Compared with animals plants lack variety in their basic resources, namely carbon dioxide and water. However, this does not mean that there is no element of choice available as to where these resources can be found. Given the relatively low levels ofcarbon dioxide now present in the atmosphere compared with some previous geological epochs, such as the early Carboniferous, and the Jurassic

(when it might have been as high as 3000 ppm), it is not surprising that many species in marginal habitats have developed the facility to access the alternative sources of carbon dioxide that reside in both the soil atmosphere and water. The development of Sphagnum magellanicum hummocks (as studied in wet conditions in the laboratory) has been found to depend on high carbon dioxide concentrations in the water in the upper layer of peat in which organic matter decomposes aerobically (the acrotelm). Sphagnum plants grown in a low dissolved carbon dioxide treatment appeared to be carbon-limited, with increased mineral nutrient concentrations and decreased ratios of carbon to other nutrients. These results demonstrate that, at least in wet conditions, atmospheric carbon dioxide alone is not sufficient to enable S. magellanicum to develop its normal hummock growth pattern. It was therefore concluded that substrate-derived carbon dioxide is an important carbon source for Sphagnum spp. (Smolders et al., 2001).

In this same context it is relevant to note that when carbon dioxide enrichment was applied in the atmosphere as in the Free-air Carbon Dioxide Enrichment (FACE) experiments on ombrotrophic peat bog lawns in Finland, Sweden, the Netherlands and Switzerland there were no significant effects on sphagna or vascular plant biomass at any of the sites even after a three-year treatment with an increased atmospheric carbon dioxide concentration at 560 ppm (Hoosbeek et al., 2001). This suggests that, just as with other nutrient-poor ecosystems, increased atmospheric carbon dioxide concentrations will have a limited effect on bog ecosystems. However, given the ability of Sphagnum magellanicum to utilize the carbon dioxide in the soil water, it is hardly surprising that carbon dioxide enrichment of the atmosphere has little effect on wetland mosses.

The season of the year is also important in assessing the role of soil respiration in contributing to the carbon resources of flowering plants. In the short growing season of the High Arctic flowering plants are metabolically active for a relatively short time and achieve the bulk of their growth and development within a few weeks of snowmelt. This episode of early growth coincides with the period that soil respiration releases carbon dioxide, much of which, like the arctic nitrogen pool, may have been derived from the photosynthetic activity of cyanobacteria growing on the soil surface. Measurements made in Spitsbergen show that the carbon dioxide flux progresses from a net carbon dioxide source of 0.3 g C m—2 d—1 during late snowmelt to a mid-summer net carbon dioxide sink of —0.39 g C m—2 d—\ returning to a net CO2 source of 0.1 g C m—2 d—1 in the early autumn (Lloyd, 2001). The low-growing, spreading and cushion plants of the Arctic are particularly well adapted to profit from this augmentation of the atmospheric carbon dioxide supply.

An estimation of the use made by these plants of soil carbon can be obtained from examination of the stable isotope ratio of the carbon sequestered in the flowering plants. The relative abundance of 13C versus 12C (expressed as d 13C %%) is frequently used as a measure of relative water efficiency over extended periods. It can also be used to distinguish the source of carbon dioxide used for photosynthesis. Carbon dioxide derived from soil respiration often has a d 13C value of around — 19%. In a sample of plants of Saxífraga oppositifolia growing in Spitsbergen a mean d 13C value of — 19.51%% ±0.32 (n = 8) suggests that carbon dioxide derived from soil respiration supplied a considerable portion of their photosynthetic needs (Crawford et al, 1995).

Forest plant communities can also benefit from carbon dioxide emitted from soils particularly where there are substantial soil reserves of organic carbon as in the boreal forest. Currently the carbon balance of these northern forests attracts much interest with regard to whether or not they will provide a sink or source for atmospheric carbon dioxide as the climate becomes warmer. Although some authors consider that the northern forests represent potentially a large sink for carbon (Myneni et al., 2001) there are other studies that demonstrate that these same regions are not only very heterogeneous in soil types but also vary from year to year and are therefore unlikely to moderate the rise in atmospheric carbon dioxide during the next century (Schlesinger & Andrews, 2000).

Comparison of the day and night vertical profiles of atmospheric carbon dioxide concentration in the lower portion of forest communities (Buchmann et al., 1996) have long been used to demonstrate the extent to which soil respiration augments the supply of atmospheric carbon dioxide to the forest plant community as a whole (Fig. 3.33). The organic matter of the forest floor provides the low-growing early spring

Fig. 3.33 Changes in atmospheric concentration in a forest during the day in relation to height above the ground. Observations taken from a mixed forest of box elder (Acer negundo) and A. grandidentatum in Red Butte Canyon 1400 m a.s.l., Utah, USA. Note the marked uptake of carbon dioxide before midday, especially in the region under 1 m above the soil surface, indicating the use made by the vegetation of carbon dioxide emitted from the soil. (Figure from Professor Nina Buchmann; see also Buchmann et al., 1996.)

Fig. 3.33 Changes in atmospheric concentration in a forest during the day in relation to height above the ground. Observations taken from a mixed forest of box elder (Acer negundo) and A. grandidentatum in Red Butte Canyon 1400 m a.s.l., Utah, USA. Note the marked uptake of carbon dioxide before midday, especially in the region under 1 m above the soil surface, indicating the use made by the vegetation of carbon dioxide emitted from the soil. (Figure from Professor Nina Buchmann; see also Buchmann et al., 1996.)

forest flora with luxury levels of carbon dioxide as compared with plants of taller status in open habitats. Rosette-shaped plants, which carpet the ground with actively photosynthesizing leaves in early spring, are therefore able to compensate for the shortness of their growing season through their ability to utilize high concentrations of carbon dioxide emanating from soil respiration.

Aquatic and amphibious plants in many instances are also able to access high levels of carbon dioxide from submerged soils due to the efficacy of their ventilation mechanisms. The aerenchyma in roots and rhizomes and the hollow stems of such species as Typha latifolia facilitate the supply of oxygen from the shoot to the root. Gaseous diffusion of carbon dioxide also operates in the reverse direction. The partial pressure of carbon dioxide (pCO2) in these aerenchyma gas spaces has been estimated to be more than 10 times atmospheric pCO2 and it appears that the source of this carbon dioxide is from microbial (soil) respiration. Over 50% of total leaf volume in T. latifolia is occupied by gas spaces and most of the total gas-space volume in plants is in the shoot. Photosynthetic rate in C3 plants, such as cattail, can increase with increasing carbon dioxide concentrations under natural conditions. For this reason, cattail and other emergent wetland plants possessing continuous gas-space pathways appear to have a significant carbon supplement as compared with other C3 plants growing in well-aerated soils (Constable et al., 1992; Constable & Longstreth, 1994).

The warming of leaves and petioles in water lilies and reeds (Phragmites australis) by solar energy leads to a net downflow of air from younger to older tissues. This process, sometimes called thermo-osmosis, depends on younger tissues having a higher resistance to diffusion than older leaves. Consequently, air travels downwards from younger leaves through submerged rhizomes and ventilates upwards through older organs. The process ventilates submerged organs and alleviates a potentially hypoxic condition in rhizomes (Armstrong et al., 1994) and in its upward movement through mature stems and leaves delivers carbon-dioxide-enriched air to aerial photosynthetic tissues.

In aquatic species a further enhancement of carbon dioxide utilization is found in those species which possess CAM and C4 metabolism. There is evidence for the occurrence of the CO2-concentrating mechanism crassulacean acid metabolism (CAM) in five genera of aquatic vascular plants, including Isoetes, Sagittaria, Vallisneria, Crassula and Littorella (Keeley, 1998). CAM is most frequently thought of as an adaptation that increases water use efficiency in drought-resistant species. However, in some aquatic habitats dissolved carbon dioxide levels can be low and CAM contributes to the carbon budget by increasing both net carbon gain and carbon recycling. Aquatic CAM plants tend to be found in shallow pools that experience low carbon dioxide levels by day and higher levels at night. CAM plants are therefore able to take advantage of the higher night-time carbon dioxide levels (Keeley, 1998).

Littorella uniflora (shoreweed) is an amphibious species which forms a shallow water turf down to a level of 4 m but flowers only when the plants are exposed above the waterline (Fig. 8.7). A study of photosynthesis in this species (Robe & Griffiths, 2000) has shown a remarkable facility for optimizing the ability to sequester carbon. When submerged the plant shows crassulacean acid metabolism and high leaf lacunal CO2 concentrations which are maintained as it emerges above the water level, suggesting a continued carbon dioxide uptake from sediments. Shore weed is an excellent example of a truly amphibious species in that it is phenotypically plastic both in morphology and physiology and is poised at any time to emerge from the submergence onto dry land without suffering any noticeable water deficit.

3.7.5 Nitrogen

It has often been assumed that ammonium is the predominant form of inorganic nitrogen nutrition in tundra soils and that it is preferentially absorbed in comparison with nitrate. Recent investigations have shown that this generalization neglects both habitat variation and the capacity of some species to access various soluble and insoluble sources of inorganic nitrogen both directly and through mycorrhizal association (Atkin et al., 1996). Although NH4+ is the predominant form of inorganic nitrogen in most arctic soils there is evidence that nitrification takes place in certain situations and producing substantial quantities of NO3~ even exceeding NH4+. In both mesic and drier soils higher temperatures can increase the presence of nitrate (Nadelhoffer et al., 1992). Faeces

Fig. 3.34 Nitrogen and phosphate enriched vegetation on bird cliffs near Grumantbyen, 78° 10' N, Spitsbergen.

deposits also provide sources for nitrate production. Consequently, as soils become warmer with earlier snowmelt at high latitudes it is probable that nitrate availability will increase. Nitrate is more mobile in soils than ammonium and the relatively luxurious vegetation that is found in the lowest regions of vegetation below bird cliffs is a consequence of the flushing of nitrate downslope from ammonium rich soils (Fig. 3.34). A possible limiting factor remains, and that is the low rates of nitrate reductase activity commonly found in arctic vegetation even when fertilized with nitrate (Atkin, 1996). However, the degree of induction of nitrate reductase activity is not uniform in arctic plants and in certain ancient bird cliffs in Spitsbergen which have been identified as having a particularly long history of being free of ice (Fig. 6.13) a surprisingly high level of induction of nitrate reductase activity has been recorded (Odasz, 1994).

The ability of arctic plants to absorb soluble organic nitrogen has now been recognized. In some arctic soils, particularly under moist or wet conditions, the concentration of free amino acids can exceed that of inorganic nitrogen. In particular glycine, aspartic and glutamic acids can be found in organic-rich soils

(Atkin, 1996). Wet and mesic soils have been particularly noted for their high concentrations of soluble nitrogen compounds. However, even in dry heath soils, water-extractable amino acids can be found. Isotope studies using C14-labelled free amino acids showed that Eriophorum vaginatum preferentially absorbed free amino acids such as glycine (Chapin et al., 1993). It has been calculated that the uptake rates of free amino acids may account for between 10% and 82% of the total N uptake in the field, depending on species and community. Deciduous shrubs had higher uptake rates than slower growing evergreen shrubs, and ectomycorrhizal species had higher amino acid uptake than did non-mycorrhizal species.

A study of the sources of nitrogen for plant growth in a coastal marsh grazed by snow geese in Manitoba, Canada, showed that amounts of nitrogen, primarily ammonium ions, increased in the latter half of the growing season and over winter, but fell to low values early in the growing season. Free amino acid concentrations relative to ammonium concentrations were highest during the period of rapid plant growth in early summer, especially in soils in the intertidal zone, where the median ratio of amino acid nitrogen to ammonium

Fig. 3.35 Comparison of increase in total dry weight at fixed nitrogen concentrations using ammonium or glycine for plants of Puccinellia phryganodes grown in hydroponic culture in the field at La Perouse Bay, Manitoba. (Reproduced with permission from Henry & Jefferies, 2002.)

nitrogen was 0.36 and amino acid concentrations exceeded those of ammonium ions in 24% of samples. Amino acid profiles, which were dominated by alanine, proline and glutamic acid, were similar to goose faecal profiles. In a continuous flow hydroponic experiment conducted in the field, growth of the salt-marsh grass Puccinellia phryganodes on glycine was similar to growth on ammonium ions at an equivalent concentration of nitrogen (Fig. 3.35). Thus, when supplies of soil inorganic nitrogen are low, amino acids represent a potentially important source of nitrogen for the regrowth of plants grazed by geese, and amino acid uptake may be as high as 57% that of ammonium ions (Henry & Jefferies, 2002).

These studies demonstrate that in a nutritionally marginal region such as the tundra, plants short-circuit the mineralization decomposition step by directly absorbing amino acids. This implies that in the organic soils of these tundra systems the following conditions occur:

(1) inorganic nitrogen is an inadequate measure of plant-available soil nitrogen

(2) mineralization rates underestimate nitrogen supply rates to plants

(3) the large differences among species in capacities to absorb different forms of N provide ample basis for niche differentiation of what was previously considered a single resource (4) by short-circuiting N mineralization, plants accelerate N turnover and effectively exert greater control over N cycling than has been previously recognized.

3.7.6 Phosphate

The essential role of phosphorus in plant metabolism might suggest that the demand for this element in the nutrition of plants would be relatively great. However, an examination of plant ash shows that the amount of phosphorus in the typical plant is lower than calcium, potassium or magnesium. Phosphates in the mineral component of soils are relatively insoluble and less readily leached than nitrate or sulphate especially if there is a good supply of calcium resulting in a high pH and the formation of insoluble calcium phosphate. In acid soils phosphoric acid combines with iron, aluminium or magnesium to form colloidal hydrates which are precipitated. Leaching is therefore minimized and phosphorus can accumulate in areas where there is or has been human habitation. This is particularly noticeable in phosphate-deficient soils formed from slow-weathering rocks as in the Northern Isles

Fig. 3.36 Nutrient retention in an oceanic landscape. Remains of prehistoric settlement on the island of Papa Stour (Shetland). Note the retention of nutrients aided by preferential grazing which has retained greater fertility than in the surrounding nutrient-deficient and phosphate-impoverished landscape.

Fig. 3.36 Nutrient retention in an oceanic landscape. Remains of prehistoric settlement on the island of Papa Stour (Shetland). Note the retention of nutrients aided by preferential grazing which has retained greater fertility than in the surrounding nutrient-deficient and phosphate-impoverished landscape.

and Outer Hebrides (Scotland; see photo of abandoned prehistoric settlement on the Shetland island of Papa Stour, Fig. 3.36). The organic component of soils can contain appreciable amounts of organic phosphates usually as inositol phosphates and in particular phytic acid (inositol hexaphosphate). Despite the retention of phosphorus in insoluble mineral and organic combination phosphate leaching is not entirely prevented. In natural plant communities phosphate is generally more limiting than nitrogen and many species which in the past were considered as classic nitrophiles have been shown experimentally to respond more to phosphorus than nitrogen. Thus nettles (Urtica dioica), dog's mercury (Mercurialis perennis) and tufted hair grass (Deschampsia caespitosa) show only a limited response to nitrogen unless they are given additional phosphate (Pigott & Taylor, 1964). In areas with hard acid rocks and high rainfall such as on the Lewisian gneiss of the Outer Hebrides (Scotland) phosphate deficiency is a problem for agriculture.

Although extreme examples of phosphate deficiency are not common, the ability of plants to access insoluble phosphate varies and has important ecological consequences. One of the most important properties of mycorrhizal systems is the marked influence they have on phosphorus uptake in areas with low phosphate availability. The most noticeable effect of vesicular-arbuscular mycorrhizae (VAM) is the growth improvement provided by the supply of phosphorus. The fungal hyphae not only render phosphate soluble but forage for supplies well beyond the zone of depletion that can normally be reached by the roots and root hairs without fungal associates. VAM are the most abundant of endo- and ectomycorrhizae and are generally the most effective in stimulating growth in the host plant. Plant species without ericoid mycorrhiza consistently show low inherent relative growth rate (RGR), low foliar N and P concentrations, and poor litter decomposition, while plant species with ectomycorrhiza had an intermediate RGR, higher foliar N and P, and intermediate to poor litter decomposability, plant species with arbuscular mycorrhizae showed comparatively high RGR, high foliar N, and P, and fast litter decomposition (Cornelissen et al., 2001).

3.7.7 Phosphate availability at high latitudes

The arctic and boreal lands with their mineral-poor, gravel soils and equally nutrient-deficient wetland peats provide a biological challenge for plant nutrition which has attracted much investigation particularly with reference to phosphorus. Phosphate uptake has also been shown to vary greatly between high latitude species. In a study which compared 15 tundra species near Toolik Lake, Alaska, the potential for phosphate absorption alone was found to vary 20-fold between species (Kielland & Chapin, 1992). Within this tundra ecosystem deciduous shrubs had the highest potential for phosphate absorption and contrasted with the evergreen shrubs which had the lowest, the difference being presumably due to their large annual nutrient requirement for producing new leaves every year. In many species mycorrhizal associations appear to enhance phosphate uptake as they do elsewhere. In arctic Canada a survey of root colonization and spore populations of arbuscular mycorrhizal fungi revealed that of 197 plant-root systems and soil rhizospheres examined, 28% were associated with arbuscular mycorrhizae (Dalpe & Aiken, 1998).

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