Legumes as dry season fallow in upland rice-based systems of West Africa

Declining fallow length in traditional upland rice-based cropping systems in West Africa results in a significant yield reduction due mainly to increased weed pressure and declining soil fertility. Promising cropping system alternatives include the use of weed-suppressing legumes as short duration fallows. N accumulation, N derived from the atmosphere (Ndfa), weed suppression, and the effects on rice yield were evaluated in 50 legumes, grown at four sites in Côte d'Ivoire with contrasting climate, soils, and rice production systems. The sites were located in the derived and the Guinea savanna and in the bimodal and the monomodal rainfall forest zones. Legume and weed biomass during the fallow were determined at bimonthly intervals. Percent Ndfa by biological N fixation was determined by ¹⁵N natural abundance. Fallow vegetation was cleared and rice seeded according to the practice of local farmers and the cropping calendar. Weed biomass and species composition were monitored at monthly intervals. Legume fallows appear to offer the potential to sustain rice yields under intensified cropping. Biomass was in most instances significantly greater in the legume fallow than in the "weedy" fallow control, and several legume species suppressed weed growth. N accumulation by legumes varied between 1–270?kg N ha^–1 with 30–90%?Ndfa. Across sites, Mucuna spp., Canavalia spp., and Stylosanthes guianensis showed consistently high N accumulation. Grain yields of rice which had been preceded by a legume fallow were on average 0.2?Mg ha^–1 or about 30% greater than that preceded by a natural weedy fallow control. At the savanna sites where fallow vegetation was incorporated, Mucuna spp. and Canavalia ensiformis significantly increased rice yield. In the bimodal forest zone, the highest rice yield and lowest weed biomass were obtained with Crotalaria anagyroides. In general, the effects of legume fallows on rice yield were most significant in environments with favourable soil and hydrological conditions.     

Soil organic matter assessment in natural regrowth,Pueraria phaseoloides and Mucuna pruriens fallow

Biological and chemical components of soil fertility were quantified under three different fallow types and related to soil quality of an Ultisol in southern Cameroon at the end of a 9-month fallow. Soil organic matter (SOM), soil exchangeable Ca²⁺, Mg²⁺ and K⁺ and available P concentrations, effective cation exchange capacity (ECEC) and, soil acidity in the 0–10 and 10–20 cm layers were evaluated under: natural regrowth mainly composed of Chromolaena odorata and the legume cover crops velvet bean (Mucuna pruriens var. utilis) and kudzu (Pueraria phaseoloides). SOM quality was assessed by C mineralisation during a 4-week incubation at 28°C in the laboratory. In addition, particulate organic matter (POM), the most active part of SOM, was fractionated by wet sieving into coarse (4000–2000 μm), medium (2000–250 μm) and fine (250–53 μm) particle size classes and analysed for C and N contents. Under Mucuna, Ca²⁺, K⁺ and P concentrations, ECEC and soil pH were higher and C mineralisation was lower than under natural regrowth and Pueraria in 0–10 cm depth. Soil under natural regrowth had a higher C mineralisation in 0–10 cm indicating more labile SOM than in Pueraria and Mucuna fallow. There was no difference in weight of total POM, for any of the fractions between the three fallow types. However, both leguminous fallow species increased POM quality through a higher N content. Compared to natural regrowth, Pueraria increased N content in coarse POM by 36% in the 0–10 cm layer and by 19% (coarse POM) and 35% (medium POM) in the 10–20 cm layer. Mucuna increased N content in the 0–10 cm layer by 12% (coarse POM), and by 19% (fine POM), compared to natural regrowth. According to the differences in nutrient concentrations, soil acidity and the biological stability of SOM, the three fallow types ranked: Mucuna≥Pueraria>natural regrowth. However, in terms of POM quality the ranking was: Pueraria>Mucuna>natural regrowth.     

Management of a previously eroded tropical Alfisol with herbaceous legumes: Soil loss and physical properties under mound tillage

A study was carried out on a previously eroded Oxic Paleustalf in Ibadan, southwestern Nigeria to determine the extent of soil degradation under mound tillage with some herbaceous legumes and residue management methods. A series of factorial experiments was carried out on 12 existing runoff plots. The study commenced in 1996 after a 5-year natural fallow. Mound tillage was introduced in 1997 till 1999. The legumes – Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides – were intercropped with maize in 1996 and 1998 while yam was planted alone in 1997 and 1999. This paper covers 1997–1999. At the end of each year, residues were either burned or mulched on respective plots. Soil loss, runoff, variations in mound height, bulk density, soil water retention and sorptivity were measured. Cumulative runoff was similar among interactions of legume and residue management in 1997 (57–151 mm) and 1999 (206–397 mm). However, in 1998, cumulative runoff of 95 mm observed for Mucuna-burned residue was significantly greater than the 46 mm observed for cowpea-burned residue and the 39–51 mm observed for mulched residues of cowpea, Mucuna and Pueraria. Cumulative soil loss of 7.6 Mg ha⁻¹ observed for Mucuna-burned residue in 1997 was significantly greater than those for Pueraria-mulched (0.9 Mg ha⁻¹) and Mucuna-mulched (1.4 Mg ha⁻¹) residues whereas in 1999 it was similar to soil loss from cowpea treatments and Pueraria-burned residue (2.3–5.3 Mg ha⁻¹). There were no significant differences in soil loss in 1998 (1–3.2 Mg ha⁻¹) whereas Mucuna-burned residue had a greater soil loss (28.6 Mg ha⁻¹) than mulched cowpea (6.9 Mg ha⁻¹) and Pueraria (5.4 Mg ha⁻¹). Mound heights (23 cm average) decreased non-linearly with cumulative rainfall. A cumulative rainfall of 500 mm removed 0.3–2.3 cm of soil from mounds in 1997, 3.5–6.9 cm in 1998 and 2.3–4.6 cm in 1999, indicating that (detached but less transported) soil from mounds was far higher than observed soil loss in each year. Soil water retention was improved at potentials ranging from −1 to −1500 kPa by Mucuna-mulched residue compared to the various burned-residue treatments. Also, mound sorptivity at −1 cm water head (14.3 cm h^−1/2) was higher than furrow sorptivity (8.5 cm h^−1/2), indicating differences in hydraulic characteristics between mound and furrow. Pueraria-mulched residues for mounds had the highest sorptivity of 17.24 cm h^−1/2, whereas the least value of 6.96 cm h^−1/2 was observed in furrow of Mucuna-burned residue. Pueraria phas eoloides was considered the best option for soil conservation on the previously eroded soil, cultivated with mound tillage.     

Effect of phosphorus management in rice–mungbean rotations on sandy soils of Cambodia

Nitrogen (N) and phosphorus (P) deficiencies are key constraints in rainfed lowland rice (Oryza sativa L.) production systems of Cambodia. Only small amounts of mineral N and P or of organic amendment are annually applied to a single crop of rainfed lowland rice by smallholder farmers. The integration of leguminous crops in the pre‐rice cropping niche can contribute to diversify the production, supply of C and N, and contribute to soil fertility improvement for the subsequent crop of rice. However, the performance of leguminous crops is restricted even more than that of rice by low available soil P. An alternative strategy involves the application of mineral P that is destined to the rice crop already to the legume. This P supply is likely to stimulate legume growth and biological N₂ fixation, thus enhancing C and N inputs and recycling N and P upon legume residue incorporation. Rotation experiments were conducted in farmers' fields in 2013–2014 to assess the effects of P management on biomass accumulation and N₂ fixation (δ¹⁵N) by mungbean (Vigna radiata L.) and possible carry‐over effects on rice in two contrasting representative soils (highly infertile and moderately fertile sandy Fluvisol). In the traditional system (no legume), unamended lowland rice (no N, + 10 kg P ha^?1) yielded 2.8 and 4.0 t ha^?1, which increased to 3.5 and 4.7 t ha^?1 with the application of 25 kg ha^?1 of urea‐N in the infertile and the moderately fertile soil, respectively. The integration of mungbean as a green manure contributed up to 9 kg of biologically fixed N (17% Nfda), increasing rice yields only moderately to 3.5–4.6 t ha^?1. However, applying P to mungbean stimulated legume growth and enhanced the BNF contribution up to 21 kg N ha^?1 (36% Nfda). Rice yields resulting from legume residue incorporation (“green manure use”–all residues returned and “grain legume use”–only stover returned) increased to 4.2 and 4.9 t ha^?1 in the infertile and moderately fertile soil, respectively. The “forage legume use” (all above‐ground residues removed) provided no yield effect. In general, legume residue incorporation was more beneficial in the infertile than in the moderately fertile soil. We conclude that the inclusion of mungbean into the prevailing low‐input rainfed production systems of Cambodia can increase rice yield, provided that small amounts of P are applied to the legume. Differences in the attributes of the two major soil types in the region require a site‐specific targeting of the suggested legume and P management strategies, with largest benefits likely to accrue on infertile soils.     

Effects of leguminous plant residues and NPK fertilizer application on the performance of yam (Dioscorea rotundata ‘c.v.’ ewuru) in south-western Nigeria

The effects of cultivating and incorporating residues of previous tropical kudzu (Pueraria phaseoloides) and soybean (Glycine max) with application of NPK fertilizer on yam performance were evaluated at the teaching and research farm, LAUTECH, Nigeria. There were nine treatments: incorporation of legume residues (5 t DM ha^?1), application of recommended fertilizer rate for yam (90–50–75 kg NPK ha^?1) in the zone or 50% of recommended rate (45–25–37.5 kg NPK ha^?1), alone and in combination with residues and a control without residues or fertilizer in a randomized complete block design. Cultivation of previous legumes reduced soil nematode population (>200%) compared with no legumes. For both years, application of Pueraria residues improved tuber yield by an average of 15.8% compared with control. Fertilizer application enhanced arbuscular mycorrhizal (AM) colonization of yam roots but AM colonization was lower (～50%) in plots where Pueraria residues were incorporated compared with other plots. Combined application of plant residues with fertilizer improved soil organic carbon, total N, exchangeable Ca and Mg compared with application of NPK fertilizer. From these results, it is concluded that half of the recommended NPK rate may be adequate and incorporation of residues with reduced NPK fertilizer application may be a sustainable soil fertility management option for continuous yam production.     

Soil pH changes during legume growth and application of plant material

During cultivation of legumes soil is acidified due to proton release from roots. As a consequence of proton release, plants accumulate organic anions which may, if returned and decomposed in the soil, neutralize the soil acids. Until now the detailed processes responsible for the change in soil pH after incorporation of plant material have not been completely understood. Using a pot experiment we studied the changes in acid and base in soil during growth of field beans (Vicia faba L. cv. Alfred) and after incorporation of the plant material into the soil. Soil pH was significantly decreased by field beans from 6.00 to 5.64 in a cultivation period of 45 days. Proton release amounted to 32.7 mmol H⁺ pot^-1, which was approximately equivalent to the accumulated alkalinity in the plant shoots (34.4 mmol). Return of field bean shoots caused a significant soil pH increase from 5.64 to 6.29. Within 7 days more than 90% of the added alkalinity was released. After 307 days incubation, soil pH decreased to 5.86 due to nitrification. In a second experiment, maize leaves (Zea mays L.), containing various concentrations of nitrogen and at various alkalinities, were incorporated into the soil. Soil pH change was positively correlated to alkalinity and malate concentration and negatively correlated to total nitrogen and water-soluble organic nitrogen of incorporated leaves. It is concluded that the soil acidification caused by legume cultivation can be partly compensated for if crop residues are returned to the soil. Addition of plant material may initially cause an increase in soil pH due to decomposition of organic anions and organic nitrogen. Soil pH may decrease if nitrification is involved. The concentrations of nitrogen and alkalinity of added plant material are decisive factors controlling soil pH change after incorporation of plant material.Dedicted to Professor J.C.G. Ottow on the occasion of his 60th birthday     

The effect of suppression treatments on the uptake of 15N by intercropped corn from labeled alfalfa (Medicago sativa)

In greenhouse experiments, we examined the N transferred to intercropped corn from ¹⁵N-labeled alfalfa shoot residue and intact roots in an undisturbed soil system in response to two different suppression treatments and complete killing of alfalfa. The alfalfa treatments included complete killing (glyphosate only), glyphosate injury + cutting, and cutting only, with alfalfa shoot residue returned to the soil surface in all three treatments. Corn was planted in each pot following application of the treatments. When alfalfa was suppressed by glyphosate injury + cutting, corn had recovered 12% of the alfalfa N by 8 weeks of growth, but with cutting only, N recovery by corn was reduced to 4.0%. The completekill treatment resulted in 8% recovery by corn of alfalfa N. In all treatments, most of the alfalfa-N remained in the soil organic pool. A second experiment tested a cutting only treatment with ¹⁵N-labeled alfalfa residue returned to the soil surface. The ¹⁵N-labeled alfalfa residue contributed 4.1% of N to corn during the 8-week growth cycle. Twice as much ¹⁵N was found in the active microbial biomass pool in the two treatments with live intereropped plants compared to the monoculture treatments with complete killing (non-intercropped) and the control treatment of alfalfa regrowth only. An analysis of the change in the ¹⁵N content of the undisturbed alfalfa roots from just before the suppression until 8 weeks later suggested that approximately 80% of the root ¹⁵N was lost from the plant suppressed by cutting. This corresponds to 28% of the total N released from the alfalfa. The results suggest that the degree of legume suppression was a key factor in the availability of legume N to the second crop. When the two species were intercropped, more of the N available from legume residues went to plant uptake and microbial biomass and was not stabilized as quickly in the soil organic pool. Appropriate management schemes must be designed to increase N availability to the second crop without yield reduction. These studies suggest severe suppression is necessary; if successful, more of the N can be maintained in active pools.     

Changes in soil N status and N supply rates in agricultural land afforested with eucalypts in south-western Australia

In south-western Australia, plantations of Eucalyptus globulus are being established on land that has previously been used for conventional agriculture. Sustaining the productivity of these tree plantations in second and subsequent rotations will depend partly on maintenance of soil fertility, especially soil nitrogen (N) supply rates. We compared soil N status and supply rates between adjacent pasture and 6-11 year old first-rotation eucalypt plantations at 31 paired sites in south-western Australia. Total soil N varied widely among sites (0.07-0.68% in the fraction <2 mm of the 0-10 cm soil layer), but concentrations averaged over all sites did not differ between land-use types. However, measurements of the indices of mineralization (mineral N produced during incubation of intact cores), potentially available N (from short-term anaerobic incubation) and model-predicted mineralization rates during 28-day aerobic incubations were generally lower in afforested soils than in pasture soils. This finding was supported by in situ field estimates of N mineralization over a full year at two contrasting paired pasture-plantation sites. At each site there was a marked reduction (2-3-fold) in net annual mineral N flux rates in soils under eucalypt plantations. Reduced N mineralization associated with tree plantations was due to both changes in soil organic matter quality and the generally lower soil moisture content under trees in comparison with pasture. These results suggest that N supply rates of pasture soils are likely to decline when the land is planted to successive crops of eucalypts. Eucalypt plantation managers will need to take account of this and implement management strategies to maintain adequate N nutrition to sustain tree growth in future rotations.     

Nitrogen availability and leaching during the terrestrial phase in a várzea forest of the Central Amazon floodplain

Availability and leaching of dissolved inorganic N (DIN = NH₄⁺ + NO₃^-) in soil were measured in a periodically flooded forest of the Central Amazon floodplain (várzea) during one terrestrial phase. Special emphasis was on the effects of a legume and a non-legume tree species. NH₄⁺-N accounted for more than 85% of DIN even at the end of the terrestrial phase although it decreased throughout the experimental period. While extractable NO₃^–-N was always low in the soil (less than 15% of DIN), the amount of leached NO₃^–-N was in the same range as NH₄⁺-N. Under the legume trees mean DIN contents of the topsoil were higher than under the non-legume trees. DIN leaching from the topsoil (0–20 cm) was significantly higher under the legume trees than at the other sites, also indicating a higher N availability. Therefore, despite considerable leaching legume trees may be an important source of N supporting a high biomass production of the várzea forest.     

Canopy Reduction and Legume Interseeding in Irrigated Continuous Corn

Abstract

Many alternative management systems have been evaluated for corn (Zea mays L.), soybeans (Glycine max L.), and wheat (Triticum aestivum L.) production, however, most have involved rotations from one year to the next. Legume interseeding systems which employ canopy reduction in corn have not been thoroughly evaluated. One such study was initiated in 1994 at the Panhandle Research Station near Goodwell, OK, on a Richfield clay loam soil, to evaluate five legume species interseeded into established corn: yellow sweet clover (Melilotus officinalis L.), subterranean clover (Trifolium subterraneum L.), alfalfa (Medicago sativa L.), arrowleaf clover (Trifolium vesiculosum L.), and crimson clover (Trifolium incarnatum L.). In addition, the effect of removing the corn canopy above the ear (canopy reduction) at physiological maturity was evaluated. Canopy reduction increased light interception beneath the corn thus enhancing legume growth in late summer, early fall, and early spring the following year prior to planting. Forage growth from legumes incorporated prior to planting were expected to lower the amount of inorganic nitrogen (N) fertilizer needed for corn production. Crimson clover appeared to be more shade tolerant than the other species, and interseeding this species resulted in the highest corn grain yields when no N was applied. In the last two years, interseeding crimson clover at physiological maturity, followed by canopy reduction resulted in a 1.32 Mg ha^?1 increase in yield compared to conventionally grown corn with no N applied. In 1999, interseeded legumes (except subterranean clover) in conjunction with the application of 56 kg N ha^?1 and crimson clover interseeded without the addition of fertilizer N (with and without canopy reduction) resulted in grain N uptake levels equal to the 112 kg N ha^?1 treatment.     

Uptake of soil mineral nitrogen by Acacia mangium and Eucalyptus urophylla × grandis: No difference in N form preference

The introduction of N₂‐fixing tree species in fast growing tree plantations is a sustainable management option aiming to reduce the risk of nitrogen (N) deficiency due to a large and frequent exportation of nutrients at harvest. Differences in soil mineral N preferences between Eucalyptus urophylla × grandis and Acacia mangium may, in addition to facilitation related to atmospheric N₂ fixation, contribute to the success of mixed‐species plantations of the two species on nutrient‐poor soils of the coastal Congolese plains. We tested whether these two species differ in their preference for nitrate or ammonium by supplying either ammonium or nitrate enriched in ¹⁵N to six‐month‐old potted trees growing in an open‐air nursery. Although the uptake of nitrate tended to be higher than that of ammonium by both species, the difference was not significant and there was no significant difference between the species regarding their preferred form of soil mineral N. Despite much lower N contents in foliage, stems, and roots of eucalypt compared to acacia, the specific rates of N uptake were up to three times higher for eucalypt than acacia, which suggests that atmospheric N₂ was the major source of N in the six‐month‐old acacias. We conclude that N₂ fixation rather than complementarity for soil mineral nitrogen alleviates the competition between species in successful mixed eucalypt and acacia plantations.     

Carbon and nitrogen mineralization dynamics in different soils of the tropics amended with legume residues and contrasting soil moisture contents

Seasonal drought in tropical agroecosystems may affect C and N mineralization of organic residues. To understand this effect, C and N mineralization dynamics in three tropical soils (Af, An₁, and An₂) amended with haricot bean (HB; Phaseolus vulgaris L.) and pigeon pea (PP; Cajanus cajan L.) residues (each at 5 mg g⁻¹ dry soil) at two contrasting soil moisture contents (pF2.5 and pF3.9) were investigated under laboratory incubation for 100–135 days. The legume residues markedly enhanced the net cumulative CO₂–C flux and its rate throughout the incubation period. The cumulative CO₂–C fluxes and their rates were lower at pF3.9 than at pF2.5 with control soils and also relatively lower with HB-treated than PP-treated soil samples. After 100 days of incubation, 32–42% of the amended C of residues was recovered as CO₂–C. In one of the three soils (An₁), the results revealed that the decomposition of the recalcitrant fraction was more inhibited by drought stress than easily degradable fraction, suggesting further studies of moisture stress and litter quality interactions. Significantly (p < 0.05) greater NH₄⁺–N and NO₃⁻–N were produced with PP-treated (C/N ratio, 20.4) than HB-treated (C/N ratio, 40.6) soil samples. Greater net N mineralization or lower immobilization was displayed at pF2.5 than at pF3.9 with all soil samples. Strikingly, N was immobilized equivocally in both NH₄⁺–N and NO₃⁻–N forms, challenging the paradigm that ammonium is the preferred N source for microorganisms. The results strongly exhibited altered C/N stoichiometry due to drought stress substantially affecting the active microbial functional groups, fungi being dominant over bacteria. Interestingly, the results showed that legume residues can be potential fertilizer sources for nutrient-depleted tropical soils. In addition, application of plant residue can help to counter the N loss caused by leaching. It can also synchronize crop N uptake and N release from soil by utilizing microbes as an ephemeral nutrient pool during the early crop growth period.     

Grain legume effects on soil nitrogen mineralization potential and wheat productivity in a Mediterranean environment

The objective of this work was to provide evidence on the effects of faba bean (Vicia faba L.) and chickpea (Cicer arietinum L.) on the dynamics of soil N availability and yield parameters of wheat (Triticum turgidum L. var. durum) in a legume–wheat rotation in comparison with the effects of the more extensively studied common vetch (Vicia sativa L.). Soil samples were taken from field plots just before wheat sowing and incubated in the laboratory to assess N mineralization potential, soil respiration and N immobilization after incorporation of legume residues. Soil after vetch cultivation showed the highest residual N and mineralization potential (120 mg N kg^?1 soil), the greatest CO₂ release and the smallest N immobilization. Smaller mineral N release (80 mg N kg^?1 soil) was shown by soil after faba bean cultivation, which, however, would be capable to support an average wheat production without fertilization. Soil after chickpea and wheat cultivation manifested no differences in residual N and mineralization or immobilization potential. Laboratory results were well correlated with grain yield and N uptake during the second season of rotation in the field. All legumes resulted in significant yield surpluses and provided N credit to the following unfertilized wheat.     

Nodulation and growth of black locust (Robinia pseudoacacia) on a desurfaced soil inoculated with a local Rhizobium isolate

Nodulation and nitrogen fixation of black locust (Robinia pseudoacacia L.), a legume tree broadly used in Argentina for urban and agricultural afforestation, was studied in hydroponic culture. The development of seedlings inoculated with a local strain of Rhizobium, highly specific for R. pseudoacacia, was also compared with respect to non-inoculated but N-fertilised seedlings. This strain produced fast nodulation and high crop yield and leaf N content. Already nodulated plants with the local Rhizobium strain were assayed for growth in a greenhouse pot experiment with soil from a field where topsoil has been removed for industrial purposes, whilst pots with non-desurfaced soil from the same field were used as control. Non-inoculated plants were also grown in either control or desurfaced soil. Inoculated plants developed better than non-inoculated plants in desurfaced soil, and in control soil as well, suggesting that the symbiosis was able to overcome the nutrient limitation of the desurfaced soil. Non-inoculated plants were nodulated by native soil born Rhizobium, either in control or desurfaced soil, but they showed low final nitrogen leaf content and low nitrogen fixation activity, suggesting that native rhizobia were ineffective.     

Nitrogen capture by grapevine roots and arbuscular mycorrhizal fungi from legume cover-crop residues under low rates of mineral fertilization

The influence of mineral fertilization on root uptake and arbuscular mycorrhizal fungi-mediated ¹⁵N capture from labeled legume (Medicago polymorpha) residue was examined in winegrapes (Vitis vinifera) in the greenhouse, to evaluate compatibility of fertilization with incorporation of cover-crop residue in winegrape production. Plants grown in marginal vineyard soil were either fertilized with 0.25× Hoagland’s solution or not. This low fertilization rate represents the deficit management approach typical of winegrape production. Access to residue in a separate compartment was controlled to allow mycorrhizal roots (roots + hyphae), hyphae (hyphae-intact), or neither (hyphae-rotated) to proliferate in the residue by means of mesh core treatments. Leaves were weekly analyzed for ¹⁵N. On day 42, plants were analyzed for ¹⁵N and biomass; roots were examined for intraradical colonization; and soils were analyzed for ¹⁵N, inorganic N, Olsen-P, X-K, and extraradical colonization. As expected, extraradical colonization of soil outside the cores was unaffected by mesh core treatment, while that inside the cores varied significantly. ¹⁵N atom% excess was highest in leaves of roots + hyphae. In comparison, leaf ¹⁵N atom% excess in hyphae-intact was consistently intermediate between roots + hyphae and hyphae-rotated, the latter of which remained unchanged over time. Fertilization stimulated host and fungal growth, based on higher biomass and intraradical colonization of fertilized plants. Fertilization did not affect hyphal or root proliferation in residue but did lower %N derived from residue in leaves and stems by 50%. Our results suggest that even low fertilization rates decrease grapevine N uptake from legume crop residue by both extraradical hyphae and roots.     

Nitrogen-fixing stem nodules on Aeschynomene afraspera

Summary Aeschynomene afraspera is a wild annual legume growing in periodically waterlogged soils in western Africa. This legume is characterized by a profuse stem nodulation. Nodules are formed on the stem at the emergence of lateral root primordia, called nodulation sites. These sites are irregularly distributed on vertical rows all along the stem and branches. Stem nodules are hemispherically shaped. Their outside is dark green and they contain a red-pigmented central zone. Stem nodules exhibit a high nitrogen-fixing potential. Acetylene reduction assays result in stem nodule activity of 309 mol C₂H₄ g^–1 dry nodule h^–1. Field-grown stem nodulated Aeschynomene accumulated more N (51 g N m^–2 in 10 weeks) than the root nodulated one. Because of this nitrogenfixing potential and its ability to grow in waterlogged conditions, A. afraspera could probably be introduced into tropical rice cropping systems.     

Mycorrhizal and common root-rot infection,and nutrient accumulation in barley grown on breton loam using N from biological fixation or fertilizer

Summary This paper presents soil biological data from a study on the functioning of three soil-plant systems on a Gray Luvisol in Cryoboreal Subhumid central Alberta. The systems were (1) an agroecological 8-year rotation, (2) a continuous grain system, both established in 1981, and (3) a classical Breton 5-year rotation established in 1930. The objectives were to (1) determine whether changes in vesicular-arbuscular mycorrhizae (VAM) populations occurred in soil under these cropping systems, (2) discover whether these cropping systems and/or VAM infection influenced the incidence of common root rot (Bipolaris sorokiniana), and (3) use nutrient translocation indices to test the hypothesis that soil quality influences non-specific physiological conditions in barley (Hordeum vulgare L.). VAM fungal propagules in soil samples and VAM infection under controlled conditions were significantly affected by the cropping system. VAM infection accounted for more than 85% of the variability in grain yield, plant biomass yield, and plant uptake of K, S, Ca, Fe, and Zn under controlled conditions. Backward-elimination regression analyses showed that under these conditions of high available P, plant P uptake was governed by the quantity of extractable P in the soil (r ²=0.82); the VAM infection contributed practically nothing when combined with available P (R ²=0.84). Neither VAM infection nor the cropping system were related to the B. sorokiniana infection in the barley. The growth of B. sorokiniana was equal, and its sporulation superior, when grown on residues of the non-host fababean (Vicia faba L.), compared with growth on residues of barley. Higher translocation of plant nutrients to the grain in the agroecological compared with the continuous grain treatments suggested that VAM and/or the soil history affected plant physiology, possible through hormonal effects. Superior barley yields in the agroecological compared with the continuous grain treatments were partly due to increased VAM colonization, greater nutrient accumulation and translocation to the grain, but not to a reduced disease incidence. These results demonstrate the benefits of a holistic systems approach while studying biological interactions involving plants and groups of soil microorganisms.(ICRISAT journal article number 1161)     

Effects of liming on carbon and nitrogen mineralization in coniferous forests

A temporary decline in tree growth has often been observed after liming in coniferous forests poor in N but seldom in forests rich in N. To test the hypothesis that the decline was caused by decreases in N supply, C and N mineralization were estimated in incubated soil: (1) after liming in the laboratory, and (2) after earlier liming in the field. Liming increased the C mineralization rate in needle litter, nor humus and 0 to 5 cm mineral soil for a period of 40 to 100 days at 15°C. After that period, liming had no effect on the CO₂ evolution rate in materials poor in N (C:N ratios 30 to 62) but increased the CO₂ evolution rate in materials rich in N (C:N ratios 24 to 28). When liming induced nitrification, the CO₂ evolution rate was reduced. Liming resulted in lower net N mineralization rate in needle litter and mor humus. The reduction was more pronounced when NH₄ ⁺ was the only inorganic form than when NO₃ ^? was the predominant form. The reason is probably that chemical fixation of NH₃ and amino compounds increases with increasing pH. Because of the fixation, the incubation technique most likely underestimated the mineralized N available to the roots. Taking this underestimation into consideration, liming initially reduced the N release in the litter layer. In the other soil layers, liming increased the N release in soils rich in N and had only small effects in soils poor in N. For the total N supply to the roots in the litter, humus and 0 to 5 cm mineral soil layers, liming caused a slight reduction in soils poor in N and a slight increase in soils rich in N. Data on tree growth corresponded with these results.The hypotheses that tree growth depressions can be caused by reduced N supply after liming and that tree growth increases can be caused by increased N supply after liming thus seem reasonable.     

Temporal variation in mineral nitrogen in soil as influenced by incorporation of legume or cereal residues under submergence or well drained conditions

A laboratory study was conducted at the Indian Agricultural Research Institute, New Delhi on a sandy clay loam soil of pH 7.9 and organic C content of 0.34% to study the effect of incorporating Sesbania or Vigna legume residues or wheat straw at 15 and 30t ha^?1 on temporal variation in ammoniacal and nitrate‐N in soil under submergence and well drained conditions. Under submergence most mineral N was present as ammoniacal‐N, while under well drained conditions it was present as Nitrate‐N. The content of ammoniacal N in soil was the highest after 30 days of incubation and declined thereafter under submergence. On the other hand under well drained conditions the mineral‐N (mostly nitrate) content in soil at 30 DAI was very little and showed increases only later, reaching the highest level at 90 DAI. Application of wheat straw specially at 301 ha^?1 level resulted in immobilization of native soil‐N. These results show that rice which is grown under submergence can be transplanted soon after incorporation of legume residues, but for wheat or other crops which are grown under well drained condition a time interval of 30 days or more needs to be provided before sowing the crop.     

Assessment of the relative uptake of added and indigenous soil nitrogen by nodulated legumes and reference plants in the 15N dilution measurement of N2 fixation: Glasshouse application of method

A method for calculating the relative uptake (R) of added N and indigenous soil N by a legume (Trifolium subterraneum) and non-legume (Lolium rigidum), growing together, was investigated in two pot experiments. In the first experiment, ¹⁵N-labelled sodium nitrate was applied to the soil surface at rates equivalent to 0.3 or 1.0kg N ha^?1. Twenty one days later, the legume had fixed about 95% of its total N and this was unaffected by N addition. There was no difference in R values between legume and non-legume at both N rates.In the second experiment using a soil of higher total N, sodium nitrate or ammonium sulphate were surface-applied at a rate equivalent to 1 kg N ha^?1 and harvests were made at 3, 6, 12 and 27 days after N addition. Fixation of atmospheric N₂ by the legume did not begin until day 12 but accounted for about 40% of the total N assimilated by the legume by day 27. There was no difference in R values between legume and non-legume throughout the growth period when sodium nitrate was applied. However, when ammonium sulphate was added to label to soil N, the uptake of added N relative to indigenous soil N was greater for the non-legume than the legume. This caused an overestimation (51 vs 43%) of the proportion on N fixed by the legume when compared with that for the control or sodium nitrate treatments.