Although several studies have pointed out the importance of P sorption in the subsoil, to our knowledge no previous study has attempted to quantify the effect of the subsoil per se on P leaching. The main objective of the present study was thus to determine whether subsoils of four Swedish agricultural soils acted as a source or sink for P leaching in particulate and/or dissolved form. Undisturbed soil columns with and without the topsoil were used for this purpose. The specific objectives of this study were (i) to study temporal and soil‐dependent variations in P leaching from four Swedish agricultural soils with contrasting texture and chemical characteristics (two clay and two sandy soils), with and without the topsoil, and to assess the causes of possible differences; (ii) to quantify the contribution of the subsoil to total P leaching in these four soils; and thereby (iii) to determine whether the subsoil had the potential to act as a source or sink for P leaching from the four soils.

Phosphorus losses from arable fields have previously been considered to occur mainly through erosion and surface runoff ( Sharpley et al., 1993 ). However, leaching of P through the soil has also been shown to be an important pathway for P losses in many areas (e.g., Sims et al., 1998 ; Djodjic et al., 1999 ). This includes several areas of Sweden where most arable land is relatively flat and rainfall intensity is moderate, facilitating good infiltration ( Ulén et al., 2007 ).

Excessive losses of phosphorus (P) from land to water are causing problems with eutrophication in many parts of the world, such as the Chesapeake Bay ( Reckhow et al., 2011 ) and the Baltic Sea ( Boesch et al., 2006 ). Agriculture is the main diffuse source of P transported to the Baltic Sea ( HELCOM, 2011 ), and approximately 40% of the Swedish anthropogenic net P load to the Baltic Sea originates from agriculture ( Brandt et al., 2009 ).

The excavation of the lysimeters created an approximately 10‐m‐long ditch. Soil samples were collected at five places in that ditch. To test the relationships between soil characteristics and P leaching from the lysimeters, each soil core was assigned the soil characteristics from the closest soil sampling spot (maximum 1 m away). Therefore, in some cases, two soil cores were assigned the same soil characteristics. Average values of Olsen P and DPS in topsoil and subsoil, respectively, were calculated for each soil core and transformed with the natural logarithm (ln) to decrease the impact of a few high values. Possible relationships with leaching of DRP from the lysimeters were tested thereafter in a log‐log regression using a Mixed model in which the sampling sites were set as a random effect.

Olsen P was divided by PSI to get an estimate of the DPS (e.g.,):with all concentrations on a molar basis. Phosphorus sorption index gives an estimate of the total amount of free P sorption sites without taking into account P already sorbed to the soil. Degree of P saturation, calculated as the ratio between Olsen P and PSI, should therefore be considered an approximate value. However, Olsen P values in Swedish soils are commonly low, and errors in this DPS calculation were therefore considered small.

Soil samples were taken at five locations per site and at five depths in the pits from which the lysimeters were taken (Mellby: 0–0.1, 0.1–0.4, 0.4–0.6, 0.6–0.8, and 0.8–1.0 m; Nåntuna, Lanna, and Bornsjön: 0–0.1, 0.1–0.3, 0.3–0.5, 0.5–0.7, and 0.7–1.0 m) simultaneously with lysimeter collection in autumn 2009. The topsoil was represented by the 0‐ to 0.4‐m (Mellby) and 0‐ to 0.3‐m layers (Nåntuna, Lanna, and Bornsjön), and the subsoil was represented by the layers below to the base of the soil columns. The topsoil was deeper at the Mellby sand site than at the other sites due to deeper plowing. After collection, the soil samples were air‐dried, crushed, and passed through a 2‐mm sieve. Soil pH was measured in water at a soil:water ratio of 1:5. Texture was determined by the pipette sedimentation procedure ( Ljung, 1987 ). Phosphorus was extracted in ammonium acetate lactate solution at pH 3.75 (P‐AL) according to the Swedish standard method ( Egnér et al., 1960 ). Concentration of P in the extract was determined by inductively coupled plasma analysis on a PerkinElmer Optima 7300 DV analyzer. Phosphorus was also extracted in 0.5 mol L −1 sodium bicarbonate at pH 8.5 (Olsen‐P) according to Olsen and Sommers (1982) , after which the concentration was measured by flow injection analysis (Tecator AB). Iron and aluminum were extracted with ammonium oxalate (Fe‐ox and Al‐ox), and the concentrations were determined by inductively coupled plasma analysis (same instrument as above).

The first water samples were collected in April 2010. However, water samples collected until the end of August 2010 (89–225 mm) were discarded to reduce the influence of preparation and storage of the soil columns and to ensure that all soil columns were at a similar water saturation level at the start of the leaching experiment. Leaching data reported here refer to the period 1 Sept. 2010 to 31 Aug. 2013, during which water samples were collected on a weekly basis if drainage water was available or after each major drain flow event. One Bornsjön clay lysimeter without topsoil was excluded from further analyses due to disproportionally low drainage amounts, most likely caused by leakage in the water collection device. Total P (TP) concentration was measured on unfiltered samples after digestion with potassium persulfate. Dissolved reactive P was measured on undigested samples after filtration through membrane filters with 0.2 μm pore size (Schleicher & Schüll, GmbH). All samples were then analyzed colorimetrically according to the molybdenum blue method ( ECS, 1996 ). The difference between TP in unfiltered and filtered samples was defined as particulate P (PP), and the difference between TP and DRP+PP was defined as residual P. The residual P fraction was not further analyzed. Mean annual P leaching loads for the study period 1 Sept. 2010 to 31 Aug. 2013 were calculated by first multiplying the concentration of the respective P fraction on each sampling occasion by the leachate volume on the same occasion and then adding up these loads for each year. Mean volume‐weighed concentrations were calculated by dividing total transport of the different P fractions by total leachate volume.

Undisturbed soil columns were collected from the four sites in autumn 2009 using a drilling technique whereby a carved‐out soil column is gently pushed into a polyvinyl chloride pipe (0.295 m i.d.) ( Persson and Bergström, 1991 ). At each site, three soil columns with topsoil and subsoil encased in 1.18‐m‐long pipes (full‐length) and three columns with only subsoil (0.9‐m‐long pipes) were collected. In the latter, topsoil was first excavated manually, after which drilling started directly on the subsoil. After collection, approximately 0.08 m of soil at the base of the lysimeters was removed and replaced with gravel, with a stainless steel mesh (pore size, 0.5 mm) on each side of the gravel layer and a fiberglass lid to facilitate drainage under gravity ( Fig. 1 ). A stainless steel mesh (pore size, 0.5 mm) was placed on top of the subsoil lysimeters to prevent heavy rainfall from having a destructive impact on the structure of the upper surface. The soil surface was set about 0.05 m below the top edge of the lysimeter casing to prevent surface flow to and from the soil columns, giving a final soil column length of 1.05 m (full‐length lysimeters) and 0.77 m (subsoil lysimeters). The subsoil lysimeters were placed on 0.28‐m‐long pipes of the same type as the lysimeter casings to bring the upper surface to the same level as in the full‐length lysimeters, and in March 2010 they were placed in an outdoor station at the Swedish University of Agricultural Sciences, Uppsala, Sweden (59°49′ N, 17°40′ E) ( Bergström, 1992 ). To eliminate the effect of crop uptake of water and P, no crop was grown in the soil columns during the experiment. To simulate common Swedish agricultural practice, in April 2011 the lysimeters were fertilized with (NH 4 )H 2 PO 4 at a rate equivalent to 22 kg P ha −1 , which is the recommended application rate to achieve P balance in Sweden ( SBA, 2013 ). The soil surface was manually weeded when required. No additional management of the soil surface was performed.

Four agricultural soils with different physical and chemical characteristics were used in this experiment: two sandy soils (Mellby: 56°29′ N, 12°59′ E; Nåntuna: 59°49′ N, 17°41′ E) and two clay soils (Lanna: 58°21′ N, 13°07′ E; Bornsjön: 59°14′ N, 17°41′ E). For the Mellby sand, Lanna clay, and Bornsjön clay, long‐term data on P leaching following different topsoil management practices are available. The Mellby sand is classified as a Fluventic Haplumbrept ( Bergström et al., 1994 ), the Nåntuna sand as a Typic Udipsamment ( Kirchmann, 1985 ), the Lanna clay as a Udertic Haploboroll ( Bergström et al., 1994 ), and the Bornsjön clay as an Inceptisol (U.S. soil taxonomy) (a more detailed soil classification has not been made). Before lysimeter collection, perennial forage crops had been grown at the Mellby site for 20 yr, at the Nåntuna site for 10 yr, at the Lanna site for 12 yr, and at the Bornsjön site for 5 yr. The Mellby and Lanna sites had not received fertilizer during the respective period, whereas the Nåntuna and Bornsjön sites had received 12 and 13 kg P ha −1 yr −1 , respectively.

Results and Discussion

Soil Chemical Properties Some selected properties of the four soils are presented in Table 1. The chemical and physical properties of the four soils are described in detail elsewhere (Andersson et al., 2013). Table 1. Some general soil properties (from ). Site Depth pH† Clay‡ Olsen P§ P‐AL Fe‐ox Al‐ox PSI Olsen P/PSI cm % mg kg−1 mmol kg−1 % Mellby 0–10 5.8 ± 0.5¶ 7 91.0 ± 4.0 294 ± 10.2 1979 ± 186 1178 ± 67 3.9 ± 0.4 76 ± 6.7 10–40 6.1 ± 0.2 6 77.8 ± 9.3 229 ± 47.0 1996 ± 198 1298 ± 168 4.6 ± 0.2 55 ± 7.3 40–60 6.0 ± 0.1 1 8.0 ± 1.7 15 ± 3.0 1805 ± 513 699 ± 131 3.6 ± 0.4 7.3 ± 2.0 60–80 5.8 ± 0.1 1 5.7 ± 2.4 11 ± 4.8 3960 ± 1994 797 ± 211 4.3 ± 0.5 4.2 ± 1.4 80–100 5.2 ± 0.4 2 10.3 ± 2.8 16 ± 4.9 1727 ± 620 432 ± 134 3.3 ± 0.5 10 ± 3.9 Nåntuna 0–10 7.0 ± 0.2 11 29.6 ± 2.9 164 ± 24.2 1710 ± 203 802 ± 92 2.3 ± 0.7 44 ± 16 10–30 7.5 ± 0.2 9 23.4 ± 2.8 126 ± 11.7 1579 ± 102 768 ± 42 2.3 ± 0.5 35 ± 11 30–50 7.6 ± 0.2 2 15.4 ± 2.1 52 ± 3.8 841 ± 179 491 ± 90 1.2 ± 0.4 48 ± 24 50–70 7.6 ± 0.2 2 16.2 ± 3.0 55 ± 7.2 1049 ± 128 465 ± 92 1.0 ± 0.3 55 ± 16 70–100 7.6 ± 0.1 6 24.6 ± 9.5 76 ± 12.3 2005 ± 426 619 ± 86 1.9 ± 0.2 44 ± 19 Lanna 0–10 6.1 ± 0.2 43 9.5 ± 3.7 33 ± 6.0 3896 ± 475 1798 ± 130 5.5 ± 0.3 5.7 ± 2.5 10–30 6.4 ± 0.1 45 7.7 ± 1.5 31 ± 6.1 3928 ± 361 1848 ± 119 2.7 ± 0.3 4.4 ± 0.9 30–50 6.8 ± 0.1 56 4.0 ± 0.0# 51 ± 24.5 3729 ± 639 2041 ± 259 7.7 ± 0.4 1.7 ± 0.1 50–70 7.0 ± 0.1 58 4.0 ± 0.0# 158 ± 5.4 3076 ± 378 1732 ± 199 6.9 ± 0.3 1.9 ± 0.1 70–100 7.2 ± 0.1 61 4.0 ± 0.0# 207 ± 15.5 2245 ± 654 1694 ± 223 6.0 ± 0.3 2.2 ± 0.1 Bornsjön 0–10 6.0 ± 0.1 60 18.2 ± 1.3 44 ± 5.8 9205 ± 1283 3140 ± 476 7.3 ± 0.4 8.1 ± 0.7 10–30 6.2 ± 0.1 60 16.4 ± 2.6 32 ± 7.5 9430 ± 611 2869 ± 289 7.8 ± 0.3 6.8 ± 0.9 30–50 6.6 ± 0.1 59 4.0 ± 0.0 9 ± 2.7 8805 ± 2770 1919 ± 313 7.3 ± 1.0 1.8 ± 0.2 50–70 6.5 ± 0.1 61 5.4 ± 2.1 12 ± 1.1 10,098 ± 6485 2069 ± 380 7.2 ± 1.4 2.4 ± 0.5 70–100 5.2 ± 0.1 54 21.0 ± 5.3 31 ± 9.5 6562 ± 831 2387 ± 219 10.5 ± 0.6 6.4 ± 1.4 There was no calcium carbonate present in any of the soil profiles. Extractable P measured as Olsen P and P‐AL was relatively high in the topsoil of the Mellby sand and lower in the subsoil but was high in the entire profile of the Nåntuna sand. In the Lanna clay, the P‐AL value increased with depth, whereas the Olsen P value decreased. Ammonium lactate can dissolve Ca‐bound P that may be present in soils with high pH. The contradictory results obtained may therefore indicate the presence of Ca‐phosphates in the soil because no calcium carbonate was detected in the analyses. In a previous study of a similar clay soil near Lanna, P was found to be bound to Ca complexes with increasing P concentrations with increasing soil depth (Ulén and Snäll, 2007). Phosphorus in these forms is much less soluble under neutral and alkaline conditions, and acid extraction with P‐AL may therefore have overestimated plant‐available P in the Lanna clay. However, the deviating extraction results in the Lanna clay were not further studied. The Bornsjön clay topsoil had a higher content of Olsen P than the Lanna clay topsoil. Both clay soils had low concentrations of Olsen P in the subsoil except for the 70‐ to 100‐cm layer in the Bornsjön clay, where Olsen P was high. This probably originated from old marine deposits of gyttja (cohesive old organic material settled in lake or marine sediments) and was thus native P. It may also explain the relatively low pH (pH 5.2) in the 70‐ to 100‐cm layer at Bornsjön. The P sorption capacity, measured as PSI, was high in the entire soil profile of the Mellby sand due to high Fe content. The high P content in the Mellby sand topsoil resulted in high DPS (65%). However, due to low P content and high P sorption capacity, DPS was very low (7%) in the subsoil of the Mellby sand. In the Nåntuna sand, DPS was high in the entire profile (45%) due to low P sorption capacity and high P content in topsoil and subsoil. Both clay soils had high P sorption capacity (PSI) and low DPS in topsoil and subsoil, although PSI was slightly higher in the Bornsjön clay than the Lanna clay, especially in the 70‐ to 100‐cm layer, most likely due to higher Fe content.

Weather Conditions and Drainage During the 3‐yr study period, total precipitation was 1718 mm, and mean air temperature was 6.4°C (Table 2). Long‐term (100‐yr) mean annual precipitation and mean air temperature at the study site are 530 mm and 5.5°C, respectively. Mean temperature was lower (slightly below freezing) in January–April in 2011 and 2013 compared with the same period in 2012 (slightly above freezing). Precipitation during this period was lower in 2012 than in the other years and mainly occurred as rain, whereas the lower temperature in 2011 and 2013 resulted in more snow. The snow cover was therefore thicker and the depth of frozen soil shallower in 2011 and 2013 than in 2012. Hence, drainage amounts at snowmelt increased rapidly in all lysimeters in March 2011 and April 2013 and reached between 80 and 110 mm for 2.5 wk, compared with 40 to 60 mm during 2.5 wk in March 2012 (Fig. 2). Drainage at snowmelt appeared at roughly the same time in lysimeters of each soil, whether with or without topsoil. Table 2. Air temperature, precipitation, and drainage from the lysimeters. Year Month Air temp. Precipitation Drainage Mellby sand Nåntuna sand Lanna clay Bornsjön clay Topsoil+subsoil Subsoil Topsoil+subsoil Subsoil Topsoil+subsoil Subsoil Topsoil+subsoil Subsoil† °C mm 2010 Sept.–Dec. 1.8 197 98 ± 2‡ 117 ± 5 118 ± 1 113 ± 19 101 ± 14 115 ± 2 105 ± 18 108 ± 1 2011 Jan.–Apr. −0.1 79 118 ± 10 128 ± 36 104 ± 43 121 ± 28 124 ± 74 126 ± 20 135 ± 23 85 ± 17 2011 May–Aug. 15.8 217 13 ± 5 101 ± 21 79 ± 2 110 ± 11 67 ± 14 85 ± 2 66 ± 11 80 ± 6 2011 Sept.Dec. 6.9 233 166 ± 9 215 ± 4 191 ± 12 216 ± 6 178 ± 9 211 ± 3 193 ± 15 223 ± 18 2012 Jan.–Apr. 0.4 162 89 ± 6 115 ± 3 143 ± 11 139 ± 9 119 ± 29 133 ± 5 128 ± 26 113 ± 20 2012 May–Aug. 14.3 343 91 ± 9 209 ± 13 167 ± 11 199 ± 14 107 ± 26 184 ± 4 141 ± 23 106 ± 93 2012 Sept.–Dec. 4.1 259 181 ± 1 173 ± 6 188 ± 15 179 ± 28 173 ± 1 200 ± 5 164 ± 34 157 ± 24 2013 Jan.–Apr. −1.6 106 119 ± 7 96 ± 9 128 ± 10 110 ± 9 112 ± 38 95 ± 8 141 ± 17 142 ± 25 2013 May–Aug. 16.0 123 2 ± 0 15 ± 11 5 ± 2 28 ± 6 3 ± 1 3 ± 1 4 ± 1 11 ± 11 Total 1718 876 ± 29 1168 ± 90 1121 ± 31 1214 ± 86 985 ± 190 1152 ± 39 1077 ± 119 1027 ± 44 Figure 2 Open in figure viewer Load of total phosphorus (TP) and drainage amounts from (left) full‐length lysimeters and (right) subsoil lysimeters from Mellby sand, Nåntuna sand, Lanna clay, and Bornsjön clay, 1 Sept. 2010 to 31 Aug. 2013. (Note different scale on y axis for Nåntuna sand.) Water holding capacity differed considerably between the soils used in this study. However, total drainage amounts were not significantly influenced by soil type (P > 0.05) due to variation in drainage volumes from lysimeters of the same soil. Total drainage amounts were higher from subsoil lysimeters than from full‐length lysimeters for all soils except the Bornsjön clay, where the opposite occurred (Table 2). However, the difference was only statistically significant for the Mellby sand (P < 0.01) and not for the other three soils (P > 0.05). The approximately 0.3 m shorter subsoil lysimeters, with lower organic matter content and thus less water holding capacity (Andersson et al., 2013), resulted in more drainage than the full‐length lysimeters. Bergström et al. (1994) reported water contents at 100 cm tension of 0.231 m3 m−3 in topsoil and 0.132 m3 m−3 in subsoil for the Mellby sand and 0.165 m3 m−3 in topsoil and 0.065 m3 m−3 in subsoil for the Nåntuna sand (no measurements at 100 cm tension were made on the clay soils included in this study). Therefore, although the soil columns were similarly water saturated at the start of the experiment, more water was needed to fill up the pore volume to field capacity in the full‐length columns after dry periods. During the study period it was noticed that the Bornsjön clay subsoil lysimeters were often ponded after heavy rainfall. This may have caused more evaporation from the surface, which could have been a contributing factor to the slightly lower drainage volumes from Bornsjön subsoil lysimeters than from the full‐length lysimeters. For full‐length soil columns, the clay soils showed larger variation in drainage amounts than the sandy soils, whereas the opposite was observed for the subsoil lysimeters (Table 2). For the clay soils, this variation can be explained by the more heterogeneous soil structure resulting in variable quantities of macropore flow, which did not occur in the sandy soils. A previous dye‐tracer lysimeter experiment reported extended macropore flow in the Lanna clay (Bergström and Shirmohammadi, 1999). In addition, a recent field plot study on pesticides suggested that macropore flow could be a common flow pattern also in the Bornsjön clay (Ulén et al., 2014). The larger variation among the sandy subsoils is more difficult to explain other than by spatial variability in properties affecting flow rates and water holding capacity.

Seasonal Variation in P Leaching Particulate‐P leaching from the soils was somewhat higher in 2012 than in the other years (Fig. 3) due to higher precipitation (Table 2) and hence higher drainage amounts (Fig. 2). Leaching of PP increased greatly in the Mellby and Nåntuna sand subsoil lysimeters at snowmelt in March 2012 and April 2013 (Fig. 3). From November 2011 to March 2012 and from September 2012 to January 2013, PP leaching also increased in subsoil lysimeters of the Lanna clay and in full‐length lysimeters of the Bornsjön clay (Fig. 3) due to increased drainage amounts (Fig. 2). Higher concentrations during these periods of PP in leachate from subsoil lysimeters than from full‐length lysimeters of the Mellby sand, Nåntuna sand, and Lanna clay may be attributable to exposure of the subsoil to precipitation and freezing/thawing processes. The organic carbon content was much higher in the topsoil (1.0–3.3%) than in the subsoil (0.1–0.4%), resulting in less stable structure in the subsoil (Andersson et al., 2013). However, the Bornsjön clay contains more organic carbon down to 1 m depth (0.7% at 70–100 cm depth) than the other soils (Andersson et al., 2013), which may have increased the stability of that soil. Nevertheless, the Bornsjön clay subsoil lysimeters were often ponded after heavy rainfall, which indicates a lack of preferential flow through macropores in the subsoil. This ponding most likely protected the soil surface from the erosive power of rain and prevented increased loss of particles from subsoil lysimeters compared with full‐length profiles. Ponding conditions in the field could trigger surface runoff, which would possibly increase surface PP losses, but no ponding was observed on any of the full‐length lysimeters in this study. Figure 3 Open in figure viewer Concentration of (left) dissolved reactive P and (right) particulate P in leachate from full‐length and subsoil lysimeters of Mellby sand, Nåntuna sand, Lanna clay, and Bornsjön clay, 1 Sept. 2010 to 31 Aug. 2013. (Note different scale on y axis for DRP in Nåntuna sand.) In contrast to PP, concentrations of DRP in leaching water were rather constant in both the sand and clay soils and thus seemed to be less affected by fluctuations in drainage amounts (Fig. 3). Fertilization with P in April 2011 did not increase P leaching in any of the soils, indicating that legacy P overshadowed the effect on leaching of a single P fertilizer application on these soils.

Comparison of P Leaching from Full‐length and Subsoil Profiles Mean annual leaching load of TP from full‐length lysimeters followed the order: Nåntuna sand > Bornsjön clay > Lanna clay > Mellby sand; the order changed for subsoil lysimeters to Nåntuna sand > Mellby sand > Lanna clay > Bornsjön clay (Table 3). Table 3. Mean annual leaching loads and mean volume‐weighted concentrations of total P, dissolved reactive P, and particulate P in leachate from full‐length and subsoil lysimeters, 1 Sept. 2010 to 31 Aug. 2013. Soil Lysimeter Volume‐weighted concentration† Mean annual load TP DRP PP TP DRP PP mg L−1 kg ha−1 yr−1 Mellby sand full‐length 0.08 ± 0.03‡ 0.04 ± 0.02 0.02 ± 0.01 0.25 ± 0.08 0.12 ± 0.05 0.06 ± 0.01 subsoil 0.16 ± 0.11 0.02 ± 0.01 0.10 ± 0.07 0.62 ± 0.42 0.08 ± 0.05 (70%)§ 0.38 ± 0.25 Nåntuna sand full‐length 1.16 ± 0.06 0.88 ± 0.08 0.06 ± 0.01 4.39 ± 0.34 3.33 ± 0.39 0.21 ± 0.04 subsoil 1.09 ± 0.65 0.81 ± 0.55 0.11 ± 0.05 4.41 ± 3.02 3.29 ± 2.52 (99%) 0.44 ± 0.19 Lanna clay full‐length 0.19 ± 0.05 0.12 ± 0.06 0.05 ± 0.03 0.61 ± 0.27 0.38 ± 0.25 0.18 ± 0.08 subsoil 0.15 ± 0.03 0.07 ± 0.04 0.07 ± 0.01 0.58 ± 0.12 0.27 ± 0.14 (70%) 0.28 ± 0.03 Bornsjön clay full‐length 0.20 ± 0.04 0.05 ± 0.02 0.14 ± 0.02 0.71 ± 0.20 0.19 ± 0.07 0.49 ± 0.11 subsoil 0.10 ± 0.02¶ 0.05 ± 0.01 0.05 ± 0.01 0.34 ± 0.09 0.17 ± 0.05 (94%) 0.15 ± 0.03 Mean annual DRP leaching load was larger from full‐length lysimeters than from subsoil lysimeters for all soils, whereas the opposite was true for PP leaching (both concentrations and load) in the Mellby sand, Nåntuna sand, and Lanna clay. However, differences in leaching of TP, DRP, and PP (concentrations and load) between lysimeters with and without topsoil were not significant (P > 0.05) for the Mellby, Nåntuna, and Lanna soils. The higher concentration of PP in leachate from subsoil lysimeters than from full‐length lysimeters of these soils may be due to direct exposure of the subsoil to precipitation and freezing/thawing processes. Conversely, mean annual leaching load of PP from the Bornsjön clay was significantly larger from full‐length lysimeters than from subsoil lysimeters (P < 0.05), indicating that most PP came from the topsoil. Leaching of DRP (concentrations and load) was significantly lower from the Mellby sand than from the Nåntuna sand (P < 0.01) despite considerably higher Olsen P content in the Mellby sand topsoil (Table 1) and small amounts of preferential flow in both soils (Bergström and Shirmohammadi, 1999; Bergström et al., 2011). Considerably higher sorption capacity in the Mellby sand subsoil due to the presence of Fe‐oxides (Table 1) efficiently functioned as a sink for P leaching. This resulted in low DRP leaching from full‐length lysimeters despite the very high P content in the topsoil and also from subsoil lysimeters. In a study on intact columns of the Mellby sand topsoil (0–0.2 m depth) collected from field plots where P fertilizer had been applied at different rates over 26 yr, Liu et al. (2012b) found that TP concentration in leachate from those columns ranged from 0.30 to 0.57 mg L−1, which is much higher than in this study (0.07–0.12 mg L−1 in full‐length lysimeters). One contributing factor may be that the field plot from which the columns in this study were collected had not been fertilized for 20 yr before lysimeter collection. Despite this, P‐AL values in the topsoil study by Liu et al. (2012b) were similar to those in our study. Thus, the lower TP concentrations in leachate measured in this study were most likely attributable to sorption of P in the subsoil. Although the Mellby sand was found to have very high sorption capacity in the subsoil, it is important to avoid heavy fertilization of this soil for extended periods because DPS in the subsoil may reach levels where P leaching starts to increase. For instance, De Bolle et al. (2013) reported an increase in DPS in 21 acid sandy subsoils in Belgium between 2001 and 2010 and concluded that this increase posed a great risk of P leaching. Similar results have been reported for Denmark, where TP and DPS in agricultural soils increased between 1987 and 1998 due to heavy P fertilization (Rubaek et al., 2013). The Nåntuna sand subsoil, on the other hand, had high P content and low P sorption capacity, which resulted in very high TP leaching from full‐length and subsoil lysimeters. For the Mellby sand, leaching of DRP from subsoil lysimeters was 70% of that from full‐length lysimeters, whereas it was 99% for the Nåntuna sand (Table 3). This clearly shows that the Nåntuna subsoil can act as a source for P leaching losses. However, one of the Nåntuna sand subsoil lysimeters had much higher TP leaching than the other two, indicating large spatial variation at the Nåntuna site. Nevertheless, when this soil column was excluded from the calculations, average TP leaching was still high from the Nåntuna sand subsoil (2.74 kg ha−1 yr−1), resulting in 57% of DRP leaching from subsoil lysimeters compared with full‐length lysimeters. Leaching of DRP (concentrations and load) was higher from the Lanna clay than from the Bornsjön clay in lysimeters with and without topsoil. This was most likely due to higher sorption capacity in the Bornsjön clay (Table 1), leading to reduced leaching losses. Although macropore flow and tile‐drainage flow have been suggested to be common flow patterns in the Bornsjön clay (Ulén et al., 2014), the water ponding observed on Bornsjön clay subsoil lysimeters in this study during wet periods suggests that macropores were not fully captured in the soil columns used for the lysimeter study. Water flow rate was thereby reduced, increasing the contact time between P in percolating soil water and soil sorption sites. This indicates that the variation in spatial distribution of macropores over the field and between seasons is probably quite large at the Bornsjön site. The leaching load of DRP from subsoil lysimeters of the Lanna clay was 70% of that from full‐length lysimeters, whereas in the Bornsjön clay it was 94%. The higher contribution to DRP leaching of Bornsjön clay subsoil than Lanna clay subsoil may be due to the high Olsen P content and low pH at 70 to 100 cm depth in the former. This may enable Ca‐bound P forms to be dissolved (Devau et al., 2011). In addition, anaerobic conditions in the uppermost layer created by ponding on Bornsjön clay subsoil lysimeters may have reduced Fe(III) to Fe(II), causing P to be released to the soil solution. However, although the ratio of DRP load from the subsoil compared with the full‐length lysimeters was greater for the Bornsjön clay than for the Lanna clay, overall DRP leaching (concentrations and load) was lower from the Bornsjön clay subsoil than from the Lanna clay subsoil, presumably due to higher sorption capacity in the former. Consequently, the difference in DRP leaching (load and concentration) between the Lanna clay and the Bornsjön clay subsoils was not significant (P > 0.05).

P Leaching in Lysimeters Compared with Tile‐Drained Plots A comparison of the data obtained for the full‐length lysimeters with long‐term field data from tile‐drained plots with various crops at three of the four sites revealed that the lysimeters and the field plots had similar chemical and physical soil properties. For example, in this lysimeter study, TP leaching averaged 0.25 kg ha−1 yr−1 in the Mellby sand, 0.61 kg ha−1 yr−1 in the Lanna clay, and 0.71 kg ha−1 yr−1 in the Bornsjön clay (Table 3). For the field plots at the Mellby sand site, reported TP leaching on an organically managed field with mean manure addition of 6.5 kg P ha−1 yr−1 averaged 0.23 kg ha−1 yr−1 over 6 yr (Torstensson et al., 2006). In a previous study at the Lanna clay site, TP leaching over 6 yr averaged 0.81 kg ha−1 yr−1 on an organically managed field without fertilizer or manure addition and 0.41 kg ha−1 yr−1 on a field where manure was added at an average rate of 7.3 kg P ha−1 yr−1 (Aronsson et al., 2007). At Bornsjön, average TP leaching measured over 6 yr on a field without P addition was 0.97 kg ha−1 yr−1 (Svanbäck et al., 2014). Hence, TP leaching was of the same magnitude in this lysimeter study as in the field plots. However, there are certain differences between measurements performed in lysimeters and those in tile‐drained field plots that need to be considered. First, no crop was grown on the lysimeters in this study, resulting in more drainage than if a crop had been present, which was the case in the field studies cited. A crop also takes up P during its growth, which would lower the P content in the soil solution and thereby reduce P leaching. In addition, in tile‐drained plots, surface runoff and/or lateral water flow (e.g., on a compacted plow pan below the topsoil) can occur if the infiltration capacity of the soil is exceeded. Moreover, tile drainage bypass can be considerable in field plots (Bergström, 1987). In contrast, in lysimeters all water reaching the soil surface that does not evaporate infiltrates into the soil. In addition, lateral water flow into the soil cannot be properly simulated in lysimeters (e.g., due to truncated macropores). In tile‐drained soils, there is also a risk of water moving directly from the topsoil into drain backfill and down to the drain pipe due to impeding layers below the topsoil, triggering lateral flow (Bergström and Johnsson, 1988). The result is less transport via water flow between the tile drains in the actual soil profile (Øygarden et al., 1997), whereas in lysimeters all water percolates through the soil profile. The moisture conditions in a lysimeter and in tile‐drained plots of the same soil may also be quite different (Bergström and Johansson, 1991), affecting the redox potential and thereby P solubility in the soil. Despite these differences, the ranking of the soils in terms of TP leaching measured in the lysimeters in this study and in the corresponding field plots demonstrate that lysimeters provide a good estimate of P leaching from the soils used here. In fact, there is reason to believe that if the objective is to determine the influence of soil properties alone on P leaching, without the influence of hydrological field conditions, unplanted and undisturbed soil columns such as those used in the present study may be preferable.

Importance of Chemical Subsoil Properties for P Leaching The data in this study were highly clustered due to high P leaching load from the Nåntuna sand. The relationships between DRP and Olsen P and DPS were therefore analyzed after natural logarithm transformation of the data (Fig. 4). This log‐log transformation showed that DRP leaching load increased with increasing Olsen P content and DPS in subsoils in full‐length and subsoil lysimeters, whereas the topsoil features had less impact on DRP leaching load. However, the relationships in full‐length profiles were not statistically significant. This lack of significance might be attributable to the small number of measurements and to the impact of the highly clustered data. Nevertheless, despite the lack of statistical significance for the relationships in the full‐length profiles, the results indicate the importance of subsoil features on DRP leaching load. These results are similar to findings by Djodjic et al. (2004) of no relationship between Olsen P or DPS in the topsoil and P leaching. The lack of a significant relationship between DRP leaching load and topsoil characteristics suggests that determination of topsoil properties is insufficient to assess the risk of P leaching and tile drainage losses from many soils. However, determination of subsoil characteristics such as Olsen P or DPS appears to be useful when estimating the risk of P leaching from soil. Figure 4 Open in figure viewer Relationship between (left) Olsen P measured in topsoil or subsoil and mean annual leaching load of dissolved reactive P (DRP) for (a) full‐length and (b) subsoil lysimeters and (right) relationship between degree of P saturation (DPS) measured in topsoil or subsoil and DRP for (c) full‐length and (d) subsoil lysimeters. *Significance at P < 0.05.