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Cross-site comparison of herbivore impact on nitrogen availability in grasslands: the role of plant nitrogen concentration

Authors

  • E. S. Bakker,

  • J. M. H. Knops,

  • D. G Milchunas,

  • M. E. Ritchie,

  • H. Olff


E. S. Bakker (l.bakker@nioo.knaw.nl) and J. M. H. Knops, School of Biological Sciences, Univ. of Nebraska, 348 Manter Hall, Lincoln, NE 68588-0118, USA. Present address for ESB: Dept of Plant–Animal Interactions, Netherlands Inst. of Ecology, Rijksstraatweg 6, NL–3631 AC Nieuwersluis, the Netherlands. – D. G. Milchunas, Forest, Range and Watershed Stewardship Dept, Colorado State Univ., Ft Collins, CO 80523, USA. – M. E. Ritchie, Dept of Biology, Syracuse Univ., 130 College Place, Syracuse, NY 13244-1270, USA. – H. Olff, Centre for Ecological and Evolutionary Studies, Univ. of Groningen, PO Box 14, NL–9750 AA Haren, the Netherlands.

Abstract

Herbivores may influence the nitrogen (N) recycling rates and consequently increase or decrease the productivity of grasslands. Plant N concentration emerged as a critical parameter to explain herbivore effects from several conceptual models, which predict that herbivores decrease soil N availability when plant N concentration is low whereas they increase it when plant N concentration is high (Hobbs 1996, Ritchie et al. 1998, Pastor et al. 2006). However, a broader cross-site comparison among published studies to test these predictions is hampered by the different methodologies used to measure soil N availability or a proxy thereof, and a lack of measurements of plant N concentration. Therefore it remains unclear whether these model predictions are generally valid across a range of grasslands.

We tested whether there is a relationship between plant N concentration and herbivore impact on soil N availability (measured with resin bags) with a study of replicate 6–8 year old exclosures (with an unfenced control) of vertebrate herbivores (>1 kg) established at each of seven grassland sites in North America and Europe. Contrary to model predictions, we found a negative relationship between the effect of herbivores on resin bag soil N availability and plant N concentration. Our study confirms the importance of plant N concentration as a predictor of herbivore effect on soil N availability across grasslands, but contradicts the models. A possible explanation may be that the results represent a transient situation as the exclosures were relatively young whereas the models may refer to an equilibrium state. Simultaneous measurements of both plant N concentration and herbivore effect on soil N availability from more grassland sites, preferably with contrasting plant N concentrations and including exclosures of different ages, should resolve the contrast between model predictions and our field measurements.

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