Nitrogen Mineralization, Plant Growth and Goose Herbivory in an Arctic Coastal Ecosystem

Seasonal partitioning of resource use and constraints on the growth of soil microbes and a forage grass in a grazed Arctic salt-marsh

“…This interpretation is consistent with the results of Jefferies (2002, 2003) who showed that exchangeable NH4 and NO3 ions fell to very low levels at the beginning of the growing season and that P. phryganodes utilized organic N rather than inorganic N, as shown in earlier studies of other Arctic and alpine species (Kielland 1994;Raab et al 1996;Schimel and Chapin 1996;Lipson and Monson 1998). Wilson and Jefferies (1996) also showed that in this salt marsh, net N mineralization rates were either very low or negative in early and mid-summer, a finding similar to the studies of Hart and Gunther (1989), Giblin et al (1991), Jonasson et al (1993Jonasson et al ( , 1996 and Schmidt et al (2007). Although these estimates are based on the use of the buried bag technique which may underestimate net N mineralization rates, as senescing roots and root hairs, which provide a source of N, are not included (Jonasson et al 2004).…”

“…At grubbed sites the N content of the surface layer of soil is low compared with soils beneath intact swards (McLaren and Jefferies 2004;Buckeridge and Jefferies 2007). In this marsh, rates of net mineralization of soil N in vegetated plots during the first two-thirds of summer are either very low or are negative (Wilson and Jefferies 1996), similar to results from other studies (Hart and Gunther 1989;Giblin et al 1991;Jonasson et al 1993Jonasson et al , 2006Schmidt et al 1999). Mineralization rates rise in the late summer months (Wilson and Jefferies 1996) but in contrast to the studies described above, graminoid swards have been intensely grazed by herbivores (lesser snow geese, Chen caerulescens caerulescens) during the post-hatch period in summer for at least 40 years (Cooke et al 1995).…”

“…There was no significant response of microbial C, N, or P to the additions of N, P and NP; values were similar to those from control plots. The sediments beneath intact swards do not show extreme temperature shifts, differences in pH, or strong seasonal trends in soil moisture content Jefferies 1995, 1996;Wilson and Jefferies 1996;McLaren and Jefferies 2004).…”

“…Frozen late autumn samples were air-freighted (frozen) to the University of Toronto, where they were processed. On each sampling date, the properties of the collected soil cores, including pH, redox potential and bulk density, were measured following established procedures Jefferies 1995, 1996;Wilson and Jefferies 1996), except experiments involving addition of carbon when only microbial carbon and plant biomass were measured. The above-ground plant biomass from the six intact cores (n=3) taken from similarly treated sub-plots was clipped, rinsed in deionised water, and oven-dried at about 50°C for 72 h. All plant biomass was re-dried at 50°C for 1 week on arrival in Toronto and weighed to the nearest milligram.…”

“…In this marsh, rates of net mineralization of soil N in vegetated plots during the first two-thirds of summer are either very low or are negative (Wilson and Jefferies 1996), similar to results from other studies (Hart and Gunther 1989;Giblin et al 1991;Jonasson et al 1993Jonasson et al , 2006Schmidt et al 1999). Mineralization rates rise in the late summer months (Wilson and Jefferies 1996) but in contrast to the studies described above, graminoid swards have been intensely grazed by herbivores (lesser snow geese, Chen caerulescens caerulescens) during the post-hatch period in summer for at least 40 years (Cooke et al 1995). High densities of faeces (< 40 m-2 wk-1; Jefferies and Rockwell 2002) result in a rapid increase in above-ground biomass within the season, much of which is consumed by geese (Bazely and Jefferies 1985).…”