- Ecosystem changes associated with grazing in subhumid grasslands - 323 Journal of Vegetation Science 17: 323-332, 2006 © IAVS; Opulus Press Uppsala. Abstract Question: What are the changes in vegetation structure, soil attributes and mesofauna associated with grazing in mesic grasslands? Location: Southern Campos of the Río de la Plata grasslands, in south-central Uruguay. Methods: We surveyed seven continuously grazed and ungrazed paired plots. Plant and litter cover were recorded on three 5-m interception lines placed parallel to the fence in each plot. We extracted soil fauna from a 10 cm deep composite sample and analysed the oribatids. Soil attributes included bulk density, water content, organic carbon (in particulate and mineral associated organic matter) and nitrogen content and root biomass at different depths. Changes in floristic, Plant Functional Types and mesofauna composition were analysed by Non-metric Multidimensional Scaling. Results: Species number was lower in ungrazed than in grazed plots. Of 105 species in grazed plots only three were exotics. Shrub and litter cover were significantly higher inside the exclosures, while the cover of Cyperaceae-Juncaceae was lower. Grazing treatments differed significantly in plant and oribatid species composition.. Grazing exclusion significantly reduced soil bulk density and increased soil water content. Carbon content in particulate organic matter was lower in the upper soil of ungrazed sites, but deeper in the profile, grazing exclosures had 8% more carbon in the mineral associated organic matter. Conclusions: Our results generally agree with previous studies but deviate from the results of previous analyses in (1) the increase of shrub cover in ungrazed sites; (2) the redistribution of the soil organic carbon in the profile and (3) the low invasibility of the prairies regardless of grazing regime. Keywords: Oribatide; Plant Functional Type; Soil attribute; Species richness; Uruguay. Abbreviations: BMRPP = Blocked multi-response permuta- tion procedures; INDVAL = Indicative values; MAOM = Mineral associated organic matter; NMDS = Non-metric multi dimensional scaling; POM = Particulate organic matter. Nomenclature: Cabrera & Zardini (1978). Ecosystem changes associated with grazing in subhumid South American grasslands Altesor, A. 1* ; Piñeiro, G. 2,4 ; Lezama, F. 1,5 ; Jackson, R.B. 3,6 ; Sarasola, M. 1,7 & Paruelo, J.M. 1,2,8 1 Departamento de Ecología, Facultad de Ciencias, Universidad de la Repúbica, Iguá 4225 Montevideo,C.P.11400, Uruguay; 2 Laboratorio de Análisis Regional y Teledetección, IFEVA-Cátedra de Ecología, Facultad de Agronomía, Universidad de Buenos Aires y CONICET, Av. San Martin 4453, C1417DSE, Buenos Aires, Argentina; 3 Department of Biology and Nicholas School of the Environment and Earth Sciences, Duke University, Box 90328, Durham, NC 27708, USA; 4 E-mail pineiro@ifeva.edu.ar; 5 E-mail felipe@ fcien.edu.uy; 6 E-mail jackson@duke.edu; 7 E-mail sarasola@fcien.edu.uy; 8 E-mail paruelo@agro.uba.ar; *Corresponding author; Fax +59 825258616; E-mail aaltesor@fcien.edu.uy Introduction The effects of grazing on the structure and function- ing of grasslands, shrublands and savannas have gener- ated controversy and debate in the literature (McNaughton 1979; Milchunas & Lauenroth 1993; Oesterheld et al. 1999; Chase et al. 2000). Grazing causes many commu- nity level changes. Prostrate growth forms tend to be- come more common at grazed sites in many grassslands (Facelli 1988; Díaz et al. 1992; Rodriguez et al. 2003). Shrub invasion of grasslands is considered a manage- ment problem in many parts of the world (Archer et al. 1988) but in other areas shrubs can decrease with graz- ing (Cipriotti & Aguiar 2004). Livestock may also af- fect the abundance and composition of soil fauna (Battigelli et al. 2003). Grazing also influences the biogeochemical and physical properties of soils. The effects of grazing on soil organic matter contents are variable, showing both increases and reductions (Milchunas & Lauenroth 1993). Grazing also induces changes in root distribution and biomass, altering water and carbon dynamics (Milchunas et al. 1989; Milchunas & Lauenroth 1992). Trampling often reduces soil bulk density and soil water holding capacity (Taboada & Lavado 1988). In summary, graz- ing promotes changes at different hierarchical levels with multiple feedbacks (Brown & Allen 1989). In this article we focus on the integrated response of grasslands at community and ecosystem levels to domestic herbiv- ore grazing. The Río de la Plata grasslands are the most extensive biogeographic unit of the prairie biome in South America; they have been extensively modified by human activities (Guerschman et al. 2003). They occupy more than 700 000 km 2 , distributed across eastern Argentina, Uruguay and southern Brazil (Soriano 1991). Steppes and prairies, though modified by grazing, still cover most of the area, mainly in the Uruguayan Campos and the flooding pampa