Introduction
Mitigating phosphorus (P) leaching from arable land is critical for improving water quality and reducing undesirable eutrophication of lakes and seas. However, P leaching is known to vary widely in time and space (e.g. Haygarth et al.
2005), hampering assessment of efforts to reduce leaching. In general, agricultural practices affect P leaching less than meteorological conditions, but P transport through the soil is also strongly dependent on soil structure (Jarvis
2007). Since soil structure may vary considerably within fields, high in-field variation in P leaching has been reported (e.g. Norgaard et al.
2013).
In Sweden, 70% of arable land is artificially drained (Elmquist
2014). An efficient drainage system can alter water quality through changes in hydrology and stabilisation of groundwater level (Skaggs et al.
1994). Dilution of solutes from the topsoil can occur when shallow groundwater is mixed in drains with infiltrating water from the soil surface. This was demonstrated in a previous 9-years’ study on 16 fields within a 1-km radius that were all tile-drained to improve infiltration of precipitation (Prec) water into the soil (Ulén et al.
2016). All fields were assumed to receive the same yearly precipitation, but measured discharge (
Q, mm year
−1) at the drain outlets varied widely between fields, probably owing to different contributions of shallow groundwater. This dilution resulted in a significant negative correlation between concentrations of dissolved reactive P (DRP) and
Q/Prec ratio. Since yearly (flow-proportionally) total P (TP) concentration was not significantly correlated to discharge,
Q could not be used as a predictor for TP leaching from the drainage system (Ulén et al.
2016).
To improve infiltration and reduce P losses from arable land, installation/renovation of drainage systems is very important (Taylor et al.
2016). To further improve infiltration, quicklime (CaO), also known as burnt lime, can be added to drain backfill (Lindström and Ulén
2003). This measure can reduce surface runoff, with an accompanying reduction in eroded soil and attached P. In studies in Lithuania, addition of burnt shell-ash (CaO, at 0.6% of soil mass) to drain backfill has been tested for reducing P losses (Šaulys and Bastienė
2007). Apart from such lime-filter ditches, structure liming (applying quicklime or slaked (hydrated) lime to the entire topsoil) can improve water infiltration over the whole field area, thereby reducing the risk of surface water ponding. Surface ponding (e.g. in micro-depressions) can increase non-equilibrium water flow and solute transport in macropores (Jarvis
2007). Macropore flow which generally leads to a fast response in tile drains with increased water flow peak flows has been suggested to be related to a high flashiness (Deelstra and Iital
2008). In contrast, contributions by shallow groundwater can damp such peaks and modify water flow fluctuations since the shallow groundwater level fluctuates much slower (e.g. Beven and Gerdman
2013).
A range of hydrological indices for flow alteration are available, considering, e.g. variation in flows (flashiness), frequency and duration of flow peaks and flow skewness. The Richards–Baker flashiness index (Baker et al.
2004) is commonly based on daily time steps (FI
day). This relative simple index has been used as an explanatory factor for nutrient leaching in a range of agricultural catchments (Deelstra et al.
2014). At farm scale, a significant relationship between FI
day and leaching of total P (TP) has been reported (Ulén et al.
2016). Concentration of topsoil P, extracted in acid ammoniumlactate (P-AL) according to Egnér et al. (
1960), was found to be another important factor in that nine-year study, whereas annual agricultural management, e.g. tillage, crop and fertilisation, did not affect TP leaching significantly for different fields (Ulén et al.
2016).
High spatial variation in both P and pesticide leaching was observed in a field plot experiment with two rows of 14 drained plots running towards a ditch at the centre of a flat valley (Ulén et al.
2013). Phosphorus and pesticide concentrations in drain water decreased with increasing distance from the ditch (Ulén et al.
2013). Accordingly, this distance was used as a predictor (covariate) in a first six-year assessment of P leaching mitigation strategies at the site (Svanbäck et al.
2014). However, differences in drain flow dynamics between plots were not considered. In the present study, we therefore tested the Richards–Baker index as an alternative and potentially more generally applicable indicator of P leaching, using results from an extended period (eight years) at the same site. A time resolution of one hour (FI
hour) was chosen to match the small area of the experimental plots, for which peak duration are commonly less than one day. Moreover, leaching of TP and particles together with FI
hour was estimated for a nearby field with similar soil type, where improved drainage system in the most crucial parts of the field possibly affected water infiltration and water flow. Previous monitoring of P leaching from the field showed that yearly losses were unaffected by crop, fertilisation and soil tillage (Ulén and Persson
1999).
The overall objective of this study was to assess yearly P leaching via drain systems from experimental plots for a prolonged period with four different management systems considering variation in flows. A second objective was to evaluate the mitigation of P leaching in a nearby field where structure lime (in a common commercial form with slaked lime mixed with milled limestone) had been applied to the topsoil of the entire field area. Additionally, the tile drainage system had been renovated and structure lime used as a backfill above drains in the middle and lower part of the field. A specific objective was to assess if FIhour could be used as an alternative predictor for P concentrations in drainage water to a previous used local factor—the distance to the receiving ditch in the valley.
Conclusions
Spatial variation in phosphorus leaching via subsurface drains was higher than the temporal (eight-year) variation at the Oxelby experimental site with gradients in clay soil content and plots with varying positions in the valley. Thus large numbers of replicates are needed in phosphorus leaching studies to ensure that such natural spatial variation is covered. Here the FIhour index used indicated an even more varied spatial P leaching than in the formerly P leaching assessment using the site-specific factor of distance to the valley centre. However, FIhour may have been affected by treatments designed to improve soil structure thus changing this hydrological signature from, e.g. SL-CT. Both factors gave similar results when used separately as predictors of phosphorus concentrations and water transport.
Long-term studies of untilled and shallow tillage plots are required, since the particles in drain water from these treatments were observed to be more P-enriched than particles from conventionally ploughed plots and since macropore flow may be enhanced in untilled soil due to higher macroporosity.
Combined treatment to improve drainage and water infiltration in different parts of fields, as tested here, seems to be a promising strategy for mitigating phosphorus leaching but any effects from the different treatments cannot be separated based on monitoring results from a single field. Long-term monitoring is advisable in order to quantify P leaching effects particularly from such a field, treated in several ways.