Forest residue management affects soil nitrogen availability and humic acid composition

Abstract

Soil humic substances are important components of soil organic matter and contain a significant portion of total soil organic carbon (C) and nitrogen (N). Solidstate 13C nuclear magnetic resonance (NMR) with cross-polarisation and magic angle spinning (CPMAS) was applied to humic acids extracted from 0-10 cm soils collected from areas under windrows of harvest residues and those areas between the windrows, 3 years after implementation of residue management in a second-rotation plantation of Araucaria cunninghamii Ait. ex D.Don (hoop pine). In addition, nitrogen availability of under-windrow and between-windrow soils was also assayed by anaerobic incubation with either water or 15N-labelled ammonium sulphate solution in the laboratory. The NMR spectra of the humic acids showed that the carbon composition of the under-windrow humic acids was different to that of the between-windrow humic acids. Potentially mineralisable nitrogen of the under-windrow soils was greater than that of the between-windrow soils, as was gross nitrogen mineralisation (ammonification, mg). Soil potentially mineralisable nitrogen was also positively correlated with humic acid-alkyl and humic acid-O-alkyl carbon (p<0.05), while gross nitrogen mineralisation was positively correlated with humcacid-aromatic carbon (p<0.01). The gross nitrogen mineralisation was 33-45 mg N/kg dry soil as determined by isotope dilution with 15N-labelled ammonium sulphate (100 mg N/kg and 99 atom% 15N excess) and was greater in under-windrow than between-windrow soil after the 7-day anaerobic incubation. In addition, gross 15N immobilised (NH4+ consumption, mi) was positively correlated with humic acidaromatic carbon (p<0.05). Humic acid-iron content was positively correlated with humic acid-alkyl and O-alkyl carbon (p<0.05).