Biodegradation of low molecular weight organic acids in rhizosphere soils from a tropical montane rain forest

Root exudation of organic acids could be an important strategy for plant acquisition of phosphorus (P) from P-deficient soils in tropical rain forests. However, the efficacy of organic acids on P mobilization in the rhizosphere could be reduced due to their rapid biodegradation by rhizosphere microorganisms. To assess the dynamics and function of organic acids in the rhizosphere soils in tropical rain forests, we examined the concentrations of oxalate, citrate, and malate in soil solution and the mineralization kinetics of ¹⁴C-radiolabelled oxalate and citrate in the rhizosphere and bulk soil fractions. We compared two tropical montane rain forests from Mt. Kinabalu, Borneo that share similar parent material (i.e., sedimentary rocks) and climate but differ in terms of soil age. The older soil (Tertiary age materials) was affected by podzolization and had less inorganic labile P compared to the younger soil (Quaternary colluvial deposits). In the P-deficient older soil, the rhizosphere soil solution contained markedly higher concentrations of oxalate, citrate, and malate than did the bulk soil, whereas in the P-rich younger soil, the levels of organic acids in the rhizosphere were lower. The higher levels of organic acids in the rhizosphere of P-deficient soils are caused by greater root exudation and the lower sorption capacity for organic acids. The results of mineralization kinetics showed that oxalate and citrate in soil solution were rapidly mineralized in both rhizosphere and bulk fractions of both P-rich and P-deficient soils, having short mean residence times (2.3–13.1 h for oxalate and 0.8–1.6 h for citrate). The mineralization rates of oxalate and citrate were highest in the rhizosphere fraction of the P-deficient soil, where the pool of organic acids was largest and rapidly replenished by root exudation. Our data indicate that consumption as well as production of organic acids in the rhizosphere could be enhanced in P-deficient soil. The efficacy of organic acids on P mobilization in the rhizosphere in tropical montane rain forests appears to vary depending on the level of soil P availability and the anion sorption capacity, attributable to soil aging with podzolization.     

Testate amoebae (protista) of an elevational gradient in the tropical mountain rain forest of Ecuador

We investigated the species composition of testate amoebae at three altitudes (1000, 2000 and 3000 m) and two horizons (L and F/H/Ah) of a tropical mountain rain forest in southern Ecuador. A total of 135 species and intraspecific taxa of testate amoebae were found (36 samples). Rarefaction plots suggest that only few more species are to be expected. Species number in the L horizon was at a maximum at 2000 m (109 species) and similar at 1000 and 3000 m (75 and 71 species, respectively). Species numbers in the F/H/Ah horizon were 29, 67 and 48 at 1000, 2000 and 3000 m, respectively. The density of testate amoebae in the L horizon increased significantly in the order 1000<3000<2000 m with 3147±129, 9198±144 and 12,497±1317 ind. g⁻¹ dry matter and in the F/H/Ah horizon with 176±25, 3118±97 and 4986±102 ind. g⁻¹ dry matter, respectively. The significant Horizon×Elevation interaction reflects the exceptionally low abundance of testate amoebae in the Ah horizon at 1000 m. The results suggest that species richness of testate amoebae does not decrease continuously with elevation; rather, it peaks at an intermediate elevation. Further, the data suggest that diversity, but not density of testate amoebae in tropical forests exceeds that in temperate forests. Morphological features of testate amoebae reflected semiaquatic habitat conditions. The great majority of testate amoebae species of the studied tropical mountain rain forests are geographically widespread, including temperate regions; however 9 of the species (i.e. 6.7%) are considered tropical, some of these species likely represent Gondwana relicts.     

Stability of hydrolytic enzyme activity and microbial phosphorus during storage of tropical rain forest soils

Storage can markedly influence microbial and biochemical properties in soils, yet recommendations for sample storage are based on studies of temperate soils that regularly experience extended cold periods. We assessed the influence of storage conditions on microbial phosphorus and the activity of four hydrolytic enzymes (phosphomonoesterase, phosphodiesterase, β-glucosidase, and N-acetyl-β-d-glucosaminidase) in three lowland tropical forest soils from the Republic of Panama that experience a constant warm temperature. The soils spanned a strong rainfall gradient and contained contrasting physical and chemical properties (pH 3.6-5.9; total carbon 26-50 g C kg⁻¹; clay 33-62%; total phosphorus 0.30-0.60 g P kg⁻¹). Storage treatments were: (i) room temperature (22 °C in the dark), (ii) refrigerated (4 °C in the dark), (iii) air-dried (10 d, 22 °C), and (iv) frozen (−35 °C). There were significant changes in enzyme activities and microbial phosphorus during refrigerated and room temperature storage, although changes were relatively small during the first two weeks. An initial marked decline in enzyme activities for one soil analyzed within 2 h of sampling was attributed to a flush of activity caused by sampling and soil preparation (sieving, etc.). For longer-term storage (>2 weeks), ambient laboratory temperature appeared preferable to freezing and cold storage, because one month of storage caused a marked decline in enzyme activities and microbial phosphorus in one soil. Freezing preserved the activities of some enzymes in some soils at rates comparable to cold or room temperature storage, but caused a marked decline in microbial phosphorus in two soils. Air-drying caused a marked decline in microbial phosphorus and the activity of all enzymes. We therefore conclude that enzyme assays and microbial phosphorus should be determined in tropical forest soils after no more than two weeks storage in the dark at ambient laboratory temperature.     

Soil properties and tree growth along an altitudinal transect in Ecuadorian tropical montane forest

In tropical montane forests, soil properties change with increasing altitude, and tree‐growth decreases. In a tropical montane forest in Ecuador, we determined soil and tree properties along an altitudinal transect between 1960 and 2450 m asl. In different vegetation units, all horizons of three replicate profiles at each of eight sites were sampled and height, basal area, and diameter growth of trees were recorded. We determined pH and total concentrations of Al, C, Ca, K, Mg, Mn, N, Na, P, S, Zn, polyphenols, and lignin in all soil horizons and in the mineral soil additionally the effective cation‐exchange capacity (CEC). The soils were Cambisols, Planosols, and Histosols. The concentrations of Mg, Mn, N, P, and S in the O horizons and of Al, C, and all nutrients except Ca in the A horizons correlated significantly negatively with altitude. The C : N, C : P, and C : S ratios increased, and the lignin concentrations decreased in O and A horizons with increasing altitude. Forest stature, tree basal area, and tree growth decreased with altitude. An ANOVA analysis indicated that macronutrients (e.g., N, P, Ca) and micronutrients (e.g., Mn) in the O layer and in the soil mineral A horizon were correlated with tree growth. Furthermore, lignin concentrations in the O layer and the C : N ratio in soil affected tree growth. These effects were consistent, even if the effect of altitude was accounted for in a hierarchical statistical model. This suggests a contribution of nutrient deficiencies to reduced tree growth possibly caused by reduced organic‐matter turnover at higher altitudes.     

Decomposition and macroinvertebrates in experimental litter along a secondary chronosequence of tropical montane forest

This paper presents the results of a decomposition experiment performed in a secondary chronosequence of tropical montane cloud forest in Mexico. The experiment was designed to explore whether the age of the forest influences the decomposition process and macroinvertebrate community independently of the quality of the decomposition resources. Fresh Pinus chiapensis needles and Persea americana leaves were set to decompose in each of four successional stages (15, 45, 75 and 100 years old). Results do not support the hypothesis that decomposition rate declines with increasing nutrient deficiency as forest succession proceeds. However, the chemical composition in decomposing leaves differed between successional stages. Higher availability of Ca in the 15-year-old forest appears to promote a positive feedback in the release of this nutrient from Persea americana leaves. Additionally, in old forests, a soil community that is more capable of breaking down recalcitrant material (acid detergent lignin) appears to have developed compared to early successional stages. The diversity of macroinvertebrates and abundance of predatory (Aranea and Diplura), detrivorous (Diplopoda) and geophagous (Enchytaeidae) taxa were different between boxes placed in different successional stages. We conclude that the decomposition and associated biota differ between successional stages. Apart from differences in litter quality, other factors associated with the age of the forest, such as small differences in soil temperature and long-lasting effects of disturbance, may also play influential roles.     

Characterization of organic matter in topsoils under rain forest and pasture in the eastern Brazilian Amazon basin

 In topsoils under forest and 7-, 12- and 17-year-old pastures, organic matter was characterized by analysing C and N distribution in particle-size fractions, the C decomposition rates of soil and particle-size fractions and by employing density-fractionation of macro-organic matter (>150 μm). The C and N associated with clay fractions increased with increasing age of pasture. The weight (%) of macro-organic matter and its heavy fractions (>1.37 g cm^–3) also increased with increasing age of pasture. However, in a long-term incubation (100 days), these changes seemed to involve an increase in the C decomposition rate in the topsoil of the oldest pasture. Using the C decomposition rates of particle-size fractions, it appeared that silt and clay contributed differently to C decomposition in the whole soil. C associated with silt contributed to the C decomposition rate during the first 40 days of incubation, while C associated with clay contributed to C decomposition in the long-term incubation (after 40 days), especially when the clay fraction appeared to reach saturation point with respect to its ability to bind organic compounds and thus protect the soil from C loss. Received: 13 March 1998     

Litter contribution to diurnal and annual soil respiration in a tropical montane cloud forest

Respiration of CO₂ from soils (R_s) is a major component of the carbon cycle of ecosystems, but understanding is still poor of both the relative contributions of different respiratory sources to R_s, and the environmental factors that drive diurnal variations in R_s. We measured total and litter-free R_s at half-hourly intervals over full 24 h periods, and thereafter twice a month for 10 months in a tropical montane cloud forest (TMCF) in Peru. Total R_s declined by about 61% during the night as a result of variations in respiration rate in the litter, which were partly correlated with the soil surface air temperature. Most of the diurnal variation of R_s in this TMCF appears to be driven by respiration in the litter layer, which contributed 37% to the total soil CO₂ efflux. Total R_s rates at this particular site would have been overestimated by 60% if derived from daytime measurements that had not been corrected for diurnal variations in R_s.     

Water flow paths in soil control element exports in an Andean tropical montane forest

We tested the hypothesis that concentrations of chemical constituents in stream water can be explained by the depth of water flow through soil. Therefore, we measured the concentrations of total organic carbon (TOC), NO₃‐N, NH₄‐N, dissolved organic nitrogen (DON), P, S, K, Ca, Mg, Na, Al and Mn in rainfall, throughfall, stemflow, litter leachate, mineral soil solution and stream water of three 8–13 ha catchments on steep slopes (1900–2200 m above sea level) of the south Ecuadorian Andes, from April 1998 to April 2003. Peak C (14–22 mg litre^?1), N (0.6–0.9 mg litre^?1), K (0.5–0.7 mg litre^?1), Ca (0.6–1.0 mg litre^?1), Mg (0.3–0.5 mg litre^?1), Al (110–390 μg litre^?1) and Mn (3.9–8.4 μg litre^?1) concentrations in stream water were associated with lateral flow (fast near‐surface flow in saturated topsoil) while the greatest P (0.1–0.3 mg litre^?1), S (0.3–0.7 mg litre^?1) and Na (3.0–6.0 mg litre^?1) concentrations occurred during low baseflow conditions. All elements had greater concentrations in the organic layer than in the mineral soil, but only C, N, K, Ca, Mg, Al and Mn were flushed out during lateral‐flow conditions. Phosphorus, S and Na, in contrast, were mainly released by weathering and (re‐)oxidation of sulphides in the subsoil. Baseflow accounted for 32% to 61% of P export, while > 50% of S was exported during intermediate flow conditions (i.e. lateral flow at the depth of several tens of cm in the mineral soil). Near‐surface water flow through C‐ and nutrient‐rich topsoil during rainstorms was the major export pathway for C, N, Al and Mn (contributing > 50% to the total export of these elements). Near‐surface flow also accounted for one‐third of total base metal export. Our results demonstrate that near‐surface flow related to storm events markedly affects the cycling of many nutrients in steep tropical montane forests.     

Controls on nitric oxide emissions from tropical pasture and rain forest soils

In field studies, forest soils in the Atlantic Lowlands of Costa Rica emitted greater amounts of nitric oxide (NO) than soils from pastures that had been actively grazed for over 20 years following their conversion from forest. We measured NO production from intact soil cores from these land uses. Laboratory tests using ammonium(NH ₄ ⁺ ), nitrate (NO ₃ ^– ), nitrite (NO ₂ ^– ), water, and acetylene (C₂H₂) additions demonstrate a response consistent with field studies.Forest soil cores produced more NO than pasture cores regardless of treatment. In forest soil the response toNH ₄ ⁺ solution was significantly greater than response to water or an ambient moisture control. Addition of 10 kPa C₂H₂ caused a marked decrease in NO production in forest soil cores. These responses suggest a nitrification-linked control over NO production. Large and rapid responses toNO ₂ ^– additions suggest that chemical decomposition of this ion may contribute to NO production. Pasture soil cores did not show a significant response to any of the treatments including NO ₂ ^– . Low porosity in the pasture soils may restrict emission of NO produced therein.     

Plot-scale manipulations of organic matter inputs to soils correlate with shifts in microbial community composition in a lowland tropical rain forest

Little is known about the organisms responsible for decomposition in terrestrial ecosystems, or how variations in their relative abundance may influence soil carbon (C) cycling. Here, we altered organic matter in situ by manipulating both litter and throughfall inputs to tropical rain forest soils, and then used qPCR and error-corrected bar-coded pyrosequencing to investigate how the resulting changes in soil chemical properties affected microbial community structure. The plot-scale manipulations drove significant changes in microbial community composition: Acidobacteria were present in greater relative abundance in litter removal plots than in double-litter plots, while Alphaproteobacteria were found in higher relative abundance in double-litter and throughfall reduction plots than in control or litter removal plots. In addition, the bacterial:archaeal ratio was higher in double-litter than no-litter plots. The relative abundances of Actinobacteria, Alphaproteobacteria and Gammaproteobacteria were positively correlated with microbial biomass C and nitrogen (N), and soil N and C pools, while acidobacterial relative abundance was negatively correlated with these same factors. Bacterial:archaeal ratios were positively correlated with soil moisture, total soil C and N, extractable ammonium pools, and soil C:N ratios. Additionally, bacterial:archaeal ratios were positively related to the relative abundance of Actinobacteria, Gammaproteobacteria, and Actinobacteria, and negatively correlated to the relative abundance of Nitrospira and Acidobacteria. Together, our results support the copiotrophic/oligotrophic model of soil heterotrophic microbes suggested by Fierer et al. (2007).     

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

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

Resource use and rarity among frugivorous birds in a tropical rain forest on Sulawesi

Relatively few studies have focussed on the relationship between resource use and rarity. Rare species could differ from common species in two ways: rare species may utilise resources that are themselves rarer or rare species may utilise a narrower range of resources than common species (i.e., they are more specialised). I investigated the relationship between local abundance and patterns of food resource use for an assemblage of 19 frugivorous birds in the tropical lowland rain forest of Sulawesi, Indonesia. Over a 10-month period I collected data on two measures of rarity, the local abundance and monthly variability in local abundance of each bird species, together with data on their feeding ecology including dietary specialisation, resource availability, resource overlap and habitat use. These measures produced 18 ecological variables, 17 of which I correlated with the two measures of rarity using univariate correlations and minimum adequate multiple regression models. These were carried out using the original data and independent contrasts that controlled for potential phylogenetic bias. Both measures of rarity were most strongly correlated with measures of resource availability. In both the original data and independent contrasts regression models of the temporal variability in bird abundance, only temporal fruit availability was included as a significant predictor. In the regression models of bird abundance, the strongest significant predictor was the amount of rare fruit in the diet (a negative relationship). Both models also included measures of resource specialisation, however, these were not significant in the absence of the measure of resource availability. On Sulawesi, rare frugivorous birds utilise rare resources. Specialisation, however, might be an important factor in species persistence once they become rare. The generality of the findings need to be tested.     

Nitrogen and phosphorus constrain labile and stable carbon turnover in lowland tropical forest soils

Tropical forests contain a large stock of soil carbon, but the factors that constrain its mineralization remain poorly understood. Microorganisms, when stimulated by the presence of new inputs of labile organic carbon, can mineralize (‘prime’) soil organic matter to acquire nutrients. We used stable carbon isotopes to assess how nutrient demand and soil properties constrain mineralization of added labile (sucrose) carbon and pre-existing (primed) soil carbon in tropical forest soils. In a series of lowland tropical forest soils from Panama, we found that the mineralization of fresh labile carbon was accelerated foremost by phosphorus addition, whereas the mineralization of pre-existing soil carbon was constrained foremost by nitrogen addition. However, there was variation in the relative importance of these nutrients in different soils and the largest effects on the acceleration of sucrose metabolism and constraint of priming occurred following the addition of nitrogen and phosphorus together. The respiration responses due to sucrose or primed soil carbon mineralization were reduced at pH below 4.8 and above 6.0. We conclude that in these tropical forest soils, phosphorus availability is more important in promoting microbial mineralization of sucrose carbon, whereas nitrogen availability is more important in constraining the priming of pre-existing soil organic carbon. This response likely arises because nitrogen is more closely coupled to organic matter cycling, whereas phosphorus is abundant in both organic and inorganic forms. These results suggest that the greatest impact of priming on soil carbon stocks will occur in moderately acidic tropical forest soils of low nitrogen availability. Given long-term changes in both atmospheric carbon dioxide and nitrogen deposition, the impact of priming effects on soil carbon in tropical forest soils may be partially constrained by the abundance of nitrogen.     

Mineral composition of leaves and bark in aluminum accumulators in a tropical rain forest in Indonesia

Mineral composition including AI, Ca, Mg, P, S, and Si and relationships between Al and other elements such as Ca, Mg, P, S, and Si in leaves and bark of trees in a tropical rain forest in West Sumatra were studied. Sixty five tree species and 12 unidentified trees were referred to as AI accumulators based on Chenery\s's definition (more than 1 g kg^-1 Al in leaves). For most of the Al accumulators, Al concentration in leaves was higher than in bark. However, some members of Euphorbiaceae, Melastomataceae, and Ulmaceae families showed a reverse trend. Most of the non-accumulators also showed a higher Al concentration in bark than in leaves. These results indicated that there was a difference in the mechanism of Al accumulation in tree bodies. Some of the Al accumulators showed an extremely high Al concentration (more than 10 g kg^-1) not only in the mature leaves, but also in the new leaves. Analysis of the relationships between the concentration of Al and the other 5 elements in leaves, revealed that Al accumulators could be separated into two groups at the Al concentration of 3 g kg^-1. This finding suggested that new criteria based on Al concentration (23 g kg^-1) or Al/Ca ratio in leaves could be proposed in order to define Al accumulators, apart from Chenery's criterion. Aluminium accumulators with an Al concentration in leaves lower than 3 g kg^-1 (AI accumulators <3 g kg^-1) showed the same trend as the non-accumulators in terms of these elemental relationships, while Al accumulators with an Al concentration in leaves higher than 3 g kg^-1 (AI accumulators 23 g kg^-1) showed a different trend from the non-accumulators. The Al accumulators 23 g kg^-1 and the other trees (AI accumulators < 3 g kg^-1 and non-accumulators) showed separately positive correlations between the concentrations of AI and Ca (or Mg) in the leaves. This observation seems to be opposite to general findings in plant nutrition, i.e. inhibition of Ca or Mg uptake by AI. A positive correlation between Al and S was also observed for all the trees. The Al accumulators ≥3 g kg^-1 showed positive correlations between the concentrations of Al and P (or Si) in the leaves, unlike the other trees. These findings suggested that Al stimulated P, S, or Si accumulation in leaves or Al was transported with P, S, or Si for the Al accumulators ≥3 g kg^-1. No negative relationships between Al and the other 5 elements in the leaves were observed for the Al accumulators ≥3 g kg^-1.     

Soil organic carbon in density fractions of tropical soils under forest – pasture – secondary forest land use changes

Our knowledge of effects of land use changes and soil types on the storage and stability of different soil organic carbon (SOC) fractions in the tropics is limited. We analysed the effect of land use (natural forest, pasture, secondary forest) on SOC storage (depth 0–0.1 m) in density fractions of soils developed on marine Tertiary sediments and on volcanic ashes in the humid tropics of northwest Ecuador. The origin of organic carbon stored in free light (< 1.6 g cm^?3) fractions, and in two light fractions (LF) occluded within aggregates of different stability, was determined by means of δ¹³C natural abundance. Light occluded organic matter was isolated in a first step after aggregate disruption by shaking aggregates with glass pearls (occluded I LF) and in a subsequent step by manual destruction of the most stable microaggregates that survived the first step (occluded II LF). SOC storage in LFs was greater in volcanic ash soils (7.6 ± 0.6 Mg C ha^?1) than in sedimentary soils (4.3 ± 0.3 Mg C ha^?1). The contribution of the LFs to SOC storage was greater in natural forest (19.2 ± 1.2%) and secondary forest (16.6 ± 1.0%) than in pasture soils (12.8 ± 1.0%), independent of soil parent material. The amount of SOC stored in the occluded I LF material increased with increasing silt + clay content (sedimentary soils, r = 0.73; volcanic ash soils, r = 0.58) and aggregation (sedimentary soils, r = 0.52; volcanic ash soils, r = 0.45). SOC associated with occluded I LF, had the smallest proportion of new, pasture‐derived carbon, indicating the stabilizing effect of aggregation. Fast turnover of the occluded II LF material, which was separated from highly stable microaggregates, strongly suggested that this fraction is important in the initial process of aggregate formation. No pasture‐derived carbon could be detected in any density fractions of volcanic ash soils under secondary forest, indicating fast turnover of these fractions in tropical volcanic ash soils.     

More sex at higher altitudes: Changes in the frequency of parthenogenesis in oribatid mites in tropical montane rain forests

We investigated changes in density, proportion of parthenogenetic species and individuals, and community structure of oribatid mites (Oribatida, Acari) along an altitudinal gradient in tropical montane rain forests in southern Ecuador. We hypothesized that the proportion of parthenogenetic species and individuals increases whereas density decreases with increasing altitude due to increased harshness of abiotic conditions known to favor parthenogenetic reproduction. In contrast to our hypothesis, the number of parthenogenetic species and individuals decreased toward higher altitudes indicating that changes in environmental conditions with altitude favor sexual rather than parthenogenetic reproduction. Low density of oribatid mites at high altitudes suggest that high frequency and density of sexual species is favored by the availability of resources and not by factors related to harsh abiotic conditions, finding mating partners or tight coupling with parasites or pathogens. Cosmopolitan decomposer taxa tended to be more frequent at higher altitudes indicating that these species are euryoecious. Overall, our data support the view that the reproductive mode of soil animals is predominantly controlled by the availability and accessibility of resources.     

Nutrient limitations to bacterial and fungal growth during cellulose decomposition in tropical forest soils

Nutrients constrain the soil carbon cycle in tropical forests, but we lack knowledge on how these constraints vary within the soil microbial community. Here, we used in situ fertilization in a montane tropical forest and in two lowland tropical forests on contrasting soil types to test the principal hypothesis that there are different nutrient constraints to different groups of microorganisms during the decomposition of cellulose. We also tested the hypotheses that decomposers shift from nitrogen to phosphorus constraints from montane to lowland forests, respectively, and are further constrained by potassium and sodium deficiency in the western Amazon. Cellulose and nutrients (nitrogen, phosphorus, potassium, sodium, and combined) were added to soils in situ, and microbial growth on cellulose (phospholipid fatty acids and ergosterol) and respiration were measured. Microbial growth on cellulose after single nutrient additions was highest following nitrogen addition for fungi, suggesting nitrogen as the primary limiting nutrient for cellulose decomposition. This was observed at all sites, with no clear shift in nutrient constraints to decomposition between lowland and montane sites. We also observed positive respiration and fungal growth responses to sodium and potassium addition at one of the lowland sites. However, when phosphorus was added, and especially when added in combination with other nutrients, bacterial growth was highest, suggesting that bacteria out-compete fungi for nitrogen where phosphorus is abundant. In summary, nitrogen constrains fungal growth and cellulose decomposition in both lowland and montane tropical forest soils, but additional nutrients may also be of critical importance in determining the balance between fungal and bacterial decomposition of cellulose.     

The effects of organic carbon on acid rain in a temperate forest in Japan

From recent studies, we noticed that stemflow had an acidity that differed from that of precipitation or throughfall. Organic substances, supplied from the tree surface, would be one of the factors that modifies the acidity of rain. The objectives of this study were to determine the DOC concentration and to clarify the influence of dissolved organic carbon (DOC) on acidity in precipitation, throughfall and stemflow. Throughfall and stemflow were measured in sugi [Cryptomeria japonica D. Don], hinoki [Chamaecyparis obutusa Endl.] and kojii [Castanopsis cuspidata (Thumb.) Schottky.] stands. All samples were analyzed for their pH, electric conductivity (EC), major inorganic anions and cations and DOC concentration.The annual average of DOC was highest in stemflow, and that of throughfall and precipitation were one-third and one-tenth of stemflow, respectively. The averages of DOC in stemflow in two coniferous, sugi and hinoki stands, were higher than that of broadleaved kojii stand. DOC concentration was low in summer and high in winter in all stands. In Stemflow, pH and DOC were negatively correlated, while EC and DOC in stemflow were positively correlated in all stands. However in throughfall, there was no evident relationship between pH, DOC and EC. This relationship was not explained by the cause of organic acid.