Abstract
To understand the responses to external disturbance such as defoliation and possible feedback mechanisms at global change in terrestrial ecosystems, it is necessary to examine the extent and nature of effects on aboveground-belowground interactions. We studied a temperate heathland system subjected to experimental climate and atmospheric factors based on prognoses for year 2075 and further exposed to defoliation. By defoliating plants, we were able to study how global change modifies the interactions of the plant-soil system. Shoot production, root biomass, microbial biomass, and nematode abundance were assessed in the rhizosphere of manually defoliated patches of Deschampsia flexuosa in June in a full-factorial FACE experiment with the treatments: increased atmospheric CO 2, increased nighttime temperatures, summer droughts, and all of their combinations. We found a negative effect of defoliation on microbial biomass that was not apparently affected by global change. The negative effect of defoliation cascades through to soil nematodes as dependent on CO 2 and drought. At ambient CO 2, drought and defoliation each reduced nematodes. In contrast, at elevated CO 2, a combination of drought and defoliation was needed to reduce nematodes. We found positive effects of CO 2 on root density and microbial biomass. Defoliation affected soil biota negatively, whereas elevated CO 2 stimulated the plant-soil system. This effect seen in June is contrasted by the effects seen in September at the same site. Late season defoliation increased activity and biomass of soil biota and more so at elevated CO 2. Based on soil biota responses, plants defoliated in active growth therefore conserve resources, whereas defoliation after termination of growth results in release of resources. This result challenges the idea that plants via exudation of organic carbon stimulate their rhizosphere biota when in apparent need of nutrients for growth.
Highlights
Soil biota plays a significant role in biogeochemical cycling and their responses to global change are considered important at the ecosystem scale (Brussaard 1998; Bradford et al 2002), but are remarkably understudied (West et al 2006; Bardgett et al 2013)
We found a negative effect of defoliation on soil biota exerted via aboveground–belowground interactions
The present study shows that exudation and belowground allocation of resources to the advantage of the soil biota does not occur when the perennial plant is in need of resources for shoot growth
Summary
Soil biota plays a significant role in biogeochemical cycling and their responses to global change are considered important at the ecosystem scale (Brussaard 1998; Bradford et al 2002), but are remarkably understudied (West et al 2006; Bardgett et al 2013). The interactions between the aboveground and the belowground spheres are complex relationships affected by biotic as well as abiotic factors. Defoliation is a disturbance of the plant–soil system by partial removal of the aboveground biomass. Defoliation effects belowground depend on abiotic factors such as climate as well as biotic factors such as plant growth phase (Guitian and Bardgett 2000; Wilsey 2001; Yeates et al 2003; Ilmarinen et al 2005; Lau and Tiffin 2009; Yeates and Newton 2009; Stevnbak et al 2012). Defoliation effects on the plant–soil interactions may relate to whether plants stimulate decomposition through the exudation of low-molecular-mass carbon compounds when in apparent need of nutrients (Griffiths and Robinson 1992).
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