Abstract
Root litter is the dominant soil carbon and nutrient input in many ecosystems, yet few studies have considered how root decomposition is regulated at the landscape scale and how this is mediated by land-use management practices. Large herbivores can potentially influence below-ground decomposition through changes in soil microclimate (temperature and moisture) and changes in plant species composition (root traits).To investigate such herbivore-induced changes, we quantified annual root decomposition of upland grassland species in situ across a landscape-scale livestock grazing experiment, in a common-garden experiment and in laboratory microcosms evaluating the influence of key root traits on decomposition.Livestock grazing increased soil temperatures, but this did not affect root decomposition. Grazing had no effect on soil moisture, but wetter soils retarded root decomposition. Species-specific decomposition rates were similar across all grazing treatments, and species differences were maintained in the common-garden experiment, suggesting an overriding importance of litter type. Supporting this, in microcosms, roots with lower specific root area (m2 g−1) or those with higher phosphorus concentrations decomposed faster.Our results suggest that large herbivores alter below-ground carbon and nitrogen dynamics more through their effects on plant species composition and associated root traits than through effects on the soil microclimate.
Highlights
Regulation of plant litter decomposition determines carbon (C) and nitrogen (N) cycling in soils
Plant height in A. capillaris and N. stricta communities is significantly reduced by grazers, whilst in J. effusus and M. caerulea dominated swards grazing reduces the abundance of tussocks without much effect on canopy height (Dennis et al, 2004; Smith et al, 2014)
Soils were significantly warmer under more intense livestock grazing in swards of all four dominant upland grass species (Fig. 1; Table 1), with the greatest treatment differences in A. capillaris swards (1.38 Æ 0.16°C (Æ 1 SD) warmer under commercial than under no grazing)
Summary
Regulation of plant litter decomposition determines carbon (C) and nitrogen (N) cycling in soils. The assumption that root and leaf decomposition rates are comparable and responsive to the processes controlling decomposition, such as climatic conditions, may lead to erroneous predictions of C cycling (see Freschet et al, 2013). This commonly held assumption underlying models currently used to predict soil C stocks (Smith et al, 1997; Jones et al, 2005; Davidson & Janssens, 2006) is challenged by the fact that the soil environment buffers climatic effects on root decomposition
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