Katarina Kyllmar

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.

Runoff and nutrient losses during winter periods in cold climates—requirements to nutrient simulation models

Large areas in Europe may experience frozen soils during winter periods which pose special challenges to modelling. Extensive data are collected in small agricultural catchments in Nordic and Baltic countries. An analysis on measurements, carried out in four small agricultural catchments has shown that a considerable amount of the yearly nutrient loss occurs during the freezing period. A freezing period was defined as the time period indicated by the maximum and minimum points on the cumulative degree-day curve. On average 6-32% of the yearly runoff was generated during this period while N-loss varied from 5-35% and P loss varied from 3-33%. The results indicate that infiltration into frozen soils might occur during the freezing period and that the runoff generating processes, at least during a considerable part of the freezing period, are rather similar compared to the processes outside the freezing period. Freeze-thaw cycles affect the infiltration capacity and aggregate stability, thereby the erosion and nutrient losses. The Norwegian catchment had a high P loss during the freezing period compared to the other catchments, most likely caused by catchment characteristics such as slope, soil types, tillage methods and fertiliser application. It is proposed to use data, collected on small agricultural dominated catchments, in the calibration and validation of watershed management models and to take into account runoff and nutrient loss processes which are representative for cold climates, thereby obtaining reliable results.

Estimating Reduction of Nitrogen Leaching from Arable Land and the Related Costs

Abstract The EU Water Framework Directive will require river-basin management plans in order to achieve good ecological status and find the most cost-efficient nitrogen (N) leaching abatement measures. Detailed scenario calculations based on modeling methods will be valuable in this regard. This paper describes the approach and an application with a coefficient method based on the simulation model SOILNDB for quantification of N leaching from arable land and for prediction of the effect of abatement scenarios for the Rönneå catchment (1900 km2) in southern Sweden. Cost calculations for the different measures were also performed. The results indicate that the individual measures—cover crop and spring plowing, late termination of ley and fallow, and spring application of manure—would only reduce N leaching by between 5% and 8%. If all measures were combined and winter crops replaced by their corresponding spring variants, a 21% reduction in N leaching would be possible. However, this would require total fulfillment of the suggested measures.

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.

Soil erosion in Nordic countries – future challenges and research needs

In Europe, water, wind and tillage erosion are common threats to soil quality (Verheijen et al., 2012). In the Nordic countries the main problem of erosion is generally that phosphorus (P), pesticides and other water pollutants attached to eroded material are lost to recipient waters. Erosion is monitored from plots, fields, and in streams and rivers (Boardman & Poensen, 2006). The amount of soil erosion measured from small agricultural catchments dominated by silty clay soils in Norway and Sweden varied largely in space (Table I). The range measured at the outlet of the small streams varied between 0.01 and 3.67 t ha 1 yr . In Lithuania, the combined variation in space, time and topography is huge and soil losses from 0.01 to 19 t ha 1 yr 1 have been measured with surface runoff from downstream slopes (Kinderiene & Karcauskiene, 2012). In general, the spatial variation between different catchments is greater (varying with a factor of more than hundred) than the variation between years (varying with a factor of 10). The total P losses in Norway and Sweden varied between 0.03 and 4.69 kg ha 1 yr 1 (Table I). In the Swedish catchments, high P losses can be related to factors such as clay and clay loam soils, medium to high precipitation and large proportions of annual crops in the catchments (Kyllmar et al., 2006). In the Norwegian catchments, the P losses differ depending on production system, soil types an water flow pathways (Bechmann et al., 2008). In arable production systems, erosion is the main P loss process. At the plot scale, Skøien and Børresen (2012) found that total P losses are closely correlated to soil losses, whereas Ulén et al. (2012a) found that particulate P concentrations can be reasonably wellpredicted from suspended sediment (SS) concentrations above base flow in a Swedish catchment. Land levelling has caused high erosion from arable fields in Norway (Øygarden et al., 2006). Roughly recalculated to arable land based on source apportionment, Norwegian studies indicate that soil losses from catchments with artificially levelled arable land may be even higher than 2 t ha 1 yr 1 (Bogen et al., 1993; Bechmann et al., 2008). At the field scale, monitoring of soil losses in surface and subsurface runoff from a land levelled arable field in Norway showed variations from 0.09 to 3 t ha 1 yr 1 (Bechmann et al., 2011). At the plot scale, measurements of soil losses showed even higher values, up to 7.6 t ha 1 yr 1 (Lundekvam, 2007; Skøien & Børresen, 2012). The tolerable rate of erosion for both agricultural production and water quality can be set to approximately 1 t ha 1 yr 1 (Verheijen et al., 2012). Standard potential erosion risk in Norway is defined for areas with the traditional autumn ploughing. Erosion risk below 500 kg ha 1 yr 1 is defined as low, from 500 to 2000 kg ha yr 1 as medium, from 2000 to 8000 kg ha 1 yr 1 as high and above 8000 kg ha 1 yr 1 as very high.

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.

Temporal trends in phosphorus concentrations and losses from agricultural catchments in the Nordic and Baltic countries

This paper in a uniform manner examines temporal trends in phosphorus (P) concentrations and losses from small and well-monitored agricultural catchments in the Nordic and Baltic countries. Thirty-four catchments (range 0.1–33 km2) in Norway (8), Denmark (5), Sweden (8), Finland (4), Estonia (3), Latvia (3) and Lithuania (3) were selected for the study. The time series ranged from 10 (2002–2011) to 21 years (1989–2009). The monthly P concentration and loss time series were tested for significant monotone trends (p < 0.05; two-sided test) using the partial Mann–Kendall test with stream discharge as an explanatory variable. The results show a large variation in concentrations and losses of total phosphorus (TP) among the 34 studied catchments, where the long-term mean annual losses varied from 0.09 to 7.5 kg TP ha−1. In addition, a large interannual variability in losses within catchments was found with up to a factor of 23 between years within the same catchment. Six catchments showed downward temporal trends in the TP loss time series. One upward trend in TP losses was detected in a catchment in south-west Sweden. Eight downward trends were detected in the TP concentration time series. Overall, our results show (1) a huge variability in mean P losses and concentrations among catchments, (2) a huge temporal variability in losses within catchments and (3) few detectable changes in P losses and concentrations over the study period. The results showcase the need for implementation of mitigation strategies towards reduced P losses from agricultural landscapes in the Nordic/Baltic Sea region in order to improve P water quality and ecology in surface waters.