The effects of forest practices on earthworm populations and soil
microbial biomass in a hardwood forest in Missouri
D. Jordan
a,*, F. Li
b, F. Ponder Jr.
c, E.C. Berry
d, V.C. Hubbard
a, K.Y. Kim
e aDepartment of Soil and Atmospheric Sciences, The School of Natural Resources, University of Missouri, Columbia, MO 65211, USAbDepartment of Agriculture, Food and Nutritional Sciences, University of Alberta, Edmonton, Alta., Canada cUSDA Forest Service at Lincoln University, Jefferson City, MO 65102, USA
dDepartment of Entomology, Iowa State University, Ames, IA 50011, USA eDepartment of Agricultural Sciences, Chonnam National University, Chonnam, South Korea
Received 24 June 1997; received in revised form 12 March 1999; accepted 17 March 1999
Abstract
Physical changes caused by forest management practices can have a dramatic effect on the soil biota in a forest ecosystem. The effects of soil compaction associated with harvesting on earthworm populations and selected soil properties were measured in a hardwood (oak-hickory) forest in Missouri. Soils in this region of Missouri are characterized by a cherty residuum that is primarily of the Clarksville series (Loamy-skeletal, mixed, mesic Typic Paledults). Earthworms were collected from a 0±15 cm depth each spring and fall for 2 years by handsorting and their populations determined on a per square meter basis. Two native earthworm species,Diplocardia omataand Diplocardia smithii, were identi®ed at this site. Regardless of species, juvenile populations accounted for a major portion of the earthworms found in spring or fall. In 1995,
Diplocardia ornatawas the dominant species present and most affected by soil compaction. In 1996, soil compaction seemed to have a less restrictive effect on earthworms. Harvesting had no effect on either earthworm populations or biomass but had a signi®cant effect on selected soil properties. Harvest levels had a signi®cant negative correlation with soil moisture, soil inorganic N, and soil microbial biomass C and N. When above-ground biomass like logs and forest ¯oor litter were removed and the soil was compacted, the standing soil microbial biomass along with soil moisture content and nutrients were reduced. Time (season of the year) had a signi®cant effect on earthworm populations and biomass and all soil properties that were measured in both 1995±1996. Future studies at this site might include a seasonal study on the ecology and reproduction of these native earthworm species.#1999 Elsevier Science B.V. All rights reserved.
Keywords:Earthworms;Diplocardiaspp.; Soil microbial biomass C and N; Soil compaction; Harvest level; Temperate hardwood forests
1. Introduction
Forest management practices that maintain sustain-able forests and productive soils are major research
initiatives and management priorities of the USDA± Forest Service. Since the 1930s, interest and concerns about how trees are harvested for commercial pur-poses have steadily increased (Woodbury, 1930; Horn-beck and Kropelin, 1982). Problems with soil quality and productivity in forests occur when management activities are improperly planned and carried out.
*Corresponding author. Tel.: 8820090; fax: +1-573-8845070; e-mail: jordandi@missouri.edu
Management practices that ensure long-term sustain-ability of forests are a national priority (Ponder and Mikkelson, 1995). To ensure that Forest Service management practices do not reduce long-term soil productivity, a network of coordinated long-term experiments have been established across the United States to monitor the effects of forest disturbance on soil productivity. This includes a study located in the hardwood region in southeastern Missouri. Along with monitoring the vegetation in the forest ecosystem, soil biological, chemical and physical properties were also measured for this particular site.
Soil productivity and quality are directly affected by management. An initial assessment of this site revealed that earthworms were a prominent part of the soil biota. Most of the soil fauna and micro¯ora that inhabit the forest ecosystem would in¯uence tree or plant productivity directly or indirectly. Earth-worms, when they are found in soils regardless of the ecosystem, may have a profound effect on soil physical (soil structure, etc.), soil chemical or nutri-ents processes (available N, etc.) and soil biological or microbial properties (enhanced or decreased standing microbial biomass populations) depending on a num-ber of interacting environmental factors (Prichett and Fisher, 1987). The `natural tilling' effects of earth-worms in soils is well known. Earthearth-worms (depending on species) may pass as much as 30 tonnes of soil per hectare through their bodies and some of this pro-cessed material is excreted in the form of casts (Pri-chett and Fisher, 1987; Edwards and Bohlen, 1996). These casts may be high in available nutrients com-pared to the bulk soil or earthworm-free soils (Edwards and Bohlen, 1996). These nutrients may be available for plant uptake. Furthermore, earth-worms eat organic material on or in the soil surface and generally promote good soil structure and aeration through their participation in soil aggregation and burrowing activities. In turn, physical manipulations (like tillage, soil compaction, harvesting practices, etc.) imposed on soils can alter the earthworm's habitat and activities (Edwards and Lofty, 1982; Pizl, 1992; Li, 1996; Jordan et al., 1997). Soil compaction from the use of heavy logging equipment in forest ecosystems have been studied for a number of years (Youngberg, 1959; Dyrness, 1965). The effects of compacted soil on plant and tree growth are well documented in the literature (Froechlich, 1979;
Lindermann et al., 1982). The removal of accumulated nutrients through harvesting during conventional log-ging has also been documented (Prichett and Fisher, 1987). Studies by Rennie (1955) generally showed that the harvest of hardwoods removed more nutrients from a site than the harvest of an equal volume of conifers. In this study, the level of harvesting and soil compaction were two key treatments on hardwoods and other tree species found at the site. From an earlier assessment of the site we found native earthworms but very few studies have been conducted on these North American species. Therefore, we selected the two most extreme treatments (no and severe compaction and minimal and maximum levels of harvested tree and litter material) to evaluate the effects of these treatments on native earthworm species, soil microbial biomass C and N (i.e., SMBC and SMBN) and selected soil properties during fall and spring for 2 years.
2. Materials and methods
The study site was located in an area of the Missouri Department of Conservation Carr Creek State Forest in Shannon County near Ellington, MO. The site is the location of a USDA Forest Service's long-term soil productivity (LTSP) study in the central USA hard-wood region (Ponder and Mikkelson, 1995). The soil at the site is a loamy-skeletal, mixed, mesic Typic Paledults (Ultisol). The soil is primarily derived from Ordovician and Cambrian dolomite with some areas of Precambrian igneous rock (Missouri Geological Sur-vey, 1979). The mean annual precipitation at this site is 112 cm and the mean annual temperature is 13.38C. The study is located on the upper northeastern-facing side slopes (20±28%) of the two parallel ridges. The oak-hickory timber is the major forest type in this central hardwood region of the USA and occurs over a variety of soils, relief, and stand conditions.
The experiment was a three factor randomized complete block design (RCBD) with two levels of treatments and three replications. Experimental fac-tors were two harvest levels (HL1and HL2) with HL1
representing plots with only the merchantable logs removed and HL2representing all above-ground
material from the plots. Litter accumulated during the study was not removed. Soil compaction included no compaction,C0and severe compaction,C2with a bulk
density of 1.8 g cm3. For compacted plots (C2), logs
were removed with a skidder and a roller was used to severely compact the soils. A skidder is a machine similar to a large rubber-tired tractor with a winch in the rear for pulling logs and shoveline blade in front for pushing or moving debris. For noncompacted plots (C0), cable logging was used which removes the logs
in a timber harvest to minimize soil disturbance. Logs were hoisted from the ground with a winch and moved along a suspended cable by a dragline mounted on a truck called a yarder. Sampling position included samples taken from the top and the bottom of the plot with slopes ranging from 20±28%.
Earthworms were collected by the handsorting method from a 15 cm depth (30.5 cm L30.5 cm W) each spring and fall for 2 years. Earthworms were anesthetized with 70% ethanol and preserved in 10% formalin and later identi®ed using the somatic key of James (1990). After identi®cation, earthworms were oven-dried at 608C for 48 h and their dry weight recorded. Dried earthworms were then ashed in a muf¯e furnace at 5008C for 4 h and the ash weight recorded. Ash-free biomass was determined by the methods of Parmelee et al., 1990. Earthworm ash free dry mass was calculated as oven dry weight minus ash weight.
Soil samples were taken from a 0±15 cm depth and kept cool until subsequent laboratory analyses. For each soil sample, soil moisture (MC, dried at 1058C for 24 h), soil inorganic N (SIN, 0.5 M K2S04extract)
and soil microbial C and N (SMBC, Horwath and Paul, 1994 and SMBN, Brookes et al., 1985a, b, respectively) were determined. Soil bulk density was determined by the core method (Blake and Hartge, 1986). Data were analyzed by ANOVA (SAS, 1989) using split plot with treatments as the
mainplot and sampling time as the subplot to account for repeated sampling of the same experimental plots. Duncan multiple range test (DMRT) and least sig-ni®cant difference (LSD) mean separation tests were used where signi®cant differences occurred. Pearson correlation was used as a simple correlation analysis.
3. Results
Initial soil properties taken at the 0±15 cm depth of the experimental area are presented in Table 1. The soil properties tested represent typical characteristics for the Clarksville soil in this region. The pH is in the typical range of 4.8±5.3. The earthworms found may be considered somewhat acid-tolerant because most species prefer a higher pH. Nutrients like P, Ca, Mg, and K are typically low compared to conventional agricultural site but within the range of values for this soil.
Two years of data on earthworm populations and biomass are presented. In 1995, Diplocardia ornata
was identi®ed as the dominant species. In 1996, earth-worm distribution could be separated into two domi-nant native species, Diplocardia ornata(Gates) and
Diplocardia smithii (MacNab and McKey Fender). Mature species and juveniles of both native earth-worms were recorded for fall and spring of 1996 only (Fig. 1). The mature species represented two native earthworms that were identi®ed by internal dissection and microscopic examination. Diplocardia juveniles dominated regardless of sampling time (fall versus spring). Juveniles accounted for more than 78% of the total earthworm population regardless of sampling time. Of the mature species identi®ed, D. ornata
was most prevalent representing greater than 50% of the population regardless of sampling time. Greater total earthworm populations were found in the fall (1996) compared to the spring (Fig. 1).
Table 1
Initial soil properties (0±15 cm) of the experimental areaa
pHs NA OM Total N Total C P Ca Mg K
(meq 100 gÿ1) (%) mg Kgÿ1
5.2 6.0 5.3 0.110 3.16 12 696 50 89
apHs: pH in 0.01 M CaCl
Soil compaction signi®cantly reduced the popula-tion (numbers) and biomass of earthworms found at this site (Fig. 2 and Table 2).Diplocardia juveniles and mature species were signi®cantly reduced when
soils were severely compacted. In spring and fall 1995, the earthworm biomass was signi®cantly reduced (Fig. 2). However, that effect was less dramatic in 1996 when no signi®cant reduction in biomass was observed.
The harvest level (HL) did not have a signi®cant effect on the earthworm population or biomass (Table 2). More signi®cant effects of the HL were observed on soil properties. In HL2, where all the
above-ground biomass was removed, a reduction in soil moisture (MC), soil inorganic N (SIN), and soil microbial biomass C and N (SMBC and SMBN) was seen (Table 3). On some of the soil properties, the HL and compaction had a signi®cant interaction (Table 2). When soils were severely compacted and all the above ground biomass removed, a signi®cant reduction in SMBC and SMBN was seen (Fig. 3). Time (fall versus spring and year of sampling) was a signi®cant factor for all earthworm and soil properties (Table 2). Generally, greater total population of earth-worms were collected in fall 1996 compared to spring and the prior year, 1995. A signi®cant interaction
Fig. 1. Diplocardia omataandDiplocardia smithii'sdistribution in spring and fall for mature and juvenile species in 1996.
Fig. 2. The effect of treatments on earthworm biomass for fall and spring of 1995±1996.
Table 2
Analysis of variance summary table for earthworm variables and soil properties, USDA-FS, 1995±1996 Source of variation d.f. Earthworma Soil propertiesb
Population AFB MC SIN SMBC SMBN Replication 2 0.8116 0.7485 0.3687 0.9712 0.9309 0.3269 Harvest level (HL) 1 0.6987 0.7466 0.0327 0.0085 0.0005 0.0006 Soil compaction 1 0.0058 0.0336 0.4862 0.8846 0.3475 0.6456 Position 1 0.0669 0.0169 0.1688 0.0365 0.2620 0.9000 HLcompaction 1 0.9076 0.5667 0.0234 0.0931 0.0294 0.0013
HLposition 1 0.8867 0.7550 0.9839 0.5038 0.7953 0.8949
HLcompactionposition 1 0.6812 0.7771 0.6691 0.1091 0.8447 0.9232
Error A 14 0.0001 0.1159 0.0001 0.0652 0.1261 0.0057 Time 3 0.0001 0.0111 0.0001 0.0001 0.0001 0.0001 HLtime 3 0.7609 0.4568 0.3826 0.4010 0.5790 0.0967
Compactiontime 3 0.0023 0.3390 0.0002 0.9733 0.6359 0.4727
Positiontime 3 0.5970 0.3315 0.6281 0.0452 0.5199 0.5817
HLcompactiontime 3 0.5098 0.6127 0.0579 0.8976 0.3789 0.7702
HLpositiontime 3 0.9252 0.9856 0.3901 0.4292 0.9714 0.9609
Compactionpositiontime 3 0.2310 0.2377 0.8964 0.1106 0.8594 0.5196 HLcompactionpositiontime 3 0.8487 0.9532 0.8127 0.7884 0.3242 0.6450 Timereplication 6 0.0462 0.6251 0.0527 0.9550 0.1679 0.6618
Error B 42
aEarthworm population and ash-free biomass (AFB) included both mature and juvenile species. bMC: moisture content; SIN: soil inorganic N; SMBC and SMBN: soil microbial biomass C and N. cSignificance probability.
Table 3
Correlation of harvest level and soil compaction with earthworm variables and soil properties, USDA-FS, 1995±1996a
Earthwormb Soil propertiesc
Population AFB MC SIN SMBC SMBN
n96
Harvest level r 0.0520 ÿ0.0381 ÿ0.2971 ÿ0.2242 ÿ0.3753 ÿ0.02878 P 0.6147 0.7126 0.0033 0.0298 0.0002 0.0047 Soil compaction r ÿ0.4277 ÿ0.2722 ÿ0.0897 ÿ0.0160 0.0811 0.0354 P 0.0001 0.0073 0.3850 0.8786 0.4322 0.7337 Population r 1 0.7176 0.3905 0.1734 0.2993 0.3885
P 0 0.0001 0.0001 0.0947 0.0031 0.0001
AFB r 0.7176 1 0.2841 0.0800 0.1984 0.3245
P 0.0001 0 0.0050 0.4435 0.0526 0.0013
MC r 0.3905 0.2841 1 ÿ0.0794 0.4620 0.4253
P 0.0001 0.0050 0 0.4467 0.0001 0.0001 SIN r 0.1734 0.0800 ÿ0.0794 1 0.3738 0.2466 P 0.0947 0.4435 0.4467 0 0.0002 0.0172
SMBC r 0.2993 0.1984 0.4620 0.3738 1 0.7379
P 0.0031 0.0526 0.0001 0.0002 0 0.0001 SMBN r 0.3885 0.3245 0.4253 0.2466 0.7379 1
p 0.0001 0.0013 0.0001 0.0172 0.0001 0
between time and compaction was observed in a few cases. With time, soil compaction became less con-straining for sampling and it's impact on earthworm number and biomass decreased.
We examined the effects of treatments on earth-worm biomass, on SMBC and SMBN. Soil compac-tion had a signi®cant impact on earthworm biomass for spring and fall 1995 (Fig. 3). When soils were severely compacted, the earthworm biomass was sig-ni®cantly reduced. Interestingly, the soil microbial biomass C and N were increased under compacted soils or not signi®cantly different from the uncom-pacted soils. This may be related to the removal of the harvested material. That is, when unharvested material was left on the plots (HL1), greater quantities of
nutrients and moisture were maintained. This may be indirectly re¯ected in a greater standing microbial biomass in these plots.
In examining the correlation data, HL had a sig-ni®cant negative correlation with the soil properties (Table 3). Soil moisture, soil inorganic N, soil micro-bial biomass C and N were reduced when merchan-table logs and forest litter were removed (HL2).
Greater MC was generally positively correlated with increased soil properties like SMBC and SMBN. Moisture content was signi®cantly correlated with earthworm population and biomass (Table 3).
4. Discussion
The genus,Diplocardia, are native surface feeders in USA, consisting of about 40 species, belonging to the Megascolecidae family.Diplocardiais widespread from southern to midwestern North America, and from Nebraska east to Delaware (James, 1990). The latitude of Detroit, Michigan is roughly the northern limit (James, 1990).Diplocardiawas ®rst identi®ed in Missouri by H.W. Olson in his 1936 study on earth-worms in Missouri (Olson, 1936).
Seasonal effects (spring versus fall) are important considerations when describing the distribution of native earthworm species in this ecosystem. Greater total earthworm populations were seen in the fall. More Diplocardia juveniles were found regardless of the time of year. This possibly suggests that a later sampling time (late April/early May) or a seasonal sampling scheme (April until late October monthly)
may be more appropriate to get an accurate assessment of which native species dominates (Diplocardia ornata or Diplocardia smithii). Diplocardia omata
was the most common shallow-dwelling species found at this site. However, a large percentage of juveniles especially in 1996, seem to suggest that further studies on the reproductive and ecology of both species should be pursued.
Forest management practices that physically manipulate (e.g., soil compaction or tillage) the soil can have negative effects on earthworms and soil properties (Li, 1996; Jordan et al., 1997). Soil com-paction had a signi®cant impact on reducing the numbers and biomass of these native earthworm spe-cies. In addition to compaction reducing pore space in the soil, it may have interfered with the earthworms' life cycle (e.g., cocoon production or juveniles killed). More detailed studies would have to be conducted to determine speci®c effects of soil compaction. Our ®ndings on the negative effects of soil compaction on earthworms were consistent with other researchers (Bostrom, 1986; Rushton, 1986; Joschko et al., 1989; Pizl, 1992; Sochtig and Larink, 1992). However, with time, soil compaction seems to become less constrain-ing to the earthworm populations. Their populations and biomass began to increase as shown by the 1996 data (Fig. 2). Their biomass was just as great as in the noncompacted plots suggesting that the earthworms probably began to overcome some of the effects by tilling and reworking the soil. Pizl (1992) also sug-gested that earthworms might have a loosening effect on compacted soils. In general, soils are recovering from the effects of compaction as the normal pro-cesses of freezing and thawing, minor shrinking and swelling, and root and litter decay occurs. These natural soil processes may also contribute to the ease at which earthworms may ameliorate the soil.
The initial removal of the whole tree and forest litter (HL2) from the plots had a negative effect on SIN,
SMBC and SMBN. The opposite effect was seen on these selected soil properties when HL1 (only
mer-chantable logs removed) was the major treatment.
5. Conclusions
com-paction or harvesting have a profound in¯uence on soil biota. In our study, compaction signi®cantly reduced earthworms especially in the ®rst year after the treat-ments were applied. It appears that with time, these native earthworm species began to overcome the effect of compaction. Studies on the effect of soil compaction on trees show, that even after many years (20±30 years), the detrimental effect on tree develop-ment is still apparent. Our preliminary data suggest that the effect of physical manipulations like severe soil compaction on earthworms may be of a shorter duration (2±5 years). The next ideal study would be to examine the interaction of the common hardwood tree species found in the area with native earthworm species under a controlled condition.
Harvest practices, which removed logs, boles and forest litter (HL2), de®nitely had an impact on
nutri-ents (this study shows this for SIN and a similar study on these plots-data not presented). With regard to the soil microbial properties, the story is unclear. It seems that the combination of severe soil compaction with HL may in some cases, substantially reduce the SMBC and SMBN and in other cases the value is the same or greater than other treatments. Further study would have to be conducted to clearly separate the differ-ences. Moisture and temperature are important envir-onmental factors that in¯uence earthworm and microbial activity in soils. In our study, soil moisture was positively correlated with increased earthworm numbers and biomass and with soil microbial biomass C and N. Although, the interaction of earthworms and soil microorganisms has not been clearly demon-strated, a likely future study would explore this area along with different moisture and temperatures for these native earthworms.
Finally, the frequency and timing of sampling are crucial to understanding more about the ecology and reproductive development of these native earthworm species in ®eld studies.
Acknowledgements
We thank Dr. Robert W. Parmelee and Dr. Samuel W. James for providing training on earthworm identi-®cation and other related technical assistance. Three anonymous reviewers are acknowledged for valuable and constructive criticisms of this manuscript. We are
grateful to all staff, student assistants, and volunteers for their help in earthworm and soil samplings. Fund-ing was provided by the USDA-Forest Service, Coop-erative Agreement Project # 94-2.
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