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Nanoscale processes in soils: The role of mycorrhizal fungi in aggregation and phosphorus acquisition

Final Report Summary - NANOSOIL (Nanoscale processes in soils: The role of mycorrhizal fungi in aggregation and phosphorus acquisition)

The project NANOSOIL investigated into small scale properties of soils important for nutrient availability and structure.
Earth’s soils are the basis of our terrestrial ecosystems and our food production. A healthy soil consists of stable aggregates containing minerals and organic matter, so that water and air can permeate the interspaces, and the aggregated particles brave erosion. Mineral nutrients for plant growth can derive from minerals or are recycled form organic matter. To increase efficiency in agriculture, mineral fertilizers have classically been applied to croplands in excessive amounts during the last decades, but Earth’s resources of especially the macronutrient phosphorus (P) are limited. Arbuscular mycorrhizal (AM) fungi are plant root symbiotic fungi essential for optimal growth and mineral nutrient supply, especially P.

The aim of the project NANOSOIL was to understand small-scale processes of nutrient foraging by AM hyphae and their effect on and interactions with the soil environment.

Charcoal, when added to soils, is a stable compound has been found to increase plant productivity. The exact mechanistic background is not yet known; changes in soil properties as pH, water holding capacity and ion exchange capacity are most likely important factors. Nothing is known about the direct interactions between AM fungi and charcoal. Different kinds of charcoals were examined as their functions as nutrient matrices for AM fungi, AM fungal hyphae’s foraging was studied with microscopy and SEM, and nutrient uptake and symbiotic transfer was quantified via radiotracers.
Experiments started in laboratory systems involving sterile cultures of AM fungal hyphae that were offered different types of biochars, and charcoals evaluated for their properties as nutrient matrices with quantification via a radiotracer. Experiments were continued under non-sterile soil conditions.

AM fungal (Rhizosphagus irregularis) hyphae were attracted to different types of biochar under sterile and under non-sterile conditions. Hyphal contact and attachment increased with increased nutrient surface loading of the biochar compared to its environmental nutrient conditions. AM fungal hyphae grew onto the surface of different biochar types, grew into cracks and pores and explore the interior surface, and firmly attached to surfaces. Direct hyphal contact to the biochar surface was essential for nutrient uptake, as AM fungal hyphae delivered six times more 33P to their host plant when they have direct surface contact to a labeled biochar compared to solely contact to the surrounding solution with diffusion of the label into solution (Fig. 1). This lead to the hypothesis that AM fungi are required to fully exploit biochar as a nutrient reservoir, as only hyphae, not plant roots, can reach the major parts of the biochar surface.

Biochar and AM fungi applied together yielded an additive positive growth response in Lactuca sativa plants compared to both treatments alone. However, no additive effect in mineral nutrition could be detected.

AM fungi are also an important player concerning soil aggregation: Unlike saprophytic fungi, AM fungi explore mineral soil with their hyphal network in the absence of any energy sources, since they are supplied with C by their host plant. They physically bind and stabilize soil particles by their extensive hyphal network and bring biomass into mineral soil, and they are therefore crucial for the primary built-up of soil structure and organic matter content. Proteins produced by AM fungi were found to contain high concentrations of iron (Fe), and Fe is a known stabilizing agent in soil aggregates, bridging between mineral particles and organic matter.

The fellow studied dynamics within AM fungi soil aggregation and the nano-structure within AM fungal-formed soil aggregates and wanted to test the hypothesis whether AM fungi increase or change the ratio of organically bound Fe/inorganic Fe fraction in a soil.
This was performed in sterile or non-sterile soil-based systems with the presence or absence of AM fungi, and analyzed with synchrotron-light based spectroscopy and spectro-microscopy methods. A first Fe-EXAFS (extended x-ray absorbance fine structure) analysis at MaxLab in Lund, Sweden, on bulk soil samples could not detect differences between soil aggregated under the influence of AM fungal hyphae compared to sole physical aggregation. Therefore, analysis were continued with Fe-XAS (X-ray absonrbance) imaging at SSRL Stanford to obtain a higher degree of spatial resolution, enabling to measure at the spots of hypha-mineral interfaces. For this, four different sample preparation techniques were tested and evaluated. Best samples derived from thin polished resin embeddings. XRF (x-ray fluorescence) maps were produced to get a spatial overview over structures and chemical distribution of the soil aggregates, and Fe-XAS analysis was performed at selected areas of ca. 3x3 µm size. Analysis was performed at the interface of hyphae to the mineral matrix, and adjacent areas without hyphal growth, or at the interface of dead organic matter to minerals. Also aggregates formed without biological influence were probed.

XRF maps reveiled a high degree of spatial heterogeneity of nutrient distribution at a micrometer scale. Especially P distributions were very patchy. At the current state of research there is an indication that there is a higher proportion of organically bound Fe at the mineral-hyphal interface than at non-biological derived aggregates.

Understanding mechanisms of soil aggregation is important to maintain healthy soils and to lead possibilities to increase carbon sequestration to soils. The urgency of a sustainable P use has recently been addressed is just starting to reach the world press. The peak in P mining is expected to be reached within 30 years, and prices are expected to rise considerably in this context. A sustainable usage of the remaining P resources as by managing AMF within agriculture is of urgent need. Charcoal additions to soils so far seem to be a promising way to reduce leaching of soil nutrients and thus can reduce fertilizer application rates. Charcoal additions to soils together with management of AM fungi can therefore in many cases, besides other positive effects, increase the nutrient exchange matrix, lead to sustainable roads in agriculture. There is a potential to economically exploit these promising results in form of charcoal based fertilizer products.

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