Plant responses to soil heterogeneity and global environmental change

Abstract

Recent evidence suggests that soil nutrient heterogeneity, a ubiquitous feature of terrestrial ecosystems, modulates plant responses to ongoing global change (GC). However, we know little about the overall trends of such responses, the GC drivers involved, and the plant attributes affected.We synthesized literature to answer the question: Does soil heterogeneity significantly affect plant responses to main GC drivers, such as elevated atmospheric carbon dioxide concentration (CO₂), nitrogen (N) enrichment and changes in rainfall regime?Overall, most studies have addressed short-term effects of N enrichment on the performance of model plant communities using experiments conducted under controlled conditions. The role of soil heterogeneity as a modulator of plant responses to elevated CO₂ may depend on the plasticity in nutrient uptake patterns. Soil heterogeneity does interact with N enrichment to determine plant growth and nutrient status, but the outcome of this interaction has been found to be both synergistic and inhibitory. The very few studies published on interactive effects of soil heterogeneity and changes in rainfall regime prevented us from identifying any general pattern.We identify the long-term consequences of soil heterogeneity on plant community dynamics in the field, and the ecosystem level responses of the soil heterogeneity × GC driver interaction, as the main knowledge gaps in this area of research.In order to fill these gaps and take soil heterogeneity and GC research a step forward, we propose the following research guidelines: 1) combining morphological and physiological plant responses to soil heterogeneity with field observations of community composition and predictions from simulation models; and 2) incorporating soil heterogeneity into a trait-based response-effect framework, where plant resource-use traits are used as both response variables to this heterogeneity and GC, and predictors of ecosystem functioning.Synthesis. There is enough evidence to affirm that soil heterogeneity modulates plant responses to elevated atmospheric CO₂ and N enrichment. Our synthesis indicates that we must explicitly consider soil heterogeneity to accurately predict plant responses to GC drivers.

Keywords: CO2, changes in rainfall regime; N enrichment; community-level studies; ecosystem functioning; global change; plant performance; plant-soil interactions; root foraging; soil nutrient heterogeneity.

Figure 1

Shoot biomass of experimental grassland communities compared across soil nutrient heterogeneity levels and atmospheric CO₂ (A), nutrient availability (B) as a measure of N enrichment, and water availability (C) as a measure of changes in rainfall regime. (A) Consisted of two levels of atmospheric CO₂: ambient (37.5 Pa) and elevated (70 Pa). (B) Consisted of two levels of nutrient availability: low (40 mg N) and high (120 mg N). (C) Consisted of three levels of water availability: low, medium and high (15.92, 31.84 and 47.76 L / m² of water per week, respectively). Data are means + SE (n = 18 for data in A, n = 32 for data in B and n = 24 for data in C). Redrawn with permission from the New Phytologist Trust (A: Maestre, Bradford & Reynolds 2005), Global Change Biology (B: Maestre & Reynolds 2006b) and Ecology (C: Maestre & Reynolds 2007b).

Figure 2

Schematic representation showing the direct and indirect effects of soil heterogeneity and global change on ecosystem functioning that are mediated by plants. NH = soil nutrient heterogeneity, CO₂ = elevated atmospheric CO₂, N enr = N enrichment, GC = global change, and CRR = changes in rainfall regime. Blue arrows represent direct effects. Square boxes depict soil nutrient heterogeneity. Ellipse boxes depict global change. Rectangle boxes depict both plant and ecosystem effects. Dashed boxes depict NH × GC interactions. Interrogation marks represent unknown or understudied effects.

Figure 3

Conceptual model proposed to include plant responses to soil heterogeneity (NH) and global change (GC) into a trait-based response-effect framework. Both NH and GC may affect the abundance and representation of a suite of response traits related with the plant-resource use strategy, with further effects on community dynamics. The subsequent and altered plant community may impact ecosystem functioning in a different way than the initial community via changes in the abundance and representation of ecosystem-effect traits. If GC and NH affect the same suite of response and effect traits, this conceptual framework will allow us to include soil heterogeneity when scaling plant responses to global change to the ecosystem level.