Factors determining soil microbial biomass and nutrient immobilization in desert soils

We examined the 10-day response of soil microbial biomass-N to additions of carbon (dextrose) and nitrogen (NH₄NO₃) to water-amended soils in a factorial experiment in four plant communities of the Chihuahuan desert of New Mexico (U.S.A.). In each site, microbial biomass-N and soil carbohydrates increased and extractable soil N decreased in response to watering alone. Fertilization with N increased microbial biomass-N in grassland soils; whereas, fertilization with C increased microbial biomass-N and decreased extractable N and P in all communities dominated by shrubs, which have invaded large areas of grassland in the Chihuahuan desert during the last 100 years. Our results support the hypothesis that the control of soil microbial biomass shifts from N to C when the ratio of C to N decreases during desertification.     

Land use intensity,rather than plant species richness,affects the leaching risk of multiple nutrients from permanent grasslands

The intensification of land use constitutes one of the main drivers of global change and alters nutrient fluxes on all spatial scales, causing landscape‐level eutrophication and contamination of natural resources. Changes in soil nutrient concentrations are thus indicative for crucial environmental issues associated with intensive land use. We measured concentrations of NO₃–N, NH₄–N, P, K, Mg, and Ca using 1,326 ion‐exchange resin bags buried in 20 cm depth beneath the main root zone in 150 temperate grasslands. Nutrient concentrations were related to land use intensity, that is, fertilization, mowing, grazing intensities, and plant diversity by structural equation modeling. Furthermore, we assessed the response of soil nutrients to mechanical sward disturbance and subsequent reseeding, a common practice for grassland renewal. Land use intensity, especially fertilization, significantly increased the concentrations of NO₃–N, NH₄–N, K, P, and also Mg. Besides fertilization (and tightly correlated mowing) intensity, grazing strongly increased NO₃–N and K concentrations. Plant species richness decreased P and NO₃–N concentrations in soil when grassland productivity of the actual year was statistically taken into account, but not when long‐term averages of productivity were used. Thus, we assume that, in the actual study year, a distinct drought period might have caused the observed decoupling of productivity from fertilization and soil nutrients. Breaking up the grassland sward drastically increased NO₃–N concentrations (+146%) but reduced NH₄–N, P, and K concentrations, unbalancing soil nutrient stoichiometry and boosting the risk of N leaching. Reseeding the sward after disturbance did not have a short‐term effect on nutrient concentrations. We conclude that renewal of permanent grassland should be avoided as far as possible and future grassland management has to strongly rise the effectiveness of fertilization. Additionally, grassland management might have to increasingly taking care of periods of drought, in which nutrient additions might not increase plant growth but potentially only facilitate leaching.     

Soil acidification as caused by the nitrogen uptake pattern of Scots pine (Pinus sylvestris)

Three-year-old Scots pine (Pinus sylvestris) trees were grown on a sandy forest soil in pots, with the objective to determine their NH₄/NO₃ uptake ratio and proton efflux. N was supplied in three NH₄-N/NO₃-N ratios, 3:1, 1:1 and 1:3, either as ¹⁵NH₄+¹⁴NO₃ or as ¹⁴NH₄+¹⁵NO₃. Total N and ¹⁵N acquisition of different plant parts were measured. Averaged over the whole tree, the NH₄/NO₃ uptake ratios throughout the growing season were found to be 4.2, 2.5, and 1.5 for the three application ratios, respectively. The excess cation-over-anion uptake value (C_a-A_a) appeared to be linearly related to the natural logarithm of the NH₄/NO₃ uptake ratio. Further, this uptake ratio was related to the NH₄/NO₃ ratio of the soil solution. From these relationship it was estimated that Scots pine exhibits an acidifying uptake pattern as long as the contribution of nitrate to the N nutrition is lower than 70%. Under field circumstances root uptake may cause soil acidification in the topsoil, containing the largest part of the root system, and soil alkalization in deeper soil layers.     

Effects of drought and N-fertilization on N cycling in two grassland soils

Changes in frequency and intensity of drought events are anticipated in many areas of the world. In pasture, drought effects on soil nitrogen (N) cycling are spatially and temporally heterogeneous due to N redistribution by grazers. We studied soil N cycling responses to simulated summer drought and N deposition by grazers in a 3-year field experiment replicated in two grasslands differing in climate and management. Cattle urine and NH₄NO₃ application increased soil NH₄ ⁺ and NO₃ ^? concentrations, and more so under drought due to reduced plant uptake and reduced nitrification and denitrification. Drought effects were, however, reflected to a minor extent only in potential nitrification, denitrifying enzyme activity (DEA), and the abundance of functional genes characteristic of nitrifying (bacterial and archaeal amoA) and denitrifying (narG, nirS, nirK, nosZ) micro-organisms. N₂O emissions, however, were much reduced under drought, suggesting that this effect was driven by environmental limitations rather than by changes in the activity potential or the size of the respective microbial communities. Cattle urine stimulated nitrification and, to a lesser extent, also DEA, but more so in the absence of drought. In contrast, NH₄NO₃ reduced the activity of nitrifiers and denitrifiers due to top-soil acidification. In summary, our data demonstrate that complex interactions between drought, mineral N availability, soil acidification, and plant nutrient uptake control soil N cycling and associated N₂O emissions. These interactive effects differed between processes of the soil N cycle, suggesting that the spatial heterogeneity in pastures needs to be taken into account when predicting changes in N cycling and associated N₂O emissions in a changing climate.     

Preference for different inorganic nitrogen forms among plant functional types and species of the Patagonian steppe

We have explored species–specific preferences for nitrate (NO₃ ^?) and ammonium (NH₄ ⁺) as an alternative niche separation in ecosystems where nitrogen (N) is present mostly in inorganic forms. The Patagonian steppe is dominated by shrubs and grasses. Shrubs absorb water and nutrients from deep soil layers, which are poor in N, while grasses have the opposite pattern, absorbing most of their water and nutrients from the upper layers of the soil. We hypothesized that the preferences of shrub and grass for inorganic N forms are different and that the rate of potential N uptake is greater in shrubs than in grasses. To test this hypothesis, we grew individuals of six dominant species in solutions of different NH₄ ⁺:NO₃ ^? concentration ratios. Nitrate uptake was found to be higher in shrubs, while ammonium uptake was similar between plant functional types. The NH₄ ⁺:NO₃ ^? uptake ratio was significantly lower for shrubs than grasses. Shrubs, which under field conditions have deeper rooting systems than grasses, showed a higher N absorption capacity than grasses and a preference for the more mobile N form, nitrate. Grasses, which had lower N uptake rates than shrubs, preferred ammonium over nitrate. These complementary patterns between grasses and shrubs suggest a more thorough exploitation of resources by diverse ecosystems than those dominated by just one functional type. The loss of one plant functional group or a significant change in its abundance would therefore represent a reduction in resource use efficiency and ecosystem functioning.     

Interactive effects of drought and N fertilization on the spatial distribution of methane assimilation in grassland soils

Soil methanotrophic bacteria constitute the only globally relevant biological sink for atmospheric methane (CH₄). Nitrogen (N) fertilizers as well as soil moisture regime affect the activity of these organisms, but the mechanisms involved are not well understood to date. In particular, virtually nothing is known about the spatial distribution of soil methanotrophs within soil structure and how this regulates CH₄ fluxes at the ecosystem scale. We studied the spatial distribution of CH₄ assimilation and its response to a factorial drought × N fertilizer treatment in a 3‐year experiment replicated in two grasslands differing in management intensity. Intact soil cores were labelled with ¹⁴CH₄ and methanotrophic activity mapped at a resolution of ～100 μm using an autoradiographic technique. Under drought, the main zone of CH₄ assimilation shifted down the soil profile. Ammonium nitrate (NH₄NO₃) and cattle urine reduced CH₄ assimilation in the top soil, but only when applied under drought, presumably because NH₄⁺ from fertilizers was not removed by plant uptake and nitrification under these conditions. Ecosystem‐level CH₄ fluxes measured in the field did show no or only very small inhibitory effects, suggesting that deeper soil layers fully compensated for the reduction in top soil CH₄ assimilation. Our results indicate that the ecosystem‐level CH₄ sink cannot be inferred from measurements of soil samples that do not reflect the spatial organization of soils (e.g. stratification of organisms, processes, and mechanisms). The autoradiographic technique we have developed is suited to study methanotrophic activity in a relevant spatial context and does not rely on the genetic identity of the soil bacterial communities involved, thus ideally complementing DNA‐based approaches.     

Soil prokaryotic community shows no response to 2 years of simulated nitrogen deposition in an arid ecosystem in northwestern China

An arid ecosystem might be sensitive to nitrogen (N) deposition, but the associated ecosystem-specific response of soil microbes is not well studied. To assess the N enrichment effects on plant and prokaryotic community diversity, we performed a two-year NH₄NO₃ treatment in a desert steppe in northwestern China. Results showed that N addition increased plant aboveground biomass and decreased plant Shannon diversity. A C₄ herb (Salsola collina) became dominant, and loss of legume species was observed. The concentrations of soil NH₄⁺-N, NO₃⁻-N, microbial biomass N, and the plant aboveground biomass N pool increased in contrast to total N, suggesting that the N input into the arid ecosystem might mainly be assimilated by plants and exit the ecosystem. Remarkably, the α-diversity and structure of the soil prokaryotic community did not vary even at the highest N addition rate. Structural equation modelling further found that the plant aboveground N pool counteracted the acidification effect of N deposition and maintained soil pH thus partially stabilizing the composition of prokaryotic communities in a desert steppe. These findings suggested that the plants and N loss might contribute to the lack of responsiveness of soil prokaryotic community to N deposition in a desert steppe.     

Occurrence and intensity of wild boar disturbances, effects on the physical and chemical soil properties of alpine grasslands

Background and aims

Physical and chemical soil properties determine local plant conditions and resources, affecting plants’ ability to respond to disturbances. In alpine grasslands, wild boar disturbances occur at different intensities, what may affect differently their soil properties. Alpine soils from five contrasted plant communities were explored within and outside disturbances, accounting for an overall and community scale effect. Additionally, we analysed the effect of disturbance intensity on soil NO₃ ^--N and NH₄ ⁺-N.

Methods

Soils were analyzed for physical (bulk density, moisture content and electrical conductivity), and chemical properties (pH, total N and C, oxidizable C, C:N ratio, available K, P, Ca²⁺, Na⁺ and Mg²⁺). Resin bags were used to compare the effect of the disturbance occurrence and intensity on soil NO₃ ^--N and NH₄ ⁺-N.

Results

Bulk density, total N and NO₃ ^--N concentration were significantly higher in disturbed areas, while soil moisture, C:N, NH₄ ⁺-N, Na⁺, Mg²⁺ and Ca²⁺ concentrations were significantly lower. However, low disturbance intensity reduced NO₃ ^--N and increased NH₄ ⁺-N concentrations.

Conclusions

Wild boar occurrence and intensity strongly alter physical and chemical conditions of alpine soils, increasing soil compaction, and altering the availability of N forms. These changes may affect most plant species, thus affecting the structure and dynamics of alpine plant communities.     

氮素类型和剂量对寒温带针叶林土壤N2O排放的影响

大气氮沉降输入会增加森林生态系统氮素有效性,进而改变土壤N_2O产生与排放,然而有关不同氮素离子(氧化态NO_3~--N与还原态NH_4~+-N)沉降对土壤N_2O排放的影响知之甚少。以大兴安岭寒温带针叶林为研究对象,构建了3种类型(NH_4Cl、KNO_3、NH_4NO_3)和4个施氮水平(0、10、20、40 kg N hm~(-2)a~(-1))的增氮控制试验,利用流动化学分析仪和静态箱-气相色谱法4次/月测定凋落物层和矿质层土壤无机氮含量、土壤-大气界面N_2O净交换通量以及相关环境因子,分析施氮类型和剂量对土壤氮素有效性、土壤N_2O通量的影响探讨氮素富集条件下土壤N_2O通量的环境驱动机制。结果表明:施氮类型和剂量均显著影响土壤无机氮含量,土壤NH_4~+-N的积累效应显著高于NO_3~--N。施氮一致增加寒温带针叶林土壤N_2O排放,NH_4NO_3促进效应最为明显,增幅为442%-677%,高于全球平均水平(134%)。土壤N_2O通量与土壤温度、凋落物层NH_4~+-N含量正相关,且随着施氮水平增加而增加。结果表明大气氮沉降短期内不会导致寒温带针叶林土壤NO_3~--N大量流失,但会显著促进土壤N_2O的排放。此外,外源性NH_4~+和NO_3~-输入对土壤N_2O排放的促进作用具有协同效应,在未来森林生态系统氮循环和氮平衡研究中应该区分对待。     

The uptake and translocation of macro- and microelements in rape and wheat seedlings as affected by selenium supply level

Plant growth in saline soils may be increased by fertilisation, but little is known about the effect of different forms of N on wheat growth in soils with different salinity levels. The aim of this study was to investigate the response of wheat (Triticum aestivum L., cv Krichauff) to (NH₄)₂SO₄ or KNO₃ or NH₄NO₃ at 0 (N0), 50 (N50), 100 (N100) and 200 (N200) mg N?kg^?1 soil in a saline sandy loam. Salinity was induced using Na⁺ and Ca²⁺ salts to achieve three EC_e levels, 2.8, 6.6 and 11.8 dS m^?1 denoted S1, S2 and S3, respectively, while maintaining a low SAR (>1). Dry weights of shoot and root were reduced by salinity in all N treatments. Addition of N significantly increased shoot and root dry weights with significant differences between N forms. Under non-saline conditions (S1), addition of NO₃???N at rates higher than N50 had a negative effect, while N100 as NH₄???N or NH₄NO₃???N increased shoot and root dry weights. At N100, shoot concentrations of N and K were higher and P, Ca, Fe, Mn, Cu and Zn were lower with NO₃???N than with NH₄???N nutrition. The concentration of all nutrients however fell in ranges did not appear to be directly associated with poor plant growth with NO₃???N. At all N additions, calculations indicated that soil salinity was highest with N addition as NO₃???N and decreased in the following order: NO₃?N > NH₄?N > NH₄NO₃?N. Addition of greater than N50 as NO₃???N, compared to NH₄???N or NH₄???NO_3, increased soil salinity and reduced micronutrient uptake both of which likely limited plant growth. It can be concluded that in saline soils addition of 100 mg N?kg^?1 as NH₄???N or NH₄NO₃???N is beneficial for wheat growth, whereas NO₃???N can cause growth depression.     

Availability of fixed NH4 + to crops

Summary Significantly lower amounts of exchaneable NH₄, soluble NO₃ and clay-fixed NH₄ forms of N were observed in the unfertilized fields with high rather than low-density cropped plots. Irrespective of planting densitites, the fixed NH₄ content in soil increased with increase in the period of crop growth. N uptake by plant and total bacterial population of rhizosphere soil were significantly higher in the plots with the high than with the low-density planting. Availability of native fixed NH₄ ⁺ to crops and biological utilization of a considerable amount of recently mineralized NH₄ ⁺ in fixed form is indicated.     

Impacts of Soil Nitrogen and Carbon Additions on Forest Understory Communities with a Long Nitrogen Deposition History

Rates of nitrogen (N) deposition have been historically high throughout much of the northeastern United States; thus, understanding the legacy of these high N loads is important for maintaining forest productivity and resilience. Though many studies have documented plant invasions due to N deposition and associated impacts on ecosystems, less is known about whether invasive plants will continue to increase in dominance with further shifting nutrient regimes. Using soil N and carbon additions, we examined the impact of both increasing and decreasing soil N on native and invasive understory plant dynamics over 4 years in a northeastern deciduous forest with a long history of N deposition. Despite applying large quantities of N, we found no difference in soil nitrate (NO₃) or ammonium (NH₄ ⁺) pools in N addition plots over the course of the study. Indicative of the potential N saturation in these forest soils, resin-available NO₃ ^? and NH₄ ⁺ showed evidence that the added N was rapidly moving out of the soil in N addition plots. Accordingly, we also found that adding N to soil altered neither invasive nor native plant abundance, though adding N temporally increased invasive plant richness. Carbon additions decreased soil N availability seasonally, but did not alter the total percent cover of invasive or native plants. Rather than being suppressed by excess N availability, native plant species in this ecosystem are primarily inhibited by the invasive species, which now dominate this site. In conclusion, understory plant communities in this potentially N-saturated ecosystem may be buffered to future alterations in N availability.     

Phytate utilization of maize mediated by different nitrogen forms in a plant–arbuscular mycorrhizal fungus–phosphate-solubilizing bacterium system

A pot experiment was conducted to investigate the organic phosphorus (P) (phytate) utilization of Zea mays L. with different nitrogen (N) forms (NH₄⁺ and NO₃^?) when both arbuscular mycorrhizal (AM) fungus (Funelliformis mosseae) and phosphate-solubilizing bacterium (PSB, Pseudomonas alcaligenes) are present. The soil was supplied with either KNO₃ or (NH₄)₂SO₄ (200 mg kg^?1 N) with or without phytin (75 mg P kg^?1). Results showed that the application of NH₄⁺ to the soil in a plant–AM fungus–PSB system decreased rhizosphere pH and increased phosphatase activity. It also enhanced the mineralization rate of phytin, which resulted in the release of more inorganic P. The application of NO₃^? promoted mycorrhizal colonization and hyphal length density in the soil. The inorganic P in the hyphosphere decreased, but more P was transferred to the plant through the mycorrhizal hyphae. Hence, in addition, the application of the two different N forms did not significantly alter the content of plant P. The plant supplied with different N fertilizers acquired P through different mechanisms associated with other microbes. NH₄⁺ application promoted phytin mineralization by decreasing soil pH, whereas NO₃^? application increased inorganic P uptake by strengthening the mycorrhizal pathway.     

Organic matter quality and management effects on enrichment of soil organic matter fractions in contrasting soils in Zimbabwe

Spatial variability of soil total nitrogen (N), available N (KCl extractable NH₄⁺ and NO₃⁻), and spatial patterns of N mineralization and nitrification at a stand scale were characterized with geostatistical and univariate analysis. Two extensive soil spatial samplings were conducted in an evergreen broadleaf forest in Sichuan province, southwestern China in June and August 2000. In a study area of 90 × 105 m², three soil samples were collected from each 5 × 5 m² plot (n = 378) in June and August, and were analyzed for total N and available N contents. Net N mineralization and nitrification were measured by in situ core incubation and the rates were estimated based on the difference of NH₄⁺ and NO₃⁻ contents between the two sampling dates. Total N, NH₄⁺, and NO₃⁻ were all spatially structured with different semivariogram ranges (from high to low: NH₄⁺, NO₃⁻, and total N). The semivariograms of mineralization and nitrification were not as spatially structured as available N. NH₄⁺ was the dominant soil inorganic N form in the system. Both NH₄⁺ and NO₃⁻ affected spatial patterns of soil available N, but their relative importance switched in August, probably due to high nitrification as indicated by greatly increased soil NO₃⁻ content. High spatial auto-correlations (>0.7) were found between available N and NH4⁺, available N and NO₃⁻ on both sampling dates, as well as total N measurements between both sampling dates. Although significant, the spatial auto-correlation between NH₄⁺ and NO₃⁻ were generally low. Topography had significant but low correlations with mineralization (r = −0.16) and nitrification (r = −0.14), while soil moisture did not. The large nugget values of the calculated semivariograms and high-semivariance values, particularly for mineralization and nitrification, indicate that some fine scale (<5 m) variability may lie below the threshold for detection in this study.     

Ammonium as a Driving Force of Plant Diversity and Ecosystem Functioning: Observations Based on 5 Years' Manipulation of N Dose and Form in a Mediterranean Ecosystem

Enhanced nitrogen (N) availability is one of the main drivers of biodiversity loss and degradation of ecosystem functions. However, in very nutrient-poor ecosystems, enhanced N input can, in the short-term, promote diversity. Mediterranean Basin ecosystems are nutrient-limited biodiversity hotspots, but no information is available on their medium- or long-term responses to enhanced N input. Since 2007, we have been manipulating the form and dose of available N in a Mediterranean Basin maquis in south-western Europe that has low ambient N deposition (<4 kg N ha⁻¹ yr⁻¹) and low soil N content (0.1%). N availability was modified by the addition of 40 kg N ha⁻¹ yr⁻¹ as a 1∶1 NH₄Cl to (NH₄)₂SO₄ mixture, and 40 and 80 kg N ha⁻¹ yr⁻¹ as NH₄NO₃. Over the following 5 years, the impacts on plant composition and diversity (richness and evenness) and some ecosystem characteristics (soil extractable N and organic matter, aboveground biomass and % of bare soil) were assessed. Plant species richness increased with enhanced N input and was more related to ammonium than to nitrate. Exposure to 40 kg NH₄ ⁺-N ha⁻¹ yr⁻¹ (alone and with nitrate) enhanced plant richness, but did not increase aboveground biomass; soil extractable N even increased under 80 kg NH₄NO₃-N ha⁻¹ yr⁻¹ and the % of bare soil increased under 40 kg NH₄ ⁺-N ha⁻¹ yr⁻¹. The treatment containing less ammonium, 40 kg NH₄NO₃-N ha⁻¹ yr⁻¹, did not enhance plant diversity but promoted aboveground biomass and reduced the % of bare soil. Data suggest that enhanced NH_y availability affects the structure of the maquis, which may promote soil erosion and N leakage, whereas enhanced NO_x availability leads to biomass accumulation which may increase the fire risk. These observations are relevant for land use management in biodiverse and fragmented ecosystems such as the maquis, especially in conservation areas.     

A link between plant diversity, elevated CO2 and soil nitrate

Interactive effects of reductions in plant species diversity and increases in atmospheric CO₂ were investigated in a long-term study in nutrient-poor calcareous grassland. Throughout the experiment, soil nitrate was persistently increased at low plant species diversity, and CO₂ enrichment reduced soil [NO₃^-] at all levels of plant species diversity. In our study, soil [NO₃^-] was unrelated to root length density, microbial biomass N, community legume contents, and experimental plant communities differed only little in total N pools. However, potential nitrification revealed exactly the same treatment effects as soil [NO₃^-], providing circumstantial evidence that nitrification rates drove the observed changes in [NO₃^-]. One possible explanation for plant diversity effects on nitrification lies in spatial and temporal interspecific differences in plant N uptake, which would more often allow accumulation of NH₄⁺ in part of the soil profile at low diversity than in more species-rich plant communities. Consequently, nitrification rates and soil [NO₃^-] would increase. Elevated CO₂ increased soil water contents, which may have improved NO₃^- diffusion to the root surface thereby reducing soil [NO₃^-]. Higher soil moisture at elevated CO₂ might also reduce nitrification rates due to less aerobic conditions. The accordance of the diversity effect on soil [NO₃^-] with previous experiments suggests that increased soil [NO₃^-] at low species diversity is a fairly general phenomenon, although the mechanisms causing high [NO₃^-] may vary. In contrast, experimental evidence for effects of CO₂ enrichment on soil [NO₃^-] is ambiguous, and the antagonistic interaction of plant species reductions and elevated CO₂ we have observed is thus probably less universal.     

Nitrogen deposition and forest nitrogen cycling along an urban–rural transect in southern China

There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δ¹⁵N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha^?1 yr^?1, while the throughfall covered a larger range of 11.7–65.1 kg N ha^?1 yr^?1. Foliar N concentration, NO₃^? leaching to stream, and soil NO₃^? concentration were low and NO₃^? production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO₃^? leaching to stream, and negatively with pH values in soil and stream water. Foliar δ¹⁵N was between ?6.6‰ and 0.7‰, and was negatively correlated with soil NO₃^? concentration and NO₃^? leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δ¹⁵N values. We proposed several potential mechanism that could contribute to the δ¹⁵N pattern, including (1) increased plant uptake of ¹⁵N‐depleted soil NO₃^?, (2) foliage uptake of ¹⁵N‐depleted NH₄⁺, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.     

Nitrogen pools and soil characteristics of a temperate estuarine wetland in eastern Australia

The nitrogen status of the soil and plant components of an estuarine wetland near Sydney were investigated over 6 months to detect seasonal and spatial changes in nitrogen content. Organic carbon, organic nitrogen and inorganic nitrogen concentrations were measured in the soil at various depths in six vegetation zones across the wetland. Carbon and nitrogen content of the plant biomass were also determined in each of the zones. Soil redox potentials and pH were measured in situ and both were found to vary with depth and inundation frequency. Organic carbon, organic nitrogen and inorganic nitrogen in the soil decreased significantly from the fringe Casuarina forest (1391 g N m^?2) through to the Avicennia mangrove zone (133 g N m^?2). Exchangeable NH₄⁺, NO₃^? and NO₂^? concentrations from less than 1% of the soil nitrogen pool and vary seasonally. The distinctive feature of the mangrove zone is that the plant component of the total nitrogen pool is large (55%). This contrasts with the saltmarsh and fringe communities, where the plant pools are small (15%) in comparison with the soil. These findings are consistent with the hypothesis that mangroves export organic nutrients whereas the saltmarsh and fringe communities act as nutrient sinks.