Methods for estimating sediment and phosphorus (P) transfer from agricultural land to surface waters are needed to mitigate their adverse impact on water quality. This study quantified sediment and P losses from five agricultural fields and evaluated the patterns observed based on the intrinsic risk of sediment and P mobilization from the soil, together with field topographical and hydrological data. The mobilization risk was estimated using the environmental soil dispersion test DESPRAL. High‐resolution LiDAR elevation data and crop management data were used to describe transport and delivery of the material mobilized. Annual flow‐weighted suspended solids (SS) concentration in drainage water ranged from 48 to 374 mg/L, total P (TP) from 0.12 to 0.39 mg/L and unreactive P (UP) from 0.08 to 0.33 mg/L. The mobilization risk, measured as turbidity in the aliquot recovered from the dispersion test, varied from 781 to 2310 nephelometric turbidity units (NTU). The method proved to be efficient in describing and differentiating sediment and P mobilization potential between fields. The topographical data also showed large differences between fields, with the length–slope (LS) parameter varying from 0.037 to 0.999. Based on assessments of source (mobilization risk) and transport factors (LS, crop management data), it was possible to characterize fields as favoured or limited in terms of source and transport and to explain the long‐term SS and P losses observed at field scale.

Eutrophication, a major problem in many fresh and brackish waters, is largely caused by nonpoint-source pollution by P from agricultural soils. This lysimeter study examined the influence of P content, physical properties, and sorption characteristics in topsoil and subsoil on P leaching measured during 21 mo in 1-m-long, undisturbed soil columns of two clay and two sandy soils. Total P losses during the period varied between 0.65 and 7.40 kg ha. Dissolved reactive P was the dominant form in leachate from the sandy soils and one clay soil, varying from 48 to 76%. Particulate P dominated in leachate from the other clay soil, where low pH (5.2) in the subsoil decreased aggregate stability and thereby probably increased the dispersion of clay particles. Phosphorus leaching was small from soils with high P sorption index (PSI) and low P saturation (<10% of PSI) in the subsoil, even though extractable P (Olsen P) in the topsoil was high, and large from a soil with low sorption capacity and high P saturation (>35% of PSI) in the profile. High sorption capacity in the subsoil was more important for P leaching in sandy soils than in clay soils with macropore flow, where the effect of high sorption capacity was reduced due to less interaction between percolating water and the soil matrix. The results suggest that P leaching is greatly affected by subsoil properties and that topsoil studies, which dominate current research, are insufficient for assessing P leaching in many soils.

The effects of soil type, crop rotation, fertilizer type and application rate on the composition of organic phosphorus (P) compounds in soils from four sites in a Swedish long‐term fertilizer experiment were investigated with 31P‐NMR. Soil textures investigated were loamy sand, sandy loam, silty clay loam and clay. Phosphorus has been added to the soils since the 1950s and 1960s at four different rates in the form of either mineral fertilizer or a combination of manure and mineral fertilizer. Results show that in soils receiving no P addition, most of the soil P was present in the form of phosphate monoesters (60–70%, depending on soil type). However, a P addition equivalent to the amount of P removed annually by harvest altered this relationship so that the soils were dominated by orthophosphate instead. This trend became more obvious with increasing P addition. At the greatest P application rate, orthophosphate comprised 70% or more of the total extracted P in all the soils. These changes in the soil were due entirely to increase in orthophosphate, because the amounts of monoesters did not change with increasing P additions. This was true both for mineral fertilizer and the combination of manure and mineral fertilizer P. Soil type and crop rotation did not influence the results. The results indicate that there is no apparent build‐up of organic P in the soils, but that P addition mainly affects the orthophosphate amounts in the soils regardless of form or amount of fertilizer.

Diffuse losses of phosphorus (P) from arable land are often related to soil P and P amendments. We follow the in situ dynamics of plant‐available [ammonium lactate/acetic acid (P‐AL)] and easily soluble (0.01 m CaCl2) P during 1 year after fertilizer application at five sites in long‐term fertility field experiments in Sweden with three different soil P levels and amendment rates and two different crop rotation systems. Clear differences between soils and P treatments were found. These can to a large extent be explained by the amount of applied of P and soil sorption capacity. The ‘bell curve’ indicating the development of plant‐available P can be described successfully using Gaussian modelling. Strong correlation between plant‐available and easily soluble P as determined by the above‐mentioned methods shows that the existing agronomic soil test P can be a good indicator even for easily soluble P, especially if other soil properties such as soil sorption capacity are taken into account. From the management standpoint, small increases in both plant‐available and easily soluble P as in the P replacement treatment indicate that such a management strategy may reduce environmental risks as a result of P amendments. Increases in P amendments above the agronomic optimum increased plant‐available P for a considerable time after P amendment (2–4 months) to indicate high environmental risk.

Abstract Identification of main contributing sources and critical source areas is a precondition for a cost-effective abatement strategy. This task is even more complicated in small ungauged catchments where inadequate amount of input, calibration, and validation data exists and thereby limit the application of process-based, parameter-heavy models. In this study, we present methodology to assess spatial variations in erosion and P losses, and based on source-apportionment and erosion modeling, to develop a base for source- and site-specific abatement countermeasures. Water quality monitoring program was developed to cover spatial variability in erosion and phosphorus (P) losses. Conducted soil survey detected high variations in soil P content within the catchment and strong correlation between soil P test and easily soluble P. A simple water balance model, calibrated for the comparable catchment with existing water flow measurements, was applied to determine water discharge, providing necessary input data for calculations of loads and source-apportionment. Source-apportionment modeling based on results from water discharge modeling, water quality monitoring program and soil survey identified P losses from arable fields as main source of P to two small lakes situated in the catchment, contributing roughly a half of total P load to the lakes. Inclusion of an assumed but previously undocumented point source considerably improved the performance of source-apportionment model. Existence of such a point-source was afterwards confirmed by results from detailed water quality monitoring program. Erosion modeling based on high-resolution digital elevation data identified areas most prone to erosion, and model results were compared to farmers' observations and analyses of 137Cs. Use of a spectrum of models and assessment tools formed a broad support for further steps in abatement efforts, and created concrete discussion base for implementation of detailed site-specific countermeasures.

Abstract Erodibility of Swedish clay soils estimated according to the existing methods is usually low, but high levels of suspended solids and attached unreactive phosphorus have been recorded in drainage water from fields and catchments dominated by clay soils. Inherent susceptibility to soil erosion is usually assessed through aggregate stability studies or dispersion tests. The latter are simple to perform and produce good results when compared against runoff lysimeter experiments. The environmental soil test to determine the potential for sediment and phosphorus transfer in runoff from agricultural land (DESPRAL) and soil suspension turbidity (SST) dispersion tests, which differ in soil–liquid ratio and shaking and settling times, were compared here for their ability to indicate the erodibility of 10 Swedish clay soils. The tests proved to be significantly correlated (r=0.78), but DESPRAL showed higher repeatability (r i =0.995) than SST (r i =0.824). Variation in soil dispersion was explained by clay, sand and organic matter content in DESPRAL and by clay and sand content in SST. An additional study on the effect of soil storage duration on dispersion (DESPRAL test) in 15 soil samples showed that storage had no effect on some soils, but significantly decreased dispersion in others after only 8 weeks. Therefore, soil dispersion tests should be performed as soon as possible after sample drying. The DESPRAL and SST tests proved to be a good option for estimating the erodibility factor K in the Revised Universal Soil Loss equation under Swedish conditions and were able to differentiate the susceptibility to sediment losses for different clay soils. They provided an indirect measure of the amounts of sediment and P mobilized, but further work is needed to calibrate them against measured values at field and catchment scale.

Abstract One problem in evaluating efforts to reduce phosphorus (P) and nitrogen (N) losses to waters is that variations in weather conditions cause nutrient concentrations and waterflow to vary. Analyses of biweekly stream water samples collected manually from two small, neighbouring Swedish agricultural catchments with clay soil (E23 and E24) demonstrated unpredictability in P and N concentrations. However, particulate P (PP) concentrations in the two separate catchments, usually sampled within 2–3 hours on the same day, were clearly correlated to each other (Spearman correlation coefficient r=0.70). Corresponding nitrate–nitrogen (NO3–N) concentrations were also correlated to each other (r=0.79). Particulate P concentrations could reasonably be predicted from suspended solid (SS) concentrations above base flow (BF) in both catchments (regression coefficient R 2=0.84 and 0.86, respectively). In the period 1993–2009, before eutrophication control programmes were introduced in catchment E23, there was no general trend in PP or SS in either catchment. Mean PP (0.13 mg L−1) predicted (R 2=0.88) from high-resolution (15 minute) turbidity concentrations was significantly higher than flow-weighted mean PP concentration estimated from discrete samples (0.10 mg L−1) collected manually at the catchment E23 outlet. Mean PP concentration estimated directly from flow-proportional sampling was also higher. High synoptic concentrations of PP (up to 0.65 mg L−1) were recorded along the open reach of the stream in the ascending limb of high-flow pulses. Using high-resolution monitoring at the catchment outlet, episodes with a clear clockwise hysteresis effect for PP concentration (seen as turbidity) were frequently observed. By contrast, the NO3–N peak appeared 4–7 hours after the flow peak and anticlockwise hysteresis was observed. Significant erosion along stream banks may take place, and the degree of erosion was estimated based both on farmers’ observations and on results from a distributed erosion model (USPED). Monitoring and erosion mapping are currently being used in practical remedial work.

Soil erosion and nutrient leaching from terrestrial systems to rivers, lakes and marine environments cause deteriorating water quality and eutrophication. In all the countries of Northern Europe, agriculture is considered to be responsible for the greatest contribution of phosphorus (P) and high contribution of nitrogen (N) to coastal waters. Recently, there has been great pressure from both the environmental and agricultural sector to target the environmental measures at the areas with the highest risk for nutrient leaching and loading. Topographic, hydrologic, geomorphologic and agronomic factors often combine to make erosion and leaching from certain areas higher and more detrimental to the aquaculture than from others. Therefore, methods to identify and prioritise agri-environmental measures on these nutrient-vulnerable areas are desirable. This report examines data availability and methodology to identify the critical source areas in the Baltic Sea Region (BSR) countries. Here critical source areas are comprised mainly of erosionand phosphorus-vulnerable areas that can often also be related to the biosecurity risk of animal husbandry. Availability, determination methods and quality in basic background data required for the inventories vary widely in the Baltic Sea basin countries. Background data should include spatially detailed information from elevation, river networks, soil (soil type, P status etc.) and agricultural management (plant cover, fertiliser rates, livestock density etc.). Risk assessments are usually made at the municipal or catchment level, depending on which regional level the statistical data are available. The differences in the soil classification systems, soil P analysis and accuracy of the data needed for the mapping prevent uniform assessments and comparisons between the countries. The accuracy of the existing risk maps is difficult to verify with water quality observations, since the observations are scarce, especially from individual risk areas. Erosion risk maps are produced mostly with USLE based methods, which are also suitable for mapping areas at risk of P leaching. In USLE-maps, the risk areas are mainly located on steeply sloped fields. USLE describes the high risk areas by surface processes. Thus, the transport of solids and P through soil matrix and via the macropores is ignored in USLE examinations. If the calculation takes into account the distance to water and if the channel map is accurate, also fields further away from the water bodies can be classified as risk areas. Meanwhile, when topographic mapping is used as the index calculation methodology, flat areas will be classified as risk areas because this method puts weight on gentle slopes with fairly large catchment areas above them. The third option is based on physical GIS-based models, which can model simultaneously hydrology and nutrient transport. In general, these models require a lot of input data and in lack of them the possibility of erroneous results increases. The P-index is often considered to be a cost-effective tool to reduce P leaching. The major challenges are lack of data (mainly on soil P status), and uncertainties and the need for additional validation of the model. Areas with high animal density and high risk of surface runoff or erosion are potential high-risk areas as regards biosecurity. By combining relevant maps, such as animal density and erosion risk areas, potential high-risk biosecurity areas can be identified. It would be important to improve the availability of more accurate, larger-scale data for the use of researchers and designers. The central issues in presenting the risk areas are accuracy, objectivity and clarity. High-risk areas and fields should be shown as objectively as possible and after solving their locations, possible mitigation measures to reduce risks in the problematic areas could be discussed and agreed with the stakeholders.

In many catchments, anthropogenic input of contaminants, and in particular phosphorus (P), into surface water is a mixture of agricultural and sewage runoff. Knowledge about the relative contribution from each of these sources is vital for mitigation of major environmental problems such as eutrophication. In this study, we investigated whether the distribution of trace elements in surface waters can be used to trace the contamination source. Water from three groups of streams was investigated: streams influenced only by agricultural runoff, streams influenced mainly by sewage runoff, and reference streams. Samples were collected at different flow regimes and times of year and analyzed for 62 elements using ICP-MS. Our results show that there are significant differences between the anthropogenic sources affecting the streams in terms of total element composition and individual elements, indicating that the method has the potential to trace anthropogenic impact on surface waters. The elements that show significant differences between sources are strontium (p < 0.001), calcium (p < 0.004), potassium (p < 0.001), magnesium (p < 0.001), boron (p < 0.001), rhodium (p = 0.001), and barium (p < 0.001). According to this study, barium shows the greatest potential as a tracer for an individual source of anthropogenic input to surface waters. We observed a strong relationship between barium and total P in the investigated samples (R(2) = 0.78), which could potentially be used to apportion anthropogenic sources of P and thereby facilitate targeting of mitigation practices.

Abstract Agricultural non-point source pollution by plant nutrients and pesticides can cause severe environmental disturbances, such as deterioration in the quality of surface water and groundwater. In order to prevent this, the development and implementation of appropriate countermeasures are necessary, which requires knowledge of critical soil functions in the vadose zone. The nutrient source, the transport pathway and the availability of solutes in soil are some important conditions that affect leaching. Our results show that organic nitrogen sources are often more susceptible to leaching than inorganic N fertilizers, due to poor synchronization between the N demand of the crop and the release of inorganic N from the organic N source. Large amounts of leachable N are left in the soil after the growing season. Preferential flow, in combination with where soil solutes occur, is critical for establishing safe loading rates. In some cases, the solute is located in smaller pores of the soil matrix, and is thereby protected against preferential flow and leaching. In other cases, especially soon after application of a fertilizer or pesticide, transient flow-peaks rapidly displace the solutes through macropores in the vadose zone, which can cause large leaching loads and associated water-quality problems.