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> Nitrogen Dynamics
- Phosphorus Dynamics
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We hypothesized that nitrogen content would increase through time as CWD decomposed; the data bore out this hypothesis. Nitrogen content in buried and surface disks increased for all species at all sites during 28 months of decomposition (Fig. 4a, Table 4). For L. racemosa and R. mangle, the nitrogen content tripled over the 28-month experiment for surface disks. Buried disks of all species had a twofold increase in N. The nitrogen content of A. germinans disks doubled in the buried and surface treatments. It is important to note that A. germinans had significantly higher initial nitrogen content (0.260% of dry weight ± 0.0159%) than L. racemosa (0.157 ± 0.0195%), but it was not significantly higher than R. mangle (0.205 ± 0.0139%). Unlike the trends for buried and surface disks, the nitrogen content of disks in the air remained unchanged throughout the study (Fig. 4a).

Rates of increase for nitrogen in surface and buried disks varied from 0.005 to 0.019% dry weight month-1 during the study. Condition did indeed have an effect on the rates of nitrogen accumulation. In the first 13 months of decomposition there were significant differences in the nitrogen accumulation rates between conditions (P < 0.0001) and among species (P < 0.001), but not sites. Nitrogen accumulation rates were higher in disks on the soil surface (0.009 ± 0.001% dry weight month-1) compared with buried disks (0.004 ± 0.001% dry weight month-1). Nitrogen accumulation rates were also different among species, increasing faster in L. racemosa (0.006 ± 0.001% dry weight month-1) than in R. mangle (0.003 ± 0.001% dry weight month-1). No significant differences between R. mangle and A. germinans were found. After 28 months of decomposition, there was no species effect on the absolute nitrogen accumulation in wood but there was a condition effect (P < 0.05). Nitrogen accumulation rates after 28 months remained higher for surface disks (0.011 ± 0.002% dry weight month-1) compared with buried disks (0.007 ± 0.001% dry weight month-1).

As hypothesized, there was a site effect on the rate of change in nitrogen content (0.10 > P > 0.05). Disks at NHB increased N content faster than disks decomposing at SD (0.013 ± 0.002 vs. 0.005 ± 0.002% dry weight month-1). Calculation of net changes in nitrogen (immobilization vs. mineralization) revealed differences among species, condition and sites (Fig. 4b). We found net mineralization of nitrogen in A. germinans and R. mangle for all three conditions and net immobilization of nitrogen for surface disks of L. racemosa during the first 13 months of decomposition. After 28 months, there was net immobilization of nitrogen in L. racemosa surface and buried disks. Averaged over condition and site, there were no net changes in A. germinans and R. mangle after 28 months. Averaged over species and condition, the sites located in more marine environments showed net immobilization of nitrogen (Fig. 4b).

graphs showing nitrogen content of decaying wood through time
Fig. 4 Nitrogen content of decaying wood through time. (a) Changes in N content as a proportion of dry mass. (b) Changes in NAI. Both figures are averaged by species, condition and site. Error bars represent 95% CI. [larger image]

screen shot of table 4
Go to Table 4

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