Nitrogen mineralization and reorganization in casts of the geophagous tropical earthworm Pontoscolex corethrurus (Glossoscolecidae)

Summary Mineral N concentrations ranged from 133.1 to 167.8 g g^-1 dry soil in fresh casts of the endogeic earthworm Pontoscolex corethrurus fed on an Amazonian Ultisol; this was approximately five times the concentration in non-ingested soil. Most of this N was in the form of NH _inf4 ^sup+ . N also accumulated in microbial biomass, which increased from a control value of 10.5–11.3 to 67.5–74.1 g g^-1 in fresh casts. During a 16-day incubation, part of the NH _inf4 ^sup+ -N was nitrified and/or transferred to the microbial biomass. Total labile N (i.e., mineral+biomas N) decreased sharply at first (ca. 50% in the first 12 h), and then more slowly. The exact fate of this N (microbial metabolites, denitrification, or volatilization) is not known. After 16 days, the overall N content of the casts was still 28% higher than that of the control soil. Incubation of the soil before ingestion by the earthworms significantly increased the production of NH _inf4 ^sup+ in casts. We calculate that in a humid tropical pasture, 50–100 kg mineral N may be produced annually in earthworm casts. Part of this N may be conserved in the compact structure of the cast where the cast is not in close contact with plant roots.     

Earthworm impacts on soil organic matter and fertilizer dynamics in tropical hillside agroecosystems of Honduras

Earthworms are important processors of soil organic matter (SOM) and nutrient turnover in terrestrial ecosystems. In agroecosystems, they are often seen as beneficial organisms to crop growth and are actively promoted by farmers and extension agents, yet their contribution to agroecosystem services is uncertain and depends largely on management. The Quesungual slash-and-mulch agroforestry system (QSMAS) of western Honduras has been proposed as a viable alternative to traditional slash-and-burn (SB) practices and has been shown to increase earthworm populations, yet the effect of earthworms on soil fertility and SOM in QSMAS is poorly understood. This study examined the role of Pontoscolex corethrurus in QSMAS by comparing their influence on aggregate-associated SOM and fertilizer dynamics with their effects under SB and secondary forest in a replicated field trial. Both the fertilized QSMAS and SB treatments had plots receiving additions of inorganic ¹⁵N and P, as well as plots with no inorganic N additions. Earthworm populations were manipulated in field microcosms at the beginning of the rainy season within each management treatment via additions of P. corethrurus or complete removal of existing earthworm populations. Microcosms were destructively sampled at harvest of Zea mays and soils were wet-sieved (using 53, 250 and 2000 μm mesh sizes) to isolate different aggregate size fractions, which were analyzed for total C, N and ¹⁵N. The effects of management system were smaller than expected, likely due to disturbance associated with the microcosm installation. Contrary to our hypothesis that earthworms would stabilize organic matter in soil aggregates, P. corethrurus decreased total soil C by 3% in the surface layer (0-15 cm), predominantly through a decrease in the C concentration of macroaggregates (>250 μm) and a corresponding depletion of C in coarse particulate organic matter occluded within macroaggregates. Earthworms also decreased bulk density by over 4%, but had no effect on aggregate size distribution. Within the two fertilized treatments, the QSMAS appeared to retain slightly more fertilizer derived N in smaller aggregate fractions (<250 μm) than did SB, while earthworms greatly reduced the recovery of fertilizer N (34% decrease) in both systems. Although management system did not appear to influence the impact of P. corethrurus on SOM or nutrient dynamics, we suggest the lack of differences may be due to artificially low inputs of fresh residue C to microcosms within all management treatments. Our findings highlight the potential for P. corethrurus to have deleterious impacts on soil C and fertilizer N dynamics, and emphasize the need to fully consider the activities of soil fauna when evaluating agroecosystem management options.     

Earthworm and microbe response to litter and soils of tropical forest plantations with contrasting C:N:P stoichiometric ratios

1

The stoichiometry of resources is increasingly acknowledged as a major control of consumer activity and abundance. Chemical properties of litter, the main source of food for decomposers, are likely to be important drivers of decomposer activity.

2

Theory predicts a high control of resource stoichiometry on the abundance of consumer organisms that maintain strict homeostasis, due to costs associated with the regulation of nutrient balance in their body tissue. Decomposer efforts in nutrient acquisition should be related to imbalances in resource stoichiometry.

3

A 21 year old experimental plantation of monospecific plots of trees with leaves of contrasting chemistry was used to test four hypotheses: (i) soil and litter nutrient stoichiometry (C, N, P) are linked; (ii); soil enzyme activity ratios and stoichiometry are linked; (iii) earthworms’ tissue stoichiometry does not depend on soil and litter stoichiometry (homeostasis); (iv) earthworm density is dependent upon phosphorus availability, the most limiting nutrient in soils at this site, and, to a lesser extent, to nitrogen availability.

4

We found (i) no relationship between litter and soil stoichiometry, (ii) microbial activity was linked to soil stoichiometry, (iii) earthworms showed strict homeostasis in their tissue and (iv) earthworm abundance increased with P availability.

5

We discuss the mechanisms that might lead to these patterns.

     

Species effects on earthworm density in tropical tree plantations in Hawaii

Summary Tree species differ in the quantity and quality of litter produced, and these differences may significantly affect ecosystem structure and function. I examined the importance of tree species in determining earthworm densities in replicated stands of Eucalyptus saligna Sm. and Albizia falcataria (L.) Fosberg, and in mixed stands (25% albizia and 75% eucalyptus). Mean earthworm densities ranged from 92 m^-2 in the pure eucalyptus, to 281 m^-2 in the mixture, and a maximum of 469 m^-2 in the pure albizia stands. Only two earthworm species were present, Pontoscolex corethrurus and Amynthas gracilis. Leaf biomass on the forest floor was highest in the pure eucalyptus and lowest in the pure albizia stands, whereas the annual fine litterfall production was lowest in the pure eucalyptus and highest in the albizia stands. The N content of fine litterfall was correlated positively with earthworm density, and the fine litterfall biomass: N ratio was correlated negatively with earthworm density. Greater leaf biomass on the forest floor under eucalyptus stands despite lower rates of litterfall suggests that litter quality, rather than litter quantity, was primarily responsible for the greater earthworm density in the albizia stands. Some biogeochemical effects of tree species in the tropics may be mediated through effects on earthworm populations.     

Five years of simulated atmospheric nitrogen deposition have only subtle effects on the fate of newly synthesized carbon in Calluna vulgaris and Eriophorum vaginatum

To understand the implications of atmospheric nitrogen deposition on carbon turnover in peatlands, we conducted a ¹³C pulse labeling experiment on Calluna vulgaris and Eriophorum vaginatum already receiving long-term (5 years) amendments of 56 kg N ha⁻¹ y⁻¹ as ammonium or nitrate. We examined shoot tissue retention, net ecosystem respiration returns of the ¹³C pulse, and soil porewater DOC content under the two species. ¹³C fixation in Eriophorum leaves was enhanced with nitrogen addition and doubled with nitrate supply. This newly fixed C appeared to be relocated below-ground faster with nitrogen fertilization as respiration returns were unaffected by N inputs. By contrast, increases in ¹³C fixation were not observed in Calluna. Instead, net ecosystem respiration rates over Calluna increased with N fertilization. There was no significant label incorporation into DOC, suggesting a conservative strategy of peatland vegetation regarding allocation of C through root exudation. Greater concentrations of total DOC were identified with nitrate addition in Calluna. Given the long-term nature of the experiment and the high N inputs, the overall impacts of nitrogen amendments on the fate of recently synthesized C in Eriophorum and Calluna in this ombrotrophic peatland were surprisingly more moderate than originally hypothesized. This may be due to N being effectively retained within the bryophyte layer, thus limiting, and delaying the onset of, below-ground effects.     

First indications for the involvement of strigolactones on nodule formation in alfalfa (Medicago sativa)

Strigolactones have recently been suggested to be phytohormones that are present in all plants. Strigolactones are released by roots into the rhizosphere, stimulating the seed germination of parasitic plants such as Striga spp. and Orobanche spp. and play a crucial role in the interaction between plants and symbiotic arbuscular mycorrhizal fungi.By applying different concentrations of the synthetic strigolactone analogue GR24 to alfalfa (Medicago sativa) inoculated with Sinorhizobium meliloti we could show that in alfalfa nodulation is positively affected by the presence of the strigolactone analogue GR24. Moreover, we could show that this increased nodulation cannot be linked with a stimulatory effect of GR24 on the growth or the expression of nod genes of S. meliloti.Putative mechanisms operating in the plant in response to the addition of GR24 and leading to increased nodule formation by rhizobia are discussed.     

The influence of inorganic nutrient fertilization on the growth,nutrient composition and vesicular-arbuscular mycorrhizal colonization of pretransplant rice (Oryza sativa L.) plants

Summary The effects of P, N and Ca+Mg fertilization on biomass production, leaf area, root length, vesiculararbuscular mycorrhizal (VAM) colonization, and shoot and root nutrient concentrations of pretransplant rice (Oryza sativa L.) plants were investigated. Mycorrhizal plants generally had a higher biomass and P, N, K, Ca, Mn, Fe, Cu, Na, B, Zn, Al, Mg, and S shoot-tissue nutrient concentrations than non-mycorrhizal plants. Although mycorrhizal plants always had higher root-tissue nutrient concentrations than non-mycorrhizal plants, they were not significantly different, except for Mn. N fertilization stimulated colonization of the root system (colonized root length), and increased biomass production and nutrient concentrations of mycorrhizal plants. Biomass increases due to N were larger when the plants were not fertilized with additional P. P fertilization reduced the colonized root length and biomass production of mycorrhizal plants. The base treatment (Ca+Mg) did not significantly affect biomass production but increased the colonized root length. These results stress the importance of evaluating the VAM rice symbiosis under various fertilization regimes. The results of this study suggest that pretransplant mycorrhizal rice plants may have a potential for better field establishment than non-mycorrhizal plants.     

Plant growth promoting properties of a strain of Enterobacter ludwigii isolated from Lolium perenne rhizosphere

The capability of native bacterial strains isolated from Lolium perenne rhizosphere to behave as plant growth promoting bacteria and /or biocontrol agents was investigated. One strain (BNM 0357) over 13 isolates from the root tips of L. perenne resulted proved to be nitrogenase positive (ARA test) and an IAA producer. Conventional tests and the API 20E diagnostic kit indicated that BNM 0357 behaves to the Enterobacteriaceae family and to the Enterobacter genus. Molecular identification by 16S rRNA sequence analysis indicated that BNM 0357 had the highest similarity to Enterobacter ludwigii (EN-119). Isolate BNM 0357 had the capability to solubilize calcium triphosphate and to antagonize Fusarium solani mycelial growth and spore germination. Strain BNM 0357 also showed the ability to improve the development of the root system of L. perenne. This study disclosed features of E. ludwigii BNM 0357 that deserve further studies aimed at confirming its putative importance as a PGPR.     

Growth promotion of plants by plant growth-promoting rhizobacteria under greenhouse and two different field soil conditions

This study was conducted with sugar beet in greenhouse and field at two soil type with different organic matter (containing 2.4 and 15.9% OM, referred as the low- and high-OM soil) conditions in order to investigate seed inoculation of sugar beet, with five N₂-fixing and two phosphate solubilizing bacteria in comparison to control and mineral fertilizers (N and P) application. Three bacterial strains dissolved P; all bacterial strains fixed N₂ and significantly increased growth of sugar beet. In the greenhouse, inoculations with PGPR increased sugar beet root weight by 2.8-46.7% depending on the species. Leaf, root and sugar yield were increased by the bacterial inoculation by 15.5-20.8, 12.3-16.1, and 9.8-14.7%, respectively, in the experiment of low- and high-OM soil. Plant growth responses were variable and dependent on the inoculants strain, soil organic matter content, growing stage, harvest date and growth parameter evaluated. The effect of PGPR was greater at early growth stages than at the later. Effective Bacillus species, such as OSU-142, RC07 and M-13, Paenibacillus polymyxa RC05, Pseudomonas putida RC06 and Rhodobacter capsulatus RC04 may be used in organic and sustainable agriculture.     

The Weedy Rice Complex in Costa Rica. I. Morphological Study of Relationships Between Commercial Rice Varieties,Wild <Emphasis Type="Italic">Oryza</Emphasis> Relatives and Weedy Types

Weedy rice is a complex of Oryza morphotypes widely distributed in commercial rice fields, which interfere with rice cultivation, seed production, industrial processing and commercialization of this crop in several countries. The objective of this study was to characterize the weedy rice complex of Costa Rica by comparing it with the cultivated and wild rice species found in the country. A collection of weedy rice accessions, representative of the morphotypes found in the country, was established and characterized. Their morphometric relationships were established by comparing 27 morphological traits with commercial rice cultivars, landraces and wild Oryza species and by performing a multivariate analysis. Twenty-one weedy rice morphotypes were identified among 735 weedy accessions by using a three-digit code based on seed characters. Three principal components (PCs) explained 66.25% of the variation observed. The first PC accounted for 36.21% of the variation and separated CCDD genome type Oryza latifolia and O. grandiglumis from AA genome species O. sativa, O. glumaepatula, O. rufipogon and O. glaberrima. The second (18.9%) and third (11.14%) PCs separated the weedy morphotype groups from the AA genome species O.sativa, O. glaberrima and O. rufipogon. The weedy morphotypes were scattered between the indica commercial rice varieties, the cluster landraces–glaberrima and O. rufipogon. Additionally, a group of morphotypes showed intermediate characteristics between O. sativa and O. rufipogon, suggesting that hybridization could have taken place in the past between these species. None of the morphotypes collected in Costa Rica clustered with the allotetraploids CCDD species or O. glumaepatula.     

Effect of plant species, nitrogen fertilizer and grass age on the dynamics of intra-aggregate SOM

Plant wax compounds (n-alkanes and alcohols) were used to trace C from the dominant pasture species to different water-stable aggregate (WSA) size classes to investigate if changes in the plant community composition caused by grass ley age and N amendment were reflected in the dynamics of intra-aggregate SOM. Age of the ley influenced the formation of aggregates, and fertilizer N application decreased %C and %N of the macroaggregates. Although changes in the plant community composition due to N amendment and ley age were reflected in the concentrations of plant wax compounds in the different WSA classes, the results of this study are more indicative of the direct effect of N fertilizer, than of the effect of dominant plant species, on intra-aggregate OM dynamics. The results of this study were found to support the suggestion that N amendments increase intra-aggregate OM dynamics especially in the smaller WSA size classes.     

Low frequency of anti-acetylcholinesterase pesticide poisoning in lesser and Eurasian kestrels of Spanish grassland and farmland populations

Lesser kestrel Falco naumanni and Eurasian kestrel Falco tinnunculus are two small raptor species commonly breeding in open agro-pastoral systems that have suffered a dramatic decline in their populations during the past few decades due to changes in land use. Direct intoxication by chemicals used in farming has been one of the causes proposed promoting population declines in both species. However, there have been no studies exploring the degree of kestrel intoxication at individual or population levels in the wild. We investigated the inter-population variation in blood levels of acetylcholinesterase (AChE) in nestlings of Spanish Lesser and Eurasian kestrels. AChE was used as an estimator of the degree of organophosphorus (OP) and/or carbamate (CB) exposure or intoxication. In addition, we measured the albumin-globulin ratio (A/G) as an index of general health status. We found only extremely low AChE levels in lesser kestrels from one of the seven studied populations. In addition, we did not find differences in nestling AChE levels between pastoral and agricultural habitats. Furthermore, the present study revealed that two of the seven lesser kestrel populations are suffering from poor health conditions, in one case due to OP-CB poisoning, and the other possibly due to a pathogen infection as suggested by the low A/G ratio observed. Thus, our study suggests that, in general, OP and/or CB pesticide poisoning in traditional agro-pastoral systems in Spain is not currently an important conservation factor for either Lesser or Eurasian kestrels chicks.     

Response of soil microbial biomass dynamics to quality of plant materials with emphasis on P availability

Soil microbial biomass is a major sink and source of plant available P and transformer of soil organic P. The dynamics of microbial biomass P (B_P) and C (B_C) and resin extractable P (P_ex) in a sandy loam soil (Kandic Rhodustalf) after amending with plant materials of diverse quality were examined in a laboratory incubation study. Quality variables of the plant materials were described by the proportion of dry matter as total P, total N, lignin (LIG), and soluble polyphenolics (Pp). The materials were added to the soil at a rate equivalent to 10 t ha⁻¹ (dry weight) and kept at 50% field moisture capacity and 25 °C. Included was an unamended control soil. During incubation of up to 56 d, soil samples were periodically analyzed for P_ex, B_P and B_C. Quality variables affecting P_ex, B_P, B_C and B_P-to-B_C ratio were total P, total N, N-to-P and C-to-P ratios. Total P appeared to be the most important quality factor affecting P_ex, B_P, B_C and B_P-to-B_C ratio. There was no consistent significant correlation between P_ex and B_P or between B_P and B_C. This was attributed to the complexity of the dynamics of P_ex and B_P pools and the absence of a significant microbial turnover due to the short incubation periods assessed in the study. By virtue of the high P content and resulting in a low B_P-to-B_C ratio, leaves of the shrubby non-legume Tithonia diversifolia (Hemsley) A. Gray, had a distinct advantage of replenishing P_ex in the short-term and yet maintain a high potential to release P through microbial turnover. The tree legume Sesbania sesban, on the other hand, resulted in a low B_P-to-B_C ratio but also had a low P content and may be useful for the build up of soil organic matter (SOM) and P release in the long term. Exploiting plant materials with quality variables exhibited by T. diversifolia might create the potentials for fast replenishment of solution P as well as improving SOM.     

The effects of earthworms on the demography of annual plant assemblages in a long-term mesocosm experiment

Earthworms have been shown to influence plant growth, survival and fecundity. They can therefore affect plant demography in plant communities changing their composition. A long term mesocosm experiment was set-up to test the effects of an endogeic (Aporrectodea caliginosa) and an anecic (Lumbricus terrestris) earthworm species on assemblages of four species of annuals: one grass (Poa annua), two forbs (Veronica persica and Cerastium glomeratum) and one legume (Trifolium dubium). The number of individuals and the biomass of each species were investigated. A. caliginosa and L. terrestris affected the density of T. dubium at each of the three monitored census dates. The other plant species responded to A. caliginosa and L. terrestris at the second and third generations. The presences of A. caliginosa and L. terrestris reduced the total number of plant individuals from the second to the third generation. At harvest (3rd generation), T. dubium and V. persica had more and larger individuals in the presence of A. caliginosa. When both earthworm species were present, T. dubium had few but larger individuals. Our study confirms that earthworms affect plant demography and plant community structure. Our results also show that accurate prediction of long-term effects of earthworms on plant communities cannot be achieved using results on their short-term effects on plant growth. This is due to the poor understanding of the effects of earthworms on plant resource allocation and demography, and also the possibility that earthworms may exert the opposite effect on the short and long-term availability of nutrients.     

Microsatellite Markers Revealed the Genetic Diversity of an Old Japanese Rice Landrace ‘Echizen’

Landraces of rice (Oryza sativa L.) are valuable sources of genetic variation that have been lost in advanced cultivars. Seeds of a rice landrace stored for almost 100 years were found on Sado Island in Niigata prefecture, Japan. This report aims to present basic data on the genetic variation of this landrace, which was known as ‘Echizen’. Five samples of ‘Echizen’, consisting of two old samples, one sample maintained on farm, and two lines regenerated from old seeds were compared with other advanced cultivars and landraces using 19 microsatellite markers. Among the five samples of Echizen, the two stored samples showed greater diversity than the other samples. Cluster analysis based on the UPGMA method also showed that old Echizen was a diverse landrace that could cover the genetic diversity of most Japanese rice cultivars.     

Lignin and cellulose degradation and nitrogen dynamics during decomposition of three leaf litter species in a Mediterranean ecosystem

Cellulose and lignin degradation dynamics was monitored during the leaf litter decomposition of three typical species of the Mediterranean area, Cistus incanus L., Myrtus communis L. and Quercus ilex L., using the litter bag method. Total N and its distribution among lignin, cellulose and acid-detergent-soluble fractions were measured and related to the overall decay process. The litter organic substance of Cistus and Myrtus decomposed more rapidly than that of Quercus. The decay constants were 0.47 year⁻¹, 0.75 year⁻¹ and 0.30 year⁻¹ for Cistus, Myrtus and Quercus, respectively. Lignin and cellulose contents were different as were their relative amounts (34 and 18%, 15 and 37%, 37 and 39% of the overall litter organic matter before exposure, for Cistus, Myrtus and Quercus, respectively). Lignin began to decrease after 6 and 8 months of exposure in Cistus and Myrtus, respectively, while it did not change significantly during the entire study period in Quercus. The holocellulose, in contrast, began to decompose in Cistus after 1 year, while in Quercus and Myrtus immediately. Nitrogen was strongly immobilized in all the litters in the early period of decay. Its release began after the first year in Cistus and Myrtus and after 2 years of decomposition in Quercus. These litters still contained about 60, 20 and 90% of the initial nitrogen at the end of the experiment (3 years). Prior to litter exposure nitrogen associated with the lignin fraction was 65, 54 and 37% in Cistus, Myrtus and Quercus, while that associated with the cellulose fraction was 30, 24 and 28%. Although most of the nitrogen was not lost from litters, its distribution among the litter components changed significantly during decomposition. In Cistus and Myrtus the nitrogen associated with lignin began to decrease just 4 months after exposure. In Quercus this process was slowed and after 3 years of decomposition 8% of the nitrogen remained associated with lignin or lignin-like substances. The nitrogen associated with cellulose or cellulose-like substances, in contrast, began to decrease from the beginning of cellulose decomposition in all three species. At the end of the study period most of the nitrogen was not associated to the lignocellulose fraction but to the acid-detergent-soluble substance (87, 88 and 84% of the remaining litter nitrogen).