Field:
One core per plot (total 12 cores) were collected in late August, 2003. The cores were immediately shipped to U of Michigan and deep froze (-80-degree C) until extraction.

Lab:
Subsamples of the cores were sequentially extracted with K2SO4 for d15N diffusion of NH4, NO3, and DON. Another set of subsamples used to determine chloroform-labile N using CHCl3 fumigation.
Note: The chroloform-labile N should not be reported as microbial N, because the conventional conversion factor was determined for temperate soils, and it is likely that the factor could be very different for Arctic soils.
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Chloroform-extractable 15N of Aug 2003 soils (Analyzed by Bill Holmes @ U. of Michigan for Yuriko Yano @ Ecosystems Ctr. MBL)

Subdivide fresh soils:
10 g to dry at 60C for total 15N analysis
12.00+/-0.02 g to extract with K2SO4 (CFE-NF, extractable NH4+ and NO3-, DON-15N)
10 g to fumigate and extract with K2SO4 (CFE-FUM)

Extractable NH4+ and NO3- and DON (CFE-NF)
extract with 0.5N K2SO4
vacuum filter through quartz fiber filter with wide Buchner funnel
microfilter extract
analyze for NH4+ and NO3- on rapid-flow analyzer (RFA)
diffuse NH4+ and NO3-
digest for 15N DON
diffuse NO3-

CFE (FUM)
fumigate soils
extract with K2SO4
vacuum filter through quartz fiber filter with wide Buchner funnel
digest extract (alkaline persulfate digest)
analyze for NH4+ and NO3- on RFA
diffuse NH4+ and NO3-

Total 15N analysis
dry soil at 60C
grind
burn
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Calculation of N pool size and 15N recovery into chloroform-extractable N pool and non-extractable pool:

We found out later that the diffusion method used above would not be able to separate DIN from DON (i.e., DON would be diffused with NH4 and NO3). Becuase "DIN" and "DON" were diffused sequencially, 15N and N pool size of DON and TDN were back calculated. A mixing model was used to claculate 15N recovery to TDN.