ALBERT

Beschreibung: Increasing atmospheric CO2 concentrations, largely due to burning of fossil fuels, may accentuate the risk of global warming. Scientists are optimistic that with appropriate management soils can function as sinks for C and contribute to CO2 abatement strategies. The objective of this study was to determine if soil C can be increased using an annual legume green manure (GM) as partial fallow replacement in a fallow-wheat (Triticum aestivum L.)-wheat (F-W-W) rotation in the Brown soil zone of Saskatchewan. In 1995 and 1996 we measured soil C fluxes in all phases of F-W-W and GM-W-W rotations, which were two of the treatments in an experiment initiated in 1987 on a medium-textured Orthic Brown Chernozem. The GM, Indianhead black lentil (Lens culinaris Medikus) was estimated to add 1800 and 1400 kg C ha−1 in 1995 and 1996, respectively. Annual inputs of C in residues of the wheat crops were two to three times those of GM. Comparison of CO2 emissions from GM with those from the fallow phase of the F-W-W system suggested that GM largely decomposed in the interval between incorporation (mid-July) and freeze-up in fall. Fluxes of CO2 from the wheat phases of GM-W-W closely matched those from the corresponding phases of F-W-W, confirming that there was little carryover of undecomposed GM to the following growing season. Our results suggest that, in a 3-yr rotation, partial fallow replacement with legume GM may have only a minor impact on C sequestration because the increase in C inputs is relatively small (~ 25% in this study) and GM decomposes rapidly in the soil due to its narrow C:N ratio (12–13). Green manuring may, however, play a more significant role in enhancing soil C levels in a F-W system, where relatively large increases in C inputs could be achieved using currently-available legume species. Key words: Carbon sequestration, carbon dioxide emissions, crop residue decomposition, wheat, summerfallow, lentil

Beschreibung: Soil organic C (SOC) is readily influenced by crop management practices, such as summerfallowing. On the Canadian prairies, the area summerfallowed has decreased significantly in recent years. Our objectives were to determine the influence of fallow frequency on the rate of change in SOC in an Orthic Brown Chernozem, and to test the effectiveness of an empirical equation developed in an earlier study for estimating SOC changes in these rotations over 33-yr period. The rotations, which were initiated in 1967, all received adequate N and P fertilizers. They were (i) fallow-spring wheat (Triticum aestivum L.) (F-W), F-W-W, F-W-W-W-W-W and W-lentil (Lens culinaris L.) (W-Lent). Soil organic C was measured in the 0- to 15-cm and 15- to 30-cm depths in 1976, 1981, 1984, 1990, 1993, 1996 and 1999. No measurements of SOC were made in 1967; we estimated SOC starting values to be 30.5 Mg ha–1 in the 0- to 15-cm depth. In the period 1967 to 1990, when growing season precipitation was near normal for this semiarid region, SOC in the four rotations approached a steady state. However, a decade of much more favourable growing season precipitation in the 1990s increased C inputs, which resulted in a marked increase in SOC in the treatments. The empirical equation suggests, and the F-W and W-Lent rotations appear to confirm, that these rotations are approaching a new steady state at a higher level of SOC, reflecting the decade of favourable precipitation. Measured SOC levels were quite variable, emphasizing the difficulty of relying on measurements made over short time frames (e.g., 5-6 yr) when quantifying SOC changes. The equation effectively simulated the trends in SOC changes in all rotations, but consistently underestimated SOC levels in the W-Lent rotation by about 2 Mg ha–1. Estimates of difference in SOC between treatments were generally similar whether expressed on a mass/fixed depth or a mass/equivalent depth basis. Based on the estimates derived by the empirical equation, we estimated rates of SOC sequestration during the 1967-1990 period to be 0.03 Mg ha–1 yr–1 for F-W, 0.10 Mg ha–1 yr–1 for F-W-W, and 0.15 Mg ha–1 yr–1 for W-Lent. If we include the decade of more favourable precipitation (1967-1999), the rates were between 0.05 Mg ha–1 yr–1 for F-W and 0.20 Mg ha–1 yr–1 for W-Lent. These values are much higher than those estimated by others using the CENTURY model. We concluded that (i) simple models, such as that used in this study, are very useful for estimating management effects on SOC changes, and (ii) we must be cautious in extrapolating C sequestration estimates based on data from short-term experiments because future weather conditions are not easily predicted and weather can have an important impact on C sequestration. Key words: C sequestration, cropping frequency, crop rotations, wheat, lentil, flax, oats

Beschreibung: Root/straw ratios for crops are urgently required by scientists wishing to estimate crop residue inputs to soil and for modeling C and N dynamics in the soil-plant-atmosphere system. In this paper we discuss the influence of moisture and rates of N application on such ratios for wheat (Triticum aestivum L.) grown in the semi-arid region of the Canadian prairies. Under natural rainfall root/straw (nongrain aboveground material), ratios decreased with increasing rates of N, but under irrigation these ratios were generally constant. We estimated root/straw ratios for roots measured at anthesis (their maximum mass) and straw measured at maturity, under natural rainfall conditions, to be 0.36–0.58 if roots are assessed for the 0 - to 120-cm depth, 0.21–0.34 for the 0 - to 30-cm depth, and 0.15–0.26 for the 0- to 15-cm depth. Under natural rainfall, if roots are measured at maturity (as is commonly done), the corresponding ratios were 0.29–0.37, 0.15–0.21, and 0.10–0.15, for the respective depths. Under irrigation, the ratios when roots were measured at anthesis were 0.36, 0.24 and 0.19 for 0- to 120-cm, 0- to 30-cm and 0- to 15-cm rooting depths, respectively; but when roots were measured at maturity these ratios were 0.30, 0.17 and 0.13, respectively. We suggest that the values based on roots measured at anthesis provide a more accurate estimate of root C available for decomposition. We propose that the ratio used should be dependent on the depth to which changes in soil C or N are being measured. Key words: Soil organic C, roots, straw, fertilizer N, moisture

Beschreibung: Light fraction of soil organic C (LFOC) represents a major portion of labile soil organic C (SOC) and is a key attribute of soil quality. Soil respiration (Cmin) is an important index depicting the potential activity of the labile SOC. Six field experiments, varying in duration (8 to 25 yr), in location (Brown, Dark Brown and Black Chernozemic soil zones of Saskatchewan) and soil texture, were conducted to evaluate the impact of tillage and crop rotations on crop production and soil quality. We sampled the 0-7.5-cm depth of soil in these experiments to determine the treatment effects on LFOC, the proportion of LFOC in the SOC (LFOC/SOC) and Cmin. Increasing the frequency of summer fallow in cropping systems decreased the LFOC in all soil zones; it also decreased the proportion of LFOC in SOC and Cmin. Tillage had little impact on LFOC in the Brown and Dark Brown Chernozemic soil zones, although it significantly decreased LFOC in the Black Chernozemic soil zone. Thus, crop rotation had a greater impact on LFOC than tillage. Tillage did not influence Cmin in any soil zone. Because adoption of no-till management increased SOC in all soil zones, we concluded that LFOC was not a sensitive indicator of the impact of tillage on this soil quality attribute for these Chernozemic soils in Saskatchewan. We also found that LFOC/SOC is directly proportional to sand content. This relationship may assist us in partitioning SOC pools with differing turnover times when modeling SOC dynamics. Key words: Soil organic C, light fraction organic C, tillage, crop rotations, texture, mineralizable C

Beschreibung: Crop management practices can have a major influence on soil fertility and soil organic C (SOC) sequestration. We need to accurately measure and estimate changes in SOC in the short term (

Beschreibung: Planting of cultivated land with perennial forages may increase C sequestration in soil organic matter and contribute to atmospheric CO2 mitigation strategies. However, little is known of the effectiveness of introduced grasses in restoring organic C in cultivated soils of the Canadian prairies. Our objective was to evaluate the C sequestration potential of crested wheatgrass (CWG) (Agropyron cristatum L. Gaertn.), a widely introduced, early-season grass. In 1995 and 1996, we measured soil CO2 fluxes, C inputs in plant material and total soil C under CWG and a fallow-wheat (Triticum aestivum L.)-wheat rotation (F-W-W). These were two of the treatments in a replicated crop rotation experiment initiated in 1987 in southwestern Saskatchewan on a medium-textured soil that had previously been under long-term wheat production. Average to above-average growing season (1 May to 31 July) precipitation in 1995–1996 resulted in annual inputs of C in wheat residues of 3000–4500 kg ha−1. Growth of CWG, which was hayed and removed, was relatively poor in both years, but especially in 1995 when dry matter yield was only 1300 kg ha−1. For the 1988–1996 period, there was a strong correlation (R2 = 0.81; P 

Beschreibung: Changes in soil C storage due to management practices are important in relation to soil quality and to the broader issue of atmospheric C sequestration. Our objective was to evaluate the effects of soil fertility management on C fluxes under two spring wheat (Triticum aestivum L.) rotations in semiarid southwestern Saskatchewan, i.e., continuous wheat (Cont W) and a rotation that included summerfallow every third year (F-W-W). Continuous wheat was grown under two fertility regimes since initiation of the experiment in 1967, i.e., fertilization with N+P (no nutrient limitation) or with P only. In F-W-W there were three fertility treatments: N+ P, N only, and P only. We measured soil CO2 emissions under all fertility treatments and rotation phases during the 1995 and 1996 cropping seasons (emissions were measured at about weekly intervals between spring and freeze-up in autumn). Inputs of C in straw were measured and a root:straw ratio of 0.59 was used to estimate root C inputs. Alleviation of nutrient limitations generally had a positive effect on wheat growth (and thus on C inputs), particularly in 1995, the wetter of the 2 yr (precipitation 14% greater than average). For example, C inputs in 1995 under Cont W were estimated at 2700 kg ha-1 in the N+P treatment compared with 1500 kg ha-1 in the P only treatment. Fertility treatments had little effect on CO2 emissions; e.g., for Cont W the mean flux for the 1995 monitoring period was 2.7 mmol CO2 m-2s-1 where N + P was applied and 2.6 mmol CO2 m-2s-1 where P only was applied. Greater C inputs, but similar outputs of CO2-C for the N + P treatment vs. the systems receiving N or P only, suggest that proper fertilization resulted in a gain in soil C. However, quantifying the fertility-induced C gain is problematic because of uncertainty regarding effects of fertility on several components of the C budget, particularly root-C inputs and the contribution of rhizosphere respiration to the measured CO2 flux. Key words: Carbon sequestration, N and P fertilization, CO2 emissions, C inputs in crop residues, spring wheat, summerfallow