D.A. Crossley

MICROBIAL AND FAUNAL INTERACTIONS AND EFFECTS ON LITTER NITROGEN AND DECOMPOSITION IN AGROECOSYSTEMS

We conducted field experiments to test the general hypothesis that the com- position of decomposer communities and their trophic interactions can influence patterns of plant litter decomposition and nitrogen dynamics in ecosystems. Conventional (CT) and no-tillage (NT) agroecosystems were used to test this idea because of their structural sim- plicity and known differences in their functional properties. Biocides were applied to ex- perimentally exclude bacteria, saprophytic fungi, and microarthropods in field exclosures. Abundances of decomposer organisms (bacteria, fungi, protozoa, nematodes, microar- thropods), decomposition rates, and nitrogen fluxes were quantified in surface and buried litterbags (Secale cereale litter) placed in both NT and CT systems. Measurements of in situ soil respiration rates were made concurrently. The abundance and biomass of all microbial and faunal groups were greater on buried than surface litter. The mesofauna contributed more to the total heterotrophic C in buried litter from CT (6-22%) than in surface litter from NT (0.4-1/1%). Buried litter decay rates (1.4-1.7%/d) were -2.5 times faster than rates for surface litter (0.5-O.7%/d). Ratios of fungal to bacterial biomass and fungivore to bacterivore biomass on NT surface litter generally increased over the study period resulting in ratios that were 2.7 and 2.2 times greater, respectively, than those of CT buried litter by the end of the summer. The exclusion experiments showed that fungi had a somewhat greater influence on the decomposition of surface litter from NT while bacteria were more important in the de- composition of buried litter from CT. The fungicide and bactericide reduced decomposition rates of NT surface litter by 36 and 25% of controls, respectively, while in CT buried litter they were reduced by 21 and 35% of controls, respectively. Microarthropods were more important in mobilizing surface litter nitrogen by grazing on fungi than in contributing to litter mass loss. Where fungivorous microarthropods were experimentally excluded, there was less than a 5% reduction in mass loss from litter of both NT and CT, but fungi- fungivore interactions were important in regulating litter N dynamics in NT surface litter. As fungal densities increased following the exclusion of microarthropods on NT surface litter, there was 25% greater N retention as compared to the control after 56 d of decay. Saprophytic fungi were responsible for as much as 86% of the net N immobilized (1.81 g /m2) in surface litter by the end of the study when densities of fungivorous microarthropods were low. Although bacteria were important in regulating buried litter decomposition rates and the population dynamics of bacterivorous fauna, their influence on buried litter N dynamics remains less clear. The larger microbial biomass and greater contribution of a bacterivorous fauna on buried litter is consistent with the greater carbon losses and lower carbon assimilation in CT than NT agroecosystems. In summary, our results suggest that litter placement can strongly influence the com- position of decomposer communities and that the resulting trophic relationships are im- portant to determining the rates and timing of plant litter decomposition and N dynamics. Furthermore, cross placement studies suggest that the decomposer communities within each tillage system, while not discrete, are adapted to the native litter placements in each.

Detritus Food Webs in Conventional and No-tillage Agroecosystems

onservation tillage-crop planting systems that leave 30% or more of crop residues on the soil surface instead of plowing them under-is becoming widely adopted in US agriculture. The total area under conservation tillage is estimated to be between 24 and 36 million hectares, or about one-third of the nations cropland, which represents an increase of about 1,25% during the past decade (Christensen and Magleby 1983). The primary reasons for this increase are that conservation

Forest hydrology and ecology at Coweeta

Coweeta is one of the oldest continuously operating laboratories of its type in the world. For the first time, a complete review and summary of more than 50 years study of the hydrological and ecological responses of baseline and managed Southern Appalachian hardwood forests at Coweeta is now supplied by this volume. The long-term research approach represents a continuum of theory, experimentation and application using watersheds as landscape units of investigation. Thus, the information encompasses a wide range of interpretations and interests. In addition to in-depth analyses of terrestrial and stream processes, the breadth of coverage includes historical perspectives and relevance of ecosystem science to management needs. In a broader sense, the Coweeta research effort is considered from a perspective of national and international forest hydrology and ecology programs.

A Hierarchical approach to evaluating the significance of soil biodiversity to biogeochemical cycling

The significance of biodiversity to biogeochemical cycling is viewed most directly through the specific biogeochemical transformations that organisms perform. Although functional diversity in soils can be great, it is exceeded to a high degree by the richness of soil species. It is generally inferred from this richness that soil systems have a high level of functional redundancy. As such, indices of species richness probably contribute little to understanding the functioning of soil ecosystems. Another approach stresses the value of identifying “keystone” organisms, that is those that play an exceptionally important role in determining the structure and function of ecosystems. Both views tend to ignore the importance of biodiversity in maintaining the numerous and complex interactions among organisms in soils and their contributions to biogeochemical cycling. We describe some of those interactions and their importance to ecosystem function.

Biological indices of soil quality: an ecosystem case study of their use

Soil quality indices can help ensure that site productivity and soil function are maintained. Biological indices yield evidence of how a soil functions and interacts with the plants, animals and climate that comprise an ecosystem. Soil scientists can identify and quantify both chemical and biological soil-quality indicators for ecosystems with a single main function, such as agricultural lands and forest plantations. However, quantifying these indices in complex ecosystems — that have multiple uses or goals such as maintaining biodiversity, aesthetics, recreation, timber production and water quality — is much more difficult. In an ecosystem context all components — plants, animals and humans — interact with the soil differently, making soil quality indices variable. These interactions result in a combination of biological processes that make each ecosystem unique. We examined the soil and site quality of five forest stands (xeric oak-pine; two mixed hardwood, cove hardwood, northern

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.

A high-efficiency, “low-technology” Tullgren-type extractor for soil microarthropods

Abstract Construction and operation of a high-gradient Tullgren-type extractor for soil microarthropods are described. The extractor is simple and inexpensive, and is designed to be operated in a refrigerator or refrigerated room. Tops of samples are heated by Christmas tree lights; bottoms of samples are protected from heat by baffles and are cooled by the refrigerated air, yielding a 20°C gradient. The design of the extractor and the operating procedures are intended to reduce contamination by soil, thus increasing efficiency and reducing sample sorting time.

Responses of trophic groups of soil nematodes to residue application under conventional tillage and no-till regimes

A laboratory and a field study were conducted to monitor the increase in numbers and 14C uptake of different trophic groups of soil nematodes in response to residue addition and to examine the relative importance of bacterivorous and fungivorous nematodes in conventional (CT) and no-till (NT) agroecosystems. In general, soil nematode numbers increased more rapidly in response to residue addition and became much more abundant (greater than five-fold) under laboratory conditions than in the field. Our results showed that bacterivorous nematodes responded to residue addition earlier than fungivorous nematodes under both CT and NT regimes in the laboratory and field studies. A depth effect was observed in NT, but not in the CT treatment; this reflected the vertical residue distribution in both tillage regimes. Soil nematodes were more abundant under NT than under CT in the field. The same pattern was observed at the beginning of the laboratory study but it reversed later. The ratios of fungivorous-to-bacterivorous nematodes (FN-to-BN) were not significantly different between CT and NT treatments at the beginning of the experiment. They were very low (less than 0.2) in both tillage regimes, indicating that bacterivorous nematodes were relatively more important than fungivorous nematodes in both tillage agroecosystems. However, the FN-to-BN ratios increased with time after residue decomposition started, particularly in the CT treatment. This suggested that the relative importance of fungivorous nematodes increased with the progress of residue decomposition. It was more pronounced in the CT treatment during the short period after residue application. In both the laboratory and field studies, the 14C specific activity of soil nematodes and the ratio of 14C bound in nematode biomass to total 14C decayed in the experiment (reported elsewhere) were significantly higher under CT than under NT, suggesting that soil nematodes use carbon more efficiently under CT than under NT. No significant difference of 14C specific activity of soil nematodes was found between the two depths under CT in both the studies; however, 14C specific activity was significantly higher in the 0–2.5 cm than in the 2.5–5.0 cm layer under NT in the laboratory study.

Biodiversity of microarthropods in agricultural soils: relations to processes

Abstract Soil microarthropods (principally mites and collembolans) are among the unseen faunal diversity in nearly all agricultural soils. Microarthropods participate in the complex food webs of soils, but their importance is seldom appreciated. Laboratory and field results show that microarthropods have impacts on organic debris, microbial decomposers, nematodes, roots and pathogenic fungi. However, their impact on primary production is only indirect. Opportunities for managing soil microarthropods in agricultural soils have been ignored.

Soil biology, soil ecology, and global change

SummaryThis overview paper addresses aspects of scaling in space and time, and scaling in relation to micro-and macrohabitats. Ecological processes in soils are examined for possible generalizations about processes and organisms, across a wide range of different habitats. Problems of scaling in space and time that have an important impact on processes associated with global change are outlined.