1Semiarid Prairie Agricultural Research Centre, Swift Current, Saskatchewan and 2 Indian Head Experimental Farm
Cereal straw is being considered by industry as a feedstock (or fibre) for making pressboard (Manitoba) and newsprint (Vulcan, AB). But, will repeated straw harvesting influence long-term grain yields or soil organic matter?
A crop rotation experiment initiated on a thin Black heavy clay soil at Indian Head, Saskatchewan in 1957, includes three fallow-wheat-wheat (F-W-W) systems, one unfertilized, two receiving N + P fertilizer based on soil tests, one of which has had about two-thirds of the straw harvested each crop year. Conventional mechanical tillage was used until 1990 when the experiment was changed to no-tillage management.
Grain yield response to fertilizer has been evident since 1970 for wheat grown on stubble, and the difference between fertilized and unfertilized treatments has been widening with time. For wheat on fallow, a response to fertilizer has only become apparent since the change to no-tillage management. Straw removal has had no influence on grain yields over the years.
In the past, we have shown that there is a direct relationship between amount of crop residues returned to the soil and soil organic matter. Further, we have shown that frequent mechanical tillage decreases soil organic matter. In 1987, we sampled the soil from the three systems but, surprisingly, could find no effect of the fertilization or straw removal on soil organic C.
Because of the change to no-tillage in 1990, we again sampled the soils in September 1996. The results depended on our method of expressing the findings. When we calculated the mass of soil C on a fixed volume basis (i.e., soil mass that reflected changes in bulk density), we found no effect of fertilizer or straw removal in 1987. However, in 1996 fertilizer increased organic C in the 0-7.5 cm depth, while straw removal had no effect. Further, we found no evidence of change in C in the unfertilized system over the 10 year period, but the two fertilized systems appeared to gain C (about 3 t ha1).
Results based on calculations using the volume basis have been criticized by some researchers as being inaccurate because they are based on equal depth of sampling rather than on an equal mass of soil. When we reanalyzed our data based on equal mass of soil for the top 7.5 cm (mass taken as 980 t ha-1), we again found no treatment effects in 1987. Although there was tendency for the fertilized systems to have higher C than the unfertilized system, the difference was not significant (P<0.05). More importantly however, was the fact that there was no increase in C due to the adoption of no-tillage. In fact, in the two fertilized systems, C remained unchanged over the 10 years, while it tended to decrease in the unfertilized system.
In conclusion, it appears that (i) producers farming on heavy-textured soils in the Black soil zone may be able to harvest and sell straw without fearing soil C or yield losses, provided they apply adequate fertilizer; (ii) the apparent gains in soil C that many of us have reported to be the consequence of improved crop management may, in some cases, be an artifact of the method of calculation; when calculated accurately (e.g. on equal mass basis) these differences may not exist; and (iii)crop roots apparently play a more important role in soil organic matter formation than does the above-ground crop residues since straw removal has had no influence on soil C.