In vitro digestibility and crude protein and mineral concentrations of guinea grass accessions in a humid tropical region of Puerto rico

Abstract

The in vitro organic matter digestibility (IVOMD), crude protein (CP) content and mineral composition of ten Panicum maximum accessions were compared between seasons in Puerto Rico and evaluated in relation to grazing ruminants requirements. On short days, PRPI 13605 was superior (P<0.05) in IVOMD to all the other accessions. In terms of the CP content, accession PRPI 3634 was above 11% in both seasons, however, without differing (P>0.05) from most accessions. The mean macroelement content of the accessions was generally adequate in relation to grazing ruminant requirements with the exception of Na and Mg. Phosphorus was less than the critical concentration (0.25%) for one‐half of the accessions in the short day season and 2 of 10 for the long day season. For all samples, Mn was high and Fe was generally adequate in relation to ruminant requirements. Most all samples contained deficient concentrations of Co, Cu, Se, and Zn. Forage Mo concentrations, therefore, did not result in conditioned Cu deficiencies. In this context, it is probable that Na, Se, Cu, Co, Zn, and perhaps Mg, will be needed even if intensive fertilization is practiced in the humid tropical region of Puerto Rico.     

In Vitro digestibility,crude protein,and mineral concentrations of Leucaena leucocephala accessions in a wet/dry tropical region of venezuela

Abstract

An experiment was conducted on a commercial farm located in the western part of Venezuela (10.5°N and 72°W; mean annual rainfall of 1000 mm; mean annual temperature of 28°C; sandy‐loam Alfisol with pH of 5.5). The purpose of the experiment was to evaluate the in vitro organic matter digestibility (IVOMD), crude protein (CP) content and mineral composition of four Leucaena leucocephala (Lam.) De Wit accessions under rotational grazing by heifers over a 6‐month period covering dry and rainy seasons, using a split‐plot experimental design with two replications. Neither accessions nor the accession x season interaction affected (P>0.05) any of the variables. The mean IVOMD was 68.6%, whereas CP content during the rainy season (26.5%) was higher (P<0.05) than in the dry season (24.3%). Average mineral content of the accessions were adequate in relation to grazing ruminant requirements with the exception of phosphorus [(P) 0.13%], sodium [(Na) 0.038%], copper [(Cu) 6.9 ppm], and zinc [(Zn) 19.7 ppm]. Forage P concentration may have been influenced by the low soil P content of the experimental site. The mean forage Ca:P ratio (11.3:1) was considerably wider than desirable. During the dry season, ash content increased (P<0.05), Na, iron (Fe), and cobalt (Co) decreased (P<0.05), but Fe and Co still remained above the critical levels. Mean concentrations of other elements were not affected (P>0.05) by season. Forage molybdenum (Mo) concentrations were low and, therefore, would not result in conditioned Cu deficiency. The four L. leucocephala accessions had similar feeding value for grazing ruminants and their quality was not markedly reduced in the dry season. Mineral supplementation of ruminants grazing this legume may be needed to correct specific deficiencies and imbalances.     

Yield,in vitro digestibility,crude protein,and mineral concentration of eight Digitaria introductions in Puerto rico

Abstract

Dry matter (DM) yield, in vitro organic matter digestibility (IVOMD), crude protein (CP) and mineral composition of eight Digitaria accessions were compared between long‐ and short‐day seasons in the semiarid southern region of Puerto Rico and evaluated in relation to grazing ruminant requirements. Digitaria milanjiana 6416 was consistently among the highest DM yielding accessions in both seasons. The mean IVOMD was similar in both seasons for most accessions. Concentrations of CP, magnesium (Mg), copper (Cu), zinc (Zn), cobalt (Co), and selenium (Se) were below the recommended levels for grazing ruminants in both seasons. This study suggests the need for livestock mineral supplementation, even under conditions of high pasture fertilization (NPK) and forage‐growing conditions in the semiarid southern region of Puerto Rico.     

Protozoa from aboveground and ground soils of a tropical rain forest in Puerto Rico

Decomposition occurs in the aboveground and ground litter and soils of tropical rain forests, but little is known about the protozoa that stimulate bacterial activity and turnover. I examined litter and ground soils, epiphytic bryophyte soils on tree trunks and branches, and adventitious roots of lianas attached to tree trunks, within 2 m above ground in the Luquillo Experimental Forest, within the Caribbean National Forest, Puerto Rico. Amoebae numbered 69,000–170,000, ciliates 1000–25,000, and testate amoebae 58,000–190,000 g⁻¹ dry wt. of litter, but were reduced by 0.25–0.5 of these abundances in the underlying soils. In the aboveground soils, amoebae numbered 64,000–145,000, ciliates 1000–8000, and testate amoebae 84,000–367,000 g⁻¹ dry wt. of soil. Eighty species of ciliates and 104 species of testate amoebae were found. About 50% of the individuals in ciliate and 33% in testate amoebae populations were small r-selected species, illustrating that functional differences between species determine community composition. Although protozoan numbers are best described as “protozoan potential” because many individuals may be dormant, the high moisture content of tropical rain forest litter and soils suggest almost continually connected soil water films (necessary for protozoan transport), and together with the large numbers and biodiversity of protozoa, suggest that a major proportion of these protozoa contribute to the bacterial decomposition channel of organic matter.     

Correlation between earthworms and plant litter decomposition in a tropical wet forest of Puerto Rico

Earthworms are recognized to play an important role in the decomposition of organic materials. To test the use of earthworms as an indicator of plant litter decomposition, we examined the abundance and biomass of earthworms in relation to plant litter decomposition in a tropical wet forest of Puerto Rico. We collected earthworms at 0–0.1 m and 0.1–0.25 m soil depths from upland and riparian sites that represent the natural variation in soils and decomposition rates within the forest. Earthworms were hand-sorted and weighed for both fresh and dry biomass. Earthworms were dominated by the exotic endogeic species Pontoscolex corethrurus Müller; they were more abundant, and had higher biomasses in the upland than in riparian sites of the forest. Plant leaf litter decomposed faster in the upland than riparian sites. We found that earthworm abundance in the upper 0.1 m of the soil profile positively correlated with decomposition rate of plant leaf litter. Ground litter removal had no effect on the abundance or biomass of endogeic earthworms. Our data suggest that earthworms can be used to predict decomposition rates of plant litter in the tropical wet forest, and that the decomposition of aboveground plant litter has little influence on the abundance and biomass of endogeic earthworms.     

Mineral concentrations of forages and soils in Benue State,Nigeria. I. Macrominerals and forage in vitro organic matter digestibility and crude protein concentrations

Abstract

Macromineral composition of different forage species and soils and forage in vitro organic matter digestibility (IVOMD) and crude protein concentrations in three agricultural zones of Benue State were studied. The zones consisted of Tiv zone (Zone A), Igala zone (Zone B), and Idoma zone (Zone C), each of which was made up of three sites. Site I was Fulani cattle migration routes across the state, site II represented grazing sites within home proximity, and site III were grazing reserves. Grazing animals were followed and forages corresponding to that consumed by them, and the corresponding soil samples were collected during the peak of the dry season (December‐January) and analyzed for nutrient composition. Forage IVOMD ranged from 21% to 79%, with values obtained in grass being lowest due to pronounced overmaturity. Only 10% of forage samples (exclusively from grasses), showed protein levels below the critical value of 7%. Most forage samples, as well as their corresponding soil samples were sufficient in calcium (Ca), magnesium (Mg), and potassium (K) concentrations. Soil concentrations of Ca, Mg, and K were particularly high compared to critical requirements in all the zones studied. Approximately 94% and 76% of forages were deficient in phosphorus (P) and sodium (Na), respectively, and the deficiencies were similar in all classes of forage and sites of study for each of the zones investigated. Results of forage‐soil relationships indicated low and nonexisting correlations; therefore soil analyzes are not of great importance in the assessment of available macromineral supplies to grazing livestock. The macrominerals most needed for supplementation of grazing livestock during the dry season are P, Na, and K.     

Applications of mineral nitrogen increase the yield and content of crude protein in narrow-leaf lupin seeds

Abstract

In a plant pot experiment with non-inoculated seed stock of narrow-leaf lupin (Lupinus angustifolius, L.) of the variety ‘Prima’ we explored the effects of nitrogen compounds applied in the form of NH₄NO₃ on different dates (before sowing, after emergence and during flowering) and at various levels (0–0.5–2–3 g N per pot) on changes in the number of pods per plant, the 1000-seed weight, seed yields and the content of crude protein in seeds. The experiment included four treatments; the first was a control treatment not fertilized with nitrogen. In the other treatments we applied nitrogen either as a single application before sowing or in three split applications. In all the nitrogen-fertilized variants the number of pods per plant, total seed yields per plant and the concentration of crude protein all increased compared to the unfertilized control variant. The highest levels of nitrogen (3 g N per pot) reduced the 1000-seed weight compared to the unfertilized control variant. The total application of 2 g N per pot split into application before sowing (0.5 g N), after emergence (0.5 g N) and during flowering (1 g N) resulted in a highly significant increase in the number of pods per plant (+78%), the 1000-seed weight (+20.1%) and therefore higher seed yields per plant (+139.8%) compared to the control treatment not fertilized with nitrogen. Increasing levels of nitrogen linearly increased the content of crude protein in seeds during harvest from 24.2% to 40.6%, thus increasing the nutritional quality of the seeds.     

Interactive effects of native and exotic earthworms on resource use and nutrient mineralization in a tropical wet forest soil of Puerto Rico

Investigation of single or mixed assemblages of native Estherella sp. and exotic Pontoscolex corethrurus from a rain forest in Puerto Rico was undertaken to understand resource use patterns, and linkages with C and N mineralization in a 19-day incubation. Resource use was explored with addition of ¹⁵N-enriched leaf litter and ¹³C-enriched glucose to reconstructed organic and mineral soil horizons. Juvenile Estherella sp. became at least 6.06‰ more enriched in ¹³C than sub-adult Estherella sp. or adult P. corethrurus. Sub-adult Estherella sp. became >3.6‰ enriched in ¹³C over P. corethrurus. '¹⁵N acquired by P. corethrurus was greater by 0.83-1.56‰ in the mixed-species than the single-species assemblages. '¹⁵N of sub-adult Estherella sp. was enriched by 0.73-0.81‰ over juvenile Estherella sp. in the single-species assemblage. Net N immobilization occurred in the organic layer of all ¹⁵N-enriched treatments. Net N mineralization in mineral soil layers was significantly greater in microcosms with P. corethrurus than in those containing only Estherella sp.. Cumulative respiration was greatest in P. corethrurus assemblages, however, assemblages with only Estherella sp. released more ¹³C in respiration. P. corethrurus assimilated different N resources when incubated with, as compared to without, native Estherella sp.. '¹³C and '¹⁵N signatures acquired by assimilation of ¹³C and ¹⁵N differed by species, developmental stage, and competitive interactions. The results showed that alone, exotic P. corethrurus induced higher mineralization rates than native Estherella sp., but that the interaction of exotic and native species impinged on resource use by P. corethrurus, reducing the effect of the exotic species on C and N mineralization. Invasion of exotic P. corethrurus may change the mineralization potentials of C and N and their biogeochemical cycling in soils.     

Frequency of defoliation and nitrogen and phosphorus fertilization on Andropogon gayanus Kunth. I. Yield,crude protein content,and in vitro digestibility

Abstract

An experiment was conducted in a semi‐arid region located in the State of Zulia, western part of Venezuela (10°32'N and 71°42'W, 600 mm average annual rainfall), to evaluate dry matter (DM) yield and in vitro organic matter digestibility (IVOMD), and crude protein (CP) content of Andropogon gayanus Kunth as affected by three frequencies of defoliation (every 42, 63, and 84 days) and fertilization with three levels of nitrogen (N) (0, 100, and 200 kg N ha^‐1‐year^‐1) and two levels of phosphorus (P) (0 and 75 kg P₂O₅ ha^‐1.year^‐1) in a factorial array using a split‐split‐plot experimental design with frequencies in the main plots, N in the sub‐plots, and P in the sub‐subplots with three replications. Soil was a sandy loam Aridisol with a pH of 5.5. Average soil calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), and P contents were 0.6, 0.3, 0.1 and 0.17 meq‐100g^‐1 soil, and 6 ppm, respectively. Data from six, four, and three cuttings for the frequencies of 42, 63, and 84 days were analyzed over the total duration of the study (252 days) as well as separately for periods with high (168 days, 384 mm) and low rainfall (84 days, 69 mm). Frequency of defoliation was the only factor that influenced (P≤0.05) the variables studied. In the overall analyses, the highest (P≤0.05) DM yield (3,656 kg#lbha^‐1.cutting^‐1) was obtained with harvests every 63 days. Mean IVOMD decreased (P≤0.05) from 54.2 to 51.7% with increasing harvest interval from 63 to 84 days, respectively. The average CP content was 7.9%. With low rainfall, mean DM yield was 2,209 kg#lbha^‐1, CP content declined (P≤0.05) from 7.5 to 3.9% as the cutting interval increased, and IVOMD decreased (P≤0.05) between 42 and 63 days. With high rainfall, the highest (P≤0.05) DM yield (4,872 kg#lbha^‐1) and IVOMD (56.5%) were found at 63 days of age. Mean CP content was 9.3%. These results confirm that A. gayanus is a highly productive forage grass. Lack of response to N and P fertilizers may be attributed partly to relatively low rainfall during the experiment, adaptation of the grass to low fertility soils, and long intervals between N applications and the next harvest.     

In vitro protein degradation of 38 sainfoin accessions and its relationship to tannin content by different assays

This study compared 38 sainfoin and 2 Lotus accessions to their respective tannin contents, N buffer solubility, and in vitro protein degradation. Tannin contents were measured by a protein precipitation method using either bovine serum albumin or Rubisco and by the colorimetric HCl/butanol method. Precipitation of bovine serum albumin and Rubisco was highly correlated (R(2) = 0.939). Correlations between the protein precipitation variants and the HCl/butanol method were relatively low (R(2) < 0.6). Protein degradation was measured at 4 h of incubation in an inhibited in vitro system and could not be explained by any of the tannin assays (R(2) < 0.03) and only partially by N buffer solubility (R(2) ≤ 0.433). Decisive factors other than the quantity of tannins or their ability to precipitate proteins must be considered. Resistance of soluble protein toward degradation can possibly be caused by tannin protein binding.     

Decomposition kinetics and organic geochemistry of woody debris in a ferralsol in a humid tropical climate

Large inputs of woody debris to soil can improve the soil. We examined the fate of woody debris buried in soil after fire‐free forest conversion to cropland in French Guiana. We measured the mass loss of woody debris > 4 mm on five sampling dates for 4 years after deforestation. Composition of the organic matter of woody debris was analysed with Rock‐Eval pyrolysis, which enabled us to distinguish a labile carbon pool (C_lab) and a resistant carbon pool (C_res). Decomposition of woody debris followed a first‐order function with a half‐life of 17.6 months. During the decomposition of woody debris > 4 mm, the C:N ratio, hydrogen index (HI) and pyrolysed carbon below 400°C (R400) decreased, suggesting that decarboxylation and dehydrogenation of woody debris occurred. Both C_lab and C_res stocks decreased with time, but the decrease in C_lab was faster. There was little humification of the debris and no long‐term biogeochemical preservation of a woody debris fraction, which accords with the first‐order decay observed. We conclude that the benefits of ligno‐cellulosic inputs for soil organic carbon contents in a tropical humid climate occur during the first year following deforestation. Résumé Les apports massifs de débris de bois au sol peuvent améliorer le sol. Nous avons examiné sur un site en Guyane le devenir de débris de bois enfouis dans le sol, suite à la conversion sans brûlis de forêt en cultures. Nous avons mesuré le stock des débris de bois > 4 mm jusqu'à 4 années après déforestation. Les transformations de la matière organique des débris ont bois ont été analysées par la pyrolyse Rock‐Eval, nous permettant de distinguer un compartiment de carbone labile (C_lab) et un compartiment de carbone résistant (C_res) dans ces débris. La décomposition des débris de bois suit une décroissance exponentielle de premier ordre, avec une demi‐vie de 17,6 mois. Durant la décomposition des débris, le rapport C/N, l'indice d'hydrogène (HI) et le carbone pyrolysé avant 400°C (R400) diminuent, suggérant des processus de décarboxylation et déshydrogénation qui se mettent en place rapidement après l'enfouissement des débris. Les stocks de C_lab et C_res diminuent tous deux au cours du temps, avec une cinétique plus rapide pour C_lab. Ces résultats indiquent une faible polymérisation de la matière organique des débris, et une absence de préservation biogéochimique d'une partie des débris, en accord avec la décroissance exponentielle observée. Cette étude suggère que les bénéfices des amendements ligno‐cellulosiques pour la matière organique des sols sont de courte durée en milieu tropical humide.     

Long‐term effects of leguminous cover crops on microbial indices and their relationships in soils of a coconut plantation of a humid tropical region

In this study, leguminous crops like Atylosia scarabaeoides, Centrosema pubescens, Calopogonium mucunoides, and Pueraria phaseoloides. grown as soil cover individually in the interspaces of a 19‐yr‐old coconut plantation in S. Andaman (India) were assessed for their influence on various microbial indices (microbial biomass C, biomass N, basal respiration, ergosterol, levels of ATP, AMP, ADP) in soils (0–50 cm) collected from these plots after 10 years. The effects of these cover crops on . CO₂ (metabolic quotient), adenylate energy charge (AEC), and the ratios of various soil microbial properties viz., biomass C : soil organic C, biomass C : N, biomass N : total N, ergosterol : biomass C, and ATP : biomass C were also examined. Cover cropping markedly enhanced the levels of organic matter and microbial activity in soils after the 10‐yr‐period. Microbial biomass C and N, basal respiration, . CO₂, ergosterol and levels of ATP, AMP, ADP in the cover‐cropped plots significantly exceeded the corresponding values in the control plot. While the biomass C : N ratio tended to decrease, the ratios of biomass N : total N, ergosterol : biomass C, and ATP : biomass C increased significantly due to cover cropping. Greater ergosterol : biomass C ratio in the cover‐cropped plots indicated a decomposition pathway dominated by fungi, and high . CO₂ levels in these plots indicated a decrease in substrate use efficiency probably due to the dominance of fungi. The AEC levels ranged from 0.80 to 0.83 in the cover‐cropped plots, thereby reflecting greater microbial proliferation and activity. The ratios of various microbial and chemical properties could be assigned to three different factors by principal components analysis. The first factor (PC1) with strong loadings of ATP : biomass C ratio, AEC, and . CO₂ reflected the specific metabolic activity of soil microbes. The ratios of ergosterol : biomass C, soil organic C : total N, and biomass N : total N formed the second factor (PC2) indicating a decomposition pathway dominated by fungi. The biomass C : N and biomass C : soil organic C ratios formed the third principal component (PC3), reflecting soil organic matter availability in relation to nutrient availability. Overall, the study suggested that Pueraria phaseoloides. or Atylosia scarabaeoides were better suited as cover crops for the humid tropics due to their positive contribution to soil organic C, N, and microbial activity.     

Methane and nitrous oxide fluxes in an acid Oxisol in western Puerto Rico: effects of tillage, liming and fertilization

Changes in land use and management of tropical systems are considered to be major factors in the recent upsurge in increases in atmospheric nitrous oxide (N₂O) and methane (CH₄). Studies were initiated in western Puerto Rico grasslands to determine the effect of plowing, or liming and fertilizing an acid Oxisol on the soil–atmosphere exchanges of N₂O and CH₄. Weekly field flux measurements and field manipulation and laboratory studies were conducted over 22 months during 1993–1995. We found that N₂O emissions from an Oxisol acidified to pH 4 were generally lower than from pH 6 Oxisol soils that were used as reference controls. Plowing the grasslands did not change mean N₂O emission rates from either pH soil. Liming the acidified Oxisol to pH 6 tended to increase N₂O emissions to the rates from the undisturbed grassland. Fertilizing the acidified grassland increased N₂O emissions but much less than when these soils were both limed and fertilized. Short-term field studies employing nitrification inhibitors in which we measured nitric oxide (NO) and N₂O emissions, demonstrated that nitrification rates generally control N₂O emissions; thus these were lower in unlimed soil. It is likely that NO was produced through the chemical decomposition of nitrite, which in turn, was a product of biological nitrification. Soil consumption of atmospheric CH₄ in the acidified Oxisol was about one-fourth of that in the pH 6 reference soil. Liming did not restore CH₄ consumption in the acid soil to rates comparable to those in the reference Oxisol. We conducted a laboratory induction study to determine if incubation of these limed or unlimed acidified soils with high concentrations of CH₄ could induce methanotrophic activity. Comparable uptake rates to the control soils were not induced by these incubations. These studies illustrate that management of soil can considerably affect the soil–atmosphere exchange of such trace gases as N₂O and CH₄ which can affect global atmospheric properties.     

Interactive influence of livestock grazing and manipulated rainfall on soil properties in a humid tropical savanna

Purpose

The effect of uncontrolled grazing and unpredictable rainfall pattern on future changes in soil properties and processes of savanna ecosystems is poorly understood. This study investigated how rainfall amount at a gradient of 50%, 100%, and 150% would influence soil bulk density (ρ), volumetric water content (θ_v), carbon (C), and nitrogen (N) contents in grazed (G) and ungrazed (U) areas.

Materials and methods

Rainfall was manipulated by 50% reduction (simulating drought—50%) and 50% increase (simulating abundance—150%) from the ambient (100%) in both G and U areas. Plots were named by combining the first letter of the area followed by rainfall amount, i.e., G150%. Samples for soil ρ, C, and N analysis were extracted using soil corer (8 cm diameter and 10 cm height). Real-time θ_v was measured using 5TE soil probes (20 cm depth). The EA2400CHNS/O and EA2410 analyzers were used to estimate soil C and N contents respectively.

Results and discussion

The interaction between grazing and rainfall manipulation increased θ_v and C but decreased N with no effect on ρ and C:N ratio. Rainfall reduction (50%) strongly affected most soil properties compared to an increase (150%). The highest (1.241?±?0.10 g cm^?3) and lowest (1.099?±?0.05 g cm^?3) ρ were in the G50% and U150% plots respectively. Soil θ_v decreased by 34.0% (grazed) and 25.8% (ungrazed) due to drought after rainfall cessation. Soil ρ increased with grazing due to trampling effect, therefore reducing infiltration of rainwater and soil moisture availability. Consequently, soil C content (11.45%) and C:N ratio (24.68%) decreased, whereas N increased (7.8%) in the grazed plots due to reduced C input and decomposition rate.

Conclusions

The combined effect of grazing and rainfall variability will likely increase soil θ_v, thereby enhancing C and N input. Grazing during drought will induce water stress that will destabilize soil C and N contents therefore affecting other soil properties. Such changes are important in predicting the response of soil properties to extreme rainfall pattern and uncontrolled livestock grazing that currently characterize most savanna ecosystems.

     

Bacterial phylogenetic diversity and a novel candidate division of two humid region, sandy surface soils

The extent of microbial community diversity in two similar sandy surface soils from Virginia and Delaware (USA) was analysed with a culture-independent small subunit ribosomal RNA (SSU rRNA) gene-based cloning approach with about 400-700 SSU rDNA clones obtained from each sample. While there were no operational taxonomic units (OTUs) having more than three individuals, about 96-99% of the OTUs had only a single individual. The clones showing less than 85% similarity to the sequences in the current databases were fully sequenced. The majority of the clones (55%) had sequences that were more than 20% different from those in the current databases. About 37% of the clones differed by 15-20% in sequence from the database, 16% of the clones differed by 10-15%, and 5% of the clones differed by only 1-10%. Phylogenetic analysis indicated that these sequences fell into 10 of the 35-40 known phylogenetic divisions. Many of the clones were affiliated with Acidobacterium (35%). While a substantial portion of the clones belong to alpha (24%) and beta (12%) Proteobacteria, a few of them were affiliated with delta (6%) and gamma (3%) Proteobacteria. About 6% of the clones belong to Planctomycetes, and 4% of the clones were related to gram-positive bacteria. About 4% of clones were related to other bacterial divisions, including Cytophaga, Green sulfur bacteria, Nitrospira, OP10, and Verrucomicrobia. Eight sequences had no specific association with any of the known divisions or candidate divisions and were phylogenetically divided into three novel division level groups, named AD1, AD2 and AD3. Candidate division AD1 represented by six clones (4%) was found in both sites and consisted of two subdivisions. The community structures were similar between these two widely separated, sandy, oligotrophic, surface soils under grass vegetation in a temperate, humid climate but somewhat dissimilar to community structures revealed in similar studies in other types of soil habitats.     

Effects of carbon additions on iron reduction and phosphorus availability in a humid tropical forest soil

In the highly weathered soils of humid tropical forests, iron (Fe) plays a key role in ecosystem biogeochemical cycling through its interactions with carbon (C) and phosphorus (P). We used a laboratory study to explore the role of C quantity and quality in Fe reduction and associated P mobilization in tropical forest soils. Soils were incubated under an ambient atmosphere headspace (room air) with multiple levels of leaf litter leachate or acetate additions. Net Fe reduction occurred in all the treatments and at every time point. The more complex mixture of organic compounds in leaf litter leachate stimulated Fe reduction as much acetate, an easily fermentable C source. At the end of the experiment, Fe reduction was generally greater with higher C additions than in the low C additions and controls. The microbial biomass P had increased significantly suggesting rapid microbial uptake of P liberated from Fe. This occurred without increases in the available (NaHCO₃) P pool. The immobilization of P by microbes during the incubation provides a P conservation mechanism in these soils with fluctuating redox potential, and may ultimately stimulate more C cycling in these highly productive ecosystems. Iron cycling appears to be an important source of P for the biota and can contribute significantly to C oxidation in upland tropical forest soils.     

Effect of fire on plant communities and soils in the humid tropical savannah of Gambela,Ethiopia

Ethiopian pastoralists and farmers use fire for regrowth of young green shoots and food crops, respectively. As a result, useful plants and wild animals are also affected. The intensity, frequency and magnitude of fires in the Gambela region (Baro-Akobo river basin) have been aggravated by the recent resettlement scheme and mechanized farming. Fires have (i) changed the physiognomy of plants; (ii) reduced the number of indispensable plant species; (iii) pushed wild animals to neighbouring areas; and (iv) brought soil erosion. If the present fire occurrences continue, this region of rich biodiversity will become desertified. © 1998 John Wiley & Sons, Ltd.     

Spatial changes of soil fungal and bacterial biomass from a sub-alpine coniferous forest to grassland in a humid, sub-tropical region

 Fungal and bacterial biomass were determined across a gradient from a forest to grassland in a sub-alpine region in central Taiwan. The respiration-inhibition and ergosterol methods for the evaluation of the microbial biomass were compared. Soil fungal and bacterial biomass both significantly decreased (P<0.05) with the shift of vegetation from forest to grassland. Fungal and bacterial respiration rates (evolved CO₂) were, respectively, 89.1 μl CO₂ g^–1 soil h^–1 and 55.1 μl CO₂ g^–1 soil h^–1 in the forest and 36.7 μl CO₂ g^–1 soil h^–1 and 35.7 μl CO₂ g^–1 soil h^–1 in the grassland surface soils (0–10 cm). The fungal ergosterol content in the surface soil decreased from the forest zone (108 μg g^–1) to the grassland zone (15.9 μg g^–1). A good correlation (R ²=0.90) was exhibited between the soil fungal ergosterol content and soil fungal CO₂ production (respiration) for all sampling sites. For the forest and grassland soil profiles, microbial biomass (respiration and ergosterol) declined dramatically with depth, ten- to 100-fold from the surface organic horizon to the deepest mineral horizon. With respect to fungal to bacterial ratios for the surface soil (0–10 cm), the forest zone had a significantly (P<0.05) higher ratio (1.65) than the grassland zone (1.05). However, there was no fungal to bacterial ratio trend from the surface horizon to the deeper mineral horizons of the soil profiles. Received: 30 March 2000