Soil, crop, and fertilizer management practices may affect quality of organic carbon (C) and nitrogen (N) in soil. A long-term field experiment (growing barley, wheat, or canola)was conducted on a Black Chernozem (Albic Argicryoll) loam at Ellerslie, Alberta, Canada, to determine the influence of 19 years (1980 to 1998) of tillage [zero tillage (ZT) and conventional tillage (CT)], straw management [straw removed (S_Rem) and straw retained (S_Ret)], and N fertilizer rate (0, 50, and 100 kg N ha⁻¹ in S_Ret and 0 kg N ha⁻¹ in S_Rem plots) on macro-organic matter C (MOM-C) and N (MOM-N), microbial biomass C (MB-C), and mineralizable C (C_min) and N (N_min) in the 0- to 7.5-cm and 7.5- to 15-cm soil layers. Treatments with N fertilizer and S_Ret generally had a greater mass of MOM-C (by 201 kg C ha⁻¹ with 100 kg N ha⁻¹ rate and by 254 kg C ha⁻¹ with S_Ret), MOM-N (by 12.4 kg N ha⁻¹ with 100 kg N ha⁻¹ rate and by 8.0 kg N ha⁻¹ with S_Ret), C_min(by 146 kg C ha⁻¹ with 100 kg N ha⁻¹ rate and by 44 kg C ha⁻¹ with S_Ret), and N_min(by 7.9 kg N ha⁻¹ with 100 kg N ha⁻¹ rate and by 9.0 kg N ha⁻¹ with S_Ret)in soil than the corresponding zero-N and S_Rem treatments. Tillage, straw, and N fertilizer had no consistent effect on MB-C in soil. Correlations between these dynamic soil organic C or N fractions were strong and significant in most cases, except for MB-C, which had no significant correlation with MOM-C and MOM-N. Linear regressions between crop residue C input and mass of MOM-C, MOM-N, C_min, and N_min in soil were significant, but it was not significant for MB-C. The effects of management practices on dynamic soil organic C and N fractions were more pronounced in the 0- to 7.5-cm surface soil layer than in the 7.5- to 15-cm subsoil layer. In conclusion, the findings suggest that application of N fertilizer and retention of straw would improve soil quality by increasing macro-organic matter and N-supplying power of soil.

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