The impact of long-term elevated CO2 on C and N retention in stable SOM pools

Elevated atmospheric CO2 frequently increases plant production and concomitant soil C inputs, which may cause additional soil C sequestration. However, whether the increase in plant production and additional soil C sequestration under elevated CO2 can be sustained in the long-term is unclear. One approach to study C-N interactions under elevated CO2 is provided by a theoretical framework that centers on the concept of progressive nitrogen limitation (PNL). The PNL concept hinges on the idea that N becomes less available with time under elevated CO2. One possible mechanism underlying this reduction in N availability is that N is retained in long-lived soil organic matter (SOM), thereby limiting plant production and the potential for soil C sequestration. The long-term nature of the PNL concept necessitates the testing of mechanisms in field experiments exposed to elevated CO2 over long periods of time. The impact of elevated CO2 and N-15 fertilization on L. perenne and T. repens monocultures has been studied in the Swiss FACE experiment for ten consecutive years. We applied a biological fractionation technique using long-term incubations with repetitive leaching to determine how elevated CO2 affects the accumulation of N and C into more stable SOM pools. Elevated CO2 significantly stimulated retention of fertilizer-N in the stable pools of the soils covered with L. perenne receiving low and high N fertilization rates by 18 and 22%, respectively, and by 45% in the soils covered by T. repens receiving the low N fertilization rate. However, elevated CO2 did not significantly increase stable soil C formation. The increase in N retention under elevated CO2 provides direct evidence that elevated CO2 increases stable N formation as proposed by the PNL concept. In the Swiss FACE experiment, however, plant production increased under elevated CO2, indicating that the additional N supply through fertilization prohibited PNL for plant production at this site. Therefore, it remains unresolved why elevated CO2 did not increase labile and stable C accumulation in these systems

Improved inoculants for lentil

Non-Peer ReviewedThe effectiveness of the available commercial inoculants for lentil, Rhizogen, Enftx-L (Esso), Nitragin 'C', and Grip (Inotec) were compared with three inoculants which contained the rhizobia strains of 99A1, ICAR 20, and 92A3. Lentil inoculated with a sterile inoculant was used as the control. Total dry matter, total grain yield, percent total N and total N was determined At seven sites, an average grain yield for lentil was observed whereas for two sites yield was below average. Inoculant containing 99A1 and ICAR 20 an overall increase in grain yield by 14 % as compared with Nitragin 'C', strain 99A3 with 9 %, and Enftx-L with 7 %. The average grain yield of all nine sites for lentil inoculated with Grip and Rhizogen showed an insignificant decrease of 2 and 3 % as compared with lentil inoculated with Nitragin 'C'

Improved inoculants for lentil

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Landscape-scale variability of N2 fixation by pea

Non-Peer ReviewedThe landscape-scale variability of N2 fixation by pulse crops is an important part in the intensity of N cycling in a hummocky terrain. A 100-gridpoint landscape-scale research design was established at a site in the thick Black Soil Zone in the spring of 1993. At the time of seeding, grid points classified as footslope landform complexes had 6.4% more water and 21.7 kg ha-1 more available N than those classified as shoulder. Pea seed yield ranged from 400 to 3750 kg ha-1 and straw yield ranged from 1900 to 12500 kg ha-1. Median seed yield on footslopes was 35% lower than that on shoulders, while median straw yield was 18% greater on footslopes in comparison to shoulders. Total N2 fixed in pea straw and seed ranged from 0 to 239 kg N ha-1. Median N2 fixed on shoulders was 116 kg N ha-1 and 91 kg N ha-1 on footslopes, despite the fact that total N did not show a spatial pattern. Spatial variability of available N, controlled by the redistribution of water, was considered to be the major process controlling the landscape-scale variability of N2 fixation

Nitrogen fertilizer use and losses in irrigated cropping systems

Non-Peer ReviewedThe fertilizer-N use efficiency of irrigated canola, softwheat, durum, and spring wheat was determined in 1988 and 1989. Crops received various amounts of N, applied at time of seeding or split between time of seeding and during the growing season. In 1989 at two sites, Outlook and Birsay both located on a field owned by farmers, N-losses due to denitrification after irrigation were determined by means of acetylene blockage. Softwheat and durum were grown at Outlook and Birsay, respectively. In 1988, all treatments received 150 kg N/ha, split equally between time of seeding and 54 days after planting. In 1989, all crops received 200 kg N/ha, applied at time of seeding or split equally between time of seeding and during the growing season. Fertilizer use efficiency (% FUE) in the crop of irrigated canola in 1988 averaged 37 % in the soil at time of final harvest. In 1989 the average FUE in canola was 27.4 %, in softwheat 42.3 % and in durum 43.0 %. The % FUE was 22 and 21 % at Birsay and Outlook, respectively. Nitrogen losses caused by denitrification were small at one site but significant at another. At the site with low input of N and irrigation water the N-losses were practically zero before the onset of irrigation, increased to a maximum loss of 50 g N/day/ha at 4 h after irrigation and declined to low levels at 10 h after irrigation. At the other site, with high N and irrigation water, the N losses from fertilized soil due to denitrification were approximately 5 kg N/day/ha, increased to 12.5 kg N/day/ha at 4 h after the application of water and declined to approximately 5 kg N/day/ha at 10 h after the water was applied

Nitrogen management under irrigated conditions

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Intercropping legumes and non-legumes in Saskatchewan

Non-Peer Reviewe

Availability of N to plants from legume and fertilizer sources: which is greater?

Non-Peer ReviewedOne benefit often cited for legumes crops is that they contribute N to subsequent crops, but the magnitude of this effect has been difficult to quantify. A study was conducted to compare how much and when N from fertilizer and residue sources was taken up by wheat Wheat straw (W), lentil straw (L), and lentil green manure (G), unenriched or enriched in 15N, were both surface placed and incorporated into microplots (10 x 40 cm) in the field in the fall of 1988. In the spring of 1989 wheat was planted in all microplots, and unenriched and enriched fertilizer N was added to microplots containing enriched and unenriched plant residues, respectively. Microplots were destructively sampled at planting and at 6, 10 and 13 weeks after planting. Approximately 29 % of added fertilizer was recovered in wheat tops by 6 weeks after planting in all treatments except incorporated W, where immobilization reduced this value to 19 %. Maximum recoveries of fertilizer 15N were 34 % by the final sampling date. The proportion of residue 15N recovered in wheat tops at the final sampling dates was 19 and 11 % from incorporated and surface-placed G, respectively, and 5.4 and 5.3 % from L and W, respectively. Surface placement of residues reduced immobilization of fertilizer N but increased losses of residue N. Comparisons of N availability based on recovery of 15N may be misleading because 15N recovery does not account for changes in mineralization of native N, which is likely to be affected unequally by the addition of different N sources

Landscape-scale variability of nitrogen fixation by pea and the availability of its residue-N for the succeeding crop

Non-Peer ReviewedThe main objectives of this study were to estimate the variability in N2-fixation across a large field and to determine whether the availability of legume residue-N to the succeeding crop was controlled by position of incorporation. In 1991, a field was selected near Blaine Lake, Saskatchewan and a 130 x 130 m grid composed of 169 sample sites was laid out. Six landform elements (upper and lower level, divergent and convergent shoulders and footslopes) were identified and the variability of N2-fixation (15N-isotope dilution) by pea was determined for 60 sites (six landforms x 10 replicates). At each site, the 15N-labeled pea residue was incorporated in a nearby unlabeled area in the spring of 1992. The percent N derived from N2-fixation (% Ndfa) by pea had a median of 57%. A difference in% Ndfa between landforms was observed with the highest% Ndfa at the divergent footslopes (69) and the lowest on the convergent shoulders (28). The total N2 fixed (seed+residue) did not show a landform effect and had a median of 57 kg N ha-1. The total N in pea residue (21-30 kg N ha-1) translated into C:N ratios ranging from 37-56. In 1992, landform differences for grain yield of spring wheat were present. Grain yield ranged from 1160 kg ha-1 on convergent footslopes to 1880 kg ha-1 on divergent shoulders. Due to the early frost, the median harvest index was low (0.24). The% Ndfr (N derived from residue) and% RUE (residue use efficiency) in the wheat grain and residue suggested that almost none of the pea residue-N had become available for wheat. The main reasons for the low N availability of the residue were: 1) incorporation of the pea residue at time of seeding (potential net N-mineralization of the residue in the fall and spring was excluded), and 2) the below average temperatures and precipitation in 1992 which would have reduced soil microbial activity and therefore net N -mineralization

Evaluation of the effectiveness of Rhizobium leguminosarum strains for pea under field conditions

Non-Peer ReviewedOne hundred and eight isolates of Rhizobium leguminosarum were screened for effectiveness for pea under controlled environments. Eight superior strains plus the commercially available pea inoculants, Nitragin 'C', Grip-Inotec, and Rhizogen were tested at Waldheim and Brooksby for effectiveness with Tipu and Trapper pea as host plants. The experiment was a RCBD, laid out as a split plot with the two pea varieties as main plot treatments and rhizobia! strain as the subplot treatments, replicated four times. At Waldheim total dry matter ranged from 2429 kg/ha for uninoculated Trapper to 4024 kg/ha for Tipu inoculated with strain 128C79. Grain yield ranged from 1046 kg/ha for Trapper inoculated with strain 175G3 to 1665 kg/ha for Tipu inoculated with strain 175G3. Total grain-N ranged from 33.7 kg/ha for uninoculated Tipu to 55.5 kg/ha for. Trapper inoculated with strain 128C56G. At this site, strain 175G3 appears to be a superior strain for Tipu but is the least effective strain tested for Trapper. At Brooks by, no significant differences due to strain or cultivar were observed. Values for total dry matter, grain yield, and total N were around 75 % of those values found at Waldheim. A survey to assess nodulation was carried out and all uninoculated pea were nodulated. This would indicate the presence of indigenous R. leguminosarum nodulating pea or possible cross contamination from adjacent plots. The below average yield and the absence of a yield response due to inoculation can largely be attributed to the extreme dry weather occurring at both sites. Nodulation of the uninoculated control has also reduced the effect of inoculation on yield