Effects of drying–rewetting frequency on soil carbon and nitrogen transformations

Abstract

Soil drying and rewetting impose a significant stress on the soil microbial community. While wetting events are common in most environments, the short and long-term effects of soil rewetting on microbial processes have not been well studied. Furthermore, it is not clear if stress history is important to consider when modeling microbial controls on ecosystem dynamics. In this experiment, we manipulated the frequency of soil rewetting events during 2 months to determine how stress history influences the response of soil microbial communities to rewetting events. Two soils were collected from the Sedgwick Ranch 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 over 2 months. Soil moisture content was adjusted so that the average moisture content 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⁺ and NO₃⁻ concentrations. Total CO₂ loss during 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. Exposure to frequent drying–rewetting events decreased the amount of CO₂ released upon rewetting and dramatically increased the activity of autotrophic nitrifier populations. For up to 6 weeks after the last drying–rewetting cycle, respiration rates in soils exposed to a history of drying–rewetting events were substantially lower than their non-stressed controls. 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 significant changes in microbial C and N dynamics and these effects can last for more than a month after the last stress. The frequency of drying–rewetting stress events has important ecosystem-level ramifications and should be incorporated into models of soil microbial dynamics.

Introduction

In most terrestrial ecosystems, surface soils experience periods of drying followed by relatively rapid rewetting. Soils of Central California, and other semi-arid, Mediterranean-type ecosystems, are particularly susceptible to drying–rewetting stresses due to the infrequency of rainfall events and the often warm, dry climate that favors rapid soil drying. Understanding the effects of these drying–rewetting events on soil microbial processes is important to our understanding of ecosystem C and N dynamics in these systems.

Both C and N mineralization rates generally increase for a few days following the rewetting of a dry soil (Birch, 1958, Bloem et al., 1992, Cui and Caldwell, 1997, Franzluebbers et al., 2000). The source of this ‘pulse’ of C and N is unclear. The rapid change in soil water potential associated with rewetting may cause microbes to undergo osmotic shock, inducing microbial cell lysis (Bottner, 1985, Van Gestel et al., 1992) or a release of intracellular solutes (Halverson et al., 2000). These labile C and N substrates could then be rapidly mineralized by the remaining microbes, yielding a pulse of C and N (Birch, 1959, Kieft et al., 1987). Alternatively, drying–rewetting may cause soil aggregates to break apart, exposing physically protected organic matter (Adu and Oades, 1978, Lundquist et al., 1999a). This previously unavailable organic matter could then be rapidly mineralized by the microbial community (Appel, 1998).

While we do know that drying–rewetting yields a 1–4 d increase in C and N mineralization rates, we know little about the longer term implications of drying–rewetting stresses for ecosystem C and N dynamics. After the short-term pulse of C and N, do soil processes return to the same pre-stress equilibrium state? If so, it is relatively unimportant to incorporate drying–rewetting dynamics into soil process models because the C and N pulse is of relatively short duration (Cabrera, 1993) and of limited magnitude in annual C and N transformations. However, if a drying–rewetting event causes the soil to assume a new equilibrium for C and N transformation rates (relative to an unstressed soil), or if the recovery to pre-stress basal rates is slow, then the incorporation of drying–rewetting events into models that adequately predict ecosystem C and N fluxes becomes more important and more difficult. Studies conducted by Clein and Schimel, 1994, Bottner, 1985 suggest that this may be the case with microbial processes affected for extended periods after a drying–rewetting event.

In the field, most soils experience, not one, but a series of drying–rewetting events throughout the course of the year. Few studies have examined how the frequency of these stress events (the rewetting stress history) controls soil processes. Schimel et al. (1999) showed that the frequency of drying–rewetting events altered microbial biomass and respiration on decomposing birch litter. Likewise, a field study by Fay et al. (2000) suggests that the length of time between rainfall events controls soil CO₂ flux. On the other hand, the majority of ecosystem models assume a roughly linear response function (below field capacity) between soil water availability and C and N mineralization rates (Rodrigo et al., 1997). These models are driven by moisture in the current time step and do not include moisture history. Thus, a linear response to moisture produces an implicit result that the average moisture over time should predict microbial activity as well as integrating the individual values at different times, regardless of the wetting and drying history. In this vein, most larger-scale biogeochemical models do assume that any variability in soil moisture content can be integrated temporally and the average moisture values can be used to adequately predict C and N mineralization rates (McGill, 1996, Parton et al., 1987). We designed this experiment partly to test these model assumptions which will increase in importance in the future with global climate change scenarios predicting a change in the seasonal pattern of rainfall events (Easterling, 1990, Houghton et al., 1990). If the frequency of drying–rewetting events controls soil processes, then the variability in rainfall during a given period, not just the average rainfall, would have to be incorporated into models of soil C and N dynamics.

The specific questions addressed by this experiment include the following. What are the short-term (1–3 d) and long-term (1 month) effects of soil drying–rewetting on C and N dynamics? Does drying–rewetting stress history, when isolated from changes in average water content, influence soil processes? Are these responses to drying–rewetting events similar between different soil types? Can longer term (months) microbial dynamics be predicted simply from knowing the average water content with time, or does the variation in moisture need to be taken into account?