Spatiotemporal variations affect uptake of inorganic and organic nitrogen by dominant plant species in an alpine wetland

Aims

Dominant plant species may coexist and maintain high productivity in alpine wetland through available nitrogen (N) niche differentiation over time and space. We tested the hypotheses that dominant plant species differ in uptake of inorganic and organic N and that such differences depend on soil depth and season.

Methods

We conducted a short-term ¹⁵N-labeling experiment in an alpine wetland on the Tibetan Plateau. The experiment used a factorial design with three N forms (nitrate, ammonium and glycine), three soil depths (0–5, 5–10 and 10–15 cm), two seasons (May and July) and three dominant species (Carex muliensis, C. lasiocarpa and Potentilla anserina).

Results

All three species took up organic N (glycine), but showed different patterns over seasons and depths. ¹⁵N uptake rate was higher in May than in July in C. muliensis and C. lasiocarpa, but lower in May than in July in P. anserina. C. muliensis took up more ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^? than glycine-¹⁵N at all soil depths. C. lasiocarpa took up more glycine-¹⁵N than ¹⁵NH₄ ⁺ or ¹⁵NO₃ at 5–10 cm depth. P. anserina showed little difference in uptake at any soil depths.

Conclusions

Dominant species in alpine wetland are able to take up both organic and inorganic N, but show different patterns depending on N form, soil depth, season and their interactions.     

Indonesia’s blue carbon: a globally significant and vulnerable sink for seagrass and mangrove carbon

The global significance of carbon storage in Indonesia’s coastal wetlands was assessed based on published and unpublished measurements of the organic carbon content of living seagrass and mangrove biomass and soil pools. For seagrasses, median above- and below-ground biomass was 0.29 and 1.13 Mg C ha^?1 respectively; the median soil pool was 118.1 Mg C ha^?1. Combining plant biomass and soil, median carbon storage in an Indonesian seagrass meadow is 119.5 Mg C ha^?1. Extrapolated to the estimated total seagrass area of 30,000 km², the national storage value is 368.5 Tg C. For mangroves, median above- and below-ground biomass was 159.1 and 16.7 Mg C ha^?1, respectively; the median soil pool was 774.7 Mg C ha^?1. The median carbon storage in an Indonesian mangrove forest is 950.5 Mg C ha^?1. Extrapolated to the total estimated mangrove area of 31,894 km², the national storage value is 3.0 Pg C, a likely underestimate if these habitats sequester carbon at soil depths >1 m and/or sequester inorganic carbon. Together, Indonesia’s seagrasses and mangroves conservatively account for 3.4 Pg C, roughly 17 % of the world’s blue carbon reservoir. Continued degradation and destruction of these wetlands has important consequences for CO₂ emissions and dissolved carbon exchange with adjacent coastal waters. We estimate that roughly 29,040 Gg CO₂ (eq.) is returned annually to the atmosphere–ocean pool. This amount is equivalent to about 3.2 % of Indonesia’s annual emissions associated with forest and peat land conversion. These results highlight the urgent need for blue carbon and REDD+ projects as a means to stem the decline in wetland area and to mitigate the release of a significant fraction of the world’s coastal carbon stores.     

Microbial immobilization drives nitrogen cycling differences among plant species

In many terrestrial ecosystems nitrogen (N) limits productivity and plant community composition is influenced by N availability. However, vegetation is not only controlled by N; plant species may influence ecosystem N dynamics through positive or negative effects on N cycling. We examined four potential mechanisms of plant species effects on nitrogen (N) cycling. We found no species differences in gross ammonification suggesting there are no changes in the ecosystem N cycling rate between the soil organic matter pool (SOM) and the plant/microbial pool. We also found weak differences among plant species in gross nitrification, thus plant species only marginally change the relative sizes of the NH₄⁺ and NO₃^? pools. Next, more than 90% of mineralized N was microbially immobilized, and microbial N immobilization was positively correlated with root biomass. Finally, while species differed in extractable soil NO3^? concentration, these differences were not related to root biomass suggesting that microbial immobilization drives net N mineralization and soil NO₃^? levels. Our results indicate that plant species do not cause feedbacks on the N cycling rate among the three major ecosystem N pools over nine years. However, plant carbon (C) inputs to the soil control microbial N immobilization and thereby change N partitioning between the plant and microbial N pools. Furthermore our results suggest that the SOM pool can act as a strong bottleneck for N cycling in these systems.     

Dissolved organic matter and nutrient dynamics of a coastal freshwater forested wetland in Winyah Bay, South Carolina

Seasonally flooded, freshwater cypress-tupelo wetlands, dominated by baldcypress (Taxodium distictum), water tupelo (Nyssa aquatica), and swamp tupelo (Nyssa sylvatica var. biflora) are commonly found in coastal regions of the southeastern United States. These wetlands are threatened due to climate change, sea level rise, and coastal urban development. Understanding the natural biogeochemical cycles of nutrients in these forested wetlands as ecosystems services such as carbon sequestration and nitrogen processing can provide important benchmarks to guide conservation plans and restoration goals. In this study, surface water and soil pore water samples were collected weekly from a cypress-tupelo wetland near Winyah Bay, South Carolina and analyzed for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), inorganic nitrogen, and phosphate during its flooding period between October 2010 and May 2011. DOC was further characterized by specific ultra-violet absorbance at 254 nm, spectral slope ratio (S_R) (ratio of two spectral slopes between 275–295 nm and 350–400 nm), E2/E3 ratio (ratio between A254 and A365), and fluorescence excitation-emission matrix. In addition, litterfall was collected on a monthly basis for a year while the biomass of the detritus layer (i.e., decomposed duff lying on the wetland floor) was determined before and after the flooding period. Results of the field study showed that concentrations of DOC, DON, NH₄ ⁺–N, and (NO₂ ^? + NO₃ ^?)–N in the surface water were generally higher during the fall, or peak litterfall season (October to December), than in the spring season (March to May). Highest concentrations of 54.8, 1.48, 0.270, and 0.0205 mg L^?1, for DOC, DON, NH₄ ⁺–N, and (NO₂ ^? + NO₃ ^?)–N respectively, in surface waters were recorded during October. Lower SUVA, but higher S_R and E2/E3 ratios of DOC, were observed at the end of the flooding season comparing to the initial flooding, suggesting the wetland system converts high aromatic and large DOC molecules into smaller and hydrophilic fractions possibly through photochemical oxidation. A similar trend was observed in soil pore water, but the pore water generally had greater and relatively stable concentrations of dissolved nutrients than surface water. No obvious temporal trend in phosphate concentration and total nitrogen to total phosphorus ratio (N:P) were found. Results of the laboratory extraction and mass balance calculation suggested fresh litter was a major source of DOC whereas decomposed duff was the source of dissolved nitrogen in surface water. In summary, the biogeochemistry of this isolated cypress-tupelo wetland is not only driven by the vegetation within the wetland system but also by hydrology and weather conditions such as groundwater table position, precipitation, and temperature.     

Wildfire Effects on Soil Gross Nitrogen Transformation Rates in Coniferous Forests of Central Idaho, USA

Forest fires often result in a series of biogeochemical processes that increase soil nitrate (NO₃ ^?) concentrations for several years; however, the dynamic nature of inorganic nitrogen (N) cycling in the plant–microbe–soil complex makes it challenging to determine the direct causes of increased soil NO₃ ^?. We measured gross inorganic N transformation rates in mineral soils 2 years after wildfires in three central Idaho coniferous forests to determine the causes of the elevated soil NO₃ ^?. We also measured key factors that could affect the soil N processes, including temperature during soil incubation in situ, soil water content, pH and carbon (C) availability. We found no significant differences (P = 0.461) in gross nitrification rates between burned and control soils. However, microbial NO₃ ^? uptake rates were significantly lower (P = 0.078) in burned than control soils. The reduced consumption of NO₃ ^? caused slightly elevated NO₃ ^? concentrations in the burned soils. C availability was positively correlated with microbial NO₃ ^? uptake rates. Despite reduced microbial NO₃ ^? uptake capacity in the burned soils, soil microbes were a strong enough N sink to maintain low soil NO₃ ^? concentrations 2 years post fire. Soil NH₄ ⁺ concentrations between the treatments were not significantly different (P = 0.673). However, gross NH₄ ⁺ production and microbial uptake rates in burned soils were significantly lower (P = 0.028 and 0.035, respectively) than in the controls, and these rates were positively correlated with C availability. Our results imply that C availability is an important factor regulating soil N cycling of coniferous forests in the region.     

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.     

蚂蚁筑巢对西双版纳热带森林土壤有机氮矿化的影响

蚂蚁作为生态系统工程师能够调节土壤微生物及理化环境,进而对热带森林土壤有机氮矿化速率及其时间动态产生显著影响。以西双版纳白背桐热带森林群落为研究对象,采用室内需氧培养法测定土壤有机氮矿化速率,比较蚁巢和非蚁巢土壤有机氮矿化速率的时间动态,揭示蚂蚁筑巢活动引起土壤无机氮库、微生物生物量碳及化学性质改变对有机氮矿化速率时间动态的影响。结果表明：（1）蚂蚁筑巢显著影响土壤有机氮矿化速率（P<0.01）,相较于非蚁巢,蚁巢土壤有机氮矿化速率提高了261%;（2）土壤有机氮矿化速率随月份推移呈明显的单峰型变化趋势,即6月最大（蚁巢1.22 mg kg^-1 d^-1、非蚁巢0.41 mg kg^-1 d^-1）,12月最小（蚁巢0.82 mg kg^-1 d^-1、非蚁巢0.18 mg kg^-1 d^-1）;（3）两因素方差分析表明,不同月份及不同处理对土壤有机氮矿化速率、NH₄-N及NO₃-N产生显著影响（P<0.05）,但对NO₃-N的交互作用不显著;（4）蚂蚁筑巢显著提高了无机氮库（NH₄-N与NO₃-N）、微生物生物量碳、有机质、水解氮、全氮及易氧化有机碳等土壤养分含量,而降低了土壤pH值;（5）回归分析表明,铵态氮和硝态氮对土壤有机氮矿化速率产生显著影响,分别解释87.89%、61.84%的有机氮矿化速率变化;（6）主成份分析表明NH₄-N、微生物生物量碳及有机质是影响有机氮矿化速率时间动态的主要因素,而全氮、NO₃-N、易氧化有机碳、水解氮及pH对土壤有机氮矿化速率的影响次之,且pH与土壤有机氮矿化速率呈显著负相关。总之,蚂蚁筑巢活动主要通过影响土壤NH₄-N、微生物生物量碳及有机质的状况,进而调控西双版纳热带森林土壤有机氮矿化速率的时间动态。研究结果将有助于进一步提高对土壤氮矿化生物调控机制的认识。     

Mass balance of nitrogen in constructed mangrove wetlands receiving ammonium-rich wastewater: Effects of tidal regime and carbon supply

A series of computer-controlled mangrove tide-tanks planted with Kandelia candel was constructed to investigate the removal and transformation of ammonium–nitrogen under two tidal regimes: (i) 12-h wet/12-h dry (long tidal regime) and (ii) 6-h wet/6-h dry/6-h wet/6-h dry daily (short tidal regime). All tanks were irrigated with NH₄Cl solution for nine water cycles (each cycle lasted for 5 weeks) at an amount of around 2.1 g NH₄Cl (equivalent to 0.52 g N) per tank per cycle. During the experiment, total Kjeldahl nitrogen (TKN), inorganic nitrogen (N) (NH₄⁺–, NO₂^?–, and NO₃^?–N) and carbon were completely removed by the mangrove system. The added NH₄⁺–N was not detected in tidal water or accumulated in sediment. The mass balance of nitrogen showed that the discharge of ammonium-rich wastewater to mangrove wetlands enhanced microbial nitrogen transformation, particularly nitrification and denitrification processes, with 15–30% of the total nitrogen inputs returned to atmosphere as N₂ gas. Growth of K. candel and macroalgae was stimulated by ammonium addition, and up to 3 and 7% of total N inputs were assimilated in plant and algal tissues, respectively. Constructed mangrove wetlands with short tidal regime had higher numbers of nitrifiers and significantly lower content of ammonium that those with long tidal regime. On the other hand, higher populations of denitrifiers and lower nitrate were found in mangroves with long tidal regime and with glucose addition.     

Dual N and O isotopes of nitrate in natural plants: first insights into individual variability and organ-specific patterns

Nitrate (NO₃ ^?) is an important form of nitrogen (N) available to plants. The measurements of NO₃ ^? concentration [NO₃ ^?] and isotopes (δ¹⁵N and δ¹⁸O) in plants provide unique insights into ecosystem NO₃ ^? availability and plant NO₃ ^? dynamics. This work investigated the variability of these parameters in individuals of a broadleaved (Aucuba japonica) plant and a coniferous (Platycladus orientalis) plant, and explored the applicability of tissue NO₃ ^? isotopes for deciphering plant NO₃ ^? utilization mechanisms. The NO₃ ^? in washed leaves showed concentration and isotopic ratios that were much lower than that in unwashed leaves, indicating a low contribution of atmospheric NO₃ ^? to NO₃ ^? in leaves. Current leaves showed higher [NO₃ ^?] and isotopic ratios than mature leaves. Moreover, higher leaf [NO₃ ^?] and isotopic enrichments (relative to soil NO₃ ^?) were found under higher soil NO₃ ^? availability for A. japonica. In contrast, leaves of P. orientalis showed low [NO₃ ^?] and negligible isotopic enrichments despite high soil NO₃ ^?. Higher [NO₃ ^?] was found in both fine and coarse roots of the P. orientalis plant, but significant isotopic enrichment was found only in coarse roots. These results reflect that the NO₃ ^? accumulation and isotopic effects decreased with leaf age, but increased with soil NO₃ ^? supply. Leaves are therefore identified as a location of NO₃ ^? reduction for A. japonica, while P. orientalis did not assimilate NO₃ ^? in leaves but in coarse roots. This work provided the first organ-specific information on NO₃ ^? isotopes in plant individuals, which will stimulate further studies of NO₃ ^? dynamics in a broader spectrum of plant ecosystems.     

Warming decreased and grazing increased plant uptake of amino acids in an alpine meadow

Organic nitrogen (N) uptake by plants has been recognized as a significant component of terrestrial N cycle. Several studies indicated that plants have the ability to switch their preference between inorganic and organic forms of N in diverse environments; however, research on plant community response in organic nitrogen uptake to warming and grazing is scarce. Here, we demonstrated that organic N uptake by an alpine plant community decreased under warming with ¹³C–¹⁵N‐enriched glycine addition method. After 6 years of treatment, warming decreased plant organic N uptake by 37% as compared to control treatment. Under the condition of grazing, warming reduced plant organic N uptake by 44%. Grazing alone significantly increased organic N absorption by 15%, whereas under warming condition grazing did not affect organic N uptake by the Kobresia humilis community on Tibetan Plateau. Besides, soil NO₃–N content explained more than 70% of the variability observed in glycine uptake, and C:N ratio in soil dissolved organic matter remarkably increased under warming treatment. These results suggested warming promoted soil microbial activity and dissolved organic N mineralization. Grazing stimulated organic N uptake by plants, which counteracted the effect of warming.     

Reassessing the nitrogen relations of Arctic plants: a mini-review

The Arctic is often assumed to be an NH₄⁺-dominated ecosystem. This review assesses the validity of this assumption. It also addresses the question of whether Arctic plant growth is limited by the ability to use the forms of nitrogen that are available. The review demonstrates that several sources of soil nitrogen are available to Arctic plants, including soluble organic nitrogen (e.g. glycine, aspartic acid and glutamic acid), NH₄⁺ and NO^?₃. In mesic Arctic soils, soluble organic nitrogen is potentially more important than either NH⁺₄ or NO^?₃. Many Arctic species are capable of taking up soluble organic nitrogen (either directly and/or in association with ectomycorrhizae), with the greatest potential for soluble organic nitrogen uptake being exhibited by deciduous species. The ability to take up soluble organic nitrogen may enable some Arctic plants to avoid nitrogen limitations imposed by the slow rate of organic matter decomposition. NO^?₃ is also present in many Arctic soils, especially calcareous soils and soils near flowing water, animal burrows and bird cliffs. Arctic species characteristic of mesic and xeric habitats are capable of taking up and assimilating NO^?₃. Even when present in lower concentrations in soils than NH⁺₄, NO^?₃ is still an important source of nitrogen for some Arctic plants. Arctic-plants therefore have a variety of nitrogen sources available to them, and are capable of using those nitrogen sources. Taken together, these findings demonstrate that the Arctic is not an NH⁺₄dominated ecosystem. Symbiotic fixation of atmospheric N₂ does not appear to be an important source of nitrogen for Arctic plants. The reliance of Arctic plants on internal recycling of nitrogen substantially reduces their dependence on soil nitrogen uptake (this is particularly the case for slow-growing evergreens). Despite the high level of internal nitrogen recycling, Arctic plant growth remains limited by the low levels of available soil nitrogen. However, Arctic plant growth is not limited by an inability to utilize any of the available forms of nitrogen. The potential effects of climatic warming on nitrogen availability and use are discussed. The question of whether the Arctic ecosystem is uniquely different from temperate nitrogen-deficient ecosystems is also assessed.     

Plant Trait Diversity Buffers Variability in Denitrification Potential over Changes in Season and Soil Conditions

Background

Denitrification is an important ecosystem service that removes nitrogen (N) from N-polluted watersheds, buffering soil, stream, and river water quality from excess N by returning N to the atmosphere before it reaches lakes or oceans and leads to eutrophication. The denitrification enzyme activity (DEA) assay is widely used for measuring denitrification potential. Because DEA is a function of enzyme levels in soils, most ecologists studying denitrification have assumed that DEA is less sensitive to ambient levels of nitrate (NO₃ ⁻) and soil carbon and thus, less variable over time than field measurements. In addition, plant diversity has been shown to have strong effects on microbial communities and belowground processes and could potentially alter the functional capacity of denitrifiers. Here, we examined three questions: (1) Does DEA vary through the growing season? (2) If so, can we predict DEA variability with environmental variables? (3) Does plant functional diversity affect DEA variability?

Methodology/Principal Findings

The study site is a restored wetland in North Carolina, US with native wetland herbs planted in monocultures or mixes of four or eight species. We found that denitrification potentials for soils collected in July 2006 were significantly greater than for soils collected in May and late August 2006 (p<0.0001). Similarly, microbial biomass standardized DEA rates were significantly greater in July than May and August (p<0.0001). Of the soil variables measured—soil moisture, organic matter, total inorganic nitrogen, and microbial biomass—none consistently explained the pattern observed in DEA through time. There was no significant relationship between DEA and plant species richness or functional diversity. However, the seasonal variance in microbial biomass standardized DEA rates was significantly inversely related to plant species functional diversity (p<0.01).

Conclusions/Significance

These findings suggest that higher plant functional diversity may support a more constant level of DEA through time, buffering the ecosystem from changes in season and soil conditions.     

Nitrogen isotope composition of tomato (Lycopersicon esculentum Mill. cv. T-5) grown under ammonium or nitrate nutrition

Studies that quantify plant δ¹⁵N often assume that fractionation during nitrogen uptake and intra-plant variation in δ¹⁵N are minimal. We tested both assumptions by growing tomato (Lycopersicon esculetum Mill. cv. T-5) at NH₄⁺ or NO^?₃ concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole-plant δ¹⁵N was not significantly different from source δ¹⁵ N for plants grown on either nitrogen form. No intra-plant variation in δ¹⁵N was observed for plants grown with NH⁺₄. In contrast. δ¹⁵N of leaves was as much as 5.8% greater than that of roots for plants grown with NO^?₃. The contrasting patterns of intra-plant variation are probably caused by different assimilation patterns. NH⁺₄ is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO^?₃ assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ¹⁵N of NO^?₃ to become enriched relative to organic nitrogen; the δ¹⁵N of NO^?₃ was 11.1 and 12.9% greater than the δ¹⁵N of organic nitrogen in leaves and roots, respectively. Leaf δ¹⁵N may therefore be greater than that of roots because the NO^?₃ available for assimilation in leaves originates from a NO^?₃ pool that was previously exposed to nitrate assimilation in the root.     

Root distribution in relation to soil nitrogen availability in field-grown tomatoes

Tomato root growth and distribution were related to inorganic nitrogen (N) availability and turnover to determine 1) if roots were located in soil zones where N supply was highest, and 2) whether roots effectively depleted soil N so that losses of inorganic N were minimized. Tomatoes were direct-seeded in an unfertilized field in Central California. A trench profile/monolith sampling method was used. Concentrations of nitrate (NO₃ ^-) exceeded those of ammonium (NH₄ ⁺) several fold, and differences were greater at the soil surface (0–15 cm) than at lower depths (45–60 cm or 90–120 cm). Ammonium and NO₃ ^- levels peaked in April before planting, as did mineralizable N and nitrification potential. Soon afterwards, NO₃ ^- concentrations decreased, especially in the lower part of the profile, most likely as a result of leaching after application of irrigation water. Nitrogen pool sizes and rates of microbial processes declined gradually through the summer.Tomato plants utilized only a small percentage of the inorganic N available in the large volume of soil explored by their deep root systems; maximum daily uptake was approximately 3% of the soil pool. Root distribution, except for the zone around the taproot, was uniformly sparse (ca. 0.15 mg dry wt g^-1 soil or 0.5 cm g^-1 soil) throughout the soil profile regardless of depth, distance from the plant stem, or distance from the irrigation furrow. It bore no relation to N availability. Poor root development, especially in the N-rich top layer of soil, could explain low fertilizer N use by tomatoes.     

Microbial N turnover processes in three forest soil layers following clear cutting of an N saturated mature spruce stand

Microbial N turnover processes were investigated in three different forest soil layers [organic (O) layer, 0–10 cm depth (M₁), 10–40 cm depth (M₂)] after the clear cutting of a nitrogen (N) saturated spruce stand at the Höglwald Forest (Bavaria, Germany). The aim of the study was to provide detailed insight into soil-layer specific microbial production and the consumption of inorganic N within the main rooting zone. Furthermore, we intended to clarify the relevance of each soil layer investigated in respect of the observed high spatial variation of seepage water nitrate (NO ₃ ^? ) concentration at a depth of 40 cm. The buried bag and the ¹⁵N pool dilution techniques were applied to determine the net and gross N turnover rates. In addition, soil pH, C:N ratio, pool sizes of soil ammonium (NH ₄ ⁺ ) and NO ₃ ^? , as well as quantities of microbial biomass carbon (C_mic) and nitrogen (N_mic) were determined. The 40 cm thick upper mineral soil was found to be the main place of NO ₃ ^? production with a NO ₃ ^? supply or net nitrification three times higher than in the considerably thinner O layer. Nevertheless, O layer nitrification processes determined via in situ field experiments showed significant correlation with seepage water NO ₃ ^? . An improved correlation noted several months after the cut may result from a transport-induced time shift of NO ₃ ^? with downstream hydrological pathways. In contrast, the soil laboratory incubation experiments found no indication that mineral soil is relevant for the spatial heterogeneity of seepage water NO ₃ ^? . The results from our study imply that in situ experiments may be better suited to studies investigating N turnover in relation to NO ₃ ^? loss via seepage water in similar ecosystems in order to gain representative data.     

Herbicide management of invasive cattail (<Emphasis Type="Italic">Typha</Emphasis> × <Emphasis Type="Italic">glauca</Emphasis>) increases porewater nutrient concentrations

Invasive wetland plants are the primary targets of wetland management to promote native communities and wildlife habitat, but little is known about how commonly implemented restoration techniques influence nutrient cycling. We tested how experimental mowing, herbicide application, and biomass harvest (i.e., removal of aboveground biomass) treatments of Typha-invaded mesocosms altered porewater nutrient (NO₃ ^?, NH₄ ⁺, PO ₄ ^?3 ) concentration and supply rate, vegetation response, and light penetration to the soil surface. We found that while herbicide application eliminated the target species, it also reduced native plant density and biomass, as well as increased porewater nutrient concentration (PO ₄ ^?3 , NO₃ ^?) and supply rates (N, P, K) up to a year after treatments were implemented. Because herbicide application promotes nutrient enrichment, it may increase the likelihood of reinvasion by problematic wetland invaders, as well as cause eutrophication and deleterious algal blooms in adjacent aquatic systems. Our data suggest that biomass harvest should be considered by managers aiming to reduce Typha abundance without eradicating native diversity, avoid nutrient leaching, as well as possibly utilizing biomass for bioenergy.     

Exotic grasses and nitrate enrichment alter soil carbon cycling along an urban–rural tropical forest gradient

Urban areas are expanding rapidly in tropical regions, with potential to alter ecosystem dynamics. In particular, exotic grasses and atmospheric nitrogen (N) deposition simultaneously affect tropical urbanized landscapes, with unknown effects on properties like soil carbon (C) storage. We hypothesized that (H1) soil nitrate (NO₃^?) is elevated nearer to the urban core, reflecting N deposition gradients. (H2) Exotic grasslands have elevated soil NO₃^? and decreased soil C relative to secondary forests, with higher N promoting decomposer activity. (H3) Exotic grasslands have greater seasonality in soil NO₃^? vs. secondary forests, due to higher sensitivity of grassland soil moisture to rainfall. We predicted that NO₃^? would be positively related to dissolved organic C (DOC) production via changes in decomposer activity. We measured six paired grassland/secondary forest sites along a tropical urban‐to‐rural gradient during the three dominant seasons (hurricane, dry, and early wet). We found that (1) soil NO₃^? was generally elevated nearer to the urban core, with particularly clear spatial trends for grasslands. (2) Exotic grasslands had lower soil C than secondary forests, which was related to elevated decomposer enzyme activities and soil respiration. Unexpectedly, soil NO₃^? was negatively related to enzyme activities, and was lower in grasslands than forests. (3) Grasslands had greater soil NO₃^? seasonality vs. forests, but this was not strongly linked to shifts in soil moisture or DOC. Our results suggest that exotic grasses in tropical regions are likely to drastically reduce soil C storage, but that N deposition may have an opposite effect via suppression of enzyme activities. However, soil NO₃^? accumulation here was higher in urban forests than grasslands, potentially related to of aboveground N interception. Net urban effects on C storage across tropical landscapes will likely vary depending on the mosaic of grass cover, rates of N deposition, and responses by local decomposer communities.     

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.     

Porewater nutrient profiles and nutrient sediment–water exchange in a tropical mangrove waterway, Mapopwe Creek, Chwaka Bay, Zanzibar

Sediments were examined in the Mapopwe Creek, a tidally dominated mangrove waterway in the Chwaka Bay mangrove forest, Zanzibar, to assess their significance in the nutrient dynamics of the mangrove forest and the adjacent bay. Porewater concentrations of dissolved ammonium and that of soluble reactive phosphorus (SRP) were generally higher during the dry season than during the wet season. NO₃^? plus NO₂^? concentration averaged 1 µm and did not vary much between the two periods. Fluxes of ammonium ranged from ?575 to 523 µm m^?2 h^?1 and those of SRP from ?55.7 to 69.5 µm m^?2 h^?1. Measurements of NO_x did not show any consistent fluxes of this dissolved nitrogen species. Variations of flux rates between the two seasons were not significant even though there were small variations in the flux direction in both nutrients. Results imply that Mapopwe sediments are a source of NH₄⁺ but act as a sink for SRP.     

Carbon sequestration in freshwater wetlands in Costa Rica and Botswana

Tropical wetlands are typically productive ecosystems that can introduce large amounts of carbon into the soil. However, high temperatures and seasonal water availability can hinder the ability of wetland soils to sequester carbon efficiently. We determined the carbon sequestration rate of 12 wetland communities in four different tropical wetlands—an isolated depressional wetland in a rainforest, and a slow flowing rainforest swamp, a riverine flow-through wetland with a marked wet and dry season, a seasonal floodplain of an inland delta—with the intention of finding conditions that favor soil carbon accumulation in tropical wetlands. Triplicate soil cores were extracted in these communities and analyzed for total carbon content to determine the wetland soil carbon pool. We found that the humid tropic wetlands had greater carbon content (P ≤ 0.05) than the tropical dry ones (96.5 and 34.8 g C kg^?1, respectively). While the dry tropic wetlands had similar sequestration rates (63 ± 10 g Cm^?2 y^?1 on average), the humid tropic ones differed significantly (P < 0.001), with high rates in a slow-flowing slough (306 ± 77 g Cm^?2 y^?1) and low rates in a tropical rain forest depressional wetland (84 ± 23 g Cm^?2 y^?1). The carbon accumulating in all of these wetlands was mostly organic (92–100%). These results suggest the importance of differentiating between types of wetland communities and their hydrology when estimating overall rates at which tropical wetlands sequester carbon, and the need to include tropical wetland carbon sequestration in global carbon budgets.