Xiaoming Zou

Global decomposition experiment shows soil animal impacts on decomposition are climate-dependent

Climate and litter quality are primary drivers of terrestrial decomposition and, based on evidence from multisite experiments at regional and global scales, are universally factored into global decomposition models. In contrast, soil animals are considered key regulators of decomposition at local scales but their role at larger scales is unresolved. Soil animals are consequently excluded from global models of organic mineralization processes. Incomplete assessment of the roles of soil animals stems from the difficulties of manipulating invertebrate animals experimentally across large geographic gradients. This is compounded by deficient or inconsistent taxonomy. We report a global decomposition experiment to assess the importance of soil animals in C mineralization, in which a common grass litter substrate was exposed to natural decomposition in either control or reduced animal treatments across 30 sites distributed from 43°S to 68°N on six continents. Animals in the mesofaunal size range were recovered from the litter by Tullgren extraction and identified to common specifications, mostly at the ordinal level. The design of the trials enabled faunal contribution to be evaluated against abiotic parameters between sites. Soil animals increase decomposition rates in temperate and wet tropical climates, but have neutral effects where temperature or moisture constrain biological activity. Our findings highlight that faunal influences on decomposition are dependent on prevailing climatic conditions. We conclude that (1) inclusion of soil animals will improve the predictive capabilities of region- or biome-scale decomposition models, (2) soil animal influences on decomposition are important at the regional scale when attempting to predict global change scenarios, and (3) the statistical relationship between decomposition rates and climate, at the global scale, is robust against changes in soil faunal abundance and diversity.

SOIL MICROARTHROPOD CONTRIBUTIONS TO DECOMPOSITION DYNAMICS: TROPICAL–TEMPERATE COMPARISONS OF A SINGLE SUBSTRATE

This study examined the effect of soil microarthropods on the decomposition of a single substrate (Quercus prinus L.) at two humid tropical forests (La Selva, Costa Rica (LAS), and Luquillo Experimental Forest, Puerto Rico (LUQ)) and one temperate forest (Coweeta Hydrologic Station, North Carolina, USA (CWT)). In this litterbag ex- periment, naphthalene was applied to reduce the microarthropod population density from half of three replicate plots established at each site. This enabled us to quantify the mass loss contributed by the fauna (MLCF) at each site and permitted an analysis of the influence of site-specific differences in the composition of the microarthropod assemblages on de- composition rates. We hypothesized that microarthropod regulation of the microbial pop- ulations involved in leaf litter decomposition would be stronger in humid tropical forests, which experience conditions of low climatic variability. In these conditions, there can be an enhanced degree of biotic interactions between microarthropods and their microbial food sources. The elevated extent of these interactions should be expressed as a greater influence of microarthropo ds at the tropical sites and could result in a site-specific effect of faunal assemblages on decomposition . Decomposition of the oak litter proceeded faster in Puerto Rican and Costa Rican forests than in a temperate forest in North Carolina, USA. Microarthropods had little effect on decomposition in the temperate forest, whereas their influence was pronounced at tropical sites. Mass loss of litter from plots with reduced microarthropod populations was similar at the tropical sites. When plots with intact faunal communities were compared, differences in the tropical sites were apparent, suggesting that there was a site-specific faunal contri- bution to decomposition at these sites. Oribatid mites constituted a dominant component (41-64%) at each of the sites. Species richness of oribatids and Fishers alpha diversity were similar in each of the three sites. The Shannon index revealed a lower diversity at LUQ. Abundance of microarthropods was lowest at LAS. Species accumulation curves for each site, though similar in form, were distinctive, as were diversity accumulation patterns in samples of increasing size. There was a positive relationship between species richness and the contribution of the fauna to litter mass loss within each site. Thus, species diversity of decomposer fauna may have important ecosystem consequences, particularly in warm moist tropical forests.

Long-term influence of deforestation on tree species composition and litter dynamics of a tropical rain forest in Puerto Rico

Abstract Understanding the long-term impact of deforestation on ecosystem structure and function of tropical forests may aid in designing future conservation programs to preserve biodiversity and sustain ecosystem productivity. We examined forest structure, tree species composition, litterfall rate, and leaf litter decomposition in a mid-successional forest (MSF) and an adjacent mature tabonuco forest (MTF) in the Luquillo Experimental Forest of Puerto Rico. Whereas the MTF site received limited human disturbance, the MSF site had been cleared for timber production by the beginning of this century and was abandoned after hurricanes struck the Luquillo Mountains in the 1920s and 1930s. We found that the MSF was dominated by successional tree species 50 years after secondary succession, and did not differ in tree basal area and litterfall rate from the MTF. Leaf decomposition rate in the MSF was higher than in the MTF, but this difference was small. Our results show that deforestation has long-term (over 50 years) influence on tree species composition and that recovery of leaf decomposition processes in secondary forest is relatively faster than that of tree species composition.

Soil macrofauna and litter nutrients in three tropical tree plantations on a disturbed site in Puerto Rico

Tree plantations are increasingly common in tropical landscapes due to their multiple uses. Plantations vary in structure and composition, and these variations may alter soil fauna communities. Recent studies have demonstrated the important role of soil fauna in the regulation of plant litter decomposition in the tropics. However, little is known about how plantation species affect soil fauna populations, which may in turn affect the biogeochemistry of the plantation system. We measured soil macroinvertebrate abundance and biomass in 9-year-old N2-fixing Leucaena leucocephala, Casuarina equisetifolia, and nonN2-fixing Eucalyptus robusta plantations on a degraded site in Puerto Rico. Nutrient concentrations and standing stocks of forest floor litter were also determined to examine the relationship between litter chemistry and soil macroinvertebrates. Leucaena plantations had significantly higher abundances and biomass of millipede species than Casuarina and Eucalyptus. Earthworm biomass did not differ among plantation treatments. Nitrogen, P, and K concentrations were generally higher in Leucaena litter, which resulted in higher standing stocks of these nutrients in fragmented, moderately decomposed litter (Oe horizon). Millipede biomass was highly correlated to N concentration and C/N ratio in the Oi litter horizon. These results suggest that plantation species differ in their influence on soil fauna, and the biomass and abundance of soil fauna can be regulated through careful selection of plantation species in degraded tropical lands. # 2002 Elsevier Science B.V. All rights reserved.

Changes in earthworm density and community structure during secondary succession in abandoned tropical pastures

Summary-Plant community succession alters the quantity and chemistry of organic inputs to soils. These differences in organic input may trigger changes in soil fertility and fauna1 activity. We examined earthworm density and community structure along a successional sequence of plant communities in abandoned tropical pastures in Puerto Rico. The chronological sequence of these plant communities were pasture, grass-vine-fern, shrub-small tree, and forest. Earthworm density was the highest in pasture (831 worms m-’ in top 0.25 m soil), decreased as secondary succession proceeded, and reached the lowest (32 wormsm-*) in the forest. Whereas only soil feeding Pontoscolex corethrurw was present in the pasture and grass-vine-fern communities, both soil and litter feeding worm species were found in the shrub-small tree and forest communities. Ground litter biomass correlated negatively with earthworm density. Soil water content differed slightly among the successional communities, but was unlikely to play an important role in triggering differences in worm density among these abandoned lands. Soil pH values did not differ along the successional changes. These results suggest that decrease in earthworm density and increase in worm community diversity during secondary succession may result from changes in the chemistry of organic inputs, rather than in soil properties and litter quantity. We conclude that successional development from grass-dominated pastures to woody species-dominated forests reduces earthworm density and diversifies worm community structure in humid tropical soils. 0 1997 Elsevier Science Ltd

Labile carbon retention compensates for CO2 released by priming in forest soils

Increase of belowground C allocation by plants under global warming or elevated CO2 may promote decomposition of soil organic carbon (SOC) by priming and strongly affects SOC dynamics. The specific effects by priming of SOC depend on the amount and frequency of C inputs. Most previous priming studies have investigated single C additions, but they are not very representative for litterfall and root exudation in many terrestrial ecosystems. We evaluated effects of (13)C-labeled glucose added to soil in three temporal patterns: single, repeated, and continuous on dynamics of CO2 and priming of SOC decomposition over 6 months. Total and (13)C labeled CO2 were monitored to analyze priming dynamics and net C balance between SOC loss caused by priming and the retention of added glucose-C. Cumulative priming ranged from 1.3 to 5.5 mg C g(-1) SOC in the subtropical, and from -0.6 to 5.5 mg C g(-1) SOC in the tropical soils. Single addition induced more priming than repeated and continuous inputs. Therefore, single additions of high substrate amounts may overestimate priming effects over the short term. The amount of added glucose C remaining in soil after 6 months (subtropical: 8.1-11.2 mg C g(-1) SOC or 41-56% of added glucose; tropical: 8.7-15.0 mg C g(-1) SOC or 43-75% of glucose) was substantially higher than the net C loss due to SOC decomposition including priming effect. This overcompensation of C losses was highest with continuous inputs and lowest with single inputs. Therefore, raised labile organic C input to soils by higher plant productivity will increase SOC content even though priming accelerates decomposition of native SOC. Consequently, higher continuous input of C belowground by plants under warming or elevated CO2 can increase C stocks in soil despite accelerated C cycling by priming in soils.

Policy and management responses to earthworm invasions in North America

The introduction, establishment and spread of non-native earthworm species in North America have been ongoing for centuries. These introductions have occurred across the continent and in some ecosystems have resulted in considerable modifications to ecosystem processes and functions associated with above- and belowground foodwebs. However, many areas of North America have either never been colonized by introduced earthworms, or have soils that are still inhabited exclusively by native earthworm fauna. Although several modes of transport and subsequent proliferation of non-native earthworms have been identified, little effort has been made to interrupt the flow of new species into new areas. Examples of major avenues for introduction of earthworms are the fish-bait, horticulture, and vermicomposting industries. In this paper we examine land management practices that influence the establishment of introduced species in several ecosystem types, and identify situations where land management may be useful in limiting the spread of introduced earthworm species. Finally, we discuss methods to regulate the importation of earthworms and earthworm-containing media so that introduction of new exotic species can be minimized or avoided. Although our focus in this paper is necessarily North American, many of the management and policy options presented here could be applicable to the problem of earthworm invasions in other parts of the world.

Variation in forest soil fungal diversity along a latitudinal gradient

In forest ecosystems, plant communities shape soil fungal communities through the provisioning of carbon. Although the variation in forest composition with latitude is well established, little is known about how soil fungal communities vary with latitude. We collected soil samples from 17 forests, along a latitudinal transect in western China. Forest types covered included boreal, temperate, subtropical and tropical forests. We used 454 pyrosequencing techniques to analyze the soil communities. These data were correlated with abiotic and biotic variables to determine which factors most strongly influenced fungal community composition. Our results indicated that temperature, latitude, and plant diversity most strongly influence soil fungal community composition. Fungal diversity patterns were unimodal, with temperate forests (mid latitude) exhibiting the greatest diversity. Furthermore, these diversity patterns indicate that fungal diversity was highest in the forest systems with the lowest tree diversity (temperate forests). Different forest systems were dominated by different fungal subgroups, ectomycorrhizal fungi dominated in boreal and temperate forests; endomycorrhizal fungi dominated in the tropical rainforests, and non-mycorrhizal fungi were best represented in subtropical forests. Our results suggest that soil fungal communities are strongly dependent on vegetation type, with fungal diversity displaying an inverse relationship to plant diversity.

Consistent effects of canopy vs. understory nitrogen addition on the soil exchangeable cations and microbial community in two contrasting forests.

Anthropogenic N deposition has been well documented to cause substantial impacts on the chemical and biological properties of forest soils. In most studies, however, atmospheric N deposition has been simulated by directly adding N to the forest floor. Such studies thus ignored the potentially significant effect of some key processes occurring in forest canopy (i.e., nitrogen retention) and may therefore have incorrectly assessed the effects of N deposition on soils. Here, we conducted an experiment that included both understory addition of N (UAN) and canopy addition of N (CAN) in two contrasting forests (temperate deciduous forest vs. subtropical evergreen forest). The goal was to determine whether the effects on soil exchangeable cations and microbial biomass differed between CAN and UAN. We found that N addition reduced pH, BS (base saturation) and exchangeable Ca and increased exchangeable Al significantly only at the temperate JGS site, and reduced the biomass of most soil microbial groups only at the subtropical SMT site. Except for soil exchangeable Mn, however, effects on soil chemical properties and soil microbial community did not significantly differ between CAN and UAN. Although biotic and abiotic soil characteristics differ significantly and the responses of both soil exchangeable cations and microbial biomass were different between the two study sites, we found no significant interactive effects between study site and N treatment approach on almost all soil properties involved in this study. In addition, N addition rate (25 vs. 50 kg N ha(-1) yr(-1)) did not show different effects on soil properties under both N addition approaches. These findings did not support previous prediction which expected that, by bypassing canopy effects (i.e., canopy retention and foliage fertilization), understory addition of N would overestimate the effects of N deposition on forest soil properties, at least for short time scale.

Dissolved Organic Carbon in Headwater Streams and Riparian Soil Organic Carbon along an Altitudinal Gradient in the Wuyi Mountains, China

Stream water dissolved organic carbon (DOC) correlates positively with soil organic carbon (SOC) in many biomes. Does this relationship hold in a small geographic region when variations of temperature, precipitation and vegetation are driven by a significant altitudinal gradient? We examined the spatial connectivity between concentrations of DOC in headwater stream and contents of riparian SOC and water-soluble soil organic carbon (WSOC), riparian soil C:N ratio, and temperature in four vegetation types along an altitudinal gradient in the Wuyi Mountains, China. Our analyses showed that annual mean concentrations of headwater stream DOC were lower in alpine meadow (AM) than in subtropical evergreen broadleaf forest (EBF), coniferous forest (CF), and subalpine dwarf forest (SDF). Headwater stream DOC concentrations were negatively correlated with riparian SOC as well as WSOC contents, and were unrelated to riparian soil C:N ratio. Our findings suggest that DOC concentrations in headwater streams are affected by different factors at regional and local scales. The dilution effect of higher precipitation and adsorption of soil DOC to higher soil clay plus silt content at higher elevation may play an important role in causing lower DOC concentrations in AM stream of the Wuyi Mountains. Our results suggest that upscaling and downscaling of the drivers of DOC export from forested watersheds when exploring the response of carbon flux to climatic change or other drivers must done with caution.