Earthworm Species Composition Research Articles

Land use change affects earthworm communities in Eastern Maryland, USA

We assessed earthworm species composition and abundance during secondary succession at the Smithsonian Environmental Research Center, Maryland, USA. Land use history is well known at this site. Adjacent forest stands of different ages and other vegetation patches were sampled in 1998–99 and in 2006. Out of the 12 species three ( Eisenoides loennbergi, Bimastos palustris, Diplocardia caroliniana) were native, the rest were European peregrine earthworms. Native species were mainly found in mature forests, otherwise the species composition of mature and successional stands was similar. Earthworm density and biomass was significantly higher in the successional forests, than in the mature forests, and this pattern appears to be stable at a decadal time scale. Two smaller stands that have never been clearcut did not harbor any earthworms. The forest floor on these sites had a thick organic layer, moder humus and low pH. Bulk density was highest in the youngest (50–70 years) forests. It appears that as the forests enter to a late successional stage (150 years), they no longer can support high abundance of litter feeding earthworms.

Effects of Earthworm Invasion on Plant Species Richness in Northern Hardwood Forests

The invasion of non-native earthworms (Lumbricus spp.) into a small number of intensively studied stands of northern hardwood forest has been linked to declines in plant diversity and the local extirpation of one threatened species. It is unknown, however, whether these changes have occurred across larger regions of hardwood forests, which plant species are most vulnerable, or with which earthworm species such changes are associated most closely. To address these issues we conducted a regional survey in the Chippewa and Chequamegon national forests in Minnesota and Wisconsin (U.S.A.), respectively. We sampled earthworms, soils, and vegetation, examined deer browse in 20 mature, sugar-maple-dominated forest stands in each national forest, and analyzed the relationship between invasive earthworms and vascular plant species richness and composition. Invasion by Lumbricus was a strong indicator of reduced plant richness in both national forests. The mass of Lumbricus juveniles was significantly and negatively related to plant-species richness in both forests. In addition, Lumbricus was a significant factor affecting plant richness in a full model that included multiple variables. In the Chequamegon National Forest earthworm mass was associated with higher sedge cover and lower cover of sugar maple seedlings and several forb species. The trends were similar but not as pronounced in Chippewa, perhaps due to lower deer densities and different earthworm species composition. Our results provide regional evidence that invasion by Lumbricus species may be an important mechanism in reduced plant-species richness and changes in plant communities in mature forests dominated by sugar maples.

Field effects of pollutants in dynamic environments. A case study on earthworm populations in river floodplains contaminated with heavy metals

In industrialized countries river floodplains can be strongly polluted with heavy metals. Published studies on effects of heavy metal pollution on soil invertebrates in floodplains, however, are inconclusive. This is unexpected since studies in other less dynamic environments reported clear effects at even lower levels of pollution. Flooding induces extra variation in invertebrate biomass and abundance which may reduce the probability to detect heavy metal effects. In this paper we combine reported data from studies on river floodplains in The Netherlands and Belgium and statistically analyze the effect of heavy metals on species composition, biomass, density and individual weight of earthworms. Interaction effects of heavy metal stress and flooding are also considered. The results suggest clear effects of zinc and copper on all variables and interaction of heavy metals and flooding for individual weight.

A classification scheme for earthworm populations (Lumbricidae) in cultivated agricultural soils in Brandenburg, Germany

AbstractEarthworm activity is observed at long‐term monitoring sites as an indicator of soil function to assess changes resulting from environmental and management conditions. In order to assess changes, characteristic values of earthworm populations under different site conditions have to be known. Therefore, a classification scheme for site‐specific earthworm populations was developed for soil in agricultural use, taking interactions between earthworm populations and soil properties into account. Characteristics of sites grouped by means of a cluster analysis after principal‐component analysis served as a basis for the derivation of the classification scheme. Soil variables found to characterize site differences with respect to earthworm populations were the texture of the topsoil, the texture of the subsoil, and the soil organic‐matter (SOM) content. The textural classes of the topsoil were divided into five groups comprising sandy soils (Ss), silty sand soils (Su), slightly loamy sand soils (Sl2), medium to strongly loamy sand soils (Sl3/Sl4), and loam and clay soils. Soil organic matter was divided into grades of equal size in a range from <1%, 1%–2% up to >6%. The variables “earthworm abundance” and “earthworm species” were selected to represent earthworm populations and were divided into six groups ranging from very low to extremely high. Defined groups of earthworm populations showed a clear structure in relation to soil textural groups and the content of SOM. From this distribution, a classification scheme was derived as basis for prognostic values of site‐specific earthworm populations, thus enabling the interpretation of changes over time. For some soil textural groups, selected variables appeared to enable the derivations of expected earthworm densities and species composition outside the range of the given database, but for some soil textural groups, broader databases will be needed to specify these derivations.

Earthworm species composition affects the soil bacterial community and net nitrogen mineralization

Knowledge of the effects of species diversity within taxonomic groups on nutrient cycling is important for understanding the role of soil biota in sustainable agriculture. We hypothesized that earthworm species specifically affect nitrogen mineralization, characteristically for their ecological group classifications, and that earthworm species interactions would affect mineralization through competition and facilitation effects. A mesocosm experiment was conducted to investigate the effect of three earthworm species, representative of different ecological groups (epigeic: Lumbricus rubellus; endogeic: Aporrectodea caliginosa tuberculata; and anecic: Lumbricus terrestris), and their interactions on the bacterial community, and on nitrogen mineralization from 15N-labelled crop residue and from soil organic matter.

Earthworm population dynamics under conservation tillage systems in south-eastern Australia

Changes in earthworm abundance, biomass, and diversity were monitored under a range of tillage and stubble management practices in a wheat/alternative crop rotation over 5 years on a Sodosol (Alfisol) in southern New South Wales, Australia. There were 3 tillage and 2 stubble management practices in a completely randomised block design with 3 replications. The 3 tillage treatments were no-tillage (NT), 1 tillage pass (1T), and 3 tillage passes (3T). Stubble management practices were stubble retained (sr) and stubble burnt (sb). Positive responses of earthworm abundance and biomass to stubble retention (>2-fold increase) were evident in the second year and to both stubble and tillage in the third year. In the latter, abundance in NT/sr was 6.6 times that found under 3T/sb (239 v. 36/m2). Higher earthworm abundance in NT/sr compared with 3T/sb prevailed for the remaining duration of the experiment. However, a drastic decline in total population (to a mean of 31/m2) was observed in the fourth year in all the treatments and this was followed by further decline to a mean abundance of 4/m2 in the fifth year. The drastic decline in abundance was also accompanied by a shift in earthworm species composition. The earthworm population was originally dominated by the exotic Lumbrid, Aporrectodea trapezoides (Lumbricidae) (~100% in composition), but by the fifth year, Microscolex dubious (Acanthodrilidae) was the dominant species, making up 75% of the earthworm population in NT/sr. Improvement in soil quality as detected in the fifth year under a conservation tillage system compared with a conventional system included higher transmitting macropores, higher labile carbon, and water-stable aggregation. The reason for the decline in earthworm abundance was not clear but was unlikely related to changes in soil quality, wheat yield, and rainfall. Instead, we suggest that it was related to the changes in insecticide applications during the course of the experiment. The study highlights the importance of judicious use of chemicals in farming systems if earthworm presence is to be encouraged.

Spatial analysis of earthworm biodiversity at the regional scale

Abstract Information about the spatial patterns of soil biodiversity at the regional scale is limited though required, e.g. for understanding regional scale effects of biodiversity on ecosystem processes. This study was conducted to determine whether earthworm biodiversity parameters display spatial patterns at the regional scale and whether they are related to soil properties. A transect study was carried out in 1996, 1997 and 1998 on tilled, sandy to loamy soils in the morainal landscape of northeast Germany. One soil block (50 cm × 50 cm × 20 cm) was sampled at 50 sites for earthworm number, biomass and species composition. Total earthworm abundance and biomass fluctuated randomly along the transect in all years studied. However, reproducible spatial patterns of earthworm biodiversity were found along the transect, in respect to the occurrence of single earthworm species and in respect to species number and species composition. Species number and species composition displayed increasing trends from south to north, in all years studied. This pattern corresponded to increasing trends of soil properties like pH, soil organic carbon, total nitrogen (Nt) and clay content. By applying state-space analysis to the 1996 data set, the number of earthworm species along the transect could be estimated with parameters pH and total nitrogen. The results show that spatial patterns of earthworm biodiversity may be identified at the regional scale. State-space analysis was shown to be an effective tool for detecting relationships between earthworm biodiversity and soil properties at this scale.

Earthworm Populations and Association with Soil Parameters in Organic and Conventional Ley Pastures

ABSTRACT Earthworm density, age structure, species composition and biomass, and soil biological, chemical and physical parameters (n = 24) were compared between pairs (n = 6) of organic and conventional ley pastures on mixed cropping farms in New Zealand. No differences in population density or biomass of the earthworm species Aporrectodea caliginosa, A. trapezoides, Lumbricus rubellus or Octolasion cyaneum were detected between the paired fields. Of the soil parameters, only nitrate- N levels differed significantly between the pairs; this was higher in conventional fields. Nitrate—N levels in organic fields were significantly and negatively correlated with previous cropping intensity. The biomass of juvenile A. caliginosa was significantly correlated with some of the same parameters in the conventional and organic fields, these were anaerobically mineralizable N (AMN), bulk density and total porosity levels. The total earthworm biomass on conventional fields was positively and significantly correlated with C, N, cation exchange capacity (CEC) and macroporosity levels, and with hot water extractable C, Mg, AMN, CEC, and microbial biomass C levels, and pasture age for total earthworm density. For the organic fields, no significant association for total earthworm biomass or density with the above parameters were observed. Only the r values for bulk density and total earthworm density on conventional (-0.82) and organic (-0.80) fields were comparable. The different associations between earthworm population attributes and a number of soil biological and chemical parameters on conventional and organic fields, provides evidence for differing relationships between these soil parameters and earthworm populations in organic and conventional ley pastures.

Field effects of ivermectin and fenbendazole on earthworm populations and the disappearance of dung pats from bolus-treated cattle

The disappearance of artificially formed dung pats of grazing cattle treated with sustained-release boluses containing either ivermectin or fenbendazole was compared with the disappearance of pats of untreated cattle through two successive grazing seasons. As earthworms play a major part in dung pat disappearance in northern temperate pastures, possible long-term effects of dung from treated cattle on worm populations were also examined. In a permanent pasture, the earthworm fauna was quantified within 50 m 2 enclosed plots provided with control dung pats, ivermectin pats or fenbendazole pats. In each plot, worms were extracted from the soil at dung-free locations and from 6-week-old dung pats+the underlying soil. Numbers, biomass and species composition of earthworms thus obtained were unaffected by the drug treatments. Whether the treatments had an impact on dung pat disappearance depended on season, weather and local differences between plots at the experimental site. The disappearance of dung with ivermectin was significantly delayed throughout the first grazing season, but this effect was only seen in spring of the following year. The disappearance of dung with fenbendazole did not show any consistent pattern compared to control pats.

Earthworm biomass and species diversity in windthrow sites of a temperate lowland forest

In temperate forests, disturbance by uprooting of trees is a key process in maintaining species richness and diversity. Nevertheless, very little is known about the changing role and abundance of earthworm communities in different windthrow microsites. The aim of the study was to find (i) whether single-tree uprooting causes significant changes in earthworm biomass and species composition and (ii) which changes in chemical and physical soil properties can be related to it. Twelve single-tree gap sites in an Ulmo-Fraxinetum aceretosum were selected, six sites locally dominated by beech (Fagus sylvatica) and six by poplar (Populus x euramericana). At each site, 7 microsites were sampled for soil chemical and soil physical properties: undisturbed soil beneath closed canopy, undisturbed soil beneath the gap in the canopy layer, the outer side of the pit, the inner side of the pit, the soil fallen from the mound into and away from the pit, and the mound itself. At six sites also earthworms were collected. The impact of microsite on earthworm biomass and species diversity was more important than the impact of tree species. A significant decrease in earthworm biomass and species number was detected from closed canopy over gaps to zones with soil disturbance and finally to windthrow mounds which never contained earthworms. The most pronounced reaction was the negative impact of uprooting on endogeic biomass. Epigeic biomass or species number of any ecological group changed more moderately. Six years after uprooting, microsites with direct soil disturbance were still adverse for earthworms though some – epigeic – species could profit from the absence of competitive species. It is concluded that in temperate lowland forests, the early uprooting phases induce changes in earthworm communities due to changes in soil physical parameters and food availability.

Earthworm communities in conventional wheat monocropping and low‐input wheat‐clover intercropping systems

SummaryA comparative study was conducted on earthworm communities in a conventional winter wheat monocropping system and a low‐input intercropping system in which successive crops of winter wheat were direct‐drilled into a permanent white clover sward. Earthworm abundance, biomass and species composition under the two cropping systems in the second and third years of successive cropping were assessed each spring and autumn in farm‐scale field plots at four sites using formalin and electrical extraction methods. The wheat‐clover cropping system supported larger earthworm communities (overall mean abundance 548 individuals m−2, 137 g biomass m−2) than conventional wheat monocropping (194 individuals m‐2, 36 g biomass m‐2). Between one and five more earthworm species were recorded in the wheat‐clover system than in the wheat system at three out of the four study sites. Wheat–clover cropping especially favoured species belonging to the epigeic and epigeic/anecic ecological groups such as Lumbricus castaneus, L. festivus, L. rubellus, juvenile Lumbricus and Satchellius mammalis. Earthworm communities in the wheat‐clover cropping system were comparable in size and species composition to communities normally found in perennial grassland‐type habitats such as pastures and grass‐legume leys.

Tillage practices, and temperature and moisture interactions affect earthworm populations and species composition

Tillage practices, and temperature and moisture interactions affect earthworm populations and species composition

Short-term effects of wood ants ( Formica aquilonia Yarr.) on soil animal community structure

Wood ants are known to affect the structure of invertebrate communities in tree canopies but it is not clear whether wood ants can affect the structure of soil animal communities. To isolate the direct effects of wood ants on soil fauna, I manipulated wood ant density in a 6-week mesocosm experiment by placing mesocosms (Ø 14 cm, height 13.5 cm) permeable to ants either on or outside the ant trails. The mesocosms contained litter, humus, and a diverse soil fauna typical of coniferous forest. I controlled earthworm species composition and biomasses by inoculating 470 mg fresh mass of epigeic lumbricid Dendrobaena octaedra (Sav.) or Dendrodrilus rubidus (Sav.) to the mesocosms. A 15-fold difference in ant density affected both the taxonomic and the trophic structure of the soil invertebrate community. The biomass of epigeic arthropod predators, Arachnida and Opiliones, was reduced in the high ant density treatment, whereas the biomass of mesofauna predators increased in the presence of ants. Although total earthworm biomass was not affected by the wood ants, reproductive output of epigeic earthworm Dendrodrilus rubidus increased under high wood ant density, while reproduction of Dendrobaena octaedra was unchanged. The overall effects of wood ants on soil fauna was relatively weak: despite the 15-fold difference in wood ant density between the low and the high density treatments, only some taxa were affected. Consequently, the direct effects of wood ants on soil animal food web is likely to be small.

Population dynamics of earthworms during crop rotation under rainfed agriculture in central Himalayas, India

The density and seasonal distribution patterns of three to five earthworm species was observed in relation to crop rotations during a period of 2 years in three localities ((a) Sun temple site; (b) Small tank site; (c) Dak bunglow site) in the rainfed upland agricultural system of central Himalayan district of Almora, India. Earthworm species composition at Sun tample site was different from the Dak bunglow site and Small tank site. Drawida sp. was dominant at the Sun tample site whereas Lennogaster yeicus and Metaphire holutii were dominant at the Dak bunglow site. Maximum earthworm density (200 m −2) was recorded in the late rainy season in the rice crop mixture and in Eleusine coracana (finger millet) crop mixture (335 m −2) sown following the harvest of winter wheat crop. Ploughing, harrowing and cult-discing the soils in the agricultural system during crop rotation had adverse effect on earthworm species populations. Ploughing back of crop byproducts along with the addition of the organic manure encouraged increase in the earthworm populations.

Effects of summer irrigation and trampling in dairy pastures on soil physical properties and earthworm number and species composition

In 1989 a replicated split-plot trial on a Krasnozem soil was established at Elliott Research Station (ERS) in the north-west of Tasmania, as well as 14 on-farm trials in newly irrigated pastures on 3 different soil types (Alluvial, Podzolic, Krasnozem) in the dairy districts of Scottsdale, Smithton, and Deloraine. There were 3 main treatments at ERS: irrigated before grazing, irrigated after grazing, and grazed and not irrigated. Part of each main plot was fenced to prevent trampling but still allowed grazing. Effects of summer irrigation and trampling by dairy cows were examined for pasture production, and soil chemical and structural properties. Summer irrigation at ERS and on-farm trials has led to a decline in soil structure indicated by slower ponded water in filtration rates on irrigated plots compared with the dryland plots. The decline in ponded water in filtration rates suggests a reduction in macroporosity, especially in the soil surface. However, other indicators for soil structural change in the top 100 mm|percentage water-stable aggregates (>2·5 mm) and bulk density|revealed no significant variation between the irrigated and dryland paddocks. There were, however, higher water in filtration rates and lower bulk densities in the untrampled areas than the trampled areas at ERS. Pasture production at ERS was about 50% more with irrigation in each of the 2 years of the study. Data collected at ERS in autumn and spring on the numbers of Aporrectodea caliginosa(Savigny) and Lumbricus rubellus (Hoffmeister) earthworms showed that they respond quite differently to irrigation. After 2 irrigation seasons, A. caliginosa numbers in irrigated plots dropped by over 50%, whereas in the dryland plots densities of this earthworm have remained around 390 earthworms/m 2. In contrast the densities of L. rubellus at ERS rose under irrigation practices, especially in the autumn{winter sampling period. Therefore, with the advent of summer irrigation at ERS, there was a shift in earthworm composition from a fauna dominated by A. caliginosa to a fauna with an increasing proportion of L. rubellus and a decreasing number of A. caliginosa. The typical dairy pasture in the on-farm trials recorded 2 main species, A. caliginosa (70%) and L. rubellus (30%). Total earthworm densities were highest in the north-west (Smithton) region of the State (293{351 earthworms/m 2) regardless of soil type, and the lowest densities were recorded in the Alluvial soils of Deloraine (96 earthworms/m 2). The north-west area also had the most diverse earthworm fauna, with 5 species recorded in one site: A. caliginosa, A. longa, Allolobophora chlorotica, L. rubellus, and O. cyaneum. Summer irrigation effects after 2 seasons on earthworm composition and abundance on dairy farms caused no significant change in A. caliginosa numbers, but there was a 45% increase in the numbers of L. rubellus in irrigated treatments. L. rubellus was considerably more active over summer in irrigated paddocks (25 earthworms/m 2) than in non-irrigated paddocks (7 earthworms/m 2). In contrast the number of A. caliginosa recorded in dryland paddocks was not statistically different to the irrigated paddocks, but the A. caliginosa in dryland paddocks were mostly inactive 8-20 mm from the soil surface.

The effects of metal contamination on earthworm populations around a smelting works: quantifying species effects

The effects of metal contaminants on the population density and species composition of earthworms were studied at 22 sites around a primary smelting works situated at Avonmouth, southwest England. All worms were absent from six sites within 1 km of the factory and numbers were also reduced significantly at an additional four sites 2 km from the plant. Total earthworm density was found to be inversely related to concentrations of metals in soils. some species of earthworms were found to be more ‘sensitive’ to metals and were absent from sites where more ‘tolerant’ species persisted. For example, Lumbricus rubellus, Lumbricus castaneus and Lumbricus terrestris were all present at sites close to the smelter where Aporrectodea rosea, Aporrectodea caliginosa and Allolobophora chlorotica were absent. The first three ‘tolerant’ species have more active calcium secretion glands in their gut than the three ‘sensitive’ worms. Calcium is known to be involved in the sequestration and elimination of various metals through the chlorogogenous tissue. Toxicity tests were carried out with a ‘sensitive’ and a ‘non-sensitive’ earthworm species ( Lumbricus rubellus and Aporrectodea rosea respectively) to examine if earthworm species distribution could be related to their susceptibility to the metals. In addition, the OECD recommended species Eisenia fetida was tested, to compare the sensitivity of this worm to the two commonly occurring soil species. Zinc was the metal used for these tests, since earlier comparisons of metal toxicity between the laboratory and field have shown that of the four metals released from the smelting works (Cd, Cu, Pb, Zn), it is zinc that is limiting the distribution of worms. Test results indicated that Aporrectodea rosea is more sensitive to the effects of zinc than Lumbricus rubellus. Thus, it appears that differences in the distribution of species of earthworms around the smelter can be related to variation in sensitivity to zinc. Both species were affected by zinc at lower concentrations than Eisenia fetida.

Earthworm abundance and species composition in abandoned tropical croplands: comparisons of tree plantations and secondary forests

Earthworm abundance and species composition in abandoned tropical croplands: comparisons of tree plantations and secondary forests

Secondary succession, soil formation and development of a diverse community of oribatids and saprophagous soil macro-invertebrates

Investigations into different stages of secondary succession (from a wheat field to a beechwood on Threstone; Northern Germany) demonstrated the formation of a carbon rich top soil in later successional stages. Parallel to changes in plant species and soil formation, there were also changes in species composition and diversity of saprophagous macro-invertebrates (Lumbricidae, Diplopoda, Isopoda) and oribatid mites (Acari: Oribatida). Diversity of diplopod and isopod species increased after cessation of cultivation, but in a late successional stage (ca 50 y-old fallow, ash-dominated wood) species number of diplopods and isopods declined strongly. In comparison with the other soil invertebrate groups, species composition of earthworms among the sites was more similar. Accumulation of soil C was assumed to be related to wood formation and occurrence of woody debris and recalcitrant leaf litter of beech trees. Incorporation of recalcitrant litter materials by earthworm species living in the upper mineral soil presumably contributed significantly to accumulation of soil C. Accumulation of soil C was accompanied by the development of an oribatid mite community rich in species. In early successional stages oribatids predominantly colonized the litter layer, while most oribatid mites of the beechwood inhabited the upper mineral soil. Maximum diversity of oribatid mites in the beechwood is assumed to be related to instability of the mineral soil caused by earthworm activity. Changes in species composition and diversity are discussed considering succession theory. Even soil invertebrates of similar trophic groups appear to respond very differently to successional changes. It is concluded that conservation strategies to maintain high diversity of soil invertebrates are most likely to be successful if a wide range of habitats of different successional stages is included.

Tillage treatments and earthworm distribution in a swiss experimental corn field

A pedobiological investigation was performed on a soil erosion research area of the Geographical Institute of the University of Basel from April to November 1989. The site belongs to, and is cultivated by, the Agricultural School Centre “Kantonale Landwirtschaftliche Schule” Stickhof/Eschikon in Lindau near Zürich (northern Switzerland). In a cropping experiment with corn ( Zea mays), the influence of four tillage systems on earthworm populations was studied. Aspects of soil physics and soil chemistry were integrated into the investigation. The earthworms were extracted by means of the hand sorting method at six dates in 1989 chosen to represent typical periods of corn cropping. The four tested tillage systems had an influence on earthworm species composition, abundance, and biomass: (1) The consequence of tillage itself was a reduction of about 50% of abundance (number of individuals m −2) and 30% of biomass (g live w m −2), but these losses were equalized during the following months. (2) The minimum tillage (strip zone tillage with a rotary cultivator and simultaneous seeding) caused a higher soil compaction, a negative selection of horizontally burrowing (“endogé”) species, and a subsequent diminution of their abundance. (3) Ploughing had a disadvantageous effect on vertically burrowing (“anecique”) species. (4) The winter catch crop cover enhanced the food supply and hence the earthworm biomass. An undersown cover had an additional favourable effect on earthworms.

Species composition and seasonal variation of earthworms and their effect on soil properties in southern Ontario, Canada

Four never cultivated, well drained forest sites representing the range of soil textures in the Lake Erie Basin of southern Ontario (approx. 42°N, 80°W) were characterized monthly during the growing season as to their earthworm populations, using the formalin extraction technique. Earthworm densities and biomasses ranged from 5 to 114 m −2, and 0.5 to 11 g dry wt m −2 respectively. There were a total of six lumbricid species at the four sites, but the anecic species, Lumbricus terrestris (L.) was the dominant contributor to faunal biomass at three of the sites. Earthworms at the fourth site were two small endogeous species that had the lowest abundance and biomass for earthworms of the four sites. The influence of faunal activity on soil properties was measured by the 137Cs (originating as fallout from atmospheric nuclear tests) distribution within 1 cm thick layers to a depth of 15 cm. Organic matter and 137Cs distribution with depth were functions of both earthworm abundance and species. Sites with anecic earthworms had 137Cs distributed throughout the surface horizons, whereas the site with endogeous species had higher 137Cs levels remaining at or near the soil surface.