Sustainable intensification through rotations with grain legumes in Sub-Saharan Africa: A review

We conducted a systematic review of literature on the residual effects of grain legumes in cereal-based systems of sub-Saharan Africa (SSA) to quantify the magnitude and variability of rotational effects, to explore the importance of environmental and management factors in determining variability and to evaluate the evidence of the different mechanisms that explain rotational effects. We retrieved 44 unique publications providing 199 observations comparing continuous cereal performance with that of a grain legume-cereal rotation. The overall mean yield increase of 0.49 t grain ha^-1, equal to an increase of 41% of the continuous cereal yield, is highly significant, but the variability in residual effects is large. Effects were more pronounced in southern Africa, the highlands of East Africa and the Guinea savannah, and less in the humid forest/derived savannah of West Africa and the Sudano-Sahelian zone. Maize showed stronger yield responses after a legume than millet and sorghum. Agro-ecological zone and cereal type were however confounded. All grain legume types significantly improved cereal yields, with stronger residual effects observed after soybean and groundnut than after cowpea. Fertiliser N application to cereals reduces the residual effects of legumes, but the response at 60-120 kg N ha^-1 still equalled 0.32 t ha^-1 or 59% of the response when no N is applied. The sustained benefits with large N applications indicate the importance of non-N effects. While mechanisms for improved soil P availability after grain legumes have been studied in some detail, it remains uncertain how important these are in farmers' fields. Grain legumes are unlikely to have a major influence on the availability of nutrients other than N and P, or on soil pH. Beneficial impacts of grain legumes on soil organic matter content can occur if legumes contribute to a greater overall cropping productivity, but studies generally report no such impacts. Evidence of impacts of grain legumes on weeds is limited to striga. Studies on the impacts on nematode pressure in cereals are inconclusive, probably because legumes act as a host for some of the key nematode genera that harm maize. The impact on the pressure of other pests and diseases in cereals is probably important, but evidence on this from SSA is lacking. Future research on N₂-fixation by grain legumes and residual N benefits should focus on explaining the wide variability observed among sites. There is a clear need for more detailed mechanistic studies to assess the occurrence and relevance of non-N effects of grain legumes, particularly in relation to common pests and diseases in cereals.

A Major Locus for Manganese Tolerance Maps on Chromosome A09 in a Doubled Haploid Population of Brassica napus L

Soil acidity poses a major threat to productivity of several crops; mainly due to the prevalence of toxic levels of Al³⁺ and Mn²⁺. Crop productivity could be harnessed on acid soils via the development of plant varieties tolerant to phytotoxic levels of these cations. In this study, we investigated the extent of natural variation for Mn²⁺ tolerance among ten parental lines of the Australian and International canola mapping populations. Response to Mn²⁺ toxicity was measured on the bases of cotyledon chlorosis, shoot biomass, and leaf area in nutrient solution under control (9 μM of MnCl₂⋅4H₂O) and Mn treatment (125 μM of MnCl₂⋅4H₂O). Among parental lines, we selected Darmor-bzh and Yudal that showed significant and contrasting variation in Mn²⁺ tolerance to understand genetic control and identify the quantitative trait loci (QTL) underlying Mn²⁺ tolerance. We evaluated parental lines and their doubled haploid (DH) progenies (196 lines) derived from an F₁ cross, Darmor-bzh/Yudal for Mn²⁺ tolerance. Mn²⁺-tolerant genotypes had significantly higher shoot biomass and leaf area compared to Mn²⁺-sensitive genotypes. A genetic linkage map based on 7,805 DArTseq markers corresponding to 2,094 unique loci was constructed and further utilized for QTL identification. A major locus, BnMn²⁺.A09 was further mapped with a SNP marker, Bn-A09-p29012402 (LOD score of 34.6) accounting for most of the variation in Mn²⁺ tolerance on chromosome A09. This is the first report on the genomic localization of a Mn²⁺ tolerance locus in B. napus. Additionally, an ortholog of A. thaliana encoding for cation efflux facilitator transporter was located within 3,991 bp from significant SNP marker associated with BnMn²⁺.A09. A suite of genome sequence based markers (DArTseq and Illumina Infinium SNPs) flanking the BnMn²⁺.A09 locus would provide an invaluable tool for various molecular breeding applications to improve canola production and profitability on Mn²⁺ toxic soils.

Divergent variations in concentrations of chemical elements among shrub organs in a temperate desert

Desert shrubs, a dominant component of desert ecosystems, need to maintain sufficient levels of nutrients in their different organs to ensure operation of various physiological functions for the purpose of survival and reproduction. In the present study, we analyzed 10 elements in leaves, stems, and roots of 24 dominant shrub species from 52 sites across a temperate desert ecosystem in northwestern China. We found that concentrations of all 10 elements were higher in leaves than in stems and roots, that non-legumes had higher levels of leaf Na and Mg than did legumes, and that Na was more concentrated in C4 leaves than in C3 leaves. Scaling relationships of elements between the photosynthetic organ (leaf) and non-photosynthetic organs (stem and root) were allometric. Results of principal components analysis (PCA) highlighted the important role of the elements responsible for osmoregulation (K and Na) in water utilization of desert shrubs. Soil properties and taxonomy explained most variation of element concentrations in desert shrubs. Desert shrubs may not be particularly susceptible to future change in climate factors, because most elements (including N, P, K, Ca, Mn, Zn, and Cu) associated with photosynthesis, osmoregulation, enzyme activity, and water use efficiency primarily depend on soil conditions.

Plant homeostasis of foliar manganese sinks: specific variation in hyperaccumulators

Plant manganese (Mn) hyperaccumulation provides unusual insight into homeostasis of this essential micronutrient, in particular its excessive storage in shoot tissues. The compartmentation of hyperaccumulated foliar Mn appears exceptional among metal hyperaccumulators, since it occurs via specific microdistribution patterns. Here, three associated Mn hyperaccumulators, Virotia neurophylla, Maytenus fournieri, and Garcinia amplexicaulis exhibiting distinctly different Mn detoxification strategies were examined. Non-invasive sample preparation in conjunction with cryo scanning electron microscopy (SEM) was used to obtain in vivo quantitative microprobe X-ray and anatomical data from fully hydrated cells. Highly vacuolated large palisade mesophyll cells in V. neurophylla leaves were found to contain around 650 mM Mn. The large non-photosynthetic hypodermal cells of M. fournieri leaves, also with high vacuolar content, and the main site for Mn disposal, had an estimated mean vacuolar Mn concentration of around 600 mM. Previous qualitative X-ray mapping had shown Mn to be almost evenly sequestered across the entire leaf cross section of G. amplexicaulis. However, quantitative data obtained here showed a marked variation in localised concentrations that ranged between ~15 and >800 mM. Notable among these were mean values of >600 mM in spongy mesophyll cells, and ~800 mM within cells of a narrow sub epidermal layer preceding the palisade mesophyll. This study demonstrated the extraordinary Mn carrying capacities of different types of leaf cell vacuoles.

Fresh water leaching of alkaline bauxite residue after sea water neutralization

Processing of bauxite to extract alumina produces a strongly alkaline waste, bauxite refining residue, which is commonly stored in engineered structures. Once full, these waste dumps must be revegetated. In many alumina refineries, the waste is separated into fine-textured red mud and coarse-textured residue sand (RS). The sand component has physical characteristics that make it a suitable plant growth medium, provided the adverse chemical characteristics can be addressed. Neutralization of the highly saline-sodic RS with sea water lowers pH, reduces Na saturation, and adds plant nutrients. However, sea water-neutralized RS remains saline sodic and needs fresh water leaching before use as a plant growth medium. Columns containing sea water-neutralized RS were leached with 30 m depth-equivalent of fresh water to evaluate the effects of rainfall on the RS and its leachate. Entrained cations were rapidly displaced by the fresh water, lowering salinity to non-plant-limiting levels (< or =0.3 dS m(-1)). The percentage of the effective cation exchange capacity (ECEC) saturated by Na decreased from 71 to 62% due to a reduction in soil solution ionic strength (causing a decrease in the ECEC) and the preferential displacement of Na(+) (and K(+)) from the exchange. Fresh water leaching increased pH (leachate pH increased from 8.0 to 10.1). This pH increase is attributed to the slow dissolution of the Na-containing mineral sodalite. Under the current experimental conditions, the application of 30 m depth-equivalent of leaching reduced the total RS sodalite content by <10%.

Nutrient dynamics and tree growth of silvopastoral systems: impact of poultry litter

Fertilizing pastures with poultry litter has led to an increased incidence of nutrient-saturated soils, particularly on highly fertilized, well drained soils. Applying litter to silvopastures, in which loblolly pine (Pinus taeda L.) and bahiagrass (Paspalum notatum) production are integrated, may be an ecologically desirable alternative for upland soils of the southeastern USA. Integrating subterranean clover (Trifolium subterraneum) into silvopastures may enhance nutrient retention potential. This study evaluated soil nutrient dynamics, loblolly pine nutrient composition, and loblolly pine growth of an annually fertilized silvopasture on a well drained soil in response to fertilizer type, litter application rate, and subterranean clover. Three fertilizer treatments were applied annually for 4 yr: (i) 5 Mg litter ha(-1) (5LIT), (ii) 10 Mg litter ha(-1) (10LIT), and (iii) an inorganic N, P, K pasture blend (INO). Litter stimulated loblolly pine growth, and neither litter treatment produced soil test P concentrations above runoff potential threshold ranges. However, both litter treatments led to accumulation of several nutrients (notably P) in upper soil horizons relative to INO and unfertilized control treatments. The 10LIT treatment may have increased N and P leaching potential. Subterranean clover kept more P sequestered in the upper soil horizon and conferred some growth benefits to loblolly pine. Thus, although these silvopasture systems had a relatively high capacity for nutrient use and retention at this site, litter should be applied less frequently than in this study to reduce environmental risks.

Outcomes of the search for new perennial and salt tolerant pasture plants for southern Australia

The potential adaptation of a range of perennial pasture species to recharge environments in southern Australia is reviewed based on their performance in 20 field nurseries in a nationally coordinated project. Species were also evaluated for their suitability to discharge sites where salt and waterlogging are major restraints. Species are ranked according to their potential to be incorporated into farming systems and the scope for further breeding and selection. Medicago sativa L. (lucerne) was the most persistent of the perennial legumes across a diversity of recharge environments. Lotus corniculatus L. (birdsfoot trefoil) showed the most promise on soils prone to waterlogging. Other legumes that showed potential included Cullen australasicum (Schltdl.) J.W. Grimes (tall verbine) and Lotononis bainesii Baker (lotononis). The herb Chicoriyum intybus L. was superior to M. sativa on more acid soils. Phalaris aquatica L. (phalaris) and summer dormant cultivars of Dactylis glomerata L. (cocksfoot), Festuca arundinacea L. (tall fescue) and Lolium perenne L. (perennial ryegrass) were among the most persistent and productive of the perennial grasses. Further exploitation of temperate perennial grass germplasm with increased summer dormancy should be a priority to increase the role of these grass species in lower rainfall, summer-dry environments. Although difficult to establish, the indigenous grasses Austrodanthonia caespitosa (Gaudich.) H.P. Linder (wallaby grass) and A. richardsonii (Cashmore) H.P. Linder were persistent and showed good recruitment. They should be a priority for low rainfall, low input environments. Other grasses that showed promise were Chloris gayana Kunth (Rhodes grass), Secale montanum Guss. (mountain rye), Microlaena stipoides (Labill.) R. Br. (weeping grass), Ehrhata calcycina Sm. (veldt grass) and Bromus stamineus E. Desv. (grazing brome). For discharge environments, Melilotus siculus (Turra) Vitman ex B.D. Jacks. was one of the most salt tolerant legumes and should be a priority for further development. Medicago polymorpha L. (burr medic) appears underutilised in discharge environments. Increasing the waterlogging tolerance of this moderately salt tolerant species would further enhance its potential. Trifolium michelianum Savi. (balansa clover) owed its success in discharge areas more to ‘salt avoidance’ rather than salt tolerance per se. Melilotus sulcatus Desf., T. tomentosum L. and Lotus tenuis Waldst. & Kit. ex Willd. also had traits that may prove advantageous for discharge environments. Within the pasture grasses, Puccinellia ciliata Bor (pucinellia) was superior at sites prone to waterlogging whereas T. ponticum performed better in moderately drained saline sites.

Residual effects from lime application on soil pH, rhizobial population and crop productivity in dryland farming systems of central New South Wales

A mildly acidic (pHCa 4.79, 0–10 cm depth) red-brown earth soil (Chromosol) at Condobolin in central-western New South Wales was cultivated and limed (once only) at six rates (range 0–4 t/ha) and sown with field peas (Pisum sativumL.) with and without inoculation (once only) with Rhizobium leguminosarum bv. viciae – the rhizobium for peas. The soil already contained a very small population of pea rhizobia (<4 per g soil). The experiment embraced two parallel rotations, each over 4 years: (1) year 1, inoculated peas; year 2, wheat; year 3, wheat; year 4, uninoculated peas; and (2) year 1, inoculated peas; year 2, wheat; year 3, inoculated chickpeas; year 4, uninoculated peas. The objectives of the work were to establish whether liming had any immediate and residual benefits for rhizobia and plants and, if so, to determine if the two events were linked. Liming had an immediate effect on soil pH (0–10 cm depth). Increases in pH were greater per unit of lime at lower rates of application than at higher rates. Although lime effects existed for the duration of the experiment (four seasons of cropping), there was a small decline in soil pH over time (mean decline in unlimed plots 0.16 pHCa units, mean decline in limed plots 0.47 pHCa units). In the first year (pea crop), there was a very large and highly significant response to inoculation on populations of rhizobia in soil and rhizosphere. The number of rhizobia that occurred naturally in uninoculated plots increased rapidly in high-lime plots until, by the third year, they were substantial and, by the fourth year, equal to those in the inoculated treatment. By the end of the experiment, the mean population of rhizobia in the 4 t/ha lime treatment was 7250 per g soil, compared with <4 rhizobia per g in the nil lime treatment. It was noteworthy that, in those years in the rotations when peas were not grown, populations of R. leguminosarum bv. viciae were sustained by their ability to colonise the rhizospheres of wheat and chickpea. In the first pea crop, eight parameters of plant production responded overwhelmingly to inoculation, while there was an underlying response to liming in two of those parameters. The positive effect of inoculation on peas in the first year carried over to the wheat crop of the second year, which was interpreted as a consequence of increased soil N in the inoculated plots. By the third and fourth years, soil populations of pea rhizobia in the plus inoculation and minus inoculation treatments were approximately equal, and inoculation was no longer a determinant of crop production. On the other hand, application of lime, which had only an underlying effect on pea production in the first year, significantly enhanced several parameters of the symbiosis and growth of the chickpea and pea crops, including legume nodulation and percentage nitrogen in the seed. R. leguminosarum bv. viciae, legumes and cereals each responded differently to increasing rates of lime application. Populations of rhizobia in soil and plant rhizospheres increased with each additional rate of liming. Legume productivity responded to additional lime up to 2 t/ha. There was no significant evidence that liming per se had any effect at any time on wheat production. The practical implications of these results are discussed.

Excess cation concentrations in shoots and roots of pasture species of importance in south-eastern Australia

Excess cation concentrations (total cations – total inorganic anions) are reported for roots and shoots of 16 plant species of importance in pastures in south-eastern Australia. This information is required for the calculation of acidification in grazed pasture systems. The excess cation concentrations for shoots at flowering were [cmol(+)/kg]: perennial grasses — Lolium perenne (perennial ryegrass) 50, Phalaris aquatic (phalaris) 51, Danthonia richardsonii (wallaby grass) 30, Dactylus glomerata (cocksfoot) 62, Holcus lanatus (Fog grass) 60; annual grasses — Lolium rigidum 29, Vulpia bromoides (vulpia) 40, Hordeum leporinum (barley grass) 46, Bromus mollis (soft brome) 59; perennial legumes — Medicago sativa (lucerne) 115, Trifolium repens (white clover) 147; annual legumes — Trifolium subterraneum (subterranean clover) 142, Medicago truncatula (barrel medic) 114, Ornithopus sativus (serradella) 137; weeds — Arctotheca calendula (cape weed) 165, Echium plantagineum (Paterson’s curse) 169. Values for roots were in the same order as shoots in vulpia and wallaby grass but lower for the other species, varying between 26 and 62% of the shoot value in grasses and 29 and 49% in legumes. For a subset of 4 legumes and 3 grasses, the excess cation concentrations in shoots were measured over the main production period in spring. Excess cation concentrations generally declined during the season, with the change being relatively larger in grasses than legumes.