Investigations into lactate-bound metals: A sustainable alternative to inorganic micronutrient fertilizers

Lee James Opdahl

doi:10.7273/000005137

Intensive farming practices have widely led to reduced micronutrient fertility and overall soil health. This trend coupled with climate change has prompted a renewed interest in sustainable and regenerative approaches to agriculture. Overall soil health and the soil microbiome have been identified as key areas to improve the agronomic performance of crops. Metal lactates are a class of fertilizer amendment that supply micronutrients as well as a labile carbon source for microbial nutrition, but no work has been done to characterize metal lactates in the context of crop production. Chapter 1 provides a comprehensive review of five metals (Zn, Mn, Cu, Ni, and Co) and their role in plant and microbial metabolism, while also discussing rhizosphere processes and microbial groups involved in plant nutrient acquisition. To gain a fundamental understanding of plant responses to metal lactates, Chapter 2 describes the uptake and toxicity of five metals in lactate and chloride form that were compared in hydroponically grown wheat (Triticum aestivum L.) using a series of sterile culture vessel experiments. Furthermore, optimal concentrations of each metal lactate were determined using primary root growth of plate-grown Arabidopsis thaliana as an indicator. Our results showed comparable uptake and utilization patterns between metal lactates and metal chlorides, and optimal concentrations for Zn (0.5-1.0 µM), Mn (0.5-1.0 µM), Cu (0.5 µM), Ni (1.0 µM), and Co (0.5 µM) lactate were identified. Our next study in Chapter 3 compared the distribution of metals in both lactate and chloride form in silt loam soil using a modified sequential extraction procedure to determine the efficacy of metal lactates to provide readily available micronutrients. We also compared the effect of metals in lactate and chloride form on microbial biomass by extracting and quantifying phospholipid fatty acids (PLFA). Our findings suggest a mostly comparable distribution of metals in both forms, with slightly greater bioavailability found for metal chlorides, while microbial PLFA analysis showed a significantly greater stimulatory effect on bacterial biomass for metal lactates compared to metal chlorides. In Chapter 4 we describe a collaborative project investigating siderophore-producing bacteria isolated from the root zone of chlorosis-afflicted ‘Concord’ grapevines. Siderophore-producing bacteria were enriched using a high-throughput chrome azurol S (CAS)-based assay, and whole genome sequencing allowed the assembly and annotation of ten full genomes. Two distinct clades among the genomes were revealed using phylogenetic analysis, and while both were closely related to the Pseudomonas genus, we identified diverse mechanisms of iron uptake and siderophore production/uptake in the protein families of the genomes. Lastly, Chapter 5 discusses future steps that could be taken to improve our understanding of metal lactates and the mechanisms in which they improve ecosystem processes. This dissertation provides foundational evidence showing the ability of metal lactates to deliver readily available nutrients and stimulate both plant and microbial growth, as well as insights into genomic approaches that investigate the link between microbial genomes and nutrient cycling processes, thus adding to the growing body of literature on alternative solutions to create healthy and sustainable soils.

Investigations into lactate-bound metals: A sustainable alternative to inorganic micronutrient fertilizers

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