Quercus ilex root growth in response to heterogeneous conditions of soil bulk density and soil NH 4 -N content Elena Cubera *, Gerardo Moreno, Alejandro Solla Ingenierı´a Te ´cnica Forestal, Universidad de Extremadura, Avenida Virgen del Puerto 2, 10600 Plasencia, Spain 1. Introduction In the western European Mediterranean countries, holm oak (Quercus ilex L.) is the most abundant forest tree and dominates the landscape in an estimated 4.1 million ha of open woodland. In these oak woodlands, the landscape is a mosaic of trees and patches of cultivated and grazed ecosystems. Since the early 1980s there has been a great concern about the severe decline of Q. ilex trees in Spain and Portugal (Brasier, 1996). Severe drought episodes, flooding and rapid fluctuations in soil water content have been reported as predisposing factors favouring tree invasion by bark borer insects and/or opportunistic fungi (Brasier, 1996; Sa ´ nchez et al., 2002). Additional decline-inducing factors sug- gested include rangeland practices generated by the changes in the traditional management of oak open woodlands, such as the use of heavy machinery for forage production, the increase of stocking rates, and the excess of N supply (Sa ´ nchez et al., 2002; Thomas et al., 2002). The effects of these recent management practices on Q. ilex health and root development have not yet been studied. Overuse of machinery, intensive cropping and trampling by grazing livestock decrease soil macropore spaces, resulting in soil compaction (Hamza and Anderson, 2005; Sa ´ nchez-Andre ´s et al., 2006). Soil compaction decreases infiltration and leads to reduced root growth and plant survival (Jordan et al., 2003; Mo ´ sena and Dillenburg, 2004; Watson and Kelsey, 2006). In Q. ilex open woodland cultivated soils, dense compact subsoils frequently underlie the loosened topsoil (Sa ´ nchez-Andre ´s et al., 2006) whereas the opposite situation occurs in areas with high grazing pressure (Coelho et al., 2004). As tillage methods changed from full width and/or uniformly applied soil disturbance to more strip-type soil manipulation, soil physical and chemical conditions became less homogeneous. Spatial patterns of physical and/or chemical properties may develop in those fields managed with controlled wheel traffic and/or localized fertilizer placement. Livestock grazing alters nutrient dynamics because large amounts of N are transferred to the soil in the excreta of animals. As the intensity of Soil & Tillage Research 103 (2009) 16–22 ARTICLE INFO Article history: Received 18 February 2008 Received in revised form 19 June 2008 Accepted 7 September 2008 Keywords: Soil bulk density Soil NH 4 content Root density Root depth Quercus ilex Oak decline ABSTRACT A greenhouse study examined the effects of heterogeneous conditions of soil bulk density and soil NH 4 -N content on the growth of holm oak (Quercus ilex L.) seedlings over an 8-month period. Heterogeneity of soil was produced inside 150 cm depth pots, by applying manual compaction or enriching different soil layers with NH 4 (vertical heterogeneity) and, by splitting the root system into two columns with differing soil bulk density or NH 4 conditions (horizontal heterogeneity). Root distribution patterns were assessed by collecting roots within layers, each 10 cm depth, and by recording both the fine (diameter <2 mm) and the coarse (diameter 2 mm) root biomass. Root systems significantly responded to the vertical gradients of soil bulk density, decreasing their maximum depth and showing lower values of fine root densities at the layers with a dry bulk density of 1.62 Mg m 3 . The split-root system showed that maximum rooting depth of the axis with a dry bulk density of 1.38 Mg m 3 was limited by the reduced rooting depth of the adjacent more compacted axis. Root systems were less dense in the NH 4 soil treatment than in the control soil treatment (34 mg dm 3 vs. 212 mg dm 3 ), even at the lowest rate used (0:1 g NH 4 L 1 soil ). This high sensitivity to NH 4 conducted to a significant decrease of shoot height and root depth (50 and 43%, respectively). The split-root system showed that unfavourable conditions of NH 4 content have local consequences on the rooting depth and systemic consequences on the fine root density. The implications of root growth restrictions for Q. ilex natural regeneration, seedling survival, and decline are discussed. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +34 927427000; fax: +34 927 425209. E-mail address: ecubera@unex.es (E. Cubera). Contents lists available at ScienceDirect Soil & Tillage Research journal homepage: www.elsevier.com/locate/still 0167-1987/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2008.09.002