02547nas a2200169 4500008004100000245012700041210006900168300001500237490000800252520195400260100001702214700001802231700002002249700001802269700001302287856007702300 2009 eng d00aCross-site comparison of herbivore impact on nitrogen availability in grasslands: the role of plant nitrogen concentration0 aCrosssite comparison of herbivore impact on nitrogen availabilit a1613 -16220 v1183 a
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.
1 aBakker, E.S.1 aKnops, J.M.H.1 aMilchunas, D.G.1 aRitchie, M.E.1 aOlff, H. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/j.1600-0706.2009.17199.x