Light-Fraction Organic Matter in Soils from Long-Term Crop Rotations

Light-fraction (LF) material, comprised largely of incompletely decomposed organic residues, may provide a sensitive indicator of the effects of cropping practices on soil organic matter. The objective of our study was to determine the influence of agronomic variables on soil LF content, and to evaluate the LF as a measure of labile organic matter. Soils from three long-term crop rotation studies in Saskatchewan, Canada, were analyzed for LF content and composition. The experiments, established at Indian Head (Udic Boroll), Melfort (Udic Boroll), and Scott (Typic Boroll), included wheat (Triticum aestivum L.) based rotations varying in fertilizer application, frequency of summer fallow, and cropping sequence. The LF of the surface soil (0–7.5 cm) accounted for 2.0 to 5.4%, 3.3 to 7.1%, and 7.1 to 17.5% of the organic C at Indian Head, Melfort, and Scott, respectively. Within each site, the LF content was generally highest in treatments with continuous cropping or perennial forages and lowest in those with a high frequency of summer fallow. Fertilizer application generally favored LF accumulation. Differences in LF content among sites and treatments were attributed to variable residue inputs and rates of substrate decomposition. The respiration rate and microbial N content of soils was strongly correlated with the LF content, suggesting that the LF is a useful indicator of labile organic matter. Nitrogen mineralization was also correlated with LF content, though the relationship was less consistent, presumably because the high C/N ratio of the LF induced temporary N immobilization. The LF content is a sensitive indicator of the effects of cropping on soil organic matter content and composition but, because of its transient nature, probably reflects primarily short-term effects.