Structure of a microbial community in soil after prolonged addition of low levels of simulated acid rain

Abstract

Humus samples were collected 12 growing seasons after the start of a simulated acid rain experiment situated in the subarctic environment. The acid rain was simulated with H2SO4, a combination of H2SO4 and HNO3, and HNO3 at two levels of moderate acidic loads close to the natural anthropogenic pollution levels of southern Scandinavia. The higher levels of acid applications resulted in acidification, as defined by humus chemistry. The concentrations of base cations decreased, while the concentrations of exchangeable H+, Al, and Fe increased. Humus pH decreased from 3.83 to 3.65. Basal respiration decreased with decreasing humus pH, and total microbial biomass, measured by substrate-induced respiration and total amount of phospholipid fatty acids (PLFA), decreased slightly. An altered PLFA pattern indicated a change in the microbial community structure at the higher levels of acid applications. In general, branched fatty acids, typical of gram-positive bacteria, increased in the acid plots. PLFA analysis performed on the bacterial community growing on agar plates also showed that the relative amount of PLFA specific for gram-positive bacteria increased due to the acidification. The changed bacterial community was adapted to the more acidic environment in the acid-treated plots, even though bacterial growth rates, estimated by thymidine and leucine incorporation, decreased with pH. Fungal activity (measured as acetate incorporation into ergosterol) was not affected. This result indicates that bacteria were more affected than fungi by the acidification. The capacity of the bacterial community to utilize 95 different carbon sources was variable and only showed weak correlations to pH. Differences in the toxicities of H2SO4 and HNO3 for the microbial community were not found.

FIG. 1

Effect of humus pH on total microbial biomass measured by SIR (a); basal respiration, where symbols indicate the mean for five replicate plots of the different treatments and bars indicate the standard error for the replicates (b); basal respiration, where symbols indicate the mean respiration level of the treatments situated within the same vegetation type, dry (dry), medium (med), or moist (moist) (c); and growth rate of fungi, estimated by [¹⁴C]acetate incorporation into ergosterol (d). Symbols for panels c and d are as described for panel a. o.m., organic matter.

FIG. 2

Score plot of PCA showing the separation of the study plots along PC 1 and PC 2 determined with humus PLFA data (a), plated-community PLFA data (CFU-PLFA) (b), Biolog GN analysis (c), and comparison of humus PLFA and plated-community PLFA analyses with PLS regression (d). Bars indicate the standard error for five replicate plots.

FIG. 3

Effect of humus pH on PLFA of bacterial origin (see Materials and Methods) (a), thymidine incorporation rate for bacteria (b), bacterial community adaptation to pH (the growth rate index is the logarithm of the ratio of thymidine incorporation at pH 3.8 to that at pH 7.2) (c), and the proportion of yellow colonies among CFU (d). o.m., organic matter.