A long-term effect of N fertilization on the 13C CPMAS NMR of de-ashed soil humin in a second-growth Douglas-fir stand of coastal British Columbia

Abstract

Carbon-13 CPMAS NMR spectroscopy was used to examine long-term effects of thinning and N fertilization on the humin fraction of soil organic matter in a second-growth Douglas-fir stand of coastal British Columbia. De-ashed organic-matter-enriched humin fractions were prepared from three mineral soil horizons of four silvicultural treatments using 1.0 M HF accompanied by a magnetic stir bar to remove ferromagnetic iron particles, followed by a density separation. With some exceptions a higher proportion of mass was recovered in the denser, light-coloured fraction, and a higher proportion of C and N in the less-dense, dark fraction. In all cases, the less-dense fraction was enriched in total C compared to the original crude humin, and had a more favourable C/Fe ratio for NMR spectroscopy. The fraction of observable C was 33–37% for 8 of the samples, but as low as 7.6% for the remaining four. Carbon-13 CPMAS NMR spectra had typical features for humins (alkyl C, O-alkyl C, di-O-alkyl C, aromatic C, and carboxyl, ester and amide C). With fertilization, there was a small but consistent decrease in the ratio of alkyl to O-alkyl C, regardless of horizon or thinning, indicating a lower extent of decomposition. Carbon-13 subspectra based on proton spin relaxation time ( T 1( 1 H) ) were obtained for one fertilized and one unfertilized sample. Subspectra of the slowly-relaxing (long T 1( 1 H) ) domain were dominated by long-chain alkyl C. For the fertilized sample, the subspectrum of the fast-relaxing (short T 1( 1 H )) domain had a higher proportion of O- and di-O-alkyl C, consistent with results from the normal CPMAS NMR spectra. Despite the uncertainties introduced by the de-ashing treatment and the low fraction of observable C, 13C CPMAS NMR showed that fertilization has a long-term effect which is reflected in this humin fraction.