Plant root and mycorrhizal fungal traits for understanding soil aggregation

Abstract

Soil aggregation is a key ecosystem process resulting in the formation and stabilization of soil structure, consisting of soil aggregates and the resulting matrix of pore spaces. As such, it significantly alters the environment of plant roots and microbes in a multitude of ways; thus, soil structure provides the basic setting in which mycorrhizas operate and have evolved. Not surprisingly, soil aggregation is important for root growth and for a wide range of soil features and ecosystem process rates, such as carbon storage and resistance to erosion (e.g. Jastrow et al., 1998; Six et al., 2006). The aggregation of soil is a complex process, regulated by a range of abiotic factors (e.g. texture) and mediated by plants and multiple biota groups and their interactions; in spite of this complexity, plant roots and their mycorrhizal symbionts are consistently found to be a crucial force in driving soil aggregation (Six et al., 2004; Rillig & Mummey, 2006; Leifheit et al., 2014). Plant and mycorrhizal fungal species, respectively, differ in their contribution to soil aggregation (Reid & Goss, 1981; Angers & Caron, 1998; Eviner & Chapin, 2002; Rillig et al., 2002; Piotrowski et al., 2004; Six et al., 2004; Duchicela et al., 2012; Graf & Frei, 2013; Pérès et al., 2013), but from previous studies it has not yet become clear what specific root (and mycorrrhizal) traits contribute to this effect. This is likely because studies typically focus on only a limited suite of physiological and architectural characteristics, and the general research focus has traditionally been more on management practices and physico-chemical factors. Therefore, we lack a mechanistic understanding of the role of organisms in soil aggregation. To this end, we argue for a shift from a classical species-based comparative approach, or the mere consideration of summary variables (e.g. root length) to a systematic trait-based approach. Summarizing ecological characteristics of species by means of traits has become an essential tool in plant ecology (Westoby & Wright, 2006), and is increasingly proposed for root–fungal associations (e.g. van der Heijden & Scheublin, 2007; Chagnon et al., 2013; Aguilar-Trigueros et al., 2014). We believe trait-approaches are a promising tool to achieve progress in our mechanistic understanding of soil aggregation from an organismic perspective, to enhance our predictive ability regarding effects of plant and fungal diversity, and to ultimately provide informed recommendations for, for example, agricultural management and environmental restoration. With such data we can then ultimately address questions such as: how can plant and mycorrhizal fungal community data enhance prediction of soil aggregation beyond the consideration of state variables such as soil type, climate and management? Is soil aggregation an intermediate driver of plant and mycorrhizal fungal community processes and thus also an integral part of plant–soil feedbacks? And, on the applied side, can we enhance ecosystem restoration by designing tailor-made mycorrhizal fungal inocula and seed mixes specifically optimized for complementarity in the ability to enhance soil aggregation? In addition to general advantages, we see specific benefits of a trait-based approach applied to the understanding of mycorrhizal and plant contributions to soil aggregation: (1) it will allow testing for the extent of root and mycorrhizal fungal functional complementarity with regard to soil aggregation, and thus whether soil aggregation is another example of an ecosystem process that positively relates to biodiversity like other more well-studied processes such as primary productivity. This is very likely true because soil aggregation is the result of multiple interlocking component processes such as aggregate formation and stabilization (both of microaggregates and macroaggregates) which might be driven by different sets of traits represented in different species. Additionally, soil aggregation may be stimulated through diversity-fueled overyielding of fungi and roots through niche complementarity (also see Table 1); (2) if we can convincingly relate soil structure to traits of plant and fungal communities, this will allow predicting which ecosystems are under most risk of deterioration and also guide towards measures to counter this process. Measure: (loge (root or hyphal lengtht0) − loge (root or hyphal lengtht1)) × (t0 − t1)−1 Rationale: these three traits represent the growth ability of hyphae or roots to encounter building materials of aggregates in soil (primary particles, organic matter particles) and then form macroaggregatesa Proxy: (number of branches) × (root or hyphal length)−1; root or hyphal branching number and angle; alternatively: directly measure ability to engulf an object Rationale: these three traits measure the capability of hyphae or roots to stabilize aggregates depending on their tearing-resistance (i.e. ability to hold material together without b