Intensive Agricultural Catchment Research Articles

Quali-quantitative water behaviour in an intensive swine production catchment in the Atlantic Forest biome, Southern Brazil.

Agriculture is an essential economic activity in Brazil. However, it is also the main source of water quality degradation. Monitoring catchments with agricultural land use is a way to generate information on a scale to identify causes and sources of water quality degradation. This work used monitoring data derived from hydrology and the quality of surface and underground water in an intensive agricultural catchment in the Atlantic Forest biome. The Fortaleza River catchment is located in the western part of Santa Catarina state in southern Brazil and has 62 km2 of drainage area. Hydrological and water quality monitoring was conducted for 7years at two fluviometric stations, three lysimeters, one meteorological station, and one piezometer. Data on precipitation, temperature, water flow, surface runoff, drainage, and water quality were used. Statistical analyses were also developed. Precipitation between 2013 and 2019 presented a homogeneous distribution in monthly and annual data, with January and July the months with the highest and lowest values, respectively. Statistical difference in the average and Q95 flows was found in upstream and downstream fluviometric sections. In terms of quality, statistical differences were identified for ammonium, nitrate, and potassium concentrations, which had higher concentrations in lysimeter runoff, indicating direct influence of agricultural activity on water quality. Principal component analysis (PCA) indicated that (i) surface water presented a positive relationship in Component 1 for the magnesium-calcium, sulphate-chloride, and acetate-bromide groups and a negative relationship for phosphate-nitrate; (ii) in lysimeters, the positive relationship occurred for Component 2 for the phosphate-chloride and sulphate-nitrate groups and was negative for ammonium-lithium and calcium-potassium-magnesium; and (iii) in piezometer, positive relationships were found for chloride-sodium and phosphate-nitrite pairs, while negative relationships were found for calcium-magnesium.

The role of ponds in pesticide dissipation at the catchment scale: The case of the Save agricultural catchment (Southwestern France)

Pesticides are a major source of pollution for ecosystems. In agricultural catchments, ponds serve as buffer areas for pesticide transfers and biogeochemical hotspots for pesticide dissipation. Some studies have highlighted the specific impact of ponds on the dynamics of pesticides, but knowledge of their cumulative effect at the watershed scale is scarce. Hence, using a modelling approach, we assessed the cumulative role of ponds in pesticide transfer in an agricultural basin (Southwest of France, 1110 km2). The Soil and Water Assessment Tool (SWAT) model was used to model the Save basin, including 197 ponds selected with a Multi-Criteria Decision Aiding Model based on their pesticide interception capacities. The daily discharge, the suspended sediment loads and two herbicide loads (i.e. S-metolachlor and aclonifen) in dissolved and particulate phases were accurately simulated from January 2002 to July 2014 at a daily time step. The presence of ponds resulted in a yearly mean reduction at the watershed outlet of respectively 61 % and 42 % of aclonifen and S-metolachlor fluxes compared to the simulations in the absence of ponds. Sediment-related processes were the most efficient for pesticide dissipation, leading to a mean dissipation efficiency by ponds of 51.0 % for aclonifen and 34.4 % for S-metolachlor. This study provides a first quantification of the cumulative role of ponds in pesticide transfer at the catchment scale in an intensive agricultural catchment.

Land management policy shift influenced seasonal variation of erosion-induced nitrogen and phosphorus outputs from intensive agricultural catchment

A shift in policy to intensive agricultural production and land management often leads to excessive fertilizer application and accelerated erosion with consequent detrimental effects to water bodies. We investigated the impact of that shift by quantifying the spatial and temporal change in sediment sources and associated total nitrogen (TN) and total phosphorus (TP) pollutants output loads in an intensive agricultural catchment in North China across one year (November 2021–November 2022). We describe the implications of this work for intensive agriculture elsewhere in China and other countries. Seasonal sediment source apportionment was estimated at the catchment outlet using Berillium-7 (7Be) combined with compound-specific stable isotope (CSSI) signatures from sources and sediments. Diagnostic ‘fingerprints’ in MixSIAR were used to discriminate sediment sources between forest and crop farmland converted from forest (F + C(F)), crop farmland (C), and vegetable farmland (V). Our study identified F + C(F) as the dominant sediment source (mean 55.24 ± 2.91 %), intermediate on V (mean 30.06 ± 2.20 %), and least on C (mean 14.70 ± 2.13 %). Sedimentation ranged from 37.98 ± 3.02 to 89.60 ± 12.68 t·ha−1·event−1 and coincided with shifted land use policy and rainfall distribution. The TN and TP in sediment were both mainly derived from F + C(F) (averaged 22.27 ± 4.26 t·event−1 and 11.62 ± 2.28 t·event−1) and least from V (averaged 1.63 ± 0.29 and 2.09 ± 0.33 t·event−1). Despite being a significant sediment source, V contributed little sediment TN and TP input for eutrophication. Our findings imply that F + C(F) are diffuse sources of catchment pollution over the short term. These results describe the successful use of CSSI and 7Be to cost-effectively quantify the seasonal variation of sediment TN and TP loads from land-use-specific sources in the catchment under shifting land management policy in China with potential for use elsewhere. These findings enable soil conservation strategies and land management practices optimized for implementing targeted pollutant abatement initiatives in intensive agriculture in China and elsewhere.

Substratum Raking Can Restore Interstitial Habitat Quality in Swedish Freshwater Pearl Mussel Streams

Functional and oxygenated stream beds provide crucial habitat for multiple endangered stream taxa, including endangered freshwater mussels, fishes, and insect larvae. Stream bed restoration measures such as substrate raking are often applied to mitigate excess fine sediment introductions and stream bed colmation, yet such measures are controversial. In this study, we conducted a systematic experiment in which sites with stream bed raking and removal of macrophytes were monitored over two years and compared with before-treatment conditions and untreated reference sites in the Swedish Brånsån stream, which still contains a population of the endangered freshwater pearl mussel Margaritifera margaritifera. The stream bed restoration resulted in improved habitat quality, as evident from decreased substrate compaction, increased redox potential, and oxygen supply into the stream bed. In contrast to previous studies in Central European catchments with more intensive agricultural catchment uses, the effects of the restoration measure were much longer, extending over two years. Consequently, stream bed raking and macrophyte removal can be considered a useful and more long-lasting restoration measure than currently assumed, especially in streams where excess input of fine sediment has already been mitigated, where catchment land use is rather extensive, and where near-natural flow regimes still prevail.

Farm size increase alters the contribution of land use types to sources of river sediment

Farm size increase (FSI) accelerates soil erosion and sediment loss into the river. However, it is not clear how large-scale management, including FSI, changes the proportion of sediment from different land use types (LUTs) into rivers in intensive agricultural catchments. Thus, the aim of this study is to quantify and elucidate the contribution of different LUTs to river sediment before and after FSI using compound-specific stable isotopes (CSSI). The study was conducted in three parallel sub-watersheds in the sugarcane plantation region of subtropical southern China, each of which included four different LUTs (eucalyptus plantation, road, sugarcane plantation and channel). Following FSI, the contribution of sediment entering the river from the four LUTs was significantly different from that before FSI. The contribution per unit area of the road, sugarcane, and river channel to river sediment was significantly higher (P < 0.05) after FSI than before FSI. Overall, 1% increase in sugarcane farm size and stream channels caused > 2% and 0.3% increase contribution to river sediment, respectively. Field and road densities significantly correlated with river sediment sources. The FSI had a significant impact on the contribution of sediment entering the river from different LUTs within the watershed. Our results demonstrate that the increase river sediment depends on the increased hydrological connectivity of the watershed due to decreased field density and increased road density. Therefore, it is important to pay attention not only to the agricultural/economic benefits of FSI, but also the negative impacts on increasing erosion and ecological/environmental degradation.

Novel sediment source fingerprinting quantifying erosion-induced total nitrogen and total phosphorus outputs from an intensive agricultural catchment, North China

Intensive farming is a primary cause of increased sediment and associated nitrogen (N) and phosphorus (P) loads in surface water systems. Determining their contributing sources, pathways and loads present major challenges in the high-intensity agricultural catchments. Herein, we quantify the sediment sources and magnitude of sediment total N and total P from different sources using a novel application of compound-specific stable isotope (CSSI) and fallout radionuclides (FRNs) of 137Cs and 210Pbex in an intensive agricultural catchment in North China. Sediment sources from surface and sub-surface soils were estimated from FRNs fingerprint and accounted for 62 ± 7% and 38 ± 7% respectively, while surface soil from land uses that originated from hillslope were identified by CSSI fingerprint. Using a novel application of FRNs and CSSI sediment fingerprinting techniques, the dominant sediment source was derived from maize farmland (44 ± 0.1%), followed by channel bank (38 ± 7%). The sedimentation rate (13.55 ± 0.30 t ha−1 yr−1) was quantified by the 137Cs cores (0–60 cm) at the outlet of this catchment. The total N and total P in sediment were both mostly derived from maize farmland and least from channel banks. The channel banks are significant sediment sources but contribute little to the input of sediment N and P for eutrophication. It implies that chemically-applied farmlands are the main hotspots for catchment erosion control and pollution prevention. The novel application of FRNs and CSSI techniques cost-effectively quantified sediment N and P loads from different sources with a single visit to the catchment, enabling rapid assessment for optimizing soil conservation strategies and land management practices. Keywords: Sediment sources, Land use, N and P loads, Compound-specific stable isotope, Fallout radionuclides.

The influence of a flood event on the potential sediment control of baseflow phosphorus concentrations in an intensive agricultural catchment

The growth of periphyton in streams is enhanced by phosphorus (P) in baseflow. The likely control of P concentrations in baseflow can be approximated by the equilibrium P concentration (EPC0) of bed sediments. However, sediment composition changes with spatial scale and flood events. It is unknown if this affects EPC0. We sampled sediments in a main stem and headwater tributary of an agricultural catchment in Reporoa, New Zealand, before and after a scouring flood event (99th percentile of flows recorded since 1962). The tributary was chosen for its low slope and predominantly single land use (intensive dairying), which minimised the number of factors likely to affect the influence of the storm event. EPC0 values were significantly correlated to dissolved reactive P (DRP) in baseflow before, and after, the flood event, despite a decrease in the proportion of fines and total P in bed sediments. Both EPC0 and DRP concentrations increased towards the catchment outlet. This increase likely reflected new P-enriched sediments from dairy-farm runoff, but hyporheic zone samples suggested that shallow groundwater may also have played a role in supplying P to the water column. Despite diel variations in dissolved oxygen, DRP concentrations showed little variation during the day and matched EPC0 estimates. This work suggests that despite changes in sediment composition due to flood events, EPC0 is a useful reflection of daytime baseflow-DRP concentrations at sites along a stream network. However, further work is required to clarify if sediment-P exchange or groundwater control baseflow-DRP concentrations. These data also inform our understanding of the influence of sediment on delaying farm and catchment efforts to decrease in-stream DRP concentrations.

Source identification and budget evaluation of eroded organic carbon in an intensive agricultural catchment

Abstract Soil erosion affects the redistribution of soil and associated soil organic carbon (SOC) from different landforms, and it has significant implications regarding the fate of eroded SOC and terrestrial carbon sequestration. Despite the importance of soil erosion, few studies have evaluated the sources and budget of eroded SOC in the Loess Plateau region, which suffers from severe soil erosion. Based on an 11.3 m check dam sediment profile, we used the 137Cs activity and extreme rainfall events as dating methods to date sediment sequences. In addition, the natural abundance levels of the stable carbon isotope (δ13C) of bulk organic matter and a two-end-member mixing model were used to discriminate the sources of eroded SOC from different siltation stages retained by the check dam in an intensive agricultural catchment of the Loess Plateau, China. The eroded SOC captured by the check dam was compared to potential source materials from different landscape units, which included sloping cropland and gully surface soils (0–5 cm). The results showed that the check dam intercepted 98.5 Gg of eroded soil and 172.6 Mg of SOC. The eroded SOC was primarily sourced from sloping cropland, which contributed to 81.3% of the total SOC retained by the check dam, whereas the gully soils contributed to 18.7% during the entire siltation stage. Additionally, the contribution of sloping cropland to eroded SOC increased from 1960 to 1990 as a result of rainfall and anthropogenic activities. A total of 89.7 Mg SOC was lost during soil erosion processes at a rate of 0.17 Mg ha−1 yr−1 and accounted for approximately 30% of the total eroded SOC exported from the eroding areas. Our results indicate that the soil erosion process has been an important net source of SOC in the study catchment. The check dam served as a carbon storage and sequestration structure for the hilly loess region due to its beneficial conditions for carbon sequestration over broad temporal and spatial distributions.

Dissolved nitrous oxide (N2 O) dynamics in agricultural field drains and headwater streams in an intensive arable catchment

Indirect nitrous oxide (N2O) emissions produced by nitrogen (N) leaching into surface water and groundwater bodies are poorly understood in comparison to direct N2O emissions from soils. In this study, dissolved N2O concentrations were measured weekly in both lowland headwater streams and subsurface agricultural field drain discharges over a two-year period (2013–2015) in an intensive arable catchment, Norfolk, UK. All field drain and stream water samples were found to have dissolved N2O concentrations higher than the water–air equilibrium concentration, illustrating that all sites were acting as a net source of N2O emissions to the atmosphere. Soil texture was found to significantly influence field drain N2O dynamics, with mean concentrations from drains in clay loam soils (5.3 µg N L-1) being greater than drains in sandy loam soils (4.0 µg N L-1). Soil texture also impacted upon the relationships between field drain N2O concentrations and other water quality parameters (pH, flow rate, and nitrate (NO3) and nitrite (NO2) concentrations), highlighting possible differences in N2O production mechanisms in different soil types. Catchment antecedent moisture conditions influenced the storm event mobilisation of N2O in both field drains and streams, with the greatest concentration increases recorded during precipitation events preceded by prolonged wet conditions. N2O concentrations also varied seasonally, with the lowest mean concentrations typically occurring during the summer months (JJA). Nitrogen fertiliser application rates and different soil inversion regimes were found to have no effect on dissolved N2O concentrations, whereas higher N2O concentrations recorded in field drains under a winter cover crop compared to fallow fields revealed cover crops are an ineffective greenhouse gas emission mitigation strategy. Overall, this study highlights the complex interactions governing the dynamics of dissolved N2O concentrations in field drains and headwater streams in a lowland intensive agricultural catchment.

Reconnecting crop and cattle farming to reduce nitrogen losses to river water of an intensive agricultural catchment (Seine basin, France): past, present and future

Nitrate and pesticide contamination of surface and groundwater has become a major problem in intensive farming regions in Europe, with nitrate concentrations reaching values above the standard defined in 2000 by the European Water Framework Directive. In the Seine basin, a major issue is the closure and abandonment of drinking-water wells, which force water managers and drinking-water producers to explore solutions for water resource protection. Organic farming has appeared as a credible alternative to conventional farming, and this study explores the potential of organic farming to reconcile agricultural production and water quality. On the basis of agricultural statistics, survey questionnaires and experimental data, the nitrogen soil surface balance (N-SSB) has been established at the scale of a small 104-km2 catchment (The Orgeval sub-basin), representative of the intensive cash crop farming in the Seine basin. The N-surplus for arable land in specialized organic cash crop systems has been found to be half that of current conventional systems (15kg Nha−1yr−1 versus 30kgNha−1yr−1, respectively). The N-yield in organic systems is 21% lower than in conventional systems, but total fertilization (mostly symbiotic N fixation) is also 26% lower. Whereas 2–3 years of forage legume (e.g., alfalfa) as a starter crop of the typical 7- to 10-year diversified rotation builds up N soil fertility and helps prevent weeds without pesticides, the existence of an outlet for this fodder production is a limiting factor for the economic sustainability and the environmental benefits of these farming systems. Therefore, we explored the possibility of a reconnection of livestock and crop farming systems in the Orgeval catchment, a traditional dairy farming and Brie cheese production region. We calculated the N-SSB for this type of a reconnected livestock and cropping system and found a value very close to the specialized organic cash crop system with full utilization of fodder production, leading to profitable animal production, essentially as milk in this farm design. This reconnected system is compared with the estimated situation in 1955 before separation of plant and livestock production. Furthermore, the N-SSB values were converted into infiltrating sub-root concentrations and used as a boundary condition to a biogeochemical model. Organic cropping and organic reconnected livestock cropping systems result in a 50% reduction of surface water nitrate concentrations, a surface water quality 20% better than that reconstructed for 1955, with an overall higher protein production.

Incidental nutrient transfers: Assessing critical times in agricultural catchments using high-resolution data

Managing incidental losses associated with liquid slurry applications during closed periods has significant cost and policy implications and the environmental data required to review such a measure are difficult to capture due to storm dependencies. Over four years (2010–2014) in five intensive agricultural catchments, this study used high-resolution total and total reactive phosphorus (TP and TRP), total oxidised nitrogen (TON) and suspended sediment (SS) concentrations with river discharge data to investigate the magnitude and timing of nutrient losses. A large dataset of storm events (defined as 90th percentile discharges), and associated flow-weighted mean (FWM) nutrient concentrations and TP/SS ratios, was used to indicate when losses were indicative of residual or incidental nutrient transfers. The beginning of the slurry closed period was reflective of incidental and residual transfers with high storm FWM P (TP and TRP) concentrations, with some catchments also showing elevated storm TP:SS ratios. This pattern diminished at the end of the closed period in all catchments. Total oxidised N behaved similarly to P during storms in the poorly drained catchments and revealed a long lag time in other catchments. Low storm FWM P concentrations and TP:SS ratios during the weeks following the closed period suggests that nutrients either weren't applied during this time (best times chosen) or that they were applied to less risky areas (best places chosen). For other periods such as late autumn and during wet summers, where storm FWM P concentrations and TP:SS ratios were high, it is recommended that an augmentation of farmer knowledge of soil drainage characteristics with local and detailed current and forecast soil moisture conditions will help to strengthen existing regulatory frameworks to avoid storm driven incidental nutrient transfers.

Improving the identification of hydrologically sensitive areas using LiDAR DEMs for the delineation and mitigation of critical source areas of diffuse pollution

Identifying critical source areas (CSAs) of diffuse pollution in agricultural catchments requires the accurate identification of hydrologically sensitive areas (HSAs) at highest propensity for generating surface runoff and transporting pollutants. A new GIS-based HSA Index is presented that improves the identification of HSAs at the sub-field scale by accounting for microtopographic controls. The Index is based on high resolution LiDAR data and a soil topographic index (STI) and also considers the hydrological disconnection of overland flow via topographic impediment from flow sinks. The HSA Index was applied to four intensive agricultural catchments (~7.5–12km2) with contrasting topography and soil types, and validated using rainfall-quickflow measurements during saturated winter storm events in 2009–2014. Total flow sink volume capacities ranged from 8298 to 59,584m3 and caused 8.5–24.2% of overland-flow-generating-areas and 16.8–33.4% of catchment areas to become hydrologically disconnected from the open drainage channel network. HSA maps identified ‘breakthrough points’ and ‘delivery points’ along surface runoff pathways as vulnerable points where diffuse pollutants could be transported between fields or delivered to the open drainage network, respectively. Using these as proposed locations for targeting mitigation measures such as riparian buffer strips reduced potential costs compared to blanket implementation within an example agri-environment scheme by 66% and 91% over 1 and 5years respectively, which included LiDAR DEM acquisition costs. The HSA Index can be used as a hydrologically realistic transport component within a fully evolved sub-field scale CSA model, and can also be used to guide the implementation of ‘treatment-train’ mitigation strategies concurrent with sustainable agricultural intensification.

Daily nitrate losses: implication on long-term river quality in an intensive agricultural catchment of southwestern france.

High nitrate concentrations in streams have become a widespread problem throughout Europe in recent decades, damaging surface water and groundwater quality. The European Nitrate Directive fixed a potability threshold of 50 mg L for European rivers. The performance of the Soil and Water Assessment Tool model was assessed in the 1110-km Save catchment in southwestern France for predicting water discharge and nitrate loads and concentrations at the catchment outlet, considering observed data set uncertainty. Simulated values were compared with intensive and extensive measurement data sets. Daily discharge fitted observations (Nash-Sutcliffe efficiency coefficient = 0.61, = 0.7, and PBIAS = -22%). Nitrate simulation (1998-2010) was within the observed range (PBIAS = 10-21%, considering observed data set uncertainty). Annual nitrate load at the catchment outlet was correlated to the annual water yield at the outlet ( = 0.63). Simulated annual catchment nitrate exportation ranged from 21 to 49 kg ha depending on annual hydrological conditions (average, 36 kg ha). Exportation rates ranged from 3 to 8% of nitrogen inputs. During floods, 34% of the nitrate load was exported, which represented 18% of the 1998-2010 period. Average daily nitrate concentration at the outlet was 29 mg L (1998-2010), ranging from 0 to 270 mg L. Nitrate concentration exceeded the European 50 mg L potability threshold during 244 d between 1998 and 2010. A 20% reduction of nitrogen input reduced crop yield by between 5 and 9% and reduced by 62% the days when the 50 mg L threshold was exceeded.

Solute transport dynamics in small, shallow groundwater-dominated agricultural catchments: insights from a high-frequency, multisolute 10 yr-long monitoring study

Abstract. High-frequency, long-term and multisolute measurements are required to assess the impact of human pressures on water quality due to (i) the high temporal and spatial variability of climate and human activity and (ii) the fact that chemical solutes combine short- and long-term dynamics. Such data series are scarce. This study, based on an original and unpublished time series from the Kervidy-Naizin headwater catchment (Brittany, France), aims to determine solute transfer processes and dynamics that characterise this strongly human-impacted catchment. The Kervidy-Naizin catchment is a temperate, intensive agricultural catchment, hydrologically controlled by shallow groundwater. Over 10 yr, five solutes (nitrate, sulphate, chloride, and dissolved organic and inorganic carbon) were monitored daily at the catchment outlet and roughly every four months in the shallow groundwater. The concentrations of all five solutes showed seasonal variations but the patterns of the variations differed from one solute to another. Nitrate and chloride exhibit rather smooth variations. In contrast, sulphate as well as organic and inorganic carbon is dominated by flood flushes. The observed nitrate and chloride patterns are typical of an intensive agricultural catchment hydrologically controlled by shallow groundwater. Nitrate and chloride originating mainly from organic fertilisers accumulated over several years in the shallow groundwater. They are seasonally exported when upland groundwater connects with the stream during the wet season. Conversely, sulphate as well as organic and inorganic carbon patterns are not specific to agricultural catchments. These solutes do not come from fertilisers and do not accumulate in soil or shallow groundwater; instead, they are biogeochemically produced in the catchment. The results allowed development of a generic classification system based on the specific temporal patterns and source locations of each solute. It also considers the stocking period and the dominant process that limits transport to the stream, i.e. the connectivity of the stocking compartment. This mechanistic classification can be applied to any chemical solute to help assess its origin, storage or production location and transfer mechanism in similar catchments.

Forecasting the decline of excess soil phosphorus in agricultural catchments

AbstractMeasures within the European Union Nitrates Directive National Action Programmes in many member states aim to reduce soil phosphorus (P) levels from excessive to agronomically optimum. This is to reduce the risk of diffuse P losses to water and ultimately help achieve the Water Framework Directive targets of good water quality status. In this study, a ‘Soil P Decline’ model was used to evaluate this expectation for soils in four intensive agricultural catchments. Realistic annual P‐balance scenario deficits (−30, −15, −7 kg P/ha) were used to estimate the average time required for soils to decline to optimum levels in two predominantly arable and two grassland catchments with excessive soil test P (STP). Depending on the STP concentration and total P reserves, for the smallest field P deficit scenario (−7 kg P/ha), the model predictions were for an average of between 5 and 20 yr for agronomically optimum levels to be reached. Under the largest P deficit scenario (−30 kg P/ha), it was forecast to take between 2 and 10 yr. These predictions highlight the likely time lag that exists between implementation of soil P mitigation regulations and the desired outcome of few or no fields with excessive soil P. Expectations for water quality improvement through diffuse P source mitigation must also factor in additional time for P decline model uncertainty, land management variability and time for P sources to transfer to and within river networks.

Seasonal exposure of fish to neurotoxic pesticides in an intensive agricultural catchment, Uma‐oya, Sri Lanka: Linking contamination and acetylcholinesterase inhibition

The annual cultivation pattern in the Uma-oya catchment in Sri Lanka is characterized by Yala and Maha rainfall periods and associated cropping. Two cultivation seasons were compared for pesticide residues: base flow, field drainage, and the runoff and supplementary sediment data for three sites in the catchment. Organophosphate and N-methyl carbamate pesticide analysis confirmed a higher concentration in the Yala season with low-flow conditions. Acetylcholinesterase (AChE) activity was measured by standard spectrometry in the brain, muscle, and eye tissues of three freshwater cyprinid fishes, Garra ceylonensis, Devario malabaricus, and Rasbora daniconius from three study sites during months overlapping two seasons in 2010 (December) and 2011 (July). Baseline AChE data were measured from fish samples from a forested reserve in the Knuckles. A 73% inhibition in muscle AChE activity in G. ceylonensis was associated with intense pesticide exposure months in the Yala season. The AChE inhibition more than 70% in G. ceylonensis eyes in both Yala (76%) and Maha (72.5%) seasons indicates particular sensitivity of eye tissue to inhibitors. The less dramatic AChE inhibition in the eye tissues in D. malabaricus and R. daniconius in both seasons indicates exemplary protective capacity of muscle AChE in fish. The highest inhibition of AChE (up to 60% in brain and up to 56% in muscle AChE activity in R. daniconius and up to 47.8% in brain and up to 64.6% in muscle AChE activity in D. malabaricus) occurred during the Yala season. Tissue AChE activity and physiological activity in fish were correlated. The results collectively indicate that AChE is a consistent biomarker for diffused contaminant exposure in agricultural catchments.

Subsurface lateral flow from hillslope and its contribution to nitrate loading in streams through an agricultural catchment during subtropical rainstorm events

Abstract. Subsurface lateral flow from agricultural hillslopes is often overlooked compared with overland flow and tile drain flow, partly due to the difficulties in monitoring and quantifying. The objectives of this study were to examine how subsurface lateral flow generated through soil pedons from cropped hillslopes and to quantify its contribution to nitrate loading in the streams through an agricultural catchment in the subtropical region of China. Profiles of soil water potential along hillslopes and stream hydro-chemographs in a trenched stream below a cropped hillslope and at the catchment outlet were simultaneously recorded during two rainstorm events. The dynamics of soil water potential showed positive matrix soil water potential over impermeable soil layer at 0.6 to 1.50 m depths during and after the storms, indicating soil water saturation and drainage processes along the hillslopes irrespective of land uses. The hydro-chemographs in the streams, one trenched below a cropped hillslope and one at the catchment outlet, showed that the concentrations of particulate nitrogen and phosphorus corresponded well to stream flow during the storm, while the nitrate concentration increased on the recession limbs of the hydrographs after the end of the storm. All the synchronous data revealed that nitrate was delivered from the cropped hillslope through subsurface lateral flow to the streams during and after the end of the rainstorms. A chemical mixing model based on electricity conductivity (EC) and H+ concentration was successfully established, particularly for the trenched stream. The results showed that the subsurface lateral flow accounted for 29% to 45% of total stream flow in the trenched stream, responsible for 86% of total NO3−-N loss (or 26% of total N loss), and for 5.7% to 7.3% of total stream flow at the catchment outlet, responsible for about 69% of total NO3−-N loss (or 28% of total N loss). The results suggest that subsurface lateral flow through hydraulically stratified soil pedons have to be paid more attention for controlling non-point source surface water pollution from intensive agricultural catchment particularly in the subtropical areas with great soil infiltration.

Seasonal and interannual variations of nitrate and chloride in stream waters related to spatial and temporal patterns of groundwater concentrations in agricultural catchments

AbstractNitrate concentrations in streamwater of agricultural catchments often exhibit interannual variations, which are supposed to result from land‐use changes, as well as seasonal variations mainly explained by the effect of hydrological and biogeochemical cycles. In catchments on impervious bedrock, seasonal variations of nitrate concentrations in streamwater are usually characterized by higher nitrate concentrations in winter than in summer. However, intermediate or inverse cycles with higher concentrations in summer are sometimes observed. An experimental study was carried out to assess the mechanisms that determine the seasonal cycles of streamwater nitrate concentrations in intensive agricultural catchments. Temporal and spatial patterns of groundwater concentrations were investigated in two adjacent catchments located in south‐western Brittany (France), characterized by different seasonal variations of streamwater nitrate concentrations. Wells were drilled across the hillslope at depths ranging from 1·5 to 20 m. Dynamics of the water table were monitored and the groundwater nitrate and chloride concentrations were measured weekly over 2 years. Results highlighted that groundwater was partitioned into downslope domains, where denitrification induced lower nitrate concentrations than into mid‐slope and upslope domains. For one catchment, high subsurface flow with high nitrate concentrations during high water periods and active denitrification during low water periods explained the higher streamwater nitrate concentrations in winter than in summer. For the other catchment, the high contribution of groundwater with high nitrate concentrations smoothed or inverted this trend. Increasing bromide/chloride ratio and nitrate concentrations with depth argued for an effect of past agricultural pressure on this catchment. The relative contribution of flows in time and correlatively the spatial origin of waters, function of the depth and the location on the hillslope, and their chemical characteristics control seasonal cycles of streamwater nitrate concentrations and can influence their interannual trends. Copyright © 2004 John Wiley &amp; Sons, Ltd.

The Impact of Agriculture On Dissolved Nitrogen and Phosphorus Cycling in Temperate Ecosystems

Although most research has focussed on inorganic nutrient forms of nitrate (NO− 3) and phosphorus (PO3 4) in runoff and receiving waters, nitrogen loss from agricultural land can also occur in organic and ammonium-nitrogen form; phosphorus losses, although often dominated by particulate transport, may occur in soluble organic and inorganic form. Furthermore, fluxes between different species may take place during transport from the land to the stream and as a result of in-stream, in-river or in-lake transformations. Knowledge of the spatial and temporal variation in all nitrogen species and phosphorus fractions in a drainage basin is therefore essential if the wider environmental significance of elevated nutrient concentrations in natural waters are to be assessed. This paper reviews recent work on N and P losses from agricultural land and presents some results from two intensive agricultural catchments: Slapton, Devon and the river Windrush catchment in the Cotswolds.