Microbial enzymatic responses to drought and to nitrogen addition in a southern California grassland

Abstract

Peer reviewed eScholarship.org Powered by the California Digital Library University of California Highlights We measured drought and N impact on litter enzyme activities and microbial biomass. Despite semi-arid conditions, the litter was bacterially dominated. Enzyme efficiencies declined with drought. Enzyme efficiencies in the N treatment suggest microbes adapt to local environment. Measuring enzyme efficiencies could help predict in-situ enzyme activity. a b s t r a c t Microbial enzymes play a fundamental role in ecosystem processes and nutrient mineralization. Therefore understanding enzyme responses to anthropogenic environmental change is important for predicting ecosystem function in the future. In a previous study, we used a reciprocal transplant design to examine the direct and indirect effects of drought and nitrogen (N) fertilization on litter decomposition in a southern California grassland. This work showed direct and indirect negative effects of drought on decomposition, and faster decomposition by N-adapted microbial communities in N-fertilized plots than in non-fertilized plots. Here we measured microbial biomass and the activities of nine extracellular enzymes to examine the microbial and enzymatic mechanisms underlying litter decomposition responses to drought and N. We hypothesized that changes in fungal biomass and potential extracellular enzyme activity (EEA) would relate directly to litter decomposition responses. We also predicted that fungal biomass would dominate the microbial community in our semi-arid study site. However, we found that the microbial community was dominated by bacterial biomass, and that bacteria responded negatively to drought treatment. In contrast to patterns in decomposition, fungal biomass and most potential EEA increased in direct response to drought treatment. Potential EEA was also decoupled from the decomposition response to N treatment. These results suggest that drought and N alter the effi-ciencies of EEA, defined as the mass of target substrate lost per unit potential EEA. Enzyme efficiencies declined with drought treatment, possibly because reduced water availability increased enzyme immobilization and reduced diffusion rates. In the N experiment, the efficiencies of b-glucosidase, b-xylosidase, and polyphenol oxidase were greater when microbes were transplanted into environments from which they originated. This increase in enzymatic efficiency suggests that microbial enzymes may adapt to their local environment. Overall, our results indicate that drought and N addition may have predictable impacts on the efficiencies of extracellular enzymes, providing a means of linking enzyme potentials with in-situ activities.

Temporal variation overshadows the response of leaf litter microbial communities to simulated global change

“…The potential activities of nine extracellular enzymes involved in carbon and nutrient cycling were assayed as described in Alster et al (2013) (Table 1). Briefly, litter samples were collected seasonally from September 2011 to March 2013 (seven sample dates in total) and frozen at − 80°C until analysis.…”

“…The homogenates were continuously stirred while dispensing 200 μl per well into 96-well microplates with eight replicate wells per sample per assay. Fluorimetric enzyme assays were performed according to the methods described in German et al (2011) and Alster et al (2013), and oxidative enzymes were measured using a colorimetric assay described in Allison and Jastrow (2006) and Alster et al (2013). The potential activities (in μmol g − 1 h − 1 ) of all nine enzymes were used to generate a Euclidian distance matrix of all samples.…”

Elemental stoichiometry of Fungi and Bacteria strains from grassland leaf litter

“…The litter layer consists of senesced plant material from primarily exotic annual grasses of the genera Avena, Bromus, and Lolium as well as annual forbs such as Erodium, Lupinus, and Vicia. Litter microbial communities are dominated by bacterial biomass, though Fungi represent up to~20% of microbial biomass in summer (Alster et al, 2013). Fungal cultures were obtained from leaf litter using a dilution to extinction method.…”

Responses of soil enzyme activity and microbial community compositions to nitrogen addition in bulk and microaggregate soil in the temperate steppe of Inner Mongolia

“…The activity of BG can serve as C acquisition investment and decomposition [28,61]. NAG and LAP can be secreted by microbes to hydrolyze chitin [6,24] and to break N-terminal amino acid of the polypeptide into leucine, respectively [2,13]. Besides the degradation of carbon-sources, microbes can also produce phosphatases (AP) to provide the nutrients for growth and enzyme synthesis [2,54].…”

“…NAG and LAP can be secreted by microbes to hydrolyze chitin [6,24] and to break N-terminal amino acid of the polypeptide into leucine, respectively [2,13]. Besides the degradation of carbon-sources, microbes can also produce phosphatases (AP) to provide the nutrients for growth and enzyme synthesis [2,54]. The effect of N input on enzyme activity in bulk soil is be universally known, but the responses of soil enzyme activity to nitrogen addition in microaggregate need to be further explored.…”