Efficiency and dimensioning of riparian buffer zones in agricultural catchments

A strong linear correlation was found between the log-transformed load and retention of nitrogen and phosphorus in riparian buffer zones (r=0.99 and 0.997, respectively). Analyses of N and P budgets in four riparian forests of varying age (two grey alder stands in Estonia and two riparian deciduous forests in USA) show a significant efficiency. Despite the different input load (72.9–110.4 kg N ha⁻¹ year⁻¹ and 2.5–3.0 kg P ha⁻¹ year⁻¹), the outputs into streams from the alder stands systems were comparably low (9.0–13.2 and 0.38–0.62 kg ha⁻¹ year⁻¹). The older forests from the USA showed less efficiency. Plant uptake of both N and P in younger stands was significantly higher than in older forests. Methods to determine the buffer zones' and buffer strips' width and their efficiency are presented. The testing of efficiency assessment in a watershed in Estonia demonstrated an expected efficiency of buffers.     

Nutrient runoff dynamics in a rural catchment: Influence of land-use changes, climatic fluctuations and ecotechnological measures

The main trend in land-use changes in the Porijõgi River catchment, south Estonia, is a significant increase in abandoned lands (from 1.7% in 1987 to 10.5% in 1997), and a decrease in arable lands (from 41.8 to 23.9%). Significant climatic fluctuations occurred during the last decades. Milder winters (increase of air temperature in February from −7.9 to −5.5°C during the period 1950–1997) and a change in the precipitation pattern have influenced the mean annual water discharge. This results in more intensive material flow during colder seasons and decreased water runoff in summer. During the period 1987–1997 the runoff of total-N, total-P, SO₄, and organic material (after BOD₅) decreased from 25.9 to 5.1, from 0.32 to 0.13, from 78 to 48, and from 7.4 to 3.5 kg ha⁻¹ year⁻¹, respectively. Most significant was a 4–20-fold decrease in agricultural subcatchments while in the forested upper-course catchment the changes were insignificant. Variations of total-N, and total-P runoff in both the entire catchment and its agricultural subcatchments are well described by the change of land use (including fertilization intensity), soil parameters and water discharge. In small agricultural subcatchments the rate of fertilization was found to be the most important factor affecting nitrogen runoff, while land-use pattern plays the main role in larger mosaic catchments. Ecotechnological measures (e.g. riparian buffer zones and buffer strips, constructed wetlands) to control nutrient flows from agricultural catchments are very important.     

Watershed modelling of nonpoint nitrogen losses from arable land to the Swedish coast in 1985 and 1994

Eutrophication problems in the Baltic Sea have drawn attention to the contribution of nutrients from surrounding countries. By using the HBV-N model in southern Sweden (145 000 km²) daily nitrogen leaching, reduction in rivers and lakes, net transport to the sea and source apportionment have been calculated in 3725 subbasins for the period 1985–1994, with calibration at 722 sites against measured time series. On average, 48% of the nonpoint losses from agriculture were reduced during the transport towards the sea, which left about 33 500 tonnes in annual mean net transport. This represents 45% of the total land-based load. Land cover and emissions for the years of 1985 and 1994 were used in two separate simulations of the 10-year period. The normalized gross leakage from arable land in 1985 was estimated to 29 kg N ha⁻¹ year⁻¹, which corresponds to 15 kg N ha⁻¹ year⁻¹ in net leakage to the sea. In 1994 these transports were reduced by 20 and 15%, and thereby the total load on the sea was decreased by 7%. This is still far from the Swedish goal of 50% reduction. The article presents the spatial variation of nitrogen leakage and retention within the southern half of Sweden, and emphasizes the importance of allocating measures where down-stream retention is low in order to achieve efficiency with respect to the sea. It is shown that the model approach may be used in the decision making process for best management practices in watersheds.     

Evaluation of Gracilaria caudata J. Agardh for bioremediation of nutrients from shrimp farming wastewater

The accelerated development of shrimp farming in Brazil in recent decades has caused negative impacts to the environment. The most evident effects resulting from this activity is the increase in organic material, the reduction in oxygen and the excessive rise in water nutrients. Thus, there is a need for finding alternative solutions that can mitigate the negative impacts caused by this activity. A potentially viable solution is the use of macroalgae to remove nutrients from the cultivation systems. This study examined in situ (shrimp pond), the growth and storage of nitrogen and phosphorous from the macroalga Gracilaria caudata. A short-term measurement experiment was also conducted to evaluate the bioremediation potential this species. These results showed positive values for biomass and growth during the study period, except at day 45 for the tubular nets and day 75 for the cages, when they reached lower values than those of the initial weight. The results obtained indicate that G. caudata may reach annual production of 59.16 ton ha⁻¹ of wet weight, which corresponds to 11.83 ton dry weight. Nitrogen and phosphorous content in the algal tissues increased with time. The mean for the period was 2.61 ± 0.26% and 0.20 ± 0.03% for the nitrogen and phosphorous, respectively. An estimate of the data showed that 1 ha of cultivated algae has the potential to remove 0.309 ton ha⁻¹ year⁻¹ of nitrogen and 0.024 ton ha⁻¹ year⁻¹ of phosphorous. The study of the biofiltration capacity of G. caudata showed a significant reduction in nutrients. The removal of NH₄–N was around 59.5%, NO₃–N 49.6% and PO₄–P 12.3% in 4 h. These results suggest that although G. caudata showed relatively modest growth rates, they can be cultivated together with shrimp and can contribute to the removal of nitrogen and phosphorous from the pond. Moreover, the capacity to efficiently remove nutrients demonstrated in laboratory experiments encourages the use of this alga as a bioremediation agent.     

Water chemistry and nutrient budgets in an undisturbed evergreen rainforest of southern Chile

In a pristine evergreen rainforest of Nothofagus betuloides, located at the Cordillera de los Andes in southern Chile (41 °S), concentrations and fluxes of nutrients in bulk precipitation, cloud water, throughfall water, stemflow water, soil infiltration and percolation water and runoff water were measured. The main objectives of this study were to investigate canopy-soil-atmosphere interactions and to calculate input-output budgets. From May 1999 till April 2000, the experimental watershed received 8121 mm water (86% incident precipitation, 14% cloud water), of which the canopy intercepted 16%. Runoff water volume amounted 9527 mm. Bulk deposition of inorganic (DIN) and organic (DON) nitrogen amounted 3.6 kg ha^–1 year^–1 and 8.2 kg ha^–1 year^–1 respectively. Occult deposition (clouds + fog) contributes for 40% to the atmospheric nitrogen input (bulk + occult deposition) of the forest. An important part of the atmospheric ammonium deposition is retained within the canopy or converted to nitrate or organic nitrogen by epiphytic bacteria or lichens. Also the export of inorganic (0.9 kg ha^–1 year^–1) and organic (5.2 kg ha^–1 year^–1) nitrogen via runoff is lower than the input to the forest floor via throughfall and stemflow water (3.2 kg DIN ha–1 year^–1 and 5.6 kg DON ha^–1 year^–1). The low concentrations of NO ₃ ^– and NH ₄ ⁺ under the rooting depth suggest an effective biological immobilization by vegetation and soil microflora. Dry deposition and foliar leaching of base cations (K⁺, Ca²⁺, Mg²⁺) was estimated using a canopy budget model. Bulk deposition accounted for about 50% of the total atmospheric input. Calculated dry and occult deposition are both of equal value (about 25%). Foliar leaching of K⁺, Ca²⁺, and Mg²⁺ accounted for 45%, 38% and 6% of throughfall deposition respectively. On an annual basis, the experimental watershed was a net source for Na⁺, Ca²⁺ and Mg²⁺.     

Plant responses to nutrient addition and predictive ability of vegetation N:P ratio in an austral fen

1. Previous studies of the N:P ratio in wetland plants have been carried out in northern hemisphere wetlands where atmospheric nitrogen deposition is higher. There is little research on foliar N:P ratio as a potential indicator of nutrient limitation in vegetation communities in southern hemisphere wetlands. This study aimed to redress this knowledge gap and answer the following questions: how well does the plant tissue nitrogen to phosphorus (N:P) ratio predict wetland plant community nutrient limitation, as indicated by vegetation standing stocks and below-ground biomass, in southern hemisphere fens? Secondly, what are the impacts of realistic upper levels of farm nutrient run-off on natural montane fen vegetation?
2. Low (35 kg ha⁻¹ year⁻¹) and high (70 kg ha⁻¹ year⁻¹) levels of nitrate-N or ammonium-N with and without P (20 kg ha⁻¹ year⁻¹) were added to 81 vegetation plots over a period of 2.75 years. Species composition, plant nutrient status, and above-ground live vegetation standing stocks were assessed after 3 years, and below-ground biomass after 2 years.
3. Plant tissue analysis suggested the community was N limited or N and P co-limited; we found greater standing stocks of vegetation in plots treated with 70 kg ha⁻¹ year⁻¹ ammonium-N, indicating N limitation. No difference between other treatments was found in above-ground standing stocks or below-ground biomass. Plant species cover increased in both high N treatments, consistent with N limitation. These changes in plant species cover were accompanied by significant decreases in species richness in both high N treatments. Native species dominated the vegetation and this was unaffected by nutrient addition (90% cover).
4. This is one of the first studies to test and find support for the N:P ratio in southern hemisphere wetlands. Observed declines in species richness after N fertilisation in an N-limited fen suggests increased N may pose risks to austral wetlands. Responses by plant communities (changes in composition, biomass) to lower levels of nutrient addition may require longer periods of fertilisation to be apparent in slow growing ecosystems.

     

Primary production and nutrient budgets of Sarcocornia perennis ssp. alpini (Lag.) Castroviejo in the salt marsh of the Palmones River estuary (Southern Spain)

Biomass, primary production and nutrient budgets associated to Sarcocornia perennis subspecies (ssp.) alpini were studied in the Palmones River estuary salt marsh (Southern Spain) to evaluate the nutrient sequestration capacity of the low marsh. Above- and belowground living and dead biomass, as well as carbon, nitrogen and phosphorus content were monitored during 1 year. Additionally, the fate of aboveground detritus was evaluated in an experiment on litter decomposition. The detritus production of S. perennis ssp. alpini was almost equivalent to its annual primary production indicating a rapid turnover of biomass. We calculated that only 12% of the aboveground detritus was exported out of the low marsh while the rest was decomposed in the sediment with a rate of 0.8 year⁻¹. Changes in concentrations of total carbon, nitrogen and phosphorus in the sediment showed patterns related to S. perennis ssp. alpini belowground biomass. Our results suggested that the sediment functions as a net sink for nutrients accumulating 550 g C m⁻² year⁻¹, 55 g N m⁻² year⁻¹, and 13 g P m⁻² year⁻¹.     

Dissolved load transported by rivers as an indicator of landscape sustainability

Investigation of the Stör River catchment in Germany shows very high annual matter losses (about 1050 kg total mineral salts per ha; excluding NaCl). Loss of base cations (e.g. Ca 263 kg ha⁻¹ year⁻¹), which are essential elements, was an important focus of this research. Compared to the stock of base cations to those minerals added by fertilising, topsoils are being depleted continuously. If this loss is not stopped, a complete breakdown of vegetation growth is expected. Strong impacts on the water cycle and a land management that is spatially and temporally unadapted leads to insufficient energy dissipation in both physical (evapotranspiration, condensation) and biological cyclic processes (photosynthesis, respiration). The resultant increased thermal and chemical potentials cause changes by way of feedback loops on the water cycle and cause matter losses. Some suggestions are presented to manage land sustainably — minimising matter losses with the help of managed wetlands and using much more perennial vegetation biomass.     

Evaluation of nutrient retention in four restored Danish riparian wetlands

During the last 15–20 years, re-establishment of freshwater riparian wetlands and remeandering of streams and rivers have been used as a tool to mitigate nutrient load in downstream recipients in Denmark. The results obtained on monitoring four different streams and wetland restoration projects are compared with respect to hydrology, i.e. flow pattern and discharge of ground or surface water, retention of phosphorus (P), and removal of nitrogen (N). Furthermore, the monitoring strategies applied for quantifying the post-restoration nutrient retention are evaluated. The four wetland restoration projects are the Brede River restoration (including river valley groundwater flow, remeandering and inundation), Lyngbygaards River restoration (groundwater flow, irrigation with drainage water, inundation with river water and remeandering), Egeskov fen (fen re-establishment and stream remeandering) and Egebjerg Meadows (fen restoration and hydrological reconnection to Store Hansted River). Retention of phosphorus varied between 0.13 and 10 kg P ha⁻¹ year⁻¹, while the removal of nitrogen varied between 52 and 337 kg N ha⁻¹ year⁻¹. The monitoring strategy chosen was not optimal at all sites and would have benefitted from a knowledge on local hydrology and water balances in the area to be restored before planning for the final monitoring design. Furthermore, the outcome concerning P retention would have benefitted from a more frequent sampling strategy.     

Dryland Soil Hydrological Processes and Their Impacts on the Nitrogen Balance in a Soil-Maize System of a Freeze-Thawing Agricultural Area

Understanding the fates of soil hydrological processes and nitrogen (N) is essential for optimizing the water and N in a dryland crop system with the goal of obtaining a maximum yield. Few investigations have addressed the dynamics of dryland N and its association with the soil hydrological process in a freeze-thawing agricultural area. With the daily monitoring of soil water content and acquisition rates at 15, 30, 60 and 90 cm depths, the soil hydrological process with the influence of rainfall was identified. The temporal-vertical soil water storage analysis indicated the local albic soil texture provided a stable soil water condition for maize growth with the rainfall as the only water source. Soil storage water averages at 0–20, 20–40 and 40–60 cm were observed to be 490.2, 593.8, and 358 m³ ha⁻¹, respectively, during the growing season. The evapo-transpiration (ET), rainfall, and water loss analysis demonstrated that these factors increased in same temporal pattern and provided necessary water conditions for maize growth in a short period. The dry weight and N concentration of maize organs (root, leaf, stem, tassel, and grain) demonstrated the N accumulation increased to a peak in the maturity period and that grain had the most N. The maximum N accumulative rate reached about 500 mg m⁻²d⁻¹ in leaves and grain. Over the entire growing season, the soil nitrate N decreased by amounts ranging from 48.9 kg N ha⁻¹ to 65.3 kg N ha⁻¹ over the 90 cm profile and the loss of ammonia-N ranged from 9.79 to 12.69 kg N ha⁻¹. With soil water loss and N balance calculation, the N usage efficiency (NUE) over the 0–90 cm soil profile was 43%. The soil hydrological process due to special soil texture and the temporal features of rainfall determined the maize growth in the freeze-thawing agricultural area.     

Mass balance on water column trace metals in a free-surface-flow-constructed wetlands in Sacramento, California

A mass balance has been performed on trace metals concentrations and hydrology observed between 1994 and 1996 at the Sacramento Demonstration Constructed Wetlands using a first-order areal plug flow model. Water losses to infiltration and evapotranspiration from a typical cell are estimated to average 35 and 7% of influent flow, respectively. The wetlands effluent metals concentrations consistently meet proposed discharge criteria. Annual total mass loadings for all trace metals average 14.0 kg ha⁻¹ yr⁻¹, 88% of which consists of zinc, copper, and nickel. Effluent metals leaving the wetland average 3.1 kg ha⁻¹ yr⁻¹, 79% of which consists of the same three metals. Annual vegetation harvest events do not appear to account for more than 5% of annual trace metals mass removal, although harvest does appear to represent a significant loss pathway for some metals like mercury, lead, nickel, and chromium. Metals mass removals resulting from first-order removal interactions within the wetland range from 27 to 81%, with the exception of arsenic and nickel which display poor mass removals in part due to their high dissolved concentrations. An average of 7.6 kg ha⁻¹ yr⁻¹, or 54% of influent metals loadings, is sequestered within the internal wetland compartments.     

Emissions of greenhouse gases from ponds constructed for nitrogen removal

Methane and carbon dioxide emission from three constructed ponds were monitored during an annual cycle. Water temperature was a good predictor of methane emission in all three ponds. In the most intensively studied pond, nitrate concentration in the bottom water could further explain the amount of methane emitted. When water temperature exceeded 15 °C between 1 and 54 mg, CH₄ m⁻² h⁻¹ was emitted on all occasions, while at temperatures below 10 °C, less than 0.6 mg CH₄ m⁻² h⁻¹ was emitted. The flux of carbon dioxide differed between the ponds and no consistent patterns were found. In a laboratory study at 20 °C, we showed that high, but naturally occurring, nitrate concentrations (8 and 16 mg NO₃⁻–N l⁻¹) constrained the production of methane compared to the treatment with no nitrate addition. Nitrous oxide production was positively correlated with nitrate concentration. Carbon dioxide production was highest at the highest nitrate concentration, which indicates that increased nitrate loading on ponds and wetlands will stimulate organic matter decomposition rates. Our conclusion is that these ponds constructed for nitrate removal emit greenhouse gases comparable to lakes in the temperate region.     

Greenhouse gas emissions and carbon sequestration potential in restored wetlands of the Canadian prairie pothole region

North American prairie pothole wetlands are known to be important carbon stores. As a result there is interest in using wetland restoration and conservation programs to mitigate the effects of increasing greenhouse gas concentration in the atmosphere. However, the same conditions which cause these systems to accumulate organic carbon also produce the conditions under which methanogenesis can occur. As a result prairie pothole wetlands are potential hotspots for methane emissions. We examined change in soil organic carbon density as well as emissions of methane and nitrous oxide in newly restored, long-term restored, and reference wetlands across the Canadian prairies to determine the net GHG mitigation potential associated with wetland restoration. Our results indicate that methane emissions from seasonal, semi-permanent, and permanent prairie pothole wetlands are quite high while nitrous oxide emissions from these sites are fairly low. Increases in soil organic carbon between newly restored and long-term restored wetlands supports the conclusion that restored wetlands sequester organic carbon. Assuming a sequestration duration of 33 years and a return to historical SOC densities we estimate a mean annual sequestration rate for restored wetlands of 2.7 Mg C ha⁻¹year⁻¹ or 9.9 Mg CO₂ eq. ha⁻¹ year⁻¹. Even after accounting for increased CH₄ emissions associated with restoration our research indicates that wetland restoration would sequester approximately 3.25 Mg CO₂ eq. ha⁻¹year⁻¹. This research indicates that widescale restoration of seasonal, semi-permanent, and permanent wetlands in the Canadian prairies could help mitigate GHG emissions in the near term until a more viable long-term solution to increasing atmospheric concentrations of GHGs can be found.     

Land treatment of olive oil mill wastewater

Experiments carried out in lysimeters filled with two calcareous clayey soils (ca 40% CaCO₃; ca 40% clay), showed that a 2m layer of soil almost completely removed the organic and inorganic components of olive oil mill wastewater (OMW) when it was applied in doses of 5000–10000m³ha⁻¹year⁻¹. This efficiency was maintained for at least 2 years. In field experiments, the application of OMW to one of these soils during three successive years at an annual rate of up to 6000m³ha⁻¹ caused changes in some chemical properties of the soil, especially in the upper layer (0–50cm). Concentrations of soil organic matter, Kjeldahl N, soluble NO₃ and available P increased enhancing soil fertility. On the other hand, soil electrical conductivity and sodium adsorption ratio also increased but below the levels representing salinization or sodification hazard for the soil. Furthermore, leaching of Na⁺ and NO₃⁻ below the 1 m layer were detected.     

Functionality of restored mangroves: A review

Widespread mangrove degradation coupled with the increasing awareness of the importance of these coastal forests have spurred many attempts to restore mangroves but without concomitant assessment of recovery (or otherwise) at the ecosystem level in many areas. This paper reviews literature on the recovery of restored mangrove ecosystems using relevant functional indicators. While stand structure in mangrove stands is dependent on age, site conditions and silvicultural management, published data indicates that stem densities are higher in restored mangroves than comparable natural stands; the converse is true for basal area. Biomass increment rates have been found to be higher in younger stands than older stands (e.g. 12 t ha⁻¹ year⁻¹ for a 12 years plantation compared to 5.1 t ha⁻¹ year⁻¹ for a 80-year-old plantation). Disparities in patterns of tree species recruitment into the restored stands have been observed with some stands having linear recruitment rates with time (hence enhancing stand complexity), while some older stands completely lacked the understorey. Biodiversity assessments suggest that some fauna species are more responsive to mangrove degradation (e.g. herbivorous crabs and mollusks in general), and thus mangrove restoration encourages the return of such species, in some cases to levels equivalent to those in comparable natural stands. The paper finally recommends various mangrove restoration pathways in a functional framework dependent on site conditions and emphasizes community involvement and ecosystem level monitoring as integral components of restoration projects.     

Emission of N2O, N2, CH4, and CO2 from constructed wetlands for wastewater treatment and from riparian buffer zones

We measured nitrous oxide (N₂O), dinitrogen (N₂), methane (CH₄), and carbon dioxide (CO₂) fluxes in horizontal and vertical flow constructed wetlands (CW) and in a riparian alder stand in southern Estonia using the closed chamber method in the period from October 2001 to November 2003. The replicates’ average values of N₂O, N₂, CH₄ and CO₂ fluxes from the riparian gray alder stand varied from −0.4 to 58 μg N₂O-N m⁻² h⁻¹, 0.02–17.4 mg N₂-N m⁻² h⁻¹, 0.1–265 μg CH₄-C m⁻² h⁻¹ and 55–61 mg CO₂-C m⁻² h⁻¹, respectively. In horizontal subsurface flow (HSSF) beds of CWs, the average N₂ emission varied from 0.17 to 130 and from 0.33 to 119 mg N₂-N m⁻² h⁻¹ in the vertical subsurface flow (VSSF) beds. The average N₂O-N emission from the microsites above the inflow pipes of the HSSF CWs was 6.4–31 μg N₂O-N m⁻² h⁻¹, whereas the outflow microsites emitted 2.4–8 μg N₂O-N m⁻² h⁻¹. In VSSF beds, the same value was 35.6–44.7 μg N₂O-N m⁻² h⁻¹. The average CH₄ emission from the inflow and outflow microsites in the HSSF CWs differed significantly, ranging from 640 to 9715 and from 30 to 770 μg CH₄-C m⁻² h⁻¹, respectively. The average CO₂ emission was somewhat higher in VSSF beds (140–291 mg CO₂-C m⁻² h⁻¹) and at the inflow microsites of HSSF beds (61–140 mg CO₂-C m⁻² h⁻¹). The global warming potential (GWP) from N₂O and CH₄ was comparatively high in both types of CWs (4.8 ± 9.8 and 6.8 ± 16.2 t CO₂ eq ha⁻¹ a⁻¹ in the HSSF CW 6.5 ± 13.0 and 5.3 ± 24.7 t CO₂ eq ha⁻¹ a⁻¹ in the hybrid CW, respectively). The GWP of the riparian alder forest from both N₂O and CH₄ was relatively low (0.4 ± 1.0 and 0.1 ± 0.30 t CO₂ eq ha⁻¹ a⁻¹, respectively), whereas the CO₂-C flux was remarkable (3.5 ± 3.7 t ha⁻¹ a⁻¹). The global influence of CWs is not significant. Even if all global domestic wastewater were treated by wetlands, their share of the trace gas emission budget would be less than 1%.     

Evaluation and prediction of nonpoint pollution in Lithuania

The transportation of nitrogen and phosphorus from agricultural fields and river basins was analysed. Evaluation and long-term (17 years) prediction of the nutrient transport from agricultural fields under corresponding soil, landscape, and climate conditions were carried out by numerical experiment (EPIC model). Due to the spatial variability of soils and precipitation the most vulnerable region is determined in Western Lithuania where annual amount of precipitation is the highest one (800–900 mm): the annual load of nitrogen up to four times higher than that in Middle Lithuania. The lowest values for both nutrients (N=15 kg ha⁻¹, P=0.15 kg ha⁻¹) have been estimated in Middle Lithuania. Contrary to the field scale estimations, the largest amount of nutrients is carried by small rivers in Middle Lithuania due to heavy mechanical constitution of soils and specific features of landscape. In the period from 1952 to the late 1980s, due to the intensive agricultural activity, increased load of nutrients is fixed. In small Lithuanian rivers, after an unprecedented decrease in the application of the mineral fertilisers and manure since 1991 (in connection with the economical situation of Lithuania), the nutrient load decreasing phase started, but due to the substantial inertia of terrestrial and aquatic ecosystems, a decreasing trend is within the accuracy limits.     

Phosphorus and nitrogen retention in five Precambrian shield wetlands

Phosphorus and nitrogen mass balances of five wetlands (two beaver ponds, two conifer-Sphagnum swamps and one sedge fen) situated in three catchments in central Ontario, Canada, were measured. Monthly and annual input-output budgets of total phosphorus (TP), total nitrogen (TN), total organic nitrogen (TON), total inorganic nitrogen (TIN), ammonium ion (NH₄ ⁺ -N), nitrate (NO ₃ ^– -N) and dissolved organic carbon (DOC) were estimated for the five wetlands during the 1982–83 and 1983–84 water years. Except for the deepest beaver pond (3.2 m) which had annual TP retention of –44% (–0.030 ± 0.015 g m^–2 yr^–1), the wetlands retained < 0.001 to 0.015 g M^–2 yr^–1 ; however, this wasless than 20% of the inputs and the estimated budget uncertainties were equal to or greater than the retention rates. Annual TN retentions ranged from –0.44 to 0.56 g m^–2 yr^–1 (–12 to 4%) but were not significantly different from zero. The wetlands transformed nitrogen by retaining TIN (16 to 80% RT) and exporting an equivalent amount as TON (–7 to 102% RT). The beaver ponds, however, retained NO ₃ ^– while NH ₄ ⁺ was passed through or the outputs exceeded the inputs. In contrast, the conifer swamps retained both NH ₄ ⁺ and NO ₃ ^– . DOC fluxes into and out of the beaver ponds were equal (–18 and 4% RT) but output from the conifer swamps exceeded input by > 90%. Marked seasonal trends in nutrient retention were observed. Nutrient retention coincided with low stream flow, increased evapotranspiration and biotic uptake during the summer. Net nutrient export occurred during the winter and spring when stream flows were highest and biotic uptake was low.     

Nitrogen export by surface runoff from a small agricultural watershed in southeast China: seasonal pattern and primary mechanism

The seasonal pattern and primary mechanism of nitrogen (N) export by surface runoff from the Wuchuan subwatershed (WCW), an agricultural upper watershed (1.88 km²) located in southeast China, were investigated based on extensive streamwater measurements in 2004–2005 under subtropical climatic conditions. The results disclosed a highly variable but strong linkage between hydrological and anthropogenic controls and N export. N export via surface runoff presented a significant seasonal pattern caused by changes in rainfall and watershed N input. Approximately 75% of the annual N export (67 kg ha⁻¹) was flushed by those storm runoff mainly occurred during the wet season (March through September). The WCW dataset of N concentrations and loads during both baseflow and stormflow implied an interactive effects of anthropogenetic N input and hydrology conditions: N export was flush-driven in late spring, summer and autumn (wet season), but highly related with soil N in winter and early spring. Compared to undisturbed watersheds under similar rainfall conditions, WCW exported a considerable amount of N due to intensive fertilizer application (a mean of 690 kg N ha⁻¹ year⁻¹, commonly as surface applications). This work provides a first characterization of a small agricultural Chinese catchment under subtropical climates and its associated N export behavior.     

Treatment of an artificial sulphide containing wastewater in subsurface horizontal flow laboratory-scale constructed wetlands

In general, treatment wetlands seem to be a potential method of tackling the sulphide problem of post-treatment of anaerobic digester effluents.Because of insufficient practical experience and lack of knowledge of sulphide removal, sulphur transformation was investigated, particularly in horizontal subsurface flow constructed wetlands (depth of 35 cm) under laboratory-scale conditions with artificial wastewater.The plants affected a clear stimulation of the sulphide and ammonia removal rates. Sulphide concentration in the range of 1.5–2.0 mg l⁻¹ was tolerated by the plants and completely removed in the planted model wetlands; sulphide concentration of >2.0 mg l⁻¹ caused instabilities in sulphide and nitrogen removal. Area-specific sulphide removal rates of up to 94 mg sulphide m⁻² d⁻¹ were achieved in the planted beds at hydraulic retention times of 2.5 d. Sulphate affected the sulphide removal. While in the unplanted control bed an almost stable removal in the range of 150–300 mg N m⁻² d⁻¹ was observed variations of hydraulic retention time, sulphide and sulphate concentrations influenced the ammonia removal rate within the planted beds in a broader range (600–1400 mg N m⁻² d⁻¹).These results showed that nitrification, sulphide oxidation, denitrification and sulphate reduction can occur simultaneously in the rhizosphere of treatment wetlands caused by dynamic redox gradients (aerobic–anaerobic) conditions.