Minerals in the rhizosphere: overlooked mediators of soil nitrogen availability to plants and microbes

Abstract

Despite decades of research progress, ecologists are still debating which pools and fluxes provide nitrogen (N) to plants and soil microbes across different ecosystems. Depolymerization of soil organic N is recognized as the rate-limiting step in the production of bioavailable N, and it is generally assumed that detrital N is the main source. However, in many mineral soils, detrital polymers constitute a minor fraction of total soil organic N. The majority of organic N is associated with clay-sized particles where physicochemical interactions may limit the accessibility of N-containing compounds. Although mineralassociated organic matter (MAOM) has historically been considered a critical, but relatively passive, reservoir of soil N, a growing body of research now points to the dynamic nature of mineral-organic associations and their potential for destabilization. Here we synthesize evidence from biogeoscience and soil ecology to demonstrate how MAOM is an important, yet overlooked, mediator of bioavailable N, especially in the rhizosphere. We highlight several biochemical strategies that enable plants and microbes /doi.org/10.1007/s10533-018-0459-5 to disrupt mineral-organic interactions and access MAOM. In particular, root-deposited low-molecularweight exudates may enhance the mobilization and solubilization of MAOM, increasing its bioavailability. However, the competitive balance between the possible fates of N monomers-bound to mineral surfaces versus dissolved and available for assimilation-will depend on the specific interaction between mineral properties, soil solution, mineral-bound organic matter, and microbes. Building off our emerging understanding of MAOM as a source of bioavailable N, we propose a revision of the Schimel and Bennett (Ecology 85:591-602, 2004) model (which emphasizes N depolymerization), by incorporating MAOM as a potential proximal mediator of bioavailable N.123 Biogeochemistry (2018) 139:103-122 https:/

Rhizosphere priming effects in soil aggregates with different size classes

“…, Jilling et al. ). Several studies have found a positive relationship between the primed gross N mineralization and RPE (Dijkstra et al.…”

“…By providing labile C to rhizosphere microbes, it has been suggested that plants can accelerate microbial decomposition of N-enriched SOM (particularly of mineral associated organic matter) and convert the unavailable organic N into available inorganic N forms to plants (i.e., priming on gross N mineralization; Cheng et al 2014, Jilling et al 2018. Several studies have found a positive relationship between the primed gross N mineralization and RPE (Dijkstra et al 2009, Bengtson et al 2012, Zhu et al 2014, Yin et al 2018).…”

“…3a, b). Rhizodeposition can be rapidly adsorbed to the surface of microaggregates due to their larger specific surface area (Totsche et al 2018), which not only can directly relieve microbial C limitation and activate microbes in microaggregates, but also can liberate mineral-protected C and N through organic acids (e.g., oxalic acid) breaking the mineral-organic compounds (Keiluweit et al 2015, Jilling et al 2018). These processes may substantially enhance SOC decomposition through microbial activation (Cheng and Kuzyakov 2005).…”

Changing perspectives on terrestrial nitrogen cycling: The importance of weathering and evolved resource‐use traits for understanding ecosystem responses to global change

“…By contrast, fertilization can directly relieve N limitation of soil microbes, which can increase C use efficiency and thus the size of the microbial biomass pool (Manzoni, Taylor, Richter, Porporato, & Agren, ; Sistla, Asao, & Schimel, ). A larger microbial biomass pool will increase inputs of microbially derived C (biomass, exudates and necromass) onto mineral surfaces, thus increasing the mineral‐associated organic matter pool (Averill & Waring, ; Cotrufo, Wallenstein, Boot, Denef, & Paul, ; Jilling et al, ). As such, predicting responses of soil C storage to N enrichment will require considering microbial nutrient limitation status in the context of other abiotic drivers of C storage, such as soil acidity and mineralogy.…”

“…New insights into the global nitrogen (N) cycle (red) overlaid on the conventional soil N cycle (black) as illustrated in the Schimel and Bennett () and Jilling et al () and other conventional N pools (boxes) and processes (arrows). (a) A biologically significant portion of N inputs to ecosystems originate from parent material; (b) microbial transformations of N depend variation in microbial resource‐use traits and abiotic soil characteristics; (c) N limitation of productivity depends on variation in plant resource‐use traits, including associations with root‐associated microbes; and (d) community‐level variation in plant resource‐use strategies mediate whether fire as a disturbance induces N losses from ecosystems.…”