Tropical Perennial Grasses Research Articles

Canopy structure and herbage intake rate of three tropical forage grasses cultivated as pure or mixed stands

Context Using forage grass species with complementary growth and resource-use strategies to enhance forage plant diversity in pastures may be an alternative to traditional monocultures in tropical regions. Aims This study aimed to determine whether a mixture of three perennial tropical forage grasses (Andropogon gayanus cv. Planaltina, Panicum maximum cv. Massai, and Brachiaria brizantha cv. BRS Piatã) could be an alternative to enhance herbage intake rates relative to their respective monocultures. Methods The treatments corresponded to three perennial tropical forage grasses cultivated as monocultures and as a mixture composed of all three species. Defoliation management corresponded to a pre-cutting height of 35 cm and post-cutting height of 17.5 cm. Key results The botanical composition of the mixture was dynamic throughout the experimental period, with variations in the proportion of species across seasons. The upper half of the canopy predominantly comprised leaves for all treatments. The canopy structure of the mixture allowed for greater herbage intake rates than monocultures during winter/early spring. Andropogon gambagrass showed lower herbage intake rates than the other treatments. Conclusions The findings of this study indicate that it is possible to combine tropical forage grass species without compromising canopy structure and grazing animal responses, compared with single-species grass pastures. Implications The selection of forage species for mixed pastures should consider their phenological cycle, growth, and resource-use strategies to achieve temporal complementarity and provide an optimal grazing environment for animals throughout the year.

Soil Carbon Fractions under Vetiver Grass in Australia and Ethiopia Relative to other Land Uses

The allocation of soil organic carbon (SOC) to its component fractions can indicate the vulnerability of organic carbon stocks to change. The impact of vetiver on the composition and distribution of SOC can provide a complete assessment of its potential to sequester carbon in soil.Purpose: This study quantified the distribution and impact of SOC under vetiver and the allocation of SOC to particulate (POC), humus (HOC) and resistant (ROC) fractions differentiated based on particle size and chemical composition under vetiver grass compared with other plant types.Methods: Carbon fractions were measured on soil samples collected from Australia and Ethiopia to a depth of 1.0 m under three plant communities (vetiver, coffee, and Australian native pastures). We used the MIR/PLSR spectra to estimate SOC fractions based on fractionated, and NMR measured values.Results: The stocks of SOC fractions indicated significant differences in the proportion of labile POC to HOC across sites and vegetation types. The dominant carbon fraction was HOC (71%) for all vegetation types. The average carbon sequestration rate under vetiver for OC was − 2.64 to + 7.69 Mg C ha− 1 yr− 1, while for the POC, HOC and ROC was 0.04 to + 1.17, -3.36 to + 4.64 and − 0.35 to + 1.51 Mg C ha− 1 yr− 1, respectively.Conclusion: Growing vetiver and undisturbed native pastures has on average a high accumulation rate of a more stable carbon (HOC) which is less vulnerable to change, and change was largely driven by the HOC fraction. We, therefore, recommend the use and promotion of perennial tropical grasses like vetiver and similar grasses and undisturbed native pastures as potential options to facilitate soil carbon sequestration.

A new perennial forage module coupled with the ECOSMOS terrestrial ecosystem model: Calibration and evaluation for Urochloa (syn. Brachiaria) brizantha

Pastures represent the main land use type in Brazil and are used for livestock production and maintenance of land ownership. Most pasture areas exhibit signs of degradation related to inadequate management, which affects pasture biomass production. Terrestrial ecosystem models are effective tools for evaluating the economic and ecological returns associated with distinct management scenarios. In this paper, a new forage module implemented in the ECOSystem MOdel Simulator (ECOSMOS) is formulated, and ECOSMOS-Forage model calibration and evaluation results were obtained for estimating tropical forage growth under continuous and rotational stocking methods subject to grazing or cutting regimes. A set of equations to simulate the daily carbon allocation, growth, senescence, and morphological characteristics of Urochloa (syn. Brachiaria) brizantha cvs. Marandu and Piatã were proposed. Biophysical and physiological parameters, including the solar radiation balance, soil water content, evapotranspiration and carbon assimilation, were calibrated and evaluated with one micrometeorological experiment. Additionally, two sets of parameters were calibrated for Marandu grass growth under continuous and rotational stocking. Based on the Marandu parameterization, a third set of parameters, thereby adjusting a few parameters, was derived for the Piatã grass under rotational stocking. The ECOSMOS-Forage model could successfully capture the energy, carbon and water fluxes and biomass dynamics during growth cycles. The net radiation, evapotranspiration and gross primary production were simulated with agreement index (d) values of 0.97, 0.89, and 0.95, respectively, during the calibration phase and 0.98, 0.90, and 0.93, respectively, during the evaluation phase. Regarding model application to simulate forage growth under rotational stocking, the model could estimate aboveground, leaf and stem dry matter accumulation levels with d values ranging from 0.91 to 0.98. Although forage growth is relatively difficult to simulate, compared with annual crops, the results suggested that the model could provide high potential for simulating tropical perennial forage grasses.

Hard-seeded temperate annual legumes establish better in a tropical grass pasture when autumn-sown than spring-sown in a summer dominant rainfall zone, Australia

ABSTRACT Temperate annual legumes can be effective companion species in tropical perennial grass pastures. These legumes are commonly sown in autumn following establishment of the grasses. However, it may be more effective to establish a seed bank before sowing the grass, or possibly sow the legumes as hard seed/pod in spring when the tropical grass pasture is sown. A study was conducted in northern New South Wales to compare establishment, regeneration and productivity of three hard-seeded legumes: bladder clover (Trifolium spumosum) cv. Bartolo, biserrula (Biserrula pelecinus) cv. Casbah and French serradella (Ornithopus sativus) cv. Margurita. The legumes were sown four times: either one or two autumns before digit grass (Digitaria eriantha) cv. Premier was established, the autumn following grass establishment or sown at the same time as the grass as either hard seed/pod segments or scarified seed (total of five treatments). While seasonal conditions influenced establishment success, autumn was the optimal time to sow temperate annual legumes to achieve a productive mixed pasture. The legumes can be sown 1–2 autumns before or in the autumn following grass establishment. Legume plant populations of spring sown hard seed/pod segments were lower than those autumn sown but they increased the following year.

Canopy leaf area and leaf mass in the upper stratum of Urochloa hybrid ‘Mavuno’ grass subjected to nitrogen fertilisation

AbstractNitrogen (N) inputs are recognised to maximise herbage mass (HM) in tropical perennial grasses, whereas less is clear on their impact on HM distribution and the effects on leaf mass (LM) and leaf area index (LAI) in the upper stratum. This 2 year study, carried out in Pirassununga, Brazil, assessed the HM distribution in the upper (>20 cm) and lower (<20 cm) strata in Urochloa hybrid ‘Mavuno’ grass maintained under similar pre‐ and post‐cutting canopy heights with contrasting N fertilisation rates applied after each cutting (no‐nitrogen, 15, 30, and 45 kg N ha−1). The relevance of specific leaf area (SLA), leaf N concentration (NLeaf), tiller weight (TW) and population density to the LM and LAI of the upper stratum were also examined. Mavuno grass expressed a stable HM < 20 cm (59%–71% during Year I and 66%–80% for Year II), and apparent N fertilisation impacts on HM > 20 cm were verified at specific regrowth cycles during Year II. Mavuno grass pastures expressed plasticity for adjustments on leaf, tiller and population attributes, which were modulated by both climatic conditions and N fertilisation. Under favourable growth conditions during Year I, fertilised pastures were able to sustain higher NLeaf and SLA but associated with lower TW, resulting in maximisation of LAI but not in LM in the upper stratum. During Year II, fertilised pastures expressed higher NLeaf, SLA, number of basal tillers, despite the lowest TW, which resulted in higher LAI and LM in the upper stratum compared with non‐fertilised pastures. Our results highlighted that adjustments on leaf and population attributes within the canopy were driven to maximise the upper stratum LAI, being positively affected by N fertilisation.

The climate benefit of sequestration in soils for warming mitigation

Soils are an enticing reservoir for nature-based climate solutions, but long timescales are required to store amounts of C of relevance to mitigate warming acknowledging its impermanence. Scientific clarity on the controlling factors in soil C persistence should help to disambiguate debates related to permanence in the climate policy domain. However, another contributing factor that is lacking in this debate is a way to compute the climate benefits of C in terrestrial ecosystems over time in the same units as greenhouse gas emissions. We use a case study approach here to demonstrate the use of the metrics of carbon sequestration (CS) and climate benefit of sequestration (CBS) with the aim of assessing the contribution of simultaneous emissions and uptake on radiative forcing. We show how this new computational framework quantifies the climate benefit achieved in two different agricultural systems, one a managed tropical perennial grass system in Hawaiʻi, USA and the other a boreal (cold-temperate, semi-humid) agricultural soil from long term amendment trials in Sweden. Using a set of computations, we show how C inputs and persistence interact to produce different levels of radiative forcing at relevant time frames, which could greatly help to clarify issues of carbon permanence discussed in climate policy. Temporary soil C storage could help to decrease peak warming provided that ambitious emission reductions are part of the portfolio of solutions; the CS and CBS framework gives us a way to quantify it based on biogeochemical understanding of soil C persistence.

Current Scenario and Perspectives for Nitrogen Fertilization Strategies on Tropical Perennial Grass Pastures: A Review

Traditional Nitrogen (N) fertilization practices for tropical perennial grass are still based on annual amounts, following flat N rates instalments. This strategy does not consider variations in climatic conditions along the growing season, their impacts on the plant’s demand and the soil N availability. At regrowth cycles where the amount of soil N released from mineralization or through animal excreta surpass the plant’s demand, most of the N may be lost (as ammonia, nitrate, and nitrous oxide), increasing production costs and environmental pollution. This paper examines current N fertilization and discusses possible gaps in knowledge for the definition of more precise fertilization guidelines in pasture-based livestock systems based on tropical perennial grasses. More precise fertilization practices, based upon site and seasonal-specific recommendations, will substantially contribute to the establishment of best fertilization guidelines. Sustainable approaches can be defined by combining the identification of regrowth cycles where high N rates are required, with enhanced efficiency fertilizers, and/or using grass species with the potential for biological nitrification inhibition. The lack of information on tropical grasses requirements, and soil and climatic factors driving the N fate into the mineralization and immobilization processes and how these factors affect plant’s N demand, still prevents opportunities for tactical applications and the establishment of best management guidelines.

Fermentation Quality of Silages Produced from Wilted Sown Tropical Perennial Grass Pastures with or without a Bacterial Inoculant

High growth rates and rapid reproductive development and associated decline in feed quality of sown tropical perennial grass pastures present management challenges for livestock producers. Conservation of surplus forage as silage could be an effective management tool. Experiments were conducted to evaluate the fermentation quality of silages produced from tropical grasses. Five species (Chloris gayana, Megathyrsus maximus, Panicum coloratum, Digitaria eriantha and Cenchrus clandestinus) were ensiled without additives after a short, effective wilt at dry matter (DM) contents ranging from 302.4 to 650.1 g kg−1. The fermentation profile of all silages in 2019 was typical for high DM silages, but in 2020 ammonia (% of total nitrogen: NH3-N), acetic acid and pH levels were higher. In 2020 M. maximus (302.4 g kg−1 DM) was poorly preserved with 20.2% NH3-N. The DM content of all other silages exceeded 350 g kg−1 and fermentation quality was generally good. In a second experiment, M. maximus was ensiled at 365 g kg−1 chopped and 447 g kg−1 DM chopped and unchopped, either without or with Pioneer 1171® (Lactobacillus plantarum and Enterococcus faecium) or Lallemand Magniva Classic® (L. plantarum and Pediococcus pentasaceus) bacterial inoculant. Inoculants increased lactic acid production, reduced pH and improved fermentation compared to Control, but D-lactate, L-lactate and acetic acid production differed between inoculants. Unchopped silages had higher pH and NH3-N and better preserved protein fraction than chopped silages at the same DM content. In both experiments, wilting increased water soluble carbohydrates by 0.5–31.5 g kg−1 DM and ensiling increased degradation of the protein fraction. We concluded that a rapid and effective wilt combined with a bacterial additive resulted in well preserved tropical grass silages.

Functional Links between Biomass Production and Decomposition of Vetiver (Chrysopogon zizanioides) Grass in Three Australian Soils.

Plant roots are primary factors to contribute to surface and deep soil carbon sequestration (SCS). Perennial grasses like vetiver produce large and deep root system and are likely to contribute significantly to soil carbon. However, we have limited knowledge on how root and shoot decomposition differ and their contribution to SCS. This study examined biomass production and relative decomposition of vetiver which was grown under glasshouse conditions. Subsequently the biomass incubated for 206 days, and the gas analysed using ANCA-GSL. The results confirmed large shoot and root production potential of 161 and 107 Mg ha−1 (fresh) and 67.7 and 52.5 Mg ha−1 (dry) biomass, respectively with 1:1.43 (fresh) and 1:1.25 (dry) production ratio. Vetiver roots decomposed more rapidly in the clay soil (p < 0.001) compared with the shoots, which could be attributed to the lower C:N ratio of roots than the shoots. The large root biomass produced does indeed contribute more to the soil carbon accumulation and the faster root decomposition is crucial in releasing the carbon in the root exudates and would also speed up its contribution to stable SOM. Hence, planting vetiver and similar tropical perennial grasses on degraded and less fertile soils could be a good strategy to rehabilitate degraded soils and for SCS.

Biological and economic responses to increasing nitrogen rates in Momba\xe7a guinea grass pastures

Nitrogen fertilization has been recognized as an essential tool towards the establishment of sustainable intensification of pasture-based livestock systems using tropical perennial grasses if, for a given ecosystem it is capable of increasing forage growth, stocking rates and animal performance. This study assessed pasture growth traits, nutritive value, animal and economic responses of Panicum maximum cv. Mombaça guinea grass pastures subjected to different levels of N fertilization (100 (N100), 200 (N200), and 300 (N300) kg N ha−1 yr−1). Pastures were managed under rotational stocking to maintain similar pre (80–90 cm) and post-grazing (45 cm) canopy heights. A partial budget and a Benefit–Cost Analysis were used to assess the economic returns on increasing N fertilization. N300 resulted in greater post-grazing herbage mass. A slightly higher neutral fiber and acid lignin detergent was observed at N100 (P < 0.05); crude protein increased linearly, and in vitro digestible organic matter reached maximum value at 265.4 kg N ha−1 yr−1. Annual averages of animal weight gain were 515, 590 and 660 g d−1, respectively, for N100, N200 and N300. There was a decrease from 3.7 to 1.9 kg of body weight gain per kg of additional N applied when increasing N rates from 100 to 200 and from 100 to 300 kg ha−1. The net profit improved with increasing N levels, but at reducing rates, reaching its maximum at the N300 level. The change from 100 to 200 kg N ha−1 presented the best return, with USD 3.73 for each additional dollar invested, while the change from 200 kg N ha−1 to 300 kg N ha−1 was economically less than optimal, recouping only USD 1.60 for each dollar. The N300 rate presented the highest net profit per hectare (accounting profit), even in a pessimist scenario (25% reduction in production). Despite being profitable, the N300 rate was less than optimal from an economic standpoint, since an additional 100 kg of Nitrogen ha−1 to change from N200 to N300 level reduced both the net returns and the Benefit–Cost ratio. Our results suggest that the economically optimal level of N fertilization for Mombaça guinea grass pasture should be between 200 and 300 kg ha−1.

Insights Into the Genetic Architecture of Complex Traits in Napier Grass (Cenchrus purpureus) and QTL Regions Governing Forage Biomass Yield, Water Use Efficiency and Feed Quality Traits.

Napier grass is the most important perennial tropical grass native to Sub-Saharan Africa and widely grown in tropical and subtropical regions around the world, primarily as a forage crop for animal feed, but with potential as an energy crop and in a wide range of other areas. Genomic resources have recently been developed for Napier grass that need to be deployed for genetic improvement and molecular dissection of important agro-morphological and feed quality traits. From a diverse set of Napier grass genotypes assembled from two independent collections, a subset of 84 genotypes (although a small population size, the genotypes were selected to best represent the genetic diversity of the collections) were selected and evaluated for 2 years in dry (DS) and wet (WS) seasons under three soil moisture conditions: moderate water stress in DS (DS-MWS); severe water stress in DS (DS-SWS) and, under rainfed (RF) conditions in WS (WS-RF). Data for agro-morphological and feed quality traits, adjusted for the spatial heterogeneity in the experimental blocks, were collected over a 2-year period from 2018 to 2020. A total of 135,706 molecular markers were filtered, after removing markers with missing values >10% and a minor allele frequency (MAF) <5%, from the high-density genome-wide markers generated previously using the genotyping by sequencing (GBS) method of the DArTseq platform. A genome-wide association study (GWAS), using two different mixed linear model algorithms implemented in the GAPIT R package, identified more than 35 QTL regions and markers associated with agronomic, morphological, and water-use efficiency traits. QTL regions governing purple pigmentation and feed quality traits were also identified. The identified markers will be useful in the genetic improvement of Napier grass through the application of marker-assisted selection and for further characterization and map-based cloning of the QTLs.

Plant regeneration via somatic embryogenesis and direct shoot organogenesis of a C4 bioenergy crop Pennisetum pedicellatum Trin.

Pennisetum pedicellatum Trin. is a C4 tropical perennial forage grass known as Deenanath grass holding great potential to address vital, puzzling issues such as bio-economy, human health and the environment. Genetic complexity not only is a big barrier in selection and breeding practices but also restricts harnessing of the full potential of this multipurpose fodder crop. By optimizing a rapid, effective and consistent in vitro regeneration protocol through callus mediated and direct shoot organogenesis, we can multiply these plants for producing a large amount of biomass. Calli were induced from mature seeds and shoot apices of two genotypes on Murashige and Skoog (MS) medium containing different levels of 9.0–22.5 µM 2,4-Dichlorophenoxyacetic acid (2,4-D) and a constant level of 2.22 µM N6-benzyladenine (BA). Seventy-one percent of the calli turned embryogenic when 2,4-D (2.25 µM) was reduced in the medium. Somatic embryogenesis was accomplished on MS medium with an increased level of either BA (4.4–17.6 µM) or kinetin (4.6–18.4 µM) along with 0.56 µM 2,4-D. Ultra-structural and histological studies indicated globular, scutellar and coleoptilar somatic embryos. A maximum of 82.7±1.6% plant regeneration was accomplished from seed explants on MS medium containing 8.8 µM BA and 0.56 µM 2,4-D. Multiple shoots were induced on MS media containing either BA (4.4–18.0 µM) or kinetin (4.6–18.4 µM) along with 0.56 µM 2,4-D using shoot apices. A maximum of 22.1±0.45 shoots per shoot apex was produced on MS medium containing 8.8 µM BA and 0.56 µM 2,4-D. Successful rooting of healthy shoots generated through both the pathways could be optimized on ½MS medium containing 0.4% charcoal. Complete plants could be hardened in plastic pots and were well established on field conditions. In vitro regeneration was established for the first time successfully, which would be very useful for genetic improvement of this apomictic species through transgenic approach.

A lucerne-digit grass pasture offers herbage production and rainwater productivity equal to a digit grass pasture fertilized with applied nitrogen

In recent years, livestock producers have widely sown tropical perennial grasses, such as digit grass (Digitaria eriantha), in the frost-prone summer dominant rainfall zone of eastern Australia. Tropical grasses require substantial nitrogen inputs to maintain water productivity and forage quality. Perennial legumes offer a potential nitrogen source for these perennial grasses when sown in a mixed sward. Lucerne (Medicago sativa) is the perennial legume most widely sown in grazing systems in south-eastern Australia. In this study, conducted over the period 2014–2018, we compared the soil water dynamics and rainwater productivity of pure stands of digit grass (fertilized with applied nitrogen), lucerne, desmanthus (Desmanthus virgatus) and leucaena (Leucaena leucocephala) and binary mixtures of digit grass (not fertilized with applied nitrogen) with each legume. We found that often growing season actual evapotranspiration (ETa) was similar among the treatments (P > 0.05), but rainwater productivity (kg DM/ha.mm) and proportion (%) of legume herbage mass were not (P < 0.05). Our results showed that the lucerne-digit grass mix was equally productive and efficient as fertilized digit grass (P > 0.05), particularly in the latter three seasons. However, lucerne dominated the herbage mass (c. > 67% legume). Leucaena production was delayed by frost and severely impacted by competition with digit grass, and generally underperformed both lucerne and desmanthus. This observation did always not hold when alfalfa mosaic virus (AMV) impacted desmanthus in specific seasons. Our experiment has shown that a mixed sward of lucerne-digit grass is highly productive and offered high rainwater productivity. Both desmanthus and leucaena, in mixes with digit grass, provided useful contributions of legume herbage mass, in specific seasons and under specific conditions, but both underperformed overall compared with lucerne.

Development and characterization of a Napier grass (Cenchrus purpureus Schumach) mapping population for flowering‐time‐ and biomass‐related traits reveal individuals with exceptional potential and hybrid vigor

AbstractNapier grass is a tropical perennial C4 grass with superior biomass yield and quality. It is an important forage crop and a promising feedstock for lignocellulosic biofuel production. However, precise phenotyping and genotyping data to support the molecular breeding of Napier grass are scarce. A Napier grass F1 mapping population (segregating pseudo‐F2 population) was generated by hybridizing genetically distant parents (N190 × N122) with contrasting flowering‐time‐ and biomass‐related traits. True F1 hybrids were confirmed with a simple sequence repeat marker, vegetatively propagated, and phenotyped in replicate field plots for 2 years (2 harvests per year) in Citra, FL, USA (29.40N). Near‐normal distributions were observed for flowering date, plant height, number of tillers, stem diameter, and leaf width, confirming the quantitative nature of these traits. The annual dry biomass yield of F1 hybrids varied between 21.0 and 41.1 t ha−1. The highest‐yielding F1 hybrids showed remarkable hybrid vigor exceeding the annual biomass production of the highest‐yielding parental accession (26.9 t ha−1) by 52% and the biomass yield of the control “Merkeron” cultivar (29.2 t ha−1) by 41% over 2 years. F1 hybrids with delayed flowering time along with significantly increased biomass yield were also identified. Late‐flowering accessions allowed maximum biomass harvest before formation and dispersal of seeds, reducing invasion risk. The developed mapping population will be an excellent resource for identifying quantitative trait loci and candidate genes for flowering‐ and biomass‐related traits in Napier grass that will accelerate the improvement of this non‐domesticated bioenergy crop.

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Tropical perennial grasses are an important forage option in the frost prone, summer dominant rainfall zone in eastern Australia. These pastures commonly lack a companion legume and are, at best, irregularly fertilised with nitrogen; therefore, their production potential is not realised. We conducted a study during 2012–16 to evaluate the productivity and persistence of a range of cultivars/lines of eight tropical and two temperate perennial legumes in mixes with digit grass (Digitaria eriantha cv. Premier) at two sites (Bingara and Manilla) in the northern inland region (North-West Slopes) of New South Wales. The sites were subjected to dry conditions during the assessment period, with the Bingara site receiving &lt;25th percentile rainfall for 2 years of the study. Lucerne (Medicago sativa) was the most productive companion legume, with mixes producing 16–18 and 38–46 t dry matter (DM)/ha at the Bingara and Manilla sites respectively. Cultivars of Desmanthus spp. were less productive, with the best performing in mixes producing 13 and 21–23 t DM/ha at Bingara and Manilla respectively. These cultivars were also persistent and readily recruited new seedlings. Their persistence was similar or superior to lucerne, which appeared to be declining, especially at the Manilla site. Mixes of burgundy bean (Macroptilium bracteatum) and digit grass produced 14–17 t DM/ha at the sites but DM tended to decline during the course of the experiment, especially at the Manilla site. Stylosanthes spp. and Chamaecrista rotundifolia had poor persistence, generally failing within 1–2 years of sowing. Therefore, we recommend lucerne and Desmanthus virgatus cvv. Marc and JCU 2 as suitable perennial legumes in mixes with digit grass during drought conditions.

Service crops improve a degraded monoculture system by changing common bean rhizospheric soil microbiota and reducing soil-borne fungal diseases.

Intensive agricultural practices have resulted in progressive soil degradation, with consequences on soil ecosystem services. The inclusion of service crops is a promising alternative to support the sustainability of the agricultural system. The aim of this study was to analyze in a six-year field experiment the effect of Brachiaria brizantha (perennial tropical grass) and Zea mays as service crops in a degraded common bean monoculture system in northwest Argentina. After six years, service crop treatments revealed a significant increase in most physical, chemical and biological properties of the soil (enzyme activities, microbial biomass, respiration and glomalin-related soil protein), compared with common bean monoculture. Also, a lower disease incidence was observed under B. brizantha treatments, associated with increased populations of Trichoderma spp. and Gliocladium spp. The phospholipid fatty acid profiles detected higher values of total microbial biomass under service crops. Our results suggest that the inclusion of several cycles of B. brizantha constitutes a promising soil management for recovering degraded agroecosystems.

Glomalin contribution to soil organic carbon under different pasture managements in a saline soil environment

ABSTRACT The glomalin produced by arbuscular mycorrhizal fungi has beneficial effects on soils. We studied the vertical profile of glomalin-related soil protein (GRSP) in pasturelands grown under saline soils and exposed to different management practices. Treatments included haying and grazing of Chloris gayana subject to either N-fertilization or not. We measured mycorrhizal colonization, spores, root biomass, total GRSP, GRSP-carbon (GRSP-C), organic carbon (OC) and total nitrogen (TN). In the meanwhile, we also developed a mathematical procedure to fit continuous power functions to binned soil data. Power decay functions adjusted excellently well the measured data across the 1-meter soil profile. Meaningful increases of root biomass, OC, TN, GRSP and GRSP-C values were associated with grazing and fertilization. Particularly, GRSP-C averaged 1,134 kg C ha−1 at the beginning experiments and increased significantly up to 1,261 kg C ha−1 in grazed and fertilized pastures at the end of the study period. Carbon accounted for between 17% and 50% of GRSP, representing in turn 0.52–1.62% of SOC stock. So, a tropical perennial grass can contribute to the carbon stock in saline soils by promoting production of GRSP. This contribution increases when inputs and cycling of nutrients are enhanced (grazing and N-fertilization).

Root depth development in tropical perennial forage grasses is related to root angle, root diameter and leaf area

Tropical perennial grasses that can rapidly establish deep roots have a greater potential to survive soil surface drying sooner after establishment. This research aimed to identify species that establish deep roots quickly and to investigate the mechanisms that drove rapid vertical root development. Perennial grass species Brachiaria brizantha, Brachiaria decumbens, Brachiaria humidicola, Brachiaria hybrid Mulato II, Brachiaria mosambicensis, Panicum maximum, Setaria sphacelata and Cynodon dactylon were grown in rhizotrons that facilitated measurement of the rate of root depth development, the rate of root length development, photosynthesis and morphological traits. Principal component analysis was employed to separate grass species according to functional traits of vertical root development and their growth strategies. P. maximum established roots more rapidly to 50 cm depth than the other species examined and accumulated biomass and leaf area faster. It was differentiated from other species by a greater percentage of fine root length (< 0.2 mm diameter), a higher photosynthetic rate and ratio of root length to leaf area according to the principal component analysis. The growing degree days required for root establishment to 50 cm depth was negatively correlated with the percentage of fine root length, and positively correlated with root angle. The exponential rate of root depth development per growing degree day increased with average root diameter in B. humidicola and B. hybrid Mulato II, indicating a conservative growth strategy. Rapid vertical root development with a narrow root angle, higher photosynthetic rate, higher ratio of root length to leaf area and higher percentage of fine roots were apparent mechanisms that enabled P. maximum to establish deep roots faster than other species. P. maximum appears to be better able to avoid water stress during surface drying by accessing profile moisture. Greater average root diameter and percentage of root length with higher diameters was related to increased exponential rate of root depth development and wide root angle.

Carbon flow through energycane agroecosystems established post‐intensive agriculture

AbstractAs part of an integrated energy and climate system, biomass production for bioenergy based on the tropical perennial C4 grass energycane can both offset fossil fuels and store soil carbon (C). We measured energycane yields, root biomass, soil C pools, and soil C stocks in a 4 year field trial and modeled C flow from plants to soils in the surface layer of no‐till energycane planted after more than a century of intensive sugarcane agriculture. Aboveground yields ranged from 16.7 to 19.0 Mg C/ha over the 4 year trial. Although total C stocks did not significantly differ in the surface layer (approx. 0–20 cm) during the study, C in free and occluded light fractions decreased, whereas C in the mineral‐rich dense fraction increased over 4 years. Belowground system inputs, estimated from measurements and informed by convergence in the final soil fraction model, were set to 2.5 Mg C ha−1 year−1. With this input value, we estimated that surface soils retained photosynthetically fixed C predominantly within the mineral‐associated organic matter pool for a mean and median transit time of 177 and 110 years, respectively. Although we did not model C flow to deep soil layers (approx. 0–100 cm), observed C accumulation (11.4 Mg C ha−1 year−1) and root growth down to 120 cm suggest that soil processes and resulting C sequestration at the surface are likely to persist deeper into the soil profile. Energycane, as a strong candidate for climate change mitigation and land degradation remediation, showed high biomass yields and allocation of resources to roots, with sequestered soil C expected to persist for over a century.

APSIM-Tropical Pasture: A model for simulating perennial tropical grass growth and its parameterisation for palisade grass (Brachiaria brizantha)

Tropical grasses are used as forage, to produce energy from biomass, for land restoration and carbon sequestration, among other applications. Many modelling approaches have been employed to simulate tropical grasses growth, but these have several limitations that must be solved by adapting them or creating new models. This study aimed to develop a tropical pasture model in the Agricultural Production Systems Simulator (APSIM) modelling framework, and to parameterise it to simulate Brachiaria brizantha ‘BRS Piatã’ growth, under grazing and cut-and-carry management. For this, three field experiments were conducted in the South-east of Brazil where pasture growth was measured in a cut-and-carry system, with irrigated and rainfed treatments, and in a rainfed grazing system. Model evaluation was performed through precision and accuracy indices and simulation errors. Under cut-and-carry management, forage productivity was estimated with R2 values from 0.89 to 0.94, Willmott agreement indices between 0.97 and 0.98, and Nash-Sutcliffe Efficiency values of 0.88 to up to 0.92. This demonstrates the capacity of the APSIM-Tropical Pasture model to simulate tropical pastures. Simulation of phenology, early growth after sowing, partitioning and senescence during flowering, and reallocation and retranslocation of plant dry matter and nitrogen were important aspects for this capacity. Then, APSIM-Tropical Pasture can be used to simulate tropical pastures, but several requirements for further improvements have been identified, such as to improve the simulations of flowering for palisade grass, N effects on pasture yield, reallocation and retranslocation processes. Under grazing management, forage productivity was estimated with R2 = 0.80, Willmott agreement index of 0.91, and Nash-Sutcliffe Efficiency value of 0.62. Despite these reasonable results, simulations presented problems, since does not take into account the effect of grazing animals on pastures. This indicates that, in its current form, APSIM-Tropical Pasture is not able to simulate pastures under grazing effectively. However, this simulation of grazed system was important to identify main modelling constraints and direct future research to improve knowledge of processes and interactions needed for pasture model development.