Quantifying short-term effects of crop rotations on soil organic carbon insouthwestern Saskatchewan

Crop management practices can have a major influence on soil fertility and soil organic C (SOC) sequestration. We need to accurately measure and estim ate changes in SOC in the short term (<20 yr). A 10-yr crop rotation experi ment, conducted on a medium-textured Orthic Brown Chernozem at Swift Curren t, in southwestern Saskatchewan, was sampled in 1990 (3 yr after initiation of the study) and in 1993 and 1996, to measure SOC changes under nine crop rotation treatments. Minimum tillage practices were used. The stubble was cut high to enhance snow trap and N and P fertilizer applied based on soil tests. Grain and straw yields of the cereals, and hay yields of the crested wheatgrass (CWG) [Agropyron cristatum (L.) Gaeertn.] were measured annuall y. An empirical equation which uses two simultaneous first order kinetic ex pressions, one to estimate crop residue decomposition and the other to esti mate soil humus C mineralization was used, together with crop residue (stra w and estimated root) C inputs, to estimate SOC changes over the 1987 to 19 96 period. The estimated SOC values for the 1990 to 1996 period were genera lly similar to the measured values (r(2) = 0.64, P < 0.0001). Significant ( P < 0.10) changes in SOC were not observed below 15 cm depth, perhaps becau se shallow tillage (10- to 12.5-cm depth) is practiced. A change from cropl and to CWG did not increase SOC, and this treatment, chemical fallow-winter wheat (Triticum aestivum L.)-spring wheat (F-WW-W), and F-high-yielding (H y) Canada Prairie Spring (CPS) wheat-Hy (F-Hy-Hy) rotations, had the lowest SOC gains among the rotations. The CPS wheat had a higher harvest index (0 .46) than hard red spring (HRS) wheat (0.39), but it increased SOC less tha n the comparable HRS wheat rotation between 1990 and 1996 indicating that h igher grain yields do not always equate to higher SOC. Weather conditions w ere favourable for cereals from 1990 to 1996 and we measured significant in creases in SOC (up to 5.5 Mg ha(-1) in 6 yr). This is encouraging for produ cers who may be contemplating participating in "C trading", although this a lso suggests that periods of less favourable weather will limit gains in SO C Summerfallowing once in 4 yr in this semiarid environment did not reduce SOC gains compared to continuous wheat (Cont W). For example, a F-W-W-W rot ation gained 4.88 Mg C ha(-1) in 6 yr while continuous wheat gained 5 Mg ha (-1). Growing Indianhead lentil (Lens culinaris Medikus) as a legume green manure crop (GM) with wheat in a GM-W-W rotation did not increase SOC more than F-W-W. The efficiencies of conversion of residue C to SOC were high, r anging between 9% for frequently fallowed systems to 29% for continuously c ropped systems, likely due to the favourable weather conditions experienced .