Forest Fertilization Research Articles

Would Forest Landowners Use Poultry Manure as Fertilizer?

Abstract When manure nutrients exceed local cropland's assimilative capacity, the potential for water quality problems exists. Concerns about water quality in Maryland have led to the passage of the Water Quality Improvement Act, which will affect the disposal of poultry litter on cropland. Because Maryland's forest soils test low for phosphorus, forest fertilization may be an alternative use for the litter. A representative sample of 402 Maryland landowners owning 40 or more acres was asked whether they would consider using poultry litter as a forest fertilizer under various incentives. When offered $20 per acre, landowners owning more acres, located in certain counties, and who were younger than the other survey respondents were most likely to consider using poultry litter. Landowners who had a forest management plan were less likely to be willing.

Nitrogen uptake and turnover in riparian woody vegetation.

The nutrient balance of streams and adjacent riparian ecosystems may be modified by the elimination of anadromous fish runs and perhaps by forest fertilization. To better understand nitrogen (N) dynamics within stream and riparian ecosystems we fertilized two streams and their adjacent riparian corridors in central Idaho. On each stream two nitrogen doses were applied to a swathe approximately 35 m wide centered on the stream. The fertilizer N was enriched in 15N to 18 per thousand. This enrichment is light relative to many previous labeling studies, yet sufficient to yield a traceable signal in riparian and stream biota. This paper reports pre-treatment differences in delta15N and the first-year N response to fertilizer within the riparian woody plant community. Future papers will describe the transfer of allochthonous litter N to the stream and its subsequent processing by stream biota. Pre-treatment delta15N differed between the two creeks (P=0.0002), possibly due to residual salmon nitrogen in one of the creeks. Pre-treatment delta15N of current-year needles was enriched compared to leaf litter, which was in turn enriched compared to needles aged 4 years and older. We conclude that fractionation due to retranslocation occurs in at least two phases. The first phase, which optimizes allocation of N in younger needle age classes, is distinctly different from the second, which conserves N prior to abscission. The delta15N difference between creeks was eliminated by the fertilization (P=0.42). In the two dominant conifer species, Abies lasiocarpa and Picea engelmannii, most fertilizer N was found in the current-year foliage; little was found in older needles and none was detected in litter (P=0.53). The only N-fixing shrub species, Alnus incana, took up only a small amount of fertilizer N [mean percent N derived from fertilizer (%Ndff) 5.0+/-1.6% (SE)]. Far more fertilizer N was taken up by other deciduous shrubs (mean %Ndff=33.9+/-4.5%). Fertilizer N made up 25% (+/-4.2%) of the N in deciduous shrub litter. These results demonstrate the feasibility of light labeling with 15N and the potential influence of riparian plant species composition on stream nutrient dynamics via allochthonous leaf litter inputs.

Coupling of the Perturbed C–N–P Cycles in Industrial Time

Coupling of the C–N–P biogeochemical cycles is effected by the dependence of the land and aquatic primary producers on the availability of N and P. In general, the Redfield ratios C:P and N:P in the reservoirs supplying nutrients for primary production on land, in the oceanic coastal zone, and in the surface ocean differ from these ratios in the land phytomass and aquatic plankton. When N:P in the source is higher than in primary producers, this results in a potential accumulation of some excess nitrogen in soil water and coastal water, and increased denitrification flux to the atmosphere. The oceanic coastal zone plays an important role in the coupled C–N–P cycles and their dynamics because of its intermediate position between the land and oceanic reservoirs. These coupled cycles were analyzed for the last 300 years of exposure to four human-generated forcings (fossil fuel emissions, land use change, chemical fertilization of land, and sewage discharge to the coastal zone) and global temperature rise. In the period from 1700 to 2000, there has been a net loss of C, N, and P primarily from the land phytomass and soil humus, despite the rise in atmospheric CO2, increased recycling of nutrients from humus, chemical fertilization, and re-growth of forests on previously disturbed land. The main mechanisms responsible for the net loss were increased riverine transport to the coastal zone of dissolved and particulate materials and, for N, increased denitrification on land. The oceanic coastal zone gained N and P, resulting in their accumulation in the organic pool of living biomass and dissolved and reactive particulates, as well as in their accumulation in coastal sediments from land-derived and in situ produced organic matter. Pronounced shifts in the rates and directions of change in some of the major land reservoirs occurred near the mid-1900s. Denitrification removes N from the pool available for primary production. It is the strongest on land, accounting for 73–83% of N removal from land by the combined mechanisms of denitrification and riverine export.

Cadmium-containing wood ash in a pine forest: effects on humus microflora and cadmium concentrations in mushrooms, berries, and needles

The cadmium (Cd) concentration of wood ash (1–30 mg·kg–1) is higher than allowed for agriculture fertilizers (3 mg·kg–1). Therefore, the objectives of this field study were to test if the Cd of wood ash has the potential to affect the coniferous forest humus microflora and if Cd enters the human food chain. These objectives were tested with ash (A) and Cd-spiked ash (ACd, 400 mg Cd·kg–1) at a fertilization rate of 3 t·ha–1. Microbial community structure, respiration, needle litter decomposition, growth rates and Cd tolerance of bacteria, and the bioavailability of Cd were measured. Also, Cd concentrations of humus, soil percolation water, mushrooms, fruits and leaves of berries, and needles were determined. The amount of Cd in the percolation water or bioavailable Cd, measured with a bacterial biosensor, and Cd tolerance of bacteria did not increase, although the ACd treatment increased the amount of humus total and extractable Cd. Only the ACd and not the A treatment caused Cd concentration increment in Lactarius rufus and berries of Empetrum nigrum. In spite of the high Cd concentration of the spiked ash, it did not have harmful effects on humus microorganisms during this 4-year study. Thus, wood ash is safe to use as a fertilizer in forests.

Effects of long-term nutrient fertilisation and irrigation on the microarthropod community in a boreal Norway spruce stand

Intensive nutrient fertilisation of forests has been suggested as a method to increase production of biofuels as a replacement for fossil fuels. We used a field experiment in a Norway spruce, Picea abies (L.) Karst., stand in northern Sweden to examine possible long-term effects of forest fertilisation on soil fauna (Oribatida, Mesostigmata, Collembola, predatory macroarthropods). Fertilisers had been applied annually for a period of 13 years, both in solid and liquid form, and N was added as ammonium nitrate (75–100 kg N ha −1 per year). For comparison, control plots and plots receiving only irrigation were included. An autumn sampling showed soil fauna decreases in plots receiving fertiliser in solid form, but increases in plots receiving liquid fertiliser. Clear shifts in community composition following both fertilisation methods were seen in Oribatida and Collembola, but species number and diversity were not significantly affected. This was probably due to increases in tolerant species that balanced decreases in other species. Liquid fertilisation had less negative effects on many species than fertilisation in solid form. Irrigation alone did not affect faunal abundances and had no effect on community composition of Oribatida and Collembola. The study indicates that intensive forest fertilisation will cause large shifts in soil microarthropod communities, but that species richness may remain unaffected. The risk of species loss will probably depend upon the size of the areas used for this purpose.

Experimental N fertilization of Scots pine: effects on soil-solution chemistry 8 years after final felling

Previous N fertilizations in boreal forests may increase the leaching of elements such as NO 3 − and Al after final felling. In order to study such effects, soil-solution chemistry was investigated during the eighth growing season after final felling a Scots pine stand in central Sweden. This stand had been subjected to experimental N fertilization, with the latest application performed 6 years before final felling. Soil solution at a depth of 50 cm was collected with Teflon and ceramic suction cups in duplicate plots to which ammonium nitrate had been applied three times, giving total N doses of 0, 360 and 1800 kg ha −1. Compared with the control, the 360 kg N ha −1 treatment showed no significant ( P>0.05) effects on soil-solution chemistry. The 1800 kg N ha −1 treatment tended to have higher concentrations of H +, total Al, Mn, NO 3 −-N, total N and F −, and lower acid-neutralizing capacity than both the control and the 360 kg N ha −1 treatment. Forest fertilization with 1800 kg N ha −1 negatively affected soil-solution chemistry after final felling as opposed to the low fertilizer dose common in Swedish silviculture. While the study provides only a snapshot in time, a final conclusion could not be drawn for the low fertilizer dose.

Increases in tree growth and nutrient supply still apparent 10 to 13 years following fertilization and vegetation control of salal-dominated cedar–hemlock stands on Vancouver Island

Cedar–hemlock forests on Vancouver Island are primarily nitrogen limited and characterized by an under story dominated by the ericaceous shrub salal (Gaultheria shallon Pursh.). In 1984, an experiment was established on regenerating cedar–hemlock forests to determine the effects of nitrogen and phosphorus (N + P) fertilization and vegetation control on conifer growth. It was hypothesized that these treatments would not only stimulate tree growth, but also improve nutrient supply, stand productivity, and site quality. To test this hypothesis, tree height growth, canopy closure, salal biomass, foliar and forest floor N and P concentrations, and seedling growth on forest floor were measured 10–13 years after treatment. Both salal control and fertilization increased tree growth and canopy cover, and reduced salal biomass. However, only fertilized plots showed changes in site quality measurable 10–13 years following N + P application. Hemlock foliar P concentrations, forest floor total N and P levels, and hemlock height increments were higher in these plots. Forest floors from the fertilized plots also supported greater growth of conifer seedlings. These results suggest that sustained changes to site quality may be achieved with N + P fertilization of cedar–hemlock forests.

NUTRIENT STATUS OF TROPICAL RAIN FORESTS INFLUENCES SOIL N DYNAMICS AFTER N ADDITIONS

Soil nitrogen (N) transformations and N-oxide emissions were measured following N additions in three tropical montane rain forests in the Hawaiian Islands that differed in substrate age and nutrient status. Nitrous oxide (N2O) and nitric oxide (NO) emissions were negligible following first-time N additions in a forest where N limits primary production, and they increased significantly following 11 yr of N fertilization. Furthermore, N-oxide fluxes in the N-limited forest were relatively low in response to a range of N additions, and all doses of N except the two highest (125 and 175 kg N/ha) resulted in net negative rates of mineralization and nitrification (i.e., soils showed significant N consumption). Short-term laboratory 15N experiments supported these trends by showing that both 15NH4+ and 15NO3− were strongly consumed in soil from both the control and long-term fertilized plots in the N-limited site. Long-term N fertilization in the N-limited forest significantly increased N availability and turnover, but it did not appreciably alter the small population size of nitrifying microorganisms found in soils of this site. In contrast, N-oxide emissions were equally large after both first-time and long-term N fertilization in forests where production is limited by N and P in combination and by P alone. Furthermore, N-oxide fluxes and net rates of N mineralization and nitrification were equally large in response to both small and large doses of N. Net N transformation and 15N assays suggested that N consumption processes in both the control and long-term N fertilized plots in the P-limited site were relatively weak (compared to the N-limited site) and not significantly different from one another. Long-term N fertilization did not significantly alter N availability or turnover in the NP-limited and P-limited forests (where pool sizes of NH4+, NO3−, and gross and net mineralization in control plots are high), but it increased rates of nitrification through an apparent increase in the number and/or activity of nitrifying microorganisms. In all forests, fluxes of N-oxides measured immediately following N additions were highly correlated with changes in the activity of nitrifying rather than denitrifying microorganisms. On average, N-oxide fluxes were predictable based on the N status of the ecosystem as estimated by pools of inorganic N and potential rates of nitrification. Combined, these results suggest that anthropogenic N inputs are processed in N-limited and P-limited tropical systems differently, such that the fate of added N in these tropical forest systems is determined in part by the relative strengths of the pathways of N retention (uptake and immobilization in plants and soil organic matter) vs. N loss (nitrification and denitrification). This work suggests that P-limited forests growing on highly weathered soils (where N cycles quickly) may respond differently to N additions than N-limited forests, with large and immediate losses of N-oxides. Corresponding Editor: J. B. Yavitt.

A mechanism of abiotic immobilization of nitrate in forest ecosystems: the ferrous wheel hypothesis

AbstractForest soils, rather than woody biomass, are the dominant long‐term sink for N in forest fertilization studies and, by inference, for N from atmospheric deposition. Recent evidence of significant abiotic immobilization of inorganic‐N in forest humus layers challenges a previously widely held view that microbial processes are the dominant pathways for N immobilization in soil. Understanding the plant, microbial, and abiotic mechanisms of N immobilization in forest soils has important implications for understanding current and future carbon budgets. Abiotic immobilization of nitrate is particularly perplexing because the thermodynamics of nitrate reduction in soils are not generally favorable under oxic conditions. Here we present preliminary evidence for a testable hypothesis that explains abiotic immobilization of nitrate in forest soils. Because iron (and perhaps manganese) plays a key role as a catalyst, with Fe(II) reducing nitrate and reduced forms of carbon then regenerating Fe(II), we call this ‘the ferrous wheel hypothesis’. After nitrate is reduced to nitrite, we hypothesize that nitrite reacts with dissolved organic matter through nitration and nitrosation of aromatic ring structures, thus producing dissolved organic nitrogen (DON). In addition to ignorance about mechanisms of DON production, little is known about DON dynamics in soil and its fate within ecosystems. Evidence from leaching and watershed studies suggests that DON production and consumption may be largely uncoupled from seasonal biological processes, although biological processes ultimately produce the DOC and reducing power that affect DON formation and the entire N cycle. The ferrous wheel hypothesis includes both biological and abiological processes, but the reducing power of plant‐derived organic matter may build up over seasons and years while the abiotic reduction of nitrate and reaction of organic matter with nitrite may occur in a matter of seconds after nitrate enters the soil solution.

Secondary Metabolite Concentrations and Terpene Emissions of Scots Pine Xylem after Long‐Term Forest Fertilization

Secondary compounds are known to be associated with the resistance of conifer xylem against insects and fungi. The effects of long-term forest fertilization with nitrogen (N) or with N, calcium (Ca), and phosphorus (P) on secondary compounds in the xylem of 50-yr-old Scots pine (Pinus sylvestris L.) trees were examined. Xylem samples were collected from trees growing in three locations in southern Finland: Vilppula, Padasjoki, and Punkaharju. Forests were fertilized every fifth (Vilppula and Padasjoki) or tenth (Punkaharju) year since the 1950s. We compared concentrations of individual and total monoterpenes and resin acids in the heartwood and sapwood of Scots pine. Terpene emissions were analyzed from the sapwood and total phenolics from the heartwood. Fertilization did not have any significant effect on the concentrations and emissions of xylem monoterpenes. Concentrations of several individual terpenes in sapwood were positively correlated with the corresponding terpene emission. The concentrations of individual resin acids (i.e., abietic and dehydroabietic) decreased significantly in Punkaharju, but increased in the sapwood of N-fertilized trees compared with control ones at Padasjoki and Vilppula. The concentrations of resin acids in the heartwood were not significantly affected by fertilization. Both fertilization treatments decreased the total phenolic concentrations in the heartwood of trees growing in Padasjoki. There was a significant positive correlation between the total phenolics and total resin acid concentration. Overall, resin acids and phenolics seemed be more responsive than monoterpenes to N treatment. These results suggest that forest fertilization might cause slight changes in secondary compound concentrations of xylem, and thus might have significance in the decay resistance of wood.

Phosphorus form and bioavailability in the pine rotation following fertilization: P fertilization influences P form and potential bioavailability to pine in the subsequent rotation

Soils in some parts of the world are known to be severely phosphorus (P) deficient. As little as 20 kg P ha −1 have produced large growth responses in forest stands. If increased tree growth and augmented P cycling are caused by a one-time P application, a valid question is whether the effect of the initial fertilization will be evident in the regenerating forest, thereby reducing the need for P fertilization early in the second rotation. The objective of this study was to evaluate the P status of the forest floor and inorganic P status of mineral surface soil of two long-term fertilizer trials in order to determine if there were increases in soil P amount and bioavailability that benefits the next rotation. Long-term fertilizer trials in Georgia, USA and New Zealand (29 and 22 years after fertilization) were the study sites. In Georgia, forest fertilization increased the mass of the F horizon by 239% and its P content by 318%. Anion exchange membranes measured up to a 47% increase in bioavailable P in the mineral soil in a fertilized treatment. While bioassays did not show significant residual P in the mineral soil under the fertilized plots, soil from an adjacent operationally fertilized stand, which had the forest floor bedded into its planting rows, showed a 100% increase in seedling P content. At the New Zealand site, the mineral soil contained 142% more P in the anion exchange membrane form than in the unfertilized soils. Bioassays indicated a 224% increase in seedling P content when grown in fertilized soil at the highest fertilization rate. Operational levels of fertilization did not yield a significant bioassay result. The data show that there can be a residual effect of fertilizer P in the rotation following fertilization. The bioassays, anion membrane exchangeable P and forest floor P from Georgia suggest that there is enough P in the soil system to support the needs of a regenerating stand of loblolly pine, while residual bioavailable P in the mineral soil from operational levels of fertilization at New Zealand site would require residual P in the forest floor or immediate fertilization to meet the demands of newly planted seedlings. Conservation of the forest floor is an important part of P management in these P deficient sites.

Growth enhancement due to global atmospheric change as predicted by terrestrial ecosystem models: consistent with US forest inventory data

AbstractSmall reported growth enhancement factors based on analyses of forest inventory data from the eastern USA (Caspersen et al. 2000, Science, 290, 1148–1151) have been interpreted as evidence against CO2 fertilization in natural forests. We show to the contrary that growth enhancement in response to rising CO2, as found in ecosystems with experimental CO2 enrichment and implemented in terrestrial ecosystem models, is consistent with the data that have been presented within their uncertainties. Comparing forest inventory data with results of an empirical model of age‐dependent biomass accumulation, we find that growth enhancement of plausible magnitude could not be detected in these data, even if it were present. Although forest regrowth due to land‐use change is recognized as an important cause of carbon uptake by eastern US forests, forest inventory data do not provide a basis for eliminating environmentally induced growth enhancement as a substantial contribution to the global terrestrial carbon sink.

Biological and Physical Constraints on Maize Production in the Humid Forest and Western Highlands of Cameroon

The aim was to identify biological and physical factors responsible for reducing maize yield in Cameroon. Two surveys were conducted in 137 fields in two agroecological zones in 1995–1997. In the Humid Forest (HF), Bipolaris maydis, Stenocarpella macrospora, Puccinia polysora, Rhizoctonia solani and soil fertility were factors that reduced maize production in 1995 and 1996. In the Western Highlands (WHL), Cercospora zeae-maydis, and the interaction between soil fertility and maize variety were the most important constraints to maize production in 1996. In 1997, C. zeae-maydis, S. macrospora, physiological spot and stem borer damage (Busseola fusca) were negatively related to ear weight. The combination of these biological factors (diseases and insects), and the physical parameter of soil fertility were responsible for reducing maize yield in these selected benchmarks of Cameroon. Maximum potential yield reductions were estimated at 68% due to B. maydis and 46% due to S. macrospora, respectively, in the HF in 1995. In 1996, maximum potential yield reductions in the HF were estimated at 34%, 41% and 30% due to S. macrospora, P. polysora and R. solani, respectively. In the WHL, C. zeae-maydis had the potential to cause a yield reduction of 79% in 1996. In the WHL in 1997, the interaction between C. zeae-maydis and B. fusca, stem diseases and the physiological spot caused potential reductions of 52%, 34% and 39%, respectively.

Juvenile amphibians do not avoid potentially lethal levels of urea on soil substrate

AbstractWe examined the effects of a forest fertilizer (urea) on newly metamorphosed terrestrial amphibians (Western toads, Bufo boreas; Cascades frogs, Rana cascadae; long‐toed salamanders, Ambystoma macrodactylum; and roughskin newts, Taricha granulosa). We examined avoidance behavior of Western toads and Cascades frogs on both paper towel and soil substrates dosed with urea (control and 100 kg N/ha and an additional treatment of 50 kg N/ha for Western toads on soil substrate) and avoidance behavior of long‐toed salamanders on soil substrate dosed with urea. We further examined the survival and feeding behavior of all four species exposed to urea on soil substrate (100 kg N/ha) for 5 d. Juvenile Western toads and Cascades frogs avoided paper towels dosed with urea but did not avoid urea‐dosed soil substrate. However, Western toads and Cascades frogs both suffered significant mortality when exposed to urea on a soil substrate for 5 d. Furthermore, after adjusting for weight, we found that urea‐exposed juvenile Western toads and Cascades frogs consumed significantly fewer prey items (crickets) compared with nonexposed control animals. Long‐toed salamanders did not discriminate against soil substrate dosed with urea, and neither long‐toed salamanders nor roughskin newts died or reduced prey consumption as a result of urea exposure. Juvenile amphibians may not be able to detect and avoid harmful levels of urea fertilizer on a natural substrate. Furthermore, anthropogenic stressors such as urea fertilizer can significantly reduce the survival and prey consumption of juvenile amphibians. These effects are important to consider in light of possible threats to the conservation status of many amphibian species.

Response of Three Young Douglas-Fir Plantations to Forest Fertilization with Low Rates of Municipal Biosolids

The growth of three young (5-6 years since planting) Douglas-fir plantations fertilized with a single low-rate application of municipal biosolids, ranging from 17-19 dry Mg ha-1, was studied. Stand measurements showed increases in DBH, height, average and total basal area, average and total volume and average and total dry-weight differences in the biosolids-fertilized vs. control. For instance, average per tree DBH was 14.7% greater than the controls, height by 2.7%, per stem basal area by 27%, per stem average volume by 33%, per stem average dry weight by 38%, basal area per hectare by 28%, volume per hectare by 32% and dry weight per hectare by 36% compared to untreated controls. Comparing differences in volume increases from initial growth periods with the latest growth period indicates that volume increases due to the biosolids treatment are continuing and apparently increasing over time. However, none of these observed differences were statistically significant (0.05 level) due to high variation within and between stands.

Ca-Mg ratios for evaluating pedogenesis in the piedmont province of the southeastern United States of America

The nature of parent materials strongly influences soil development in the southeastern US Piedmont. In this region, most parent materials (saprolite) form from metamorphic and igneous felsic and mafic rocks. Although most soils in felsic terrains are acid and kaolinitic, soils formed from mafic parent materials range from kaolinitic Ultisols to Alfisols with smectite. Because it is sometimes difficult in soils developed in mixed terrains to determine parent material influence, exchangeable Ca/Mg ratios were used to elucidate soil-parent material relationships. Establishment of Ca/Mg signatures diagnostic of soil-parent material suites could facilitate soil mapping activities and forest fertilization regimes. Characterization data collected as part of the National Cooperative Soil Survey (mainly Georgia and Alabama) were evaluated, and soils were grouped by parent material type. The Ca/Mg ratios did not prove to be diagnostic for separating parent materials; however, significant depth trends existed. In most of the groupings, Ca/Mg ratios decreased with depth in the solum, which was most likely due to selective retention of Ca2+ and the effects of bio-cycling. The ratio either increased below the solum (mafic soils), or continued to decrease (felsic soils). Thus, these ratios are useful for identifying solum depth in some of these soils. The (Ca + K)/Mg ratios were more diagnostic with regard to parent material. These data suggest soils possessing ratios of (Ca + K)/Mg < 2 within the solum (excluding surface horizons) have been significantly influenced by mafic parent materials. Key words: Piedmont, Ca-Mg ratios, Ultisols, Alfisols, pedogenesis

Ion leakage after liming or acidifying fertilization of Swedish forests — a study of lysimeters with and without active tree roots

Forests in southern Sweden are exposed to acidic deposition, which may ultimately become detrimental to the vitality and growth of the forests. The aim of the present work was to study the leaching of a range of physiologically important ions after different treatments representing high loads of N and other acidifying substances, as well as the effects of treatments aimed to counteract soil acidification (additions of lime, ash and vitality fertilizer). We compared the leaching from two types of lysimeters (“ingrowth” with, and “trenched” without, active tree roots) at two sites in the south and two sites in the north of Sweden. Generally, considerably larger amounts of ions were leached from trenched than ingrowth cores, and from the southern as compared to the northern sites. Additions of N caused, in the presence of tree roots, increased leaching of inorganic N in the north, and in the south of not just N, but also Ca, Mg, Mn and Al. Liming caused increased NO 3 − leaching only in the absence of roots at an N-rich site. Indications of soil Mg depletion were found after limestone addition. Vitality fertilizer caused no change in the leaching of ions in ingrowth cores. The study emphasizes the importance of tree root uptake, i.e. of healthy forest stands, to counteract the acidifying leaching of base cations, as well as Al and NO 3 − from mineral soils.

Sensitivity to urea fertilization in three amphibian species.

Forest fertilization with granular urea is a well-established management practice in many forested regions of the world. We hypothesize that chemical forest fertilizers may be affecting forest-dwelling wildlife. In the laboratory, we studied the effects of fertilization doses of granular urea on three species of forest-dwelling amphibians (Plethodon vehiculum, Rhyacotriton variegatus, and Taricha granulosa). In avoidance experiments, the three species avoided a substrate treated with a dose of 225 kg N/ha urea. In toxicity experiments, we exposed amphibians to urea at doses of 225 kg N/ha and 450 kg N/ha for 4 days. The observed effects increased with time and dose, and there were significant differences in sensitivity among the species. Both treatment levels had an acute effect on survival of P. vehiculum and R. variegatus. At 24 h, mortality at the highest dose was 67% for P. vehiculum, and 47% for R. variegatus. In contrast, there was no mortality for T. granulosa at these concentrations. We suggest that environmental levels of urea could be affecting behavior and survival of some amphibians species in fertilized forests.

Forest fertilization with wood ash: effect on the distribution and storage of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs).

Before wood ash can be safely used as a fertilizer in forests, possible negative effects such as input of organic contaminants or remobilization of contaminants already stored in the soil must be investigated. The objective of this study was to examine the effects of wood ash application on concentrations, storage, and distribution of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) in a Swiss forest soil. In May 1998, we added 8 Mg wood ash ha(-1) to a forest soil. We determined 20 PAHs and 14 PCBs in the organic layer, in the bulk mineral soil, and in soil material taken from preferential flow paths and from the matrix before and after the wood ash application. In the control plots, the concentrations of PAHs in the organic layer indicated moderate pollution (sum of 20 PAHs: 0.8-1.6 mg kg(-1)), but sum of PCB concentrations was high (21-48 microLg kg(-1)). The wood ash had high concentrations of PAHs (sum of 20 PAHs: 16.8 mg kg(-1)), but low concentrations of PCBs (sum of 14 PCBs: 3.4 microg kg(-1)). The wood ash application increased the PAH concentrations in the organic horizons up to sixfold. In contrast, PCB concentrations did not change in the Oa horizon and decreased up to one third in the Oi and Oe horizons. The decrease was probably caused by the mobilization of stored PCBs because of the high pH of the wood ash. This probably results in a higher mobility of dissolved organic matter, acting as PCB carrier. In the mineral soil, the preferential flow paths of the A horizon contained more PAHs and PCBs (+20 +/- 15% and +43 +/- 60%, respectively) than the matrix. This was particularly true for higher molecular weight compounds (molecular weight > 200 g mol(-1)). Below 50 cm depth, concentrations of PAHs and PCBs were smaller in the preferential flow paths, suggesting that in deeper depths, processes acting as sinks dominated over inputs in the preferential flow paths.

Nitrogen Oxide Fluxes and Nitrogen Cycling during Postagricultural Succession and Forest Fertilization in the Humid Tropics

The effects of changes in tropical land use on soil emissions of nitrous oxide (N2O) and nitric oxide (NO) are not well understood. We examined emissions of N2O and NO and their relationships to land use and forest composition, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and patterns of carbon (C) cycling in a lower montane, subtropical wet region of Puerto Rico. Fluxes of N2O and NO were measured monthly for over 1 year in old (more than 60 years old) pastures, early- and mid-successional forests previously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertilized tabonuco forest. N2O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg N · ha−1· y−1). Among the unfertilized sites, the expected pattern of increasing emissions with stand age did not occur in all cases. The mid-successional forest most dominated by leguminous trees had the highest emissions (9.0 kg N · ha−1· y−1), whereas the mid-successional forest lacking legumes had the lowest emissions (0.09 kg N · ha−1· y−1). N oxide fluxes from late-successional forests were higher than fluxes from pastures. Annual N oxide fluxes correlated positively to leaf litter N, net nitrification, potential nitrification, soil nitrate, and net N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissions. We conclude that (a) N oxide fluxes were substantial where the availability of inorganic N exceeded the requirements of competing biota; (b) species composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) the established ecosystem models that predict N oxide loss from positive relationships with soil ammonium may need to be modified.