Carbon, nitrogen and sulphur cycling following incorporation of canola residue of different sizes into a nutrient-poor sandy soil

No information is available on the role of particle size of canola (Brassica napus) residue in altering C mineralization and nutrient (N, S) cycling in soil. We studied decomposition of canola residue (at 20±1 °C temperature and 10% moisture (w/w) for 6 months to elucidate the effect of its particle size (<1, 5-7, and 20-25 mm) on dynamics of C, N and S turnover following incorporation into a nutrient-poor sandy soil.Averaged over time, particle size of canola residue did not significantly affect C mineralization rate, the size of microbial-C and microbial-N pools, or the extent of CaCl₂-extractable S immobilization, but altered the extent of mineral-N immobilization and water-soluble organic C (W-SOC) depletion. A rapid decrease in C mineralization rate in the first week matched the rapid depletion of W-SOC, especially for the <1 mm residue treatment. Over 6 months, mineral-N in the amended soils rarely increased beyond the starting level (0.8-1 mg kg⁻¹ soil for all the treatments), whereas nitrate-N increased 19-fold in the non-amended soil. This suggests an occurrence of strong N immobilization in the amended soils; such immobilization was high for the <1 mm residue treatment. On a cumulative basis, 33-35% of C added in canola residues to the soil was respired in 6 months. The microbial-C and microbial-N pools peaked by day 4 for all the residue treatments (compared to time zero, 58-122% increase for microbial-C and 36-57% for microbial-N). Averaged over time, amended soils contained approx. 40% more microbial-C and microbial-N than the non-amended soil. An addition of canola residue (regardless of the size) to soil increased the extractable S significantly (3.4-fold) on day 0; this initially increased S level decreased by one-third over 6 months. In conclusion, particle size of canola residue did not affect temporal pattern of C and S mineralization in a nutrient-poor sandy soil, but altered N cycling.