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Written by Administrator   
Sunday, 06 November 2005
Earthworm (Lumbricidae) survey of North Dakota fields placed in the U.S. Conservation Reserve Program

1/1/2003

Journal of Soil and Water Conservation

By R. A. Utter

Twenty-three field sites in North Dakota, where highly erodible soil is placed under permanent vegetation in the U.S. Conservation Reserve Program (CRP) from five to eight years, were surveyed for the presence or absence of earthworms. Soils were sampled to determine chemical and physical properties, and soil cores were collected to estimate earthworm populations. Earthworm species identified at 12 CRP sites were Aporrectodea tuberculata (Eisen), Aporrectodec trapezoids (Duges), Aporrectodea caliginosa (Savigny), Dendrobaena octaedra (Savigny), and Lumbricus rubellus (Hoffmeister). Sites with earthworms were associated with organic matter levels of greater than 2.5%. Sand content of the 11 sites without earthworms averaged 67% ([+ or -]13), and the soil usually contained what appeared to be sharp shiny crystals or grains that might not be ideal for earthworm survival. Dendrobaena octaedra and Lumbricus rubellus were found at sites with the highest soil organic matter and nitrate-N levels plus low sa nd percent. Soil P, K, pH and EC levels were not related to the presence or absence of earthworms in these CRP sites. Total earthworm population estimates from five CRP sites averaged 6.3 million [ha.sup.*1] ([+ or -]4.7), with adults, juveniles, and cocoons at 0.6 ([+ or -]0.4), 4.5 ([+ or -]3.1), and 1.2 ([+ or -]2.0) million [ha.sup.*1], respectively. Earthworm populations along a 90-meter transect from the edge of the CRP field were similar when averaged over the five sites. An estimate of population at the other seven earthworm sites was not possible because environmental stress as earthworms tended to migrate only to areas in the field where taproot plant species were located. The presence of wetlands or tree habitat in these CRP fields could not be used as criteria for determining the presence of earthworms.

Keywords: Aporrectodea, Conservation Reserve Program, CRP, earthworms, Lumbricidae, soil properties

Some soils in the United States have been degrading at a rapid rate because of serious soil erosion problems and loss of organic matter, usually through extensive tillage practices. These poor soil-conservation practices have reduced the quality of the soil by changing the chemical, physical, or biological conditions in the soil and have raised water-quality concerns. The Conservation Reserve Program (CRP) in the United States, one provision of the Food Security Act of 1985, was established to address such concerns. This program emphasizes removing from production cultivated fields with soils that were highly erodible and placing them under permanent vegetation (mainly grass) for at least 10 years. CRP, with slight modifications, has continued as part of the 1990 Food, Agriculture, Conservation, and Trade Act (FACTA) and the 1996 Federal Agriculture Improvement and Reform Act (FAIR). States in the Northern Great Plains have placed a large number of acres under CRP. North Dakota has enrolled more than 3.1 million acres (CAST 1990). It is estimated that the CRP initiative has decreased net erosion in the United States by 650 million tons per year (CAST 1990), thus improving water quality. The long-term impact that CRP can have on improving the quality of the soil for placement back into production is being evaluated. Baer et al. (2000) reported that long-term enrollment had a greater impact on some soil chemical and physical properties than did short-term enrollment. Karlen et al. (1999) reported that several soil-quality indicators were improved by placing cropland into perennial grass in Iowa, Minnesota, North Dakota, and Washington. They indicated that soil biological indicators were changed more than soil chemical or physical properties. Deibert (1997) proposed that the health of the soil could be determined by measuring earthworm populations because their presence or absence is influenced by agricultural management practices.

Although earthworm research is limited in the Northern Great Plains, some believe that most species found in this area were introduced from other countries because the native species did not survive the last glaciation. This hypothesis was first proposed by Heimburger (1914) and expanded by Gates (1970). Lee (1985) later summarized research to support this belief in a chapter on earthworm dispersal. Barley (1961) was one of the first researchers to describe the importance of earthworms to agricultural land. Buckerfield (1992), Deibert and Utter (1994), and Kladivko and Timmenga (1990) related the different soil-management practices (residue management or tillage system, crop rotation, and the presence or absence of legumes) to earthworm populations.

Earthworm distributions and species in the Northern Great Plains may also be controlled by annual precipitation that ranges from 25 cm (10 in) in the west to 50 cm (20 in) in the east. A recent survey in this precipitation range, with pictures of earthworm species and their distribution was presented by Utter et al. (1995a). Although Edwards (1998) edited a book on current earthworm research, little information has been reported on earthworm populations in fields that have been converted to CRP. The survey reported in this paper was designed to determine: (1) which earthworm species are present in CRP fields and (2) what impact placing fields in CRP has on earthworm populations, particularly in relation to differences in chemical and physical properties of the soil, position in the landscape, and presence or absence of other habitat such as wetlands or trees.

Methods and Materials

Twenty--three field sites were selected in six counties of North Dakota with the largest acreages of CRP land. Site information variables--including mapped soil series from the most recent soil survey, site habitats (presence or absence of wetlands or trees), landscape positions and aspect, year placed in CRP, previous crops grown on the field, and current plant species in the CRP field--were recorded. Soil samples from the 0 to 15 cm (0 to 6 in) depth were collected with a shovel from 15 to 20 areas, selected at random, to represent the field at each site (Franzen and Cihacek 1998). Samples at each site were combined in a bucket and mixed, and sub-samples were placed in separate, marked, 2 qt plastic containers, one container for each site. Sample containers with soil were placed in a cooler, returned to the lab, air-dried, and ground to pass a 2 mm sieve. Soil analyses, including organic matter (OM), nitrate N, phosphorus (P), potassium (K), 1:1 pH, and electrical conductivity (EC) were determined at the ND SU Soil Testing Lab using standard analytical procedures Brown 1998). Part-icle-size analysis (sand, silt, clay) was determined by the hydrometer method (Gee and Bauder 1986). Each field site was initially surveyed once in 1994 for the presence or absence of earthworms during April, May or June. Soil was extracted to 30 cm (12 in) depth with a spade (shovel) at random locations across the field and examined. A positive site indicated that earthworms were found somewhere in the CRP field, while a negative site indicated no earthworms were found anywhere in the CRP field or in adjacent trees, wetlands, or grass areas along the road ditches. When earthworms were present, the adults were placed in labeled plastic bags containing soil, which were placed in a cooler with ice for storage. Earthworm samples were returned to the laboratory fixed with 10% formalin, and later identified using a guide by Schwert (1990).

Based on previous research, the authors found that a core method provided better estimates of earthworm populations because both small juveniles and cocoons could be included in the total population. Also the standard formalin method (Raw 1960) did not work well in some soils, especially clays, and the researchers wanted to limit exposure to the hazardous chemical formalin. Thus, intact soil cores were taken to make earthworm population estimates. Based on the initial sampling described previously, cores were only taken at those sites where the authors, based on experience, felt adequate earthworms were present throughout the field. Single cores were taken along a transect at 30 m intervals from the edge of the field. The transect was located at least 50 m from any trees or wetland areas in the field, A 20 cm (8 in) diameter by 15 cm (6 in) deep metal irrigation pipe was pounded into the soil. The core, with soil and earthworms, was extracted and placed in a plastic mesh bag inside a plastic bucket for storag e and transport. Landscape position and aspect of each core was recorded because previous research suggested that location on the landscape might influence populations. Soil from each core was hand-sorted to collect earthworms. Subsequently, the sorted soil was washed over a 1 mm sieve to recover cocoons and juvenile earthworms missed during hand-sorting. Number of adults (recognizable from genital markings such as clitellum), juveniles (no genital markings), and cocoons were counted and recorded. The estimated reproductive capacity of the adult earthworms, found at sampling time, was determined by summing the number of cocoons plus juveniles.

Averages, analysis of variance (ANOVA), and simple or multiple correlations were performed on the data using SAS procedures (Freund and Littell 1981). Standard deviations (SD) are provided for variable averages. Significant differences among means or significant correlations were determined at the 0.05 level of probability Analyses were performed on either all 23 sites combined or by a grouping of CRP sites (12 positive and 11 negative earthworm sites). The three core distances along the transect were considered replications in the five sites for populations estimates. Multiple correlations analysis included soil properties (sand, silt, clay, OM, P, K, pH, EC), habitat (trees, wetlands), and earthworm species (number of species, A. tuberculata, A. trapezoides, A, caliginosa, Dendrobaena octaedra, L. rubellus).

Results and Discussion

The location of each CRP earthworm sampling site within the state of North Dakota and mean annual precipitation (Ramirez 1973) is depicted in Figure 1. Specific site number, year established, plant species present, and previous crops for each CRP site are summarized in Table 1. Previous crops rotated on the sites before placing the land into CRP included wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), durum (Triticum turgidum L.), oats (Avena sativa L.), rye (Secale cereale L.), sunflower (Helianthus annuus L.), corn (Zea mays L.), soybean (Glycine max Merrill), dry edible bean (Phaseolus vulgaris L.), potato (Solanum tuberosum L.), and summer fallow. The three predominant crops previously found at nearly all CRP sites, in considering a four-year rotation, included small grain at 50% of the time followed by fallow (20%) and sunflower (16%). Plant species seeded on the CRP fields included tall wheat grass (Agropyron elongatum), smooth brome grass (Bromus inermis, crested wheat grass (Agropyron deser torum), alfalfa (Medicago sativa L.), slender wheat grass (Agropyron trachycaulum), sweet clover (Melilotus officianalis Lam.), western wheat grass (Agropyron smithu), intermediate wheat grass (Agropyron intermedium), pubescent wheat grass (Agropyron trichophorum), and switch grass (Panicum virgatum). Plant cover on the CRP sites was mainly grass with smooth brome (17 sites) and slender wheat (9 sites) the main grasses that sometimes included intermittent legumes plants such as alfalfa (19 sites) and sweet clover (13 sites). Weeds species identified on some, but not all, of the sites included dandelion (Taraxacum officinale), quackgrass (Agropyron repens), yellow goats beard (Tragopogon pratensis), purple coneflower (Brauneria angustifolia, curled dock (Rumex crispus), prickly lettuce (Lactuca virosa) and Canada thistle (Cirsium arvense).

Soil properties for each site are listed in Table 2 with both the positive and negative earthworm sites indicated. Particle-size analysis indicated that sand content ranged from 8% to 81%, silt 6% to 45%, and clay 10% to 48%. The average sand content was 60% ([+ or -]18) with both silt and clay fractions at 20% ([+ or -]10). A separation into positive (12 sites) and negative (11 sites) earthworm sites indicated that, generally, sites with less than 60% sand contained earthworms, while those exceeding 60% sand were absent of earthworms. The average sand content (53%) for soils with earthworms places the textural class in the loam or sandy clay loam category, while average sand content (67%) on the sites with no earthworms places the texture as sandy loam. Some specific site exceptions occurred, which we believe were related to soil particle shapes. When earthworms were absent in the soil, we observed a portion of the soil particles were shiny with what appeared to be distinct sharp flat edges. We speculate tha t these sharp particles would be harmful when passing through the earthworm gut or that their outer skin might be damaged as they moved through the soil. Thus earthworms would tend to avoid or not survive in these soils, and any introduced earthworms would not survive for long or reproduce under these conditions. Lee (1985) cited several instances where coarse-textured particles affected earthworm species and distribution in New Zealand and Egypt. The four positive earthworm sites (sites 3, 14, 17, 23) with sand content greater than 60% had no observable sharp soil particles. The exact nature of these sharp particles, or crystal grains, and their effect on earthworms requires further investigation.

Only a few significant correlations (n = 12 and P < 0.05) were found between earthworm numbers and soil particle size. A negative correlation (r = -0.74) occurred between increased sand content and both Dendrobaena octaedra and Lumbricus rubellus, with a subsequent positive correlation based on increased silt (r = +0.77) and clay (r = +0.56) content. This is not surprising because these two species are usually associated with high soil water content, which occurs in soils with higher silt and clay content. Nordstrom and Rundgren (1974) found a positive correlation between earthworm abundance (L. rubellus and A. caliginosa) and clay content at depths below 20 cm (8 in) but pointed out that this may be because of increased soil water at the lower depths as a result of increased clay content.

The OM content of the soils from all CRP sites ranged from 0.9% to 5.3% with an average of 3.2% ([+ or -]0.9). Although more OM tends to encourage earthworm development as a result of more available food, an OM content exceeding 2.5% was adequate to support earthworms in these CRP sites. Those sites without earthworms and with more than 2.5% OM were either associated with the presence of sharp soil particles, previously discussed, or no earthworm source was available or introduced as a result of human activity (earthworms transported with soil attached to plant roots, tillage implements, or dumping earthworms left after fishing). Nitrate-N (all sites) ranged from 1 to 14 ppm, P from 3 to 40 ppm, and K from 100 to 605 ppm. Although the average OM, nitrate N, P, and K amounts on the sites with earthworms (12 sites) were higher than those with no earthworms (11 sites), no significant correlations were obtained among earth worms and P or K. Positive correlations, higher OM (r = +0.63) and nitrate-N (r = +0.87), w ere found between these two variables and the presence of both Dendrobaena octaedra and L. rubellus. A positive correlation was found between the number of earthworm species found and higher nitrate N levels (r = +0.70). The pH levels across all sites ranged from 6.2 to 7.9 with similar values or soils with or without earthworms and little relationship between pH and the presence or absence of earthworms. EC values, except at two sites, were usually less than 1.0 mmhos/cm with a range of 0.16 to 2.30. High EC values were positively correlated with both Dendrobaena octaedra and Lumbricus rubellus species (r = +0.87). Lower average EC values in soils no earthworms would be expected because these sites also have higher sand content, which would allow easier leaching of salts to below the 15 cm sample depth.

Different species of earthworm were found at each CRP site (Table 3). Thirteen sites contained trees, but only eight of these sites contained earthworms. These earthworm species were probably introduced via the roots or soil when the trees were established in the field. The five tree sites without earthworms suggest that earthworms were never present or that any introduced earthworms did not survive either because of dry soil conditions or the sharp soil particles. The presence of earthworms on four sites without ;rees, three with wetlands, indicates that the earthworms were transported to the field from another area by tillage implements or carried by water or birds. A. caliginosa, Dendrobaena octaedra and L. rubeilus were found at only one site. This is not surprising as these three species were reported in only a few areas in a North Dakota earthworm survey by Utter et al. (1995b). The main earthworm species, A. tuberculata and A. trapezoids, occurred at 10 and 5 sites, respectively. These two species were identified by Utter et al. (1992) as the predominant earthworm species in North Dakota cultivated fields, shelterbelts and prairie grasses.

In seven of the positive earthworm sites, the presence or number of earthworms was very low across the field. This may be related to either the dry soil conditions associated with low precipitation or the depletion of soil water caused by dense vegetative growth. Earthworms, to survive at these sites, tended to migrate or concentrate only in the field near plants with tap roots, including alfalfa, sweet clover, dandelion, goats beard, and prickly lettuce. Evidently the food materials or root exudates and/or moisture or nutrients sustained the earthworms during these environmental stress periods. This may offer a clue to sampling for earthworms under stress conditions. These same low earthworm number CRP sites were checked in subsequent years, but the numbers recovered still remained low.

Estimates of populations at five positive earthworm sites are presented in Table 4. Populations of cocoons, juveniles, adults and total earthworms varied with CRP site. High or low populations did not appear to be related to landscape position or aspect, as distance that the core was taken from the edge of the field showed no significant difference among populations. The adult population (range 0 to 1.5 million) averaged about 640,000 [ha.sup.-1] ([+ or -] 39,000). These populations are smaller than adult populations reported under grass, trees, or reduced-tillage systems by other research in North Dakota and Canada (Utter and Deibert 1998, Clapperton et al. 1997). The reproductive capacity of earthworms in soils under CRP with an adequate food supply, surface cover, and soil water is well-demonstrated in these sites. The average number of cocoons approached 1.2 million [ha.sup.-1], while the number of juveniles was about 4.5 million [ha.sup.-1]. Total reproduction (sum of cocoons plus juveniles) ranged from a low of 925,000 [ha.sup.-1] (site 9 upper slope), where adult population was also low at about 300,000 [ha.sup.-1], to a high of more than 1.8 million [ha.sup.-1] (site 7 midslope) where the adult population exceeded 1.5 million [ha.sup.-1]. The average earthworm reproduction exceeded 5.7 million [ha.sup.-1], or about 9 cocoons plus juveniles for every adult earthworm. The average total earthworm population at these five sites approached 6.4 million [ha.sup.-1] with a range of 1.2 million (site 9 upper slope) to almost 20 million [ha.sup.-1] (site 7 midslope). When the population data were averaged over all five sites, no significant differences were found for cocoons, juveniles, adults, or total earthworm populations among sample core distance from the edge of the field. These results were not expected, as previous unpublished population surveys by the authors on no-till and organic fields suggested that the lower landscape positions contained higher populations than the upper slopes because of greater soil water. The effect of landscape position on earthworm populations warrants further investigation in CRP fields.

Summary and Conclusion

Our survey provides information on the distribution and population of earthworm species in CRP fields in North Dakota, a dryland area of the Northern Great Plains region. This information will provide a better understanding of the distribution and population of earthworm species in various agroecosystems such as CRP that was designated as a research imperative by Edwards et al. (1995). Our results also provide some initial information on the relationships of earthworms to texture (particle size), selected chemical soil properties, landscape position, and other habitat in CRP fields. The predominant earthworm specie found at the CRP sites was A. tuberculata, located at 10 of the 12 sites. Multiple species were found at only five sites. About 80% of the soils sampled for earthworms in this survey were sandy loam or sandy clay loam texture. Texture, or at least sand content, did not appear to be the determining factor for the presence or absence of earthworms as earthworms were found in 12 of 23 CRP sites with s and content that ranged from 8% to 74%. However, sites with no earthworms usually had sand content above 60% sand and sharp soil grains or crystals that would not be conducive for earthworm survival. An OM content of 2.6% or more was measured on all earthworm sites. Five sites without earthworms contained soil OM levels below 2.6%. Although the average [NO.sub.3]-N, P, K, pH, and EC levels in the soil were higher in positive compared with negative earthworm sites, little relationship was found among these chemical soil properties and the presence or absence of earthworms. The presence of tree or wetland habitat was not a good indicator of positive earthworms in the CRP sites in this survey. Wetland areas in the CRP field without trees but with earthworms suggest that the earthworms were previously transported to the field from another area, either by tillage implements, water erosion, or birds. Earthworm populations measured at five sites for cocoons, juveniles, and adults averaged about 1.19 million [ha.sup. -1], 4.54 million [ha.sup.-1], and 0.64 million [ha.sup.-1], respectively On seven earthworm sites, earthworms were only found where they tended to migrate because of environmental stress. Estimates of populations at these sites, based on the core method of sampling, would be unreliable. Insight is given in this survey for sampling earthworms under environmental stress conditions, often exhibited under dense vegetation in CRP fields, low precipitation, and course-textured soils that would normally have low water-holding capacity.

Table 1 
 
North Dakota CRP earthworm sampling sites, year established, plant 
species, and previous crop information. 
 
      Year CRP plant  CRP species           Previous 
Site   established    present +             crops + 
 
   1       1987       SB,IW                 sg-fa 
   2       1987       SW,SB,AL              cn-sg-db-cn 
   3       1987       SB,CW,SW,AL,SC        sg-sg-sf 
   4       1986       SB                    sf-sg-db-sg 
   5       1986       CW,SC,SB,AL           sg-sf-sg-fa 
   6       1986       TW,SB,AL              sg-db-sg-po 
   7       1989       IW,CW,AL              sg-fa 
   8       1988       WW,SB,AL              sg-fa 
   9       1986       SW,CW,SC,SB,AL        sg-sg-fa 
  10       1988       SB,AL,SC              sg-sg-sf 
  11       1987       SW,SB,AL              sf-cn 
  12       1989       SB,AL,SC              sg-cn-sf-cn 
  13       1987       SB,AL,SC              sg-cn-sf-sg 
  14       1986       IW,CW,AL,SC           sg-sg-sf 
  15       1988       SB,AL                 sg-fa 
  16       1987       PW,TW,SW,AL,SC        sg-fa-sf 
  17       1988       SW                    fa-sg-sg 
  18       1987       IW,SB                 sg-sg-sf 
  19       1987       IW,TW,AL,SC           sg-sg-sf 
  20       1987       IW,TW,SC,AL           fa-sf-sg 
  21       1987       SW,WW,IW,PW,WH,AL,SC  sg-sg-fa 
  22       1986       SW,IW,AL,SC           fa-sf-sg-cn 
  23       1986       TW,SW,SB,AL,SC        sg-sf-fa 
 
+ Plant species: TW = tall wheat grass; SB = smooth brome grass; CW = 
crested wheat grass; AL = alfalfa; SW = slender wheat grass; SC = sweet 
clover; WW = western wheat grass; IW = intermediate wheat grass; PW = 
pubescent wheat grass; WH = switch grass. 
 
++ Previous crops rotated before CRP: sg = small grain (wheat, barley, 
durum, oats, rye); sf = sunflower; fa = summer fallow; cn = corn; sb = 
soybean; db = dry edible bean; po = potato. 
 
 
Table 2 
 
North Dakota CRP earthworm sampling sites, mapped soil series, particle 
sizes, and various chemical properties in the 0-15 cm (6-inch) soil 
depth. 
 
                                                             Organic 
                    Mapped     Sand      Silt      Clay      Matter 
                    soil 
      Site +        series                        % 
 
        1           Lihen       77         6        17         1.9 
        2           Embden      81         9        11         2.3 
        3 +         Arvilla     74        16        10         3.6 
        4 +         Inkster     53        21        27         4.0 
        5 +         Inkster     31        21        48         4.5 
        6 +         Bearden      8        45        46         5.3 
        7 +         Belfleld    56        18        26         2.7 
        8           Beisigi     63        15        22         2.4 
        9 +         Shambo      51        25        24         3.3 
       10           Barnes      36        32        33         4.6 
       11           Renshaw     62        21        17         3.2 
       12           Arvilla     80         7        13         1.9 
       13           Arvilla     62        24        14         3.4 
       14 +         Calvin      73        16        11         2.6 
       15           Arvilla     62        19        19         2.6 
       16           Arvilla     63        17        20         2.7 
       17 +         Wyndmere    73        13        14         2.6 
       18           Fossum      81         9        10         1.8 
       19           Karlsruhe   72        13        15         2.8 
       20 +         Barnes      41        32        27         4.0 
       21 +         Sioux       59        27        14         4.1 
       22 +         Swenoda     53        28        19         3.7 
       23 +         Fossum      64        22        14         2.8 
                    Average     60        20        20         3.2 
                    SD ++       18        10        10         0.9 
Average (positive)              53        24        23         3.6 
      SD ++                     19         9        13         0.8 
Average (negative)              67        16        17         2.7 
      SD ++                     13         8         6         0.8 
 
 
                    NO3-N     P          K        pH           EC 
 
      Site +                     ppm                1:1  mmhos/cm 
 
        1             4        7        280       7.8         0.20 
        2             1       12        210       6.9         0.16 
        3 +           4        9        170       7.3         0.23 
        4 +           3       40        605       6.2         0.25 
        5 +           2       19        340       6.2         0.30 
        6 +          14        8        210       7.7         2.30 
        7 +           6       16        500       7.6         0.40 
        8             7        9        280       6.8         0.25 
        9 +           4        7        275       7.6         0.37 
       10             3        4        515       7.8         0.45 
       11             6        4        100       7.7         0.25 
       12             1        4        160       7.4         0.18 
       13             1        8        125       6.8         0.18 
       14 +           1        3        200       7.3         0.19 
       15             2        3        200       7.8         0.25 
       16             3        7        220       7.0         0.20 
       17 +           6        4        265       8.0         0.25 
       18             1        5        100       7.8         0.18 
       19             1        8        270       7.9         0.50 
       20 +           5        8        390       6.9         0.31 
       21 +           5       17        340       7.1         0.30 
       22 +           4        6        365       7.7         1.35 
       23 +           6       11        330       7.4         0.32 
                      4        9        280       7.3         0.41 
                      3        8        131       0.5         0.48 
Average (positive)    5       12        332       7.2         0.55 
      SD ++           3       10        126       0.6         0.63 
Average (negative)    3        6        224       7.4         0.25 
      SD ++           2        3        118       0.4         0.11 
 
+ Number followed by a + indicates a site testing positive for 
earthworms. 
 
++ Standard deviation. 
 
 
Table 3 
 
North Dakota CRP earthworm sampling sites, Lumbricidae specie, and 
habitat information. 
 
                                  Species 
 
          Aporrectodea  Aporrectodea  Aporrectodea  Dendrobaena 
Site +     callginosa   tuberculata   trapezoides    octaedra 
 
  1            0 +           0             0             0 
  2            0             0             0             0 
  3+           0             X             X             0 
  4+           0             X             0             0 
  5+           0             X             0             0 
  6+           0             X             0             X 
  7+           0             X             0             0 
  8            0             0             0             0 
  9+           0             X             X             0 
 10            0             0             0             0 
 11            0             0             0             0 
 12            0             0             0             0 
 13            0             0             0             0 
 14+           0             X             0             0 
 15            0             0             0             0 
 16            0             0             0             0 
 17+           0             X             X             0 
 18            0             0             0             0 
 19            0             0             0             0 
 20+           X             X             0             0 
 21+           0             0             X             0 
 22+           0             0             X             0 
 23+           0             X             0             0 
 
            Specie            Other habitat 
                               persent 
          Lumbricus 
Site +    rubellus     Wetlands       Trees 
 
  1           0            0            0 
  2           0            0            X 
  3+          0            0            X 
  4+          0            0            X 
  5+          0            X            X 
  6+          X            0            X 
  7+          0            X            X 
  8           0            0            0 
  9+          0            X            0 
 10           0            X            0 
 11           0            0            X 
 12           0            0            X 
 13           0            0            0 
 14+          0            0            X 
 15           0            X            0 
 16           0            0            0 
 17+          0            0            X 
 18           0            X            X 
 19           0            X            X 
 20+          0            X            0 
 21+          0            0            0 
 22+          0            X            0 
 23+          0            0            X 
 
+ Number followed by a + indicates a site testing positive for 
earthworms. 
 
+ Code: 0 = specie or habitat not present at site; X = specie or habitat 
present at site. 
 
 
Table 4 
 
North Dakota CRP earthworm sites, sampling dates, core distances, 
landscape position-aspects, and Lumbricidae populations. 
 
                       Core         Landscape         Cocoons + 
          Sample    distance +      position 
 Site      date     in meters      and aspect           million 
                                                      [ha.sup.-1) 
 
   6      5/18/94       30      Nearly flat-South       3.084 (ss) 
                        60      Nearly flat-South       0.308 
                        90      Nearly flat-South       0.617 
                     Average                            1.336 
   7      6/21/94       30      Lower slope-East        1.233 
                        60      Mid slope-East          8.017 
                        90      Upper slope-East        0.925 
                     Average                            3.392 
   9      6/22/94       30      Upper slope-South       0.308 
                        60      Mid slope-South         1.542 
                        90      Lower slope-South       0.000 
                     Average                            0.617 
  17      4/19/94       30      Upper slope-West        0.308 
                        60      Mid slope-West          0.617 
                        90      Lower slope-West        0.617 
                     Average                            0.514 
  23      5/24/94       30      Lower slope-North       0.308 
                        60      Mid slope-North         0.000 
                        90      Upper slope-North       0.000 
                     Average                            0.103 
Average                 30                              1.048 
                        60                              2.097 
                        90                              0.432 
                     Average                            1.192 
                        SD (n)                          2.048 
 
                       Core     Juveniles  Adults     Total 
          Sample    distance + 
 Site      date     in meters         million [ha.sup.-1) 
 
   6      5/18/94       30        4.009    0.308      7.401 
                        60        1.542    0.308      2.158 
                        90        4.317    0.617      5.551 
                     Average      3.289    0.411      5.036 
   7      6/21/94       30        7.092    0.617      8.942 
                        60       10.176    1.542     19.735 
                        90        4.009    0.925      5.859 
                     Average      7.092    1.028     11.512 
   9      6/22/94       30        0.617    0.308      1.233 
                        60        1.850    0.925      4.317 
                        90        2.158    0.000      2.158 
                     Average      1.542    0.411      2.570 
  17      4/19/94       30        1.850    0.617      2.775 
                        60        2.467    0.617      3.701 
                        90       10.176    0.617     11.410 
                     Average      4.831    0.617      5.961 
  23      5/24/94       30        8.017    0.308      8.633 
                        60        4.934    0.617      5.551 
                        90        4.934    123.3      6.167 
                     Average      5.961    0.719      6.784 
Average                 30        4.317    0.432      5.797 
                        60        4.194    0.802      7.092 
                        90        5.119    0.678      6.229 
                     Average      4.543    0.637      6.373 
                        SD (n)    3.069    0.394      4.664 
 
+ Distance the soil core was taken from the CRP field edge. 
 
++ Cocoons = number of cocoons with one or more viable embryos; juvenile 
= small earthworms with no recognizable genital markings; adults = 
earthworms with either genital markings or developed citellum; Total = 
sum of cocoons+ juveniles+adults. 
 
(ss) Value time 100 equals the number of earthworms found in one square 
meter. 
 
[n] Standard deviation. 
 

Acknowledgements

The authors would like to thank the North Dakota cooperators for allowing their CRP fields to be sampled and those specific personnel at the Carrington Research/Extension Center or Natural Resource Conservation Service who helped in some part to locate sites and/or provide general site information.

References Cited

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Edward J. Deibert is a professor and Rodney A. Utter is a research specialist with the Soil Science Department at North Dakota State University in Fargo, North Dakota.

 
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