Melany C. Fisk

Ecosystem Consequences of Exotic Earthworm Invasion of North Temperate Forests

The invasion of north temperate forests by exotic species of earthworms is an important issue that has been overlooked in the study and management of these forests. We initiated research to address the hypothesis that earthworm invasion will have large consequences for nutrient retention and uptake in these ecosystems. In this special feature of Ecosystems, we present five papers describing results from our experiment. In this paper, we (a) introduce our experimental approach and conceptual model of how earthworms influence forest ecosystem processes, (b) describe the characteristics of the study areas and earthworm communities at our two study locations, and (c) provide a brief overview and synthesis of the main findings. The most dramatic effect of earthworm invasion was the loss of the forest floor at an undisturbed forest site, which altered the location and nature of nutrient cycling activity in the soil profile. Invasion changed soil total carbon (C) and phosphorus (P) pools, carbon–nitrogen (C:N) ratios, the loss and distribution of different soil P fractions, and the distribution and function of roots and microbes. Response to invasion varied with site characteristics and earthworm species. Our results suggest that exotic earthworm invasion is a significant factor that will influence the structure and function of northern temperate forest ecosystems over the next few decades. Regional evaluations of these forests will need to consider the presence or absence of earthworms along with other important ecosystem drivers, such as pollution, climate, and underlying soil characteristics.

TOPOGRAPHIC PATTERNS OF ABOVE‐ AND BELOWGROUND PRODUCTION AND NITROGEN CYCLING IN ALPINE TUNDRA

Topography controls snowpack accumulation and hence growing-season length, soil water availability, and the distribution of plant communities in the Colorado Front Range alpine. Nutrient cycles in such an environment are likely to be regulated by interactions between topographically determined climate and plant species composition. We investigated variation in plant and soil components of internal N cycling across topographic gradients of dry, moist, and wet alpine tundra meadows at Niwot Ridge, Colorado. We expected that plant production and N cycling would increase from dry to wet alpine tundra meadows, but we hypothesized that variation in N turnover would span a proportionately greater range than productivity, because of feedbacks between plants and soil microbial processes that determine N availability. Plant production of foliage and roots increased over topographic sequences from 280 g·m−2·yr−1 in dry meadows to 600 g·m−2·yr−1 in wet meadows and was significantly correlated to soil moisture. Contra...

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO2-C g−1 d−1 in fertilized plots, compared to 270 µg CO2-C g−1 d−1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO3− immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH4+ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO2, respiration per unit biomass) were higher in fertilized plots (56 µg CO2-C mg C−1 d−1) than control (48 µg CO2-C mg C−1 d −1) or litter removal (45 µg CO2-C mg C−1 d−1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.

SEASONAL PARTITIONING OF NITROGEN BY PLANTS AND SOIL MICROORGANISMS IN AN ALPINE ECOSYSTEM

The seasonal dynamics of plant N assimilation and microbial N immobili- zation were studied in an alpine ecosystem to evaluate temporal patterns of plant and microbial N partitioning and the potential for plant vs. microbial competition for N. Plant N uptake was higher in the first half of the growing season than later in the season, as indicated by changes in biomass N and by 15 N uptake. Microbial N pools were low during the first half of the growing season (9.5 g N/m 2 on 1 June) and increased late in the season, from 11.4 g N/m 2 on 1 August 1991 to 38.6 g N/m 2 on 14 October 1991. Two different measures of N availability were highest in the midseason. Ion exchange resin bag N uptake was greatest in July (86.0 m gN · g

Influence of Earthworm Invasion on Redistribution and Retention of Soil Carbon and Nitrogen in Northern Temperate Forests

We analyzed soil organic matter distribution and soil solution chemistry in plots with and without earthworms at two sugar maple (Acer saccharum)–dominated forests in New York State, USA, with differing land-use histories to assess the influence of earthworm invasion on the retention or loss of soil carbon (C) and nitrogen (N) in northern temperate forests. Our objectives were to assess the influence of exotic earthworm invasion on (a) the amount and depth distribution of soil C and N, (b) soil δ13C and δ15N, and (c) soil solution chemistry and leaching of C and N in forests with different land-use histories. At a relatively undisturbed forest site (Arnot Forest), earthworms eliminated the thick forest floor, decreased soil C storage in the upper 12 cm by 28%, and reduced soil C:N ratios from 19.2 to 15.3. At a previously cultivated forest site with little forest floor (Tompkins Farm), earthworms did not influence the storage of soil C or N or soil C:N ratios. Earthworms altered the stable isotopic signature of soil at Arnot Forest but not at Tompkins Farm; the alteration of stable isotopes indicated that earthworms significantly increased the loss of forest floor C but not N from the soil profile at Arnot Forest. Nitrate (NO3−) concentrations in tension and zero-tension lysimeters were much greater at Tompkins Farm than Arnot Forest, and earthworms increased NO3− leaching at Tompkins Farm. The results suggest that the effect of earthworm invasion on the distribution, retention, and solution chemistry of soil C and N in northern temperate forests may depend on the initial quantity and quality of soil organic matter at invaded sites.

Physiological and production responses of plant growth forms to increases in limiting resources in alpine tundra: implications for differential community response to environmental change

Physiological and growth measurements were made on forbs and graminoids following additions of water and N+water in a graminoid-dominated dry meadow and a forb-dominated moist meadow, to determine if the community-level response was related to differential responses between the growth forms. Graminoids had higher photosynthetic rates and lower transpiration rates and foliar N concentrations than forbs, and consequently maintained higher photosynthetic N- and water-use efficiencies. Photosynthetic rates, stomatal conductance, and transpiration rates increased significantly only in response to N fertilization and only in moist meadow species. The increase in photosynthetic rates was unrelated to variation in foliar N concentration, but instead correlated with variation in stomatal conductance. Growth based N-use efficiency was higher in moist meadow graminoids than in moist meadow forbs, but did not differ between the growth forms in the dry meadow. The moist meadow community had higher biomass and N standing crops, but the relative increase in these factors in response to N fertilization was greater in the dry meadow. Graminoids had a greater relative increase in biomass and N accumulation than forbs following N fertilization, but moist meadow graminoids exhibited a greater response than dry meadow graminoids. The difference in the growth response between the dry meadow and moist meadow graminoids to N fertilization was correlated with more conservative leaf gas exchange responses in dry meadow species, presumably related to a higher frequency of soil water deficits in this community. Community-level response to the resource additions was therefore mediated by the plant growth form response, corresponding with differences between the growth forms in physiological factors related to resource acquisition and use.

Exotic Earthworm Invasion and Microbial Biomass in Temperate Forest Soils

Invasion of north temperate forest soils by exotic earthworms has the potential to alter microbial biomass and activity over large areas of North America. We measured the distribution and activity of microbial biomass in forest stands invaded by earthworms and in adjacent stands lacking earthworms in sugar maple-dominated forests in two locations in New York State, USA: one with a history of cultivation and thin organic surface soil horizons (forest floors) and the other with no history of cultivation and a thick (3–5 cm) forest floor. Earthworm invasion greatly reduced pools of microbial biomass in the forest floor and increased pools in the mineral soil. Enrichment of the mineral soil was much more marked at the site with thick forest floors. The increase in microbial biomass carbon (C) and nitrogen (N) in the mineral soil at this site was larger than the decrease in the forest floor, resulting in a net increase in total soil profile microbial biomass in the invaded plots. There was an increase in respiration in the mineral soil at both sites, which is consistent with a movement of organic matter and microbial biomass into the mineral soil. However, N-cycle processes (mineralization and nitrification) did not increase along with respiration. It is likely that the earthworm-induced input of C into the mineral soil created a microbial “sink” for N, preventing an increase in net mineralization and nitrification and conserving N in the soil profile.

Influence of earthworm invasion on soil microbial biomass and activity in a northern hardwood forest

Recent invasion and activity of exotic earthworms has profoundly altered the chemical and physical environment of surface soils in northern hardwood forests that previously had mor humus horizons. We investigated the influence of earthworm invasion on soil microbial biomass and activity in surface soils of Allegheny northern hardwood forests in central New York state. Earthworm activity in these sites had transformed surface soils with clear Oi, Oe, and Oa horizons (forest floor) overlying mineral soil, to more uniformly mixed organic-enriched A horizons. The highest concentrations of microbial biomass and activity occurred in the forest floor. Microbial biomass (assayed by chloroform fumigation-extraction) nearly doubled in surface (0-5 cm) mineral soils in response to earthworm activity, an effect that corresponded directly to redistribution of organic matter from forest floor into the mineral soil. Microbial activity in surface mineral soils was even more sensitive to the presence of earthworms than microbial biomass. For example, substrate-induced respiration (or maximum initial respiratory rate, MIRR) was 6.7-fold greater, basal respiration was 5-fold greater, and microbial respiration per unit microbial biomass (metabolic quotient, qCO(2)) was almost 3-fold greater in surface mineral soils where earthworms were present than in earthworm-free sites. Of the activity indices, only MIRR was higher when expressed on an organic matter basis. Surface mineral soils where earthworms were,9 present thus appear to retain a high proportion of the microbial biomass and activity found in mor organic horizons. Our findings suggest that earthworm activity stimulates the activity of soil microorganisms, probably by enhancing organic C availability via processing and mixing of litter. The relative pattern in microbial properties did not change over the growing season; however, there were some seasonal changes in the proportional differences between worm and no-worm soils. Our results indicate interactions among earthworms, organic matter, and soil microbial activity that should alter the carbon and nutrient balance of northern hardwood forest surface soils, relative to non-invaded soils

Effects of Exotic Earthworms on Soil Phosphorus Cycling in Two Broadleaf Temperate Forests

We compared the biogeochemical cycling of phosphorus (P) in northern hardwood forest plots invaded by exotic earthworms versus adjacent uninvaded reference plots. In three of the six pairs of plots, earthworm invasion resulted in significantly more total P in the upper 12 cm of soil. The finding of increased amounts of unavailable and occluded inorganic P forms in the invaded plots suggests that earthworm activity mobilized unweathered soil particles from deeper layers of the soil, increasing the stocks of total P in surface soils. In two pairs of plots, the earthworm-invaded soils had less total P than the reference soils. In these plots, earthworm activity resulted in augmented rates of P cycling and alteration of the physical structure of the soil that increased loss of P in leaching water, reducing the total amount of P. We hypothesize that the different effects of earthworm invasion on the soil P cycle result from unique characteristics of the ecological groups of earthworms dominating each site. The invaded plots with increased total P were dominated by the anecic species Lumbricus terrestris, a large earthworm that constructs deep, vertical burrows and is effective at moving soil materials from and to deeper layers of the profile. In contrast, the earthworm-invaded plots where the total P in the surface soil decreased were dominated by the epi-endogeic species L. rubellus, which feeds and lives in the upper organic layers of the soil. In these plots, earthworms significantly increased the amount of readily exchangeable P in the soil, increasing the loss of this element in leaching water.

Earthworm Invasion, Fine-root Distributions, and Soil Respiration in North Temperate Forests

AbstractThe efflux of carbon from soils is a critical link between terrestrial ecosystems and the atmosphere. Current concerns about rising atmospheric carbon dioxide (CO2) concentrations highlight the need to better understand the dynamics of total soil respiration (TSR, sum of root and heterotroph respiration) in changing environments. We investigated the effects of exotic earthworm invasion on TSR, fine-root distributions, and aboveground litterfall flux in two sugar maple-dominated forests in two locations in New York State, USA. The Arnot Forest in central New York was harvested in the late 19th century and has no history of cultivation. Tompkins Farm in eastern New York regenerated following abandonment from cultivation approximately 75 years ago. Arnot had 20% higher total soil CO2 efflux (880 g C m−2year−1) than Tompkins (715 g C m−2year−1). The presence of earthworms had no influence on TSR at either location. However, fine-root (< 1 mm diameter) biomass in earthworm plots (350 g/m2) was significantly lower than in worm-free reference plots (440 g/m2) at Arnot. Fine-root nitrogen (N) concentrations were not influenced by earthworms, and total fine-root N content was significantly reduced in the presence of earthworms at Arnot. Our results indicate that the presence of exotic earthworms is not presently affecting net C emission from soil in these forests. They also suggest a change in root function in earthworm plots that is not associated with higher fine-root N concentration, but that increases efficiency of nutrient uptake and also may enhance the belowground supply of C for heterotroph metabolism.