Document: NOA-3-58-2

The influence of drying-rewetting events and stress history on soil microbial processes.

FIERER, N.G.* and J.P.SCHIMEL

University of California, Santa Barbara, CA 93106 ¹

Abstract:
Soil drying and rewetting impose a significant stress on the soil microbial community. While wetting events are common, the short and long-term effects of soil rewetting on microbial processes and communities are still not well understood. Furthermore, it is not clear if stress history is important to consider when modeling microbial controls on ecosystem dynamics. We manipulated the frequency of soil rewetting events over a two month period to determine how stress history influences the response of soil microbial communities to rewetting events. Two soils were collected from the Sedgwick Natural Reserve in Santa Ynez, CA, one from an annual grassland, the other from underneath an oak canopy. Soils were incubated in the lab and went through either 0, 1, 2, 4, 6, 9, or 15 drying-rewetting cycles during a two month period, after which soils were incubated at optimal moisture for a further two months. Soil moisture content was adjusted so the average moisture over the course of the incubation was the same for all samples, compensating for the number of drying-rewetting cycles. Soils were analyzed for respiration rate, substrate utilization efficiency, nitrification potential, microbial biomass, and NH₄⁺/NO₃^- concentrations. Total CO₂ loss over the course of the incubation significantly increased with number of rewetting events for oak soils but not for grass soils, where a large number of rewetting events decreased total CO₂ loss. After the drying-rewetting cycles ended, respiration rates showed some recovery in grassland soils, but not in oak soils. Nitrification potentials increased with increasing number of drying-rewetting events. One day after the final stress event, the ratio of carbon allocated to CO₂ versus biomass was higher in stressed samples than the control. In all cases, the effects of the rewetting stress were greater in oak than in grass soils. The results indicate that drying-rewetting events can induce short-term and long-term changes in microbial processes and community composition, and the frequency of such stress events must be incorporated into models of microbial dynamics.

Keywords: drying-rewetting, stress, soil respiration, nitrification

This abstract is being presented at: 8:00 AM in session:
Oral Session #71: Soil Microbial Biomass and Soil Respiration.