The potential for rhizobial inoculation to increase soybean grain yields on acid soils in Ethiopia

In Ethiopia, inoculation of soybean with rhizobial inoculants is not common practice, but could provide an option to increase grain yields in the low nitrogen (N) acidic soils. In these acid soils, the selection of acid tolerant rhizobia is one strategy that may increase the performance of soybean. In this study, rhizobial strains isolated from Ethiopian soils were evaluated for their acid tolerance and symbiotic N fixation efficiency with soybean, in controlled environments. Following this, four isolated rhizobial strains were evaluated in six field experiments in major soybean growing areas of Ethiopia. Inoculation with the commercial strain or with one of two locally-sourced isolates, that were developed as inoculants, improved soybean yield. The yield increase due to inoculation with the commercial strain was consistent and greater than other treatments, while the increase due to the two locally-sourced strains was comparable to, or greater than, application of 46 kg N/ha in soils, where the resident rhizobial population was ≤ 1.4 × 103 cfu/g soil. For soils with high background rhizobial populations, there was no response to inoculation. In one of the experimental sites (Bako), the percentage of N fixed (%Ndfa) was 55 for the commercial strain and 35 for a local strain, ES3. This study demonstrated that field validation is a necessary step in the selection of acid tolerant strains of rhizobia to increase soybean production for Ethiopia.Daniel Muleta, Maarten H. Ryder and Matthew D. Dento

Plant nitrogen and phosphorus resorption in response to varied legume proportions in a restored grassland

Published: 1 March 2020An in-depth assessment of plant nutrient resorption can offer insights into understanding ecological processes and functional responses to biotic and abiotic changes in the environment. The legume proportion in a mixed grassland can drive changes in the soil environment and plant relationships, but little information is available regarding how the legume proportion influences plant nutrient resorption in mixed grasslands. In this study, three mixed communities of Leymus chinensis (Trin.) Tzvel. and Medicago sativa L. differing in legume proportion (Low-L, with 25% legume composition; Mid-L, with 50% legume composition; High-L, with 75% legume composition) were established with four replicates in a degraded grassland. Four years after establishing the mixed grassland, the quantity of biological N2 fixation by M. sativa, the availabilities of water and nitrogen (N) and phosphorus (P) in soil were examined, and the concentrations and resorption of leaf N and P for both species were measured during forage maturation and senescence. The results showed Mid-L had greater biological N2 fixation and soil N availability than Low-L and High-L, while the High-L had lower soil water and P availability, but a greater soil available N:P ratio compared with Low-L and Mid-L. Legume proportion did not alter N or P concentrations of mature leaves. However, in Mid-L N resorption was reduced by 8 to 16% for the two mixed-species compared with Low-L and High-L. High-L enhanced P resorption by 20 to 24% in both plant species compared with Low-L. The L. chinensis and M. sativa responded differently to varied legume proportion in terms of P resorption. It was concluded that legume proportion drove changes in soil nutrient availability of mixed communities, which primarily altered plant nutrient resorption during senescence, but had no influence on the nutrient concentrations of mature plants. A moderate legume proportion reduced N resorption, and increased senesced leaf N concentration of grass and legume species. The difference in P resorption by two mixed-species significantly changed the interspecific difference of senesced leaf P concentration and the N:P ratio with varied legume proportion.Qiang Li, Daowei Zhou and Matthew D. Dento

High soil temperatures alter the rates of nitrification, denitrification and associated N(2)O emissions

Published online: 11 January 2019Purpose: The responses of nitrification and denitrification are not well characterised at temperatures above 35 °C, which is the focus of our study. Materials and methods: Soils collected from two dairy pastures (Victoria, Australia) were incubated at 10 to 45 °C in the dark for 5 to 10 days following amendment with 100 μg N g−1 either as NH4NO3, 14NH415NO3 or 15NH415NO3 (10 atom% 15N excess) at 50% water-filled pore space. To detect N2O from heterotrophic nitrification, acetylene (0.01% v/v) was used in a subset of samples amended with 15NH415NO3. Atom% 15N enrichments of NO3ˉ, N2O and N2 were measured during the experiment to evaluate the responses of nitrification and denitrification to temperature. Results and discussion: N2O production from the two soils increased with rising temperature and peaked between 35 and 40 °C. N2O production from nitrification and denitrification both had similar thermal responses, which were different to N2 production. The N2O/N2 ratio decreased from > 4 at 35–40 °C to 0.5 at 45 °C, due to greater N2 than N2O production in the Dermosol. Heterotrophic nitrifiers oxidised NH4+ and released N2O at 35–40 °C, suggesting a role for heterotrophs in N cycling under warm climates. Topt for nitrification was between 35 and 40 °C, which is higher than reported previously. A short-term effect of high temperatures could provide NH4+ for the growth of crops but may also decrease soil C pools. Conclusions: Increasing temperature above 35 °C altered the rates of nitrification, denitrification associated N2O and N2 production. Nitrification and denitrification peaked at 35–40 °C in the Chromosol and Dermosol. The production of N2 increased rapidly above 40 °C, which may be related to high soil respiration rates that likely decreased O2 availability, thus expanding the anaerobic microsites; such circumstances increased the reduction of N2O to N2 production from the Dermosol.Thang V. Lai, Ryan Farquharson, Matthew D. Dento

Contrasting water use patterns of two important agroforestry tree species in the Mt Elgon region of Uganda

Published online: 22 Jan 2019.Lack of information on water use of key agroforestry species is an obstacle to understanding their influence on crop productivity. Cordia africana and Albizia coriaria are the dominant tree species of smallholder farming systems in the Mt Elgon region of Uganda and have multiple uses in agroforestry systems. This study deployed six sap flow meters on stems of three selected trees each of C. africana and A. coriaria on-farm. The objective of the study was to assess the daily water use patterns of these agroforestry tree species at different times of the year. We measured the daily sap flow of these two species using the heat ratio method over a period of 18 months. There was a significant main effect of the interaction between tree species and season on daily water use. The two species show contrasting patterns of seasonal water use across leaf shedding stages characterised by episodes of reverse flow in A. coriaria at specific periods of the year. We propose that reverse flows in A. coriaria were triggered by leaf shading while the zero flows in C. africana, which occurred during rainfall events, could have resulted from a lag phase, an indication that the two species may have different water-use strategies. Although C. africana uses 12–15 l day−1 and A. coriaria uses 20–32 l day−1 based on the study trees, C. africana generally uses 12% more water than A. coriaria on a standardised daily basis. Albizia coriaria exhibited radial variation of sap velocities between the inner and outer thermocouples at different periods of measurement, a phenomenon worth investigating further. The leaf shedding patterns of the two trees provide an opportunity for maximising the temporal complementarities of agroforestry systems where these trees exist. This knowledge of C. africana and A. coriaria tree water use provides critical insight for developing successful long-term tree monitoring and management programs in agroforestry systems.J. Buyinza, C. W. Muthuri, A. Downey, J. Njoroge, M. D. Denton and I. K. Nuber

Glass Transition of Hard Sphere Systems: Molecular Dynamics and Density Functional Theory

The glass transition of a hard sphere system is investigated within the framework of the density functional theory (DFT). Molecular dynamics (MD) simulations are performed to study dynamical behavior of the system on the one hand and to provide the data to produce the density field for the DFT on the other hand. Energy landscape analysis based on the DFT shows that there appears a metastable (local) free energy minimum representing an amorphous state as the density is increased. This state turns out to become stable, compared with the uniform liquid, at some density, around which we also observe sharp slowing down of the $alpha$ relaxation in MD simulations.Comment: 5 pages, 5 figure

Inoculating Legumes: Practice and Science

It is envisaged that this revised edition of the preceding handbook, Inoculating Legumes – A Practical Guide will be easily accessible to those needing information for their own purposes or who are giving advice to growers. We hope that it will be a one-stop shop for information on rhizobia and legume inoculation. It is also intended that this handbook will be a comprehensive resource for agronomists and other agricultural scientists in the preparation of seminars and training workshops for growers and advisers

Soil salinity determines the assembly of endophytic bacterial communities in the roots but not leaves of halophytes in a river delta ecosystem

Available online 31 March 2023Although soil and rhizosphere microbiomes in highly saline environments have been well-studied, the role of soil salinity in the ecological processes affecting endophyte colonization and persistence remain largely unclear in halophytic plants. The present study sampled young and mature plants of the halophyte Suaeda salsa from 42 sites in the Yellow River Delta, China that varied in soil salinity. Soil physicochemical properties, root and leaf microbiomes, phylogenetic variation among plant ecotypes, and leaf metabolites were analysed. In the roots of both young and mature plants, soil salinity significantly influenced the composition of the endophytic microbiota (r = 0.29 ~ 0.45, P < 0.001), and negatively correlated with endophyte alpha-diversity (r = -0.75 ~ -0.78, P < 0.001). Leaf microbiome dissimilarity increased with geographic distance (r = 0.17 ~ 0.26, P < 0.001), based on a distance-decay model, and was associated with plant phylogenetic variation (r = 0.15, P = 0.015 for young plants only). Additionally, leaf microbiome diversity and composition were correlated with soil age, pH, P content, and certain leaf metabolite compounds, but not with soil salinity. The dominant genera observed in young roots were Mesorhizobium spp. and Rhodomicrobium spp., while Pelagibius spp. was dominant in mature roots, and Pseudomonas spp. and Kushneria spp. were dominant in leaves. Soil salinity exerted a strong deterministic effect on the diversity and composition of the root endophyte community, while the acquisition and assembly of the leaf microbiome was affected by the dispersal effects, and the leaf metabolism of the host halophyte.Yi Zhou, Yanli Wei, Maarten Ryder, Hongmei Li, Zhongjuan Zhao, Ruey Toh, Peizhi Yang, Jishun Li, Hetong Yang, Matthew D Dento

Strategies to acquire and use phosphorus in phosphorus-impoverished and fire-prone environments

Published online: 19 May 2022Background Unveiling the diversity of plant strategies to acquire and use phosphorus (P) is crucial to understand factors promoting their coexistence in hyperdiverse P-impoverished communities within fire-prone landscapes such as in cerrado (South America), fynbos (South Africa) and kwongan (Australia). Scope We explore the diversity of P-acquisition strategies, highlighting one that has received little attention: acquisition of P following fires that temporarily enrich soil with P. This strategy is expressed by fire ephemerals as well as fast-resprouting perennial shrubs. A plant’s leaf manganese concentration ([Mn]) provides significant clues on P-acquisition strategies. High leaf [Mn] indicates carboxylatereleasing P-acquisition strategies, but other exudates may play the same role as carboxylates in P acquisition. Intermediate leaf [Mn] suggests facilitation of P acquisition by P-mobilising neighbours, through release of carboxylates or functionally similar compounds. Very low leaf [Mn] indicates that carboxylates play no immediate role in P acquisition. Release of phosphatases also represents a P-mining strategy, mobilising organic P. Some species may express multiple strategies, depending on time since germination or since fire, or on position in the landscape. In severely P-impoverished landscapes, photosynthetic P-use efficiency converges among species. Efficient species exhibit rapid rates of photosynthesis at low leaf P concentrations. A high P-remobilisation efficiency from senescing organs is another way to use P efficiently, as is extended longevity of plant organs. Conclusions Many P-acquisition strategies coexist in P-impoverished landscapes, but P-use strategies tend to converge. Common strategies of which we know little are those expressed by ephemeral or perennial species that are the first to respond after a fire. We surmise that carboxylate-releasing P-mobilising strategies are far more widespread than envisaged so far, and likely expressed by species that accumulate metals, exemplified by Mn, metalloids, such as selenium, fluorine, in the form of fluoroacetate, or silicon. Some carboxylate-releasing strategies are likely important to consider when restoring sites in biodiverse regions as well as in cropping systems on P-impoverished or strongly P-sorbing soils, because some species may only be able to establish themselves next to neighbours that mobilise P.Hans Lambers, Patrícia de Britto Costa, Gregory R. Cawthray, Matthew D. Denton, Patrick M. Finnegan, Patrick E. Hayes, Rafael S. Oliveira, Simon C. Power, Kosala Ranathunge, Qi Shen, Xiao Wang, Hongtao Zhon

The preceding root system drives the composition and function of the rhizosphere microbiome

Background: The soil environment is responsible for sustaining most terrestrial plant life, yet we know surprisingly little about the important functions carried out by diverse microbial communities in soil. Soil microbes that inhabit the channels of decaying root systems, the detritusphere, are likely to be essential for plant growth and health, as these channels are the preferred locations of new root growth. Understanding the microbial metagenome of the detritusphere, and how it responds to agricultural management such as crop rotations and soil tillage, is vital for improving global food production. Results: This study establishes an in-depth soil microbial gene catalogue based on the living-decaying rhizosphere niches in a cropping soil. The detritusphere microbiome regulates the composition and function of the rhizosphere microbiome to a greater extent than plant type: rhizosphere microbiomes of wheat and chickpea were homogenous (65–87% similarity) in the presence of decaying root (DR) systems but were heterogeneous (3–24% similarity) where DR was disrupted by tillage. When the microbiomes of the rhizosphere and the detritusphere interact in the presence of DR, there is significant degradation of plant root exudates by the rhizosphere microbiome, and genes associated with membrane transporters, carbohydrate and amino acid metabolism are enriched. Conclusions: The study describes the diversity and functional capacity of a high-quality soil microbial metagenome. The results demonstrate the contribution of the detritusphere microbiome in determining the metagenome of developing root systems. Modifications in root microbial function through soil management can ultimately govern plant health, productivity and food security.Yi Zhou, David R. Coventry, Vadakattu V.S.R. Gupta, David Fuentes, Andrew Merchant, Brent N. Kaiser, Jishun Li, Yanli Wei, Huan Liu, Yayu Wang, Shuheng Gan, and Matthew D. Dento

Soil mineral nitrogen benefits derived from legumes and comparisons of the apparent recovery of legume or fertiliser nitrogen by wheat

Nitrogen (N) contributed by legumes is an important component of N supply to subsequent cereal crops, yet few Australian grain-growers routinely monitor soil mineral N before applying N fertiliser. Soil and crop N data from 16 dryland experiments conducted in eastern Australia from 1989–2016 were examined to explore the possibility of developing simple predictive relationships to assist farmer decision-making. In each experiment, legume crops were harvested for grain or brown-manured (BM, terminated before maturity with herbicide), and wheat, barley or canola were grown. Soil mineral N measured immediately before sowing wheat in the following year was significantly higher (P < 0.05) after 31 of the 33 legume pre-cropping treatments than adjacent non-legume controls. The average improvements in soil mineral N were greater for legume BM (60 ± 16 kg N/ha; n = 5) than grain crops (35 ± 20 kg N/ha; n = 26), but soil N benefits were similar when expressed on the basis of summer fallow rainfall (0.15 ± 0.09 kg N/ha per mm), residual legume shoot dry matter (9 ± 5 kg N/ha per t/ha), or total legume residue N (28 ± 11%). Legume grain crops increased soil mineral N by 18 ± 9 kg N/ha per t/ha grain harvested. Apparent recovery of legume residue N by wheat averaged 30 ± 10% for 20 legume treatments in a subset of eight experiments. Apparent recovery of fertiliser N in the absence of legumes in two of these experiments was 64 ± 16% of the 51–75 kg fertiliser-N/ha supplied. The 25 year dataset provided new insights into the expected availability of soil mineral N after legumes and the relative value of legume N to a following wheat crop, which can guide farmer decisions regarding N fertiliser use.Mark B. Peoples, Antony D. Swan, Laura Goward, John A. Kirkegaard, James R. Hunt, Guangdi D. Li, Graeme D. Schwenke, David F. Herridge, Michael Moodie, Nigel Wilhelm, Trent Potter, Matthew D. Denton, Claire Browne, Lori A. Phillips, and Dil Fayaz Kha