Soil microbes produce and consume large quantities of greenhouse gases (GHGs) while cycling nitrogen (N) and carbon (C), with production and consumption being roughly in balance over geological time. The availability of metals that are utilized as cofactors for microbial processes can cause dramatic shifts in cycling of C and N, which can in turn affect the production and consumption of GHGs. Copper, in particular, is a cofactor for important enzymes including ammonia monooxygenase and (one variety of) methane monooxygenase, which catalyze the consumption of ammonia and methane. However, previous studies on the effects of copper and GHGs have only examined methane, and none have included microbial community structure.
The goals of this study were to: (1) quantify GHG production, (2) measure changes in N and C, and (3) examine changes in soil microbial community composition, all in response to the addition of copper. To achieve these goals, we incubated aliquots of scrubland soil in a growth chamber for one week under various copper concentrations. Daily GHG flux was measured by gas chromatography, and randomly chosen samples were removed and used for assessment of C and N following potassium chloride extraction and of microbial community composition by 16S sequencing.
In comparing the microbial communities using the UniFrac distance metric, each of the four conditions (no treatment (dry soil), water-only, low copper concentration, and high copper concentration) clustered tightly within itself, indicating strong replicability. Furthermore, the high copper concentration ([Cu2+]) showed the greatest separation from the controls, with the low [Cu2+] intermediate between the two extremes. In addition, the low and high concentrations showed the largest shift in the microbial communities over the course of the week-long trials, although there were notable shifts in the water-only treatment compared to the dry soil. The gas flux data show that the three GHGs under study (nitrous oxide [N2O], carbon dioxide [CO2], and methane [CH4]) were all also strongly affected by the addition of copper in these soils, again with the largest changes corresponding to the highest copper concentrations. In comparison to the control (water-only) condition, increasing [Cu2+] sharply reduced the amount of N2O and CO2 being produced within 24 h post-wet-up, and the increase lasted at least a week. Overall, these data indicate that increased levels of copper can have major effects on microbial communities’ structure and function related to GHG cycling.