POTENTIAL USE OF N2-FIXING CYANOBACTERIA FOR ESTABLISHING RENEWABLE BIOLOGICAL SOIL CRUSTS AND MODULATING SOIL NITROGEN IN AGROECOSYSTEMS
Open Access
- Author:
- Peng, Xin
- Graduate Program:
- Ecology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 27, 2016
- Committee Members:
- Mary Ann Bruns, Dissertation Advisor/Co-Advisor
Mary Ann Bruns, Committee Chair/Co-Chair
Katriona Shea, Committee Member
Armen Ricardo Kemanian, Committee Member
Zhibiao Zhao, Outside Member - Keywords:
- cyanobacteria
biological soil crusts
N2-fixation
soil nitrogen cycling
agroecosystem
microbial succession - Abstract:
- Large amounts of synthetic nitrogen (N) fertilizer are used in modern agriculture to increase crop yields, but N use efficiency is low due to high N losses and poor soil N retention. Biological N fixation by free-living cyanobacteria has been utilized extensively in flooded rice paddy systems, but rarely in agriculture of temperate regions like the United States. The objective of this dissertation research was to develop a renewable means of delivering N to soils that will be environmentally sustainable and economically beneficial through reducing the need for commercial fertilizer and increasing soil stability. The approach involves the establishment of renewable biological soil crusts (BSCs) using filamentous N2-fixing cyanobacteria, which takes advantage of their capabilities to photosynthesize, fix atmospheric N2, and stabilize soil surfaces. The overarching hypothesis of this research is that cyanobacteria will survive and promote BSC succession after being applied to agricultural soils, as they increase soil carbon and N content. Cyanobacteria were enriched from naturally formed BSCs on agricultural soils and their growth was compared to commercial strains in soil microcosm tests under N-limited conditions. Selected cyanobacteria were further evaluated for potential N2 fixation and soil N retention at different N concentrations, using 15N2 isotope labeling of cells and simulated rainfall experiments, respectively. A succession model was developed to simulate the microbial dynamics and N contributions of BSCs following the artificial application of N2-fixing cyanobacteria to soil. The model simulation results were compared with one year of observations of naturally occurring BSCs in agricultural fields. Additional cyanobacterial properties, including cyanotoxin content, drought resistance and large-scale biomass production in photobioreactors (PBRs) were also evaluated in consideration of future field applications. First, these results showed that artificial BSCs formed by DG1 - a cyanobacterial enrichment obtained from local agricultural soil - exhibit high biomass density and stability on N-limited soils (both sandy soil and silt loam soil). Second, DG1 could fix 0.1 mg N g-1 dry biomass day-1 from atmospheric N2 in N-free medium, but fixed less N2 in the presence of inorganic N. In addition, when soil nitrate was at adequate levels, application to soils of a small amount of living DG1 biomass (0.88 g dry biomass m-2) resulted in more soil nitrate retention under simulated rainfall than non-inoculated controls. Thus DG1 has potential to reduce N loss from soil. Third, based on the dynamics of cyanobacteria, green algae, and moss compartments of BSCs in model simulations and field observations, application of cyanobacteria (i.e. DG1) could accelerate the growth and succession of BSCs, and improve N retention in soil, especially when the inoculum is added to soils with high available inorganic N. Finally, DG1 has several additional properties that are favorable for field application, including negative cyanotoxin test results, rapid recovery after eight weeks under air-dry conditions, preliminary evidence of UV protection, and capability for growth in large-scale PBRs. In conclusion, this study demonstrates the feasibility of developing high-performance cyanobacterial enrichments for future applications in temperate agriculture. The results showed that the selected cyanobacterial enrichment DG1 has potential to be used as an agricultural soil amendment to accelerate BSC formation, improve soil stability and reduce application rates of synthetic N fertilizer.