Responses In Microbes And Plants To Changed Temperature, Nutrient, And Light Regimes In The Arctic

Ecology, Sept, 1999 by Sven Jonasson, Anders Michelsen, Inger K. Schmidt, Esben V. Nielsen

INTRODUCTION

Numerous studies of arctic and subarctic ecosystems in both North America and Europe have shown that plant community composition and plant productivity are extremely sensitive to changes in nutrient availability, particularly of N and P (e.g., Shaver and Chapin 1980, 1991, 1995, Chapin and Shaver 1985, 1996, Jonasson 1992, Chapin et al. 1995, Jonasson and Michelsen 1996, Jonasson et al. 1996). However, as the climatic severity increases along latitudinal or altitudinal gradients, temperature limitations seem to play an increasing role, shown by progressively enhanced growth as the temperature rises (Havstrom et al. 1993, Wookey et al. 1993, Callaghan and Jonasson 1995a, b, Jonasson et al. 1996, Graglia et al. 1997). This temperature control is important because the predicted strong potential effect on air temperature in the Arctic, caused by increased emission of trace gases (Mitchell et al. 1990, Maxwell 1992), will feed back on the biological processes at various levels of resolution (e.g., Chapin et al. 1992, Chapin and Korner 1995).

Warming may affect the plants directly, if temperature controls growth processes (Korner and Larcher 1988), but also indirectly because warming is likely to accelerate litter and soil organic matter decomposition and nutrient mineralization (Nadelhoffer et al. 1992, Robinson et al. 1997), which will lead to increased productivity in presently nutrient-limited communities. However, soil microbes may also sequester nutrients and buffer changes during the growing season, when they presumably build up their populations and create a strong nutrient sink (Nadelhoffer et al. 1992, Jonasson et al. 1996, Jonasson 1997) at the same time as the plants' need for nutrient uptake is at a maximum. Such sequestering is a probable reason for the very low and, in many cases, even negative net nutrient mineralization measured during the arctic growing season (Chapin et al. 1988, Giblin et al. 1991, Nadelhoffer et al. 1991, 1992, Jonasson et al. 1993). By contrast, nutrients are generally released during the nongrowing season (Giblin et al. 1991, Hobbie and Chapin 1996), when the soil microbial populations presumably decline and the microbial communities lose part of the sequestered nutrients. The plants may therefore have their main access to nutrients in autumn, when the microbial populations decline, and before the microbial populations recover in spring (Chapin et al. 1978, Nadelhoffer et al. 1992).

The potential for strong nutrient sequestering by microorganisms is evident, as they contain a large proportion of the soil pool of plant limiting nutrients, particularly in nutrient-poor soils. For instance, Walbridge (1991) reported that the microbes could contain [greater than]50% of the soil P pool in highly organic, nutrient-poor pocosin soils. High contents of N and particularly of P have also been reported in microbes of arctic soils. Jonasson et al. (1996) found a microbial pool of [approximately]7% of the total soil N and 35% of the soil P in nutrient-poor shrub heaths in subarctic northern Sweden. Due to the low plant biomass, the amount of microbial N approached that in the plant biomass, and the microbial P greatly exceeded the pool of plant P. Furthermore, the retention times of N and P in the microbial pool, estimated by dividing the soil microbial N and P pools by the annual input to the soil of N and P from aboveground litter, were 7.5 yr and [greater than]30 yr, respectively. By contrast, the retention time of C was [less than]1 yr (Jonasson et al. 1999). The differences show that, as the organic matter decomposed and C was released to the atmosphere through microbial respiration, the nutrients were retained and circulated within the microbial biomass for years (N) or decades (P) without being taken up by plants. Hence, there is a realistic possibility for strong competition for nutrients between plants and soil microbes during the growing season (Schimel et al. 1989, Harte and Kinzig 1993, Chapin and Schimel 1996, Norton and Firestone 1996, Kaye and Hart 1997), which gives strength to the hypothesis that plant nutrient uptake may be restricted during the time of the year when microbial productivity is high.

The objective of the present study was to investigate responses of soil microbes and plants to changed temperature, nutrient, and light levels in two contrasting subarctic ecosystems. These systems had the same dominant plant functional types. Effects on ecosystem processes such as productivity and nutrient sequestering were measured after five years of manipulations. We measured responses in both plants and microbes at one time, which has not been done in previous studies. This provided for analyses of simultaneous responses in the entire plant-microbe-soil system, giving much more detailed information than previously reported on ecosystem function and the sensitivity of tundra to moderate changes in the environment.

MATERIALS AND METHODS

Site description

The sampling took place in the summer of 1993 in two plant communities dominated by Cassiope tetragona (L.) D. Don at Abisko in northern Swedish Lapland. One community is a subalpine heath just above the forest line at 450 m above sea level (a.s.l.) and the other is a fellfield at 1150 m a.s.l. The climate is subarctic montane, with a growing season of [approximately]3 mo, lasting from mid-late June to early-mid September; see Havstrom et al. (1993) for more detailed accounts of the climate, and Michelsen et al. (1996a) for information on the plant community composition at the two sites.