Nitrogen cycle of tropical perennial crops under shade trees

The distribution and fluxes of nitrogen in some parts of a coffee plantation under shade were studied at a typical mountain (1380 m a sl) location in Venezuela. The amounts of nitrogen in the soil to 60 cm depth are by far the largest nitrogen store, reaching a total of 49 000 kg ha^?1. The nitrogen flow associated with litterfall was dominated by the shade-tree fraction accounting for a transfer of 86 kg ha^?1 yr^?1 of the total 189 kg ha^?1 yr^?1. The rapid decomposition of this litter, although showing a phase of nitrogen accumulation, is an important source of nitrogen to the roots of coffee which occupy preferentially the upper 30 cm of soil and even the litter layer itself. Some evidence of synchrony was found between the peaks of nitrogen transfer to the soil by litter and the periods of high nitrogen demand by the crop plants. It is proposed that the system can amply compensate the nitrogen outputs by harvest (17 kg ha^?1 yr^?1) with a subsidy from the shade trees.Erythrina sp. andInga sp. are potential nitrogen fixers although we found no active sites during the dry period sampled. The average litter decomposition constant, k, expressed in terms of nitrogen, was estimated as 4.5, equivalent to a half-life of approximately two months.     

Plant species control and soil faunal involvement in the processes of above‐ and below‐ground litter decomposition

Among the factors determining litter decomposition rates, the role of soil fauna as decomposers still remains unclear, especially for how they are involved in decomposing below‐ground root litter compared to their relatively‐known contributions to decomposing above‐ground leaf litter. We conducted a litterbag experiment using two sizes of meshes and pursued the leaf and root decomposition of six major tree species in a Japanese temperate forest over 411‐days to test the interactive effects of soil mesofauna and litter quality addressed based on two features (litter types and species) on the process. Moreover, given a possible correlation between litter traits of the leaves and roots, we examined whether soil mesofauna alters the relationship between leaf and root decomposition across species. We found that the effects of plant species identity was stronger than that of soil mesofauna for determining the litter mass loss rate and the microbial respiration rate in both above‐ground and below‐ground decomposition. In addition, we found a significant positive correlation between leaf and root litter decomposition processes, regardless of the involvement soil mesofauna. On the other hand, the presence of soil mesofauna increased microbial respiration rates in the early stage of leaf decomposition; however, soil mesofauna did not affect root microbial respiration rates during the experiment. Such differential involvement of mesofauna in the leaf and root litter decomposition may drive the general patterns of faster and slower decomposition of plant leaves and roots in the soil, respectively.     

Decomposition of <Superscript>15</Superscript>N-labelled beech litter and fate of nitrogen derived from litter in a beech forest

The decomposition and the fate of ¹⁵N- labelled beech litter was monitored in a beech forest (Vosges mountains, France) over 3 years. Circular plots around beech trees were isolated from neighbouring tree roots by soil trenching. After removal of the litter layer, ¹⁵N-labelled litter was distributed on the soil. Samples [labelled litter, soil (0–15 cm depths], fine roots, mycorrhizal root tips, leaves) were collected during the subsequent vegetation periods and analysed for total N and ¹⁵N concentration. Mass loss of the ¹⁵N-labelled litter was estimated using mass loss data from a litterbag experiment set up at the field site. An initial and rapid release of soluble N from the decomposing litter was balanced by the incorporation of exogenous N into the litter. Fungal N accounted for approximately 35% of the N incorporation. Over 2 years, litter N was continuously released and rates of N and mass loss were equivalent, while litter N was preferentially lost during the 3rd year. Released ¹⁵N accumulated essentially at the soil surface. ¹⁵N from the decomposing litter was rapidly (i.e. in 6 months) detected in roots and beech leaves and its level increased regularly and linearly over the course of the labelling experiment. After 3 years, about 2% of the original litter N had accumulated in the trees. ¹⁵N budgets indicated that soluble N was the main source for soil microbial biomass. Nitrogen accumulated in storage compounds was the main source of leaf N, while soil organic N was the main source of mycorrhizal N. Use of ¹⁵N-labelled beech litter as decomposing substrate allowed assessment of the fate of litter N in the soil and tree N pools in a beech forest on different time scales. Received: 3 May 1999 / Accepted: 3 January 2000     

Root decomposition across a barrier island chronosequence: Litter quality and environmental controls

A root decomposition study using the litterbag approach was conducted along a dune and swale chronosequence on the Virginia Coast Reserve-Long Term Ecological Research Site in Virginia, USA to evaluate how environmental and substrate quality factors influence belowground decay and associated nutrient dynamics. Gradients in moisture levels and nitrogen availability associated with the chronosequence provided the experimental framework. Spartina patens roots were buried at all sites as a standard substrate to evaluate environmental influences. Roots native to each site were buried to evaluate community decay dynamics and the influence of litter quality. Spartina decay was reduced in the wet, anoxic soils of swale sites (k = 0.21–0.33 yr^-1) relative to decay in dunes soils (k = 0.52–0.72 yr^-1). Increasing soil nitrogen availability from younger to older sites had no effect on the rate of Spartina root decay. Native root decay across the Hog Island chronosequence exhibits certain trends expected in response to nitrogen limitation and moisture availability. Increased nitrogen content of root material corresponds to increased soil nitrogen availability. Among dune sites, native root decay increased in concert with increased root nitrogen (6 year k = 0.34 yr^-1, 120 year dune: k = 0.97 yr^-1). Litter quality, alone, does not explain this trend since Spartina roots decayed more slowly than native dune roots and had a higher initial nitrogen content. Among swales, increased moisture levels and associated soil anoxia inhibited native root decomposition and minimized the effects of litter quality on decay. In general, phosphorus was rapidly lost from decaying roots while nitrogen immobilization was low to nonexistent. The low nitrogen immobilization of decaying roots in a nitrogen limited ecosystem warrants further study and may reveal that belowground decay increases the rate of nutrient cycling relative to decay aboveground.     

The effect of plant species,weather variables and chemical composition of plant material on decomposition in a tropical grassland

Summary The decomposition of litter and roots ofChenopodium album, Desmostachya bipinnata and mixed grass samples for a period of 402 days and ofDichanthium annulatum andSesbania bispinosa for a period of 278 days was studied in a tropical grassland. Litter bags positioned at midcanopy height, soil surface and at five cm depth below the soil surface and root bags placed at 5, 15, 25 and 35 cm depths belowground were used. For the total study period, the cumulative weight loss in litter bags was: Chenopodium=76–100%; Desmostachya=33–98%; Dichanthium=26–96%; mixed grass=43–99% and Sesbania=25–99%. The weight loss in root bags was: Chenopodium=93–100%; Desmostachya=47–56%; Dichanthium=71–87%; mixed grass=61–82%; Sesbania=87–100%. The nature of plant species affected decomposition rates. The position of litter/root bags also affected the decomposition rates. The mean relative decomposition rates of litter as well as of root material were found to be highest in rainy season and lowest in winter months. Rainfall, particularly the frequency of rainfall, was an important factor affecting decomposition rates. The litter species characterized by highest concentration of nitrogen, ash, acid detergent cell wall component and lowest concentration of carbon, cellulose and lignin, decomposed rapidly. In the case of roots, the material having high nitrogen, carbon, cellulose and ash content and low C/N ratio and lignin content decomposed rapidly.     

Nitrogen fixation in a Mexican coffee plantation

Fertilizer studies in Mexico indicate that coffee production can be stimulated by added nitrogen. One traditional method of coffee cultivation employs leguminous trees for shade, but these species may also play an important role in coffee production by biologically fixing nitrogen. The presence and importance of nitrogen fixation was evaluated in four systems: coffee only, coffee plus the leguminous shade treeInga jinicuil Schletchter, coffee plus the leguminous treeInga vera H.B. and K., and coffee plus banana and orange trees. In all systems coffee leaves with epiphylls, wood litter, soil, roots, and root nodules were assayed for nitrogen fixing activity with the acetylene reduction technique. All components of these systems exhibited activity except roots. Total apparent fixation was highest in theInga jinicuil site, and equivalent to >40 kg N ha^?1 yr^?1 assuming a 3∶1 C₂H₂∶N₂ ratio. The activity was primarily associated withInga jinicuil nodules. Apparent fixation in the other three sites was less than 1 kg N ha^?1 yr^?1. Nitrogen fixed in theI. jinicuil site was 53% of the average amount of fertilizer nitrogen applied annually, suggesting that fixation by non-crop legumes can be an important nitrogen source for coffee agro-ecosystems.     

Nutrient dynamics within amazonian forests

Summary Relationships between fine root growth, rates of litter decomposition and nutrient release were analysed in a mixed forest on Tierra Firme, a Tall Amazon Caatinga and a Low Bana on podsolized sands near San Carlos de Rio Negro. Fine root growth in the upper soil layers (root mat+10 cm upper soil) was considerably higher in the Tierra Firme forest (1117 g m^-2 yr^-1) than in tall Cattinga (120) and Bana (235). Fine root growth on top of the root mat was stimulated significantly by added N in Tall Caatinga and Low Bana forests, by P in Tierra Firme and Bana forests, and by Ca only in the Tierra Firme forest. Rate of fine root growth in Tierra Firme forest on fresh litter is strongly correlated with the Mg and Ca content of litter. Rate of litter decomposition was inversely related to % lignin and the lignin/N ratio of litter. Litter contact with the dense root mat of the Tierra Firme increased rates of disappearance for biomass, Ca and Mg as compared with litter permanently separated or lifted weekly from the root mat to avoid root attachment. Nitrogen concentration of decomposing litter increased in all forests, net N released being observed only in Caryocar glabrum and Aspidosperma megalocarpum of the Tierra Firme forest after one year of exposure. Results emphasize the differences in limiting nutrients in amazonian forest ecosystems on contrasting soil types: Tierra Firme forests are particularly limited by Ca and Mg, while Caatinga and Bana forests are limited mainly by N availability.     

Growth dynamics of superficial roots in Portuguese plantations ofEucalyptus globulus Labill. studied with a mesh bag technique

Summary The variation in growth of the fine roots of blue gum (Eucalyptus globulus labill. ssp.globulus) in the 0–40 cm soil layer was studied from March 1982 to March 1983 at Quinta do Furaduoro, Óbidos, Portugal. A mesh bag method was used; bags of nylon net were inserted into a clay soil and a sandy soil and filled with root-free soil. They were resampled after 2, 4, 6 and 12 months in both places and, in a separate series in the sandy soil every second month throughout the year.The ingrowth of roots was high during the winter months but there was also a surprisingly high ingrowth during the spring-early summer period. There was also some root growth during the driest part of the yearviz. July–September.The amount of fine roots reached a maximum of about 260 g dw m^–2 after about 6 months in the sandy soil, whereas it took at least 12 months to reach the somewhat higher level of 450 g dw m^–2 in the clay soil. At that level the decomposition of dead roots was expected to equal the formation of new roots. Dead roots appeared after only 2 months. There was a higher proportion of dead roots in the clay soil than in the sandy soil, 35% as compared with 20% on an average, which indicates a slower decomposition or a higher mortality at equal decomposition rates in the clay than and in the sandy soil. The present data gives an indication of a minimum fine root production in mature Eucalyptus stands of at least 600 g dw m^–2 yr^–1.     

Long-Term Simulated Atmospheric Nitrogen Deposition Alters Leaf and Fine Root Decomposition

Atmospheric nitrogen deposition increases forest carbon sequestration across broad parts of the Northern Hemisphere. Slower organic matter decomposition and greater soil carbon accumulation could contribute to this increase in carbon sequestration. We investigated the effects of chronic simulated nitrogen deposition on leaf litter and fine root decomposition at four sugar maple (Acer saccharum)-dominated northern hardwood forests. At these sites, we previously observed that nitrogen additions increased soil organic carbon and altered litter chemistry. We conducted a 3-year decomposition study with litter bags. Litter production of leaves and fine roots were combined with decomposition dynamics to estimate how fine roots and leaf litter contribute to soil organic carbon. We found that nitrogen additions marginally stimulated early-stage decomposition of leaf litter, an effect associated with previously documented changes in litter chemistry. In contrast, nitrogen additions inhibited the later stages of fine root decomposition, which is consistent with observed decreases in lignin-degrading enzyme activities with nitrogen additions at these sites. At the ecosystem scale, slower fine root decomposition led to additional root mass retention (g m^?2), and this greater retention of root residues was estimated to explain 5–51% of previously documented carbon accumulation in the surface soil due to nitrogen additions. Our results demonstrated that simulated nitrogen deposition created contrasting effects on the decomposition of leaf litter and fine roots. Although previous nitrogen deposition studies have focused on leaf litter, this work suggests that slower fine root decomposition is a major driver of soil organic carbon accumulation under elevated nitrogen deposition.     

Longevity and turnover of roots in the shortgrass steppe: influence of diameter and depth

We used minirhizotrons to determine patterns of root longevity andturnover for the perennial bunchgrass Bouteloua gracilisinthe shortgrass steppe of eastern Colorado, USA. We hypothesized that rootlongevity would be partially controlled by root diameter, following previouslyobserved patterns in woody plants. In addition, we hypothesized that rootturnover would be greatest in surface soil horizons and decrease with depth dueto variation in soil moisture availability and temperature. Root longevity wascorrelated with root diameter. Median life span of roots > 0.4mm was approximately 320 days, while roots < 0.2mmhad a median life span of 180 days. There was approximately a 6%decreasein the likelihood of mortality with a 0.10-mm increase inroot diameter, controlling for the effect of depth in the soil profile. Rootlength production and mortality were highest in the upper20 cm of the soil profile and decreased with depth.However,because root length density also decreased with depth, there were nosignificantdifferences in turnover rate of root length among sampling intervals. Turnoverwas approximately 0.86 yr^–1 based on root length production,while turnover was 0.35 yr^–1 using root length mortality as ameasurement of flux. The imbalance between turnover estimates may be aconsequence of the time the minirhizotrons were in place prior to imaging or mayresult from our lack of over-winter measures of mortality. Our worksuggests that Bouteloua gracilis roots have complex lifehistory strategies, similar to woody species. Some portion of the root systemishighly ephemeral, while slightly larger roots persist much longer. Thesedifferences have implications for belowground carbon and nitrogen cycles in theshortgrass steppe.     

Controls over leaf and litter calcium concentrations among temperate trees

Four-fold variation in leaf-litter Ca concentration among 14 tree species growing in a common garden in central Poland was linked to variation in soil pH, exchangeable Ca, soil base saturation, forest floor turnover rates, and earthworm abundance. Given the potential importance of tissue Ca to biogeochemical processes, in this study we investigated potential controls on leaf Ca concentrations using studies of both laboratory seedlings and 30-year-old trees in the field. We first assessed whether species differences in Ca concentration of green leaves and leaf litter were due to differences in Ca uptake, plant growth, or Ca translocation to different organs, by measuring seedlings of 6 of the 14 species grown under controlled conditions of varying Ca supply. We also investigated whether trees species with high Ca concentrations in green leaves and leaf litter access soil Ca to a greater extent than low-Ca species by growing more fine roots in high-Ca soil horizons. Root distribution in the field was determined in all 14 tree species by profile wall mapping and soil sampling of excavated pits. There was no correlation between horizon root count density (number of roots m⁻²) and exchangeable soil Ca, nor was there a correlation of stand-level leaf litter Ca with density of roots 45–100 cm deep in the soil, suggesting that a deeper root distribution does not result in greater Ca acquisition among these species. Variation among species in leaf Ca concentration of greenhouse seedlings was positively correlated with leaf Ca concentrations of mature trees, indicating that the same ranking in leaf Ca among species existed under controlled Ca supply. Species also differed in seedling growth response to Ca supply. Tilia, the species with the highest leaf Ca in the field, generated only 10% as much biomass and height at low relative to high Ca supply, whereas the other species exhibited no significant differences. Species exhibited differences in (i) partitioning of whole plant Ca and biomass to leaf, stem and root organs and (ii) the pattern of such partitioning between high and low Ca treatments. Our data support the hypothesis that although soil Ca supply can contribute to variation among trees in leaf and litter Ca concentration, innate physiological differences among species also can be a major cause for species variation.     

The soil fauna of a beech forest on limestone: trophic structure and energy budget

Summary The soil fauna of a mull beech forest on lime-stone in southern Lower Saxony (West Germany) was sampled quantitatively. Biomass estimates, trophic characteristics, and measurement and calculation of the energetic parameters of the constituent animal populations were used to construct an energy budget of the total heterotrophic subsystem of the forest. Mean annual zoomass amounted to about 15 g d wt m^–2; earthworms (about 10 g d wt m^–2) and other groups of the macrofauna were dominant. Protozoa constituted about 1.5 g d wt m^–2. Relative distribution of zoomass among the trophic categories was 50% macrosaprophages, 30% microsaprophages, 12% microphytophages, and 4% zoophages. Total annual consumption rate of the saprophagous and microphytophagous soil fauna (6328 and 4096 kJ m^–2 yr^–1, respectively) was of the same order of magnitude as annual litter fall (canopy leaves 6124 kJ m^–2 yr^–1, flowers and fruits 944 kJ m^–2 yr^–1, herbs 1839 kJ m^–2 yr^–1, fine woody material 870 kJ m^–2 yr^–1, tree roots 3404 kJ m^–2 yr^–1, without coarse woody litter). Primary decomposers (macrosaprophages) were the key group for litter comminution and translocation onto and into the soil, thus contributing to the high decomposition rate (k=0.8) for leaf litter. Consumption rates of the other trophic groups were (values as kJ m^–2 yr^–1): bacteriophages 2954, micromycophages 416, zoophages 153. Grazing pressure of macrophytophages (including rhizophages) was low. Faeces input from the canopy layer was not significant. Grazing pressure on soil microflora almost equalled microbial biomass; hence, a large fraction of microbial production is channelled into the animal component. Predator pressure on soil animals is high, as a comparison between consumption rates by zoophages and production by potential prey — mainly microsaprophages, microphytophages and zoophages — demonstrated. Soil animals contributed only about 11% to heterotrophic respiration. However, there is evidence that animals are important driving variables for matter and energy transfer: key processes are the transformation of dead organic material and grazing on the microflora. It is hypothesized that the soil macrosaprophages are donor-limited.     

The invasive species Alliaria petiolata (garlic mustard) increases soil nutrient availability in northern hardwood-conifer forests

The invasion of non-native plants can alter the diversity and activity of soil microorganisms and nutrient cycling within forests. We used field studies to analyze the impact of a successful invasive groundcover, Alliaria petiolata, on fungal diversity, soil nutrient availability, and pH in five northeastern US forests. We also used laboratory and greenhouse experiments to test three mechanisms by which A. petiolata may alter soil processes: (1) the release of volatile, cyanogenic glucosides from plant tissue; (2) the exudation of plant secondary compounds from roots; and (3) the decomposition of litter. Fungal community composition was significantly different between invaded and uninvaded soils at one site. Compared to uninvaded plots, plots invaded by A. petiolata were consistently and significantly higher in N, P, Ca and Mg availability, and soil pH. In the laboratory, the release of volatile compounds from the leaves of A. petiolata did not significantly alter soil N availability. Similarly, in the greenhouse, the colonization of native soils by A. petiolata roots did not alter soil nutrient cycling, implying that the exudation of secondary compounds has little effect on soil processes. In a leaf litter decomposition experiment, however, green rosette leaves of A. petiolata significantly increased the rate of decomposition of native tree species. The accelerated decomposition of leaf litter from native trees in the presence of A. petiolata rosette leaves shows that the death of these high-nutrient-content leaves stimulates decomposition to a greater extent than any negative effect that secondary compounds may have on the activity of the microbes decomposing the native litter. The results presented here, integrated with recent related studies, suggest that this invasive plant may change soil nutrient availability in such a way as to create a positive feedback between site occupancy and continued proliferation.     

Fine root growth phenology,production, and turnover in a northern hardwood forest ecosystem

A large part of the nutrient flux in deciduous forests is through fine root turnover, yet this process is seldom measured. As part of a nutrient cycling study, fine root dynamics were studied for two years at Huntington Forest in the Adirondack Mountain region of New York, USA. Root growth phenology was characterized using field rhizotrons, three methods were used to estimate fine root production, two methods were used to estimate fine root mortality, and decomposition was estimated using the buried bag technique. During both 1986 and 1987, fine root elongation began in early April, peaked during July and August, and nearly ceased by mid-October. Mean fine root ( 3 mm diameter) biomass in the surface 28-cm was 2.5 t ha^–1 and necromass was 2.9 t ha^–1. Annual decomposition rates ranged from 17 to 30% beneath the litter and 27 to 52% at a depth of 10 cm. Depending on the method used for estimation, fine root production ranged from 2.0 to 2.9 t ha^–1, mortality ranged from 1.8 to 3.7 t ha^–1 yr^–1, and decomposition was 0.9 t ha^–1 yr^–1. Thus, turnover ranged from 0.8 to 1.2 yr^–1. The nutrients that cycled through fine roots annually were 4.5–6.1 kg Ca, 1.1–1.4 kg Mg, 0.3–0.4 kg K, 1.2–1.7 kg P, 20.3–27.3 kg N, and 1.8–2.4 kg S ha^–1. Fine root turnover was less important than leaf litterfall in the cycling of Ca and Mg and was similar to leaf litterfall in the amount of N, P, K and S cycled.     

Above- and belowground organic matter storage and production in a tropical pine plantation and a paired broadleaf secondary forest

The distribution of tree biomass and the allocation of organic matter production were measured in an 11-yr-old Pinus caribaea plantation and a paired broadleaf secondary forest growing under the same climatic conditions. The pine plantation had significantly more mass aboveground than the secondary forest (94.9 vs 35.6 t ha^-1 for biomass and 10.5 vs 5.0 t ha^{-1 for litter}), whereas the secondary forest had significantly more fine roots (⩽2 mm diameter) than the pine plantation (10.5 and 1.0 t ha^-1, respectively). Standing stock of dead fine roots was higher than aboveground litter in the secondary forest. In contrast, aboveground litter in pine was more than ten times higher than the dead root fraction. Both pine and secondary forests had similar total organic matter productions (19.2 and 19.4 t ha^-1 yr^-1, respectively) but structural allocation of that production was significantly different between the two forests; 44% of total production was allocated belowground in the secondary forest, whereas 94% was allocated aboveground in pine. The growth strategies represented by fast growth and large structural allocation aboveground, as for pine, and almost half the production allocated belowground, as for the secondary forest, illustrate equally successful, but contrasting growth strategies under the same climate, regardless of soil characteristics. The patterns of accumulation of organic matter in the soil profile indicated contrasting nutrient immobilization and mineralization sites and sources for soil organic matter formation.     

南亚热带米老排人工林碳贮量及其分配特征

米老排是我国南方速生用材树种,研究米老排(Mytilaria laosensis)人工林碳贮量与碳分配的规律,可为评价米老排人工林的固碳能力与发展人工林多目标经营提供科学依据。采用样地测定的方法,对南亚热带米老排人工林不同器官碳浓度、碳贮量及分配特征进行了研究。结果表明:不同器官碳浓度均值的变化范围为:51.73%—55.75%,各器官碳浓度大小为:新叶>新枝>老叶>树干>老枝>根桩>粗根>细根;凋落物碳浓度为未分解层>半分解层;0—100 cm土壤碳浓度随深度增加而降低,变化范围为0.62%—4.10%。20年生米老排人工林总碳贮量为331.61 t/hm2,乔木层,凋落物层和土壤层的碳贮量分别为154.07、2.74和174.80 t/hm2。年均总固碳量为14.77 t/hm2,折合CO2量为54.16 t,其中乔木层、凋落物层和土壤层所占比例分别为60.73%,6.16%和33.11%。     

Leaf litter decomposition and nutrient dynamics in a subtropical forest after typhoon disturbance

Decomposition of typhoon-generated and normal leaf litter and their release patterns for eight nutrient elements were investigated over 3 yr using the litterbag technique in a subtropical evergreen broad-leaved forest on Okinawa Island, Japan. Two common tree species, Castanopsis sieboldii and Schima wallichii, representative of the vegetation and differing in their foliar traits, were selected. The elements analyzed were N, P, K, Ca, Mg, Na, Al, Fe and Mn. Dry mass loss at the end of study varied in the order: typhoon green leaves > typhoon yellow leaves > normal leaves falling for both species. For the same litter type, Schima decomposed faster than Castanopsis. Dry mass remaining after 2 yr of decomposition was positively correlated with initial C:N and C:P ratios. There was a wide range in patterns of nutrient concentration, from a net accumulation to a rapid loss in decomposition. Leaf litter generated by typhoons decomposed more rapidly than did the normal litter, with rapid losses for N and P. Analysis of initial quality for the different litter types showed that the C:P ratios were extremely high (range 896 – 2467) but the P:N ratios were < 0.05 (range 0.02 – 0.04), indicating a likely P-limitation for this forest. On average 32% less N and 60% less P was retranslocated from the typhoon-generated green leaves than from the normal litter for the two species, Castanopsis and Schima. An estimated 2.13 g m^–2 yr^–1 more N and 0.07 g m^–2 yr^–1 more P was transferred to the soil as result of typhoon disturbances, which were as high as 52% of N and 74% of P inputted from leaf litter annually in a normal year. Typhoon-driven maintenance of rapid P cycling appears to be an important mechanism by which growth of this Okinawan subtropical forest is maintained.     

Arbuscular mycorrhizal fungi colonize decomposing leaves of<Emphasis Type="Italic"> Myrica parvifolia</Emphasis>,<Emphasis Type="Italic"> M. pubescens</Emphasis> and<Emphasis Type="Italic"> Paepalanthus</Emphasis> sp.

Hyphae and vesicles of arbuscular mycorrhizal fungi (AMF) were found within the decomposing leaves of Myrica parvifolia, M. pubescens and Paepalanthus sp. at three montane sites in Colombia. Hyphae, vesicles, and arbuscule-like structures were also found within scale-like leaves of the rhizomes of Paepalanthus sp. The litter found in the vicinity of the roots was divided into three decomposition layers. The highest AMF colonization occurred in the most decomposed leaves, which were in close association with roots. In contrast, there were no differences in AMF colonization of roots present in the different decomposition layers. Colonization of decomposing leaves by AMF did not differ between the two closely related species M. parvifolia and M. pubescens, nor between two sites (Guatavita and Zipacón, Colombia) differing in soil fertility. Occurrence of vesicles in decomposing leaves was correlated with abundant AMF extraradical hyphae among the leaves. We propose that AMF enter decomposing leaves mechanically through vascular tissue. As a consequence, AMF are well positioned to obtain and efficiently recycle mineral nutrients released by decomposer microorganisms before their loss by leaching or immobilization in soil.     

Mass loss and nitrogen dynamics in decomposing acid forest litter in the Netherlands at increased nitrogen deposition

Litterbag experiments were carried out in five forest ecosystems in the Netherlands to study weight loss and nitrogen dynamics during the first two years of decomposition of leaf and needle litter. All forests were characterized by a relatively high atmospheric nitrogen input by throughfall, ranging from 22–55 kg N ha^–1 yr^–1.Correlation analysis of all seven leaf and needle litters revealed no significant relation between the measured litter quality indices (nitrogen and lignin concentration, lignin-to-nitrogen ratio) and the decomposition rate. A significant linear relation was found between initial lignin-to-nitrogen ratio and critical nitrogen concentration, suggesting an effect of litter quality on nitrogen dynamics.Comparison of the decomposition of oak leaves in a nitrogen-limited and a nitrogen-saturated forest suggested an increased nitrogen availability. The differences in capacities to retain atmospheric nitrogen inputs between these two sites could be explained by differences in net nitrogen immobilization in first year decomposing oak leaves: in the nitrogen-limited oak forest a major part (55%) of the nitrogen input by throughfall was immobilized in the first year oak leaf litter.The three coniferous forests consisted of two monocultures of Douglas fir and a mixed stand of Douglas fir and Scots pine. Despite comparable litter quality in the Douglas fir needles in all sites, completely different nitrogen dynamics were found.     

Respiration from coarse wood litter in central Amazon forests

Respiration from coarse litter (trunks and large branches >10 cm diameter) was studied in central Amazon forests. Respiration ratesvaried over almost two orders of magnitude (1.003–0.014 µg Cg^–1 C min^–1, n = 61), and weresignificantly correlated with wood density (r² _adj= 0.42), and moisture content (r² _adj= 0.39). Additional samples taken from a nearby pasture indicatedthat wood moisture content was the most important factor controllingrespiration rates across sites (r² _adj =0.65). Based on average coarse litter wood density and moisture content,the mean long-term carbon loss rate due to respiration was estimated tobe 0.13 yr^–1 (range of 95% prediction interval(PI) = 0.11–0.15 yr^–1). Comparing meanrespiration rate with mean mass loss (decomposition) rate from aprevious study, respiratory emissions to the atmosphere from coarselitter were predicted to be 76% (95% PI =65–88%) of total carbon loss, or about 1.9 (95% PI= 1.6–2.2) Mg C ha^–1yr^–1. Optimum respiration activity corresponded toabout 2.5 g H₂O g^–1 dry wood, and severelyrestricted respiration to < 0.5 g H₂O g^–1dry wood. Respiration from coarse litter in central Amazon forests iscomparable in magnitude to decomposing fine surface litter (e.g. leaves,twigs) and is an important carbon cycling component when characterizingheterotrophic respiration budgets and net ecosystem exchange(NEE).