Effectiveness of buffer strips without added fertilizer to reduce phosphorus loads from flat fields to surface waters I. G. A. M. Noij 1 , M. H einen 2 , H. I. M. H eesmans 2 , J. T. N. M. T hissen 3 & P. G roenendijk 1 1 Alterra, Climate and Water Centre, Wageningen-UR, PO Box 47, 6700 AA Wageningen, the Netherlands, 2 Alterra, Soil Science Centre, Wageningen-UR, PO Box 47, 6700 AA Wageningen, the Netherlands, and 3 Biometris, Plant Research International, Wageningen-UR, PO Box 100, 6700 AC, Wageningen, the Netherlands Abstract Buffer strips with no added fertilizers are a mitigation measure to reduce P loads from agricultural land to surface water. However, the experimental evidence on their effectiveness (BSE) has been from sloping locations with shallow flow and surface runoff. The aim of this experimental study was to quantify BSE for P on flat agricultural lowland, predominantly with deep flow. We selected sites characteristic of five major hydrogeological classes in the Netherlands and in each experimental field installed paired 5-m-wide unfertilized grass (BS) and reference treatments (REF) that abutted the ditch. The REF treatments were managed similar to the adjacent grass or maize field, but BS treatments were only harvested. Each treatment had a reservoir in the ditch to collect and measure discharge and flow- proportional P concentration for 3 or 4 yr. We also measured net P withdrawal, the P status of the soil and P concentration in upper groundwater. We found a significant BSE for P of 61% on the site with the shallowest flow and steepest slope (2%). At the other sites, BSE was low and statistically insignificant. We conclude that harvested unfertilized buffer strips reduce P loads from flat fields only in specific areas with high surface runoff and ⁄ or shallow flow, especially in combination with a high original soil P status. Keywords: Buffer strip, buffer strip effectiveness, hydrogeology, phosphorus, surface water quality Introduction Surface water quality is often a major concern in areas with intensive agriculture owing to runoff of nutrients. Excess nitrogen (N) and phosphorus (P) loads hinder water use and have negative ecological impacts. Mitigation measures are currently being considered both in the Netherlands (Hoogervorst, 2009) and in other EU countries (Schoumans et al., 2011) not to exceed water quality standards (EU, 1991) and to achieve the ecological goals of the Water Framework Directive (EU, 2000). Unfertilized buffer strips are a widely recognized mitigation option to reduce N and P transport from fields to surface water (Heinen et al., 2011; Dorioz et al., 2006 for P; Mayer et al., 2005, 2007 for N). However, their relative N and P reduction in load, or buffer strip effectiveness (BSE), varies greatly, ranging from below zero to almost 100% (e.g. White & Arnold, 2009). BSE depends on the nutrient input load, width, vegetation, maintenance (nutrient removal with sediment or biomass), time since installation and site conditions. The key site factors governing BSE are slope and hydrogeology (Noij et al., 2011), but there is no available experimental evidence from well-drained, virtually flat fields with deep permeable soils that occur over most of the Netherlands. Newly installed unfertilized buffer strips on flat fields may behave very differently from existing ones on sloping land next to natural streams. Nutrient loads in surface runoff and groundwater flowing from sloping land to the upper layers of riparian buffer strips are subject to infiltration (N, P), sedimentation of solids (P, N), denitrification (NO 3 -N), plant uptake (N, P), adsorption to the soil matrix (PO 4 -P, NH 4 -N) and storage of organic matter (N, P), but the flow paths from a deep permeable flat field towards a ditch might be below the active topsoil of the recently installed buffer strip. We provided experimental evidence for a low BSE for N on five characteristic Dutch agricultural lowland sites (Noij et al. (2011). The BSE for P will probably be different because the transport processes and behaviour of P in the soil differ from those of N (e.g. Ga¨chter et al., 2004). The prime Correspondence: I. G. A. M. Noij. E-mail: gert-jan.noij@wur.nl Received September 2011; accepted after revision July 2012. Soil Use and Management, March 2013, 29 (Suppl. 1), 162–174 doi: 10.1111/j.1475-2743.2012.00443.x 162 ª 2013 The Authors. Soil Use and Management ª 2013 British Society of Soil Science Soil Use and Management