Because crop management has a strong influence on soil C, we analyzed results of a 30-yr crop rotation experiment, initiated in 1967 on a medium textured Orthic Brown Chernozem at Swift Current, Saskatchewan, to determine the influence of cropping frequency, fertilizers and crop types on soil organic C (SOC) changes in the 0- to 15-cm depth. Soil organic C in the 0- to 15-cm and 15- to 30-cm depths were measured in 1976, 1981, 1984, 1990, 1993, and 1996, but results are only presented for the 0- to 15-cm depth since changes in the 15- to 30-cm depth were not significant. We developed an empirical equation to estimate SOC dynamics in the rotations. This equation uses two first order kinetic expressions, one to estimate crop residue decomposition and the other to estimate soil humus C mineralization. Crop residues (including roots) were estimated from straw yields, either measured or calculated from grain yields. The parameter values in our equation were obtained from the scientific literature or were based on various assumptions. Carbon lost by wind and water erosion was estimated using the EPIC model. We found that (i) SOC was increased most by annual cropping with application of adequate fertilizer N and P; (ii) that frequent fallowing resulted in lowest SOC except when fall-seeded crops, such as fall rye (Secale cereale L.), that reduce erosion were included in the rotation, and (iii) the fallow effects are exacerbated when low residue yielding flax (Linum usitatissimum L.) was included in the rotation. Some of the imprecision in SOC values we speculated to be related to variations in soil texture at the test site. In the first 10 yr of the experiment, SOC was low and constant for fallow-spring wheat (Triticum aestivum L.) (F-W) and F-W-W rotations because this land was managed in this manner for the previous 50 yr. However, in rotations that received N + P fertilizer and were cropped annually [continuous wheat (Cont W) and wheat-lentil (Lens culinaris L.)], or that included fall-seeded crops (e.g., F-Rye-W),SOC appeared to increase sharply in this period. In the drought period (1984â??1988) SOC was generally constant, but large increases occurred in the wet period (1990 to 1996) in response to high residue inputs. The efficiency of conversion of residue C to SOC for the 30-yr experimental period was about 10â??12% for F-W, F-W-W and Cont W (+P) systems, and it was about 17â??18% for the well fertilized F-Rye-W, Cont W, and W-Lent systems. The average annual SOC gains (Mg haâ??1 yrâ??1) between 1967 and 1996 were 0.11 for F-W (N + P), 0.09 for the mean of the three F-W-W rotations (N + P, + N, + P), 0.23 for F-Rye-W (N + P), 0.32 for Cont W (N + P), 0.12 for Cont W (+ P), and 0.28 for W-Lent (N + P). The corresponding mean estimated (by our equation) annual SOC gains for these rotations, were 0.06, 0.10, 0.16, 0.22, 0.14, and 0.22 Mg haâ??1 yrâ??1, respectively. Because soil C measurements are usually so variable, we recommend that calculations such as ours may be employed to assist in the interpretation of measured C trends and to test if they seem reasonable.
