Kim, K. 2021. Change of soil micro-environments during plant decomposition and its effect on carbon and nitrogen dynamics. Dissertation, Michigan State University, East Lansing MI.
Detritusphere is one of the most important hotspots; it consists of soil surrounding dead organic materials and is affected by their decomposition and recycling. When assessing C and N dynamics associated with decomposition, micro-environments created near decomposing residues should be considered because the microbial processes are affected by the condition of detritusphere micro-environments, not that of the bulk soil. This could possibly overcome current inaccuracy of global greenhouse gas emission and biogeochemical cycle models. The goal of my Ph.D. research was to investigate the change of soil micro-environmental conditions within detritusphere during plant residue decomposition and to understand their role in and interactions with C and N transformation and dynamics.
In Chapter 1, I evaluated the water absorption by decomposing plant roots, based on the finding of water absorption by leaves (a.k.a. sponge effect). In addition to this ‘sponge effect’ in root residues, I assessed the soil moisture gradient created by it by using micro-computed tomography. The study found that the moisture redistribution near decomposing roots depends on the initial soil moisture content and the pore characteristics nearby the roots. It also suggested that the anaerobic micro-environment formed near the roots might influence the N2O emission in the early stage of the decomposition process.
In chapter 2, I hypothesized that the influence of moisture redistribution on the N2O emission found in previous chapter is mediated by reduced O2 availability near plant residues. I measured O2 and N2O concentrations in the pores adjacent to leaf and root residues by using electrochemical microsensors. The leaf residues had lower O2 availability near them due to greater water absorption and microbial O2 consumption. Both N2O production and emission were negatively correlated to O2 availability, supporting the initial hypothesis.
In Chapter 3, I investigated the fate of C and N during the decomposition of switchgrass roots grown in contrasting soil pores, to test if the micro-environmental characteristics described in former chapters have significant influence on decomposition dynamics. Comprehensive assessments of CO2 and N2O emissions, priming effects, and C and N remaining in soil were performed using dual-isotope labeling (13C and 15N) techniques. There were enhanced influences of soil pore sizes on plant-driven N2O emission, N2O priming, and enzyme activity in in-situ grown root systems. The study also confirmed that detritusphere micro-environments formed in large-pore soils are more favorable for microbial activity and denitrification processes.
My dissertation contributed to the characterization of micro-environmental conditions in detritusphere, and their relevance to C and N cycling. It stresses the importance of hotspot micro-environments in predicting greenhouse gas emission and related microbial processes and urges further research to understand the full mechanism and incorporate those in greenhouse gas prediction models.
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