Organic C accumulation in soil over 30 years in semiarid southwestern Saskatchewan - Effect of crop rotations and fertilizers

Because crop management has a strong influence on soil C, we analyzed resul ts of a 30-yr crop rotation experiment, initiated in 1967 on a medium textu red Orthic Brown Chemozem 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-c m and 15- to 30-cm depths were measured in 1976, 1981, 1984, 1990, 1993, an d 1996, but results are only presented for the 0- to 15-cm depth since chan ges in the 15- to 30-cm depth were not significant. We developed an empiric al equation to estimate SOC dynamics in the rotations. This equation uses t wo first order kinetic expressions, one to estimate crop residue decomposit ion and the other to estimate soil humus C mineralization. Crop residues (i ncluding roots) were estimated from straw yields, either measured or calcul ated 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 foun d that (i) SOC was increased most by annual cropping with application of ad equate fertilizer N and P; (ii) that frequent fallowing resulted in lowest SOC except when fall-seeded crops, such as fall rye (Secale cereale L.), th at reduce erosion were included in the rotation, and (iii) the fallow effec ts are exacerbated when low residue yielding flax (Linum usitatissimum L.) was included in the rotation. Some of the imprecision in SOC values we spec ulated 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 wa s managed in this manner for the previous 50 yr. However, in rotations that received N + P fertilizer and were cropped annually [continuous wheat (Con t W) and wheat-lentil (Lens culinaris L.)], or that included fall-seeded cr ops (e.g., F-Rye-W), SOC appeared to increase sharply in this period. In th e drought period (1984-1988) SOC was generally constant, but large increase s occurred in the wet period (1990 to 1996) in response to high residue inp uts. The efficiency of conversion of residue C to SOC for the 30-yr experim ental period was about 10-12% for F-W, F-W-W and Cent W (+P) systems, and i t was about 17-18% for the well fertilized F-Rye-W, Cent W, and W-Lent syst ems. The average annual SOC gains (Mg ha(-1) yr(-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 Cent W (N + P), 0.12 f or Cent W (+ P), and 0.28 for W-Lent (N + P). The corresponding mean estima ted (by our equation) annual SOC gains for these rotations, were 0.06, 0.10 , 0.16, 0.22, 0.14, and 0.22 Mg ha(-1) yr(-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 trend s and to test if they seem reasonable.