As a prerequisite for quantification of annual N2O emissions at a regional scale, this study was conducted to determine the landscape‐scale patterns and seasonal fluctuations of N2O emission and to demonstrate the linking relationships between large‐scale controllers and proximal factors of N2O emission. An area of the Black soil zone of central Saskatchewan, Canada, was stratified into three main textural areas: clay loam, fine sandy loam, and sandy. Within each textural area, representative sites were selected based on land use: unfertilized and fertilized cropland, fallow, pasture, and forest sites. A consistent landscape‐scale pattern of N2O emission was observed; footslope positions had higher N2O fluxes than shoulder positions. The role of topography is attributed to its strong influence on the hydrologic and pedologic processes in the landscape, which, in turn, regulate the soil factors controlling N2O emission at the microscale level. The seasonal fluctuation of N2O emission was influenced by precipitation. Pulses of activity were observed during summer, after rainfall events following N fertilizer application, and during spring thaw. At the regional scale, the sandy area had lower N2O emissions than the fine‐textured areas. The general order of N2O evolved among the land uses was forest < pasture < fallow ≈ unfertilized < fertilized cropland. Our results showed the importance of developing spatially based, predictive relationships between N2O emission and its controlling factors. Linking these relationships with large‐scale integrative variables, such as soil texture and land use, provides a means for extrapolating N2O fluxes from landscape to regional scale.

The nitrogen and non-nitrogen rotation benefits ofpea to succeeding crops. Can. J.plantSci.76:735-T45.Theinclusionofapulsecropinarotationoftenleadstogreaterseedyieldsinthesucceedingcereal crop. Two rotations were established at three sites in 1993 to examine the N and non-N rotation benefits of pea (Pisum satiwm L.) to the subsequent wheat (Triticum aestiwm L.) then oilseed crops. Wheat seed yield was 43oh greater (rotation benefit) when preceded by pearather than wheat, a consistent.oponr. among sites. Six to fourteen kg hrl of the extra 27 kghtt of N accumulated by wheat in the pea--wheat rotation was dirived from the additional N derived from pea residue' The additional soil N availability in the pea-wheat rotration, as indicated by the A-value, explained 8% ofthe rotation effect on seed yield (N benefit). The remaining 926/o of the yield advantage in the pea-wheat rotationwas attributed to non-N rotation benefit. The yield of the oilseed crop f6llowing the pea-wheat phise of the'rotation did not differ from that following the wheat-wheat phase. The influence of gro'wing conditions^and cropping history on the magnitude of the N to non-N rotation benefits, and the contribution of different non-N effects, should be investigated further.Key words: Rotation benefit, pea, wheat, residue N, non-N benefit The rotation benefit of a legume to the succeeding cereal kg ha-| . Typically, the C:N of pulse crop residues ranges crop is the yield advantage for the cereal relative to a cere-from 25:1 to 40: l, while the range of C:N for cereal residues al-cereal rotation. For example, seed yield of barley is 70:l to 100:1. The narrower C:N is believed to promote (Hordeumvulgarel.) (Evans et cereal crops (Bremer and Van Kessel 1992a; Jensen al. 1991). Wright (1990a) found that 100 kg ha-r of N fer-l994a,b). Therefore, the N contribution of pea to the suctilizer was required by barley following barley to produce a cgeding crop was much smaller than expected. Most of the yield comparable to unfertilized barley following pea.lsN that was not recovered by wheat was assimilated by the Evans et al. (1989) and Armstrong et al. (1994) showed microbes and consequently incorporated into a more passive that the N contribution of pea to the soil (N2 fxed in the fraction of the soil organic matter (Bremer and Van Kessel residue minus soil N accumulated in the seed) was about 20 1992b; Jensen 1994b

Lentil (Lens culinaris Medikus) is being grown increasingly on the Canadian prairies as a pulse or green manure crop, and may increase N availability to a succeeding crop. This study was designed to compare the effects of lentil green manure, lentil straw, and wheat (Triticum aestivum L.) straw on plant‐available N during the growing season after application. The fate of 15N from fall‐applied (1988) lentil green manure, lentil straw, and wheat straw and spring‐applied (1989) fertilizer (NH4)2 SO4 was determined four times during the 1989 growing season at a field site located at Outlook, Saskatchewan, Canada, on a Bradwell sandy loam (Typic Boroll). Denitrification and leaching losses of 15N from added lentil and wheat straw were negligible, but 24 and 30% of the 15N in lentil green manure and fertilizer, respectively, were lost in the 6‐wk period after planting (8 May 1989). By wheat harvest (8 Aug. 1989), 7% of the 15N in lentil and wheat straw and 37% of the 15N in lentil green manure were mineralized. Addition of green manure increased net mineralization of indigenous soil N at the time of planting by 0.4 g m−2, equivalent to 10% of added green manure N. Immobilization of soil and fertilizer N was similar for lentil and wheat straw. The smaller fraction of 15N assimilated from green manure (19%) than from fertilizer (34%) by wheat was due solely to less net mineralization of green‐manure N rather than net immobilization of fertilizer N. Of the 15N added in lentil and wheat straw, 5.5% was assimilated by wheat. Thus, lentil straw was not a significant source of N in this study, while ≈40% of the N in lentil green manure was potentially available for plant uptake.

ABSTRACIUsing a split-root technique, roots of soybean plants were divided between two pots. In one of the two pots, two maize plants were grown and half of those pots were inoculated with the vesicular arbuscular mycorrhizal (VAM) fungus, Glomus fasciculatas. Fifty-two days after planting, "5N-labeled ammonium sulfate was applied to the pots which contained only soybean roots. Forty-eight hours after application, significantly higher values for atom per cent 'IN excess were found in roots and leaves of VAM-infected maize plants as compared with the non-VAM-infected maize plants. Results indicated that VAM fungi did enhance N transfer from one plant to another.ined VAM mediated transfer ofN from a soybean to maize using highly labeled ('5NH4)2SO4 and a split-root technique. MATERIALS AND METHODSSurface sterilized (2 min in 3% NaOCI seeds of soybeans (Glycine max [L.] Merr) and maize (Zea mays L.) were germinated in vermiculite. The last 0.5 cm of the soybean tap roots were removed and the two seedlings were placed in two separate plastic elbows (pvc elbow, 21 mm o.d., 13 mm diameter hole) (16). At the same time, two pregerminated maize seedlings were planted in pot B (Fig 1).Half of the B pots were inoculated with a VAM fungi, Glomus fasciculatus, maintained in a course sand pot culture with each inoculated pot receiving 30 g ofinoculum which contained small Vesicular arbuscular mycorrhizal fungi are ubiquitous and infect plant roots of most species under a wide variety of soil conditions (8). The fungi form a symbiosis with host plants in which the plant provides carbon for VAM2 growth and in turn the VAM fungi provide plant nutrients, especially phosphorus, from the soil solution (1 1). Growth responses of host plants to infection by VAM may be dramatic in nutrient-poor environments (7). Hyphae of mycorrhizae may also spregd from one infected plant and enter the roots of one or more other plants (9). It has been shown that assimilates may be transported from one plant to another through VAM hyphal connections. Transfer of '4C photosynthate from one plant to another was primarily through VAM hyphae rather than leakage from the roots of the donor plants (2,6,14). Similar results were obtained in a 3p experiment where hyphal linkage between plants was the dominant factor for transferring P (3, 17).Leguminous plants infected with both Rhizobium and VAM showed an increase in nodulation and N2-fixation as compared with VAM-uninfected legumes (4, 5). The increase in total N has been explained mainly by an increase in N2-fixation as a result of a higher P uptake through the VAM hyphae rather than increased soil N uptake (15). Although the role of VAM on N uptake and transport has been studied, the results are inconclusive. Rhodes and Gerdemann (15)

Organic resources (ORs) are important nutrient inputs in tropical agriculture. Combined with mineral fertilizers, they form the backbone of integrated soil fertility management. This study was conducted to determine the medium‐ to long‐term influence of OR quality and quantity on maize productivity and to evaluate the occurrence of additive benefits in terms of extra grain yield produced by the combined application of ORs and N fertilizers. Farmyard manure, high quality Mexican sunflower [Tithonia diversifolia (Hemsl.) A. Gray], intermediate quality calliandra (Calliandra calothyrsus Meisn.) and maize (Zea mays L.), and low quality silky‐oak (Grevillea robusta A. Cunn. ex R. Br.) sawdust were incorporated into the soil at equivalent rates of 1.2 and 4 Mg C ha−1 yr−1 in Embu (clayey) and Machanga (sandy soil), together with a control to which no OR was added. All plots were split, with one half receiving 120 kg N ha−1 season−1 as CaNH4NO3. The ORs, except sawdust and maize, improved maize grain yields compared with the control at both sites. Greatest mean maize yields (i.e., 4.9 and 2.3 Mg ha−1 season−1, in Embu and Machanga, respectively) over 10 seasons were observed with the high rate of Mexican sunflower, but was not significantly different from calliandra and manure. Generally, maize yields were greater with higher than lower OR rates, except for maize and sawdust. Although N fertilizer additions to the ORs improved grain yields in Embu, the increase was marginal; resulting in negative interactive effects of applying ORs with N fertilizers, especially with high‐N ORs. Thus high‐N ORs should not be applied in combination with N fertilizers, especially at such high fertilizer N rates.