David A. McCartney

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.

Movement of N from decomposing earthworm tissue to soil, microbial and plant N pools

A microcosm experiment was made to determine the fate of nitrogen released from 15 N-labelled decomposing earthworms (Lumbricus terrestris) in soil in the presence or absence of ryegrass seedlings (Lolium perenne). Earthworm tissue (2.0% 15 N atom enriched) was added to each microcosm. Nitrogen movement from earthworm tissue to soil N [mineral N (NH4-N+NO3N), dissolved organic N (DON) and organic N], microbial biomass N and plant shoot N pools was determined by destructive sampling at 1, 2, 4, 8 and 16 d. Earthworm tissues decomposed rapidly, and no tissue was visible after 4 d. Initially in pots without plants, most of the N from earthworm tissue was found in the organic N pool, however, as much as 55% of the N from decomposing earthworm tissue was incorporated into microbial biomass after 2 d. Much less of the N from earthworm tissue was transformed into DON and mineral N forms after 2 d. The DON and mineral N pools contained 13‐18% and 4‐7% of the N from earthworm tissue, respectively, from d 2 to 16. By the end of the experiment, N from earthworm tissue in the microbial biomass N pool declined to 29% while the amount of N from earthworm tissue in the organic N pool increased to 49%. The increase in the organic N may have resulted from the production of new organic compounds such as microbial by-products. In pots with plants, N from earthworm tissue was rapidly incorporated into microbial biomass, and by d 2, the microbial biomass N pool contained 40% of the N from earthworm tissue. Mineral N, DON and microbial biomass N concentrations were lower in pots with ryegrass seedlings compared to pots without plants, and after d 2 declined to almost undetectable amounts because of rapid plant uptake. Between 42‐52% of the N from earthworm tissue was found in the organic N pool from d 1 to 8, and then declined to 19% by d 16. After 16 d, over 70% of the N added as earthworm tissue was incorporated into plant shoot biomass. Our results demonstrate that the movement of N from dead earthworm tissue into microbial biomass was extremely rapid, and in pots without plants, much of this N was transformed into organic N forms, while in pots with ryegrass, most of the N from earthworm tissue accumulated in ryegrass shoots. # 1999 Elsevier Science Ltd. All rights reserved.

CHARACTERISTICS OF MACROPOROSITY IN A REDUCED TILLAGE AGROECOSYSTEM WITH MANIPULATED EARTHWORM POPULATIONS: IMPLICATIONS FOR INFILTRATION AND NUTRIENT TRANSPORT

Abstract The effects of macroporosity on the potential for nutrient transport has been extensively studied for no-tillage agroecosystems. The present study was undertaken to quantify macroporosity and to demonstrate the potential for nutrient transport in reduced-tillage systems. Soil macropore area and numbers were quantified by image analysis into three size classes (1–8, 8–16 and > 16 mm 2 ) at three depths (10, 20 and 30 cm) at two locations (between-row, within-row) in corn agroecosystem enclosures with manipulated earthworm populations (reduction, not manipulated, addition). A dilute solution of latex paint was surface-applied to determine pathways for water infiltration. All macropore sizes contributed to infiltration. Earthworm-treatments had no significant effects on infiltration rates, but rates were significantly faster within crop rows than between rows. In the earthworm-addition plots the area of macropores was significantly greater in the surface soil (10 cm depth) then in the other treatments, indicating re-formation of continuous flow pathways destroyed by tillage practices. The majority of the > 16 mm 2 size pores were recognized as Lumbricus terrestris (L.) burrows, which represented the greatest per cent area of all size classes at the 10 cm depth. The area of these large macropores was significantly greater in addition-treatments than in the other plots at all depths and locations except for 30 cm-deep between-row locations. The absence of an earthworm effect at this location is attributed to the existence of pre-existing burrows that were not disrupted by tillage or root activity and is due to earthworms concentrating their activity in the root-zone, in the within-row location. By increasing soil macroporosity and creating transport pathways of preferential flow, earthworms potentially affect the nutrient transport in leachate and nutrient loss from the agroecosystem.

EARTHWORM EFFECTS ON SOIL RESPIRATION IN CORN AGROECOSYSTEMS RECEIVING DIFFERENT NUTRIENT INPUTS

Abstract Available evidence suggests that earthworms enhance the mineralization of carbon in soil, but there are few data from field experiments that demonstrate that earthworms increase soil respiration under natural environmental conditions. We measured soil respiration (CO2 flux) during 1993–1994 in 20-m2 field enclosures in which earthworm populations had been decreased, increased, or left unmodified (the latter serving as a control). The enclosures were in corn agroecosystems receiving one of three different nutrients: legume cover crop, cow manure or inorganic fertilizer (NH4NO3). Soil respiration was measured in the enclosures by the static diffusion method. Earthworms had significant effects on soil respiration, but their effects varied seasonally and were influenced by environmental conditions. There were significant differences in respiration rates among earthworm treatments on seven of the 24 sampling dates, and where significant differences did occur, respiration rates were greates in plots with increased populations and lowest in plots with decreased populations. Most of the significant effects of earthworms on soil respiration were observed during the growing season (June–August) of 1994. A severe drought in the summer of 1993 decreased overall respiration rates relative to 1994, and also inhibited earthworm activity. Soil respiration was significantly greater, during the growing season, in the organically-amended plots than in plots treated with inorganic fertilizer; there were no differences in soil respiration among nutrient treatments in the autumn or in the spring before amendments were added. Our results show that earthworms had a significant influence on soil respiration in the field, but that their influence was seasonal, depended on environmental conditions, and was affected by temporal patterns in C supply.

Earthworm effects on crop and weed biomass, and N content in organic and inorganic fertilized agroecosystems

Abstract Results are reported from an experiment comparing the effects of earthworm manipulations and agroecosystem fertility treatments on corn (maize, Zea mays ) and weed biomass, and on nitrogen content. The experimental design consisted of inorganic (ammonium nitrate) and organic (cover crop and manure) fertility treatments. Within each fertility treatment, earthworm manipulations consisted of ambient, augmented and reduced populations. Both ambient and augmented earthworm population treatments resulted in greater weed biomass compared to earthworm reductions. Early season crop biomass was significantly influenced by earthworm reductions. Early season crop biomass was significantly influenced by earthworm × N source interactions, with greater maize biomass in the earthworm reduction treatment. In fertilizer and manure treatments, grain yields were higher in the reduced earthworm treatment compared to either augmented or ambient earthworm treatments. This effect on yields was probably related to interactions with the weeds and damage to the maize by an insect pest.

Organic matter dynamics in maize agroecosystems as affected by earthworm manipulations and fertility source

Abstract We quantified the mass of five size classes (>6, 2–6, 0.25–2, 0.053–0.25 and 6 mm SOM. The reduced EW treatment had higher 0.053–0.25 mm SOM only in May just prior to spring amendment addition. Fertility treatment effects were not significant overall except for an increased amount in the >6 mm SOM for the manure treatment. There were significant date effects for all SOM size classes with increases at the times of spring amendment and residue incorporation and fall root senescence followed by rapid declines.