The influence of goethite and gibbsite on soluble nutrient dynamics and microbial community composition

Iron and aluminum (oxyhydr)oxides are ubiquitous in the soil environment and have the potential to strongly affect the properties of dissolved organic matter. We examined the effect of oxide surfaces on soluble nutrient dynamics and microbial community composition using an incubation of forest floor material in the presence of (1) goethite and quartz, (2) gibbsite and quartz, and (3) quartz surfaces. Forest floor material was incubated over a period of 154 days. Aqueous extracts of the incubations were harvested on days 5, 10, 20, 30, 60, 90, and 154, and concentrations of P, N, PO₄ ^3?, NO₂ ^?, NO₃ ^?, and organic C were measured in the solutions. Microbial community composition was examined through pyrosequencing of bacterial and fungal small subunit ribosomal RNA genes on selected dates throughout the incubation. Results indicated that oxide surfaces exerted strong control on soluble nutrient dynamics and on the composition of the decomposer microbial community, while possibly having a small impact on system-level respiration. Goethite and gibbsite surfaces showed preferential adsorption of P-containing and high molar mass organic solutes, but not of N-containing compounds. On average, organic C concentrations were significantly lower in water extractable organic matter (WEOM) solutions from oxide treatments than from the control treatment (P = 0.0037). Microbial community composition varied both among treatments and with increasing time of incubation. Variation in bacterial and fungal community composition exhibited strong-to-moderate correlation with length of incubation, and several WEOM physiochemical characteristics including apparent (weight averaged) molar mass, pH and electrical conductivity. Additionally, variation in bacterial community composition among treatments was correlated with total P (r = 0.60, P < 0.0001), PO₄ ^3? (r = 0.79, P < 0.0001), and organic C (r = 0.36, P = 0.015) concentrations; while variation in fungal communities was correlated with organic C concentrations (r = ?0.48, P = 0.0008) but not with phosphorus concentrations. The relatively small impact of oxide surfaces on system-level microbial respiration of organic matter despite their significant effects on microbial community composition and WEOM dynamics lends additional support to the theory of microbial functional redundancy.