Responses in Microbes and Plants to Changed Temperature, Nutrient, and Light Regimes in the Arctic

Abstract

Previous research has shown that experimental perturbations of arctic ecosystems simulating direct and indirect effects of predicted environmental changes have led to strong responses in the plant communities, mostly associated with increased plant nutrient availability. Similarly, changes in decomposition and nutrient mineralization are likely to occur if the soil warms and the soil moisture conditions are altered. Plant and microbial responses have usually been investigated separately, and few, if any, studies have addressed simultaneous responses to environmental changes in plants and soil microorganisms, except in models.We measured simultaneous responses in biomass, nitrogen (N), and phosphorus (P) incorporation in plants and microorganisms after five years of factorial fertilizer addition, air warming, and shading. We expected increased N and P uptake by microorganisms after fertilizer addition and also after warming, due to increases in mineralization rates in warmer soils. Plant productivity and N and P uptake were expected to increase after fertilizer addition but less after warming, because microbes were expected to absorb most of the extra released nutrients. Shading was expected to decrease plant production and also microbial biomass, due to the reduced production of labile carbon (C) in plant root exudates associated with reduced photosynthesis.We found that the plants responded strongly to fertilizer addition by increased biomass accumulation and N and P uptake. They responded less to warming, but more than expected, showing a decline in N and P concentrations in many cases. There were few significant responses to shading. The strongest response was found in combined fertilizer addition and warming treatments. All functional vascular plant groups responded similarly. However, mosses declined under those conditions when vascular plant growth was most pronounced.Contrary to our expectation, microbial C, N, and P did not increase after warming, but microbial N and P increased after shading. As expected, fertilizer addition led to increased microbial P content, whereas microbial N either increased or did not change. In general, microbial C did not change in any treatment. The microbes accumulated extra N and P only when soil inorganic N or P levels increased, suggesting that the soil microorganisms absorbed extra nutrients only in cases of declining N and P sink strength in plants.

Two Decades of Experimental Manipulations of Heaths and Forest Understory in the Subarctic

“…Consequently, if these responses persist in the fellfield, the changes could lead to lower tissue quality for herbivores and reduced decomposition, because of higher recalcitrance of litter. Nutrient addition led to increased vascular plant biomass in several subarctic ecosystem types (Press et al 1998;Jonasson et al 1999), but plant biomass changes may be dampened due to immobilization of nutrients in microbial biomass ). These results suggest that the strong microbial nutrient sink in subarctic and alpine heaths may mitigate changes in the plant biomass even if warming increases the mineralization rate.…”

“…In addition to warming treatments, nutrients have frequently been added to experimental plots (Press et al 1998;Jonasson et al 1999). In high doses, responses to such additions increase our understanding of how nutrients act as drivers in these, often nutrient-poor ecosystems.…”

Dwarf shrubs are stronger competitors than graminoid species at high nutrient supply in peat bogs

“…This is not surprising as vascular plants in bogs need to keep up with the rising Sphagnum surface to avoid being overgrown (Malmer et al 1994). Additionally, some studies suggest that shading, enhanced by increased shoot growth, can have a negative impact on Sphagnum growth (Jonasson et al 1999;Berendse et al 2001) and even can increase N deposition by canopy interception (Lamers 2000). This would give the dwarf shrub a competitive advantage over Sphagnum at increased shoot growth rates.…”

Long‐term sensitivity of a High Arctic wetland to Holocene climate change

“…Soil moisture exerts a control on tundra ecophysiology through production, decomposition and nutrient cycling (Miller et al 1984), and a lowered water-table and increased thaw might be expected to accelerate the rate of soil decomposition (CO 2 source) over photosynthesis (CO 2 sink), so that the balance in tundra soils shifts from one of C-input, or storage, to C-output (Billings et al 1982(Billings et al , 1983Johnson et al 1996). If the effect of decomposition were to increase nutrient availability, there may be an additional uptake of CO 2 owing to higher rates of photosynthesis (Shaver & Chapin 1986;Shaver et al 1998;Johnson et al 2000), although as sink strength in vascular plants decreases, productivity may be offset by substrate-controlled or nutrient-limited CO 2 loss from soil respiration by microorganisms (Nadelhoffer et al 1991;Hobbie 1996;Jonasson et al 1999). Larger sinks of CO 2 are accordingly associated with lower respiration rates in wetter habitats (Vourtilis et al 2000), while short-term experiments designed to explain the net effect of climate warming on soil moisture and the C-balance of tundra plots (Johnson et al 1996) support observational data demonstrating that a shift from net C-input to C-output accompanies the recent drying of tundra habitats (Oechel et al 1993Weller et al 1995).…”