha^–1 yr^–1) than at the spruce control site (7.2 kg N ha^–1 yr^–1), the magnitude of total gaseous N losses, i.e. N₂-N + NO-N + NO₂-N + N₂O-N, could be calculated for the first time for a forest ecosystem. Total gaseous N-losses were 14.0 kg N ha^–1 yr^–1 at the spruce control site and 15.5 kg N ha^–1 yr^–1 at the beech site, respectively. In view of the huge interannual variability of N-trace gas fluxes and the pronounced site differences in N-gas emissions it is concluded that more research is needed in order to fully understand patterns of microbial N-cycling and N-gas production/emission in forest ecosystems and mechanisms of reactions of forest ecosystems to the ecological stress factor of atmospheric N-input.