Colonisation of seminal roots of wheat and barley by egg-parasitic nematophagous fungi and their effects on Gaeumannomyces graminis var. tritici and development of root-rot

This study provides evidence that egg-parasitic nematophagous fungi, Pochonia chlamydosporia, Pochonia rubescens and Lecanicillium lecanii, can also reduce root colonisation and root damage by a fungal pathogen. Interactions of nematophagous fungi with the take-all fungus, Gaeumannomyces graminis var. tritici (Ggt), and their influence on severity of the root disease it causes were studied in laboratory and pot experiments. In Petri dish experiments the three nematophagous fungi reduced colonisation of barley roots by Ggt and also reduced necrotic symptoms. On the contrary, root colonisation by nematophagous fungi was unaffected by Ggt. In growth tube experiments, the three nematophagous fungi again reduced Ggt root colonisation and increased effective root length of barley seedlings. This was true for both simultaneous and sequential inoculation of nematophagous fungi versus Ggt. In the pot experiments the inoculum of the tested fungi in soil was applied in the same pot, as a mixture or in layers, or in coupled pots used for wheat grown with a split-root system. The nematophagous fungi P. chlamydosporia (isolate 4624) and L. lecanii (isolate 4629), mixed with Ggt or in split root systems with the pathogen, promoted growth of wheat (i.e. increased shoot weight), although no disease reduction was found. In split root systems, lower levels of peroxidase activity were found in seedlings inoculated with Ggt in combination with the nematophagous isolates 4624 and 4629 than when the take-all fungus was applied alone.Our results show that nematophagous fungi reduce root colonisation by Ggt, root damage and stress induced senescence in Ggt-inoculated plants.     

Impact on soil fauna of sheep faeces containing a range of parasite control agents

To compare the impact of parasite control agents in sheep faeces, 1 kg quantities of fresh faeces were spread uniformly over 1 m² pasture plots in June 2001 (winter; a time of high earthworm activity). Faecal treatments applied to five replicate plots were C− (none), C+ (from untreated sheep), B (from sheep with an intra-ruminal bolus releasing a benzimidazole anthelmintic—‘albendazole’), ML (from sheep with a bolus releasing a macrocyclic lactone anthelmintic—‘ivermectin’), F (from sheep receiving a daily feed supplement containing chlamydospores of the nematophagous fungus, Duddingtonia flagrans). The disappearance of faeces was assessed visually over the 50 days following faecal application, then soil samples were taken to assess: (a) populations of earthworms and other soil macrofauna, (b) nematodes and other soil microfauna, and (c) the presence of D. flagrans in soil. Faecal disappearance was greatest in F and C+ plots and least in ML and B plots at 12 and 23 days (P < 0.05). Earthworm casting after 23 and 50 days was greater (P < 0.05) in plots with faeces (C+, ML, F, but not B) than in plots without faeces (C−). Greater earthworm activity in plots with faeces was reflected in greater numbers of earthworms, cocoons and greater biomass m⁻² than in C− plots. On the basis of faecal dry weight applied, F plots had most earthworms and ML plots the least. After 50 days total nematodes in 0–5 cm soil showed a treatment effect (P < 0.001), being more abundant in F, C+ and B than in C− and ML plots; enchytraeids, rotifers, tardigrades and copepods showed no treatment effects. A few nematode taxa (Acrobeles, Alaimus, Pungentus, Tylencholaimus) showed significant treatment effects. The greatest effect among nematodes was in nematode channel ratio (NCR) (P < 0.008), with a decrease in F plots; changes in NCR may reflect the impact of earthworm activity on soil processes rather than a direct effect of the fungal treatment on nematodes. D. flagrans did not become established in the soil. During the trial conditions were favourable for earthworms and their activity was high in all treatments receiving faeces, with F and ML plots being the extremes. There was an apparent shift towards fungal-mediated decomposition in F plots. At the end of the 50-day trial, in a period when earthworms were active, there was no evidence of differential effects of any of the anthelmintic treatments on environmental indicators.     

Karlheinz richter zum 65. geburtstag

Studies were carried out on spatio-temporal variations of arbuscular mycorrhizal (AM) fungi associated with Carica papaya L. growing in two different geographic localities in Goa, India – Western Ghats and coastal area, differing in soil characteristics and plantation status. The study recorded considerable variation in root colonization, spore density and distribution of AM fungi in the selected sites. The mean total root colonization was at a maximum in the month of July, while spore density was highest in April. The study recorded a total 33 species of AM fungi. Western Ghats recorded a relatively higher diversity of AM fungi compared to coastal area. Species richness of AM fungi was at a maximum in April and coincided with maximum mean spore density. Edaphic as well as climatic factors influenced the AM fungal parameters. The study recorded the existence of seasonality in AM fungi.     

The relationship between vesicular-arbuscular mycorrhiza and lime: Associated effects on the growth and nutrition of brachiaria grass (Brachiaria decumbens)

Summary We studied the effects of limining on growth and nutrient concentrations of Brachiaria decumbens inoculated with five vesicular-arbuscular mycorrhizal (VAM) fungal assemblages which orginated from soils with different acidity. Liming increased plant growth when applied at rates up to 3 g kg^-1 soil and depressed growth at higher rates. Mycorrhizal plants grew better than non-mycorrhizal ones in unlimed soil and also liming rates of 4.5 and 6.0 g kg^-1 soil. The growth amelioration effects of VAM in highly acid or over-limed soils were related to nutrient uptake. VAM fungi isolated from an acidic soil exhibited a high symbiotic effectiveness and were better adapted to unlimed soil than those that originated from non-acidic soils. VAM root colonization, 90 days after planting, was little affected by liming. Fungal spore production and species compositions were highly affected by liming. A mixture of Glomus diaphanum and Glomus occultum predominated in unlimed soils inoculated with VAM assemblages isolated from non-acidic soils. In these fungal assemblages, an increased liming rate favored Glomus etunicatum over the other VAM fungi. Gigaspora margarita sporulated abundantly when introduced into unlimed soils, but rarely in limed soils. VAM appear to be crucial for the establishment of brachiaria pastures in the nutrient-deficient acidic soils of Central Brazil. It is suggested that liming may cause striking shifts in VAM populations which may, in turn, have a long-term impact on agricultural productivity in the tropics.     

Bacterial oxidation of manganous ions as affected by organic substrate concentration and composition

The ability of a soil Arthrobacter sp. to oxidize manganous ions in agar media containing various proportions and concentrations of sucrose and yeast extract as substrates was examined. Oxidation occurred readily on media ranging from those containing no added substrate to those containing as much as 2 per cent substrate. A variety of patterns of manganese oxide deposition was observed in colonies on various media. On media enriched with yeast extract the oxide was confined to the colonies and consisted of numerous small specks. Enrichment of media with sucrose resulted in the oxide being deposited as large discrete specks both within the colonies and in the agar below and around them. Oxidation did not occur on media which became and remained more acid than pH 6.0 or on those that rapidly became alkaline (pH 8.0). Within these limits increasing substrate concentration improved both growth and oxidation. Both the direction and rate of pH change, and hence the onset of oxidation, were affected by the proportion and concentration of sucrose and yeast extract. Non-oxidizing colonies, on media that had become alkaline, rapidly became oxidizing when exposed to CO₂-enriched air. The location of oxidation on agar plates and the time required for the first visible evidence of oxidation on various media are explained on the basis of pH changes during growth.These results are discussed in relation to the selection of media for the isolation and enumeration of manganese-oxidizing micro-organisms, to manganese oxidation by fungi and to the availability of manganese in soils.     

Microbial populations in pullman clay loam receiving large applications of cattle feedlot waste

One way to disperse waste from large commercial feedlots is to spread large amounts of the waste on limited areas nearby. The effects of this practice on the soil microbial populations was assessed. Feedlot waste (FLW) was applied for 5 yr at rates of 0, 22, 67, 134, and 269 t ha^?1 yr^?1. Additional treatments were 538 t ha^?1 yr^?1 for 1 and 3 years and applications of N and NPK fertilizer. Soil cores from plots were sampled for microbial analysis before, during, and after the fifth growing season. April, July. and December soil samples were analyzed to 180-, 20-, and 480-cm depths, respectively. The following utritional and physiological groups of microorganisms were counted: soil fungi on Rose Bengal agar; bacteria on a basal mineral salts medium, on nutrient agar (both aerobically and in BBL GasPak jars), and on EMB agar (Escherichia coli-type colonies and total counts); and nitrifying and denitrifying organisms. Little effect due to FLW application rates was found, and organisms producing coliform-type colonies on EMB agar did not persist in the soil. The results indicated that applying large amounts of feedlot waste will not deleteriously affect soil microorganisms.     

Distribution and Sorption of Phosphate in a Savanna Soil Under Improved Pastures in Northern Nigeria

Abstract

Land use patterns affect soil nutrient transformation and availability. The study determined the distribution of phosphorus (P) fractions and sorption in five pasture fields composed of Andropogon gayanus, Brachiaria decumbens, Chloris gayana, Digitaria smutsii, and Stylosanthes guianensis. The objectives were to characterize P fractions in improved pastures and to determine the effect of forage species on soil P lability. Total P (P_t) across the pastures was not significantly different. Organic P (P_o) accounted, on the average, for 64% of P_t. Resin‐P, considered the plant‐available P, ranged from 4 to 10 mg kg^?1, suggesting acute P deficiency in the pastures. The sum of P fractions extracted by 0.5 M NaHCO₃, 0.1 M NaOH, and 1.0 M HCl, together with the resin‐P, accounted for less than 35% of P_t. Factor analysis indicated that plant‐available P approximated by resin‐P was furnished by ?HCO₃‐P_o mineralization and HCl‐P. The highest concentrations of ?HCO₃‐P_o and ?OH‐P_o were maintained by Brachiaria decumbens. Grouping P_i and P_o fractions into labile and nonlabile fraction showed that Brachiaria decumbens maintained the greatest concentration of labile P as a proportion of its P_t. The pasture soils sorbed between 31 and 65% of added P from a standard concentration of 50 mmol kg^?1. Phosphorus sorbed by soils from the pasture fields was in the order: Digitaria smutsii=Stylosanthes guianensis>Brachiaria decumbens=Chloris gayana>Andropogon gayanus, whereas resin recovery of sorbed P was greater in Brachiaria decumbens than other pastures. Between 82 and 92% of sorbed P was bound irreversibly. It was concluded that the relatively high concentration of labile P maintained by soil under Brachiaria decumbens was probably related to its capacity to sequester more carbon than the other pastures.     

Transformations of nitrogen and phosphorus in sheep faeces located in saltbush rangeland and on irrigated pasture

Sheep faeces enclosed in terylene mesh bags were placed in two contrasting microenvironments within a semi-arid rangeland dominated by Atriplex vesicaria Hew ex Benth., and one within an irrigated pasture. Groups of bags were retrieved and their contents analysed at 3-monthly intervals over a total of 20 months. Sheep faeces from semi-arid rangeland and irrigated pastures were used at each site. Irrespective of source, microenvironment, and season, the amount of faecal material remaining on the semi-arid rangeland sites decreased consistently by approximately 2% month^?1. The rate of disappearance for faeces placed under irrigated pastures was 2.5% month^?1.The retention of N in the faeces was proportionately greater than the retention of faecal material. The presence of faecal pellets increased the mineral and total N of the soil beneath bags in the betweenbush microenvironment, but not the mineral-N content beneath bags placed under bushes of A. vesicaria in semi-arid rangeland.After 20 months, the proportion of total and inorganic P remaining in the faecal material placed under bushes was 50 and 40% respectively, and in faecal material placed between bushes the corresponding figures were 30 and 20% More Truog “available” P accumulated in soil under bags placed between bushes than under bags placed below bushes. The amount of total and inorganic P remaining after 20 months in the faecal pellets on the irrigated pastures was 20 and 10% respectively. The small loss of organic P from the faecal material indicated an absence of mineralization.     

Mycoparasitism by Pythium oligandrum and P. acanthicum

Isolates of the reported mycoparasites Pythium oligandrum, P. acanthicum and P. periplocum markedly reduced growth and cellulolysis by Botryotrichum piluliferum, grew rapidly across agar plates precolonized by Phialophora radicicola var radicicola (sensu Deacon) and, where tested (not P. periplocum), were non-pathogenic towards higher plants. Isolates of P. echinulatum, P. mamillatum. P. megalacanthum, P. spinosum, P. ultimum and one isolate of P. acanthicum behaved differently from the mycoparasites and could, themselves, be placed in two groupings in these tests. It is suggested that the ability or otherwise to grow on Phialophora-precolonized agar plates may help to distinguish broad biological groupings within the genus Pythium, but these groupings may cut across conventional taxonomic ones.One isolate of P. acanthicum was tested for its effects on a range of cellulolytic fungi: it reduced their growth to different extents, as did P. oligandrum.Plates of potato-dextrose agar precolonized by Phialophora radicicola were used to isolate selectively P. oligandrum and similar fungi from soils, but the use of hemp seed baits in conjunction with precolonized plates was less selective for these fungi.Straw pieces precolonized by P. oligandrum and buried in soil decomposed at the same rate as virgin straws or those precolonized by P. ultimum or Mucor hiemalis. Subsequently, Stachyholrys atra appeared to colonize straws more frequently from soil, and Fusarium spp. less so, in the presence of P. oligandrum than in its absence. In the laboratory, P. oligandrum was antagonized by Slachyholrys, whereas Fusarium spp. were frequently overgrown by the Pythium.     

Microfungal communities in soil,litter and casts of Lumbricus terrestris L. (Lumbricidae): a laboratory experiment

The anecic earthworm Lumbricus terrestris L. was kept in laboratory microcosms containing beech forest soil without litter, with beech leaf litter or with lime leaf litter. The structure of microfungal communities in soil, litter and fresh and aged (100 days) earthworm faeces was analysed using the washing and plating technique. The passage of mineral soil through the gut of L. terrestris affected the structure of the fungal community only little. In contrast, in the litter treatments the structure of the fungal community in fresh earthworm casts significantly differed from that in soil and litter. The majority of soil and litter inhabiting fungi survived passage through the gut of L. terrestris and the fungal community in casts consisted of a mixture of soil and litter inhabiting fungi. However, the frequency of Cladosporium spp., Alternaria spp., Absidia spp., and other taxa was strongly reduced in fresh casts. The degree of colonization of litter particles (number of isolates per number of plated particles) also decreased, but some fungi (mainly Trichoderma spp.) benefited from gut passage and flourished in fresh casts. During ageing of cast material the dominance structure of the fungal community changed. Both the degree of colonization of organic particles and the species diversity increased and approached that in soil. However, the structure of the fungal community in casts remained cast specific even after 100 days of incubation. It is concluded that the feeding and burrowing activity of L. terrestris accelerates the colonization of litter by the edaphic mycoflora but also extends the range of occurrence of litter-associated fungi into mineral soil layers.     

Effets de quelques pesticides sur la colonisation d'un substrat par le Trichoderma harzianum rifai en presence des autres champignons du sol

The effects of some pesticides upon substrate colonization by Trichoderma harzianum Rifai in the presence of other soil fungi.A herbicide (propyzamide) and 5 fungicides (benomyl, quintozene, vinclozolin. thiram, prothiocarb) were added at various concentrations to two Trichoderma harziunum-enriched garden soils. After thorough mixing, followed by solidifying with water agar, discs of these mixtures were cut with a cork-borer and transferred to a non specific nutrient medium (PDA). The development of T. harzianum on the agar, around each implant, was estimated after incubation for 1 week at 28°C.In the presence of other soil fungi, T. harzianum development was scarcely or not at all modified by prothiocarb, propyzamide and quintozene. It was highly depressed by benomyl. Vinclozolin was only slightly inhibitory. In our experimental conditions, T. harzianum development was strongly enhanced by thiram, even at a high dosage (300 μg ml^?1).The limits and the advantages of the method are estimated and the results are discussed in view of practical applications.     

Interactions of the microflora from nodulation problem and non-problem soils towards Rhizobium spp on agar culture

Microorganisms (348 fungi, 388 actinomycetes and 319 bacteria) were isolated from a nodulation problem soil, a non-problem virgin soil, a cultivated problem soil and the rhizosphere of clover plants grown in the problem soil. Rhizobium trifolii TA 1 which failed to establish in problem soils was inhibited on laboratory media by a greater number of these soil microorganisms than the better soil colonizing R. trifolii (WU95 and WU290) and R. lupini (WU425). R. lupini was not inhibited or stimulated on agar by many soil or rhizosphere isolates. R. meliloti showed greater stimulation than either R. trifolii or R. lupini and was inhibited by relatively few soil microorganisms so that its poor soil survival was thought to be due to chemical or physical soil conditions rather than to biotic factors. The greatest incidence of rhizobial inhibitors, mainly associated with TA 1, was found among the isolates from the clover rhizosphere. There was a reduction in the relative numbers of rhizobial inhibitors isolated from the cultivated soil compared with the virgin problem soils, a result possibly due to the changed soil environment changing with cultivation, altered vegetation and the addition of superphosphate. Inhibitors of rhizobia were more frequent amongst the bacteria than fungi or actinomycetes. Strong stimulation was more commonly shown by fungi than by actinomycetes or bacteria. The interaction on agar between rhizobia and the soil microflora is related to soil colonization and persistence.     

Mycorrhiza and phosphate protection of tropical grass species against heavy metal toxicity in multi-contaminated soil

In this paper, the effects of arbuscular mycorrhizal (AM) fungi and phosphate amendments on protection of the tropical grass Brachiaria decumbens Stapf. against metal toxicity caused by Zn, Cd, Cu, and Pb were studied in a sterilized soil. Plants inoculated with a mixture of AM fungi (Acaulospora morrowiae, Gigaspora albida, and Glomus clarum) isolated from a heavy-metal-contaminated site or amended with P (added as triple superphosphate) exhibited marked positive growth responses, indicating the ameliorating effects of these two factors. Soil metal concentrations needed to inhibit plant growth by 50% were around twofold higher for AM plants as compared to those for non-inoculated ones. Similarly, phosphate showed ameliorating effects for B. decumbens, but its effects were not related to mycorrhizal conditions. Although mycorrhiza and phosphate act independently, their protecting effects were additive. Metal bioaccumulation factor of B. decumbens is high, especially for Cd; but AM inoculation prevents metal transference from roots to shoots, retaining these metals in the roots. AM fungus and phosphate represent a promising tool for enhancing ground vegetation in heavy-metal-contaminated sites.     

Arbuscular mycorrhizal fungal dynamics under Mitragyna parvifolia (Roxb.) Korth. in Thar Desert

The temporal and spatial dynamics of arbuscular mycorrhizal fungi (AMF) were investigated in Indian Thar Desert. Soil samples under Mitragyna parvifolia were collected from July 2003 to June 2004. AMF colonization and spore density were used to compare the responses of AMF to different abiotic parameters. The mean percent colonization and spore density of AMF reached maximal values in rainy and summer seasons, respectively. Vesicular and hyphal colonizations were positively correlated with soil organic carbon content. AMF spore density was positively correlated with soil pH and negatively correlated with Olsen P content. A high Shannon–Weiner diversity index of AMF was observed in Thar Desert. A total of fifteen AMF species were associated with M. parvifolia. Percent spore density and species richness suggest that the genus Glomus was the predominant AMF under Thar Desert environment. The reasons for the observed variations are discussed.     

Root herbivory by Tipula paludosa larvae increases colonization of Agrostis capillaris by arbuscular mycorrhizal fungi

The objective of this study was to determine if root-feeding by insect larvae affects subsequent colonization of roots by arbuscular mycorrhizal fungi. We investigated grazing by larvae of Tipula paludosa on colonization of Agrostis capillaris by two species of fungi, Glomus mosseae and G. intraradices. Host plants were subjected to 7 days of grazing only, continuous grazing for 42 days, or no herbivory. Those plants with no herbivory had significantly lower levels of colonization by arbuscules and hyphae compared to plants which were grazed for 7 or 42 days. The effect only occurred in the upper parts of the root system, where larvae were active. We suggest that this effect was most likely mediated by a change in quantity and composition of root exudates.     

Cumulative effects of tree-based intercropping on arbuscular mycorrhizal fungi

In tree-based intercropping system (agroforestry), the role of perennial trees in maintaining active populations and mycelial networks of arbuscular mycorrhizal fungi (AMF) is well documented. Agroforestry positively influences the AMF community, but complete studies regarding mycorrhization in such systems are scarce. The present study was conducted to assess the effect of tree introduction in agriculture fields on mycorrhization. In particular, we investigated the effect of trees on AMF colonization of intercrops and vice versa, the effect of canopy management of trees on their root colonization, and the cross-infectivity of AMF isolated from tree rhizosphere in intercrops and vice versa. The results of the field study suggest that in agroforestry systems, trees acted as AMF inoculum reservoir for intercrops, especially during the rainy season. Intercropping (Phaseolus mungo and Triticum aestivum in the rainy and winter seasons, respectively) increased mycorrhization, i.e., root colonization and spore population in the rhizosphere of Albizia procera and Eucalyptus tereticornis. Canopy management, i.e., shoot pruning, reduces root colonization in A. procera, Anogeissus pendula, Dalbergia sissoo, Hardwickia binata, and Tectona grandis, especially in April 2005 (late spring), but during subsequent periods, differences among the treatments were at par. Results from greenhouse suggest that AMF are nonspecific in their selection of host since species isolated from tree rhizosphere could colonize the roots of crops and vice versa.     

Integrated management reduces Globodera rostochiensis abundance and enhances nematode community composition

The potato cyst nematode is a major threat to potato production worldwide. In the highlands of Veracruz State, Mexico, potato cultivation has been the main economic activity during the last seven decades, and it is severely affected by the high abundance of Globodera rostochiensis. In this study, the field efficacy of chemical nematicides, nematophagous fungi, crop rotation and the combination of the latter two strategies were assessed on G. rostochiensis abundance while also considering their effects on the free-living nematode community. Integrated management combining the use of nematophagous fungi and crop rotation was the most effective strategy for decreasing the abundance of G. rostochiensis (up to 13%). Additionally, high diversity, low disturbance, and a slightly higher structure were observed in the nematode community in the integrated management treatment. Although nematophagous fungi and crop rotation separately were less effective in reducing G. rostochiensis abundance (41% and 23%, respectively), the free-living nematode community was not drastically affected. Chemical control was not only less effective in reducing the abundance of G. rostochiensis (up to 37%) but also affected the free-living nematode community to a greater extent. Our results suggest integrated management not only reduces G. rostochiensis abundance but also enhances free-living nematode community.     

Tempo-spatial dynamics of arbuscular mycorrhizal fungi under clonal plant Psammochloa villosa Trin. Bor in Mu Us sandland

Tempo-spatial dynamics of AM fungi within the rhizome system of Psammochloa villosa (Poaceae) were investigated in Mu Us sandland, northwest China. Soil samples in the annual and perennial ramet rhizospheres of P. villosa were collected in 2007. AM fungal percent colonization reached maximal values in the rainy season and spore number in the dry season. Spore number exhibited positive correlation with soil pH and available phosphorous (P) (P < 0.01), and negative correlation with available nitrogen (N) (P < 0.05). Vesicular, arbuscular, hyphal and total colonization were positively correlated with soil organic matter and available P (P < 0.01), and negatively correlated with available N (P < 0.01). Fourteen species of AM fungi in four genera were isolated. The same AM fungal taxa were found in the annual and perennial ramet rhizospheres, although the last ones had higher fungal colonization and spore number. A high Shannon-Weiner diversity index of AM fungi was observed. Spore number and species richness indicated that Glomus was the predominant AM fungi, especially the small-spored taxa. AM fungal dynamics under P. villosa are highly seasonal: different aged ramets and nutrient availability have effects on AM fungal development and abundance in Mu Us sandland.     

Sheep excreta cause no positive priming of peat-derived CO2 and N2O emissions

Large areas of peatlands in Germany and the Netherlands are affected by drainage and high nitrogen deposition. Sheep grazing is a common extensive management activity on drained peatlands, in particular on nature protection areas. However, input of easily mineralisable material such as sheep excrements could enhance degradation of soil organic carbon (C_org), thereby increasing the effect of these ecosystems on national GHG budgets. Thus, a microcosm experiment on the influence of sheep excreta on GHG emissions from a histic Gleysol with strongly degraded peat was set up. The ¹⁵N and ¹³C stable isotope tracer technique was used to partition sources of CO₂ and N₂O. Labeled sheep faeces and urine were obtained by feeding enriched material. Undisturbed soil columns were treated with surface application of urine, faeces or mixtures of both in different label combinations to distinguish between direct effects and possible priming effects. Incubation was done under stable temperature and precipitation conditions. Fluxes as well as ¹⁵N and ¹³C enrichment of N₂O and CO₂, respectively, were measured for three weeks. Addition of sheep excreta increased emission of total CO₂ in proportion to the added carbon amounts. There was no CO₂ priming in the peat. No effect on CH₄ and N₂O was observed under the aerobic experimental conditions. The N₂O–N source shifted from peat to excreta, which indicates negative priming, but priming was not significant. The results indicate that sheep excreta do not significantly increase GHG emissions from degraded peat soils. Considering the degraded peatland preserving benefits, sheep grazing on peatlands affected by drainage and high nitrogen deposition should be further promoted.     

Fertility of tropical soils under different land use systems—a case study of soils in Tabasco,Mexico

Increasing deforestation in Mexico in the past 40 years has led to significant land use changes. It is important to establish land use systems that allow for the necessities of an increasing population and the conservation of soil fertility in the long term. In this study, we investigated the influence of different land use forms on soil fertility in Tabasco, SE Mexico. We chose two different commonly used pastures (Cynodon plectostachyus and Brachiaria decumbens) and a succession forest. We characterised soil fertility by physico-chemical parameters (texture, density, pH, P, Corg., Ntot., cation exchange capacity (CEC)) as well as by biological parameters, such as litter decomposition, microbial biomass and earthworm community. To estimate litter decomposition we used leaves of Gliricidia sepium, a common fodder tree in the region. The three land use systems had very similar soil chemical characteristics. All three can be characterised as acidic (pH between 4.1 and 5.3) with a high content of organic matter and total nitrogen. However, the three land use systems differed significantly with respect to their soil biological characteristics. Earthworm density as well as litter decomposition were significantly lower under B. decumbens than in the other soils. In all land use systems, the participation of macrofauna and mesofauna accelerated litter decomposition rate significantly as compared with decomposition with microfauna and microflora alone.We extracted two components of the pool of data by main component analysis. The acidity component explained mainly the microbial litter decomposition rate. The rate of litter decomposition – with participation of soil meso- and macrofauna – could be explained by the humus component. We assume that biological parameters were more suitable to characterise differences between the different land use systems. The use of C. plectostachyus and succession forest showed a positive effect on soil fertility.