Functional responses of soil microbial communities to sugar amendment across a five year time sequence in Puget lowland prairies

Background/Questions/Methods

Introduced plant species have invaded many habitats with elevated soil nitrogen. Making this nitrogen less available through carbon additions such as sawdust, activated charcoal, and sugar have met with mixed success in enhancing the success of native over introduced plant species. These amendments generally succeed in making nitrogen less available, but this outcome only sometimes results in increased success of native plant species. Despite the hypothesized mechanism that carbon boosts the microbial populations, which then sequester soil nitrogen, little work has explored the direct effects of carbon additions on the soil microbial community. Moreover, recent work (Kirkpatrick & Lubetkin 2011) has demonstrated that initial negative responses by introduced plant species to sugar additions in western Washington lowland prairies was later reversed, such that introduced species were more abundant five years after sugar addition than on control soils. We have begun to describe the qualitative ways that sugar addition alters the soil microbial community function using Biolog® Ecoplates, which assess the ability of a given microbial community to metabolize 31 carbon substrates.  Using principal components analysis, we compared soil community function in samples gathered from matched sets of sugar-treated and control plots from prairies treated with sugar in different years. 

Results/Conclusions

Sugar treatment systematically displaced the microbial communities in the PCA analyses, with treatments applied six weeks prior to analysis showing a marked shift, treatments applied two years prior showing a lesser but marginally significant shift in the opposite direction, and treatments applied five years prior showing a slight non-significant shift back. Microbial community function in soils recently treated with sugar showed significantly more variability in function than in matched control soils. Among the 31 carbon sources, microbial communities of recently sugar-treated soils metabolized more carbohydrates than those of control soils, and metabolized fewer polymers and carboxylic acids. Complex carbon sources such as polymers are often assumed to be metabolized by the fungal community more readily than by the bacterial community. Taken together, these results suggest that 1) sugar amendment has both immediate and relatively long-term effects on the soil microbial community, 2) although these effects may be generally similar and predictable, some communities change in unpredictable and persistent ways, and 3) sugar amendment may enhance bacteria at the expense of fungi in the soil community.