Faruk Djodjic

Future agriculture with minimized phosphorus losses to waters: Research needs and direction

Abstract The series of papers in this issue of AMBIO represent technical presentations made at the 7th International Phosphorus Workshop (IPW7), held in September, 2013 in Uppsala, Sweden. At that meeting, the 150 delegates were involved in round table discussions on major, predetermined themes facing the management of agricultural phosphorus (P) for optimum production goals with minimal water quality impairment. The six themes were (1) P management in a changing world; (2) transport pathways of P from soil to water; (3) monitoring, modeling, and communication; (4) importance of manure and agricultural production systems for P management; (5) identification of appropriate mitigation measures for reduction of P loss; and (6) implementation of mitigation strategies to reduce P loss. This paper details the major challenges and research needs that were identified for each theme and identifies a future roadmap for catchment management that cost-effectively minimizes P loss from agricultural activities.

Topsoil and Subsoil Properties Influence Phosphorus Leaching from Four Agricultural Soils

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.

Conditional Phosphorus Index as an Educational Tool for Risk Assessment and Phosphorus Management

Abstract Phosphorus index (PI) is a risk-assessment tool that combines phosphorus (P) source factors and transport factors to rank the vulnerability of fields to P losses. Here we present the structure and concepts of conditional PI, developed as an educational and P-management tool adjusted for Swedish conditions. Because the significance of certain factors for P losses depends on their interplay with other factors, conditional rules are needed for a more accurate process description and quantification. Accounting for P losses through the soil profile, separate calculations for reactive and unreactive P and a changed approach to P loss assessment from erosion losses are some of the new features included in the tool presented here. The performance of the tool was tested by comparing the calculated PI values with measured annual P transport from seven observation fields included in a Swedish water quality monitoring program. This first test indicated that the tool could be used successfully for P loss risk assessment.

Turnover and Losses of Phosphorus in Swedish Agricultural Soils: Long-Term Changes, Leaching Trends, and Mitigation Measures

Transport of phosphorus (P) from agricultural fields to water bodies deteriorates water quality and causes eutrophication. To reduce P losses and optimize P use efficiency by crops, better knowledge is needed of P turnover in soil and the efficiency of best management practices (BMPs). In this review, we examined these issues using results from 10 Swedish long-term soil fertility trials and various studies on subsurface losses of P. The fertility trials are more than 50 years old and consist of two cropping systems with farmyard manure and mineral fertilizer. One major finding was that replacement of P removed by crops with fertilizer P was not sufficient to maintain soil P concentrations, determined with acid ammonium lactate extraction. The BMPs for reducing P leaching losses reviewed here included catch crops, constructed wetlands, structure liming of clay soils, and various manure application strategies. None of the eight catch crops tested reduced P leaching significantly, whereas total P loads were reduced by 36% by wetland installation, by 39 to 55% by structure liming (tested at two sites), and by 50% by incorporation of pig slurry into a clay soil instead of surface application. Trend analysis of P monitoring data since the 1980s for a number of small Swedish catchments in which various BMPs have been implemented showed no clear pattern, and both upward and downward trends were observed. However, other factors, such as weather conditions and soil type, have profound effects on P losses, which can mask the effects of BMPs.

The role of subsoil as a source or sink for phosphorus leaching.

The importance of subsoil features for phosphorus (P) leaching is frequently mentioned, but subsoil effects are still poorly documented. This study examined whether the subsoil of four agricultural Swedish soils (two sand and two clay) functioned as a source or sink for P leaching by measuring P leaching from intact soil columns with topsoil (1.05 m deep) and without topsoil (0.77 m deep) over 3 yr. One sandy soil with high topsoil P content (Olsen P, 84 mg kg) and high subsoil sorption capacity (P sorption index [PSI], 3.7 mmol kg) had low leaching of dissolved reactive P (DRP) from full-length and subsoil lysimeters (0.12 and 0.08 kg ha yr, respectively). The other sandy soil, with high Olsen P content in the topsoil and subsoil (27 and 19 mg kg, respectively) and low PSI in the subsoil (1.4 mmol kg), had high DRP leaching from full-length and subsoil lysimeters (3.33 and 3.29 kg ha yr, respectively). High P content at depth (Olsen P, 21 mg kg) in one clay soil resulted in relatively higher subsoil DRP contribution (89%) to total leaching than observed in the other clay soil (71%). These results indicate that the subsoil can act as source or sink for P leaching, depending on P content, degree of P saturation, and P sorption capacity, and therefore subsoil properties should be considered when selecting mitigation measures to reduce P leaching.

Long-term temporal dynamics and trends of particle-bound phosphorus and nitrate in agricultural stream waters

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.

Process Based Modelling of Phosphorus Losses from Arable Land

Improved understanding of temporal and spatial Phosphorus (P) discharge variations is needed for improved modelling and prioritisation of abatement strategies that take into account local conditions . This study is aimed at developing modelling of agricultural Phosphorus losses with improved spatial and temporal resolution, and to compare the accuracy of a detailed process-based model with a rainfall-runoff coefficient-based model. The process-based SWAT model (Soil and Water Assessment Tool) was implemented for five river basins in central Sweden, and results compared with the rainfall-runoff coefficient-based model WATSHMAN (Watershed Management System) for one of these river basins. Parameter settings and attribute values were adapted to Scandinavian soil conditions, crops and management practices. Model performance regarding flow dynamics was overall satisfactory. Comparable results were achieved at several scales. The modelled P load was of high accuracy for the days when monitoring data were available for validation, generally once a month. Modelled monthly P load did not fit as well with averaged monthly monitoring load values, mainly since monthly monitoring often partly or entirely misses the peak flows. The comparison of SWAT and WATSHMAN gave slightly better results for the process-based model (SWAT). Better spatial resolution for input data such as Soil-P content and agricultural management practices will be required to reach modelling results that enable identification of measures adapted to local conditions.

Barium as a Potential Indicator of Phosphorus in Agricultural Runoff

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.

Screening risk areas for sediment and phosphorus losses to improve placement of mitigation measures

Identification of vulnerable arable areas to phosphorus (P) losses is needed to effectively implement mitigation measures. Indicators for source (soil test P, STP), potential mobilization by erosion (soil dispersion), and transport (unit-stream power length-slope, LS) risks were used to screen the vulnerability to suspended solids (SS) and P losses in two contrasting catchments regarding topography, soil textural distribution, and STP. Soils in the first catchment ranged from loamy sand to clay loam, while clay soils were dominant in the second catchment. Long-term SS and total P losses were higher in the second catchment in spite of significantly lower topsoil STP. A higher proportion of areas in the second catchment were identified with higher risk due to the significantly higher risk of overland flow generation (LS) and a significantly higher mobilization risk in the soil dispersion laboratory tests. A simple screening method was presented to improve the placement of mitigation measures.

Assessing soil erodibility and mobilization of phosphorus from Swedish clay soils – Comparison of two simple soil dispersion methods

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.