Robert W. Parmelee

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

Ecosystem processes along an urban-to-rural gradient

In order to understand the effect of urban development on the functioning of forest ecosystems during the past decade we have been studying red oak stands located on similar soil along an urban-rural gradient running from New York City ro rural Litchfield County, Connecticut. This paper summarizes the results of this work. Field measurements, controlled laboratory experiments, and reciprocal transplants documented soil pollution, soil hydrophobicity, litter decomposition rates, total soil carbon, potential nitrogen mineralization, nitrification, fungal biomass, and earthworm populations in forests along the 140 × 20 km study transect. The results revealed a complex urban-rural environmental gradient. The urban forests exhibit unique ecosystem structure and function in relation to the suburban and rural forest stands; these are likely linked to stresses of the urban environment such as air pollution, which has also resulted in elevated levels of heavy metals in the soil, the positive effects of the heat island phenomenon, and the presence of earthworms. The data suggest a working model to guide mechanistic work on the ecology of forests along urban-to-rural gradients, and for comparison of different metropolitan areas.

Decay Rates, Nitrogen Fluxes, and Decomposer Communiies of Single‐ and Mixed‐Species Foliar Litter

Decomposition rates, N fluxes, and abundances of decomposer organisms were quantified in mixed-species litterbags (containing leaves of two or three of the following tree species: Acer rubrum, Cornus florida, and Quercus prinus) and in litterbags containing leaves of a single species. Data from single-species litterbags were used to generate predicted decay rates, N fluxes, and abundances of decomposer organisms for mixed-species litter- bags, against which observed values could be compared to determine if significant inter- action effects occurred when litter of different species, and different resource quality, was mixed. Decay rates of-mixed-species litterbags during the 1-yr study were not significantly different than predicted from decay rates of individual component species. However, there were significant interaction effects on N fluxes and abundances of decomposer organisms. In the C. florida-A. rubrum and C. florida-A. rubrum-Q. prinus litter combinations there were significantly greater initial releases of N and lower subsequent N immobilization than predicted. In the A. rubrum-Q. prinus and C. florida-A. rubrum-Q. prinus litter combi- nations, lengths of fungal hyphae were significantly less than predicted on at least half the collection dates. Bacterial numbers in the mixed-litter combinations were also generally less than predicted. Nematode abundances, especially fungivores, were generally greater than predicted in mixed-species litterbags until the last sample date. Observed mean abun- dances of nematodes over all dates were 20-30% greater than predicted. Microarthropod abundances were more variable, but tended to be lower than predicted. Our results indicate that measurement of N flux in single-species litterbags may not reflect actual N flux in the field, where leaves of several tree species are mixed together. The differences in N flux between single- and mixed-species litterbags can affect ecosystem-level estimates of N release or accumulation in decomposing litter. For example, estimates of ecosystem-level N fluxes at our field site, based on data from single-species litterbags, resulted in a 64% underestimate of N released by day 75 and a 183% overestimate of N accumulated in the litter by day 375, relative to estimates based on data from mixed-species litterbags. We suggest that the deviation of observed N fluxes in mixed-species litterbags from those predicted using single-species litterbags are the result of differences in the decomposer community, such as lower microbial and microarthropod densities and higher nematode densities, resulting when litter of varied resource quality is mixed together. Longer term studies will be needed to determine if the differences between observed and predicted decomposer communities in mixed-species litter combinations influence the latter stages of decomposition where invertebrate-microbial interactions may have a greater effect on decay rates and nutrient release.

Earthworm abundance and nitrogen mineralization rates along an urban-rural land use gradient

Preliminary observations of glaciated regions in North America suggest that forest stands associated with urban areas may support high populations of non-native species of earthworms relative to forests in rural areas. Moreover, the presence of these non-native species of worms may be moderating the effects of pollutant deposition on litter quality, or the decomposability of litter, and subsequently nutrient cycling processes in the urban stands. In this study we quantified earthworm abundance and biomass in urban and rural oak forest stands along a 130 × 20 km urban-rural transect in New York City, USA metropolitan area. We also evaluated the effects of earthworms on potential net N mineralization and nitrification in a laboratory microcosm study. Earthworm abundance and biomass along the transect was significantly higher in urban (25.1 individuals m−2 and 2.16 g m−2) than in rural (2.1 individuals m−2 and 0.05 g m−2) stands. In a microcosm study, potential net N mineralization rates (0.15 mg N kg−1 d−1) were significantly higher in urban soil with earthworms than in urban soil without earthworms, which exhibited a net immobilization of N. Rural soil with earthworms had significantly higher rates (0.57 mg N kg−1 d−1) than urban soil with earthworms and rural soil without earthworms (0.28 mg N kg−1 d−1). Nitrification rates in urban were surprisingly high given the relatively low litter quality and rates of N mineralization in these soils. The results suggest that earthworms may play an important role in forest ecosystems embedded within urban areas by enhancing nitrogen cycling processes and thereby compensating for the effects of air pollution on litter quality and decomposition.

EARTHWORM EFFECTS ON CARBON AND NITROGEN DYNAMICS OF SURFACE LITTER IN CORN AGROECOSYSTEMS

We examined the influence of earthworms on surface litter decomposition in corn (Zea mays) agroecosystems in Wooster, Ohio. We employed a split-plot experimental design with 12 main plots, each 20 × 30 m and containing three 4.5 × 4.5 m field enclosures in which earthworm populations were (1) increased, (2) decreased, or (3) unmodified. The main plots received one of three nutrient treatments (cow manure, legume cover crop, inorganic fertilizer) with four replicates. The three earthworm population treatments were randomly assigned to the three field enclosures within each main nutrient-treatment plot. We added corn litter to the soil surface in each of the treatment combinations in the field enclosures in November 1992 and collected remaining litter after 19, 85, 135, 161, and 191 d. We separated out small piles of surface litter (i.e., “middens”) associated with the entrance to burrows of Lumbricus terrestris from the rest of the litter to determine if they differed from each other in C and N content and...

Changes in soil N pools in response to earthworm population manipulations in agroecosystems with different N sources

Abstract Responses of soil N pools to field manipulation of earthworm populations (reduced, unaltered or increased each spring and autumn) were evaluated within each of three agroecosystems based on different N sources: NH4NO3 fertilizer, cow manure or a legume-rye winter cover crop. Our objectives were to determine the effects of earthworms on soil N dynamics in agroecosystems based on different organic or inorganic sources of N, and to examine potential interactive effects of agroecosystem treatments and field-scale earthworm manipulations on soil N pools and potential N losses. Earthworm manipulations began in spring 1991, and were repeated each spring and fall. Soil microbial biomass N was determined by fumigation-extraction on six dates in 1992 and four dates in 1993. Extractable inorganic soil N (0–15 cm) was measured in January and approximately every 2 weeks during the growing seasons of 1992 and 1993. Additionally, the post-growing season vertical distribution (0–15, 15–30, and 30–45 cm) of extractable soil NO3N was evaluated in November of 1992 and 1993. Earthworm manipulations affected microbial biomass N and extractable inorganic N pools in bulk soil samples. Microbial biomass N was significantly higher in the earthworm reduction treatments. There were significant earthworm × agroecosystem interactions affecting soil NO3. In the inorganically fertilized system, earthworm additions resulted in elevated amounts of extractable NO3 during the growing season of both years. Extractable NH4 concentrations were increased by earthworm additions in 1993, but only in the inorganically fertilized system. Earthworm additions also increased the concentration of soil NO3 at lower depths after the growing season, especially in the inorganically fertilized system. These results suggest that earthworms can alter N cycling processes in agroecosystems, and that these changes are sufficient to be detected by bulk soil sampling. Our results also indicate that the net effects of earthworm activity can vary with agroecosystem management practices. Earthworms may increase N availability by reducing microbial immobilization and enhancing mineralization. However, increased amounts of soil NO3 at the end of the growing season, and increased concentrations in lower soil horizons, could lead to increased leaching losses from inorganically fertilized systems. The implications of these changes for ecosystem-level nutrient fluxes will require further investigation.

Earthworms Increase Nitrogen Leaching to Greater Soil Depths in Row Crop Agroecosystems

Many biological functions of soil organisms are replaced in intensive agricultural systems, but earthworms and other soil invertebrates may continue to have significant effects on nutrient cycling in these disturbed systems. We investigated the influence of earthworms on leaching of water and nitrogen in corn (Zea mays L.) agroecosystems in a long-term (6-year) field experiment in Wooster, Ohio, USA. We employed a split-plot experimental design in which main plots received one of three nutrient treatments (cow manure, legume–grass mixture, inorganic fertilizer) and contained three 4.5 × 4.5-m field enclosures in which earthworm populations were increased, decreased, or unmodified. We installed zero-tension lysimeters beneath enclosures with increased or decreased populations and collected leachates regularly in 1996, analyzing them for water volume and concentrations of NH4+, NO3−, and dissolved organic nitrogen (DON). Earthworms did not influence concentrations of inorganic N or DON but greatly increased leachate volume. The total flux of N in soil leachates was 2.5-fold greater in plots with increased earthworm populations than in those with decreased populations. Earthworm population density was positively correlated with total N leaching flux (r2 = 0.49). Leaching losses of N to a depth of 45 cm were greater in the inorganically fertilized than in the organically fertilized plots, possibly due to greater inorganic N concentrations and lower immobilization potential in inorganically fertilized systems. Our results indicate that earthworms can increase the leaching of water and nitrogen to greater soil depths, potentially increasing N leaching from the system.

Population dynamics of earthworm communities in corn agroecosystems receiving organic or inorganic fertilizer amendments

Abstract The dynamics of earthworm populations were investigated in continuously-cropped, conventional disk-tilled corn agroecosystems which had received annual long-term (6 years) amendments of either manure or inorganic fertilizer. Earthworm populations were sampled at approximately monthly intervals during the autumn of 1994 and spring and autumn of 1995 and 1996. The dominant earthworm species were Lumbricus terrestris L. and Aporrectodea tuberculata (Eisen), which comprised 50–60% and 8–13%, respectively, of the total annual earthworm biomass. Lumbricus rubellus (Hoffmeister) and Aporrectodea trapezoides (Dugés) were much less abundant and contributed a small fraction of total earthworm biomass. Earthworm numbers and biomass were significantly greater in manure-amended plots compared to inorganic fertilizer-treated plots during the majority of the study period. Seasonal fluctuations in earthworm numbers and biomass were attributed to changes in soil temperature and moisture, and cultivation. Unfavorable climatic conditions in the summer and autumn of 1995 caused earthworm abundance and biomass to decline significantly. Mature L. terrestris, L. rubellus and A. tuberculata were most abundant in May and June of 1995 and 1996, and cocoon production was greatest in June and July 1995 and June 1996. Recruitment of juveniles of Lumbricus spp. and Aporrectodea spp. into earthworm communities occurred primarily in the autumn. Long-term amendments of manure or inorganic fertilizer did not change the species composition of earthworm communities in these agroecosystems. The earthworm populations in both manure and inorganic fertilizer plots have declined significantly after 5 years of continuously-cropped corn.

An ecosystem approach to soil toxicity testing: a study of copper contamination in laboratory soil microcosms

An ecosystem approach to soil toxicity testing allows for integration of the effects of chemical contaminants on different components of the soil food web (system structure) and ecosystem-level processes (system function). We used this approach to study copper contamination in small laboratory soil microcosms. Microcosm soils were treated with CuSO4 at the following concentrations: 0, 50, 100, 200, 400, and 800 mg Cu kg−1 soil. Five, 10, 20 and 40 days after soil treatment, we made the following organism-level measurements: microbial biomass N, substrate-induced respiration (SIR) and soil urease activity; total nematode numbers; earthworm mortality, growth and body accumulation of Cu. Our process-level measurements were net N mineralization and litter decomposition. SIR was the most sensitive of the parameters measured with significant effects observed at Cu concentrations as low as 50 mg kg−1. Microbial biomass N and earthworm growth showed intermediate sensitivity with effects at 200 mg kg−1 Cu. The least sensitive organism-level parameters were soil urease activity and nematode abundance, both showing significant effects only at 800 mg kg−1 Cu. At the process-level, there was an inhibition of litter decomposition starting at 100 mg kg−1 Cu, and a sharp increase in net N mineralization at 800 mg kg−1 Cu. By examining both the structure and function of the soil system, we were able to link the direct effects of copper on organisms to indirect effects on ecosystem-level processes and were able to suggest mechanisms to account for our results. The release of nitrogen from microbial cells killed by direct toxicity of Cu at 800 mg kg−1 resulted in a transient increase in dissolved organic N, followed by a flush of N mineralization, resulting in large increase in NH4N compared with the untreated control. Microbial mortality also apparently led to the inhibition of litter decomposition. Measuring the effects of contamination at different trophic levels simultaneously, and linking them to ecosystem processes, provided insights into the ecological mechanisms of the observed effects. This makes the ecosystem approach particularly valuable for analyzing the highly complex soil system. We suggest that the information obtained in a laboratory test based on the ecosystem approach is the most appropriate method for extrapolation to field situations.