Interactions between Litter Lignin and Nitrogenitter Lignin and Soil Nitrogen Availability during Leaf Litter Decomposition in a Hawaiian Montane Forest

Previous work in a young Hawaiian forest has shown that nitrogen (N) limits aboveground net primary production (ANPP) more strongly than it does decomposition, despite low soil N availability. In this study, I determined whether (a) poor litter C quality (that is, high litter lignin) poses an overriding constraint on decomposition, preventing decomposers from responding to added N, or (b) high N levels inhibit lignin degradation, lessening the effects of added N on decomposition overall. I obtained leaf litter from one species, Metrosideros polymorpha, which dominates a range of sites in the Hawaiian Islands and whose litter lignin concentration declines with decreasing precipitation. Litter from three dry sites had lignin concentrations of 12% or less, whereas litter from two wet sites, including the study site, had lignin concentrations of more than 18%. This litter was deployed 2.5 years in a common site in control plots (receiving no added nutrients) and in N-fertilized plots. Nitrogen fertilization stimulated decomposition of the low-lignin litter types more than that of the high-lignin litter types. However, in contrast to results from temperate forests, N did not inhibit lignin decomposition. Rather, lignin decay increased with added N, suggesting that the small effect of N on decomposition at this site results from limitation of decomposition by poor C quality rather than from N inhibition of lignin decay. Even though ANPP is limited by N, decomposers are strongly limited by C quality. My results suggest that anthropogenic N deposition may increase leaf litter decomposition more in ecosystems characterized by low-lignin litter than in those characterized by high-lignin litter. Received 26 October 1999; accepted 2 June 2000.     

Grasshoppers (Orthoptera: Acrididae) select vegetation patches in local‐scale responses to foliar nitrogen but not phosphorus in native grassland

Abstract Key elements such as nitrogen (N) and phosphorus (P) are often limiting relative to the nutritional needs of herbivores that feed on them. While N often limits insect herbivores in natural terrestrial ecosystems, the effect of P is poorly studied in the field, even though compelling hypotheses from the ecological stoichiometry literature predict its importance. We evaluated small‐scale spatial distributions of, and herbivory by, grasshoppers among neighboring plots that vary in foliar‐N and ‐P in tallgrass prairie. Grasshopper densities were 67% greater in N‐fertilized plots but detected no effect to grasshopper densities from P‐fertilizer. Leaf damage to the dominant grass Andropogon gerardii was 32% greater in N‐fertilized plots, but no response to foliar‐P was detected. Herbivore damage to a common forb, goldenrod (Solidago missouriensis), was not strongly linked by fertilizer treatments, although there was increased leaf damage in N‐fertilizer treatments when no P was applied (a significant N × P interaction). Under field conditions at local scales, we conclude that spatially heterogeneous distributions of grasshoppers are primarily affected by foliar‐N in host plants with little evidence that P‐levels contribute to the spatial patterns.     

Galling by Rhopalomyia solidaginis alters Solidago altissima architecture and litter nutrient dynamics in an old-field ecosystem

Plant–insect interactions can alter ecosystem processes, especially if the insects modify plant architecture, quality, or the quantity of leaf litter inputs. In this study, we investigated the interactions between the rosette gall midge Rhopalomyia solidaginis and tall goldenrod, Solidago altissima, to quantify the degree to which the midge alters plant architecture and how the galls affect rates of litter decomposition and nutrient release in an old-field ecosystem. R. solidaginis commonly leads to the formation of a distinct apical rosette gall on S. altissima and approximately 15% of the ramets in a S. altissima patch were galled (range: 3–34%). Aboveground biomass of galled ramets was 60% higher and the leaf area density was four times greater on galled leaf tissue relative to the portions of the plant that were not affected by the gall. Overall decomposition rate constants did not differ between galled and ungalled leaf litter. However, leaf-litter mass loss was lower in galled litter relative to ungalled litter, which was likely driven by modest differences in initial litter chemistry; this effect diminished after 12 weeks of decomposition in the field. The proportion of N remaining was always higher in galled litter than in ungalled litter at each collection date indicating differential release of nitrogen in galled leaf litter. Several studies have shown that plant–insect interactions on woody species can alter ecosystem processes by affecting the quality or quantity of litter inputs. Our results illustrate how plant–insect interactions in an herbaceous species can affect ecosystem processes by altering the quality and quantity of litter inputs. Given that S. altissima dominates fields and that R. solidaginis galls are highly abundant throughout eastern North America, these interactions are likely to be important for both the structure and function of old-field ecosystems.     

Effects of elevated [CO2] on forest growth and carbon storage: a modelling analysis of the consequences of changes in litter quality/quantity and root exudation

Many researchers have proposed that the stimulus of plant growth under elevated [CO₂] observed in short-term experiments will be moderated in the longer term by a reduction in soil nitrogen (N) availability linked to decreased litter quality and/or increased litter production. However, these negative feedbacks may be offset to some extent by a stimulus in N fixation linked to increased root exudation. The aim of this modelling study is to examine how changes in litter quality/quantity and root exudation –- if they occur –- will affect the CO₂ responses of net primary productivity and ecosystem carbon (C) storage on different timescales. We apply a model of C and N cycling in forest ecosystems (G’DAY) to stands of Norway spruce (Picea abies, L. Cast) growing at a N-limited experimental site at Flakaliden, Sweden, and draw the following conclusions: (1) in the absence of changes in litter quality and root exudation, the short-term CO₂ stimulus of litter quantity leads to only a minimal CO₂ stimulus of productivity or C storage in the medium term (≈ 20 years) and long term (≈ 200 years), because of constraints on soil N availability; (2) increasing plant nitrogen use efficiency (via a decrease in the N:C ratio of new litter) makes little impact on these results; (3) a significant CO₂ response in the medium term requires a substantial decrease in the N:C ratio of older litter, when it is approaching stabilisation as soil organic matter, although the long-term CO₂ response remains small; and (4) an increase in N fixation leads to a small effect on productivity in the short term, but a very large effect on both productivity and C storage in the long term. These results suggest that soil N constraints on the long-term CO₂-fertilisation effect can be overcome to a significant extent only by increases in N acquisition, although only modest increases may be required. This revised version was published online in June 2006 with corrections to the Cover Date.     

Organic matter inputs shift soil enzyme activity and allocation patterns in a wet tropical forest

Soil extracellular enzymes mediate organic matter turnover and nutrient cycling yet remain little studied in one of Earth’s most rapidly changing, productive biomes: tropical forests. Using a long-term leaf litter and throughfall manipulation, we explored relationships between organic matter (OM) inputs, soil chemical properties and enzyme activities in a lowland tropical forest. We assayed six hydrolytic soil enzymes responsible for liberating carbon (C), nitrogen (N) and phosphorus (P), calculated enzyme activities and ratios in control plots versus treatments, and related these to soil biogeochemical variables. While leaf litter addition and removal tended to increase and decrease enzyme activities per gram soil, respectively, shifts in enzyme allocation patterns implied changes in relative nutrient constraints with altered OM inputs. Enzyme activity ratios in control plots suggested strong belowground P constraints; this was exacerbated when litter inputs were curtailed. Conversely, with double litter inputs, increased enzymatic investment in N acquisition indicated elevated N demand. Across all treatments, total soil C correlated more strongly with enzyme activities than soluble C fluxes, and enzyme ratios were sensitive to resource stoichiometry (soil C:N) and N availability (net N mineralization). Despite high annual precipitation in this site (MAP ~5 m), soil moisture positively correlated with five of six enzymes. Our results suggest resource availability regulates tropical soil enzyme activities, soil moisture plays an additional role even in very wet forests, and relative investment in C, N and P degrading enzymes in tropical soils will often be distinct from higher latitude ecosystems yet is sensitive to OM inputs.     

Non-additive effects of leaf litter and insect frass mixture on decomposition processes

Although there is a growing body of evidence that herbivorous insects have a significant impact on decomposition and soil nutrient dynamics through frass excretion, how mixtures of leaf litter and insect frass influence such ecosystem processes remains poorly understood. We examined the effects of mixing of leaf litter and insect frass on decomposition and soil nutrient availability, using a study system consisting of a willow, Salix gilgiana Seemen, and a herbivorous insect, Parasa consocia Walker. The chemical characteristics of insect frass differed from those of leaf litter. In particular, frass had a 42-fold higher level of ammonium–nitrogen (NH₄ ⁺–N) than litter. Incubation experiments showed that the frass was decomposed and immobilized with respect to N more rapidly than the litter. Furthermore, litter and frass mixtures showed non-additive enhancement of decomposition and reduction of NH₄ ⁺–N, depending on the litter–frass mixing ratio. These indicate that, while insect frass generally accelerated decomposition, the effect of frass on soil nutrient availability was dependent largely on the relative amounts of litter and frass.     

Palatability, decomposition and insect herbivory: patterns in a successional old-field plant community

We tested the hypothesis that selective feeding by insect herbivores in an old‐field plant community induces a shift of community structure towards less palatable plant species with lower leaf and litter tissue quality and may therefore affect nutrient cycling. Leaf palatability of 20 herbaceous plant species which are common during the early successional stages of an old‐field plant community was assayed using the generalist herbivores Deroceras reticulatum (Mollusca: Agriolomacidae) and Acheta domesticus (Ensifera: Gryllidae). Palatability was positively correlated with nitrogen content, specific leaf area and water content of leaves and negatively correlated with leaf carbon content and leaf C/N‐ratio. Specific decomposition rates were assessed in a litter bag experiment. Decomposition was positively correlated with nitrogen content of litter, specific leaf area and water content of living leaves and negatively correlated with leaf C/N‐ratio. When using phylogentically independent contrasts the correlations between palatability and decomposition versus leaf and litter traits remained significant (except for specific leaf area) and may therefore reflect functional relationships. As palatability and decomposition show similar correlations to leaf and litter traits, the correlation between leaf palatability and litter decomposition rate was also significant, and this held even in a phylogenetically controlled analysis. This correlation highlights the possible effects of invertebrate herbivory on resource dynamics. In a two‐year experiment we reduced the density of above‐ground and below‐ground insect herbivores in an early successional old‐field community in a two‐factorial design by insecticide application. The palatability ranking of plants showed no relationship with the specific change of cover abundance of plants due to the reduction of above‐ or below‐ground herbivory. Thus, changes in the dominance structure as well as potentially associated changes in the resource dynamics are not the result of differences in palatability between plant species. This highlights fundamental differences between the effects of insect herbivory on ecosystems and published results from vertebrate‐grazing systems.     

Leaf litter production and decomposition in a poplar short-rotation coppice exposed to free air CO2 enrichment (POPFACE)

The capacity of forest ecosystems to sequester C in the soil relies on the net balance between litter production above, as well as, below ground, and decomposition processes. Nitrogen mineralization and its availability for plant growth and microbial activity often control the speed of both processes. Litter production, decomposition and N mineralization are strongly interdependent. Thus, their responses to global environmental changes (i.e. elevated CO₂, climate, N deposition, etc.) cannot be fully understood if they are studied in isolation. In the present experiment, we investigated litter fall, litter decomposition and N dynamics in decomposing litter of three Populus spp., in the second and third growing season of a short rotation coppice under FACE. Elevated CO₂ did not affect annual litter production but slightly retarded litter fall in the third growing season. In all species, elevated CO₂ lowered N concentration, resulting in a reduction of N input to the soil via litter fall, but did not affect lignin concentrations. Litter decomposition was studied in bags incubated in situ both in control and FACE plots. Litter lost between 15% and 18% of the original mass during the eight months of field incubation. On average, litter produced under elevated CO₂ attained higher residual mass than control litter. On the other end, when litter was incubated in FACE plots it exhibited higher decay rates. These responses were strongly species‐specific. All litter increased their N content during decomposition, indicating immobilization of N from external sources. Independent of the initial quality, litter incubated on FACE soils immobilized less N, possibly as a result of lower N availability in the soil. Indeed, our results refer to a short‐term decomposition experiment. However, according to a longer‐term model extrapolation of our results, we anticipate that in Mediterranean climate, under elevated atmospheric CO₂, soil organic C pool of forest ecosystems may initially display faster turnover, but soil N availability will eventually limit the process.     

<Emphasis Type="Italic">Hedychium gardnerianum</Emphasis> Invasion into Hawaiian Montane Rainforest: Interactions Among Litter Quality,Decomposition Rate,and Soil Nitrogen Availability

Few studies have examined the invasion of understory species into closed-canopy forests and, despite inter-specific differences in litter quality and quantity between understory and dominant canopy trees, the influence of understory invasions on soil nitrogen (N) cycling remains unknown. This paper examines litter quality and decomposition of kahili ginger (Hedychium gardnerianum), an invasive understory herb, to determine the influence of this species on N cycling in a Hawaiian montane rainforest. To examine the potential feedback between increased soil N availability and litter decomposition, litter from the invasive ginger, a native tree, and native tree fern was collected from unfertilized and fertilized plots and decomposed in a reciprocal transplant design. Hedychium litter decomposed faster than litter from the two native species. Across species, decomposition rates were negatively correlated with litter lignin content. Despite rapid decomposition rates of Hedychium litter, soil nitrogen availability and rates of net mineralization in the soil were similar in invaded and uninvaded plots. Nitrogen cycling at this site may be more strongly influenced by native species, which contribute the most to overall stand biomass. A negative effect of fertilization on the decomposition of Hedychium litter suggests that a negative feedback between litter quality and soil N availability may exist over longer timescales.     

Effects of Single Chinese Fir and Mixed Leaf Litters on Soil Chemical, Microbial Properties and Soil Enzyme Activities

The quality of leaf litter can control decomposition processes and affect the nutrient availability for plant uptake. In this study, we investigated the effect of single leaf litter (Chinese fir – Cunninghamia lamcealata (Lamb.) Hook) and mixed leaf litters (C. lamcealata, Liquidamba formosana Hance and Alnus cremastogyne Burk) on soil chemical properties, soil microbial properties and soil enzyme activities during 2 years decomposition. The results showed that soil microbial biomass C, the ratio of soil microbial biomass C to total soil organic C (soil microbial quotient, Cmic/Corg) and soil enzymes (urease, invertase, dehydrogenase) activities increased significantly in mixed leaf litters treatments whereas soil chemical properties remained unchanged. However, soil microbial metabolic quotient (qCO₂) values and soil polyphenol oxidase activity were higher in the single Chinese fir leaf litter treatment that had a higher C:N (carbon:nitrogen) ratio (79.53) compared with the mixed leaf litter (C:N ratios of 76.32, 56.90, 61.20, respectively). Our results demonstrated that the mixed leaf litter can improve forest soil quality, and that soil microbial properties and soil enzyme activities are more sensitive in response to litter quality change than soil chemical properties.     

Browsing-induced Effects on Leaf Litter Quality and Decomposition in a Southern African Savanna

We investigated the linkages between leaf litter quality and decomposability in a savanna plant community dominated by palatable-spinescent tree species. We measured: (1) leaf litter decomposability across five woody species that differ in leaf chemistry; (2) mass decomposition, nitrogen (N); and carbon (C) dynamics in leaf litter of a staple browse species (Acacia nigrescens) as well as (3) variation in litter composition across six sites that experienced very different histories of attack from large herbivores. All decomposition trials included litter bags filled with chopped straw to control for variation in site effects. We found a positive relationship between litter quality and decomposability, but we also found that Acacia and straw litter mass remaining did not significantly vary between heavily and lightly browsed sites. This is despite the fact that both the quality and composition of litter returned to the soil were significantly different across sites. We observed greater N resorption from senescing Acacia leaves at heavily browsed sites, which in turn contributed to increase the C:N ratio of leaf litter and caused greater litter N immobilization over time. This, together with the significantly lower tree- and herb-leaf litter mass beneath heavily browsed trees, should negatively affect decomposition rates. However, estimated dung and urine N deposition from both browsers and grazers was significantly greater at high- than at low-herbivory sites. We hypothesize that N inputs from dung and urine boost litter N mineralization and decomposition (especially following seasonal rainfall events), and thereby offset the effects of poor leaf litter quality at chronically browsed sites. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

云南药山自然保护区黄背栎林和巧家五针松林生态化学计量特征

以云南药山自然保护区黄背栎林和巧家五针松林的4块样地为研究对象,旨在揭示这2种森林生态系统营养元素含量状况和土壤养分的供给能力。通过分析4块样地"叶片-凋落物-土壤"C、N、P含量、生态化学计量特征及其相关性,结果表明:(1)不同样地同一组分的C、N含量差异显著,P含量差异不显著,同一样地各组分间C、N含量差异显著,均为叶片凋落物土壤,P含量则为叶片土壤凋落物;(2)不同样地叶片C∶P、N∶P比值和凋落物与土壤C∶N比值差异不显著,其余指标差异均显著。同一样地叶片、凋落物、土壤的C∶N、C∶P、N∶P比值差异显著,均为凋落物叶片土壤;(3)黄背栎林叶片-土壤C含量、C∶P比值和凋落物-叶片N∶P比值呈极显著或显著相关,巧家五针松林凋落物-叶片C、N含量和叶片-土壤P含量、C∶N、N∶P比值呈极显著或显著相关;(4)土壤N元素缺乏是限制植物生长的主要因素,P元素主要源于土壤矿物风化释放,而非生物小循环。     

Removal of invasive shrubs alters light but not leaf litter inputs in a deciduous forest understory

The establishment and spread of non‐native, invasive shrubs in forests poses an important obstacle to natural resource conservation and management. This study assesses the impacts of the physical removal of a complex of woody invasive shrub species on deciduous forest understory resources. We compared leaf litter quantity and quality and understory light transmittance in five pairs of invaded and removal plots in an oak‐dominated suburban mature forest. Removal plots were cleared of all non‐native invasive shrubs. The invasive shrubs were abundant (143,456 stems/ha) and diverse, dominated by species in the genera Ligustrum, Viburnum, Lonicera, and Euonymus. Annual leaf litter biomass and carbon inputs of invaded plots were not different from removal plots due to low leaf litter biomass of invasive shrubs. Invasive shrub litter had higher nitrogen (N) concentrations than native species; however, low biomass of invasive litter led to low N inputs by litter of invasive species compared to native. Light transmittance at the forest floor and at 2 m was lower in invaded plots than in removal plots. We conclude that the removal of the abundant invasive shrubs from a native deciduous forest understory did not alter litter quantity or N inputs, one measure of litter quality, and increased forest understory light availability. More light in the forest understory could facilitate the restoration of forest understory dynamics.     

Changes in Asymbiotic, Heterotrophic Nitrogen Fixation on Leaf Litter of Metrosideros polymorpha with Long-Term Ecosystem Development in Hawaii

We measured nitrogenase activity (acetylene reduction) of asymbiotic, heterotrophic, nitrogen-fixing bacteria on leaf litter from the tree Metrosideros polymorpha collected from six sites on the Hawaiian archipelago. At all sites M. polymorpha was the dominant tree, and its litter was the most abundant on the forest floor. The sites spanned a soil chronosequence of 300 to 4.1 million y. We estimated potential nitrogen fixation associated with this leaf litter to be highest at the youngest site (1.25 kg ha^-1 y^-1), declining to between 0.05 and 0.22 kg ha^-1 y^-1 at the oldest four sites on the chronosequence. To investigate how the availability of weathered elements influences N fixation rates at different stages of soil development, we sampled M. polymorpha leaf litter from complete, factorial fertilization experiments located at the 300-y, 20,000-y and 4.1 million–y sites. At the youngest and oldest sites, nitrogenase activity on leaf litter increased significantly in the plots fertilized with phosphorus and “total” (all nutrients except N and P); no significant increases in nitrogenase activity were measured in leaf litter from treatments at the middle-aged site. The results suggest that the highest rates of N fixation are sustained during the “building” or early phase of ecosystem development when N is accumulating and inputs of geologically cycled (lithophilic) nutrients from weathering are substantial. Received 4 February 1999; accepted 29 March 2000.     

Top-down effects of insect herbivores during early succession: influence on biomass and plant dominance

We tested the hypothesis that phytophagous insects would have a strong top-down effect on early successional plant communities and would thus alter the course of succession. To test this hypothesis, we suppressed above-ground insects at regular intervals with a broad-spectrum insecticide through the first 3 years of old-field succession at three widely scattered locations in central New York State. Insect herbivory substantially reduced total plant biomass to a similar degree at all three sites by reducing the abundance of meadow goldenrod, Solidago altissima. As a result, Euthamia graminifolia dominated control plots whereas S. altissima dominated insecticide-treated plots by the third year of succession. S. altissima is the dominant old-field herbaceous species in this region but typically requires at least 5 years to become dominant. Past explanations for this delay have implicated colonization limitation whereas our data demonstrate that insect herbivory is a likely alternative explanation. A widespread, highly polyphagous insect, the xylem-tapping spittlebug, Philaenus spumarius, appeared to be the herbivore responsible for the reduction in standing crop biomass at all three sites. Insect herbivory typically caused little direct leaf tissue loss for the ten plant species we examined, including S. altissima. Consequently, the amount of leaf area removed was not a reliable indicator of the influence of insect herbivory on standing crop biomass or on early succession. Overall, we found a strong top-down effect of insect herbivores on biomass at several sites, so our results may be broadly applicable. These findings run counter to generalizations that top-down effects of herbivores, particularly insects, are weak in terrestrial systems. These generalizations may not apply to insects, such as spittlebugs, that can potentially mount an effective defense (i.e., spittle) against predators and subsequently reach relatively high abundance on common plant species. Our results suggest that insect herbivory may play an important but often overlooked role during early old-field succession. Received: 26 December 1998 / Accepted: 3 April 1999     

Augmentation of soil detritus affects the spider community and herbivory in a soybean agroecosystem

If soil detritivores provide a significant prey source for predators in the vegetation, then augmentation of the soil community could affect the grazing food web. Specifically, increases in predator density could enhance any top‐down effects and reduce herbivory. We tested this hypothesis by providing detrital subsidies in the form of composted vegetable matter to 36 m² plots in soybean, Glycine max (L.) Merr. (Fabales: Fabaceae), fields that were managed using either conventional or conservation tillage practices. The foliage‐dwelling spiders, insect predators, and leaf‐chewing insects were censused and the body size of one large spider species, Argiope trifasciata (Forskål) (Araneae: Araneidae), was measured. In addition, the density and size of the plants were assessed and leaf damage was quantified. Any effects of treatments on the palatability of soybean plants to herbivores were determined in two laboratory experiments. Compost increased the density of foliage dwelling spiders and the abdomen size of A. trifasciata. We uncovered no treatment effects on insect predators, herbivorous insects, or plant characteristics except that compost addition reduced leaf damage. In addition, there was a negative correlation across plots between spider abundance and soybean leaf damage and abdomen width of A. trifasciata and weed herbivory levels across plots. These results suggest a connection between the soil community and the foliage food web, but the spiders appear to have exerted a top‐down effect without a shift in herbivore abundance. Further study of the specific seasonality of the herbivores and their behavior in the presence of spiders are needed to uncover the underlying mechanism. Nevertheless, these results provide evidence for complex linkage between the soil and grazing food webs that may be important to biological control.     

Combined effects of elevated temperatures and reduced leaf litter quality on the life-history parameters of a saprophagous macroarthropod

Because soil macroinvertebrates strongly modify decomposition processes, it is important to know how their abundance will respond to global change. We investigated in laboratory microcosms, the effects of elevated temperatures and reduced leaf litter quality on the life‐history traits of a saprophagous macroarthropod (development time, growth, survival and reproduction). Millipedes (Polydesmus angustus) from an Atlantic temperate forest were reared throughout their life cycle (≥16 months) under two temperature regimes differing on average by 3.3 °C; in a factorial design, they were fed either on Atlantic leaf litter or on Mediterranean leaf litter with a higher C : N ratio; humidity was consistently high. The components of the population growth rate (r) were affected positively by the temperature rise and negatively by the switch from Atlantic to Mediterranean leaf litter. When both treatments were combined, litter effects offset temperature effects. These results show that the short‐term response of saprophagous macroarthropods to warming is positive but depends on the availability of high‐quality litter, which is difficult to predict in the global change context. In a parallel experiment, conspecific millipedes from a Mediterranean population, which have evolved for a long time in a warmer climate and on poor‐quality litter, were reared at elevated temperatures on Mediterranean leaf litter. All components of r were higher than in the Atlantic population under the same conditions. This suggests that in the longer term, macroarthropods can overcome detrimental trophic interactions. Based on our study and the literature, we conclude that for decades the positive effects of warming on saprophagous macrofauna should exceed the negative effects of changes in litter quality. The abundance of those organisms in temperate forests could increase, which is confirmed by latitudinal patterns in Europe. Studies aimed at predicting the impacts of global change on decomposition will need to consider interactions with soil macroinvertebrates.     

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.     

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     

Contrasting effects of rabbit exclusion on nutrient availability and primary production in grasslands at different time scales

Herbivores influence nutrient cycling and primary production in terrestrial plant communities. However, both empirical and theoretical studies have indicated that the mechanisms by which herbivores influence nutrient availability, and thus their net effects on primary production, might differ between time scales. For a grassland in southeast England, we show that the effects of rabbits on primary production change over time in a set of grazed plots paired with exclosures ranging from 0 to 14 years in age. Herbivore exclusion decreased net aboveground primary production (APP) in the short term, but increased APP in the long term. APP was closely correlated with N mineralization rates in both grazed and ungrazed treatments, and accumulation of litter within the grazing exclosures led to higher N mineralization rates in exclosures in the long run. Rabbit grazing did not influence litter quality. The low contrast in palatability between species and the presence of grazing-tolerant plants might prevent rabbits from favoring unpalatable plant species that decompose slowly, in contrast to results from other ecosystems. Our results indicate that it is essential to understand the effects on N cycling in order to predict the effect of rabbit grazing on APP. Rabbits might decrease N mineralization and APP in the long term by increasing losses of N from grasslands.