Correlations among indices of forest soil nutrient availability in fertilized and unfertilized loblolly pine plantations

Summary Various laboratory indices of N and P availability in forest soils correlated poorly among themselves and with on-site ion exchange resin (IER) estimates in both unfertilized and N+P fertilized loblolly pine plantations. IER nutrient availability estimates had greatest within-site variability than laboratory indices. Net nitrification was minimal in laboratory incubation of the mineral soil despite high rates of ammonification. In contrast, IER NO₃–N values were usually of the same magnitude as IER NH₄–N values. In both fertilized and unfertilized stands, at least one N availability index was negatively correlated with at least one P index. Soil N and P availabilities were generally higher on fertilized plots than on unfertilized plots 3.5 years after fertilization, and IER estimates showed the greatest number of plots with increased levels. The greater ability of the IER method to distinguish between fertilized and unfertilized plots indicated the method was affected by on-site factors that the laboratory methods do not assess.     

Fine root production and nutrient content in wet and moist arctic tundras as influenced by chronic fertilization

We used ingrowth cores to estimate fine root production in organic soils of wet sedge and moist tundra ecosystems near Toolik Lake on Alaska's North Slope. Root-free soil cores contained in nylon mesh tubes (5 cm diameter, 20–30 cm long) were placed in control and chronically fertilized (N plus P) plots in mid-August 1994 and were retrieved 1 year later. Estimated fine root production in control plots was 75 g m^–2 year^–1 in wet sedge and 56 g m^–2 year^–1 in moist tussock tundra. Fine root production in fertilized plots was 85 g m^–2 year^–1 in wet sedge and 67 g m^–2 year^–1 in moist tussock tundra. Although our estimates of fine root production were higher on fertilized than control plots, differences were not statistically significant within either tundra type. Comparisons between our estimates of fine root production and other estimates of aboveground (plus rhizome) production on the same (wet sedge tundra) or similar (moist tussock tundra) plots suggest that fine root production was about one-third of total net primary production (NPP) under non-fertilized conditions and about one-fifth of total NPP under chronic fertilization. Fine root N and P concentrations increased with fertilization in both tundra types, but P concentrations increased more than N concentrations in wet sedge tundra, whereas relative increases in N and P concentrations in moist tundra roots were similar. These data are consistent with other studies suggesting that NPP in wet sedge tundra is often P limited and that co-limitation by N and P is more important in moist tussock tundra.     

Nitrogen transformations in limed and nitrogen fertilized soil in Norway spruce stands

Nitrogen transformations in the soil, and the resulting changes in carbon and nitrogen compounds in soil percolate water, were studied in two stands of Norway spruce (Picea abies L.). Over the last 30 years the stands were repeatedly limed (total 6000 kg ha^–1), fertilized with nitrogen (total about 900 kg ha^–1), or both treatments together. Both aerobic incubations of soil samples in the laboratory, and intact soil core incubations in the field showed that in control plots ammonification widely predominated over nitrification. In both experiments nitrogen addition increased the formation of mineral-N. In one experiment separate lime and nitrogen treatments increased nitrification, in the other, only lime and nitrogen addition together had this effect. In one experiment immobilization of nitrogen to soil microbial biomass was lower in soil only treated with nitrogen. Soil percolate water was collected by means of lysimeters placed under the humus layer and 10 cm below in the mineral soil. Total N, NH₄-N and NO₃-N were measured, and dissolved organic nitrogen was fractioned according to molecular weight. NO₃-N concentrations in percolate water, collected under the humus layer, were higher in plots treated with N-fertilizer, especially when lime was also added. The treatments had no effect on the N concentrations in mineral soil. A considerable proportion of nitrogen was leached in organic form.     

Vertical transport of dissolved organic C and N under long-term N amendments in pine and hardwood forests

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental additions of NH₄NO₃ (0, 5 and 15 g N m^–2 yr^–1) have been made in two forest stands:Pinus resinosa (red pine) and mixed hardwood. In the seventh year of the study, we measured solute concentrations and estimated solute fluxes in throughfall and at two soil depths, beneath the forest floors (Oa) and beneath the B horizons.Beneath the Oa, concentrations and fluxes of dissolved organic C and N (DOC and DON) were higher in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxes of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendments. DOC fluxes were not strongly related to N amendment rates, but ratios of DOC:DON often decreased.The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, making the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the solum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m^–2 yr^–1), consistent with other manifestations of nitrogen saturation. Exports of N from the solum in the pine forest decreased in the order NO₃-N> NH₄-N> DON, with exports of inorganic N 14-fold higher than exports of DON. In the hardwood forest, in contrast, increased sinks for inorganic N under N amendments resulted in exports of inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.     

Cation distribution,cycling, and removal from mineral soil in Douglas-fir and red alder forests

Overstory species influence the distribution and dynamics of nutrients in forest ecosystems. Ecosystem-level estimates of Ca, Mg, and K pools and cycles in 50-year old Douglas-fir and red alder stands were used to determine the effect of overstory composition on net cation removal from the mineral soil, i.e. cation export from the soil in excess of additions. Net cation removal from Douglas-fir soil was 8 kg Ca ha^–1 yr^–1, 1 kg Mg ha^–1 yr^–1, and 0.3 kg K ha^–1 yr^–1. Annual cation export from soil by uptake and accumulation in live woody tissue and O horizon was of similar magnitude to leaching in soil solution. Atmospheric deposition partially off-set export by adding cations equivalent to 28–88% of cation export. Net cation removal from red alder soil was 58 kg Ca ha^–1 yr^–1, 9 kg Mg ha^–1 yr^–1, and 11 kg K ha^–1 yr^–1. Annual cation accumulation in live woody tissue and O horizon was three times greater than in Douglas-fir, while cation leaching in soil solution was five to eight times greater. The lack of excessive depletion of exchangeable cations in the red alder soil suggests that mineral weathering, rather than exchangeable cations, was the source of most of the removed cations. Nitric acid generated during nitrification in red alder soil led to high rates of weathering and NO₃-driven cation leaching.     

Effect of sub-optimal root zone temperatures at varied nutrient supply and shoot meristem temperature on growth and nutrient concentrations in maize seedlings (Zea mays L.)

Maize seedlings were grown for 10 to 20 days in either nutrient solution or in soils with or without fertilizer supply. Air temperature was kept uniform for all treatments, while root zone temperature (RZT) was varied between 12 and 24°C. In some treatments the basal part of the shoot (with apical shoot meristem and zone of leaf elongation) was lifted up to separate the indirect effects of root zone temperature on shoot growth from the direct effects of temperature on the shoot meristem.Shoot and root growth were decreased by low RZT to a similar extent irrespective of the growth medium (i.e. nutrient solution, fertilized or unfertilized soil). In all culture media Ca concentration was similar or even higher in plants grown at 12 as compared to 24°. At lower RZT concentrations of N, P and K in the shoot dry matter decreased in unfertilized soil, whereas in nutrient solution and fertilized soil only the K concentration decreased.When direct temperature effects on the shoot meristem were reduced by lifting the basal part of the shoot above the temperature-controlled root zone, shoot growth at low RZT was significantly increased in nutrient solution and fertilized soil, but not in unfertilized soil. In fertilized soil and nutrient solution at low RZT the uptake of K increased to a similar extent as plant growth, and thus shoot K concentration was not reduced by increasing shoot growth rates. In contrast, uptake of N and P was not increased, resulting in significantly decreased shoot concentrations.It is concluded that shoot growth at suboptimal RZT was limited both by a direct temperature effect on shoot activity and by a reduced nutrient supply through the roots. Nutrient concentrations in the shoot tissue at low RZT were not only influenced by availability in the substrate and dilution by growth, but also by the internal demand for growth.     

Potassium uptake by Chinese cabbage (Brassica pekinensis Rupy.) from fused potassium silicate,a slow-releasing fertilizer

Fused potassium silicate (FPS), which contains K₂Ca₂Si₂O₇, has been prepared as a slow-releasing potassium fertilizer. Moreover, it is difficult to estimate the proportion of nutrients utilized by plants that come from the soil versus the slow-releasing fertilizer applied. To trace the uptake of potassium (K) by plants from FPS supplied to the soil, the fertilizer K was partially replaced with rubidium (Rb). The growth and K+Rb uptake (moles) of Chinese cabbage (Brassica pekinensis Rupy. cv. Kekkyu) plants in sand culture experiments were not affected by the replacement of K with Rb. In pot experiments using a volcanic ash Ando soil, Chinese cabbage was grown with no application of K, with K and Rb salts, or with Rb-containing FPS for three cycles of 40 days each. The amounts of fertilizer-derived K in the shoots estimated by the Rb-tracer method were smaller than those estimated by the difference of K accumulation between K fertilized and unfertilized plants. Such result suggests the involvement of `K priming', a process by which the addition of K fertilizers enhances plant K uptake from the soil. The amount of K absorbed from FPS, calculated by the Rb-tracer method, indicated that the sparingly soluble K, that is soluble in 0.2 g L^–1 citric acid solution but not in water, was absorbed at least partly through direct contact with the roots without prior exchange with soil K. Moreover, the plant absorption of sparingly soluble K from FPS was also confirmed by the difference method in K uptake.     

Benthic meiofauna responses to five forest harvest methods

Benthic meiofauna were collected from the pools of minute (0 order) streams in the Ouachita National Forest, Arkansas during March 21–23, 1996 to see if benthic communities responded to forest harvest methods in a similar manner as plankton communities collected two years prior. The study streams and their watersheds (2–6 ha) were located in 14–16 ha forest stands that were selected for comparability of stands. Five treatment stands were paired with adjacent undisturbed reference stands (10 total). Treatment stands were subjected to one of five harvest methods listed in order of decreasing severity of harvest disturbance to the stands: (1) clearcut; (2) pine seed-tree; (3) pine shelterwood; (4) pine-hardwood group selection; and (5) pine single-tree selection. The mean number of taxa per site was 14 with a range of 9–20 taxa including rotifers, copepods, nematodes, dipterans, ostracods and `other' meiofauna. Densities of total meiofauna (mean=2449 No. l ^–1) were significantly higher (p= 0.002) in treated sites. Highest densities occurred in single-tree and clearcut treatments. Rotifers were significantly more numerous at the single-tree treatments (p=0.03) and nematodes were significantly greater at the clearcut treatments (p=0.03). We conclude that benthic meiofauna in these headwater streams are sensitive to silviculture practices and that the impact of forest harvest persists for at least 2.5 years.     

Soil nutrient dynamics in response to irrigation of a Panamanian tropical moist forest

We measured concentrations of soil nutrients (0–15 and 30–35 cm depths) before and after the dry season in control and dry-season irrigated plots of mature tropical moist forest on Barro Colorado Island (BCI) in central Panama to determine how soil moisture affects availability of plant nutrients. Dry-season irrigation (January through April in 1986, 1987, and 1988) enhanced gravimetric soil water contents to wet-season levels (ca. 400 g kg^–1 but did not cause leaching beyond 0.8 m depth in the soil. Irrigation increased concentrations of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), but it had little effect on concentrations of inorganic N (NH₄ ^+C, NO₃ ^– and S (SO₄ ^2–). These BCI soils had particularly low concentrations of extractable P especially at the end of the dry season in April, and concentrations increased in response to irrigation and the onset of the rainy season. We also measured the response of soil processes (nitrification and S mineralization) to irrigation and found that they responded positively to increased soil moisture in laboratory incubations, but irrigation had little effect on rates in the field. Other processes (plant uptake, soil organic matter dynamics) must compensate in the field and keep soil nutrient concentrations at relatively low levels.     

Effects of N-free fertilization on ectomycorrhiza community structure in Norway spruce stands in Southern Sweden

The application of N-free fertilizer (i.e. lime combined with nutrients such as P, Ca, K and Mg) has been suggested as one way of compensating for nitrogen-caused eutrophication and losses of base cations due to atmospheric pollution. To study the effects of such a treatment on mycorrhizal fungi, fine-root samples were collected from the LFH-layer in four Norway spruce stands in southern Sweden. One stand was part of a larger experiment (Skogaby) and had four replicates. It was fertilized twice in 1988–89 (P:K:Ca:Mg:S 48:43:218:46:75 kg ha^-1), and sampling was carried out once yearly during 1991–93. The other three stands were fertilized once in 1988–89 (P:K:Ca:Mg:S|25:62:33:12:54 kg ha^-1) and sampled in 1992.Ectomycorrhizal fine-roots were classified into morphotypes on the basis of the structure and colours of their external hyphae and fungal mantle. The fungal biomass was estimated in 1992 using ergosterol analysis. In Skogaby, N-free fertilizer had no apparent effects on fungal biomass or on the total number of ECM types. Similar results were obtained for the other three stands. Previously reported 50% reductions in sporocarp production on the fertilized plots at Skogaby can probably be explained by a decrease in carbon allocation to the roots and by a decline in the abundance of a single morphotype which accounted for 3% of the total number of root tips, but ca. 30% of the sporocarp biomass in the control plots in the present study. It is concluded that moderate levels of N-free fertilization are not likely to drastically affect the community structure of the dominating ectomycorrhizal fungi. This result should be interpreted with some caution, however, since it remains to be determined whether the fertilizer treatments affect the function of the nutrient-absorbing soil mycelium of the mycorrhizal fungi.     

Scarification,fertilization and herbicide treatment effects on planted conifers and soil fertility

The influences of soil surface modification (blade scarification and plastic mulching), fertilization and herbicide application on soil nutrient and organic carbon content and tree growth and foliar nutrient status were examined after seven years in a study located within the Great Lakes-St. Lawrence forest region of Canada. Plots had been planted with white pine (Pinus strobus L.) and white spruce (Picea glauca [Moench] Voss) seedlings. Light (PAR), soil moisture and temperature were monitored and recorded throughout the growing season. Forest floor and soil mineral (0–20 cm layer) samples were collected from all experimental plots, except those which had plastic mulching. Foliar samples were collected in autumn and analysed for N, P and K and storage compounds. Seedling mortality was 20% higher in unscarified plots. Combined silvicultural treatments increased productivity as much as 14 times, but scarification reduced soil carbon and nutrient capital 2–3 fold. Herbicide application reduced soil carbon by at least 20 %. Foliar nutrient, protein, starch and lipid contents in autumn were little affected by treatment. The future management of such stands in Canada probably will include more shelterwood harvesting and crop rotations, silvicultural systems that are more closely aligned with natural forest succession.     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.     

Growth response of Pinus densiflora seedlings to different fertilizer compound ratios in a recently burned area in the eastern coast of Korea

This study was conducted to determine fertilizer compound ratios suitable for soil conditions and tree seedling growth in recently burned areas in Korean forests. Currently, the conventional fertilizer ratio applied to undisturbed forests in Korea is N:P:K 3:4:1. In this study, Japanese red pine (Pinus densiflora S. et Z.) seedlings planted in the burned area were fertilized over four growing seasons with the following NPK compound ratios: unfertilized (CON), 3:4:1, 6:4:1, 3:8:1, and 3:4:2. Fertilization generally increased current-year needle nutrient concentrations of the seedlings, and the chlorophyll a:b ratio in CON was significantly lower than in all fertilized plots. Fertilization significantly affected the growth of the pine seedlings, which had 71–87 % more height growth and 29–67 % increased root collar diameter (RCD) compared to CON. The increases in height and RCD were significantly higher with the 6:4:1 and 3:8:1 ratios than with the 3:4:1 ratio. The 3:4:2 and 3:4:1 fertilizer ratios had no effect on the RCD growth of seedlings. This suggests that the early growth of pine seedlings could be improved by providing high N and P supplies to areas affected by forest fires rather than the conventional fertilizer ratio of 3:4:1 in Korean forest soils. Therefore, application of the suitable fertilization ratio may be a very effective way to reduce reforestation cost as well as to shorten restoration period.     

Lysimeter and field studies on15N in a tropical soil

Twenty four plots, each 2.0 m² in area, were established on St. Augustine loam soil series as field plots and microplots (containing lysimeters) in a completely randomised block design of four treatments (mulched fertilized, unmulched fertilized microplots; mulched fertilized and unmulched fertilized field plots), replicated three times. Labelled (¹⁵N) and unlabelled (NH₂)₂CO fertilizer were applied at rates of 400 kg N ha^–1 and CaH₂PO₄ and KCl were applied at rates of 100 and 150 g ha^–1 respectively to the field plots and microplots. Mulch (bagasse) was maintained to a depth of two cm and the plots were kept bare with regular applications of gramoxone.The maximum depth of leaching as measured by diffusion of NO ₃ ^– –¹⁵N in both the dry and wet seasons was 30 cm. The potential for downward movement of water and NO ₃ ^– –¹⁵N was low in the wet season because high intensity rainfall followed high soil moisture contents. Effects of mulching, on the mobility of applied N fertilizers were inconclusive. Infiltration rates were significantly (P=0.25) inversely correlated with soil moisture content, supporting the hypothesis that high intensity rainfall on a saturated soil surface is more likely to result in NO ₃ ^– –¹⁵N dispersion than NO ₃ ^– –¹⁵N leaching.     

Dissolved organic matter under native Cerrado and Pinus caribaea plantations in the Brazilian savanna

The transformation of native Cerrado into Pinus caribaea Morelet plantations changes the DOM dynamics including changed rates of mineralisation, denitrification, and C export to the groundwater. To examine the differences in quantity, temporal dynamics, and quality of DOM between Cerrado and Pine plantations we collected rainfall, throughfall, stemflow, litter leachate (under pine only) and soil solution at 15, 80, and 200 cm depth in weekly intervals during the rainy seasons 1997/98 and 1998/99. We determined total dissolved organic carbon (DOC) concentrations and assessed DOM quality by separating hydrophilic and hydrophobic fractions and by NMR analysis of organic layer extracts. The rainfall had a mean DOC concentration of 2.6 mg L^–1. The mean concentrations of DOC in the throughfall of the pine plantations (5.0–10.5 mg L^–1) were significantly above those of Cerrado (2.6–4.9 mg L^–1). During the first part of the rainy seasons (October–December), the concentrations of DOC in the soil solution (15–200 cm depth) under Cerrado and pine did not differ significantly. During the second part of both rainy seasons (January–April), the concentrations of DOC in the soil solution under Cerrado (4.4–5.1 mg L^–1) exceeded those under PI (1.4–2.7 mg L^–1). Possible explanations of the latter include higher DOM input into the Cerrado soil and a stronger retention and/or faster mineralisation of the pine DOM than of the Cerrado DOM in the mineral soil. As the structural composition of DOM extracted from the organic layer under Cerrado and pine did not differ significantly, faster mineralisation was the most likely explanation for partly lower DOC concentrations in the soil solution under pine than under Cerrado. This assumption was supported by increasing contributions of hydrophobic DOM to total DOM with increasing depth under pine while, under Cerrado, the DOM composition did not change with depth. The reason for DOM mineralisation under pine was probably the higher N availability because total N concentrations were 11–23 times higher under pine than under Cerrado.     

Needle chemistry in young Norway spruce stands after application of crushed wood ash

Nutrient concentrations in current and 1-year-old needles were analyzed annually for 5 years after application of hardened wood ash in 1–4-year-old Norway spruce (Picea abies (L.) Karst.) stands within a range of climate and fertility gradients. At each site, 3000 kg ha^–1 hardened wood ash of two types, Nymölla and Perstorp, was applied in a randomized block design. Wood ash Nymölla contained 12 kg ha^–1 P, 30 kg ha^–1 K, 891 kg ha^–1 Ca, 72 kg ha^–1 Mg and wood ash Perstorp contained 12 kg ha^–1 P, 60 kg ha^–1 K, 486 kg ha^–1 Ca, and 60 kg ha^–1 Mg. The ash was intended to compensate for nutrients removed at the preceding harvest when logging residues were collected and removed from the site (whole-tree harvesting). The climate gradient included four climate zones throughout Sweden and each of these included a fertility gradient of three sites classified according to their ground vegetation type. There were no effects on nutrient concentrations in the needles 1 year after the application of wood ash. Five years after ash application, the concentrations of P, K and Ca in current and 1-year-old needles were higher than in the control plots. The results were consistent over all stands, irrespective of climate zone and fertility status. P and K concentrations were higher in spruce needles from plots treated with Perstorp wood ash, whereas Ca concentrations were higher in those of Nymölla treated plots. Analyses across all study sites revealed a treatment effect in terms of increased ratios of P:N, K:N and Ca:N in 1-year-old needles. The ratio P:N tended to increase with time in the Perstorp wood ash treatment compared with the control. The needle concentrations of Mg and S were not affected by the ash applications. The increase in needle nutrient concentrations after application of hardened wood ash suggests that wood ash recycling could be used in order to replace nutrients removed at whole-tree harvesting.     

Soil Organic Carbon and Water Retention after Conversion of Grasslands to Pine Plantations in the Ecuadorian Andes

Tree plantations in the high elevations of the tropics constitute a growing land use, but their effect on ecosystem processes and services is not well known. We examined changes in soil organic carbon (C) and water retention in a chronosequence of Pinus radiata stands planted in páramo grasslands in Cotopaxi province, Ecuador. Water retention at 10, 33, and 1,500 kPa declined with stand age, with soils in the oldest pine stands retaining 39%, 55%, and 63% less water than grassland soils at the three pressures tested. Soil organic C in the 0–10-cm depth also declined with stand age, from 5.0 kg m^–2 in grasslands to 3.5 kg m^–2 in 20–25-year-old pine stands (P < 0.001); at greater depth in the A horizon, C contents decreased from 2.8 to 1.2 kg m^–2 (P = 0.047). There were no significant differences among age classes in the AC and C horizons (P = 0.15 and P = 0.34, respectively), where little or no weathering of the primary material has occurred. Inputs of C may be affected by the significantly higher carbon–nitrogen (C:N) ratio of the litter under older pine stands (P = 0.005), whereas outputs are influenced by substrate quality as well as soil environmental factors. Soil ratios at the 0–10 cm depth were significantly higher in grasslands and young pine stands (P < 0.001), whereas carbon–phosphorous (C:P) ratios at 0–10-cm depth followed a similar but not significant trend. However, there was no significant difference in short-term decomposition rates (P = 0.60) when the soils were incubated under uniform temperature and moisture conditions. In páramo ecosystems, where high soil moisture plays an important role in retarding decomposition and driving high C storage, the loss of water retention after afforestation may be the dominant factor in C loss. These results suggest that soil C buildup and water retention respond rapidly to changes in biota and need to be assessed with regard to implications for C sequestration and watershed management.     

Bioremediation (Natural Attenuation and Biostimulation) of Diesel-Oil-Contaminated Soil in an Alpine Glacier Skiing Area

We investigated the feasibility of bioremediation as a treatment option for a chronically diesel-oil-polluted soil in an alpine glacier area at an altitude of 2,875 m above sea level. To examine the efficiencies of natural attenuation and biostimulation, we used field-incubated lysimeters (mesocosms) with unfertilized and fertilized (N-P-K) soil. For three summer seasons (July 1997 to September 1999), we monitored changes in hydrocarbon concentrations in soil and soil leachate and the accompanying changes in soil microbial counts and activity. A significant reduction in the diesel oil level could be achieved. At the end of the third summer season (after 780 days), the initial level of contamination (2,612 ± 70 μg of hydrocarbons g [dry weight] of soil⁻¹) was reduced by (50 ± 4)% and (70 ± 2)% in the unfertilized and fertilized soil, respectively. Nonetheless, the residual levels of contamination (1,296 ± 110 and 774 ± 52 μg of hydrocarbons g [dry weight] of soil⁻¹ in the unfertilized and fertilized soil, respectively) were still high. Most of the hydrocarbon loss occurred during the first summer season ([42 ± 6]% loss) in the fertilized soil and during the second summer season ([41 ± 4]% loss) in the unfertilized soil. In the fertilized soil, all biological parameters (microbial numbers, soil respiration, catalase and lipase activities) were significantly enhanced and correlated significantly with each other, as well as with the residual hydrocarbon concentration, pointing to the importance of biodegradation. The effect of biostimulation of the indigenous soil microorganisms declined with time. The microbial activities in the unfertilized soil fluctuated around background levels during the whole study.     

An assessment of the contribution of net mineralization to N cycling in grass swards using a field incubation method

Measurements of net mineralization using a field incubation method were made over a full growing season (180 d). Soil cores, taken from cut swards which for many years had been previously grazed by cattle, were placed in jars in the field for successive incubation periods of 14 d. Acetylene was added to the incubation jars to inhibit nitrification in the soil cores and thereby prevent losses of N through denitrification. Net mineralization over 180 d amounted to 415, 321 and 310 kg N ha^–1 under grass/clover, unfertilized grass and grass receiving 420 kg N ha^–1 y^–1, respectively. At the start of the growing season, an index of potentially mineralizable N in the soil was estimated by a chemical extraction method, but this index was <50% of the estimates obtained by field incubation. The amount of N in herbage harvested regularly from the swards also under-estimated the supply of N from the soil, with apparent recoveries of 53, 82 and 74% and total yields of N of 240, 263 and 538 (kg N ha^–1) from grass/clover, unfertilized grass and fertilized grass, respectively. Mineralization rates varied significantly with seasonal soil temperature fluctuations, but the incubation method was apparently less sensitive in relation to changes in soil water content. Rates of N-turnover (as % of total soil N) were highest under grass/clover (9%), but similar under fertilized and unfertilized grass swards (approximately 5%).     

Lead dynamics in the forest floor and mineral soil in south-central Ontario

Recent studies have suggested that the residence time of Pb in the forest floor may not be as long as previously thought, and there is concern that the large pulse of atmospheric Pb deposited in the 1960s and early 1970s may move rapidly through mineral soils and eventually contaminate groundwater. In order to assess Pb mobility at a woodland (JMOEC) in south-central Ontario, a stable Pb isotope tracer ²⁰⁷Pb (8 mg m^–2) was added to the forest floor in white pine (Pinus strobus) and sugar maple (Acer saccharum) stands, respectively, and monitored over a 2-year period. Excess ²⁰⁷Pb was rapidly lost from the forest floor. Applying first-order rate coefficients (k) of 0.57 (maple) and 0.32 (pine) obtained from the tracer study, and estimates of Pb deposition in the region, current predicted Pb concentrations in the forest floor are 1.5–3.1 and 2.1–5.8 mg kg^–1 in the maple and pine plots, respectively. These values compare favorably with measured concentrations (corrected for mineral soil contamination) of 3.1–4.3 mg kg^–1 in the maple stand and 2.6–3.6 mg kg^–1 in the pine stand. The response time (1/^k) of Pb in the forest floor at the sugar maple and white pine plots was estimated to be 1.8 and 3.1 years, respectively. The rapid loss of Pb from the forest floor at the JMOEC is much greater than previously reported, and is probably due to the rapid rate of litter turnover that is characteristic of forests with mull-type forest floors. In a survey of 23 forested sites that border the Precambrian Shield in south-central Ontario, Pb concentrations in the forest floor increased exponentially with decreasing soil pH. Lead concentrations in the forest floor at the most acidic survey sites, which exhibited mor-type forest floors, were approximately 10 times higher (80 mg kg^–1) than at the JMOEC, and pollution Pb burdens were up to 25 times greater. Despite the rapid loss of Pb from the forest floor at the JMOEC, the highest pollution Pb concentrations were found in the upper (0–1 cm) mineral soil horizon. Lead concentrations in the upper 30 cm of mineral soil were strongly correlated with organic matter content, indicating that pollution Pb does not move as a pulse down the soil profile, but instead is linked with organic matter distribution, indicating groundwater contamination is unlikely.