Solid-state 13C NMR spectroscopy: characterization of soil organic matter under two contrasting residue management regimes in a 2-year-old pine plantation of subtropical Australia

Abstract

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, with cross-polarisation (CP) and magic angle spinning (MAS), was used to characterize soil organic matter (SOM) in a 2-year-old exotic pine plantation of subtropical Queensland, Australia, under two contrasting harvest residue management regimes. Soil samples were collected from the 0–10 cm depth of experimental plots receiving either no harvest residues (no harvest residues) or the double quantity of harvest residues applied (double harvest residues). Carbon-13 CP and dipolar dephasing (DD) NMR techniques were able to detect differences in SOM composition and quality under the two contrasting residue treatments. The SOM under no harvest residues displayed an increased extent of decomposition, as determined by the alkyl C/ O-alkyl C (A/O-A) ratio, and lower potentially mineralizable nitrogen (PMN), organic C, total P and total N contents. The CP spectra displayed little evidence of strong aromatic signals derived from lignin or tannin structures. This was confirmed by the DD spectra, which rapidly lost signal in the methoxyl and alkyl C regions, indicating protein and amide structures with little mobility might be dominant in the aromatic spectral region. The DD spectra also indicated that SOM under double harvest residues might have a small amount of condensed tannin structures, which did not exist in the SOM under no harvest residues. The carbonyl C region displayed resonances indicative of oxalate, carboxyl, amide and ester C in both treatments. Overall, the results of this study indicate that residue removal following harvest of exotic pine plantations on low-fertility soils in subtropical Australia can remove valuable nutrients from the site, which in turn may increase the extent of decomposition, leading to decreased SOM quality in subsequent rotations.