Performing Department
(N/A)
Non Technical Summary
Plant facilitation of rhizosphere microbes promotes plant nutrient acquisition from soil organic matter (SOM), providing a basis for soil health and sustainable fertility. Microbial activity is primed by plant exudates and this rhizosphere priming is a major determinant of SOM dynamics. However, priming mechanisms remain poorly described because they operate through complex multipartite interactions between plants, microbes, and soils. We hypothesize that phenolic acid bacteria are catalysts of rhizosphere priming, stimulated by exudates to metabolize SOM and enhance plant access to nutrients locked in soil. We predict that this priming response requires: i) proliferation of phenolic acid bacteria, and ii) induction of phenolic acid metabolism. We predict that both root exudation and SOM dynamics can promote proliferation and induction, causing priming to vary as a function of plant host and soil health. Objective 1 employs a field experiment investigating plant capacity to promote proliferation and induction of phenolic acid bacteria. Objective 2 determines the distribution of phenolic acid bacteria in agricultural systems and the degree to which these microbes respond to SOM and management. Objective 3 implements a greenhouse experiment that will include plants that differ in exudate production, bacterial inocula derived with or without phenolic acid enrichment, and inocula derived from synthetic communities with or without inclusion of phenolic acid bacteria. The phenolic acid hypothesis explains plant-microbe interactions in relation to soil properties and SOM dynamics. This mechanism has the potential to explain a major component of biological soil health that underlies sustainable soil fertility in diverse agricultural settings.
Animal Health Component
(N/A)
Research Effort Categories
Basic
100%
Applied
(N/A)
Developmental
(N/A)
Goals / Objectives
Plant facilitation of rhizosphere microbes promotes plant nutrient acquisition from soil organic matter (SOM), providing a basis for soil health and sustainable fertility. Microbial activity is primed by plant exudates and this rhizosphere priming is a major determinant of SOM dynamics. However, priming mechanisms remain poorly described because they operate through complex multipartite interactions between plants, microbes, and soils. We hypothesize that phenolic acid bacteria are catalysts of rhizosphere priming, stimulated by exudates to metabolize SOM and enhance plant access to nutrients locked in soil. We predict that this priming response requires: i) proliferation of phenolic acid bacteria, and ii) induction of phenolic acid metabolism. We predict that both root exudation and SOM dynamics can promote proliferation and induction, causing priming to vary as a function of plant host and soil health. Objective 1 employs a field experiment investigating plant capacity to promote proliferation and induction of phenolic acid bacteria. Objective 2 determines the distribution of phenolic acid bacteria in agricultural systems and the degree to which these microbes respond to SOM and management. Objective 3 implements a greenhouse experiment that will include plants that differ in exudate production, bacterial inocula derived with or without phenolic acid enrichment, and inocula derived from synthetic communities with or without inclusion of phenolic acid bacteria. The phenolic acid hypothesis explains plant-microbe interactions in relation to soil properties and SOM dynamics. This mechanism has the potential to explain a major component of biological soil health that underlies sustainable soil fertility in diverse agricultural settings.
Project Methods
Objective 1A.Characterize the root exudate profiles of winter annual cover crops and their ability to recruit phenolic acid bacteria. We will characterize the response of the soil microbial community to differences in plant root exudate composition by establishing a 2-year split-plot field experiment that includes six over-wintered cover crop species that differ in root architecture, functional characteristics, and exudate profiles. We have selected cover crops because the use of cover crops is a common strategy used for organic management of SOM and soil nutrients, and because their impacts on SOM dynamics remain poorly constrained. This experiment will have two fertilization levels (low and high) as main plots, seven split-plot treatments (6 crops and control), and four replicate blocks, totaling fifty-six plots. The fertilized main plots will be used to determine whether exudation and proliferation of phenolic acid bacteria is reduced in nutrient replete conditions (see Obj. 1B).Objective 1B.Investigate the enrichment of phenolic acid bacteria in the rhizosphere as a function of soil nutrient status and plant nutrient acquisition. We will leverage the aforementioned split-plot experiment to examine our hypothesis that priming of phenolic acid bacteria depends on plant exudate responses to nutrient availability and endogenous rates of SOM turnover. We selected the alfalfa field in Musgrave Research Farm due to our expectation that this site will be limited in soluble P resulting from five years of biological nitrogen fixation by alfalfa [80]. Main plots will receive a mineral fertilizer treatment that is designed to exceed the recommended nutrient levels for cover crop growth to inhibit priming-induced mineralization of SOM. We expect that plant nutrient acquisition in unfertilized split plots will require exudate-induced mineralization of native SOM by phenolic acid bacteria.Objective 2A.Survey publicly available datasets to map the distribution of phenolic acid bacteria across agricultural landscapes. We will leverage published 16S amplicon sequence datasets to determine the distribution of Paraburkholderia and relatives across global agricultural landscapes using updated reference databases. In particular, we are interested in understanding: 1) the distribution of Paraburkholderia in agricultural soils with respect to geography and inherent soil features such as pH and texture, 2) the relationship between Paraburkholderia and soil chemo-biological variables relevant to soil health, including SOM and total carbon, and 3) the degree to which Paraburkholderia are associated with particular management regimes and cropping systems.Objective 2B.Quantify pobA abundance and phenolic acid community structure in relation to soil health indicators such as POXC. To characterize the abundance of phenolic acid degrading bacteria varies with soil health, we will leverage our relationship with the Cornell Soil Health Lab to survey a geographically diverse set of soil samples and determine whether phenolic acid bacteria correlate with soil health metrics. We will also collect soil from a long term field experiment that has developed a strong SOM gradient over the past fifty years, and which should, if our hypothesis is correct, also contain a gradient in the abundance of phenolic acid bacteria [93].Objective 3A.Determine the response of phenolic acid bacteria to incubation with artificial phenolic root exudates. We will test whether soil enrichment with phenolic acids enhances the abundance of phenolic acid bacteria and promotes a plant priming response. Microcosm treatments will include five biological replicates of each incubation substrate: glucose, a sterile water control, and an artificial phenolic acid exudate solution with and without added glucose (n = 20). The composition of the phenolic acid solution will be chosen to reflect representative exudate profiles of high priming plants identified in Obj. 1. Soil for the microcosm (Obj. 3A) and inoculation experiments (Obj. 3B, 3C) will be collected from the tillage x residue harvest plots at the Chazy site. Because this soil has a low total health score and is deficient in SOM, we expect to observe dramatic increases in soil respiration, enzyme activity, and phenolic acid bacteria abundance in response to phenolic acid exudate addition. Substrates will be added such that 0.4 mg of C is added per gram dry weight of soil, which we have shown is sufficient to observe differences in community composition over a 48-day incubation period [108].Objective 3B.Evaluate the contribution of phenolic acids to the priming response and recruitment of phenolic acid bacteria to the rhizosphere. Inoculations will be prepared using incubated soil from the microcosm experiment (Obj. 3A) and applied in a potted greenhouse experiment. Leveraging our results from the field experiment in Obj. 1, we will select two plant species that exhibit significant differences in root exudate profiles, priming activity, and response to fertilization. These plants will be grown from surface-sterilized seeds germinated in the lab and transplanted into sterile field soil in pots after one week. Soil for the potted greenhouse experiments will be collected from the tillage x residue harvest plots in the Chazy long term experimental site. Due to the low SOM content and poor soil health rating of this soil, we expect to observe significant effects of priming-sufficient communities on plant nutrient uptake and growth. In brief, soil will be dried, gently homogenized, and autoclaved twice. To ensure adequate initial plant growth across all treatments, a ½ formulation of Hoagland's nutrient solution will be applied during the first two weeks of growth for all treatments. To evaluate the interactive effects of nutrient availability and bacterial enrichment by phenolic acids on the priming effect, plants will be grown at two levels of nutrient availability after the second week: 1X Hoaglands and low-N & P Hoaglands. Thus, we will include factorial combinations of plant species, enriched soil inoculant, and nutrient level with four biological replicates (n = 64).Objective 3C.Determine the requirement for phenolic acid bacteria in RPE using SynComms. We will carry out an identical experimental design as described in Obj. 3B, this time using synthetic communities (SynComms) as an inoculant for plants grown in low SOM soil. SynComms are a powerful reductionist tool to understand the functional implications of plant-microbiome interactions. We are in the process of developing SynComms that lead to net mineralization ('priming sufficient') or net stabilization ('priming deficient') of SOM based on the presence or absence of phenolic acid bacteria, respectively. This work is currently supported by a USDA-NIFA Postdoctoral Fellowship (award number 2023-67012-39838). Our group has also developed a system for inoculating and growing plants with SynComms in sterile soil to evaluate plant-microbe interactions [110]. We will adapt these systems to enable experimental evaluation of root priming in the two winter cover crops chosen for Obj. 3B. We predict that the removal of phenolic acid degrading bacteria from the SynComm will severely diminish the root priming effect evidenced by reduced rhizosphere enzyme activity and plant growth.