Soil Feeders Research Articles

Termite (Insecta: Isoptera) Species Composition in a Primary Rain Forest and Agroforests in Central Amazonia

ABSTRACTTermites play important roles in organic matter decomposition, nutrient cycling, and soil structure in tropical rain forests. When forests are replaced by agriculture, termite species richness, abundance, and function often decline. We compared the termite assemblage of a primary forest site with that of a low plant diversity, palm‐based agroforest (five plant species) and a high plant diversity, home‐garden agroforest (10 plant species) using a rapid biodiversity assessment protocol. In comparing the primary forest termite species composition to previously published studies, we found soil feeders and the Apicotermitinae to be more dominant than previously reported in Amazonia. Thirty percent of the species belonged to the Apicotermitinae, and an unusually high percentage (57%) of species were soil feeders. Unexpectedly, the palm‐based agroforest, despite its lower plant diversity, was closer to primary forest in termite species composition, rate of species accumulation, and proportions of species in taxonomic and functional classes than was the home‐garden agroforest. This suggests that particular plant attributes may better determine the termite assemblage than plant diversity alone in these agroecosystems. Unlike other agroecosystems reported in the literature, Apicotermitinae and soil feeders were proportionally more abundant in these agroforests than in primary forest. The ability of agroforests to support populations of soil feeders has a potentially positive effect on soil fertility in these agroecosystems; insomuch as feeding guild is a proxy for function, these closed‐canopy agroforests may be able to sustain the same termite‐mediated functions as primary forest.

Abundance and diversity of termites in a savanna woodland reserve in tropical Australia

AbstractThis study contributes to the paucity of knowledge on the structure of Australian tropical savanna termite assemblages, compared with that available for tropical savannas elsewhere in the world. Termite species diversity, frequency of occurrence and functional diversity in Charles Darwin National Park, a savanna woodland reserve in tropical northern Australia, were assessed. In four 50 × 50 m sites, a direct search and bait sampling protocol was used to sample mounds, standing and lying dead wood, litter, soil and arboreal nest microhabitats for termites between April and June 2003. Results were compared with a priori predictions of termite community structure based on the work by Braithwaite et al. (1988). Sixteen species from nine genera and three families (Mastotermitidae, Rhinotermitidae and Termitidae) were recorded, including the first record of Ephelotermes taylori (Hill) from open savanna woodland habitat and the first official record of Macrognathotermes errator Miller from north Australian savanna. The family Termitidae represented 69% of termite occurrences. From this family, the wood/soil interface feeder Nasutitermes eucalypti (Mjöberg) represented 30% of encounters. High levels of termite activity were recorded at baits within 6 weeks. Four nesting habits were represented: epigeal (mounds above ground), hypogeal (subterranean nests), nests within wood and arboreal nests. Termites were most frequently encountered in epigeal mounds and lying dead wood. Wood and wood/soil interface feeders were the most common. The findings partially supported the a priori predictions proposed. The termite assemblage structure was represented by species commonly found in other Northern Territory savanna woodland habitats. However, species diversity was lower; soil feeders and live wood feeders were less common and grass‐harvesting species were absent. Soil nitrogen and phosphorus and species richness and patchiness of ground vegetation may contribute to these differences. The lower species diversity recorded contrasts with the richer termite faunas of tropical savannas recorded elsewhere.

Ascomycete diversity in soil-feeding termite nests and soils from a tropical rainforest.

Molecular microbial ecology has revealed remarkable biodiversity - prokaryotic and eukaryotic - in numerous soil environments. However, no culture-independent surveys of the termitosphere exists, although termites dominate tropical rainforests. Here, we focused on soil feeders, building nests with their soil-born faeces, enriched with clay-organic complexes, thus contributing to the improvement of soil fertility. In order to assess the fungal community composition of these termitaries compared with soils not foraged by termites, samples of the two types were collected in the Lopé rainforest, Gabon, and processed for generation of fungal internal transcribed spacer (ITS) clone libraries. Although primers were universal, most of the recovered sequences represented Ascomycete that were previously uncharacterized and the proportions of which reached 72.5% in soils and 80% in termitaries. Their affiliation with identified fungi was analysed in performing a phylogenetic tree based on 5.8S rDNA. Furthermore, the ascomycete communities of soil-feeding termitaries and soils shared only 6.3% of sequences. This discrepancy of composition between soil and nest may result from the building behaviour of termites, as the organic matter in the nest is chemically modified, and some vacant ecological microniches are available for more specialized fungi.

Dual stable isotope analysis (δ 13C and δ 15N) of soil invertebrates and their food sources

More research is required to validate and refine natural abundance stable isotope ratio techniques as a tool for the investigation of the feeding ecology of soil animals and trophic relations in soil food webs. Isotope ratios of C (δ 13C) and N (δ 15N) were measured in herbivorous and detritivorous invertebrate groups, namely lumbricid earthworms (7 species), enchytraeid worms (3 species), slugs (3 taxa), and their potential food sources in an arable system. Intrapopulation δ 15N variation in the slug Deroceras reticulatum ( n=52) was large (range 4.2‰), possibly reflecting spatial variability in the food sources. Significant correlations between C:N ratios and isotope ratios in earthworms suggest that factors other than feeding may influence isotopic patterns. One enchytraeid species, Enchytraeus buchholzi, was enriched in 13C and strongly depleted in 15N compared to all other groups. Invertebrates formed a continuum when considered in relation to C and N separately, but fell into two distinct groups on the basis of combined C and N isotope ratios. The less enriched group represents herbivorous and litter-feeding species, while the more enriched group represents soil feeders. It is concluded that δ 13C measurements could provide a means of assigning separate baseline δ 15N values to primary and secondary decomposers, which in turn could improve the inference of higher trophic levels, omnivory and intraguild predation.

Evolution of termite functional diversity: analysis and synthesis of local ecological and regional influences on local species richness

AbstractAim To (1) describe termite functional diversity patterns across five tropical regions using local species richness sampling of standardized areas of habitat; (2) assess the relative importance of environmental factors operating at different spatial and temporal scales in influencing variation in species representation within feeding groups and functional taxonomic groups across the tropics; (3) achieve a synthesis to explain the observed patterns of convergence and divergence in termite functional diversity that draws on termite ecological and biogeographical evidence to‐date, as well as the latest evidence for the evolutionary and distributional history of tropical rain forests.Location Pantropical.Methods A pantropical termite species richness data set was obtained through sampling of eighty‐seven standardized local termite diversity transects from twenty‐nine locations across five tropical regions. Local‐scale, intermediate‐scale and large‐scale environmental data were collected for each transect. Standardized termite assemblage and environmental data were analysed at the levels of whole assemblages and feeding groups (using components of variance analysis) and at the level of functional taxonomic groups (using correspondence analysis and canonical correspondence analysis).Results Overall species richness of local assemblages showed a greater component of variation attributable to local habitat disturbance level than to region. However, an analysis accounting for species richness across termite feeding groups indicated a much larger component of variation attributable to region. Mean local assemblage body size also showed the greater overall significance of region compared with habitat type in influencing variation. Ordination of functional taxonomic group data revealed a primary gradient of variation corresponding to rank order of species richness within sites and to mean local species richness within regions. The latter was in the order: Africa > south America > south‐east Asia > Madagascar > Australia. This primary gradient of species richness decrease can be explained by a decrease in species richness of less dispersive functional taxonomic groups feeding on more humified food substrates such as soil. Hence, the transects from more depauperate sites/regions were dominated by more dispersive functional taxonomic groups feeding on less humified food substrates such as dead wood. Direct gradient analysis indicated that ‘region’ and other large‐scale factors were the most important in explaining patterns of local termite functional diversity followed by intermediate‐scale geographical and site variables and, finally, local‐scale ecological variables.Synthesis and main conclusions Within regions, centres of termite functional diversity lie in lowland equatorial closed canopy tropical forests. Soil feeding termite evolution further down food substrate humification gradients is therefore more likely to have depended on the long‐term presence of this habitat. Known ecological and energetic constraints upon contemporary soil feeders lend support for this hypothesis. We propose further that the anomalous distribution of termite soil feeder species richness is partly explained by their generally very poor dispersal abilities across oceans. Evolution, radiation and dispersal of soil feeder diversity appears to have been largely restricted to what are now the African and south American regions. The inter‐regional differences in contemporary local patterns of termite species richness revealed by the global data set point to the possibility of large differences in consequent ecosystem processes in apparently similar habitats on different continents.

Feeding group responses of a Neotropical termite assemblage to rain forest fragmentation.

Biomass collapse and its associated microclimatic stresses within recently isolated rain forest fragments may negatively affect species diversity of most resident taxa. However, for some decomposer organisms, increased resource availability via accompanying tree die-off may effect positive responses, at least for a time, with implications for rates of nutrient cycling and greenhouse gas release. This study investigates the early effects of forest fragmentation on a Neotropical termite assemblage. Numbers of encounters (surrogate for relative abundance) and species richness of wood and leaf-litter feeders, soil feeders, and the whole assemblage, were studied across true forest islands and mainland sites at a hydroelectric reservoir in French Guiana. Results showed no overall effect of fragmentation on either total termite encounters or species richness. However, numbers of encounters and species richness of wood and leaf-litter feeders showed positive responses to forest fragmentation. By contrast, soil feeders showed a negative response for numbers of encounters and no significant effect for species richness. Environmental data suggest that increased tree die-off, and other edge effects associated with biomass collapse, were underway at the time of sampling. Resulting increase in resource availability may therefore explain the positive influence on wood and leaf-litter feeders. A possible decrease in predation pressure from ants with decrease in island size was not tested for, but was a likely effect of the flooded matrix habitat. Fragmentation effects on soil feeder encounters may be due to the energetic and microclimatic constraints of feeding lower down the humification gradient of termite food substrates, but were not sufficient to affect species richness. The patterns revealed suggest that rates of wood decomposition following tree die-off, and of soil nutrient cycling, under different rain forest fragmentation scenarios, merit further study.

Termite diversity across an anthropogenic disturbance gradient in the humid forest zone of West Africa

Data are presented for termite assemblages across an anthropogenic disturbance gradient in the humid forest zone of West and Central Africa. Sampling was by standardised 100 m×2 m transects in: primary forest, several ages of regenerating forest, agroforestry plots, short fallows, mixed food crop fields, and mechanically cleared plots. Most sites were in southern Cameroon—two additional transects were conducted in primary forest in Congo (Brazzaville). Species richness was negatively correlated with the disturbance gradient, although transects in areas with a complete or near-complete canopy were broadly similar in species richness. The strongest negative correlation was found for termite species feeding furthest down the humification gradient (group IV soil feeders) probably because these species are energetically constrained from living in non-forest habitats. In contrast species feeding and nesting in dead wood (group I wood feeders) termites, which live in the most protected micro-habitats, were significantly positively correlated with the disturbance gradient. Species composition was also strongly correlated with disturbance level in multivariate redundancy analyses. Pest species were an insignificant element of the assemblage, in part because of the distance of the sites from savanna source pools. Overall, it appears that termite-mediated wood and litter decomposition may be relatively unaffected by light to moderate disturbance intensity, but that soil-conditioning by termites might be greatly reduced in agricultural land cleared from tropical forest.

The termites of the Mayombe Forest Reserve, Congo (Brazzaville): transect sampling reveals an extremely high diversity of ground-nesting soil feeders

Sampling using a replicated standard transect method in an extensive undisturbed primary hill rain forest in Congo (Brazzaville) revealed a very high species richness of termites, especially soil-feeding soldierless termites (Apicotermitinae). The assemblage, as estimated by the transects, resembled that previously characterized in comparable forest in southern Cameroon, but with some species turnover reflecting the gamma diversity of the Guinean-Congolese block as a whole. Species richness was three or four times that shown by a previous study addressed to mound populations alone, emphasizing the importance of sampling soil-feeding termites neither building nor living in mounds.

Use of RAPD markers for the study of microbial community similarity from termite mounds and tropical soils

In this study, we test the use of the RAPD (Random Amplified Polymorphic DNA) molecular markers as a way to estimate the similarity of the microbial communities in various termite mounds and soils. In tropical ecosystems, termite activities induce changes in the chemical and physical properties of soil. The question then arises as to whether or not termites affect the presence of natural microbial communities. Successful 16S rDNA amplifications provided evidence of the occurrence of bacterial DNA in termite constructions including both soil feeder and fungus grower materials. A phenetic dendrogram using the similarity distance calculated from pairwise data including 88 polymorphic RAPD markers was reconstructed and bootstrap scores mapped. The microbial communities of the mounds of the four soil-feeding termites were clustered in the same clade, while those of the mounds of the fungus-growing species were distinct like those of control soils. Microbial changes in nests result from termite building behavior, depending on whether they include feces in their constructions for soil-feeders or use saliva as particle cement for fungus-growers. It is argued that RAPDs are useful markers to detect differences in microbial community structure not only between termitaries and control soils but also between mounds of soil-feeders.

Earthworm [formula omitted] and [formula omitted] analyses suggest that putative functional classifications of earthworms are site-specific and may also indicate habitat diversity

Natural abundances of the stable isotope pairs 13C/ 12C and 15N/ 14N ( δ 13 C and δ 15 N ) were measured from earthworms sampled from six sites with contrasting habitats (deciduous and coniferous woodland, arable and permanent pasture). Knowledge about the function of earthworms is important to the understanding of their ecology. The hypothesis, that endogeic (primarily soil and organic matter feeders) and epigeic (surface litter feeders, ingesting little or no soil) earthworms would be isotopically distinct and that isotopic values for anecic (surface litter and soil feeders) earthworms would fall between the other two groups based on their feeding strategies, was rejected. Earthworm δ 13 C and δ 15 N values from six sites indicated that classifying earthworms into the functional groups epigeic, anecic and endogeic is site-dependent. In contrast, δ values clearly separated earthworms into humic formers and humic feeders. Average 13C-enrichment (3.9‰) between earthworm and putative dietary source (vegetation) across all sites was larger than the typically reported enrichment (1‰) between a single trophic level suggesting that earthworms, as expected, derive nutrition from a number of sources, not just living vegetation. Enrichments of 13C and 15N in earthworms, relative to diet, could be developed as a tool for assessing habitat diversity.

Successional response of a tropical forest termite assemblage to experimental habitat perturbation

Summary 1. Research into the successional responses of tropical forest communities following disturbance has potential applications for habitat restoration. Currently little is known of how these responses relate to the recovery of biodiversity and ecosystem processes. Succession of assemblages of decomposer arthropods is essential for the recovery of the soil community and nutrient cycling processes. 2. This study investigated the successional response of a termite assemblage to the experimental perturbation of forest habitat in southern Cameroon, examining the implications for tropical forest restoration. A randomized block design consisting of four experimental perturbations of differing severity was established in an old secondary forest in the Mbalmayo Forest Reserve. Isolated control sites were left in undisturbed forest. Recovery of the termite assemblage was assessed by measuring termite species richness and abundance at regular intervals over the subsequent 12 months. 3. The speed of recovery of the termite assemblage varied with the type and extent of perturbation. In treatments involving severe soil and canopy disturbance, termite species richness and abundance recovered more rapidly when dead wood was left on the ground following perturbation. The availability of dead wood also resulted in recolonization by a subset of the termite assemblage that was distinct compositionally from that sampled from all other treatments. This subset at sites with additional dead wood included not only certain wood‐feeding species, but also soil feeders. 4. The positive effects upon the termite assemblage of leaving substantial dead wood on the ground has implications for the restoration of tropical forests following human‐induced disturbances such as logging. The accelerated recovery of termite diversity and assemblage composition is a significant component of soil community recovery and the restoration of nutrient cycles. These benefits are expected to influence soil fertility and, ultimately, forest regeneration. The duration and persistence of these effects will depend crucially on the type, scale and intensity of the original disturbance. The impact of termites on soil properties, and vice versa, clearly deserves more attention in studies of tropical forest regeneration and recovery.

Methane and hydrogen production in a termite‐symbiont system

Methane and hydrogen emission rates and the δ13C of CH4 were observed for various termites in Australia, Thailand and Japan. Combined with the already reported emission rates of CH4 in the literature, the phylogenetic trend was examined. Emission rates of the observed termites were categorized into five groups: group I with high CH4 and low H2 emission rates with a CH4/H2 ratio of typically 10/1; group II with high CH4 and high H2 emissions with a CH4/H2 ratio of 4/1–1/2; group III with low emission rates of CH4 and H2; group IV with high H2 and insignificant CH4 emissions; and group V with insignificant emissions for both CH4 and H2. In lower termites, there are both colonies infected and uninfected with methanogens even in the same species, and no specific trend in CH4 and H2 emissions was observed within a genus. Whether protozoa in the hindgut of termites are infected with methanogens or not and the differences in species compositions of protozoa are possibly responsible for the inter‐colonial variations. The proportions of infected colonies were possibly small for the family Kalotermitidae (dry wood feeders), and relatively large for families of wet or damp wood feeders. The hydrogen emission rate possibly depends on the locality of methanogens: namely, whether they are intracellular symbionts of protozoa or whether they are attached to the hindgut wall. Emission rates of higher termites were classified into groups according to genera and the diet. Most species of soil or wood/soil interface feeders classified into group I, while the soil feeders Dicuspiditermes in Thailand and Amitermes in Australia were classified into groups with high H2 emission rates. Typical wood‐feeding termites and fungus‐growing termites were classified into group III. The results indicate that higher termites tend to increase the CH4 emission rate during dietary evolution from wood‐ to soil‐feeding, and two types of the system with different efficiencies of interspecies transfer of H2 have been formed. The δ13C of CH4 was discernible with a difference in the decomposition process in the termite–symbiont system among lower termites, fungus‐growing termites and other higher termites.

Body Size and Energy Use in Termites (Isoptera): The Responses of Soil Feeders and Wood Feeders Differ in a Tropical Forest Assemblage

Studies of log body size/log energy use in local animal assemblages reveal a range of distributional patterns from those with a negative upper bound slope (NUBS), showing energy use to be dominated by smaller bodied species, to those with a positive upper bound slope (PUBS), indicating that larger bodied species dominate energy use. We investigated the log body size/log energy use relationships for a local termite assemblage at the Mbalmayo Forest Reserve in Cameroon, and compared the patterns for wood and soil feeding groups. We measured the biomass densities of 75 species of termites across three forest sites. Termite species metabolic rates were derived either directly (25 species), or by regression estimates (50 species), from Warburg manometry measurements of termite xO 2 at 28°C. The combined data from the three sites revealed a marked difference in log body size/log energy use distributions between wood and soil feeding groups. While wood feeders showed a polygonal distribution, with a negative upper bound slope (NUBS), soil feeders showed a (statistically very significant) positive distribution. The data for soil feeders are also exceptional, in comparison with the vast majority of other animal assemblage data, in showing significantly greater 'total' energy use for larger, compared with smaller, body size categories. Furthermore, the modal body size category for number of individual termites does not coincide with that for number of termite species. We interpret these results in the context of the likely constraints imposed by the food substrates of termites and the economies of scale. Specifically, soil feeders utilise a low energy, universally abundant substrate compared with wood feeders which feed on a patchily distributed, higher energy resource. Digestive efficiency is likely to be at a premium in soil feeders, driving selection towards larger body size (and hence larger gut volume and transit time), compared with wood feeders.

PH profiles of the extremely alkaline hindguts of soil-feeding termites (Isoptera: Termitidae) determined with microelectrodes

In three of the four subfamilies of higher termites, the anterior hindgut is characterized by an unusually high pH. This phenomenon is especially prominent among the soil (humus) feeders, which represent more than half of all known termite species, but whose digestive physiology is still largely obscure. In this study, we used microelectrodes to determine axial pH profiles in intact guts of the soil-feeding termites Thoracotermes macrothorax, Crenetermes albotarsalis, Noditermes indoensis and two Cubitermes species (all Termitidae: Termitinae). In all species, the pH of the gut contents increased sharply from circumneutral in the midgut to highly alkaline between the midgut-hindgut junction and the first proctodeal dilation (P1). This location coincided exactly with the extent of the mixed segment, a morphologically unique gut region found exclusively among higher termites. The most alkaline pH values occurred in the P1 (pH 11–12.5) and were equal to the highest values ever encountered in biological systems. The second proctodeal dilation (P3) was also alkaline (pH >10), and only in the posterior part of the following segment (P4b) did the gut contents regain a pH close to neutrality. The rectum (P5) was slightly acidic, as was the foregut. Sharp drops of 1–3 pH units were observed between the major gut regions, most notably between P1 and P3, i.e. across the short enteric valve (P2), and also along the P4a. Individual profiles within a species were highly reproducible, as were species-specific differences. The high-resolution profiles presented here show that guts of soil-feeding termites are even more alkaline than reported previously, and they provide sorely needed basic information on microenvironments existing within guts of an extremely important group of terrestrial humivores.

Roles of oxygen and the intestinal microflora in the metabolism of lignin-derived phenylpropanoids and other monoaromatic compounds by termites

Prompted by our limited understanding of the degradation of lignin and lignin-derived aromatic metabolites in termites, we studied the metabolism of monoaromatic model compounds by termites and their gut microflora. Feeding trials performed with [ring-U-(sup14)C]benzoic acid and [ring-U-(sup14)C]cinnamic acid revealed the general ability of termites of the major feeding guilds (wood and soil feeders and fungus cultivators) to mineralize the aromatic nucleus. Up to 70% of the radioactive label was released as (sup14)CO(inf2); the remainder was more or less equally distributed among termite bodies, gut contents, and feces. Gut homogenates of the wood-feeding termites Nasutitermes lujae (Wasmann) and Reticulitermes flavipes (Kollar) mineralized ring-labeled benzoic or cinnamic acid only if oxygen was present. In the absence of oxygen, benzoate was not attacked, and cinnamate was only reduced to phenylpropionate. Similar results were obtained with other, nonlabeled lignin-related phenylpropanoids (ferulic, 3,4-dihydroxycinnamic, and 4-hydroxycinnamic acids), whose ring moieties underwent degradation only if oxygen was present. Under anoxic conditions, the substrates were merely modified (by side chain reduction and demethylation), and this modification occurred at the same time as a net accumulation of phenylpropanoids formed endogenously in the gut homogenate, a phenomenon not observed under oxic conditions. Enumeration by the most-probable-number technique revealed that each N. lujae gut contained about 10(sup5) bacteria that were capable of completely mineralizing aromatic substrates in the presence of oxygen (about 10(sup8) bacteria per ml). In the absence of oxygen, small numbers of ring-modifying microorganisms were found (<50 bacteria per gut), but none of these microorganisms were capable of ring cleavage. Similar results were obtained with gut homogenates of R. flavipes, except that a larger number of anaerobic ring-modifying microorganisms was present (>5 x 10(sup3) bacteria per gut). Neither inclusion of potential cosubstrates (H(inf2), pyruvate, lactate) nor inclusion of hydrogenotrophic partner organisms resulted in anoxic ring cleavage in most-probable-number tubes prepared with gut homogenates of either termite. The oxygen dependence of aromatic ring cleavage by the termite gut microbiota is consistent with the presence, and uptake by microbes, of O(inf2) in the peripheral region of otherwise anoxic gut lumina (as reported in the accompanying paper [A. Brune, D. Emerson, and J. A. Breznak, Appl. Environ. Microbiol. 61:2681-2687, 1995]). Taken together, our results indicate that microbial degradation of plant aromatic compounds can occur in termite guts and may contribute to the carbon and energy requirement of the host.

A New Carpet Soiling Test

With the addition of a soil feeder, a laboratory carpet walker has been adapted to provide controlled soiling to evaluate carpet soil resistance. The degree of soiling is measured with a Hunter ColorQuest spectrocolorimeter.

Guild distribution of some termites from cerrado vegetation in south-east Brazil

ABSTRACTThe distributional patterns and food preferences of some cerrado vegetation termites were studied in an area of 2500 m2in south-east Brazil. All the termitaria were mapped and opened for identification of the species present. Their spatial distribution was studied using the nearest neighbour method (Clark &amp; Evans 1954). We found 46 termite species in 450 nests. The majority of them were wood feeders. The general distribution of termitaria was regular. The distributional patterns of grass and litter feeders were not significantly different from a random distribution. Soil and organic matter feeders were contagiously distributed. The distribution of wood feeders was also contagious. Their spatial distribution was largely influenced by the distribution of the speciesNasutitermessp. 1 andArmitermes euamignathus. These two species occurred in two distinct patches, suggesting resource partitioning.

Influence of reduced rates of water and fertilizer application using daily intermittent drip irrigation on the water requirements, root development and responses of avocado trees (cv. Fuerte)

SummaryAvocado trees (cv. Fuerte) were drip irrigated daily at two rates of intermittent water application, linked to daily evaporation measured with an U.S. Weather Bureau Class A standard evaporation pan. The pan coefficients used were 0.46 (‘dry treatment’) and 0.64 (‘wet treatment’). These were compared with a third treatment of sprinkling every four days, using a pan coefficient of 0.64. In the dry treatment, soil was maintained for extended periods at very low water content, with little change even during irrigation. Percolation to soil deeper than 90 cm ceased in both treatments by 1981. Roots were densest in the dry treatment, particularly with feeder roots of diameter smaller than 2 mm: in the upper 60 cm of soil feeder roots were 45% denser than in the wet treatment, 52% denser than in the sprinkling treatment. In the zone with the greatest potential for over-wet conditions (less than 10 cm from the dripper in the top 30 cm of soil) roots were denser than in any location in the other treatments,...

Toxic nitrate accumulation in the sorghums

The sorghums are valuable crop plants to the cattle industry. Their use as forages is varied and widespread throughout the United States. As pasture they may become hazardous to grazing ruminants if toxic accumulations of cyanide and/or nitrates exist. As hay the major toxic threat is excessive nitrates since cyanide is readily lost as gaseous HCN during the curing process.&#x0D; The sorghum plant is a hardy soil feeder which allows for tremendous dry matter production in a relatively short period of time. This feature also allows for excessive nitrate accumulations when there is a luxury of soil nitrogen or if the plant becomes stressed. An understanding of sorghum culture and the plant’s growth processes is essential to the prevention and management of toxic nitrate accumulations.