Dichanthium Aristatum Research Articles

Pollen allergen skin test and specific IgE reactivity among Filipinos: a community-based study

BackgroundDespite the clinical importance of pollen allergens among Filipinos, few studies delve into the sensitization profiles of Filipinos against pollen allergens. This study determined the sensitization profile of Filipinos to pollen using skin prick test (SPT) and pollen-specific ELISA.MethodsPollen from fifteen selected plant sources was collected and extracted for use in sensitization tests. Volunteers were interviewed for their clinical history prior to blood sampling and SPT. The blood samples collected were assessed using Enzyme-Linked Immunosorbent Assay (ELISA).ResultsThe best panel of pollen allergens for the skin prick test was Mangifera indica (64%), Acacia auriculiformis (28%), Mimosa spp. (25%) Amaranthus spinosus (22%), Lantana camara (20%), Pilea microphylla (16%) and Dichanthium aristatum (15%). Young adults had more sensitizations to pollen than among early childhood and elderly. There were more allergic subjects that have rhinitis (61%) than asthma (42%) and atopic dermatitis (35%). Pollen-specific IgE levels show low percent reactivity as compared to the skin test with Cocos nucifera obtaining the highest IgE reactivity (21%).ConclusionsPollen allergens from both arboreal and herbaceous plants used in this study yielded positive reactivities for both skin tests and specific IgE tests.

Invasive grasses in South Texas rangelands: historical perspectives and future directions

AbstractSouth Texas is home to a high diversity of species due to its location at the confluence of subtropical, desert, and coastal ecoregions. Historical overgrazing of South Texas rangelands transformed the savanna and prairie to a landscape dominated by woody plants and shrubs interspersed with low seral grass species and bare ground. During the first half of the 20th century, exotic grass species, coupled with the application of industrial agricultural practices appeared to be the future of forage production in South Texas and elsewhere. Several of these exotic species, namely King Ranch bluestem [Bothriochloa ischaemum(L.) Keng], Kleberg bluestem [Dichanthium annulatum(Forssk.) Stapf], Angelton bluestem [Dichanthium aristatum(Poir.) C.E. Hubbard], buffelgrass [Pennisetum ciliare(L.) Link], guineagrass [Urochloa maxima(Jacq.) R. Webster], Lehmann lovegrass (Eragrostis lehmannianaNees), and Bermudagrass [Cynodon dactylon(L.) Pers.], have escaped pasture cultivation. Additionally, the native grass tanglehead [Heteropogon contortus(L.) P. Beauv. ex Roem. & Schult.] has begun displaying invasive behaviors. The monoculture growth habit of these species simplifies vegetation structure, reduces biodiversity, and decreases habitat for many species of wildlife. These grasses also alter natural fire regimes and nutrient cycling. This landscape-level transformation of vegetation composition and structure requires monitoring to quantify and assess the spatial and temporal distributions of invasive species as a basis to inform management practices. Current advances in remote sensing technologies, such as very high spatial resolution coupled with daily satellite imagery and unmanned aerial vehicles, are providing tools for invasive vegetation monitoring. We provide a synthesis of the natural history of these grasses, including their introductions, an overview of remote sensing applications in South Texas, and recommendations for future management practices.

Colonización de micorrizas arbusculares en tres especies de pasturas del departamento de Sucre

El presente estudio se llevó a cabo durante el segundo semestre de 2015 en fincas ganaderas de cinco localidades del departamento de Sucre, establecidas con pastos colosuana (Bothrichloa pertusa (L) A. Camus), angletón (Dichanthium aristatum, Benth) y mombaza (Panicum maximum Jacq), con el objetivo de evaluar la colonización de hongos formadores de micorrizas arbusculares (HMA) en raíces de estas pasturas. El muestreo se realizo aleatoriamente en cada finca ganadera, recolectando muestra de suelos y raíces, determinándose el porcentaje de infección mediante técnica de coloración de raíces por HMA. Los resultados muestran que los porcentajes de colonización fueron de 52,73% entre el pasto colosoana y 50,46% para angletón, siendo estadísticamente similares; mientras que el pasto mombaza tuvo menor colonización 40,92%. El mayor valor de colonización de HMA por localidad correspondieron a Chapinero 70,06% y los menores para Cantagallo 37,06% y el Mamón 32,7%. El presente estudio confirmó la presencia de HMA en tres especies de pasturas de interes económico para las explotaciones ganaderas del departamento de Sucre. Así mismo, la colonizacón de HMA de raíces en el presente trabajo dependió del tipo de pastura y la localidad.

Colonización de hongos formadores de micorrizas arbusculares en raíces de pasto colosoana y angletón en cinco localidades del departamento de Sucre

El presente estudio se realizó durante el segundo semestre de 2015 en fincas ganaderas de cinco localidades del departamento de Sucre, Colombia, establecidas con pastos colosoana (Bothriochloa pertusa (L) A. Camus) y angletón (Dichanthium aristatum, Benth), con el objetivo de determinar el porcentaje de colonización de hongos formadores de micorrizas arbusculares (HMA) en raíces de estas dos especies de pasturas. Las muestras de suelos con presencia de raíces fueron colectadas de cinco localidades del departamento de Sucre, a partir de la cuales se determinó el porcentaje de colonización en raíces por HMA. Los resultados de análisis de varianza para porcentaje de colonización en raíces de pastos colosoana y angletón (p value = 0,000), respectivamente), encontrándose los mayores porcentaje de colonización para el pasto colosoana (36.9±79.09%) con respecto al pasto angletón (33.3±72.72%) en las cinco localidades evaluados. Los resultados obtenidos demuestra que ambas pasturas esta colonizadas por HMA.

Effects of increasing rates of molasses supplementation and forage quality on the productivity of steers

This study consisted of several experiments that investigated the feed intakes, and growth rates of steers fed low-quality Angleton grass (Dichanthium aristatum) supplemented with urea and sulfur (dry matter (DM) digestibility, 45%) and medium-quality Rhodes grass (Chloris gayana) (DM digestibility, 58%) when fed either alone or with molasses mixtures (MM, molasses plus nitrogen and phosphorus). Some digestion characteristics of the basal forage diets were also measured. Addition of MM at 1.2 times calculated maintenance energy requirements (M) was associated with about a four-fold increase in microbial crude protein production and a 42% decrease in neutral detergent fibre digestibility. In the first growth experiment, MM at 0M, 0.45M, 0.90M and 1.35M was offered and mostly consumed before forage was offered ad libitum. The initial mean weight of the steers was 240 kg. Consumed MM comprised 0%, 21%, 38% and 50% of the total DM intake for Angleton grass and 0%, 19%, 33% and 41% for Rhodes grass. The corresponding mean liveweight (LW) gains were 0.06, 0.31, 0.73 and 0.83 kg/day for steers fed Angleton grass and 0.61, 0.90, 1.02 and 1.13 kg/day for steers fed Rhodes grass. In a second experiment, MM was offered to steers (initially 370 kg) at the same time as forage. Offered MM comprised 0, 0.40, 0.80 and 1.20M of the calculated metabolisable energy (ME) requirements for maintenance. Consumed MM comprised 0%, 20%, 36% and 48% of the total DM intake for Angleton grass diets and 0%, 18%, 28% and 32% for Rhodes grass diets. The corresponding mean LW gains were –0.09, 0.36, 0.58 and 0.70 kg/day for Angleton grass diets and 0.63, 0.67, 0.80 and 0.94 kg/day for Rhodes grass diets. The method of presentation of the MM had only minor effects on the amounts and proportions of forage and the MM consumed and on the LW gains of the steers, expressed as g/kg0.75 LW per day to allow for differences in initial LW between experiments. Using modifications to the scheme of SCA (1990) to predict maintenance energy requirements, a value for efficiency of use of ME for growth was calculated from growth rates of steers fed Rhodes grass. When MM with an assumed content of 11 MJ ME/ kg DM was added to the forage diets, the calculated ME content of forage decreased with increased MM content, consistent with depression of fibre digestion. The relationships for the separate experiments indicated that at 50% MM in the diet, the magnitudes of the depression in the calculated ME content of the forages were 20–37% for Rhodes grass and 18–33% for Angleton grass.

Treatment of tropical forages with exogenous fibrolytic enzymes: effects on chemical composition and in vitro rumen fermentation

The effects of three treatments of fibrolytic enzymes (cellulase from Trichoderma longibrachiatum (CEL), xylanase from rumen micro-organisms (XYL) and a 1:1 mixture of CEL and XYL (MIX) on the in vitro fermentation of two samples of Pennisetum clandestinum (P1 and P2), two samples of Dichanthium aristatum (D1 and D2) and one sample of each Acacia decurrens and Acacia mangium (A1 and A2) were investigated. The first experiment compared the effects of two methods of applying the enzymes to forages, either at the time of incubation or 24h before, on the in vitro gas production. In general, the 24h pre-treatment resulted in higher values of gas production rate, and this application method was chosen for a second study investigating the effects of enzymes on chemical composition and in vitro fermentation of forages. The pre-treatment with CEL for 24h reduced (p<0.05) the content of neutral detergent fibre (NDF) of P1, P2, D1 and D2, and that of MIX reduced the NDF content of P1 and D1, but XYL had no effect on any forage. The CEL treatment increased (p<0.05) total volatile fatty acid (VFA) production for all forages (ranging from 8.6% to 22.7%), but in general, no effects of MIX and XYL were observed. For both P.clandestinum samples, CEL treatment reduced (p<0.05) the molar proportion of acetate and increased (p<0.05) that of butyrate, but only subtle changes in VFA profile were observed for the rest of forages. Under the conditions of the present experiment, the treatment of tropical forages with CEL stimulated their in vitro ruminal fermentation, but XYL did not produce any positive effect. These results showed clearly that effectiveness of enzymes varied with the incubated forage and further study is warranted to investigate specific, optimal enzyme-substrate combinations.

Variables física y quimicas del suelo y su relacción con la colonización de micorrizas arbusculares en raices del pasto angleton (Dichanthium aristatum Benth)

The objective of this study was to determine the colonization of arbuscular mycorrhizal fungi (AMF), associated with the roots of the angleton grass, related to physicochemical soil factors in livestock farms of four agroecological zones of the sub-region Sabana of Sucre, Colombia. The percentage of AMF colonization of roots was estimated by the staining technique, which allowed to observe under the microscope the presence of colonizing fungal structures and the subsequent determination of the percentage of colonization for sample, farms and area. With the results of the colonization percentage and the physicochemical soil analysis a correlation between th ese variables, using statistical methods and multivariate analysis, was performed. The analysis of simple correspondence showed a relationship between the presence of colonizing structures and the soil texture. The results of analysis between multiple types of colonizing structures and physicochemical soil variables showed the presence of vesicles and arbuscules in soils with high contents of calcium, magnesium and potassium and low values of nitrogen and phosphorus. There is a need for further studies in other geographical areas and other grasses to establish correlation between physicochemical soil variables and the magnitude of AMF colonization in tropical pastures in Colombia.

Arbuscular mycorrhizal symbioses in a cut-and-carry forage production system of legume tree Gliricidia sepium and fodder grass Dichanthium aristatum

Arbuscular mycorrhizal (AM) symbioses may alter the competitive abilities of plant species and facilitate positive interactions such as nutrient transfer between plants. They are therefore particularly interesting components in agroforestry systems. We studied spatial variation of AM colonisation on a cut-and-carry fodder production site (agroforestry plot) of the legume tree Gliricidia sepium and the fodder grass Dichanthium aristatum. Roots of the two plant species were sampled under the tree canopies and on the adjacent grass plot at 1 and 3.5 m from the first tree row where G. sepium roots also occur. Roots of D. aristatum were also sampled on a nearby grass monocrop. Colonisation of arbuscules, vesicles and hyphae in root samples was visually determined, and AM fungal species were identified by DNA sequencing. Colonisation and frequency of types of AM formations varied statistically significantly between the species and sampling points. Arbuscular colonisation in G. sepium roots was higher under the tree canopies than on the adjacent grass plot. Soil nutrient content, particularly P and N, and interspecies competition are the most probable explanations for the observed variation in AM colonisation. Both arbuscular colonisation and arbuscule:vesicle ratio in D. aristatum roots was lower on the D. aristatum monocrop than on the agroforestry plot under or near the tree canopies. Intercropping could stimulate AM symbiosis in D. aristatum. Both plant species formed symbiosis with Rhizophagus intraradices, indicating potential for interplant N transfer via common mycelial networks of AM-forming fungi.

Diversidad de micorrizas arbusculares en agroecositemas de pastura del departamento de sucre

El objetivo del presente estudio fue comparar la diversidad de micorrizas arbusculares asociadas al pasto colosoana (Bothriochloa pertusa (L) A. Camus) y angletón (Dichanthium aristatum, Benth) presentes en fincas ganaderas de los municipios de Corozal y Tolú, Departamento de Sucre, Colombia. Se determinó la densidad de esporas/100gr suelo y el porcentaje de infección en raíces en ambas especies de pasto. A partir de los morfotipos aislados se procedió a la identificación de especies de MA, utilizando claves taxonómicas. Los resultados del análisis de varianza para densidad de esporas/100 g de suelo de muestran diferencia altamente significativa entre pasturas y localidad. La mayor densidad de esporas encontrada fue de 1160 ± 6233 y 444 ± 2643 esporas/100 g de suelo, respectivamente para colosoana y angletón en el municipio de Corozal. EL porcentaje de colonización mostró diferencia altamente significativa entre pastura y localidad. La prueba de Tukey mostró diferencia altamente significativa entre el porcentaje de colonización para la especie colosoana y angletón en los municipios de Corozal y de Tolú. Un total de 32 morfotipos MA fueron aislados, 15 para el municipio de Corozal y el resto en el municipio de Tolú. Existe una alta diversidad de MA asociadas a estas dos pasturas en condiciones edafoclimaticas diferentes, del departamento pero aún faltan estudios que permita predecir los patrones de dispersión y localización de estas micorrizas en agroecosistemas específicos de pasturas en el departamento de Sucre.

Bioecología del picudo del tallo del maíz Linogeraeus capillatus (LeConte) (Coleoptera: Curculionidae) en el departamento de Córdoba-Colombia

En el Caribe colombiano Linogeraeus capillatus a tomando importancia dentro del agroecosistema maíz reportándose en los municipios de Codazzi-Cesar en 1975, en San Onofre-Sucre y en los municipios de Montería, Cereté, Ciénaga de Oro, San Carlos, San Pelayo, Cotorra, Lorica, Chimá en el departamento de Córdoba. El objetivo de la presente investigación fue conocer sus aspectos bioecológicos abarcando su ciclo de vida y la caracterización morfológica de sus instares, para esto se contó con 350 larvas discriminadas en los diferentes instares. Parejas recién emergidas se confinaron en jaulas entomológicas de 50x50 cm, sobre plantas de maíz completamente sanas y mediante muestreo destructivo se confirmó la densidad de larvas vivas presentes. Bajo condiciones de laboratorio a temperatura de 29,23°C, el huevo presentó una duración de 5,3 días, las larvas de 158,2 días y la pupa 34,44 días. La cantidad de huevos por oviposición oscila entre 22-24 huevos (4,4 a 4,8 huevos/planta), con longitud promedio de 1 mm. Las larvas presentan cinco instares, los cuales se desarrollan dentro de la planta. Las arvenses Sorghum halepense y Dichantium aristatum fueron las principales plantas hospederas para los adultos de L. capillatus. Como enemigos naturales de adultos de L. capillatus se registraron los depredadores del orden Araneae representadas por las familias Dipluridae y Lycosidae, mientras que la acción depredadora de los estados larvales la ejercieron hormigas del género Solenopsis.

Two new Indian gall midges of the genus Contarinia Rondani (Cecidomyiidae: Diptera) reared from Amaranthaceae &amp; Poaceae

Two new species of Genus Contarinia Rondani viz. C. asperae reared from leaf galls of Achyranthes aspera L. (Family Amaranthaceae) and C. dichanthiumae reared from ear heads of Dichanthium aristatum Hub.( Family : Poaceae) are described and illustrated.

Determinación de un modelo logístico para evaluación in situ de la colonizacion de micorrizas en pasto dichanthium aristatum (l)

El objetivo fue evaluar la colonización de micorrizas arbusculares nativas, asociadas a raíces de la especie de pasto Dichanthium aristatum (L) en función de factores químicos del suelo, en fincas ganaderas, de la subregión fisiográfica de Sabanas. La determinación del porcentaje de colonización en raíces coloreadas, fue realizada por medio del conteo de micelio, esporas, células auxiliares, vesículas y arbusculos en el microscopio. El modelo o función de probabilidad creada fue utilizado para medir las posibilidades de ocurrencia de la colonización como respuesta a la presencia de ciertos factores químicos del suelo. Para obtener esta función logística se determinó la importancia de cada uno de estos factores de riesgo y sus posibles interacciones. Fueron muestreadas 60 fincas ganaderas. El proceso de modelación logística encontró que los factores que mejor explican el fenómeno de colonización de hongos formadores de micorrizas asociados a rizosfera de pasto angletón fueron el Nitrógeno, Calcio y el Potasio.

Field studies of the effects of pre-sowing weed control and time of sowing on tropical perennial grass establishment, North-West Slopes, New South Wales

Two separate experiments on the North-West Slopes of New South Wales investigated: (1) the effects of differing lengths of pre-sowing weed control (2 years, 1 year, and 3–4 months in the spring immediately before sowing) on the soil seed density of liverseed grass (Urochloa panicoides) from November 2005 to November 2007 and its seedling density at establishment (December 2007), as well as on subsequent herbage mass and plant frequency of this species and Panicum coloratum var. makarikariense cv. Bambatsi (Expt 1); and (2) the effects of cultivar, time of sowing (November 2006, January 2007, and March 2007), and defoliation in the autumn after sowing, on the over-wintering ability of 5 tropical perennial grasses (Expt 2). The cultivars sown in Expt 2 were Bambatsi, Dichanthium aristatum cv. Floren, Chloris gayana cv. Katambora, Digitaria eriantha var. eriantha cv. Premier, and Bothriochloa bladhii var. glabra cv. Swann. At sowing (November 2007), liverseed grass seed and seedling densities in Expt 1 were lowest (P &lt; 0.05) in the 2-year pre-sowing weed control treatment (15 and 13/m2, respectively), and the subsequent herbage mass of Bambatsi (January and October 2008) was highest (P &lt; 0.05) in this treatment. In Expt 2, 59 frosts occurred in 2007, and Floren, Bambatsi, and Swann sown in March of that year over-wintered poorly and had lower (P &lt; 0.05) herbage mass and plant frequency in the following spring compared with the other sowing times. These differences were also evident in autumn and spring 2008, up to 23 months after sowing.

Effects of depth and time of sowing and over-wintering on tropical perennial grass seedling emergence in northern New South Wales

Two studies were conducted in northern NSW to examine some of the factors affecting the successful emergence of seedlings of five species of tropical perennial grasses [Panicum coloratum var. makarikariense cv. Bambatsi (panic), Digitaria eriantha ssp. eriantha cv. Premier (digit), Chloris gayana cv. Katambora (Rhodes grass), Dichanthium aristatum cv. Floren (bluegrass), and Bothriochloa bladhii ssp. glabra cv. Swann (forest bluegrass)]. The first experiment investigated the effects of depth and time (month) of sowing on tropical perennial grass seedling emergence, while a second series of studies examined the effects of contact of dispersal/sowing units with the soil, particularly over winter, on subsequent seed germination and seedling emergence. No seedling emergence of any species occurred in September 2006 and June–August 2007. From October 2006 to May 2007 there were significant effects (P &lt; 0.001) of species, sowing time, depth, and their interaction on predicted emergence. From December to March, predicted emergence was always lower for surface-sown caryopses compared with those sown at 10 and 25 mm depth, and Bambatsi was the only species that had &gt;50% emergence from a depth of 50 mm. Recovery of intact caryopses from 300 μm nylon mesh bags stored in the soil for 6 and 12 months was low for all species. Similarly, predicted germination of caryopses from field soil surface and buried locations (2007 and 2008) and those from florets and coated seeds (2008) was low, indicating that these species had a poor ability to over-winter in moist soil.

A novel approach to planting grass - legume pastures in the mixed farming zone of southern inland Queensland, Australia

Reliable establishment of pasture systems with a desirable composition of grasses and legumes remains a challenge in the highly variable climates of the northern grain belt of southern inland Queensland. In this paper, an alternative system is proposed whereby legumes and grasses are planted simultaneously in alternative strips. A 4-year study tested the establishment, production, and botanical composition of the tropical legume species Macroptilium bracteatum cvv. Cardaga and Juanita, Lablab purpureus cv. Endurance, and Clitoria ternatea cv. Milgarra sown as pure stands or with grass strips. The grass strips, and a grass-only treatment, both contained a mix of Panicum maximum cv. Petrie, Dichanthium aristatum cv. Floren, and Bothriochloa insculpta cv. Bisset. L. purpureus was relatively unproductive, yielding ~2000 kg/ha dry matter (DM) produced in each of the first 2 years. M. bracteatum produced 2050 and 3300 kg/ha DM in Years 1 and 2, with declining plant populations and DM in subsequent years. C. ternatea produced 960 kg/ha DM in Year 1, 2730 kg/ha DM in Year 2, and continued to persist throughout the trial, albeit at low DM production levels. The grass-only treatment was dominated by the sown grass species, while the legume-based treatments were dominated by a colonising native species, Dichanthium sericeum, in the fourth and final year. Sowing grass strips adjacent to the legume areas proved a successful strategy, with the proportion of sown grasses in the legume strips increasing to &gt;20% of total DM by Year 4. Soil carbon changes did not differ between treatments, but total C in the top 0.15 m increased from 0.99% in Year 1 to 1.13% in Year 4, representing a net gain of 6.5 t/ha in C over 3 years.

Spatial and temporal variation of below-ground N transfer from a leguminous tree to an associated grass in an agroforestry system

Nitrogen (N) transfer from leguminous trees can be a major N source for the associated crop in low-input agroforestry systems. The aim of this study was to identify the main climatic and soil factors controlling N transfer from the leguminous tree Gliricidia sepium (Jacq.) Walp to the associated grass Dichanthium aristatum (Poir.) C.E. Hubb, in a 16-year-old tropical agroforestry system. Nitrogen transfer was estimated using the natural 15N abundance method. Before tree pruning, total N transfer represented 57% of the N uptake of the grass, including 31% coming from N 2 fixation. The spatial variation induced by the tree was well described by soil organic N content (ON). In this system, ON is an index of soil available N as well as of tree root density. Rainfall (R) and evapotranspiration (ETP) were the main climatic factors controlling N transfer. Multiple regression analysis indicated that R, ETP and ON explained 79% of the temporal and spatial variation of N transfer. Transferred N cannot be estimated after pruning because of the change in the isotopic signature of the soil N source. This was related to N release from root turnover. The results suggest that grass showed a preferential uptake of N coming from the tree, which could be due to a lower energy cost compared to obtaining absorbed N from the clayey soil used in this work.

Effect of sulfur supplements on cellulolytic rumen micro-organisms and microbial protein synthesis in cattle fed a high fibre diet

To examine the effect of sulfur-containing compounds on the growth of anaerobic rumen fungi and the fibrolytic rumen bacteria Ruminococcus albus, Ruminococcus flavefaciens and Fibrobacter succinogenes in pure culture and within the cattle rumen. The effect of two reduced sulfur compounds, 3-mercaptopropionic acid (MPA) or 3-mercapto-1-propanesulfonic acid as the sole S source on growth of pure fibroyltic fungal and bacterial cultures showed that these compounds were capable of sustaining growth. An in vivo trial was then conducted to determine the effect of sulfur supplements (MPA and sodium sulfate) on microbial population dynamics in cattle fed the roughage Dichanthium aristatum. Real-time PCR showed significant increases in fibrolytic bacterial and fungal populations when cattle were supplemented with these compounds. Sulfate supplementation leads to an increase in dry matter intake without a change in whole tract dry matter digestibility. Supplementation of low S-containing diets with either sodium sulfate or MPA stimulates microbial growth with an increase in rumen microbial protein supply to the animal. Through the use of real-time PCR monitoring, a better understanding of the effect of S supplementation on discrete microbial populations within the rumen is provided.

Nitrogen transfer from a legume tree to the associated grass estimated by the isotopic signature of tree root exudates: A comparison of the 15N leaf feeding and natural 15N abundance methods

Nitrogen (N) transfer from legume trees to associated crops is a key factor for the N economy of low-input agroforestry systems. In this work, we presented a new approach to estimate N transfer based on the 15N content of root exudates and N released by root turnover of the donor plant ( Gliricidia sepium) and the temporal change of the 15N content of the receiver plant ( Dichanthium aristatum). The study was carried out in greenhouse using two isotopic methods: 15N leaf feeding (LF) and the natural 15N abundance (NA). Measurements of exudate 15N were made at several dates before and after tree pruning. A time-dependent box model was devised to quantify N transfer in time and to make comparisons between the isotopic methods. In NA, although tree roots and exudates presented a similar 15N signature before tree pruning, exudates were strongly depleted in 15N after pruning. In LF, exudates were always depleted in 15N in relation to tree roots. Hence, the current assumption used in N transfer studies concerning the equal 15N/ 14N distribution in tissues of the donor plant and in its excreted N was not confirmed in our study. Before pruning, N transfer functioned as a two-N-source system (soil N and exudates N) and both isotopic methods provided similar estimates: 11–12% for LF and 10–15% for NA. Calculations performed with the model indicated that N transfer occurred with small or nil fractionation of 15N in exudates. After pruning, there was a third N source associated with N released from tree root turnover. During this period, the isotopic signature of the receiver plant showed a transient state due to the progressive decrease of 15N content of that N source. The amount of N derived from the tree represented 65% of the total N content of the grass at the end of the experiments.

Transfer of N fixed by a legume tree to the associated grass in a tropical silvopastoral system

Below-ground transfer of nitrogen (N) fixed by legume trees to associated non-N 2-fixing crops has received little attention in agroforestry, although the importance of below-ground interactions is shown in other ecosystems. We used 15N natural abundance to estimate N transfer from the legume tree Gliricidia sepium (Jacq.) Kunth ex Walp. to C 4 grass Dichanthium aristatum (Poir.) C.E. Hubb. in a silvopastoral system, where N was recycled exclusively by below-ground processes and N 2 fixation by G. sepium was the sole N input to the system. Finding a suitable reference plant, a grass without contact with tree roots or litter, was problematic because tree roots invaded adjacent grass monocrop plots and soil isotopic signature in soil below distant grass monocrops differed significantly from the agroforestry plots. Thus, we used grass cultivated under greenhouse conditions in pots filled with agroforestry soil as the reference. A model of soil 15N fractionation during N mineralization was developed for testing the reliability of that estimate. Experimental and theoretical results indicated that 9 months after greenhouse transplanting, the percentage of fixed N in the grass decreased from 35% to <1%, due to N export in cut grass and dilution of fixed N with N taken up from the soil. The effect of soil 15N fractionation on the estimate of the reference value was negligible. This indicates that potted grass is a suitable reference N transfer studies using 15N natural abundance. About one third of N in field-grown grass was of atmospheric origin in agroforestry plots and in adjacent D. aristatum grassland invaded by G. sepium roots. The concentration of fixed N was correlated with fine root density of G. sepium but not with soil isotopic signature. This suggests a direct N transfer from trees to grass, e.g. via root exudates or common mycorrhizal networks.

Role of root inputs from a dinitrogen-fixing tree in soil carbon and nitrogen sequestration in a tropical agroforestry system

Agroforestry is often mentioned as a suitable technology for land rehabilitation in the tropics and for mitigation of climate change because this land-use favours nutrient recycling and C sequestration. The aim of this work was to estimate soil C sequestration in a 12-year-old tropical silvopastoral system composed of a legume tree (Gliricidia sepium) and a C4 fodder grass (Dichanthium aristatum), and to link it with tree root biomass and N status in the soil. The site was under cut-and-carry management, i.e. tree pruning residues and cut grass were removed from the field and fed to stabled animals elsewhere. Thus, main sources for tree C and N inputs were root activity and turnover. Organic C derived from the trees and tree root biomass were determined based on natural 13C abundance. For the 0–0.2 m soil layer, the biomass of tree roots ≤2 mm diameter was 2.4 Mg/ha when the trees were pruned every 6 months (SS6), and 0.6 Mg/ha when pruned every 2 months (SS2). Both C (R2 = 0.39, P &lt; 0.05) and N (R2 = 0.82, P &lt; 0.05) sequestration were correlated with tree root biomass. The trees and grass contributed 18 and 8 Mg C/ha to soil, respectively, over the 12-year experiment in SS6. The net increase of 2.5 Mg N/ha in soil, originating from the trees, contributed to the net soil C sequestration. In SS2, trees contributed 16 Mg C/ha to soil over 12 years, but grass-derived C was reduced by 2 Mg C/ha because of the small amount of grass litter. The increase of 1.7 Mg N/ha in soil, derived from the trees, was not large enough to avoid C loss in this plot. Differences in soil C and N sequestration between plots were due to differences in system management, which affected the amount and the C/N ratio of inputs and outputs.