Early stages of root and leaf decomposition in Hawaiian forests: effects of nutrient availability

We examined the effects of soil nutrient availability and tissue chemistry on decomposition of both fine roots (<2 mm diameter) and leaves in three sites along a forest chronosequence in the Hawaiian Islands. These sites form a natural fertility gradient, with the youngest and oldest sites having lower nutrient availability than the intermediate-aged site. Nitrogen (N) limits aboveground net primary productivity (ANPP) in the youngest site, while phosphorus (P) limits ANPP in the oldest site. Both root and leaf litter decomposed most slowly in the 4.1-Myear-old site. We also investigated root decomposition in fertilized plots at the youngest and oldest sites; when roots were produced and decomposed in fertilized plots, root decomposition rates increased with N and P additions at the 4.1-Myear-old site. At the 300-year-old site, however, root decomposition rates did not respond to N or P additions. Roots decomposed faster than leaves at the more infertile sites, in part because of lower lignin-to-nitrogen ratios in roots than in leaf litter. Decomposing roots immobilized more nutrients than did decomposing leaves, and may serve an important role in retaining nutrients in these forests. Received: 30 November 1998 / Accepted: 12 August 1999     

Organic matter and nitrogen accumulation and nitrogen fixation during early ecosystem development in Hawaii

We used a chronosequence comprised of 10 y, 52 y and 142 yold `a'a lava flows on Mauna Loa, Hawaii, to determine theaccumulation of organic matter and nitrogen and rates of nitrogenfixation through time. The mass of organic matter (live and deadbiomass and soil organic matter) on the 1984, 1942 and 1852 lavaflows was 0.6, 2.2 and 7.6 kg m^{– 2}, respectively, while total N was 4.8, 10.9 and 85.7 g m^{– 2}.We estimated the total rates of nitrogen fixation for thethree different aged ecosystems using an acetylene reduction assaycalibrated with ¹⁵N incubations. While mean rates of total N fixation remained largely constant across the three sites – between2.0 and 3.1 kg ha^{– 1} y^{– 1} – the most important sources of N fixation changed. On the 10 y flow, the most important fixer was the pioneering cyanolichen, Stereocaulon vulcani. After 52 years ofecosystem development, the most important N fixer was a cyanoalga,while after 142 years, the predominant N fixers were heterotrophicbacteria associated with leaf litter, twigs and detritus. The totalamount of N accumulated after 52 years of ecosystem development wasequivalent to cumulative inputs through biological N fixation. After 142 years, however, cumulative inputs from N fixation couldonly account for between 27–59% of the total nitrogen accrued.We used fertilizer additions of all essential nutrients otherthan N to test whether the availability of lithophilic nutrientsregulated rates of N fixation in early ecosystem development. Ratesof nitrogen fixation by the lichen, S. vulcani, approximately doubled when fertilized on the 1984 and 1942 flows. Rates of N-fixation by heterotrophic nitrogen fixing bacteria on leaf litter ofMetrosideros polymorpha also increased significantly when fertilized with lithophilic nutrients. These findings suggest that weathering rates of lava in part regulate rates of nitrogen fixation in these young ecosystems.     

Nutrient Limitation to Nitrogen Fixation in Young Volcanic Sites

I used measures of ¹⁵N natural abundance and of nitrogenase activity (acetylene reduction) to examine whether the supply of non-N nutrients limits rates of N₂ fixation on young volcanic substrates in Hawaii. Leaves of the dominant tree (Metrosideros polymorpha, a nonfixer) were strongly depleted in ¹⁵N in control plots (–10.8 to –11.1⁰/₀₀). More than 5 y of repeated fertilization with P increased δ¹⁵N to –8.9 to –9.9⁰/₀₀, and the addition of all other essential plant nutrients (except N) together with P further increased ¹⁵N to –8.1 to –9.3⁰/₀₀. This pattern is consistent with enhanced N₂ fixation, because newly fixed N would have a δ¹⁵N near 0⁰/₀₀. Assays of nitrogenase activity in the experimental plots demonstrated that potential N fixation associated with nonvascular plants and with tree and fern litter were increased significantly by additions of P and by the combined nutrient treatment; when these were added together, the increase in nitrogenase activity was 6- to 11-fold over control plots. The supply of P and other weathering-derived nutrients constrains rates of N₂ fixation in these young volcanic sites and thereby contributes to the maintenance of N limitation to primary production and other ecosystem processes. Received 7 January 1999; accepted 3 May 1999.     

Polyphenols in litter from tropical montane forests across a wide range in soil fertility

In nutrient-poor ecosystems high polyphenol concentrations in plant litter have been proposed to influence soil nutrient availability in benefit of the plants. We addressed the question whether litter polyphenol concentrations vary across a soil chronosequence of almost identical geology, climate and plant species composition, but of a wide range in nitrogen (N) and phosphorus (P) availability in the Hawaiian Islands. Concentrations of total phenolics (TPh) and proanthocyanidins (PA) in leaf litter of the dominant tree species Metrosideros polymorpha were higher at the oldest, P-limited site compared to the youngest, N-limited site, with intermediate values at the two relatively fertile sites co-limited by N and P. Polyphenol concentrations in fine root litter differed considerably from those observed in leaf litter and varied differently across the soil age gradient. Long-term fertilization did not significantly alter polyphenol concentrations in Metrosideros litter at either site. Moreover, green leaves and leaf litter of Metrosideros showed similar relative differences among sites when compared between natural populations and plants from the same populations but grown in a common garden. These results suggest that polyphenol concentrations inherently vary among populations of the dominant tree species in Hawaiian montane forests possibly indicating an adaptation to ecosystem properties such as substrate age related differences in soil fertility. The combined above- and below-ground input rate of TPh ranged from 62.4 to 170.8 g/m²/yr and was significantly higher at the P-limited than at the N-limited site. Root-derived polyphenols contributed a much higher absolute and relative amount of phenolic input at the N-limited than at the P-limited site. The differences in amount, quality, and pathways of input might suggest specific interactions with soil processes and nutrient cycling among the Hawaiian rainforests studied here.     

Soil Phosphorus Fractions and Symbiotic Nitrogen Fixation across a Substrate-Age Gradient in Hawaii

We evaluated soil phosphorus (P) fractions, other soil characteristics, and rates of symbiotic N₂ fixation across a substrate-age gradient in Hawaii that was dominated by the leguminous tree Acacia koa (koa). Patterns of soil P observed on this gradient were compared to those on a slightly wetter gradient dominated by the nonfixer Metrosideros polymorpha (ohia). Along both gradients, concentrations of primary-mineral P fell sharply between the young and intermediate-aged sites, while labile inorganic P declined most steeply between the intermediate-aged and old sites. The most marked difference between the two gradients was that total soil carbon (C), nitrogen (N), and P, as well as nonoccluded organic P, were more variable across the ohia gradient, increasing to the intermediate-aged sites, then declining sharply at the old site. On the koa gradient, specific nitrogenase activity, measured by the acetylene-reduction (AR) assay, decreased three- to eightfold between the young site and the intermediate-aged and old sites, respectively. Nodule biomass showed no clear pattern. N₂ fixation rates, estimated by combining AR activity and nodule biomass measurements, were up to 8 kg N · ha⁻¹ · y⁻¹ at the young site and no more than 2 kg N · ha⁻¹ · y⁻¹ at the older sites, suggesting that koa may be a modest source of N in these Hawaiian forests. Received 26 September 2000; accepted 15 February 2002     

An invasive frog, Eleutherodactylus coqui, increases new leaf production and leaf litter decomposition rates through nutrient cycling in Hawaii

A frog endemic to Puerto Rico, Eleutherodactylus coqui, invaded Hawaii in the late 1980s, where it can reach densities of 50,000 individuals ha⁻¹. Effects of this introduced insectivore on invertebrate communities and ecosystem processes, such as nutrient cycling, are largely unknown. In two study sites on the Island of Hawaii, we studied the top-down effects of E. coqui on aerial, herbivorous, and leaf litter invertebrates; herbivory, plant growth, and leaf litter decomposition rates; and leaf litter and throughfall chemistry over 6 months. We found that E. coqui reduced all invertebrate communities at one of the two study sites. Across sites, E. coqui lowered herbivory rates, increased NH₄⁺ and P concentrations in throughfall, increased Mg, N, P, and K in decomposing leaf litter, increased new leaf production of Psidium cattleianum, and increased leaf litter decomposition rates of Metrosideros polymorpha. In summary, E. coqui effects on invertebrates differed by site, but E. coqui effects on ecosystem processes were similar across sites. Path analyses suggest that E. coqui increased the number of new P. cattleianum leaves and leaf litter decomposition rates of M. polymorpha by making nutrients more available to plants and microbes rather than through changes in the invertebrate community. Results suggest that E. coqui in Hawaii has the potential to reduce endemic invertebrates and increase nutrient cycling rates, which may confer a competitive advantage to invasive plants in an ecosystem where native species have evolved in nutrient-poor conditions.     

Exotic grass invasion alters potential rates of N fixation in Hawaiian woodlands

Exotic grass invasion promotes fire which drives the conversion of native woodlands to exotic grasslands in the seasonally dry submontane forests of the island of Hawai'i. We compared potential rates of N fixation in an unburned forest site and a converted grassland site using the acetylene reduction assay. In addition to measuring rates of N fixation on separate and mixed substrates in each site, we tested the effect of abiotic factors on rates of N fixation of specific substrates. We hypothesized that rates of N fixation would be higher in the converted grassland site. N fixation estimates were 4.9 kg N ha⁻¹ year⁻¹ for the unburned forest, and 0.10 kg N ha⁻¹ year⁻¹ for the grassland site, so our hypothesis was rejected. The N fixation in the unburned forest occurs mostly on the leaf litter of native woody species. These substrates are absent from the grassland site, except for wood debris which was not consumed during the fires. No nitrogenase activity was detected in the rhizosphere and litter of grasses, the rhizospheres of shrubs or in soil. Although wood debris is not a significant contributor to the N fixed in the unburned forest, it contributes the majority of N fixed in the grassland. The response of nitrogenase activity to varying conditions of moisture and temperature suggests that microclimatic differences between sites do not control differences in N fixation activity; rather, these differences are due to the abundance of N-fixing substrates. The substantial decrease in N fixation activity after the conversion from woodland to grassland implies that ecosystem-level rates of N accretion are decreased by fire in these sites so much that the N lost during volatilization due to fire is not replenished over the long term by N fixation. Received: 10 January 1997 / Accepted: 7 August 1997     

Nutrient Movement in Litter Fall and Precipitation Components for Central Himalayan Forests

The annual total litter fall in six Central Himalayan forestsranged from 2.1 to 3.8 t C ha^–1, of which 54 to 82 percent was leaf litter, 9–20 per cent wood litter and 6–14per cent other litter. In all forests the order of relativeabundance of nutrients (kg ha^-1 year^-1) in litter fall was Ca(50.8–91.6) > N (47.7–72.2) > K (22.8–37.1)> P (4.1–6.4). Leaf litter accounted for 63–95per cent of the total nutrients returned through litter fall. In these forests throughfall ranged from 71.3 to 81.4 per cent,stemflow from 0.50 to 2.16 per cent and canopy interceptionfrom 17.7 to 28.2 per cent of the gross rainfall. In the incidentrainfall the concentration and annual input of Ca was the greatestand of P the least. Canopy precipitation was richer in all nutrientscompared to incident rainfall. Net gain of nutrients from thecanopy ranged from 0.16 kg ha^-1 year^-1, for P, to 17.77 kg ha^-1year^-1 for K. Leaching was greatest for K and least for N. Ofthe total quantity of nutrients returned to the soil, 11 to46 per cent was accounted for by precipitation components. Thusprecipitation inputs play a significant role in nutrient cyclingof these forests. Himalaya, forest, litter fall, precipitation components, nutrients     

Nitrogen Oxide Fluxes and Nitrogen Cycling during Postagricultural Succession and Forest Fertilization in the Humid Tropics

The effects of changes in tropical land use on soil emissions of nitrous oxide (N₂O) and nitric oxide (NO) are not well understood. We examined emissions of N₂O and NO and their relationships to land use and forest composition, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and patterns of carbon (C) cycling in a lower montane, subtropical wet region of Puerto Rico. Fluxes of N₂O and NO were measured monthly for over 1 year in old (more than 60 years old) pastures, early- and mid-successional forests previously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertilized tabonuco forest. N₂O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg N · ha⁻¹· y⁻¹). Among the unfertilized sites, the expected pattern of increasing emissions with stand age did not occur in all cases. The mid-successional forest most dominated by leguminous trees had the highest emissions (9.0 kg N · ha⁻¹· y⁻¹), whereas the mid-successional forest lacking legumes had the lowest emissions (0.09 kg N · ha⁻¹· y⁻¹). N oxide fluxes from late-successional forests were higher than fluxes from pastures. Annual N oxide fluxes correlated positively to leaf litter N, net nitrification, potential nitrification, soil nitrate, and net N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissions. We conclude that (a) N oxide fluxes were substantial where the availability of inorganic N exceeded the requirements of competing biota; (b) species composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) the established ecosystem models that predict N oxide loss from positive relationships with soil ammonium may need to be modified. Received 22 February 2000; accepted 6 September 2000.     

Nitrogen uptake in a Norway spruce stand following ammonium sulphate application,fertigation, irrigation,drought and nitrogen-free-fertilisation

The above-ground accumulation of N,N uptake and litter quality resulting from improved or deteriorated availability of water and nutrients in a 25 year old Norway spruce stand in SW Sweden (as part of the Skogaby project) is presented. Treatment include irrigation; artificial drought; ammonium sulphate addition; N-free-fertilisation and irrigation with liquid fertilisers including a complete set of nutrients according to the Ingested principle (fertigation). At start of the experiment the stand contained 86.5 t dry mass and 352 kg N ha⁻¹. The following three years the annual N uptake in untreated trees was 32 kg N ha⁻¹ to be compared with the annual N throughfall of 17 kg ha⁻¹. Simultaneously, the treatment with ammonium sulphate and liquid fertilisation resulted in 48 and 56 kg ha⁻¹ y⁻¹, respectively, in treatment specific N-uptake following an application of 100 kg N ha⁻¹ y⁻¹. Addition of a N-free fertiliser resulted in improved N-uptake by 19 kg N ha⁻¹ y⁻¹ and irrigation by 10 kg N ha⁻¹ y⁻¹, compared to control. A linear relation between total above-ground dry mass production and N-uptake was found for trees growing with similar water availability. Dry mass production increased with increased water availability given the same N-uptake. It is concluded that the studied stand this far is not N saturated', as N fertilisation resulted in both increased N uptake and increased growth. Addition of a N-free-fertiliser resulted in increased uptake of N compared to the control, indicating an increased mineralisation rate or uptake capacity of the root system. The linear relation between N uptake and biomass production shows that at this study site N is a highly limiting factor for growth.     

Decomposition and Fragmentation of Coarse Woody Debris: Re-visiting a Boreal Black Spruce Chronosequence

We re-visited a seven-stand boreal chronosequence west of Thompson, Manitoba, Canada, in which coarse woody debris (CWD) and its instantaneous decomposition were measured in 2000. New CWD measurements were performed in 2007, and tree inventories updated to provide mortality and snag failure data. These data were used to model CWD changes, compare methods of estimating decomposition, and infer possible fragmentation rates. Measured CWD was between 9.7 (in both the 77- and 43-year-old stands) and 80.4 (in the 18-year-old stand) Mg ha⁻¹ in 2007. Spatial variability was high; at most stands CWD levels had not changed significantly from 2000 to 2007. Tree mortality was a significant flux only in older stands, whereas snag fall rate varied by an order of magnitude, from 2.9% y⁻¹ (0.2 Mg ha⁻¹ y⁻¹) in the 9-year-old stand to 9.8% y⁻¹ (2.3 Mg ha⁻¹ y⁻¹) in the 12-year-old stand. A one-pool model based on these inputs underestimated actual 2000–2007 CWD decomposition in the younger stands, suggesting that fragmentation could be an important part of the carbon flux exiting the CWD pool. We compared three independent measures of annual decomposition (k): direct measurements of CWD respiration, rates based on the 7-year re-sampling effort described here, and rates inferred from the chronosequence design itself. Mean k values arrived at via these techniques were 0.06 ± 0.03, 0.05 ± 0.04, and 0.05 ± 0.05 y⁻¹, respectively. The four-pool model suggested that the transition rate between decay classes was 0.14–0.19 y⁻¹; the model was most sensitive to initial CWD values. Although the computed k values implied a problem with chronosequence site selection for at least one site, the overall CWD trend was consistent with a larger number of sites surveyed in the region.     

Foliar and Litter Nutrients, Nutrient Resorption, and Decomposition in Hawaiian Me t rosideros polymorpha

The native tree Metrosideros polymorpha dominates Hawaiian forests across a very wide range of soil fertility, including both sites where forest production is limited by nitrogen (N) and others where it is limited by phosphorus (P). Five long-term fertilization experiments have further broadened the range of nutrient availabilities experienced by Metrosideros. Adding P to P-limited sites increased foliar P concentrations threefold and litter P concentrations up to 10-fold; lignin concentrations decreased, and the decomposability of leaf litter increased from 32%–35% to 36%–46% mass loss in the first year. Adding N to N-limited sites increased leaf and litter N concentrations by only 15%–20%, with little or no effect on the decomposability of tissue. Received 22 January 1998; accepted 4 May 1998.     

Nutrients and mass in litter and top soil of ten tropical tree plantations

The importance of litter to nutrient and organic matter storage and the possible influence of species selection on soil fertility in ten stands each consisting of a separate tree species were examined in this study. The plantations had been grown under similar conditions in an arboretum in the Luquillo Experimental Forest, Puerto Rico. The species involved were: Anthocephalus chinensis, Eucalyptus × patentinervis, E. saligna, Hernandia sonora, Hibiscus elatus, Khaya nyasica, Pinus caribaea var. hondurensis, P. elliottii var. densa, Swietenia macrophylla, and Terminalia ivorensis. After 26 yr, litter mass ranged from 5 mg ha^-1 in the H. sonora stand to 27.2 Mg ha^-1 in the P. caribaea stand. Nutrients in the litter (N, P, K, Ca, and Mg) also varied widely, but stands were ranked in different order when ranked by nutrients in the litter than then ranked according to accumulation of mass. Only E. saligna and A. chinensis stands were ranked similarly in accumulation of both nutrients and mass, and the stand of H. elatus was ranked higher with respect to nutrient accumulation than to accumulation of mass. The nutrient concentration in standing leaf litter generally increased in the order of recently fallen <old intact< fragmented. Nutrient concentration of standing leaf litter appears to increase with age and depth in the litter layer. The amount of nutrients stored in the litter compartment of these plantations was in the same order of magnitude as the quantity of available nutrients in the top 10-cm of mineral soil. Total litter mass was negatively correlated with the mass-weighted concentration of N, K, and Mg. The same relationship was found for Ca in the leaf litter and N in the fine wood litter compartments. In some stands (notably P. caribaea, P. elliottii, and E. saligna), leaf litter derived from species other than the species planted in that particular stand had higher nutrient concentration than leaf litter from the planted species. Soils of the 10 stands were classified in the same soil series and had similar texture (clay soils). However, significantly different chemical characteristics were found. Results obtained by analysis of covariance and by limiting comparisons to adjacent stands with similar soil texture, indicate that different species have had different influences on the concentration of available nutrients in soil.     

Dry matter and nutrient inputs through litter fall in a dry tropical forest of India

The present paper elucidates the pattern of leaf and non-leaf fall and quantifies of the total annual input of litter in a dry tropical forest of India. In addition, concentration of selected nutrients in various litter species and their annual return to the forest floor are examined. Total annual input of litter measured in litter traps ranged between 488.0–671.0 g m^-2 of which 65–72% was leaf litter fall and 28–35% wood litter fall. 73–81% leaves fall during the winter season. Herbaceous litter fall ranged between 80.0–110.0 g m^-2 yr^-1. The annual nutrient return through litter fall amounted (kg ha^-1): 51.6–69.6 N, 3.1–4.3 P, 31.0–40.0 Ca, 14.0–19.0 K and 3.7–5.0 Na, of which 71–77% and 23–29% were contributed by leaf and wood litter fall, respectively for different nutrients. Input of nutrients through herbaceous litter was: 13.0–16.6 for N, 1.0–1.4 for P, 4.0–5.0 for Ca, 7.9–10.5 for K and 0.8–1.0 kg ha^-1 yr^-1 for Na.     

NONSYMBIOTIC AND SYMBIOTIC NITROGEN FIXATION IN A WEAKLY MINEROTROPHIC PEATLAND

The acetylene reduction assay was used to measure nonsymbiotic and symbiotic nitrogen fixation in a weakly minerotrophic peatland throughout the ice-free season. Nonsymbiotic nitrogen fixation was found in surface materials and subsurface peat. In surface materials, nitrogenase activity measured in the field contributed about 0.6 kg N ha^-1 yr^-1, was closely associated with Sphagnum, but was not correlated with temperature between 12 and 27 C. No cyanobacteria were found in association with Sphagnum. In subsurface peat, nitrogenase activity measured in situ contributed no more than 0.4 kg N ha^-1 yr^-1 and was closely correlated with temperature between 7 and 21 C. There were uncertainites in these measurements due to presence of ethylene oxidizing activity and a long time lag. Symbiotic nitrogen fixation was found only in actinomycete-induced root nodules of Myrica gale L. Legumes were absent and the few lichens present lacked nitrogenase activity. Based on acetylene reduction assays, Myrica gale fixed about 35 kg N ha^-1 yr^-1. Nitrogenase activity in Myrica gale showed a strong seasonal pattern which varied little during three consecutive years even though water levels varied substantially. Nitrogen input to the peatland from nonsymbiotic nitrogen fixation was only 15% the amount contributed by bulk precipitation. Symbiotic fixation, in contrast, contributed approximately six times the amount in bulk precipitation.     

Does nitrogen availability control rates of litter decomposition in forests?

The effects of increased exogenous N availability on rates of litter decomposition were assessed in several field fertilization trials. In a jack pine (Pinus banksiana Lamb.) forest, needle litter decomposed at the same rate in control plots and in plots fertilized with urea and ammonium nitrate (1350 kg N ha^-1) with or without P and K. Mixed needle litter of western hemlock (Tsuga heterophylla (Raf.) Sarg.), western red cedar (Thuja plicata Donn) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) incubated in plots recently amended with sewage sludge (500 kg N ha^-1) lost less weight during 3 years than did litter in control plots. Forest floor material also decomposed more slowly in plots amended with sewage sludge. Paper birch (Betula papyrifera Marsh.) leaf litter placed on sewage sludge (1000 kg N ha^-1), pulp sludge, or sewage-pulp sludge mixtures decomposed at the same rate as leaf litter in control plots. These experiments demonstrate little effect of exogenous N availability on rates of litter decomposition.The influence of endogenous N availability on rates of litter decomposition was examined in a microcosm experiment. Lodgepole pine (Pinus contorta var. latifolia Engelm.) needle litter collected from N-fertilized trees (525 kg N ha^-1 in ammonium nitrate) were 5 times richer in N than needles from control trees (1.56% N versus 0.33% N in control trees), but decomposed at the same rate. Green needles from fertilized trees contained twice as much N as needles from control trees (1.91% N versus 0.88% N), but decomposed at the same rate. These experiments suggest that N availability alone, either exogenous or endogenous, does not control rates of litter decomposition. Increased N availability, through fertilization or deposition, in the absence of changes in vegetation composition, will not alter rates of litter decomposition in forests.     

Differential production yet chemical similarity of dissolved organic matter across a chronosequence with contrasting nutrient availability in Hawaii

Dissolved organic matter (DOM) is a critical phase in terrestrial carbon and nutrient cycling forming the basis of many ecosystem functions, yet the primary drivers controlling its flux from organic horizons and resultant chemical composition remain only partially understood. We studied dissolved organic matter production and chemistry from organic soil horizons across a 4.1 My old well-constrained chronosequence in Hawaii. Controlled soil column irrigation and leaching experiments were conducted on field moist organic soil horizons to quantify microbial activity, DOM production and chemistry. Both microbial activity (defined as CO₂ production per unit substrate C) and DOM production were found to be lowest in the youngest (0.3 ky) and oldest (4.1 My) sites of the chronosequence, where nutrients (N and P respectively) were most limiting. By contrast, DOM production and microbial activity was greatest at the intermediate-aged (20–350 ky) sites where nutrients were least limiting, unrelated to the mass of organic matter found in the organic horizons. While differences in production rates were found, ¹³C NMR spectroscopic results indicated that there was a convergence of chemistry from the solid to the dissolved phase at all sites. In particular, all DOM samples were found to have a high proportion of aromatic acids. With supporting data from a diverse range of ecosystems, we postulate that chemical homogenization of DOM relative to source material is a common feature of many ecosystems due to two microbially mediated processes: (1) similar extracellular enzymatic oxidation conferring solubility to a subset of degradation products; and (2) the rapid selective consumption of the more labile organic compounds in the soil solution.     

Growth and N2 fixation in an Alnus hirsuta (Turcz.) var. sibirica stand in Japan

To estimate the N₂ fixation ability of the alder (Alnus hirsuta (Turcz.) var. sibirica), we examined the seasonal variation in nitrogenase activity of nodules using the acetylene reduction method in an 18-year-old stand naturally regenerated after disturbance by road construction in Japan. To evaluate the contribution of N₂ fixation to the nitrogen (N) economy in this alder stand, we also measured the phenology of the alder, the litterfall, the decomposition rate of the leaf litter, and N accumulation in the soil. The acetylene reduction activity per unit nodule mass (ARA) under field conditions appeared after bud break, peaked the maximum in midsummer after full expansion of the leaves, and disappeared after all leaves had fallen. There was no consistent correlation between ARA and tree size (dbh). The amount of N₂ fixed in this alder stand was estimated at 56.4 kg ha^?1 year^?1 when a theoretical molar ratio of 3 was used to convert the amount of reduced acetylene to the amount of fixed N₂. This amount of N₂ fixation corresponded to the 66.4% of N in the leaf litter produced in a year. These results suggested that N₂ fixation still contributed to the large portion of N economy in this alder stand.     

Production and Resource Use Efficiencies in N- and P-Limited Tropical Forests: A Comparison of Responses to Long-term Fertilization

At two sites at the extreme ends of a soil development chronosequence in Hawaii, we investigated whether forest responses to fertilization on young soils were similar to those on highly weathered soils and whether the initial responses were maintained after 6–11 years of fertilization. Aboveground net primary production (ANPP) was increased by nitrogen (N) application at the 300-year-old site and phosphorus (P) application at the 4.1-million-year-old site, thus confirming earlier results and their designations as N- and P-limited forests. Along with ANPP, application of the limiting element consistently increased leaf area index (LAI), radiation conversion efficiency (RCE), and foliar and litter nutrient concentrations. Fertilization did not consistently alter N or P retranslocation from senescent leaves at either site, but a comparison with other sites on the chronosequence and with a common-garden study suggests that there is a genetic basis for low foliar and litter nutrients and higher retranslocation at infertile sites vs more fertile sites. N limitation appears to be expressed as limitation to carbon gain, with long leaf lifespans and high leaf mass per area. P limitation results in high P-use efficiency and disproportionally large increases in P uptake after fertilization; a comparison with other studies indicates large investments in acquiring and storing P. Although the general responses of ANPP, LAI, and RCE were similar for the two sites, other aspects of nutrient use differ in relation to the physiological and biogeochemical roles of the two elements. Received 2 June 2000; Accepted 4 April 2001.     

Leaf litter decomposition of Piper aduncum,Gliricidia sepium and Imperata cylindrica in the humid lowlands of Papua New Guinea

No information is available on the decomposition and nutrient release pattern of Piper aduncum and Imperata cylindrica despite their importance in shifting cultivation systems of Papua New Guinea and other tropical regions. We conducted a litter bag study (24 weeks) on a Typic Eutropepts in the humid lowlands to assess the rate of decomposition of Piper aduncum, Imperata cylindrica and Gliricidia sepium leaves under sweet potato (Ipomoea batatas). Decomposition rates of piper leaf litter were fastest followed closely by gliricidia, and both lost 50% of the leaf biomass within 10 weeks. Imperata leaf litter decomposed much slower and half-life values exceeded the period of observation. The decomposition patterns were best explained by the lignin plus polyphenol over N ratio which was lowest for piper (4.3) and highest for imperata (24.7). Gliricidia leaf litter released 79 kg N ha^–1, whereas 18 kg N ha^–1 was immobilised in the imperata litter. The mineralization of P was similar for the three species, but piper litter released large amounts of K. The decomposition and nutrient release patterns had significant effects on the soil. The soil contained significantly more water in the previous imperata plots at 13 weeks due to the relative slow decomposition of the leaves. Soil N levels were significantly reduced in the previous imperata plots due to immobilisation of N. Levels of exchangeable K were significantly increased in the previous piper plots due to the large addition of K. It can be concluded that piper leaf litter is a significant and easily decomposable source of K which is an important nutrient for sweet potato. Gliricidia leaf litter contained much N, whereas imperata leaf litter releases relatively little nutrients and keeps the soil more moist. Gliricidia fallow is more attractive than an imperata fallow for it improves the soil fertility and produces fuelwood as additional saleable products.