ALBERT

Beschreibung: An 11-yr study was conducted on a coarse-textured Brown Chernozemic soil in the semiarid prairie of southwestern Saskatchewan. Soil was sampled after 3, 7 and 11 yr, and the results were used to assess the influence of fallow frequency and tillage on selected soil quality attributes [e.g., total soil organic C and N, microbial biomass C (MB-C) and microbial biomass N (MB-N), C mineralization (Cmin) and N mineralization (Nmin), and specific respiratory activity (SRA)] in the 0- to 7.5-cm and 7.5- to 15-cm depths. Although it took 11 yr before we observed significant treatment effects on total organic C or N, effects on Cmin and Nmin were observed in 7 yr in the 0- to 7.5-cm depth and by 11 yr, MB and SRA also showed significant treatment effects in this depth. Generally, soil quality attributes were greater in no-tillage (NT) systems than in conventional mechanical tillage (CT) or minimum tillage (MT), and greater in continuous wheat (Triticum aestivum L.) (Cont W) than in fallow-wheat (F-W) systems. With time, the labile constituents tended to increase under the Cont W cropping, but to decrease when F-W was coupled with MT. After 11 yr there was a strong, direct association between the labile attributes (viz., Cmin, Nmin and MB-C) in the 0- to 7.5-cm depth and the mean annual straw produced (kg ha−1yr−1) in the four cropping systems tested. Of the soil quality attributes tested, Cmin and Nmin were the most sensitive indices to tillage and fallow frequency effects. Key words: Mineralizable C, mineralizable N, microbial biomass, specific respiratory activity, crop residues

Beschreibung: Measurements of seasonal changes in soil biochemical attributes can provide valuable information on how crop management and weather variables influence soil quality. We sampled soil from the 0- to 7.5-cm depth of two long-term crop rotations [continuous wheat (Cont W) and both phases of fallow-wheat (F–W)] at Swift Current, Saskatchewan, from early May to mid-October, 11 times in 1995 and 9 times in 1996. The soil is a silt loam, Orthic Brown Chernozem with pH 6.0, in dilute CaCl2. We monitored changes in organic C (OC) and total N (TN), microbial biomass C (MBC), light fraction C and N (LFC and LFN), mineralizable C (Cmin) and N (Nmin), and water-soluble organic C (WSOC). All biochemical attributes, except MBC, showed higher values for Cont W than for F–W, reflecting the historically higher crop residue inputs, less frequent tillage, and drier conditions of Cont W. Based on the seasonal mean values for 1996, we concluded that, after 29 yr, F–W has degraded soil organic C and total N by about 15% compared to Cont W. In the same period it has degraded the labile attributes, except MBC, much more. For example, WSOC is degraded by 22%, Cmin and Nmin by 45% and LFC and LFN by 60–75%. Organic C and TN were constant during the season because one year's C and N inputs are small compared to the total soil C or N. All the labile attributes varied markedly throughout the seasons. We explained most of the seasonal variability in soil biochemical attributes in terms of C and N inputs from crop residues and rhizodeposition, and the influences of soil moisture, precipitation and temperature. Using multiple regression, we related the biochemical attributes to soil moisture and the weather variables, accounting for 20% of the variability in MBC, 27% of that of Nmin, 29% for LFC, 52% for Cmin, and 66% for WSOC. In all cases the biochemical attributes were negatively related to precipitation, soil moisture, temperature and their interactions. We interpreted this to mean that conditions favouring decomposition of organic matter in situ result in decreases in these attributes when they are measured subsequently under laboratory conditions. We concluded that when assessing changes in OC or TN over years, measurements can be made at any time during a year. However, if assessing changes in the labile soil attributes, several measurements should be made during a season or, measurements be made near the same time each year. Key words: Microbial biomass, carbon, nitrogen, mineralization, water-soluble-C, light fraction, weather variables

Beschreibung: 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 (Cmin and Nmin), 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., R2 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., Cont W (Fert) and F–W–W–H–H–H) than in F–W or Cont 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, Cmin and Nmin 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 Cmin have increased markedly in the more fertile treatments [e.g., Cont W (Fert), F–W–W–H–H–H], but have hardly changed in the less fertile treatments such as F–W or Cont W (Unfert). In this period the less labile attributes (e.g., OC) have hardly changed in any treatment. Key words: Microbial biomass, carbon, nitrogen, mineralization, water-soluble C, light fraction, weather variables

Beschreibung: Society is interested in increasing C storage in soil to reduce CO2 concentration in the atmosphere, because the latter may contribute to global warming. Further, there is considerable interest in the use of straw for industrial purposes. Using soil samples taken from the 0- to 7.5-cm and 7.5- to 15-cm depths in May 1987 and September 1996, we determined organic C and total N in five crop rotations (nine treatments) using automated Carlo Erba combustion analyzer. The experiment was managed using conventional mechanical tillage from 1957 to 1989; it was changed to no-tillage management in 1990. Our objective was to determine: (a) if change to no-tillage management had changed soil C and N storage, and (b) if method of calculating organic C and N change would influence interpretation of the results. All three methods of calculation confirmed the efficacy of employing best management practices (e.g., fertilization based on soil tests, reducing summerfallow, including legumes in rotations) for increasing or maintaining soil organic matter, and showed that the latter was directly associated with the amount of crop residues returned to the soil. Where bulk density was significantly different between sampling times, the often used mass per fixed depth (MFD) (i.e., volume basis) calculation can lead to erroneous conclusions. When the recently recommended mass per equal depth (MED) method of calculation was used, it showed that 6 yr of no-tillage did not increase soil organic C or total N. However, in unfertilized systems, where crop yields are gradually decreasing since the change, there is an accompanying decrease in organic matter, while fertilized, or high-fertility systems that include legume hay crops, in which wheat yields have been maintained have tended to maintain the organic matter level over time. When the MFD calculation was used, there was no change in C over time when straw was harvested in the F–W–W system; however, the MED calculation and concentrations tend to show a decrease in soil C and N. This suggests that in time, industrial use of straw may have negative consequences for soil conservation. We concluded that concentrations may be as effective as MED for assessing changes in organic matter, provided "amounts" are not required. Key words: Straw removal, fertilizers, legumes, cropping frequency, C mass calculation