Decomposition of Erythrina poeppigiana leaves in 3-, 9-, and 18-year-old alleycropping systems in Costa Rica

Timing the application of organic residues and therefore the release of nutrients during decomposition may be critical to the growing crop in tropical alleycropping agroforestry systems. Field experiments were carried out in Turrialba, Costa Rica, to determine differences in Erythrina poeppigiana (Walp.) O.F. Cook leaf decomposition in 3, 9 and 18-year alleycropped agroforestry systems. Treatments consisted of mulch-only, and mulch plus Arachis pintoi Krapov. and W. Gregory var. CIAT 18347 in 3 and 9-year old alleycrops under no-till cultivation. The 18-year old site consisted of treatments with mulch-only and mulch plus chicken manure under disk plow cultivation. Litterbags, filled with E. poeppigiana leaves from 3, 9 and 18-year old trees, were placed on the soil surface and collected over a period of 84 days. Results showed no significant differences in the amount of plant residues remaining after 84 days in the 3-, 9-, and 18-year-old systems, or between the manure and mulch-only treatments. Comparing mulch-only treatments, leaves in the 18-year old system decomposed most rapidly which may be due to disk-plow cultivation practices where litterbags were in direct contact with the soil as opposed to the no-till system in the younger alleycrops.     

Dynamics and stratification of protozoa in the organic layer of a Scots pine forest

Fluctuations in soil biota abundance in different organic layers of a Scots pine forest in The Netherlands were studied by bimonthly counts during 2.5 years. The counts were made using litterbags which were placed in the litter (L), fragmentation (F) and humus (H) layers at the start of the experiment. Results from the L layer were also compared with results from litter which was renewed every 2 months (L′) to study colonisation. In this study the results for amoebae, flagellates and ciliates are presented. The highest numbers of soil protozoa were found in the L layer during most sampling occasions. The H layer contained the lowest numbers. The L layer also showed higher numbers than the L′ litterbags which were renewed every 2 months. Fluctuations in abundance could partly be explained by fluctuations in moisture content. Moisture content in the litterbags was rather constant throughout the experiment, although occasionally moisture contents of 10% and 80% were observed. Fluctuations in moisture content in the L layer were often larger than in the F and H layers. Flagellates were the most abundant group, reaching numbers of several hundred thousands to several millions per gram fresh weight on various occasions. Amoebae often reached numbers of between tens of thousands and several hundred thousands. Ciliates only reached numbers of up to several thousands. Received: 26 June 1997     

Dynamics and stratification of functional groups of nematodes in the organic layer of a Scots pine forest in relation to temperature and moisture

Nematode trophic groups were studied in litterbags in a Pinus silvestris forest using sampling periods of 8 weeks during 2.5 years. Temperature, moisture relationships and annual periodicity of nematodes were analyzed in the litter (L), fragmentation (F) and humus (H) horizons. Litterbags containing L, F and H material were placed in stacks and buried in the organic layer. Undisturbed core samples were used to compare the nematode abundance under normal field conditions with that in the litterbags. Time dependence of population growth and colonization was also studied in separate litterbags that were replaced every 8 weeks. During the first 4 to 6 months of the experiment, nematodes in the litterbag stacks grew rapidly to circa 5×10⁶m^–2. After that period, abundance gradually decreased to about 2.5×10⁶m^–2. These abundances were similar to those found in undisturbed cores. Nematode abundance during the first year was most pronounced in the top (L) litterbags; subsequently densities were more or less the same in the three organic horizons, reflecting the gradual change of L to F material. On average, during 2.5 years, bacterial feeding nematodes were the dominant group in the organic horizons (73%), with 17% hyphal feeders and 9% plant feeders. There were dissimilarities between layers and in the course of time. The number of hyphal feeding nematodes differed significantly between layers. In the first 2 to 4 months, hyphal feeding nematodes equalled the bacterial feeders in the L layer. Later bacterial feeders became dominant. The highest number of plant feeding nematodes was found in the F litterbags. Significant effects of temperature and moisture were mainly found on bacterial feeding nematodes. Regression coefficients for trophic group abundances and moisture were generally positive. Temperature was negatively correlated with the three functional groups in the L horizon only. Bacterial and hyphal feeding nematodes showed a significant decrease with time in the L layer, reflecting diminishing substrate quality (and food availability) during decomposition. A significant annual periodicity could be demonstrated for bacterial feeders in L litterbags and plant feeding nematodes in the H material. Received: 26 June 1997     

Effects of litter quality and microarthropods on N dynamics and retention of exogenous 15N in decomposing litter

Summary Surface additions of (¹⁵NH₄)₂SO₄ were used to measure the immobilization and subsequent movement of exogenous N added to two litter types of contrasting quality (Cornus florida and Quercus prinus). Litterbaskets were used to measure the litter mass loss and N dynamics and to follow the movement of the ¹⁵N label through litter, F layer, and soil pools. Half of the litterbaskets of each species were treated with naphthalene to reduce microarthropod densities. The faster decomposing C. florida litter maintained a higher excess atom % ¹⁵N, and a greater relative concentration of the labeled input (g ¹⁵N g^–1) than did Q. prinus litter. In both litter types the excess atom % ¹⁵N, relative concentration (g ¹⁵N g^–1), and absolute amount of label recovered in the litter declined over time. This occurred during a period of net accumulation of total litter N, implying simultaneous release of the initial input and immobilization of N from other sources. The concentration of ¹⁵N in the soil increased over time, while the F layer apparently acted as an intermediary in the transfer of ¹⁵N from litter to soil. Naphthalene effectively reduced microarthropod numbers in all horizons of the litterbaskets and significantly reduced the decay rates of Q. prinus, but not C. florida litter. Naphthalene did not appear to affect total N dynamics in the litter. However, with all horizons taken together, the naphthalene-treated litterbaskets retained more total ¹⁵N than the control litterbaskets. Naphthalene also changed the vertical distribution of ¹⁵N within litterbaskets, so that the litter retained less of the ¹⁵N-labeled input and the F layer and soil horizons retained more of the labeled input than in control litterbaskets. Our major conclusions are: (1) the N pool of decomposing litter is dynamic, with simultaneous N release and immobilization activating N turnover even during the net accumulation phase; (2) litter quality is an important determinant of immobilization and retention of exogenous N inputs and, therefore, turnover of the litter N pool; and (3) microarthropod activity can significantly affect the incorporation and retention of exogenous N inputs in decomposing litter, although these changes are apparently not reflected in net N accumulation or release during the 1st year of decomposition. However, the naphthalene may have affected microbially mediated N dynamics and this possibility needs to be considered in interpreting the results.     

Decomposition of plant tissue submerged in an extremely acidic mining lake sediment: phenolic CuO-oxidation products and solid-state C NMR spectroscopy

In extremely acidic mining sediments of the Lusatian mining district, the alkalinisation process relies on organic C, which can serve as electron donor for microbially induced sulfate reduction. Plant material of the pioneer plant Juncus bulbosus is an important organic matter source in lake sediments. Therefore, decomposition of the plant tissue was assessed during the exposure of litterbags for 30 months in the 0-5 cm layer of waterlogged mining sediments, which have a pH between 2.5 and 3. The ash free dry weight (AFDW) and elemental content of the plant tissue were recorded several times during the exposure. Changes in chemical structure were analyzed by solid-state ¹³C cross polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy and the lignin component characterized by wet-chemical CuO oxidation. The AFDW accounted for about 34% of initial biomass after field exposure for 30 months. Mass loss of biomass occurred in two phases with decomposition rates varying between 30 and 430 mg AFDW d⁻¹. The mass loss increased considerably after 5-7 months when litterbags were invaded by fresh J. bulbosus plants. With respect to higher mass loss, ¹³C CPMAS NMR spectroscopy, showed slight changes of the bulk chemical composition after 11 months, indicating that microorganisms present in the sediments or in the rhizosphere degrade plant material as a whole, rather than selectively. During the second phase from about 11 months until the end of the exposure period, contribution of O-alkyl C most probably assignable to easily degradable polysaccharides decreased. In contrast, the contribution of alkyl, aromatic and carboxyl C increased. CuO oxidation showed that the lignin component of J. bulbosus is degraded oxidatively during field exposure. Our results indicate that the exposed plant material is decomposed in the sediment due to changes in sediment conditions that followed plant invasion of the litterbags. It is suggested that the rhizosphere of J. bulbosus by its influence on the redox potential, pH and the microbial component plays a crucial role in organic matter degradation in acidic mining sediments.     

Soil mesofauna dynamics,wheat residue decomposition and nitrogen mineralization in buried litterbags

The effect of soil microarthropods and enchytraeids on the decomposition of wheat straw in buried litterbags was studied by selective admission and exclusion. Litterbags with 20 m mesh size admitted nematodes, but excluded microarthropods, although temporarily. After 27 weeks of incubation part of these litterbags were colonized, probably through egg-deposition of mainly fungivorous Collembola and mites. When litterbags with a complete microarthropod community (1.5 mm mesh size) were compared to litterbags with strongly reduced microarthropod numbers (20 m mesh size), no differences between decomposition rates were found. However, in colonized 20-m mesh bags, we found reduced decomposition rates compared to the coarse mesh litterbags, probably due to overgrazing of the fungal population by large numbers of fungivorous microarthropods. These large numbers might be caused by the absence of predators. Extraction of microarthropods as well as enchytraeids and nematodes from the coarse mesh litterbags showed a distinct succession during decomposition. The decomposition process was dominated in the first phase by bacterivorous nematodes, nematophagous and bacterivorous mites, and in the later phase by fungivorous nematodes, fungivorous and omnivorous mites and Collembola, and predatory mites. This succession is indicative of a sequence from bacterial to fungal dominated decomposition of the buried organic matter. The results indicate that the decomposition rate is predator controlled.     

Applications of microarthropod life-history tactics in nature management and ecotoxicology

Examples of the practical use of microarthropod life-history tactics in nature management and ecotoxicology were selected. A key to the tactics was developed to facilitate the use of the life-history tactics. Examples of the microarthropod distribution over these tacties were selected from a forest, a grassland, and a salt marsh. The process of decomposition of grass leaves in litterbags was described using life-history tactic diagrams. The effects of various disturbances were described in terms of shifts in the distribution of life-history tactics of species. Irregular unpredictable disturbances lead to an increased representation of phoresy tactics. Regular disturbances lead to an increased representation of synchronization tactics. Permanent and persistent pollution, finally, leads to an increased representation of tactics with thelytokous reproduction. The consequences of the latter for ecotoxicology were examined. It was concluded that microarthropod life-history tactics meet the criteria described, permitting comparisons between effects of management measures and pollution in different biotopes and countries.     

Aboveground–belowground relationships in temperate forests: Plant litter composes and microbiota orchestrates

The effects of plant diversity on ecosystem functioning have been mainly studied for fast growing systems such as grassland. These studies have stressed the context dependence of aboveground and belowground relationships (e.g. soil fertility, field vegetation or soil biota). Over the last few years, with the growing need for indicators for sustainable forest management, there has been a considerable increase in studies on AG–BG relationships within forest ecosystems. Nevertheless, given the specific characteristics of forest systems (long-term dynamics, the single tree effect) our mechanistic understanding of these relationships remains poor.     

Site of leaf origin affects how mixed litter decomposes

Recent studies have demonstrated that mass loss, nutrient dynamics, and decomposer associations in leaf litter from a given plant species can differ when leaves of that species decay alone compared to when they decay mixed with other species’ leaves. Results of litter-mix experiments have been variable, however, making predictions of decomposition in mixtures difficult. It is not known, for example, whether interactions among litter types in litter mixes are similar across sites, even for litter mixtures containing the same plant species. To address this issue, we used reciprocal transplants of litter in compartmentalized litterbags to study decomposition of equal-mass litter mixtures of sugar maple (Acer saccharum Marshall) and red oak (Quercus rubra L.) at four forest sites in northwestern Connecticut. These species differ significantly in litter quality. Red oak always has higher lignin concentrations than maple, and here C:N is lower in oak leaves and litter, a pattern often observed when oak coexists with maple. Overall, we observed less mass loss and lower N accumulation in sugar maple and red oak litter mixtures than we predicted from observed dynamics in single-species litterbags. Whether these differences were significant or not depended on the site of origin of the leaves (P<0.02), but there was no significant interaction between sites of decay and the differences in observed and predicted decomposition (P>0.2) . Mixing of leaf litter types could have significant impacts on nutrient cycling in forests, but the extent of the impacts can vary among sites and depends on the origin of mixed leaves even when the species composition of mixes is constant.     

Green mulch decomposition and nitrogen release from leaves of two Inga spp. in an organic alley-cropping practice in the humid tropics

Inga edulis Mart and Inga samanensis Uribe are promising yet little studied legume trees for use in agroforestry on acidic soils. The objective of this study was to analyze the decomposition and N release processes of green mulch from these species. Litterbags filled with leaves from each species were placed on the ground in an organic maize (Zea mays L.) alley-cropping experiment at the time of maize sowing and collected every 2 weeks over a 20 week period, and measured for dry matter, N, hemicellulose, cellulose, and lignin content. Three types of models were applied to the data, according to the characteristics of each component, to analyze the decomposition dynamics of whole leaves and leaf components: a negative exponential decay function, an inverted Michaelis-Menten function, and a linear regression. Initial decay of I. samanensis mulch was faster than I. edulis mulch. However, the recalcitrant fraction was about half of the initial litter mass in both Inga spp. Hemicellulose disappeared almost completely from the litter during the 20-week incubation period, while no significant lignin decay occurred. After a slow start, cellulose partially decayed following linear kinetics. The half-life of labile N, estimated as a Michaelis-Menten parameter, was 10 weeks in I. samanensis and ca. 24 weeks in I. edulis litter. Polyphenol content was significantly higher in I. edulis. Litter of I. edulis and I. samanensis may be classified as ‘low-quality’ and ‘medium-quality’ mulch, respectively. Due to the relatively large recalcitrant mulch fraction, both Inga spp. may promote C sequestration and long-term N accumulation in soil.