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Published in Agron J 99:1514-1520 (2007)
DOI: 10.2134/agronj2006.0371
© 2007 American Society of Agronomy
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Pasture Management

Soil Seed Bank Composition in Pastures of Diverse Mixtures of Temperate Forages

Matt A. Sandersona,*, Sarah C. Gosleea, Keith D. Klementb and Kathy J. Sodera

a USDA-ARS Pasture Systems and Watershed Management Research Unit, Bldg. 3702 Curtin Rd., University Park, PA 16802-3702
b USDA-ARS U.S. Sheep Experiment Station, Dubois, ID 83423

* Corresponding author (matt.sanderson{at}ars.usda.gov)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Seed banks may contribute useful or weedy species that fill gaps in pastures. In a previous study, pastures planted to complex mixtures of forages had a lesser proportion of weedy species in the aboveground vegetation. In this study, we relate changes in the species composition of the seed bank to changes in the aboveground vegetation. In August 2001, four mixtures [two, three, six, and nine species of temperategrasses, legumes and chicory (Cichorium intybus L.)] were established in replicated 1-ha pastures (eight total) in central Pennsylvania. Pastures were grazed by dairy cattle from April to September in 2002 and 2003. Soil cores (1.88-cm diam. by 5-cm depth) were taken in April and October in 2002 and 2003, and in April 2004 to determine the density of germinable seeds. Soil samples were placed in a greenhouse under natural light and controlled temperatures for 12 to 18 mo and germinated seedlings counted regularly. The total density of germinable seeds from all species did not differ among mixtures (P = 0.08). Annual nonleguminous forbs accounted for 79% of the germinable seeds. Yellow woodsorrel (Oxalis stricta L.) was the dominant annual forb. There were significant differences among pastures planted to different mixtures in the density of germinable annual forb seeds; however, these differences likely occurred because of preexisting spatial variation in seed bank composition. Seeded species contributed fewer than 1000 seeds m–2 total in the 3 yr. Kentucky bluegrass (Poa pratensis L.) and white clover (Trifolium repens L.) were the most common forage species in the seed bank. There was little relation between the species composition of the seed bank and the composition of the aboveground vegetation. Data from this study indicate that previous land management had larger effects on the soil seed bank than did planting diverse mixtures of forages.

Soil Seed Bank Composition in Pastures of Diverse Mixtures of Temperate Forages

Matt A. Sandersona,*, Sarah C. Gosleea, Keith D. Klementb and Kathy J. Sodera

a USDA-ARS Pasture Systems and Watershed Management Research Unit, Bldg. 3702 Curtin Rd., University Park, PA 16802-3702
b USDA-ARS U.S. Sheep Experiment Station, Dubois, ID 83423

* Corresponding author (matt.sanderson{at}ars.usda.gov)

Received for publication December 30, 2006.
Seed banks may contribute useful or weedy species that fill gaps in pastures. In a previous study, pastures planted to complex mixtures of forages had a lesser proportion of weedy species in the aboveground vegetation. In this study, we relate changes in the species composition of the seed bank to changes in the aboveground vegetation. In August 2001, four mixtures [two, three, six, and nine species of temperategrasses, legumes and chicory (Cichorium intybus L.)] were established in replicated 1-ha pastures (eight total) in central Pennsylvania. Pastures were grazed by dairy cattle from April to September in 2002 and 2003. Soil cores (1.88-cm diam. by 5-cm depth) were taken in April and October in 2002 and 2003, and in April 2004 to determine the density of germinable seeds. Soil samples were placed in a greenhouse under natural light and controlled temperatures for 12 to 18 mo and germinated seedlings counted regularly. The total density of germinable seeds from all species did not differ among mixtures (P = 0.08). Annual nonleguminous forbs accounted for 79% of the germinable seeds. Yellow woodsorrel (Oxalis stricta L.) was the dominant annual forb. There were significant differences among pastures planted to different mixtures in the density of germinable annual forb seeds; however, these differences likely occurred because of preexisting spatial variation in seed bank composition. Seeded species contributed fewer than 1000 seeds m–2 total in the 3 yr. Kentucky bluegrass (Poa pratensis L.) and white clover (Trifolium repens L.) were the most common forage species in the seed bank. There was little relation between the species composition of the seed bank and the composition of the aboveground vegetation. Data from this study indicate that previous land management had larger effects on the soil seed bank than did planting diverse mixtures of forages.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SEED BANKS provide a reservoir of seeds with potential to contribute to or replace the aboveground vegetation if it is disturbed or destroyed (Simpson et al., 1989). Buried seed in pasture soils may contribute useful species to grazing management, but often are reservoirs of less desirable weedy species (Rice, 1989). Seed banks in pastures of the northeastern USA are dominated by annual and perennial forbs (Tracy and Sanderson, 2000). Bluegrass (Poa spp.) and white clover are the dominant forage species found in northeastern USA pasture soil seed banks. Although the species composition of seed banks and the aboveground vegetation frequently are not closely related (Tracy and Sanderson, 2000; Thompson and Grime, 1979; Marriott et al., 2002), species composition of the vegetation may affect the abundance of seed in the seed bank. For example, Tracy et al. (2004) reported that the seed bank formed under tall fescue- [Lolium arundinaceum (Schreb.) S.J. Darbyshire] dominated swards had less germinable seed than that found under smooth bromegrass (Bromus inermis Leyss.).

Many studies of vegetation change in pastures as a result of management treatments do not include assessments of changes in the soil seed bank. Knowledge of the seed bank composition of pastures is helpful to estimate what species may dominate the sward after disturbances that open gaps in the sward. These data are also useful to help understand changes in plant community composition and species diversity with time in pastures.

Grazed pastures of the northeastern USA are commonly seeded to only a few species (e.g., white clover and orchardgrass, Dactylis glomerata L.). Recently, landowners and researchers have focused more attention on multispecies forage mixes for grazing animals (Sanderson et al., 2004). These mixes may allow a larger diet selection for foraging animals; however, little is known about the sustainability and soil seed bank composition (Tracy and Sanderson, 2000) of these complex pastures.

In previous studies we measured changes in aboveground vegetation structure, composition, and productivity (Sanderson et al., 2005); animal performance (Soder et al., 2006); and sward structure (Sanderson et al., 2006) of four mixtures of forages differing in the number of planted species (two, three, six, or nine species). In this study, we relate changes in the composition of the seed bank to change in the aboveground vegetation composition of plant and nonplanted species during 3 yr. We reported differences among the mixtures in the abundance of unplanted species (Sanderson et al., 2005) and hypothesized that the forage mixtures influenced seed bank composition as well.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Details on establishment of the pastures and grazing management are in Sanderson et al. (2005) and Soder et al. (2006). We summarize the main points here.

We conducted the research at the Dairy Cattle Research and Education Center of the Pennsylvania State University in University Park. In July 2001, existing vegetation at the site was killed with glyphosate [N-(phosphonomethyl)glycine] and dicamba (3,6-dichloro-2-methoxybenzoic acid) herbicides both of which were applied at 1 kg ha–1 active ingredient. On 29 Aug. 2001, four mixtures of forage species (two-, three-, six-, and nine-species mixtures) were no-till planted in replicated 1-ha pastures. The mixtures were (i) orchardgrass and white clover; (ii) orchardgrass, white clover, and chicory; (iii) orchardgrass, tall fescue, perennial ryegrass (Lolium perenne L.), red clover (Trifolium pratense L.), birdsfoot trefoil (Lotus corniculatus L.), and chicory; and (iv) mixture iii plus white clover, alfalfa (Medicago sativa subsp. sativa), and Kentucky bluegrass. The experimental design was a randomized complete block with two replicates (pastures) of each mixture.

Land use history of the experimental site was varied with a mixture of row crops, hay, pasture, and liquid manure application in the previous 5 yr. Soil at the site is a Hagerstown silt loam (fine, mixed, semiactive, mesic, Typic Hapludalfs). Soil tests (to a 15-cm depth) in 2001 indicated a pH of 6.5, 220 kg ha–1 available P, and 210 kg ha–1 of available K, thus no fertilizer was required. A portable meteorological station at the site monitored solar radiation, air temperature, and rainfall from April 2002 to October 2003.

The pastures were subdivided into smaller paddocks and stocked rotationally with lactating Holstein cows from April through August in 2002 and 2003. Five cows grazed each treatment. Cows were confined to a fresh area of pasture after each milking, which took place each morning at 0500 h and afternoon at 1800 h. Lactating cows were not available after 1 August of each year; therefore, pastures were mob grazed with 21 dry cows for 2 d in mid-August 2003 and early September 2002 and 2003 to complete the grazing season. Pastures were grazed five (2002) or six (2003) times from April to September. If necessary, pastures were clipped to a 10-cm stubble height after grazing. The experiment was conducted under the approval of the Penn State Animal Care and Use committee. Herbage and animal production data are reported in Sanderson et al. (2005) and Soder et al. (2006).

Aboveground plant community composition was sampled in October 2001, April and October in 2002 and 2003, and in April 2004. We used a modified Whittaker plot to assess species richness at a range of spatial scales (Stohlgren et al., 1995; Goslee, 2006). Two plots were established in each pasture. All species present within a 20- by 50-m area were recorded, and percentage canopy cover was visually estimated within 10 1-m2 quadrats (2 by 0.5 m) distributed throughout the larger plot. Although species present in subplots of two intermediate sizes (2 by 5 m and 5 by 20 m) were also recorded; here we will focus on species abundance from the 1-m2 quadrats, and species richness at the 1-m2 and 1000-m2 scales. The Bray-Curtis dissimilarity index (percentage difference; Legendre and Legendre, 1998) was used to describe the differences in plant species abundance data within and among samples of the aboveground vegetation and the soil seed bank. The Jaccard dissimilarity index was used for presence-absence data (Legendre and Legendre, 1998).

Soil samples for analysis of the seed bank were taken in April and October in 2002 and 2003, and in April 2004. Two soil cores (1.88-cm diam. by 5 cm deep) were taken at each of 16 locations within each modified-Whittaker plot in each pasture. We did not have seed bank samples before the site was planted in August 2001; therefore, we used the April 2002 sampling date as an indicator of the existing seed bank before planting, assuming little change in seed bank composition between August 2001 and April 2002.

We used the direct germination method (Thompson and Grime, 1979) to determine the germinable seed composition in the soil. Soil from the 32 cores from each modified Whittaker plot was pooled and uniformly mixed. Thus, there were two composite soil samples from each pasture. Immediately after collection in spring or fall, each composite soil sample was spread onto a plastic tray (25 by 50 by 6 cm) containing 2 cm of sterile potting soil. Trays were placed in a greenhouse (25 ± 2°C) and under natural light conditions. Trays were watered daily with tap water and rotated monthly to different parts of the greenhouse to reduce any effects of differential conditions. As seedlings emerged they were identified, counted, and removed from the trays. The soil in each tray was stirred about every 3 mo to stimulate further germination. Soil samples from April 2002, October 2002, and April 2003 were monitored for 18 mo. Samples from October 2003 were monitored for 15 and 11 mo for the April 2004 samples. Few seedlings emerged after 12 mo in the first three samplings, so different lengths of monitoring times did not affect results. Seedling counts were totaled from the two composite soil samples in each pasture for statistical analysis.

Species were grouped into annual forbs, perennial forbs, biennial forbs, annual grasses, perennial grasses, annual legumes, and perennial legumes for statistical analysis of germinable seed counts. We use the term forb to mean nonleguminous forbs. Total counts of germinable seeds from all species and from the species groups were square-root transformed to stabilize variances for analysis. Untransformed means are presented in tables and graphs, but significance tests were based on analysis of transformed data. Seed count data for each period across years were analyzed as repeated measures in a randomized complete block design with the mixed models procedure in SAS (Littell et al., 1996). Treatments and periods were considered fixed effects and blocks were random effects. A compound symmetry covariance structure best fit the data (lowest AIC value). Denominator degrees of freedom were calculated with the Kenward-Rogers option.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Across all pasture mixture treatments, 72 species of plants germinated from the soil sampled from 2002 to 2004. Ninety-four species were identified in the aboveground vegetation during the same time, with 52 species common to both aboveground vegetation and the seed bank.

Within the four mixture treatments, 43 to 46 species of plants were identified from the seed bank (Table 1 ). Most of the species identified (43%) were annual forbs with only 3 to 18% of species found in the other species classes. Mixture treatments did not differ in the number of species identified in the seed bank.


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  		Table 1. Total number of species identified in the soil seed bank of four mixtures compared in an experiment at University Park, PA, during 2002 to 2004.

 
There was no difference among pasture mixtures (P = 0.08) in the total density of germinated seeds averaged for sampling periods (Fig. 1a ). Pasture mixtures differed (P = 0.04) in the number of annual forb seeds that germinated (Fig. 1b). Fewer annual forbs germinated from the seed bank of the pastures planted to three- and nine-species mixtures compared with pastures planted to the two-species mixture. There was no pasture mixture treatment x sampling date interaction (P > 0.05) for total seed density or seed density of species groups. This indicates that the differences in species seed density and composition patterns among treatments existed before planting the pastures and did not change in response to the forage species mixture planted or grazing management during the 3 yr.


Figure 1
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  		Fig. 1. (a) Number of germinated seeds from all species and (b) from annual nonleguminous forbs in the soil seed bank of four pasture mixtures compared in an experiment at University Park, PA. Data are averaged for two replicates and five sampling periods. Means with different letters above the bars differ (P = 0.04). Significance tests were based on analysis of square-root transformed data. Analysis of untransformed data yielded approximately the same results; thus, untransformed means are presented along with an overall standard error for clarity.

 
The total density of seeds from all species and from annual forbs varied with sampling period (P = 0.01; Fig. 2 ). Peak counts of germinable seed occurred from soil samples taken in October 2002 and 2003. Yellow woodsorrel and common chickweed (Stellaria media L.) influenced seed counts the most at these times.


Figure 2
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  		Fig. 2. (a) Number of germinated seeds from all species (solid line) and from annual nonleguminous forbs (dashed line) in the soil seed bank at five sampling times at University Park, PA. (b) Density of perennial forbs, biennial forbs, perennial grasses, and perennial legumes. (c) Density of yellow woodsorrel, common chickweed, and white clover seeds during the same sampling times [bars indicate ±1 SE). Data are averaged for two replicates and four mixtures. There was a significant sampling period effect (P = 0.04) for each species group in (a), for the biennial forb and perennial grass groups in (b), and each species in (c). Significance tests were based on analysis of square-root transformed data. Analysis of untransformed data yielded approximately the same results; thus, untransformed means are presented.

 
Annual forbs clearly dominated the seed bank in each mixture treatment. Eighteen to 20 annual forb species were identified, and these species accounted for 79% of the total germinable seed density (Fig. 2, Table 2 ). Yellow woodsorrel was the dominant annual forb and had lower (P < 0.05) germinable seed density from the three-species mixture compared with other mixtures (Fig. 3 ). Yellow woodsorrel can be either annual or perennial and reproduces mainly by seed (University of Illinois, 1981). It is a summer annual in cooler climates with plants dying back to the ground in winter (Uva et al., 1997). Pigweed (Amaranthus retroflexus L.), common chickweed, and shepherd's-purse [Capsella bursa-pastoris (L.) Medik.] also were among the most frequent and abundant annual forbs. Germinable seed density of shepherd's-purse was lower (P < 0.05) from the three- and nine-species mixtures compared with the other pasture mixtures (Fig. 3). A few other species were abundant and frequent in specific mixtures. Hairy bittercress (Cardamine hirsuta L.) occurred commonly in the three- and six-species mixtures.


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  		Table 2. Number of germinable annual nonleguminous forb seeds identified from soil samples of four pasture mixtures (two, three, six, or nine species) compared in an experiment at University Park, PA, during 2002 to 2004. Data are totals from two composite samples of 32 cores each from two pastures per treatment and five sampling periods (20 total potential observations).{dagger}

 

Figure 3
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  		Fig. 3. Number of germinated seeds of shepherd's-purse and yellow woodsorrell in the soil seed bank of four pasture mixtures at University Park, PA. Bars with different letters differ at P < 0.05. Significance tests were based on analysis of square-root transformed data. Analysis of untransformed data yielded approximately the same results; thus, untransformed means are presented along with standard errors for clarity.

 
Seven to nine species of perennial forbs germinated from the seed bank (Table 3 ). The density of germinable perennial forb seeds decreased with time (Fig. 2). Dandelion (Taraxacum officinale Weber in Wiggers) dominated the perennial forbs and was common in all treatments.


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  		Table 3. Number of germinable perennial and biennial nonleguminous forb seeds identified from soil samples of four pasture mixtures (two, three, six, or nine species) compared in an experiment at University Park, PA, during 2002 to 2004. Data are totals from two composite samples of 32 cores each from two pastures per treatment and five sampling periods (20 total potential observations).{dagger}

 
There were only a few species found in the remaining groups of plants, and mixture treatments did not differ in the density of germinable seeds from these groups. The density of biennial forbs and perennial grass seeds decreased greatly from April 2002 to April 2004 (Fig. 2). Fleabane (Erigeron philadelphicus L.) was the most common biennial forb (Table 3). Barnyard grass [Echinochloa crus-galli (L.) Beauv.] was the most common annual grass, whereas Kentucky bluegrass was the most common perennial grass (Table 4 ).


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  		Table 4. Number of germinable grass, legume, and sedge seeds identified from soil samples of four pasture mixtures (two, three, six, or nine species) compared in an experiment at University Park, PA, during 2002 to 2004. Data are totals from two composite samples of 32 cores each from two pastures per treatment and five sampling periods (20 total potential observations).{dagger}

 
Seeded species contributed few or no germinable seeds in the soil seed bank when compared with annual forbs (Tables 3 and 4). Of the forage species planted, only Kentucky bluegrass and white clover occurred relatively frequently and abundantly (Table 4). White clover was not seeded in the six-species mix hence the lower amounts of germinable white clover seed in the seed bank. Chicory occurred only in the pastures where it was planted, but it was not very abundant or frequent in the seed bank (Table 3).

The species composition of the seed bank was similar among treatments. The four treatments had 47 to 57% of their species in common (determined as 1 minus the Jaccard dissimilarity value). When abundances were considered, the treatments ranged from 58 to 75% similarity (determined as 1 minus the Bray-Curtis dissimilarity value). The aboveground vegetation varied less among treatments: 64 to 74% similar at the 1000-m2 scale and 52 to 69% similar at the 1-m2 scale. There was no significant relationship between species composition or abundance in the seed bank and the planted treatments.

As found often in seed bank studies in pastures, there was little relationship between the species composition of the seed bank and the composition of the aboveground vegetation. A weak but significant relationship was found between presence of species in the seed bank and abundance and presence of those species at the 1-m2 scale (Mantel r = 0.21, P < 0.001 for abundance; Mantel r = 0.16, P < 0.001 for presence) but not for presence at the 1000-m2 scale. Quackgrass [Elytrigia repens (L.) Desv. ex Nevski] was the most common unplanted species found in hand sorts of the aboveground vegetation during 2002 and 2003 (Sanderson et al., 2005). In the seed bank, however, quackgrass was rare (Table 3) and yellow woodsorrel was the dominant species.

Our results agree with a previous survey of seed banks in pastures of the northeastern USA (Tracy and Sanderson, 2000). In that survey, annual and perennial forbs dominated the seed bank, dandelion was common to all farms, and bluegrass and white clover were the most abundant forage species found. The composition of species in the annual forb group differed between the survey and the current study. Although many of the same species were found in the survey and the current study, purslane speedwell (Veronica peregrina L.), yellow rocket (Barbarea vulgaris L.), wild mustard (Brassica kaber L.), and common lambsquarters (Chenopodium album L.) were the dominant species of annual forbs in the previous survey. Annual bluegrass (Poa annua L.) and yellow foxtail (Setaria glauca L.) were the most common annual grasses in the survey, but occurred less abundantly in the current study. Tracy and Sanderson (2000) conducted their regional survey during the summer, whereas in the current study sampling was done at a specific site during spring and fall, which may account for some of the differences between studies.

We hypothesized that because we measured lower amounts of nonplanted species in the aboveground vegetation of some treatments (Sanderson et al., 2005) we would also observe differences among treatments in the seed bank. We observed differences among pastures in the seed bank density of annual forbs, yellow woodsorrel, and shepherd's-purse. The reasons for these differences, however, are not clear. In fact, the similarities in seed bank composition and abundance among the pastures and treatments were much greater than the differences.

Grazing management could affect soil seed bank composition by altering seed input from plants of different functional groups (Sternberg et al., 2003). In our study, most of the annual forbs in the seed bank were either rare or absent in the aboveground vegetation and the pastures were grazed regularly, which limited flowering of most plants, thus differential seed input probably did not occur.

Soil disturbance through tillage (Cavers and Benoit, 1989) or grazing animals (Renne et al., 2006; Renne and Tracy, 2007) can have a large effect on soil seed banks. The pastures used in our study were planted via no-till techniques, thus soil disturbance from tillage should have been minimal. We did not observe any differences among pastures in the degree of soil disturbance by trampling or hoof impact of the grazing animals.

Spatial variation in seed banks can be extreme and may obscure cultural effects. The pasture site had a mixed history of cropping, including row crop, hay, and grazing along with liquid manure applications. These previous activities could have strongly influenced the composition and spatial variation of seed bank. The lack of a mixture treatment x sampling date interaction supports this speculation and suggests that differences in the seed banks among pastures used for the treatments did not result from planting different mixtures of forage species.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Pasture mixture treatment did not affect the total number of germinable seeds in the soil seed bank during the 3 yr of this study. There was little relationship between the species composition of the seed bank and the composition of the aboveground vegetation. Annual forbs accounted for most of the germinable seeds in pasture soil seed banks. Planting functionally diverse mixtures of forages (e.g., grasses, legumes, and forbs) may reduce the proportion of nonplanted species in the aboveground vegetation of intensively managed pastures but it does not alter the density or species composition of the soil seed bank.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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