Adopting zero tillage management: Impact on soil C and N under long-term crop rotations in a thin Black Chernozem

Society's desire to sequester C in soils, thereby reducing the net loss of CO2 (a greenhouse gas) to the atmosphere, is well known. It is also accepte d that the choice of appropriate agricultural management practices adopted by producers will affect this goal. However, quantification of the extent a nd rate at which it can be achieved is uncertain. A crop rotation experimen t that was initiated in 1957 on a thin Black chernozemic clay soil at India n Head, Saskatchewan, was managed using conventional tillage until changed to zero tillage in 1990. Soil was sampled (0- to 7.5- and 7.5- to 15-cm dep ths) in May 1987 and 1997 to determine the effects of treatments on soil or ganic C (SOC) and total N. The rotations were: fallow-wheat (Triticum aesti vum L.) (F-W), F-W-W, continuous wheat (Cont W), legume green manure (GM)-W -W, and F-W-W-hay (legume-grass)-hay-hay (F-W-W-H-H-H). The monoculture cer eal rotations were either fertilized with N and P based on soil tests or un fertilized, while the legume systems were both unfertilized. There was also a F-W-W (N+P) treatment in which the straw was baled and removed. When the experiment was changed to zero tillage management in 1990, the fertilizer protocol was changed to satisfy the "moist soil" criteria. Consequently, hi gher rates of N and P were added thereafter to the fallow crop, resulting i n a positive yield response of wheat,grown on fallow, where before there wa s no response to fertilizer. Over the 10-yr period (1987-1997) fertilized s oil gained C and N, but unfertilized soil did not. For example fertilized F -W, F-W-W and Cent W gained about 4, 5 and 2 Mg C ha(-1) in the 10-yr perio d. During this period, C emissions from manufacture and transportation of N fertilizer was 0.28, 0.53 and 0.90. Mg ha(-1) for these three rotations, r espectively. These results suggest that without adequate fertility, convers ion to zero tillage may not always result in an increase in soil C or N. By 1997, fertilizer increased soil C and N in F-W-W and Cent W, and soil C an d N were greater in F-W-W-H-H-H than in GM-W-W and lowest in F-W-W tall unf ertilized). Straw removal had no significant effect on C or N. The analysis showed that C inputs from crop residues was the main factor influencing SO C changes.