dc.description.abstract
Soil organisms are organized in highly diverse communities that provide
numerous ecosystem services and contribute decisively to the productivity and
resilience of agricultural systems. Over the last decades, however, agricultural
intensification has led to a loss of biodiversity, compromising the beneficial functions
performed by soil communities. To counteract this decline, a number of agrienvironmental
schemes have been implemented to maintain and promote
biodiversity. For example, spatial diversification (e.g., flower strips) can effectively
promote aboveground biodiversity, whereas little is known on the impact of such
measures on belowground communities. Earthworms are an integral component of
soil communities as they perform key ecological functions. Earthworms are negatively
affected by intensive agricultural management, especially intensive soil management.
Consequently, the implementation of perennial structures into agricultural systems
(e.g., perennial flower strips and tree rows through agroforestry) is expected to benefit
earthworm communities. However, field-based studies validating this assumption,
remain scarce. Here, we conducted two studies (Chapters 2 and 3) to evaluate the
impact of flower strips and alley-cropping agroforestry on earthworm communities.
For that purpose, we sampled earthworms using chemical extraction with
allyl isothiocyanate (AITC) under different flower strip mixtures (two annual and two
perennial mixtures) and a grassy field margin vegetation at three different sites in
Germany (Chapter 2). We found that perennial flower strip mixtures harbored greater
earthworm population density and biomass than field margin vegetation, whereas
population density and biomass were lower in annual flower strip mixtures as
compared to the field margins and perennial flower strip mixtures. The absence of
tillage in the field margins and the perennial flower strips as well as high plant diversity
of the perennial flower strips are expected to cause the promotion of earthworms.
Similar effects of soil management were observed in an alley-cropping
agroforestry system in Germany (Chapter 3). Here, we used AITC extraction to
sample earthworms in the tree rows, at different distances from the trees into the crop
row, and in an adjacent cropland monoculture without trees. We found increased
earthworm population density and biomass as well as an altered community
composition under the trees as compared to the crop row and the monoculture
cropland. The absence of tillage under the trees was most likely the main beneficial
factor influencing earthworm communities. In addition, increased above- and
belowground litter input in close proximity to the trees might also have promoted
earthworms, as some of the recorded positive effects also extended into the crop row.
Despite our findings, several knowledge gaps regarding the impact of spatial
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diversification measures on earthworms remain (e.g., influence of different flower strip
mixtures, tree row orientation, and age of the perennial structures).
To fill these knowledge gaps, more field-based studies are required.
However, commonly used methods for earthworm sampling and species
determination are demanding and expensive. The standardized sampling method for
earthworms requires hand sorting of the excavated topsoil and subsequent chemical
extraction (e.g., with AITC) of the subsoil. Although this method offers high recovery
rates of earthworms, hand sorting is labour-intensive, time-consuming, and
destructive towards the sampling site. In Chapter 4 we, therefore, compared this
standardized method to a method using only AITC extraction without hand sorting at
eleven different sites in Germany. We found AITC extraction without hand sorting to
be a viable alternative for investigations regarding anecic earthworms and overall
species richness as well as for on-site comparisons of the whole community.
Following earthworm sampling, determination of the collected individuals on
species level is a necessary step in order to draw conclusions regarding earthworm
functions. Species determination is mostly carried out through morphological
identification, which is time-consuming, requires taxonomical expertise, and is usually
not suitable for the identification of juveniles and cryptic species. Molecular
approaches such as DNA barcoding, however, are expensive and hence not
commonly used. In Chapter 5, we investigated the potential of high-resolution melting
(HRM) curve analysis as a cost-saving alternative to DNA barcoding. In our study,
HRM curve analysis enabled the distinction between eight earthworm species
commonly found in European agricultural soils. We were also able to distinguish
different haplotypes of the earthworm species Allolobophora chlorotica using HRM
curve analysis, which indicates the potential of the method to differentiate between
cryptic species. Additionally, HRM curve analysis is suitable for the identification of
juveniles and damaged individuals and could thus serve as a complementary tool to
morphological identification.
Overall, the results presented in this thesis show that spatial diversification
through perennial flower strips and agroforestry systems generally benefits earthworm
communities. Furthermore, it can be concluded that for certain research questions,
AITC extraction and HRM curve analysis are viable options to facilitate field-based
earthworm research. By this, we hope that remaining knowledge gaps regarding the
response of earthworm communities to agricultural management practices can be filled
and thereby further practises that preserve the integrity of earthworm communities in
agricultural soils can be identified and implemented into agri-environmental schemes.
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