D.C. Coleman

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.

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.

Nonadditive effects of leaf litter species diversity on breakdown dynamics in a detritus-based stream.

Since species loss is predicted to be nonrandom, it is important to understand the manner in which those species that we anticipate losing interact with other species to affect ecosystem function. We tested whether litter species diversity, measured as richness and composition, affects breakdown dynamics in a detritus-based stream. Using full-factorial analyses of single- and mixed-species leaf packs (15 possible combinations of four dominant litter species; red maple [Acer rubrum], tulip poplar [Liriodendron tulipifera], chestnut oak [Quercus prinus], and rhododendron [Rhododendron maximum]), we tested for single-species presence/absence (additive) or species interaction (nonadditive) effects on leaf pack breakdown rates, changes in litter chemistry, and microbial and macroinvertebrate biomass. Overall, we found significant nonadditive effects of litter species diversity on leaf pack breakdown rates, which were explained both by richness and composition. Leaf packs containing higher litter species richness had faster breakdown rates, and antagonistic effects of litter species composition were observed when any two or three of the four litter species were mixed. Less-consistent results were obtained with respect to changes in litter chemistry and microbial and macroinvertebrate biomass. Our results suggest that loss of litter species diversity will decrease species interactions involved in regulating ecosystem function. To that end, loss of species such as eastern hemlock (Tsuga canadensis) accompanied by predicted changes in riparian tree species composition in the southeastern United States could have nonadditive effects on litter breakdown at the landscape scale.

Labile soil carbon pools in subtropical forest and agricultural ecosystems as influenced by management practices and vegetation types

Abstract Carbon storage in agricultural and forest soils has attracted attention recently due to its potential as a substantial carbon sink. Labile soil C pools are especially important because they are more vulnerable to climatic change and disturbance and play vital roles in nutrient cycling. Southern Appalachian forest soils and those from conventional tillage (CT), no-tillage (NT) and fescue sods at three sites in the Georgia piedmont were analyzed for total C, total N, carbohydrates, and microbial biomass C. The sizes of soil labile C pools (carbohydrates and microbial biomass) and their contributions to the total soil C pool differed significantly among ecosystems. The highest carbohydrate contents and microbial biomass C were found in forest soils, but agricultural soils had a significantly higher proportion of the soil organic matter present as carbohydrates and as microbial biomass. This difference probably reflects the quality of soil organic matter. Soil microbial biomass C was more sensitive to changes in management regimes than soil carbohydrates. Management practices signfiicantly affected organic C, carbohydrate contents, microbial biomass C and organic C turnover rates in agricultural soils, whereas differences in the quality of organic input due to different vegetation types substantially influenced soil labile C pools in forest soils. High mannose-to-xylose ratios in highly sandy agricultural soils indicate that plant-derived materials are rapidly metabolized by microorganisms and that organic C protection in sandy soils is largely dependent on reducing microbial access through effective residue management such as surface placement.

Transformations of added and indigenous nitrogen in gnotobiotic soil: a comment on the priming effect

When 15N-labelled fertilizer is added to soil, it frequently appears that more indigenous soil-N is mineralized-a phenomenon called the “priming effect”. We have made an experimental study of the interaction of added and indigenous N during laboratory incubations with well-defined N concentrations, microbial populations and incubation conditions. Populations of a single species each of bacteria and amoebae were inoculated into propylene-oxide sterilized soil and their growth, respiration and N-mineralization were monitored for 34 days. In one experiment, bacterial numbers doubled (from 17.2 to 36.7 × 108 g−1) and respired C nearly doubled (from 966 to 1892 μg g−1) as added N (ammonium sulfate) increased from 0 to 70 μg g−1. In a second experiment, mineral-N following incubation increased from 2.6 to 55.9 μg g−1 and mineral-N of soil origin increased from 2.6 to 16.3 μg g−1 as added N increased from 0 to 100 μg g−1. However, in the same experiment, the amount of unlabelled soil-N required to dilute the 15N-labelled ammonium to the observed atom% 15N decreased from 72.1 to 41.2 μg g−1. Mineralization of soil-N was enhanced (an apparent priming effect) in spite of decreased interaction with unlabelled-N. This “priming effect” resulted from increased net N-mineralization that accompanied increased N-fertilization so long as mineral-N concentrations remained low enough to limit soil microbial activity.

Nitrogen mineralization and nitrification following land conversion in montane Ecuador

Abstract The lower montane zone of northwestern Ecuador, like many parts of the tropics, is undergoing rapid conversion from native forest vegetation to crop and pastureland. The current landscape is a mosaic of agricultural land, forest fragments and second-growth vegetation in various stages of development. While there is abundant research documenting the effects of land-use change in the lowland tropics, such information is scarce for montane regions on young volcanic soils. We compared foxtail pasture (Setaria sphacelata, [(Schumach.) Stapf and C.E. Hubb.]) and traditional, mixed-species pasture with undisturbed old-growth forest, 15–20 y-old secondary forest and 5–10 y-old shrubby regrowth. At two replicates of each vegetation class, we measured soil nitrogen and carbon pools, in situ net nitrogen mineralization and nitrification, soil respiration and soil physical properties. Setaria pasture decreased soil NO−3-N pools and net mineralization and nitrification rates compared to mature forest, secondary vegetation, and mixed-species pasture. Soil NO−3-N in Setaria pastures, during wet and dry seasons, was 40 and 25% of amounts measured in other vegetation types. Net nitrification rates were also lower beneath Setaria during both seasons, the greatest difference occurring during the wet season. Net nitrification rates increased considerably (two-fold and greater) under wet season conditions beneath all vegetation types except Setaria pasture. Soils beneath both pasture types were wetter and had higher bulk density than mature and second-growth forests. Conversion to pasture produces widely varying effects on soil N dynamics depending on characteristics of the pasture species, such as Setaria’s extremely dense root system. Reduced soil N availability beneath Setaria pasture will affect both long-term pasture productivity and subsequent forest regeneration in abandoned pastures.

Characterization of a substrate-induced respiration method for measuring fungal, bacterial and total microbial biomass on plant residues

Abstract A substrate-induced respiration (SIR) method is described to measure the contributions of fungi and bacteria to total glucose-induced microbial respiration on plant residues of differing composition. Relationships between fungal, bacterial and total SIR and biomass were used to develop regression equations for predicting microbial biomass C from measures of SIR. Total SIR rates (100–2000 μg CO2-C g−1 h−1) and biomass-specific SIR rates (64–72 ng CO2-C h−1 μg−1 biomass C) from plant residues were considerably greater than those calculated from the literature for soils. Results of longer term decomposition studies indicate that the C:N ratios of plant residues through time account for the greatest amount of the variation in total SIR. Annual decomposition rate constants (k) for plant residues were positively correlated (r2=0.99) to overall mean estimates of total SIR. The plant residue SIR method has advantages over conventional direct count methods because it distinguishes a physiologically active component of the microbial biomass. Furthermore, it allows separation of fungal and bacterial components that may aid in understanding microbial controls on plant residue decomposition.

Soil carbohydrates in aggrading and degrading agroecosystems: influences of fungi and aggregates

Abstract Differences in soil organic carbon, nitrogen and carbohydrates as a result of different tillage practices under continuous cropping were studied in a 12 year old sorghum/rye rotation experiment in the Georgia piedmont. Soil organic C and N concentrations in an aggrading agroecosystem (no-tillage, NT) were significantly higher than in a degrading agroecosystem (conventional tillage, CT) at all dates. Soil carbohydrates followed a pattern similar to total organic C. Carbohydrate concentrations in macroaggregates (>250 μm) from NT surface soils were significantly higher than in microaggregates ( 2000 μm aggregates contained the lowest total organic C concentration. A significantly higher mannose to xylose ratio in microaggregates than in macroaggregates suggested that organic matter (OM) in microaggregates was strongly metabolized by soil microorganisms, but OM in macroaggregates was much less processed. The inhibition of fungi by the fungicide captan significantly reduced the concentrations of soil C and acid-hydrolyzable carbohydrates in NT soils but not in CT soils, indicating that fungi play a more significant role in organic C retention in the NT system than in the CT system.

Budgets for root-derived C and litter-derived C: comparison between conventional tillage and no tillage soils

Placement of plant residues in conventional tillage (CT) and no-tillage (NT) soils affects organic matter accumulation and the organization of the associated soil food webs. Root-derived C inputs can be considerable and may also influence soil organic matter dynamics and soil food web organization. In order to differentiate and quantify C contributions from either roots or litter in CT and NT soils, a 14 C tracer method was used. To follow root-derived C, maize plants growing in the field were 14 C pulse-labeled, while the plant litter in those plots remained unlabeled. The 14 C was measured in NT and CT soils for the different C pools (shoots, roots, soil, soil respiration, microbial biomass). Litter-derived C was followed by applying 14 C labeled maize litter to plots which had previously grown unlabeled maize plants. The 14 C pools measured for the litter-derived CT and NT plots included organic matter, microbial biomass, soil respiration, and soil organic C. Of the applied label in the root-derived C plots, 35‐55, 6‐8, 3, 1.6, and 0.4‐2.4% was recovered in the shoots, roots, soil, cumulative soil respiration, and microbial biomass, respectively. The 14 C recovered in these pools did not differ between CT and NT treatments, supporting the hypothesis that the rhizosphere microbial biomass in NT and CT may be similar in utilization of root-derived C. Root exudates were estimated to be 8‐13% of the applied label. In litter-derived C plots, the percentage of applied label recovered in the particulate organic matter (3.2‐82%), microbial biomass (4‐6%), or cumulative soil respiration (12.5‐14.7%) was the same for CT and NT soils. But the percentage of 14 C recovered in CT soil organic C (18‐69%) was higher than that in NT (12‐43%), suggesting that particulate organic matter (POM) leaching and decomposition occurred at a higher rate in CT than in NT. Results indicate faster turnover of litter-derived C in the CT plots. q 2001 Elsevier Science Ltd. All rights reserved.

Long-term effects of earthworms on microbial biomass nitrogen in coarse and fine textured soils

Abstract We conducted field studies of the effects of earthworms on microbial biomass-N in sandy clay loam vs. sandy soils under no-tillage management on Ultisols of the southern Appalachian Piedmont in Georgia, USA. 15 N -labeled crop residue was applied to the surface of plots with or without the addition of earthworms (principally Lumbricus rubellus and Aporrectodea caliginosa). Microbial biomass N and 15 N levels were measured at intervals over five years. Microbial N concentrations increased in both soils but more so at the surface of the sandy soil and in deeper soil layers in the clay soil. Microbial N concentrations were consistently lower in earthworm treatments than in controls in the clay soil and to a lesser extent in the sandy soil. In deeper layers of the clay soil, earthworm additions increased 15 N enrichment of the microbial pool, suggesting that earthworms increased transport of crop residue N into the subsoil. In the sandy soil the microbial pool was half as large as in the clay soil, but showed a 100–300% increase in 15 N enrichment during the first year, indicating substantially higher microbial turnover. Although earthworm activity reduced standing stocks of microbial biomass, particularly in the fine-textured soil, it appeared to increase the turnover of the microbial-N pool as indicated by 15 N measurements. Observed changes in microbial biomass reflected rapid cycling of labile organic matter pools in response to biological activity, soil texture and soil management.