Re-evaluation of the liming-fertilization interaction in grasslands on poor and acid soils V. Poozesh*, P. Castillon†, P. Cruz* and G. Bertoni‡ *UMR AGIR, Institut National de la Recherche Agronomique, Castanet-Tolosan, France, †ARVALIS, Institut du Ve ´ge ´tal, Bazie `ge, France, and ‡UMR DYNAFOR, Institut National de la Recherche Agronomique, Castanet- Tolosan, France Abstract The effect of surface liming on herbage production in permanent grasslands is rather uncertain. To better understand the effect of liming on the grassland, a study was made with a field experiment and a pot experiment with soil from the same field. In the field, the effects of liming and NPK fertilization on the production and composition of the vegetation were studied. In pots, the effects of liming and phosphate fertilization on different grass species were analysed. The effect of NPK fertiliza- tion (+3Æ96 t ha )1 ) on the production of the original grassland was greater than that of liming (+0Æ68 t ha )1 ), which was only observed (P <0Æ05) on the unfertilized plots. Liming increased the total number of species and the proportion of dicotyledons. After replacing the semi-natural community with Dactylis glomerata L., the effects of liming (+2Æ37 t ha )1 ) and fertilization (+6Æ52 t ha )1 ) were increased. These results, together with those of the pot trial, show the important role of phosphorus in the fertilization effect, and are inter- preted as a protective effect of P against aluminium toxicity. Keywords: soil acidity, root growth, phosphorus, alu- minium Introduction On soils where pH (pH water ) is below 5Æ5, acidity can lead to conditions unfavourable for plant growth (Von Uexku ¨ ll and Mutert, 1995). In areas that have a temperate climate, acidity is frequently observed in high-rainfall regions, such as northern Europe (Falken- gren-Grerup and Tyler, 1993; Tyler, 1996; Austrheim et al., 2005; Fystro and Bakken, 2005), Galicia in Spain (Antuna et al., 1980), the Vosges (Fichter et al., 1998) and the Alps (Baize and Roque, 1998) in France. On acid soils, aluminium and manganese toxicity (Calba and Jaillard, 1997; Bolan et al., 2003; Kochian et al., 2005) and suboptimal supply of nutrients, notably of phosphorus (Hocking, 2001), are the major growth limitations. In grasslands, these limitations lead to the proliferation of grass species of limited or intermediate productivity and low nutritional value for livestock (Boeker, 1969; Antuna et al., 1980; Johnston, 1997). These grasses, such as Deschampsia flexuosa, Festuca rubra, Molinia caerulea and Sieglingia decumbens, have a low relative growth rate (<210 g kg )1 d )1 ), a high leaf dry matter (DM) content (>310 g kg )1 ) and resistance to Al toxicity ({Al} 50 > 10 lM; Poozesh et al., 2007). Due to the selection of the better land for other crops, grasslands are increasingly restricted to fields of poor productivity, often likely to be abandoned, such as poor acid grassland on hillsides (Marriott et al., 2004). Fer- tilization can bring these grasslands up to the yield level sought by the farmer, but pose problems, notably as regards the environment, sustainability and biodiversity (Plantureux et al., 2005). Faced with these problems, surface liming of acid grasslands is highly recommended agronomically, but its consequences for forage production are debatable. The favourable effect of liming on grass production has been established for Italian ryegrass grown in pots (Curtin and Smillie, 1986) and in grassland sown mainly with perennial ryegrass: the total grass yield was increased by 45% (8Æ1 vs. 5Æ6 t DM ha )1 ) in Euskadie (Spain) on soils that had an initial pH (water) of 4Æ8 (Pinto et al., 1995) and by 13% (10Æ9 vs. 9Æ6 t ha )1 ) in Ireland, for soils that had initial pH (water) between 5Æ1 and 5Æ5 (Stevens and Laughlin, 1996). However, these results were obtained on sown grass- lands and, moreover, with perennial ryegrass (Lolium perenne L.) which is sensitive to Al toxicity (Wheeler et al., 1992; Poozesh et al., 2007). More recently, Fystro and Bakken (2005) have shown herbage yield increases in response to surface liming of established leys based Correspondence to: Dr G. Bertoni, Ecole Nationale Supe ´ rieure Agronomique, Institut National Polytechnique de Tou- louse, BP 32607 – F 31326 Castanet-Tolosan, France. E-mail : bertoni@ensat.fr Received 24 November 2009; revised 25 February 2010 Ó 2010 Blackwell Publishing Ltd. Grass and Forage Science, 65, 260–272 doi: 10.1111/j.1365-2494.2010.00744.x 260