1098 available P was determined according to the Bray 1 method. The pH of soils was measured according to the water method (Anonymous 1990). The soil nitrogen mineralization rates were determined in only eight kraal—control pairs over a 28-day period in the wet season. We used a ﬁeld incubation method to quantify the in situ net N mineralization rate (Robertson et al. 1999; Grant and Scholes 2006; Coetsee et al. 2008). For this purpose, six stainless steel pipes (length 15 cm, diameter 5 cm) were implanted per site so that 10 cm of the pipe length was buried. The pipe tops were covered and an aboveground side hole allowed gas exchange. Net miner- alization rates were calculated as the difference between nitrate and ammonium concentrations at the start and end of the incubation period, after correcting for soil moisture (Robertson et al. 1999; Coetsee et al. 2008). Start-incuba- tion soil samples were taken within 30 cm radius of implanted incubation pipes. Soil samples were pooled per site and 10 g sub-samples, contained in plastic bags, were kept on ice until extraction with KCl (Robertson et al. 1999; Coetsee et al. 2008). Extracts were analyzed for nitrate and ammonium concentration by the Institute for Soil, Climate and Water of the Agricultural Research Council, Pretoria (Coetsee et al. 2008). The texture of soil sub-samples was determined following Sammis (1996). A bioassay with maize Zea mays was carried out to validate the ﬁndings of the soil chemical analysis. Soil sub- samples from kraal (n = 9) and control sites (n = 9), were contained in nursery bags, and ﬁve maize seeds (com- mercial supply) were planted per bag. The maize plants were kept in a nursery and rainfall was supplemented with frequent watering. After 2 months, the aboveground bio- mass was harvested, dried (70 C) to constant mass and weighed. Aboveground biomass was taken as an indicator of soil nutrient status. Vegetation biomass and forage quality The aboveground herbaceous biomass of kraal and control sites was estimated by taking 100 readings per site with a standard Disc Pasture Meter during February 2007 (Bransby and Tainton 1977; Waldram et al. 2008). We used the disc settling height—herbaceous biomass calibration curve, derived for swards in the KNP, of Zambatis et al. (2006). Herbaceous swards in the study area have similar species composition and biomass as swards in the KNP (10–60 km east). The woody layer was assessed in 200 m2 plots, or in the case of smaller kraal sites, the largest plot area that ﬁtted into kraal sites (minimum, 60 m2). Live woody plants rooted within plots were counted. To determine if forage quality was higher on kraal sites than on control sites, we analyzed leaf samples of the 123 Oecologia (2011) 165:1095–1107 common grass Urochloa mosambicensis, which occurred on all sites except one control site. Leaf material from a random selection of U. mosambicensis tufts were collected on each site, and the samples were dried at 70 C and milled through a 1-mm sieve. N, P, Ca, Mg, Na and K concen- tration were measured with a Skalar San-plus autoanalyzer at the laboratory of the Wageningen University (The Netherlands) after destruction with a mixture of H2SO4, Se and salicylic acid (Novozamsky et al. 1983). Herbivore use of kraal sites The use of kraal and control sites by large herbivores was assessed by (1) calculating dung accumulation rates in marked plots (Augustine 2003), (2) using exclosure cages to assess consumption (Ydenberg and Prins 1981; McNaughton et al. 1996) and (3) assessing the frequency of herbaceous tuft use (Grant and Scholes 2006). First, dung piles were counted in rectangular plots with the maximum area that ﬁtted within kraal sites (20 9 30 m, but depending on kraal area), while a default of 20 9 50 m plot size was used for control sites. Dung piles were marked with small painted stones at the beginning of the study period (December 2006). After a 33- to 45-day per- iod, new dung piles (unmarked) were counted per species. The identiﬁcation of dung was aided by experienced ﬁeld rangers. Second, 1 9 1.5 m exclosures (n = 16 wire netted cages) were installed in December 2006. After 8 weeks, the herbaceous vegetation in two 0.5 9 0.5 m quadrates were clipped close to ground level inside the grazer-pro- tected cage and two quadrates alongside cages (controls). The cuttings from cages and controls were separately pooled, dried and weighed. Third, at each site, we assessed the closest herbaceous plant to 100 points spaced 1 m apart (two or more parallel transects) for presence or absence of recent grazing signs following Grant and Scholes (2006). Plant utilization was expressed as the percentage of tufts utilized. Habitat manipulation experiment We used data from a concomitant habitat manipulation experiment to test whether impala in the study area respond to changes in vegetation structure alone, without associated changes in grazing quality. As part of a larger experiment, six circular plots (34 m diameter) were marked in a 500 9 800 m closed Mopane woodland in the northern parts of the study area. In May 2007, three of the six plots were cleared of all woody vegetation by cutting stems with a chainsaw close to the ground (\5 cm height) and treating stumps with an arboricide. The cut plant material was removed from the plots and scattered in the surrounding area. From June 2007 to January 2009, new impala dung