Long-term tillage, straw management and N fertilization effects on quantity and quality of organic C and N in a Black Chernozem soil

Soil, crop and fertilizer management practices may affect the amount and quality of organic C and 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 (1980 to 1998) or 27 years (1980 to 2006) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [SRem]and straw retained [SRet]) and N fertilizer rate (0, 50 and 100 kg N ha−1 in SRet and 0 kg N ha−1 in SRem plots) on total organic C (TOC) and N (TON), and light fraction organic C (LFOC) and N (LFON) in the 0–7.5 and 7.5–15 cm or 0–5, 5–10 and 10–15 cm soil layers. The mass of TOC and TON in soil was usually higher in SRet than in SRem treatment (by 3.44 Mg C ha−1 for TOC and 0.248 Mg N ha−1 for TON after 27 years), but there was little effect of tillage and N fertilization on these parameters. The mass of LFOC and LFON in soil tended to increase with SRet (by 285 kg C ha−1 for LFOC and 12.6 kg N ha−1 for LFON with annual rate of 100 kg N ha−1 for 27 years), increased with N fertilizer application (by 517 kg C ha−1 for LFOC and 36.0 kg N ha−1 for LFON after 27 years), but was usually higher under CT than ZT (by 451 kg C ha−1 for LFOC and 25.3 kg N ha−1 for LFON after 27 years). Correlations between soil organic C or N fractions were highly significant in most cases. Linear regressions between crop residue C input and soil organic C or N were significant in most cases. The effects of tillage, straw management and N fertilizer on soil were more pronounced for LFOC and LFON than TOC and TON, and also in the surface layers than in the deeper layers. Tillage and straw management had little or no effect on C:N ratios, but the C:N ratios in light organic fractions significantly decreased with increasing N rate (from 20.06 at zero-N to 18.91 at 100 kg N ha−1). Compared to the 1979 results, in treatments that did not receive N fertilizer (CTSRem0, CTSRet0, ZTSRem0 and ZTSRet0), CTSRem0 resulted in a net decrease in TOC concentration (by 1.9 g C kg−1) in the 0–15 cm soil layer in 2007 (after 27 years), with little or no change in the CTSRet0 and ZTSRem0 treatments, while there was a net increase in TOC concentration (by 1.2 g C kg−1) in the ZTSRet0 treatment. Straw retention and N fertilizer application at 50 and 100 kg N ha−1 rates showed a net positive effect on TOC concentration under both ZT (ZTSRet50 by 2.3 g C kg−1 and ZTSRet100 by 3.1 g C kg−1) and CT (CTSRet50 by 3.5 g C kg−1 and CTSRet100 by 1.6 g C kg−1) treatments in 2007 compared to 1979 data. In conclusion, the findings suggest that retention of straw, application of N fertilizer and elimination of tillage would improve soil quality, and this might increase the potential for N supplying power of the soil and sustainability of crop productivity.