Microbial biomass, functional capacity, and community structure after 12 years of soil warming

We examined the effect of chronic soil warming on microbial biomass, functional capacity, and community structure in soil samples collected from the Soil Warming Study located at the Harvard Forest Long-term Ecological Research (LTER) site. Twelve years of chronic soil warming at 5 C above the ambient temperature resulted in a significant reduction in microbial biomass and the utilization of a suite of C substrates which included amino acids, carbohydrates, and carboxylic acids. Heating significantly reduced the abundance of fungal biomarkers. There was also a shift in the mineral soil microbial community towards gram positive bacteria and actinomycetes. 2008 Elsevier Ltd. All rights reserved. Long-term changes in soil temperature regimes resulting from climate warming are expected to alter soil properties and processes. Numerous warming studies have been initiated over the past decade to examine the ecosystem-scale effects of rising temperature (Rustad et al., 2001). Most of these studies, from a soil perspective, have focused on soil respiration. While soil CO2 efflux is initially stimulated by warming, an increasingly common observation is that this effect diminishes over time. That is, acclimation of soil respiration occurs in response to warming (Oechel et al., 2000; Luo et al., 2001; Bradford et al., 2008). For example, in a soil warming experiment at Harvard Forest located in central Massachusetts, warming accelerated CO2 fluxes to the atmosphere in the first few years of the study (Melillo et al., 2002, 2004). However, this stimulatory effect significantly decreased after 5–6 years of warming and there was no response to heating between years 7 and 12. Similar results have been observed by others (Oechel et al., 2000; Luo et al., 2001). Proposed mechanisms underlying this observation include reduced plant production leading to lower root respiration rates, reduced microbial activity induced by soil drying, and substrate limitation due to losses of labile soil C (Oechel et al., 2000; Luo et al., 2001; Melillo et al., 2002). Changes in microbial community structure may be another important, but understudied, mechanism influencing soil C cycling. Soil warming studies that have examined microbial community dynamics can be divided into field or mesocosm experiments of variable duration (1–15 years) with a modest increase in soil temperature (1–3 C; Kandeler et al., 1998; Bardgett et al., 1999; Deslippe et al., 2005; Zhang et al., 2005; Sowerby et al., 2005; Rinnan et al., 2007) and lab incubations of short duration (6–16 weeks) with soils incubated over a wide range of temperatures (5– 40 C; Zogg et al., 1997; Andrews et al., 2000; Waldrop and Firestone, 2004). Of the field studies, only two examined microbial community structure. Zhang et al. (2005) observed a 20–60% increase in the fungal:bacterial ratio at a tallgrass prairie site exposed to a w2 C increase in temperature over a three year period. Warming did not alter microbial biomass or respiration. After 15 years of soil warming (1–2 C) in northern Sweden, the only response was a significant reduction in the relative abundance of fungi (Rinnan et al., 2007), the opposite effect as the one observed by Zhang et al. (2005). We still lack a clear understanding of how warming will affect the soil microbial community and in turn, how changes in the community will feedback to influence nutrient cycling dynamics. Our objective was to examine how microbial biomass, functional capacity, and community structure have responded to longterm soil warming in a northeastern forest. Samples were collected 12 years after establishment of the Soil Warming Study located at the Harvard Experimental Forest in Petersham, MA (42.5 N, * Corresponding author. Department of Natural Resources and the Environment, University of New Hampshire, 215 James Hall, 56 College Road, Durham, NH 03824, USA. Tel.: þ1 603 862 3880; fax: þ1 603 862 4976. E-mail address: [email protected] (S.D. Frey).