The effects of burning on in situ extractable nitrogen
(NH+4-N+NO-3-N)
and net Nmineralisation following scheduled fuel reduction burns in repeatedly
burnt dry and wet sclerophyll forest sites in south-east Queensland were
assessed. In addition, soil organic matter composition in the wet sclerophyll
site was assessed by 13C NMR spectroscopy. The results
showed that at the dry sclerophyll site, extractable N and net N
mineralisation for 1 year were largely unaffected by burning, while at the wet
sclerophyll site, these parameters decreased. 13C NMR
analysis of soil samples from the wet sclerophyll site revealed that there was
a significant reduction in the proportion of O-alkyl (alkoxy/carbohydrate)
C with increasing burning frequency. Statistically significant effects on the
other chemical shift regions were not detected. The ratio of alkyl C to
O-alkyl C, a proposed index of organic matter decomposition, increased with
increasing burning frequency. A high ratio of alkyl C to O-alkyl C suggests
low amounts of carbohydrates relative to waxes and cutins, which could in turn
lead to slower mineralisation. The findings are in accord with this
hypothesis. There were significant linear relationships between cumulative N
mineralisation for 1 year and the proportions of alkyl C and O-alkyl C, and
the ratio of alkyl C/O-alkyl C. Thus, in addition to reductions in
substrate quantity (low organic C and total N for burnt soils), there was also
an alteration of substrate quality as revealed by 13C
NMR spectroscopy which is reflected in low N mineralisation.
Adsorption of dissolved organic matter on clay minerals as assessed by infra-red, CPMAS 13C NMR spectroscopy and low field T1 NMR relaxometry
