Seasonal trends in soil biochemical attributes: Effects of crop management on a Black Chernozem

Knowledge of the response of soil biochemical attributes to crop management and growing season weather is important for assessing soil quality and fertility. Long-term (38-39 yr) crop rotations on a Black Chernozem at Indian Head, Saskatchewan, were sampled (0- to 7.5-cm depth) between early May and mid-October, 11 times in 1995 and 9 times in 1996. We assessed the effect of cropping frequency [fallow-wheat (Triticum aestivum L.) (F-W) vs. F-W-W, vs. Continuous (Cont) W], fertilizers (unfertilized vs. N + P applied), straw harvesting, legume green-manure (GM) in GM-W-W (unfertilized), and legume-grass hay (H) in F-W-W-H-H-H (unfertilized) systems. Changes in organic C and total N (OC, TN), microbial biomass C (MBC), light fraction C and N (LFC and LFN), mineralizable C and N (C-min and N-min), and water-soluble organic C (WSOC) were monitored. Organic C and TN were constant and unaffected by rotation phase during the season, but most of the other more labile soil biochemical attributes varied during the season. Much of this temporal variability was associated with changes in soil moisture, temperature and precipitation, and with rhizodeposition in some cases. Whenever conditions favoured rapid decomposition in situ (e.g., high moisture, temperature and/or precipitation) we obtained lower values for the more labile attributes in subsequent laboratory measurements. Seasonal trends in the more labile attriutes were more pronounced in 1995 (a much wetter year) than in 1996, and the proportion of the variability attributable to weather conditions was greater in 1995 than in 1996 (viz., R-2 ranged from 20 to 44% in 1996 and from 37 to 60% in 1995). Seasonal variability was greater in the more fertile treatments [e.g., Cent W (Fert) and F-W-W-H-H-H) than in F-W or Cent W (Unfert). Seasonal variability in LF was unaffected by cropping. Light fraction was lower in 1995 than 1996 because of faster decomposition in 1995 (335 mm of growing season precipitation compared to 157 mm in 1996). Microbial biomass was not influenced by cropping in 1996, but in 1995 it was higher in cropped than in fallow phases of the rotations, suggesting a positive effect of rhizodeposition. Water-soluble organic C was greatest in the more fertile treatments and in cropped than in fallow phases. Mineralizable C and N were greater in cropped than in fallow rotation phases in 1995, but unaffected by cropping in 1996. Further, C-min and N-min were higher in 1996 than in 1995, likely due to more rapid decomposition in the wetter 1995. Over the last 10 yr of this experiment LFC and C-min have increased markedly in the more fertile treatments [e.g., Cent W (Fert), F-W-W-H-H-H], but have hardly changed in the less fertile treatments such as F-W or Cent W (Unfert). In this period the less labile attributes (e.g., OC) have hardly changed in any treatment.