The first nitrogen translocation measurements in Sphagnum, reported in this issue (Aldous (2002); pp. 241–253), are of vital importance in developing our understanding of nutrient constraints in peatlands and, potentially, even global carbon budgets. In making the connection, this article is not only a commentary on excellent research, but also a homage to the importance of mosses within the genus Sphagnum. These small, delicate, nonvascular plants dominate the cover and productivity of many peatlands globally, particularly those that are more ombrotrophic (i.e. ‘rain-fed’) or bog-like. Although peatlands occupy only approx. 2–3% of the terrestrial land surface (Bridgham et al., 2001a), the imbalance between production and decomposition over thousands of years has led to an accumulated soil carbon pool that has been estimated to be one-third of the global total (Gorham, 1995). Within bogs, this peat is largely derived from partially decomposed Sphagnum tissue. Sphagnum tissue decomposes slowly because it has low concentrations of nitrogen and high concentrations of inhibitory compounds (Aerts et al., 1999). The often-seen successional transition from minerotrophic (i.e. ‘ground-water fed’) fens to bogs is initiated by the increasing dominance of Sphagnum mosses that occur in raised hummocks above the water table (Vitt & Kuhry, 1992). Proton exchange from unesterified polyuronic acids in Sphagnum (Clymo, 1963), organic acid production during decomposition, and isolation of the peat surface from ground-water exchange lead to increasing acidification of peatlands. The increasing water stress on the nonvascular Sphagnum plants limits the growth of peat above the local water-table surface (Weltzin et al., 2001). The extent to which succession in peatlands is driven by autogenic (internally driven) processes makes them unique among the world's ecosystems, and Sphagnum mosses are the primary plant species which drive these processes.
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Adaptations of Sphagnum to low-nutrient environments
Bogs are extremely nitrogen-deficient ecosystems (Bridgham et al., 2001b), and the ability of bog plants to efficiently utilize nitrogen is essential to their survival. Sphagnum mosses have very low tissue nitrogen concentrations and high nutrient use efficiency (Aerts et al., 1999). The Sphagnum moss carpet scavenges from 50 to 90% of atmospherically deposited nitrogen (Li & Vitt, 1997), and Aldous demonstrated that vascular plants received < 1% of added atmospheric nitrogen at realistic dosage rates. Thus there may be effective nutrient partitioning between mosses and vascular plants, allowing coexistence (Pastor et al., 2002). Interestingly, most Sphagnum species also have very low nitrogen tolerances, sometimes with a positive growth effect from fertilization with low levels of nitrogen, but a toxic nitrogen effect after only a few years of even moderate nitrogen additions (Aerts et al., 1992; Thormann & Bayley, 1997; Chapin, 1998; S. Bridgham, unpublished). Bog shrub species are more strongly nutrient limited, so upon fertilization one quickly sees a shift to shrub-dominated communities. In fact, high nitrogen deposition in more industrialized areas of England has been implicated in the loss of Sphagnum mosses from peatlands (Lee et al., 1987). Thus, in many ways Sphagnum mosses represent an extreme in terms of plant adaptation to low-nutrient environments.
It is against this background that Aldous reports the first nitrogen translocation measurements in Sphagnum. This had previously been done using radioisotopes for carbon and phosphorus (Rydin & Clymo, 1989) but, for the reasons already described, nitrogen is probably the important element to examine. In a well designed experiment, Aldous followed the movement of a 15NH415NO3 tracer in Sphagnum capillifolium over the course of 2 yr in two bog sites in northern New York, USA with high atmospheric nitrogen deposition (1.0–1.3 g N m−2 yr−1 wet deposition) and two bogs in eastern Maine, USA with low deposition (0.2–0.4 g N m−2 yr−1 wet deposition). To calculate nitrogen translocation, she has developed a simple, but elegant model of the proportion of 15N in old and new Sphagnum tissue over time. She hypothesized that nitrogen translocation would be greater in the Maine site with low nitrogen deposition, but found the opposite result, with the effect of atmospheric nitrogen deposition possibly confounded by differences in water-table dynamics among the sites.
Most importantly, Aldous unequivocally demonstrated the importance of nitrogen translocation in this important nonvascular plant. She found from 11 to 32% of 15N was translocated into newly growing Sphagnum tissue over 1 yr in the low deposition Maine site, whereas from 64 to 83% of 15N was translocated in the high deposition New York sites.
Aldous further estimated that between 0.5 and 11% of the annual nitrogen requirement of S. capillifolium was met by translocation. However, her calculations assume that all ‘new’ uptake for Sphagnum is derived from atmospheric deposition (her equations 9 and 10). While uptake of mineralized nitrogen in shallow peat has not been estimated yet for Sphagnum, given the translocation of 15N in live stems shown in this study, it is reasonable to assume that Sphagnum mosses can be very effective competitors with microbes and vascular plant roots for available soil nitrogen in shallow peat. Aldous examined translocation in only the top 2 cm of stem, but Rydin & Clymo (1989) demonstrated translocation of phosphorus and carbon over 7 cm of stem. Of course, the zone of active uptake of soil nitrogen would vary among species of Sphagnum, as different species visually have quite different lengths of apparently live stem. If one assumes no discrimination between 15N and 14N, multiple sources of available nitrogen, and that translocation is independent of the source of nitrogen, the larger values of translocation (i.e. 11–32% in Maine and 64–83% in New York) given by Aldous are more appropriate to build nitrogen budgets for this species. Given that approx. 9% of the nitrogen demand of Sphagnum is supplied each year by atmospheric nitrogen deposition (calculated from the data given in the paper), in the low deposition Maine site about one-third of the nitrogen requirements of S. capillifolium is supplied by either the atmosphere or translocation. Even more impressively, in the high deposition New York site, 73–92% of the nitrogen requirement is satisfied by these pathways. Thus, efficient scavenging of atmospheric nitrogen and translocation of internal nitrogen can supply almost all of the nutrient demand of this important species under at least some circumstances. An important consequence of this is that Sphagnum mosses will have spatially distinct nutrient pools from deeper rooted vascular plants, allowing coexistence (Pastor et al., 2002).
Aldous presents a pivotal report on the nutrient dynamics of this important moss genus. Overall, one begins to get a coherent picture of Sphagnum nutrient dynamics. They have very low nutrient demands due to low tissue nutrient concentrations, high nutrient use efficiency, tight nutrient cycling (including translocation, despite the lack of vascular tissue), and spatially distinct nutrient pools from vascular plant, but they also have a very low toxic threshold for high atmospheric nitrogen deposition. Sphagnum mosses are a keystone species on an ecosystem scale. Understanding their ecological strategies to low nutrient availability is essential not only for predicting peatland dynamics but also for predicting their future role in global carbon budgets and global change.