Carbon and nitrogen contents of different-sized light fraction organic matter as influenced by tillage and residue management

The light fraction (LF) has a variable elemental content because it comprises a pool of soil organic matter that is in transition between fresh residues and stable, humified organic matter. Our aim was to assess the influence of time, tillage (CT vs. NT) and straw management (removed or retained) practices on the C and N contents of two particle sizes of LF materials from a Gray Luvisol in Alberta. The LF C and N concentrations were not affected by tillage and straw treatments. The C concentration was higher in LF > 1 mm (coarse LF) than in the < 1 mm LF (fine LF), while the converse was observed for N concentration, resulting in C:N ratios of 45-59 in the coarse fraction and 18-19 for the finer materials. The C concentration of the fine LF decreased and the N concentration increased with time. After 4 yr, LF C and N stocks were higher under NT than under CT mainly because of faster decomposition of litter under CT. Retaining straw resulted in bigger increases in C and N stocks in the coarse LF compared with straw removal; with the fine LF, the C stock decreased more quickly and the N stock increased less rapidly with straw removal. Our results show that time strongly affected the LF C and N stocks and concentrations, and that separating the fraction by size can lead to a more meaningful interpretation of those data.Key words: Light fraction, carbon, nitrogen, tillage, crop residue, straw management