Decomposition of plant material in Australian soils .IV. Decomposition in situ of ¹⁴C labeled and ¹⁵N labeled legume and wheat materials in a range of southern Australian soils

Abstract

14C- and 15N-labelled wheat straw, and tops or roots of a pasture legume (either Medicago littoralis or Trifolium subterraneum) were incorporated into topsoils at 12 field sites in southern Australia. These sites were representative of soil types widely used for wheat growing in each region. The soils varied markedly in their physical and chemical properties (e.g. pH, texture and organic matter content). Based on amounts of residual I4C (averaged for all sites), the legume tops decomposed more extensively than did wheat straw, especially soon after incorporation. To a lesser extent the legume tops decomposed more extensively than legume roots, and T. subterraneum tops more than M. littoralis tops; root decomposition for both legumes was similar. For example, after 1 year, the residual organic 14C from wheat straw, M. littoralis tops, T. subterraneum tops and legume roots accounted for 48%, 41%, 38% and 54% of their respective inputs. After two years, residual 14C of wheat straw accounted for 30% of the input. Differences in decomposition due to climate and soil properties were generally small, but at times were statistically significant; these differences related positively with rainfall and negatively with soil clay content, but showed no relationship with pH or soil organic C and N. Some N was mineralized from all plant materials, the greatest from legume tops, the least from wheat straw. After 1 year, residual organic 15N accounted for 56%, 63% and 78% respectively of input l5N from legume tops and roots and from wheat straw. The influence of climate and soil properties on amounts of residual organic I5N was small and generally was consistent with those found for residual 14C. AS an exception, the residual organic 15N from wheat straw was negatively related to soil organic N levels, whereas residual I5N of legume tops and roots and residual 14C of all plant materials were not influenced by soil organic matter levels. These results are discussed in terms of the turnover of N in soils amended with isotope labelled plant materials of different available C:N ratios.