In aboveground litter decay, nitrogen immobilization occurs often and may be enhanced in nitrogen limited environments. It was hypothesized that similar patterns would occur during belowground decay in a nitrogen limited ecosystem. Litter bags containing native roots were buried along the dune chronosequence. Nitrogen availability increased with dune age. In addition, each dune site was fertilized with 15 g N/m2/year. After 111 days, nitrogen was immobilized in the oldest 120 year old dune (108% of initial N). A net loss in absolute amounts of nitrogen occurred in the younger 24 and 36 year old dune sites (65-79% of initial N). In addition, rates of decay were highest in the oldest dune and lowest in the younger 24 year old dune. Absolute amounts of nitrogen stabilized in all sites after 218 days (66-75% initial N remaining) while mass loss continued. In the younger sites, rates of nitrogen release were greater in control plots than in fertilized plots. In addition, decay rates were increased by nitrogen addition. No differences in percent initial nitrogen remaining or decay rates occurred between fertilized and control plots in the 120 year old site during early stages of decomposition. Both exhibited net nitrogen immobilization (107-110% initial N after 150 days). Beyond this point, the 120 year old fertilized plots continued to retain greater amounts of nitrogen than the control plots.
Differences in litter quality between the three dune sites were correlated with differences observed in decay rates and nitrogen immobilization potentials. Greater decay rates in the oldest dune may be related, in part, to the significantly greater nitrogen content in roots from the oldest dune (0.80% vs. 0.59% in youngest dune) producing an inherently more decomposable litter. Nitrogen immobilization was expected in the poorest quality litter, however, this did not occur. Nitrogen immobilized in native litter incubated in the oldest dune may be a result of the greater lignin content found in these roots (20%) in comparison to roots from younger dunes (9-13%). Lignin content is known to be positively correlated with nitrogen immobilization, presumably due to increased physicochemical complexing between various nitrogen species and lignin components. Contrary to nitrogen limited aboveground systems, overall nitrogen immobilization potential was low to non- existent.
The interaction between landform age, topographic position and environmental regime along the chronosequence was used to study environmental controls on belowground decomposition rates. Notable differences in hydrology and soil redox potential were evident between dune and swale sites. Mean water table position dropped from younger to older sites and was higher in swales (4.8 cm aboveground to 14.7 cm belowground) than in dunes (91.2 cm to 116.5 cm belowground). Mean soil redox levels exhibited no differences between dunes (423 to 573 Mv) and were lower in swales (-35 to 239 Mv). Older swales had higher soil redox levels. Roots collected from one site were incubated in all dune and swale sites to factor out the influences of litter quality. Decomposition of the standard root type was greater in dunes (40.8 - 57.5 % mass remaining) than in swales (74.2 - 86.3 % mass remaining). Multiple regression analysis demonstrated hydrology and soil redox potential were strongly correlated with belowground decomposition rates.
Hydrology appears to strongly influence belowground decomposition rates, both directly and indirectly through modification of other environmental parameters. For example, soil temperatures were cooler in the swales and soil redox potentials reflected depth and duration of saturation. Hydrologic influences in swale regions contribute to lower turnover time of organic matter and its subsequent accumulation. This is in contrast to the more rapid turnover of organic matter observed in the dry, well-drained dune sites. In addition, more nitrogen and phosphorus was released from decaying litter in swales, possibly due to leaching processes mediated by a high, fluctuating water table.