Phosphorus Loss in Drainflow from Intensively Managed Grassland Soils P. S. Hooda,* M. Moynagh, I. F. Svoboda, A. C. Edwards, H. A. Anderson, and G. Sym ABSTRACT The loss of P in subsurface drainage from lysimeters (0.5 ha), managed as either monoculture grass or grass-clover for the last 9 yr have been quantified. Both systems received two to three cattle slurry applications annually and were cut two to three times before being grazed by dairy cattle. Mineral fertilizer-P was applied only to the grass-clover (about 25 kg P ha -1 yr-1). After 9 yr, NaHCO3-soluble P in the topsoil (0-10 cm)averaged 38 and 47 mg P kg -~ for the grass and grass-clover respectively, giving an average increase of 1.0 mg OIsen-P kg -~ yr -t in the grass-clover. Drainage-weighted molybdate- reactive phosphorus(MRP) and total phosphorus(TP) concentrations ranged from0.16 to 0.38 mg P L -~ and 0.45 to 0.79 mg P L -1, respec- tively during the 2-yr study period. The MRP and TPlosses in subsur- face-flow from the grass-clover (1.68-2.03 and 3.47-5.03 kg P ha -1 yr -~, respectively) were significantly larger than those fromthe grass (1.27-1.34 and 2.97-3.58 kg P ha -~ yr-~, respectively). Averaged across years and pastures, MRP accounted for 42% of the TP loss, while a non-MRP form accounted for 41% of the TP loss through field drains. Particulate-associated P represented about 17% of the TP loss. The P losses in subsurface runoff measured in the present study were much larger than previous estimates. The results also showed that, despite the subsoil having much larger P-sorption capacity than the topsoil, significant amounts of P losses could occur through preferen- tial hydrological pathways. rath et al., 1995; Haygarth and Jarvis, 1997; Beauchemin et al., 1998) have indicated that losses/concentrations of P in subsurface flow are larger than previously thought. It is also commonly thought that P present in the percolating water is retained in the subsoil as the water moves through the soil profile. However, soils with arti- ficial drainage systems such as tile or mole drains may yield greater P outputs in subsurface flow than naturally drained soils, because the accelerated subsurface flow in such soils may shorten the contact-time between subsoil and percolating water, or in extreme cases may effec- tively bypass the subsoil altogether. Lowland grassland farms in the UK and other northwestern European countries are commonly artificially drained, but little is known about the P-leaching outputs from such soils. Clearly, the loss of P attributable to such subsurface hydrological pathway needs to be quantified. The pri- mary objectives of this investigation were therefore to: (i) estimate P losses in drainflow from intensively man- aged grassland soils, (ii) study the effects of repeated inputs either as slurry or slurry and mineral fertilizer on soil extractable-P and loss in drainflow, and (iii) examine the various chemical forms of P in drainflow. p IHOSPHORUS inputs to grassland soils that receive in- tensive application of livestock slurry/manure and are regularly fertilized with mineral-P can exceed crop requirements. This practice often results in excessive P accumulation in the soil (Uunk, 1991; Daniel et al., 1993), and such soils represent a potential diffuse source of pollution as P plays a key role in the eutrophication of surface, estuarine, and somecoastal waters (Sharpley and Withers, 1994; Sims et al., 1998). Recent extensive literature reviews concluded that annual outputs of P in surface runoff from agricultural soils were generally <2 kg P ha -I (Sharpley and Withers, 1994; Sims et al., 1998). Since P in surface runoff is transported predominantly in the particulate phase (Sharpley and Menzel, 1987), its concentration in sub- surface flow is generally considered to be muchsmaller than that in surface runoff; implicit to this belief is the importance of surface-solution P chemistry and a gen- eral strong retention in soils. However, there is little quantitative data from field-scale studies capable of sup- porting the commonlyheld view of P leaching being insignificant source of diffuse pollution. Although based on short sampling periods, recent field studies (Heck- P.S. Hooda, M. Moynagh, I.F. Svoboda, and G. Sym, Scottish Agricul- tural College, Auchincruive, Ayr KA6 5HW, UK; A.C. Edwards and H.A. Anderson, Macaulay Land Use Research Inst., Craigiebuckler, Aberdeen AB15 8QH, UK. P.S. Hooda current address: School of Biological and Molecular Sciences, Oxford Brookes Univ., Oxford OX3 0BP, UK. Received 23 Mar. 1998. *Corresponding author (pshooda@brookes.ac.uk). Published in J. Environ. Qual. 28:1235-1242 (1999). MATERIAL AND METHODS ExperimentalSite, Soil, and Grass Production Systems The experimental site is situated at the Crichton Royal Farmin Dumfries, southwest Scotland (55°N, 4°30’W,15-m MSL). Average annual precipitation at the experimentalsite is 1054 mm. Average daily meanand maximum temperatures are 8.6 and 12.4°C, respectively. Long-term meteorological records suggestthat soils in this area return to field capacity by about mid-October and cease draining by early April, and that the majorityof soil drainage is likely to occur from No- vember to March. The soil at the experimental site is a noncal- careous gley of the Stirling Association (FAO - Eutric gleysol), witha silty clayloam topsoil overa silty clay subsoil, developed on estuarine and lacustrine raised beach silts and clays. The profile was relatively stone-free to 30 cm, with weathered sandstone and conglomerate stones up to 15 cmbelow this point. The whole profile had vertical worm channels appar- ently filled by darker Ap horizon material, offering an obvious potential bypass hydrological pathway and prolonged contact with soil relatively enriched in P. Typical properties of the soil are givenin Table1. Two 36-ha units, monoculture grass and grass-clover, were established by plowing and reseeding the existing pastures in July 1987. The grass-clover unit was reseeded with a mixture of ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), and the monoculture grass unit with only ryegrass. Abbreviations: MRP, molybdate reactive phosphorus; TP, total phos- phorus; TDP,total dissolved phosphorus; PP, particulate phosphorus; DOP, dissolved organic phosphorus; ICP-AES, inductively coupled plasma atomic emission spectroscopy; LSD, least significant dif- ference. 1235