Pre-exposure to drought increases the resistance of tropical forest soil bacterial communities to extended drought

Abstract

Global climate models project a decrease in the magnitude of precipitation in tropical regions. Changes in rainfall patterns have important implications for the moisture content and redox status of tropical soils, yet little is known about how these changes may affect microbial community structure. Specifically, does exposure to prior stress confer increased resistance to subsequent perturbation? Here we reduced the quantity of precipitation throughfall to tropical forest soils in the Luquillo Mountains, Puerto Rico. Treatments included newly established throughfall exclusion plots (de novo excluded), plots undergoing reduction for a second time (pre-excluded) and ambient control plots. Ten months of throughfall exclusion led to a small but statistically significant decline in soil water potential and bacterial populations clearly adapted to increased osmotic stress. Although the water potential decline was small and microbial biomass did not change, phylogenetic diversity in the de novo-excluded plots decreased by ∼40% compared with the control plots, yet pre-excluded plots showed no significant change. On the other hand, the relative abundances of bacterial taxa in both the de novo-excluded and pre-excluded plots changed significantly with throughfall exclusion compared with control plots. Changes in bacterial community structure could be explained by changes in soil pore water chemistry and suggested changes in soil redox. Soluble iron declined in treatment plots and was correlated with decreased soluble phosphorus concentrations, which may have significant implications for microbial productivity in these P-limited systems.

Figure 1

Phylogenetic distance for control and rainfall exclusion plots over time: (a) June 2009; (b) August 2009; (c) April 2010. The curves represent the average (±s.d.) of the Faith's PD statistic over time.

Figure 2

(a) Weighted and (b) unweighted correspondence analysis of the individual control and treatment plots over time: (a, d) June 2009, (b, e) August 2009 and (c, f) April 2010. The CA plots depict the ordination of weighted unifrac distance matrices showing the distinction of rainfall exclusion plots from the control plots following 10 months of the exclusion. Site markers represent replicates within the different treatments.

Figure 3

Differences in the relative abundance of different taxa between the treatment and control plots following 10 months throughfall exclusion. The different bars are colored by phylum (with the exception of the Proteobacteria that are differentiated at the class level). The data is drawn from the OTU tables and the labels identify the most prominent taxa down to highest taxonomic level available. Bars are the average of five replicate plot/treatment. Significant differences (ANOVA, P⩽0.05) between the two groups are signified using red dots adjacent to a given bar.

Figure 4

Principal components analysis detailing the relationship between plot community composition and edaphic factors following 10 months of rainfall exclusion. The principal components analysis plot is based on correlations between biological and physicochemical variables. Biological site markers are the same as that for Figure 2. Abbreviated physicochemical variables: Ca, calcium; Cl, chloride; DOC, dissolved organic carbon; Fe, iron; K, potassium; Mg, magnesium; Mo, molybdenum; Na, sodium; NH₄, ammonium; NO₃, nitrate; P, phosphate; SO₄, sulfate; Ψ, water potential; WC, water content.