478 F. S. Chapin III et al. 



nutrients become bound in peat (Moore and Bellamy 1974). However, 

 slow decomposition of mosses is probably due more to low nutrient con- 

 tent and tundra environmental conditions than to any inherent resistance 

 of mosses to decay (Heal et al. 1978, Coulson and Butterfield 1978). 

 Therefore, the idea that mosses are a major path of nutrients to soil or- 

 ganic matter deserves further critical examination. 



The importance of animals in the coastal tundra at Barrow is much 

 greater than is indicated by their biomass, owing to their ability to recycle 

 nutrients, as shown by high turnover rates (Table 12-5). Lemmings recy- 

 cle 80 to 90% of the ingested plant phosphorus directly back to the sol- 

 uble inorganic pool, whereas most phosphorus contained in standing 

 dead material must recycle through the soil organic pool at a much lower 

 rate. Thus herbivores can short-circuit the decomposition process, just as 

 cattle do in the shortgrass prairie (Dean et al. 1975). The annual phos- 

 phorus turnover in vascular plants more than doubles in years of peak 

 lemming abundance. The annual absorption of phosphorus by vascular 

 plants that would be necessary to balance this loss to herbivores exceeds 

 the calculated annual uptake rate by 25%. In contrast, most of the calci- 

 um recycled through lemming feces is contained in undigested cell walls 

 and must still cycle largely through the decomposition process before be- 

 coming available again to plants. Soil invertebrates also apparently in- 

 crease the rate of nutrient release from organic matter, perhaps by graz- 

 ing upon microbes (Chapter 11). 



A given group of organisms may play very different roles in cycling 

 of different elements. The roles of mosses and vascular plants in cycling 

 of nitrogen and of carbon are quite different (Figures 12-1 and 12-2). In 

 the case of carbon, mosses and vascular plants function in parallel, both 

 fixing carbon photosynthetically and then passing it on to herbivore- or 

 saprovore-based trophic systems. In the case of nitrogen, mosses act in 

 series with vascular plants. Mosses probably receive a large proportion of 

 their nitrogen as leachate from vascular plant leaves and are thus an in- 

 termediate step in the movement of nutrients from vascular plants to the 

 soil. Moreover, because blue-green algae associated with mosses are re- 

 sponsible for the bulk of the nitrogen fixation at Barrow, mosses repre- 

 sent a major point of entry of nitrogen into the ecosystem. Mosses un- 

 doubtedly retain some elements more effectively than others, e.g. dival- 

 ent cations more than monovalent cations. 



LONG-TERM CHANGES IN COASTAL TUNDRA 



Lakes and ponds, which constitute 30% or more of the surface area 

 of arctic coastal tundra, are continually forming, enlarging and draining 

 in a cyclic process which encompasses thousands of years (Chapter 1). 



i 



