Then and now: Revisiting nutrient export in agricultural watersheds within southern Ontario’s lower Great Lakes basin

An enhanced understanding of nonpoint source (NPS) nutrient export to the lower Great Lakes is needed to inform land use and land management decisions within southern Ontario. However, this understanding is limited by a lack of long-term, temporally-intensive monitoring. To address this knowledge gap, we revisit six agriculturally-dominated subwatersheds in southern Ontario, which were intensively studied during the mid-1970s, to assess changes in hydrology and NPS nutrient contributions. We compared 1975–1977 to 2016–2018 stream runoff, nutrient export (kg/day∙km²), and flow-weighted mean concentrations (FWMCs) of total phosphorus (TP), total dissolved phosphorus (TDP), total nitrogen (TN), nitrates (NO₃^–+NO₂^–) and Total Kjeldahl Nitrogen (TKN). Relative to the 1970s, runoff increased at three of six watersheds (by ~20–35%) while TP and TDP export increased at five watersheds (by ~50–125%). The increases in TP and TDP FWMCs were lower relative to phosphorus export changes at the three watersheds with increased runoff, suggesting that hydrology is an important driver of phosphorus export at these sites. Interestingly, export of TN and nitrates increased while TKN export decreased at most watersheds. We further note a shift in the timing of nutrient export at most sites, with ~40–70% of export now occurring during the winter and fall seasons whereas ~40–85% of past export occurred during spring and summer. These findings support an enhanced importance of non-growing season nutrient export from agricultural watersheds since the mid-1970s and stresses the need for targeted best management practices specific to the fall and winter seasons.     

Nutrient and suspended-sediment concentrations in the Maumee River and tributaries during 2019 rain-induced fallow conditions

Above average precipitation from October 2018 through July 2019 in the Maumee River (R.) Basin resulted in 29% of cropland left fallow, providing a glimpse of potential effects from decreased nutrient application. Ongoing monitoring at 15 water-quality sites on the Maumee R. upstream from Defiance enabled comparison with 2017, which was hydrologically similar to 2019 in precipitation and streamflow. In 2019, nitrate (as nitrogen; NO₃-N) for March-July was significantly less than previous years (2015–2018), but the response for phosphorus (P) was more complicated. Relative to 2017, total P (TP) was lower at 7 of 15 sites, but higher at 7, reflecting higher suspended sediment (SS). Dissolved P (DP) was generally lower, but less different than NO₃; DP was higher at 3 sites. DP-P:NO₃-N was generally higher in 2019, DP-P:TP was lower, and there was less TP relative to SS. Overall, less P was in the system in 2019. However smaller streams showed a large range of difference between 2019 and 2017 for all constituents, indicating variability in land management and physiography. In contrast, all constituents were lower in 2019 in larger (>5000 km²) streams, including the Maumee R. near Defiance, where the difference in NO₃ (−37%) exceeded that for TP (−16%), DP (−10%), and SS (−20%). Differences in these relations among N, P, and SS indicate that P was available from legacy sources that are more difficult to distinguish during typical agricultural production years and that some material from 2019 was stored in the system upstream from the largest sites.     

Phosphorus loading to Lake Erie from the Maumee,Sandusky and Cuyahoga rivers: The importance of bioavailability

Lake Erie has undergone re-eutrophication beginning in the 1990s, even though total phosphorus (TP) loads to the lake continued to slowly decline. Using our 1982 and 2007–10 studies of the bioavailability of dissolved and particulate phosphorus export from major Ohio tributaries, together with our long-term TP and dissolved reactive phosphorus (DRP) loading data, we estimated long-term annual export of dissolved and particulate bioavailable phosphorus. DRP was found to adequately represent dissolved bioavailable export while 26–30% of the particulate phosphorus (PP) was extractable by 0.1 N NaOH, a frequently used indicator of PP bioavailability. During the period of re-eutrophication (1991–2012), DRP export from nonpoint sources in the Maumee and Sandusky rivers increased dramatically while NaOH-PP export had a slight decline for the Maumee and a small increase in the Sandusky. For the Cuyahoga River, both DRP and NaOH-PP increased, but these changes were small in relation to those of the Maumee and Sandusky. During this period, whole lake loading of both non-point and point sources of phosphorus declined. This study indicates that increased nonpoint loading of DRP is an important contributing factor to re-eutrophication. Although nonpoint control programs in the Maumee and Sandusky have been effective in reducing erosion and PP export, these programs have been accompanied by increased DRP export. Future target loads for Lake Erie should focus on reducing bioavailable phosphorus, especially DRP from nonpoint sources. Agricultural P load reduction programs should address both DRP and PP, and take into account the lower bioavailability of PP.     

A Bayesian hierarchical spatially explicit modelling framework to examine phosphorus export between contrasting flow regimes

We examine the ability of a SPARROW-based model (SPAtially Referenced Regression On Watershed attributes) to assess regional P export coefficients that can assist with evaluation of nutrient mitigation projects and support adaptive watershed management. Limitations in number of tributary monitoring stations were overcome by assembling multi-agency water quality data from provincial, municipal, citizen science, and academic programs. We introduced a Bayesian hierarchical framework designed to guide parameter estimation from tributary nutrient loading in southern Georgian Bay drainage basin during contrasting flow regimes, such as dry and wet years. Agriculture was identified as a major non-point P source representing between 30 and 48% of delivered P loading. Our source apportionment predicted TP loss rates from croplands that exceeded those from forested areas by 320% during dry years and by 360% during wet years, while low intensity agricultural areas (hay and pasture) exceeded P export from forests by a mere 20% and 30%, respectively. Our study identified urban runoff as another significant non-point nutrient source displaying the highest variability between dry and wet years. In particular, owing to the extensive urbanization in the Lake Simcoe watershed, urban runoff contributed nearly half of delivered P loading from tributaries into the lake. The nutrient loading management plan for Lake Simcoe calls for a reduction in P loading by ～40% from a long-term average of 72 t P y^?1 in 2002–2007 to 44 t P y^?1 by 2045. Our analysis emphasizes the importance of mitigating urban non-point sources together with efforts to control agricultural runoff.     

Assessment of site-specific agricultural Best Management Practices in the Upper East River watershed,Wisconsin, using a field-scale SWAT model

The Great Lakes “Priority Watershed” effort targeted the Upper East River watershed, a 116.5-km² tributary watershed to Wisconsin's Green Bay, to reduce its sediment and nutrients loads from agricultural sources. A Soil and Water Assessment Tool (SWAT) model was created to determine the effectiveness of agricultural Best Management Practices (BMPs) funded through the Great Lakes Restoration Initiative. The model was calibrated at the monthly time-step for flow, sediment, dissolved reactive phosphorus (DRP), total phosphorus (TP), nitrate, and total nitrogen (TN). Field- and watershed-scale sediment and nutrient reductions were calculated due to the implementation of 74 BMP combinations on dairy and cash grain rotations. Modeling results indicated that when multiple BMPs were placed on a field, especially those including filter strips and grassed waterways, sediment and nutrient loads generally were reduced more than single BMP implementation. The most effective in-field practice at reducing DRP and TP on dairy fields was a combination of 5 different BMPs: cover crops, crop rotation, nutrient management plan, reduced tillage, and a filter strip. Conservation cover was the single most effective practice at reducing sediment and nutrient yields. Sediment and nutrient loads decreased at the watershed scale as the quantity and coverage of BMPs increased. When all contracted BMPs were simulated at the watershed scale, sediment loads were reduced 2%, while TP, DRP, TN and nitrate loads were reduced 20%, 9%, 24%, and 17%, respectively. Modeling scenarios also indicated that over-winter manure storage was important to keep soluble nutrients out of waterways.     

The Contribution of Conservation Practices in Reducing Runoff, Soil Loss, and Transport of Nutrients at the Watershed Level

The lack of land use planning and the absence of conservation practices in a watershed can contribute to increased runoff, soil loss, and nutrient transport, which compromise the environmental quality in a watershed, especially the water resources. The objective of this study was to assess the contribution of conservation practices in reducing runoff and soil and nutrient losses using the Soil and Water Assessment Tool (SWAT) in the S?o Bartolomeu Stream Watershed, which is a significant watershed in Brazil. The modeling allowed us to identify critical areas regarding sediment yield, runoff, and nutrient loss. After that, conservation practices aimed at reducing the impacts of such processes were simulated. We also identified the most sensitive model parameters to simulate changes in management practices. Simulation results showed an average annual runoff (R) of 35?mm, average annual sediment yield (SY) of 51?t ha^-1?year^-1, 3.6?t ha^-1?year^-1 of total nitrogen (TN), and 1.6?t ha^-1?year^-1 of total phosphorus (TP). When considering the adoption of conservation practices, results showed an increase in water infiltration in the watershed and reductions of 18?%, 66?%, 25?%, and 30?% for R, SY, TN, and TP, respectively. Interventions which prioritize adequate management practices can be highly efficient and avoid changes in consolidated land uses.     

Non-Point Source Pollution from Abandoned Agricultural Land in the Great Lakes Basin

The contribution of abandoned farm fields to non-point source pollution in the Great Lakes Basin was studied on a 7.73 ha old field watershed near East Lansing, Michigan, that had been abandoned 18 years prior to the study. Exports of nitrogen were low and approached values expected for undisturbed forests in the Great Lakes Basin. Exports of phosphorus were slightly elevated compared to undisturbed forests. Exports and concentrations of both N and P were similar to values listed for cleared, unproductive land for the Great Lakes region by the recent nationwide survey of EPA. Annual exports of total P, PO₄–P, NO₃–N, NO₂–N, NH₄–N, Organic–N, Cl, Na, Ca, and suspended solids are given. Most exports of all constituents occurred during rainfall or snow-melt generated runoff events during the spring runoff period. Soil and soil-water nutrient concentrations were low, reflecting the low fertility of the study watershed. Nutrient concentrations in runoff reflect this low fertility and would be higher from recently abandoned farm land. The contribution of abandoned fields to nutrient loading of the Great Lakes would decrease from levels typical of agricultural runoff at time of abandonment to levels typical of undisturbed forests within 15 to 20 years following abandonment.     

Quantification of cormorant litter and nutrient deposition to Great Lakes island ecosystems

Ornithogenic nutrients derived from waterbirds such as the double-crested cormorant (Phalacrocorax auritus, Lesson) have been linked to habitat change within nesting colonies. For the islands of Lake Erie, where increasing cormorant populations and subsequent habitat change have spurred management activity, estimates of the quantity and chemical characteristics of avian-derived contributions are lacking. To evaluate the quantity and chemical characteristics of ornithogenic litterfall beneath a double-crested cormorant colony on a western Lake Erie island we investigated the mass of material and nutrient composition (PO₄^3 −, NO₃⁻ and NH₄⁺) reaching the forest floor under three nest densities (Low: 1–96 nests ha^− 1; Medium: 97–255 nests ha^− 1 and High: > 255 nests ha^− 1). As expected, litterfall (total mass) input differed among nest densities with the most substantial input (225.05 g/m² week^− 1) measured under High nest density conditions. Nutrient concentrations also showed increases with nest density to a point, where mean PO₄^3 − and NH₄⁺ concentrations showed no differences between Medium and High nest density sites. As well, NO₃⁻ concentrations were highest under Medium density, with no differences in this nutrient observed between Low and High density. Collectively, litterfall nutrient composition was similar to those linked to habitat changes in other waterbird colonies. Similarities in the concentrations of several nutrients between Medium and High nest density categories suggest that management actions aimed at reducing allochthonous nutrient contributions should try to sustain nest density at or below 96 nests ha^− 1.     

Periphytic algal biomass as a bioindicator of phosphorus concentrations in agricultural headwater streams of southern Ontario

Algal blooms in Lake Erie have worsened in recent decades and are driven by diffuse export of phosphorus (P) from a large stream network that drains predominately agricultural land. Given the diffuse nature of nonpoint source pollution, best management practices (BMPs) must target areas where P levels are high. This requires long-term watershed-wide monitoring programs that do not currently exist in many jurisdictions. Instead of conventional nutrient analyses that can be costly and time-consuming, we propose the use of periphyton biomass as a bioindicator of trophic status in low-order streams, where agricultural runoff first enters watercourses. We carried out 2-week in-stream bioassays to measure periphytic algal biomass (CHL_peri) in 19 low-order streams in southern Ontario across an agricultural gradient (8 % to 89 %). CHL_peri was significantly related to total P (TP) concentration (r² = 0.46; p = 0.0015) but was not significantly related to soluble reactive P (SRP). A relationship between TP and turbidity (r² = 0.52; p = 0.0007) is consistent with previous observations of increasing SRP uptake in streams draining agriculturally-dominated landscapes. Stream temperature (°C) was correlated with the proportion of agricultural land (R = 0.55; p = 0.019) and may reflect the warming effects of the sun in unshaded agricultural streams. This method involving substrate rods (Peristix) is cost-effective, requires very little training, and yielded data that were significantly related to TP concentrations in agricultural streams. We recommend that environmental agencies and landowners use this bioassay to identify areas for implementing BMPs to reduce P export from the Lake Erie watershed.     

农业非点源污染物在水塘景观系统中的空间变异性研究

农业非点源污染是流域不同景观结构对降雨的一种综合响应过程，景观格局影响非点源污染物的产生和运移。本文研究安徽鲍家塘子流域中水田、秧田、水塘、荒地、沟渠等不同土地利用类型的总磷(TP)、速效磷(DRP)在土壤中的空间分布，以及降雨径流过程中TP、DRP、溶解态总磷(DTP)、悬浮物(TSS)在多水塘景观结构中的动态变化。结果表明：养分在流域不同景观组分土壤中的空间变化，是人为干扰景观异质性的综合结果；磷素输出以颗粒态磷(TP-DTP)为主，在不同景观中具有显著性差异；施肥水田是最主要的“源”景观，在降雨径流过程大量释放养分和悬浮物；水塘、荒地、没有施肥水田持留养分和悬浮物，是流域的“汇”景观，水塘有效地持留颗粒态磷；水塘系统中的沟渠是非点源污染物的主要运移通道。因此，通过对流域土地利用和景观格局的优化调整，达到对流域景观中养分过程的有效管理，实现对农业非点源污染的控制。     

Nitrogen cycling in large temperate floodplain rivers of contrasting nutrient regimes and management

Hydraulic connection between channels and floodplains (“connectivity”) is a fundamental determinant of ecosystem function in large floodplain rivers. Factors controlling material processing in these rivers depend not only on the degree of connectivity but also on the sediment conditions, nutrient loads, and source. Nutrient cycling in the nutrient‐rich upper Mississippi River (MISS) is relatively well studied, whereas that of less eutrophic tributaries is not (e.g., St Croix River; SACN). We examined components of nitrogen cycling in 2 floodplain rivers of contrasting nutrient enrichment and catchment land use to test the hypothesis that N‐cycling rates will be greater in the MISS with elevated nutrient loads and productivity in contrast to the relatively nutrient‐poor SACN. Nitrate (NO₃^?‐N) concentrations were greatest in flowing habitats in the MISS and often undetectable in isolated backwaters except where groundwater inputs occurred. In the SACN, NO₃^?‐N concentrations were greatest in the flowing backwater where groundwater inputs were high. Ambient nitrification in the MISS was twice that in the SACN and tended to be lowest in the main channel. Denitrification was 3× greater in the MISS than that in the SACN, N‐limited in both rivers. Community production/respiration was >1 in the MISS and likely provisioned labile C to fuel microbial metabolism and dissimilatory NO₃^?‐N reduction, whereas the heterotrophic (production/respiration < 1) nature of the SACN likely limited microbial metabolism and NO₃^?‐N dissimilation. It appears that N‐cycling in the SACN was driven by groundwater, whereas that in the MISS was supported mainly by water column N‐sources.     

妫水河流域农业非点源污染负荷估算与分析

本文利用改进的输出系数模型对妫水河流域农业非点源污染负荷进行了估算,并且分别通过三分法,即分区、分类、分期的方法对结果进行了分析。结果表明:(1)2017年妫水河流域农业非点源污染物TN和TP的负荷量分别为1 402 214.9 kg/a,279 629.1 kg/a;(2)流域内各乡镇TN和TP的负荷量差异较大,旧县镇、永宁镇所贡献的TN、TP负荷量最多且单位负荷强度最大,属于重点治理区域;(3)流域内对TN的贡献最大的污染源为农业种植,对TP的贡献最大的污染源为农村生活污染,这两个污染源属于优先控制的污染源;(4)TN、TP负荷与降雨量呈现正相关关系,降雨冲刷是导致的污染物负荷增加的一个主要因素。农业非点源污染的三分研究结果为妫水河流域非点源污染治理与农业结构调整提供了科学依据。     

张家口冬奥会核心区非点源污染负荷估算

确定非点源污染负荷量对于张家口冬奥会核心区水环境安全具有重要意义。利用输出系数法,结合数字高程模型,根据2015年土地利用遥感数据及2007—2016年统计年鉴数据,计算了非点源污染输出负荷量和空间分布,分析了不同类型污染负荷的贡献。结果表明:(1)2007—2016年非土地利用因素污染源年输出的TN污染负荷量为1 154.25~17 540.39 kg;年输出COD污染负荷量为2 114.76~34 552.41 kg;年输出TP污染负荷量为51.29~842.54 kg;年输出NH₄+-N污染负荷量为34.40~514.88 kg。(2)土地不同利用类型输出的TN污染负荷量为33 372.94 kg;COD污染负荷量为21 453.45 kg;TP的污染负荷量为512.82 kg;NH₄+-N的污染负荷量为1 129.78 kg。(3)农业人口生活和耕地为输出非点源污染负荷量的主要污染源。(4)污染物输出负荷量的空间分布不均匀,东沟上游子流域和下游出口子流域的输出量比较高,太子城河中下游子流域输出负荷量相对较高。     

Nutrient Loading of Southern Lake Michigan by Dry Deposition of Atmospheric Aerosol

Aerosol samples and micrometeorological data were collected at 87°00’W 42°00’N in the mid southern Lake Michigan basin from May through September, 1977, to determine total phosphorus and nitrate-nitrite nitrogen particulate (TP + N) loadings. Hi-volume samplers with cellulose fiber filters and a meterological data collection system were operated on board the USEPA's R/V Roger R. Simons. A diabatic drag coefficient method was used to estimate aerosol deposition velocity (V_d), which had a mean value of 0.65 cm/s. TP + N concentration and V_d were determined for 42 distinct sampling periods of at least three to four hours each. A climatological data base was used to compute weighted average loading rates for TP + N. Dry deposition loading for the southern basin was found to be 0.15 to 0.18 × 10⁶ kg/year for P; N loading was 3.5 to 5.1 × 10⁶ kg/year. TP + N inputs via dry deposition account for 15% or more of all atmospheric nutrient inputs, and are significant nutrient sources to midlake waters.     

Sediment‐bound nutrient export from five small reservoir catchments and its implications for the Sudan savanna zone of Ghana

A study was carried out in the Sudan savanna zone in the Upper East Region of Ghana to assess the rate of sediment‐bound nutrient export (NE) into five small reservoirs (Dua, Doba, Zebilla, Kumpalgogo and Bugri) and to analyse the implications of this export. The catchment soils and reservoir sediments from the various study sites were sampled and analysed for their bulk density, particle size distribution and nutrient content. Assessment of the nutrient concentrations indicated that the reservoir sediments were richer not only in nutrients and organic carbon, but also in clay and silt, than the catchment soils, having enrichment ratios >1. Nutrient export rates (NE; kg ha^?1 year^?1) from the reservoir catchments ranged from 0.755 (±0.264) for OC, 0.104 (±0.0245) for N, 0.0020 (±0.0003) for P, 0.016 (±0.0038) for K, 0.009 (±0.0024) for Na, 0.113 (±0.017) for Ca and 0.027 (±0.0093) for Mg. These rates were lower than those of other studies, likely due to the low nutrient content in the catchment soils. The relationships established between NE and specific sediment yield (SSY) indicated the NE was positively correlated with SSY (R² = 0.66–0.98). The derived empirical equations can be satisfactorily used to predict the quantity of sediment‐bound plant nutrients lost from similar catchments and subsequently stored in the reservoir sediments. The study results also suggest the need for sustainable land management practices to forestall erosion in the catchment areas and to reduce reservoir sedimentation, for enhancement of the livelihoods of the communities in the study area.     

Snowmelt and its role in the hydrologic and nutrient budgets of prairie streams

Small watersheds in the Canadian Prairies are characterized by seasonally disconnected hydrologic networks whereby stream channels are hydrologically connected during snowmelt but have disconnected reaches throughout the remainder of the year. Snowmelt is the most significant hydrological event in the Canadian Prairies, yet few studies have investigated the role of snowmelt in the nutrient budget of prairie streams. We quantified hydrologic and nutrient dynamics during snowmelt for ten agricultural subwatersheds distributed along a gradient of human activity in the Red River Valley, Canada, to evaluate the timing of nitrogen (N) and phosphorus (P) export. Elevated concentrations of total P (TP) and total N (TN) were observed during the snowmelt peak, with maximum concentrations reaching 3.23 mg TP L(-1) and 18.50 mg TN L(-1). Dissolved P and N dominated the total nutrient pool throughout snowmelt, likely due to reduced erosion and sediment transport resulting from the combination of the flat topography, frozen soil and stream banks, and gradual snow cover melt. Significant correlations were observed between snowmelt N load (r=0.91; p<0.05) and both agricultural land cover and fertilizer usage, with a weaker correlation between snowmelt P load (r=0.81; p<0.05) and agricultural area. Our results showed that snowmelt plays a key role in nutrient export to prairie aquatic ecosystems and this may have serious impacts on downstream ecosystems. Land use management practices need to consider the snowmelt period to control nutrient loads to Lake Winnipeg and other waterbodies in the Great Plains.     

生态塘对稻田降雨径流中氮磷的拦截效应研究

农田养分的大量流失已成为农业面源污染的主要来源之一,生态塘兼具排水和生态湿地双重功效,研究其对稻田排水氮磷的拦截效应对于防治农业非点源污染具有重要意义。针对降雨径流条件下生态塘对降雨径流中氮磷的动态拦截效应以及降雨径流结束后氮磷在静水中的去除效应有待明了的需求,本文选取太湖西岸何家浜流域典型农田作为研究对象,并将该区域的塘堰改造为生态塘,研究了生态塘对水稻生长期内的三场降雨径流氮磷的拦截去除效果及降雨径流结束后氮磷在静水中的去除效应。研究结果表明:(1)在三场降雨过程中,生态塘对总氮(TN)的平均去除率为34.7%,总磷(TP)的平均去除率为34.8%;(2)生态塘对降雨径流中不同形态氮磷的去除率大小排序为氨氮(NH_4~+-N)颗粒态氮(PN)硝态氮(NO_3~--N),颗粒态磷(PP)溶解态磷(DP),且径流状态下水体垂向分层氮磷浓度分布随降雨进行而变化,总体分布规律为底层氮磷浓度大于表层氮磷浓度;(3)降雨径流结束后,TN在生态塘中的去除率为50.4%,TP在生态塘中的去除率为52.3%,塘2对TN、TP的去除率大于塘1与塘3,生态塘表现了较强的抗冲击自修复性。     

Geostatistically Derived Great Lakes USLE Monthly Rainfall Erosivity Factors

The R factor, an index of rainfall erosivity in the universal soil loss equation (USLE), fundamentally governs water related soil loss from agricultural plots and is based on well studied empirical relations. Soil particles and adsorbed contaminants from agricultural runoff inevitably end up in water-courses and ultimately the Great Lakes system, disturbing natural habitat, reducing water clarity and quality. We here use over 22 years of records containing hourly precipitation recordings for 453 meteorological-recording sites in Ontario, southwestern Quebec, Michigan, Ohio, Pennsylvania, and New York to estimate the R factor surrounding the lower Laurentian Great Lakes. We generate annual and monthly R factor maps for the major growing season months using geostatistical interpolation. Our annual and seasonal maps can help in land use planning within the regions of intense agriculture surrounding Lakes Huron, Erie, and Ontario. Moving north of the Great Lakes, we find greater proportional errors on estimated R values because of low meteorological-recording site density. We find a strong northeast trend in decreasing erosivity of rainfall in the study area. The mean R value for 453 sites was 1,599.13 MJ mm⁻¹ ha⁻¹ yr⁻¹, with a standard deviation of 591.74 MJ mm⁻¹ ha⁻¹ yr⁻¹. Results are in general agreement with other published work but show some spatial differences due to climatic and physiographic variations across our study area.     

生态沟渠对水稻不同生长期降雨径流氮磷的拦截效应研究

农田养分的大量流失已成为农业面源污染的主要来源之一,研究生态沟渠对稻田降雨径流氮磷拦截效应具有重要意义。针对水稻不同生长期内的降雨以及降雨的不同时段下生态沟渠对稻田径流氮磷的动态拦截效应研究缺乏的现状,本文选取太湖西岸何家浜流域典型农田作为研究对象,将该流域的自然排水沟渠改造为生态沟渠。研究了生态沟渠对水稻不同生长期内的3场降雨径流的氮磷去除效果。研究结果表明:(1)在3场不同强度的降雨过程中,生态沟渠对TN(总氮)的平均去除率为31.4%,TP(总磷)的平均去除率为40.8%;(2)生态沟渠对降雨径流不同形态氮磷的去除率大小为NH4+-N(氨氮)PN(颗粒态氮)NO3--N(硝态氮),PP(颗粒态磷)DP(溶解态磷),且PN和PP的去除率随沟渠径流量的增大而呈现下降趋势;(3)生态沟渠底泥总氮、总磷浓度在水稻的生长周期内呈现先增加后降低的趋势,说明生态沟渠具有一定的自净能力,对氮磷的拦截去除具有可持续性。     

Nutrient release from inundated land of a tropical reservoir (Nanhua reservoir,Taiwan)

Nutrient release from land inundated by creation of a tropical reservoir (Nanhua Reservoir, Taiwan) is investigated. Both the magnitudes of nitrogen and phosphorus which might be released from inundated land are determined on the basis of measurements of biomass and nutrients content of terrestrial vegetation and the nutrients content of topsoil. The fresh biomass of the terrestrial vegetation was estimated at 48600 kg/ha, with an equivalent dry biomass of 15500 kg/ha. After complete decay following submergence, the terrestrial vegetation could potentially contribute 242 kg of nitrogen and 37 kg of phosphorus per hectare. The kinetics of nutrient release from topsoil is determined on a laboratory scale for 80 days, which showed that the phosphorus release rate is averaged at 5.25 mg TP/m²/day in anaerobic conditions and 1.34 mg TP/m²/day in aerobic conditions, also the nitrogen release rate is averaged at 78.31 mg TKN/m²/day in anaerobic condition and 35.71 mg TKN/m²/day in aerobic conditions. Based on the actual operation status of Nanhua Reservoir during the initial operation period of 21 months after construction (from August, 1993 to Apnl, 1995) and nutrient release kinetics, the accumulated nutrient loads originating from the topsoil of the inundated land are about 81342 kg of nitrogen and 4701 kg of phosphorus.