Shrub Expansion of <Emphasis Type="Italic">Alnus viridis</Emphasis> Drives Former Montane Grassland into Nitrogen Saturation

The N₂-fixing shrub Alnus viridis is currently encroaching on montane grasslands in the Alps as a result of reduced land management and complete abandonment. Alnus introduces large amounts of nitrogen (N) into these formerly N-poor grasslands and restricts the succession to montane forests. We studied pools and fluxes of N and the associated C pools in pastures (controls) and adjacent Alnus shrublands at two elevations (1650 versus 1950 m a.s.l.) in three valleys in the Swiss central Alps. The total N and C pools stored in 50-year-old Alnus shrubland did not exceed those in adjacent pastures with a total of approximately 610 g N m^?2 in phytomass plus soil (down to 30 cm) at both elevations. In Alnus stands, reduced soil N pools balanced the gain in phytomass N pools, a likely result of a faster turnover of soil N. The soil solution under Alnus was continuously enriched with nitrate, with a total N leaching of 0.79 g N m^?2 season^?1 (June–October) under 50-year-old stands at both elevations and the highest flux of 1.76 g N m^?2 season^?1 in 25-year-old shrubland at low elevation, clearly indicating an excess of available N in Alnus shrubland. In contrast, N leaching across all pastures was close to zero (0.08 g N m^?2) throughout the season. At the catchment scale, streamlet water showed increased nitrate concentrations with typical flushing peaks in spring and autumn, provided more than one fifth of the catchment area was covered by Alnus shrubs. We conclude that the expansion of Alnus rapidly converts centuries-old, N-poor grassland into N saturated shrubland, irrespective of elevation, and it reduces the C storage potential of the landscape because the Alnus dominance constrains re-establishment of a natural montane forest.     

High precipitation causes large fluxes of dissolved organic carbon and nitrogen in a subtropical montane <Emphasis Type="Italic">Chamaecyparis</Emphasis> forest in Taiwan

Fluxes of dissolved organic carbon (DOC) and nitrogen (DON) may play an important role for losses of C and N from the soils of forest ecosystems, especially under conditions of high precipitation. We studied DOC and DON fluxes and concentrations in relation to precipitation intensity in a subtropical montane Chamaecyparis obtusa var. formosana forest in Taiwan. Our objective was, to quantify DOC and DON fluxes and to understand the role of high precipitation for DOC and DON export in this ecosystem. From 2005 to 2008 we sampled bulk precipitation, throughfall, forest floor percolates and seepage (60 cm) and analyzed DOC, DON and mineral N concentrations. Average DOC fluxes in the soil were extremely high (962 and 478 kg C ha^?1 year^?1 in forest floor percolates and seepage, respectively) while DON fluxes were similar to other (sub)tropical ecosystems (16 and 8 kg N ha^?1 year^?1, respectively). Total N fluxes in the soil were dominated by DON. Dissolved organic C and N concentrations in forest floor percolates were independent of the water flux. No dilution effect was visible. Instead, the pool size of potentially soluble DOC and DON was variable as indicated by different DOC and DON concentrations in forest floor percolates at similar precipitation amounts. Therefore, we hypothesized, that these pools are not likely to be depleted in the long term. The relationship between water fluxes in bulk precipitation and DOC and DON fluxes in forest floor percolates was positive (DOC r = 0.908, DON r = 0.842, respectively, Spearman rank correlation). We concluded, that precipitation is an important driver for DOC and DON losses from this subtropical montane forest and that these DOC losses play an important role in the soil C cycle of this ecosystem. Moreover, we found that the linear relationship between bulk precipitation and DOC and DON fluxes in forest floor percolates of temperate ecosystems does not hold when incorporating additional data on these fluxes from (subtropical) ecosystems.     

Context-dependent tree species effects on soil nitrogen transformations and related microbial functional genes

Although it is generally accepted that tree species can influence nutrient cycling processes in soils, effects are not consistently found, nor are the mechanisms behind tree species effects well understood. Our objectives were to gain insights into the mechanism(s) underlying the effects of tree species on soil nitrogen cycling processes, and to determine the consistency of tree species effects across sites. We compared N cycling in soils beneath six tree species (ash, sycamore maple, lime, beech, pedunculate oak, Norway spruce) in common garden experiments planted 42 years earlier at three sites in Denmark with distinct land-use histories (forest and agriculture). We measured: (1) net and gross rates of N transformations using the ¹⁵N isotope pool-dilution method, (2) soil microbial community composition through qPCR of fungal ITS, bacterial and archaeal 16S, and (3) abundance of functional genes associated with N cycling processes—for nitrification the archaeal and bacterial ammonia-monooxygenase genes (amoA AOA and amoA AOB, respectively) and for denitrification, the nitrate reductase genes nirK and nirS. Carbon concentrations were higher in soils under spruce than under broadleaves, so N transformation rates were standardized per g soil C. Soil NH₄⁺ parameters (gross ammonification, gross NH₄⁺ consumption, net ammonification (net immobilization in this case), and NH₄⁺ concentrations, per g C) were all lowest in soils under spruce. Soils under spruce also had the lowest gene abundance of bacteria, bacterial:fungal ratio, denitrifying microorganisms, ammonia-oxidizing archaea and ammonia-oxidizing bacteria. Differences in N-cycling processes and organisms among the five broadleaf species were smaller. The ‘spruce effect’ on soil microbes and N transformations appeared to be driven by its acidifying effect on soil and tighter N cycling, which occurred at the previously forested sites but not at the previously agricultural site. We conclude that existing characteristics of soils, including those resulting from previous land use, mediate the effects of tree species on the soil microbial communities and activities that determine rates of N-cycling processes.     

Estimating Ecosystem Nitrogen Addition by a Leguminous Tree: A Mass Balance Approach Using a Woody Encroachment Chronosequence

Difficulty in quantifying rates of biological N fixation (BNF), especially over long time scales, remains a major impediment to defining N budgets in many ecosystems. To estimate N additions from BNF, we applied a tree-scale N mass balance approach to a well-characterized chronosequence of woody legume (Prosopis glandulosa) encroachment into subtropical grasslands. We defined spatially discrete single Prosopis clusters (aged 28–99 years), and for each calculated BNF as the residual of: soil N (0–30 cm), above- and below-ground biomass N, wet and dry atmospheric N deposition, N trace gas and N₂ loss, leaching loss, and baseline grassland soil N at time of establishment. Contemporary BNF for upland savanna woodland was estimated at 10.9 ± 1.8 kg N ha^?1 y^?1, equal to a total of 249 ± 60 kg N ha^?1 over about 130 years of encroachment at the site. Though these BNF values are lower than previous estimates for P. glandulosa, this likely reflects lower plant density as well as low water availability at this site. Uncertainty in soil and biomass parameters affected BNF estimates by 6–11%, with additional sensitivity of up to 18% to uncertainty in other scaling parameters. Differential N deposition (higher rates of dry N deposition to Prosopis canopies versus open grasslands) did not explain N accrual beneath trees; iterations that represented this scenario reduced estimated BNF estimates by a maximum of 1.5 kg N ha^?1 y^?1. We conclude that in this relatively well-constrained system, small-scale mass balance provides a reasonable method of estimating BNF and could provide an opportunity to cross-calibrate alternative estimation approaches.     

Soil nitrogen turnover in proximal and distal stem areas of European beech trees

In European beech (Fagus sylvatica L.) forests, a large proportion of the water and ion input to the soil results from stemflow which creates a soil microsite of high element fluxes proximal to the tree trunk. The soil proximal to the stem is considered to have different rates of nitrogen turnover which might influence the estimation of N-turnover rates at the stand scale. In a previous study we reported high nitrate fluxes with seepage proximal to the stems in a forest dominated by European beech in Steigerwald, Germany. Here, we investigated the soil nitrogen turnover in the top 15 cm soil in proximal (defined as 1 m² around beech stems) and distal stem areas. Laboratory incubations and in situ sequential coring incubations were used to determine the net rates of ammonification, nitrification, and root uptake of mineral nitrogen. In the laboratory incubations higher rates of net nitrogen mineralization and nitrification were found in the forest floor proximal to the stem as compared to distal stem areas. No stem related differences were observed in case of mineral soil samples. In contrast, the in situ incubations revealed higher rates of nitrification in the mineral soil in proximal stem areas, while net nitrogen mineralization was equal in proximal and distal areas. In the in situ incubations the average ratio of nitrification/ammonification was 0.85 in proximal and 0.34 in distal stem areas. The net nitrogen mineralization was 4.4 g N m^-2 90 day^-1 in both areas. Mineralized nitrogen was almost completely taken up by tree roots with ammonium as the dominant nitrogen species. The average ratio of nitrate/ammonium uptake was 0.69 in proximal and 0.20 in distal areas. The higher water content of the soil in proximal stem areas is considered to be the major reason for the increased rates of nitrification. Different nitrogen turnover rates in proximal stem areas had no influence on the nitrogen turnover rates in soil at the stand scale. Consequently, the observed high nitrate fluxes with seepage proximal to stems are attributed to the high nitrogen input by stemflow rather than to soil nitrogen turnover. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of non-native Asian earthworm invasion on temperate forest and prairie soils in the Midwestern US

Effects of invasive European earthworms in North America have been well documented, but less is known about ecological consequences of exotic Asian earthworm invasion, in particular Asian jumping worms (Amynthas) that are increasingly reported. Most earthworm invasion research has focused on forests; some Amynthas spp. are native to Asian grasslands and may thrive in prairies with unknown effects. We conducted an earthworm-addition mesocosm experiment with before–after control-impact (BACI) design and a complementary field study in southern Wisconsin, USA, in 2014 to investigate effects of a newly discovered invasion of two Asian jumping worms (Amynthas agrestis and Amynthas tokioensis) on forest and prairie litter and soil nutrient pools. In both studies, A. agrestis and A. tokioensis substantially reduced surface litter (84–95 % decline in foliage litter mass) and increased total carbon, total nitrogen, and available phosphorus in the upper 0–5 cm of soils over the 4-month period from July through October. Soil inorganic nitrogen (ammonium– and nitrate–N) concentration increased across soil depths of 0–25 cm, with greater effects on nitrate–N. Dissolved organic carbon concentration also increased, e.g., 71–108 % increase in the mesocosm experiment. Effects were observed in both forest and prairie soils, with stronger effects in forests. Effects were most pronounced late in the growing season when earthworm biomass likely peaked. Depletion of the litter layer and rapid mineralization of nutrients by non-native Asian jumping worms may make ecosystems more susceptible to nutrient losses, and effects may cascade to understory herbs and other soil biota.     

Fluxes of nitrous oxide, methane and carbon dioxide during freezing–thawing cycles in an Inner Mongolian steppe

Combined measurements of nitrification activity and N₂O emissions were performed in a lowland and a montane tropical rainforest ecosystem in NE-Australia over a 18 months period from October 2001 until May 2003. At both sites gross nitrification rates, measured by the BaPS technique, showed a strong seasonal pattern with significantly higher rates of gross nitrification during wet season conditions. Nitrification rates at the montane site (1.48?±?0.24–18.75?±?2.38 mg N kg^?1 day^?1) were found to be significantly higher than at the lowland site (1.65?±?0.21–4.54?±?0.27 mg N kg^?1 day^?1). The relationship between soil moisture and gross nitrification rates could be described best by O’Neill functions having a soil moisture optimum of nitrification at app. 65% WFPS. At the lowland site, for which continuous measurements of N₂O emissions were available, nitrification was positively correlated with N₂O emission. Nitrification contributed significantly to N₂O formation during dry season (app.85%) but less (app. 30%) during wet season conditions. In average 0.19‰ of the N metabolized by nitrification was released as N₂O. The N₂O fraction loss for nitrification was positively correlated with changes in soil moisture and varied slightly between 0.15 and 0.22‰. Our results demonstrate that combined N₂O emission and microbial N turnover studies covering prolonged observation periods are needed to clarify and quantify the role of the microbial processes nitrification and denitrification for annual N₂O emissions from soils of terrestrial ecosystems.     

Nitrogen cycling in forest soils across climate gradients in Eastern China

A ¹⁵N tracing study was carried out to investigate the potential gross nitrogen (N) dynamics in thirteen forest soils in Eastern China ranging from temperate to tropical zones (five coniferous forests, six deciduous broad-leaf forests, one temperate mixed forest, one evergreen broad-leaf forests ecosystems), and to identify the major controlling factors on N cycling in these forest ecosystems. The soil pH ranged from 4.3 to 7.9 and soil organic carbon (SOC) ranged from 6.6 g?kg^?1 to 83.0 g?kg^?1. The potential gross N transformation rates were quantified by ¹⁵N tracing studies where either the ammonium or nitrate pools were ¹⁵N labeled in parallel treatments. Gross mineralization rates ranged from 0.915 μg N g^?1 soil day^?1 to 2.718 μg N g^?1 soil day^?1 in the studied forest soils. The average contribution of labile organic-N (M _Nlab ) to total gross mineralization (M _Nrec +M _Nlab ) was 86% (58% to 99%), indicating that turnover of labile organic N plays a dominant role in the studied forest ecosystems. The gross mineralization rates in coniferous forest soils were significantly lower (ranging between 0.915 and 1.228 μg N g^?1 soil day^?1) compared to broad-leaf forest soils (ranging from 1.621 to 2.718 μg N g^?1 soil day^?1) (p?<?0.01). Thus, the dominant vegetation may play an important role in regulating soil N mineralization. Nitrate production (nitrification) occurred via two pathways, oxidation of NH ₄ ⁺ and organic N the forest soils. Correlations with soil pH indicated that this is a key factor controlling the oxidation of NH ₄ ⁺ and organic N in theses forest ecosystems. NH ₄ ⁺ oxidation decreased with a decline in pH while organic N oxidation increased. The climatic conditions (e.g. moisture status) at the various sites governed the NO ₃ ^? -N consumption processes (dissimilatory NO ₃ ^? reduction to NH ₄ ⁺ (DNRA) or immobilization of NO ₃ ^? ). Total NO ₃ ^? consumption and the proportion of total NO ₃ ^? consumption to total NO ₃ ^? production decreased with an increase in the drought index of ecosystems, showing that strong interactions appear to exist between climatic condition (e.g. the drought index), N mineralization and the rate of DNRA. Interactions between vegetation, climatic conditions govern internal N cycling in these forests soils.     

Nutrient addition affects net and gross mineralization of phosphorus in the organic layer of a tropical montane forest

In tropical ecosystems with highly weathered soils, transformation of organic phosphorus (P) to bioavailable inorganic P plays a crucial role for the nutrition of organisms. In these ecosystems, P is suspected to be growth-limiting and might therefore be affected by atmospheric nutrient deposition occurring even in remote areas such as montane rainforests. We assessed effects of P and nitrogen (N) addition on net and gross P mineralization rates, and microbial P immobilization in the organic layer along an altitudinal gradient of a tropical montane rainforest in Ecuador. Net P mineralization rates amounted to 1.8 ± 0.9 (at 1000 m a.s.l.), 3.7 ± 0.6 (at 2000 m) and 2.0 ± 0.4 (at 3000 m) mg P kg^?1 day^?1. Altitudinal differences led to an increased microbial P immobilization at 1000 m compensating for a higher gross P mineralization. P addition increased net P mineralization rates at 2000 and 3000 m, suggesting a higher P demand at 1000 m. At higher altitudes, P likely was released as a by-product during organic matter decomposition. Gross P mineralization, determined by means of the isotopic dilution approach, could not be calculated for control and N treatments due to rapid microbial P immobilization. For P (+N) treatments, gross P mineralization rates were lowest at the 1000 m site towards the end of the long-term incubation period. Atmospheric P deposition in the tropics might lead to P fertilization effects through direct input as well as through acceleration of P release from organic matter, thereby increasing P availability for organisms.     

Control of Nitrous Oxide Emissions in European Beech,Norway Spruce and Scots Pine Forests

Elevated nitrogen deposition has increased tree growth, the storage of soil organic matter, and nitrate leaching in many European forests, but little is known about the effect of tree species and nitrogen deposition on nitrous oxide emission. Here we report soil N₂O emission from European beech, Scots pine and Norway spruce forests in two study areas of Germany with distinct climate, N deposition and soils. N₂O emissions and throughfall input of nitrate and ammonium were measured biweekly during growing season and monthly during dormant season over a 28 months period. Annual N₂O emission rates ranged between 0.4 and 1.3 kg N ha^?1 year^?1 among the stands and were higher in 1998 than in 1999 due to higher precipitation during the growing season of 1998. A 2-way-ANOVA revealed that N₂O fluxes were significantly higher (p<0.001) at Solling than at Unterlüß while tree species had no effect on N₂O emissions. Soil texture and the amount of throughfall explained together 94% of the variance among the stands, indicating that increasing portions of silt and clay may promote the formation of N₂O in wet forest soils. Moreover, cumulative N₂O fluxes were significantly correlated (r² = 0.60, p<0.001) with cumulative NO ₃ ^? fluxes at 10 cm depth as an indicator of N saturation, however, the slope of the regression curve indicates a rather weak effect of NO ₃ ^? fluxes on N₂O emissions. N input by throughfall was not correlated with N₂O emissions and only 1.6–3.2% of N input was released as N₂O to the atmosphere. Our results suggest that elevated N inputs have little effect on N₂O emissions in beech, spruce and pine forests.     

Impact of nonnative feral pig removal on soil structure and nutrient availability in Hawaiian tropical montane wet forests

Conservation and restoration of ecosystems impacted by nonnative ungulates increasingly involves their removal and exclusion. While the influence of nonnative ungulate removal on plant communities is commonly monitored, impacts on underlying ecological processes are seldom quantified. Here we examined how nonnative feral pig (Sus scrofa) removal from Hawaiian tropical montane wet forests affects soil physical and chemical properties. Unique to this study, measurements were taken in paired sites inside and outside of five feral pig removal units representing a ~20 year, highly constrained chronosequence where other potentially confounding variables are held constant. Additional targeted measurements were taken inside and outside of a single exclosure in areas characterized by ‘low’ versus ‘high’ feral pig activity. Overall, nonnative feral pig removal increased stable soil aggregates and porosity, and decreased bulk density, water-filled pore space, and soil moisture content. Further, feral pig removal increased soil nutrient regeneration as evidenced by increased extractable cations, increased resin available NO₃ ^? and total inorganic N, and enriched foliar δ¹⁵N. Increasing time since feral pig removal was positively related to net nitrification and total net inorganic N mineralization, and negatively related to pH and net ammonification. Results from both the chronosequence and targeted sampling were consistent in direction and support a central role of feral pig removal in modifying soil physical and chemical properties. Changes in soil properties following ungulate removal coincided with large increases in understory vegetation cover, highlighting the need to better understand aboveground-belowground linkages following nonnative ungulate removal.     

Perennial Grass Bioenergy Cropping on Wet Marginal Land: Impacts on Soil Properties,Soil Organic Carbon,and Biomass During Initial Establishment

The control of soil moisture, vegetation type, and prior land use on soil health parameters of perennial grass cropping systems on marginal lands is not well known. A fallow wetness-prone marginal site in New York (USA) was converted to perennial grass bioenergy feedstock production. Quadruplicate treatments were fallow control, reed canarygrass (Phalaris arundinaceae L. Bellevue) with nitrogen (N) fertilizer (75 kg N ha^?1), switchgrass (Panicum virgatum L. Shawnee), and switchgrass with N fertilizer (75 kg N ha^?1). Based on periodic soil water measurements, permanent sampling locations were assigned to various wetness groups. Surface (0–15 cm) soil organic carbon (SOC), active carbon, wet aggregate stability, pH, total nitrogen (TN), root biomass, and harvested aboveground biomass were measured annually (2011–2014). Multi-year decreases in SOC, wet aggregate stability, and pH followed plowing in 2011. For all years, wettest soils had the greatest SOC and active carbon, while driest soils had the greatest wet aggregate stability and lowest pH. In 2014, wettest soils had significantly (p?<?0.0001) greater SOC and TN than drier soils, and fallow soils had 14 to 20% greater SOC than soils of reed canarygrass + N, switchgrass, and switchgrass + N. Crop type and N fertilization did not result in significant differences in SOC, active carbon, or wet aggregate stability. Cumulative 3-year aboveground biomass yields of driest switchgrass + N soils (18.8 Mg ha^?1) were 121% greater than the three wettest switchgrass (no N) treatments. Overall, soil moisture status must be accounted for when assessing soil dynamics during feedstock establishment.     

Tissue-specific turnover rates of the nitrogen stable isotope as functions of time and growth in a cyprinid fish

Ecological applications of stable isotope data require knowledge on the isotopic turnover rate of tissues, usually described as the isotopic half-life in days (T _0.5) or the change in mass (G _0.5). Ecological studies increasingly analyse tissues collected non-destructively, such as fish fin and scales, but there is limited knowledge on their turnover rates. Determining turnover rates in situ is challenging, with ex situ approaches preferred. Correspondingly, T _0.5 and G _0.5 of the nitrogen stable isotope (δ¹⁵N) were determined for juvenile barbel Barbus barbus (5.5 ± 0.6 g starting weight) using a diet-switch experiment. δ¹⁵N data from muscle, fin and scales were taken during a 125 day post diet-switch period. Whilst isotopic equilibrium was not reached in the 125 days, the δ¹⁵N values did approach those of the new diet. The fastest turnover rates were in more metabolically active tissues, from muscle (highest) to scales (lowest). Turnover rates were relatively slow; T _0.5 was 84 (muscle) to 145 (scale) days; G _0.5 was 1.39 × body mass (muscle) to 2.0 × body mass (scales), with this potentially relating to the slow growth of the experimental fish. These turnover estimates across the different tissues emphasise the importance of estimating half-lives for focal taxa at species and tissue levels for ecological studies.     

A δ<Superscript>15</Superscript>N assessment of nitrogen deposition for the endangered epiphytic orchid <Emphasis Type="Italic">Laelia speciosa</Emphasis> from a city and an oak forest in Mexico

Atmospheric nitrogen deposition poses a major threat to global biodiversity. Tropical epiphytic plants are especially at risk given their reliance on atmospheric sources of nutrients. The leaf, pseudobulb, and root carbon and nitrogen content, C:N ratio, as well as the nitrogen isotopic composition were studied for individuals of Laelia speciosa from a city and from an oak forest in Mexico. The nitrogen content of leaves was similar between the city and the oak forest, reaching 1.3 ± 0.2 % (dry mass). The δ¹⁵N of leaves, pseudobulbs, and roots reached 5.6 ± 0.2 ‰ in the city, values found in sites exposed to industrial and vehicular activities. The δ¹⁵N for plant from the oak forest amounted to –3.1 ± 0.3 ‰, which is similar to values measured from sites with low industrial activities. Some orchids such as Laelia speciosa produce a single pseudobulb per year, i.e., a water and nutrient storage organ, so the interannual nitrogen deposition was studied by considering the ten most recent pseudobulbs for plants from either site formed between 2003 and 2012. The C:N ratio of the ten most recent pseudobulbs from the oak forest, as well as that of the pseudobulbs formed before 2010 for plants in the city were indistinguishable from each other, averaging 132.4 ± 6.5, while it was lower for the two most recent pseudobulbs in the city. The δ¹⁵N values of pseudobulbs from the oak forest averaged ?4.4 ± 0.1 ‰ for the entire series. The δ¹⁵N ranged from 0.1 ± 1.6 ‰ for the oldest pseudobulb to 4.7 ± 0.2 ‰ for the pseudobulb formed in the city from 2008 onwards. Isotopic analysis and the C:N ratio for L. speciosa revealed that rates of nitrogen deposition were higher in the city than in the forest. The δ¹⁵N values of series of pseudobulbs showed that it is possible to track nitrogen deposition over multiple years.     

Responses of nitrous oxide fluxes and soil nitrogen cycling to nutrient additions in montane forests along an elevation gradient in southern Ecuador

Tropical montane forests are commonly limited by N or co-limited by N and P. Projected increases in N deposition in tropical montane regions are thought to be insufficient for vegetation demand and are not therefore expected to affect soil N availability and N₂O emissions. We established a factorial N- and P-addition experiment (i.e., N, P, N + P, and control) across an elevation gradient of montane forests in Ecuador to test these hypotheses: (1) moderate rates of N and P additions are able to stimulate soil-N cycling rates and N₂O fluxes, and (2) the magnitude and timing of soil N₂O-flux responses depend on the initial nutrient status of the forest soils. Moderate rates of nutrients were added: 50 kg N ha^?1 year^?1 (in the form of urea) and 10 kg P ha^?1 year^?1 (in the form of NaH₂PO ₄ ^. 2H₂O) split in two equal applications. We tested the hypotheses by measuring changes in net rates of soil–N cycling and N₂O fluxes during the first 2 years (2008–2009) of nutrient manipulation in an old-growth premontane forest at 1,000 m, growing on a Cambisol soil with no organic layer, in an old-growth lower montane forest at 2,000 m, growing on a Cambisol soil with an organic layer, and an old-growth upper montane rainforest at 3,000 m, growing on a Histosol soil with a thick organic layer. Among the control plots, net nitrification rates were largest at the 1,000-m site whereas net nitrification was not detectable at the 2,000- and 3,000-m sites. The already large net nitrification at the 1,000-m site was not affected by nutrient additions, but net nitrification became detectable at the 2,000- and 3000-m sites after the second year of N and N + P additions. N₂O emissions increased rapidly following N and N + P additions at the 1,000-m site whereas only smaller increases occurred at the 2,000- and 3,000-m sites during the second year of N and N + P additions. Addition of P alone had no effect on net rates of soil N cycling and N₂O fluxes at any elevation. Our results showed that the initial soil N status, which may also be influenced by presence or absence of organic layer, soil moisture and temperature as encompassed by the elevation gradient, is a good indicator of how soil N cycling and N₂O fluxes may respond to future increases in nutrient additions.     

Inorganic nitrogen immobilization in live and sterile soil of old-growth conifer and hardwood forests: implications for ecosystem nitrogen retention

Rapid immobilization of inorganic nitrogen (N) in soil contributes to ecosystem N accumulation, even in old-growth and chronically-fertilized forests once thought to have poor N retention capacity. In old-growth conifer and hardwood stands in Pennsylvania, we tested the hypotheses that biotic and abiotic N immobilization are regulated by N form and forest type. We added ¹⁵NH₄ ⁺, ¹⁵NO₂ ^?, and ¹⁵NO₃ ^? to sterile (γ-irradiated) and live organic-horizon soil and define N immobilization as the mass of added ¹⁵N remaining in soil following extractions conducted 15 min, 24 h, and 21 days later. Immobilization of NO₂ ^? (19–25% of added N) occurred in sterile soils within 15 min and was little changed thereafter. Tracer NO₃ ^? immobilization was not observed, although soils had been pretreated (refrigerated) so as to quantify the lower limit of immobilization potential. Immobilization of NH₄ ⁺ (27%) occurred in live conifer soils by 21 days but not in other treatments. In 21-day incubations, tracer N immobilization was greater in NO₃ ^?-poor and humic-rich soils. Immobilization was greater in sterile than in live soil, perhaps owing to artifacts of sterilization. Conifer stands exhibited more massive O-horizons, so NO₂ ^? immobilization per unit area was greater in conifer (1.46 mg N m^?2) than hardwood (0.43 mg N m^?2) stands, possibly accounting for lower N leaching from conifer forests. Areal immobilization rates appear to be fast enough to retain all N transformed to NO₂ ^?, so NO₂ ^? production may be a limiting step in soil N retention in old-growth ecosystems.     

Effects of phosphorus addition with and without nitrogen addition on biological nitrogen fixation in tropical legume and non-legume tree plantations

It is well documented that phosphorus (P) input stimulates biological nitrogen (N) fixation (BNF) in tropical forests with non-legume trees. However, in tropical legume forests with soil N enrichment and P deficiency, the effects of P availability and its combination with N on BNF remain poorly understood. In this study, we measured BNF rate in different compartments, i.e., bulk soil, forest floor, rhizosphere, and nodules, in two tropical plantations with legume trees Acacia auriculiformis (AA) versus non-legume trees Eucalyptus urophylla, (EU) in southern China after 4 years of P addition and combined N and P additions. The objective was to investigate how P addition and its combination with N addition regulate BNF in a tropical legume plantation, and to compare the effects with those in a non-legume plantation. Our results showed that total BNF rates were significantly higher in the P-addition plots than in the control plots by 27.4 ± 4.3 and 23.3 ± 1.7 % in the EU and AA plantations, respectively. Total BNF rates were significantly higher in the NP-addition plots than in the control plots by 27.7  ± 5.0 and 8.5 ± 1.4 % in the EU and AA plantations, respectively, which contrasted to our previous result that total BNF rates were significantly lower in N-addition plots than in the control plots in the AA plantation. These findings suggest that P input can stimulate BNF in tropical forest biome dominated by legume trees, even in consideration of elevated atmospheric N deposition. Thus, our study revealed the important role of P in regulating biological N input, which should be taken into account in the modeling of biogeochemical cycles in the future.     

The role of gross and net N transformation processes and NH4 + and NO3 − immobilization in controlling the mineral N pool of a temperate mixed deciduous forest soil

In many forests of Europe and north-eastern North America elevated N deposition has opened the forest N cycle, resulting in NO₃ ^? leaching. On the other hand, despite this elevated N deposition, the dominant fate of NO₃ ^? and NH₄ ⁺ in some of these forests is biotic or abiotic immobilization in the soil organic matter pool, preventing N losses. The environmental properties controlling mineral N immobilization and the variation and extent of mineral N immobilization in forest soils are not yet fully understood. In this study we investigated a temperate mixed deciduous forest, which is subjected to an average N deposition of 36.5 kg N ha^?1 yr^?1, but at the same time shows low NO₃ ^? concentrations in the groundwater. The aim of this study was to investigate whether the turnover rate of the mineral N pool could explain these low N leaching losses. A laboratory ¹⁵N pool dilution experiment was conducted to study gross and net N mineralization and nitrification and mineral N immobilization in the organic and uppermost (0–10 cm) mineral layer of the forest soil. Two locations, one at the forest edge (GE) and another one 145 m inside the forest (GF1), were selected. In the organic layers of GE and GF1, the gross N mineralization averaged 10.9 and 11.1 mg N kg^?1 d^?1, the net N mineralization averaged 6.1 and 6.8 mg N kg^?1 d^?1 and NH₄ ⁺ immobilization rates averaged 3.8 and 3.6 mg N kg^?1 d^?1. In the organic layer of GE and GF1, the average gross nitrification was 3.8 and 4.6 mg N kg^?1 d^?1, the average net nitrification was ?25.2 and ?31.3 mg N kg^?1 d^?1 and the NO₃ ^? immobilization rates averaged 29.0 and 35.9 mg N kg^?1 d^?1. For the mineral (0–10 cm) layer the same trend could be observed, but the N transformation rates were much lower for the NH₄ ⁺ pool and not significantly different from zero for the NO₃ ^? pool. Except for the turnover of the NH₄ ⁺ pool in the mineral layer, no significant differences were observed between location GE and GF1. The ratio of NH₄ ⁺ immobilization to gross N mineralization, gross N mineralization to gross nitrification, and NO₃ ^? immobilisation to gross nitrification led to the following observations. The NH₄ ⁺ pool of the forest soil was controlled by N mineralization and NO₃ ^? immobilization was importantly controlling the forest NO₃ ^? pool. Therefore it was concluded that this process is most probably responsible for the limited NO₃ ^? leaching from the forest ecosystem, despite the chronically high N deposition rates.     

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. Received 22 February 2000; accepted 6 September 2000.     

Impacts of climate and land use on N2O and CH4 fluxes from tropical ecosystems in the Mt. Kilimanjaro region,Tanzania

In this study, we quantify the impacts of climate and land use on soil N₂O and CH₄ fluxes from tropical forest, agroforest, arable and savanna ecosystems in Africa. To do so, we measured greenhouse gases (GHG) fluxes from 12 different ecosystems along climate and land‐use gradients at Mt. Kilimanjaro, combining long‐term in situ chamber and laboratory soil core incubation techniques. Both methods showed similar patterns of GHG exchange. Although there were distinct differences from ecosystem to ecosystem, soils generally functioned as net sources and sinks for N₂O and CH₄ respectively. N₂O emissions correlated positively with soil moisture and total soil nitrogen content. CH₄ uptake rates correlated negatively with soil moisture and clay content and positively with SOC. Due to moderate soil moisture contents and the dominance of nitrification in soil N turnover, N₂O emissions of tropical montane forests were generally low (<1.2 kg N ha^?1 year^?1), and it is likely that ecosystem N losses are driven instead by nitrate leaching (~10 kg N ha^?1 year^?1). Forest soils with well‐aerated litter layers were a significant sink for atmospheric CH₄ (up to 4 kg C ha^?1 year^?1) regardless of low mean annual temperatures at higher elevations. Land‐use intensification significantly increased the soil N₂O source strength and significantly decreased the soil CH₄ sink. Compared to decreases in aboveground and belowground carbon stocks enhanced soil non‐CO₂ GHG emissions following land‐use conversion from tropical forests to homegardens and coffee plantations were only a small factor in the total GHG budget. However, due to lower ecosystem carbon stock changes, enhanced N₂O emissions significantly contributed to total GHG emissions following conversion of savanna into grassland and particularly maize. Overall, we found that the protection and sustainable management of aboveground and belowground carbon and nitrogen stocks of agroforestry and arable systems is most crucial for mitigating GHG emissions from land‐use change.