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Forage legumes are used to enhance soil fertility of the agroecosystem. Understanding their effect on the agroecosystem during their growing period is essential. The objective of this field study was to evaluate annual clovers intercropping and earthworm activity on the growth of weeds. The field experiment was carried out during the 2006–2007 growing seasons at the research farm of the Seed and Plant Improvement Institute, Karaj (Iran) involving various mixed cropping ratios of berseem clover (Trifolium alexandrinum L., B) and Persian clover (Trifolium resupinatum L., P) (B : P = 1 : 0, 3 : 1, 1 : 1, 1 : 3) with and without earthworm Pheretima sp. inoculation. Nitrogen content of plants and weeds as well as biomass was measured. Forage yield was higher in 2006 than 2007 but had similar response to cropping systems and earthworm inoculation. Total forage yield was highest in mixed cropping system (MCS). While in the first cut, the berseem sole crop (SC) tended to greatly suppress the growth of weeds, in the second cut Persian clover sole crop further suppressed weeds. The greater total crop biomass had an even higher weed suppression. Earthworm activity did not affect cut 1 but increased forage and weed biomass yield in cut 2. However in cut 1, berseem tended to greatly suppress the growth of weeds, which may be explained by the greater nitrogen accumulation in monocultures and intercrops, Persian clover in cut 2 had greater suppression on weed biomass production. Nitrogen accumulation of crops and weed increased under earthworm activity in the second cut.
Earthworms are keystone species in the process of soil formation. The present paper reports on comparative studies of earthworm activity at a semi-natural park site and a typical farmland in Northern Poland. Of the six species found, Aporrectodea caliginosa, A. rosea, Allolobophora chlorotica, and Lumbricus terrestris occurred at all sites. The epigeic species L. casteneus and D. octaedra were missing at the farmland sites. Least species rich was the farmland site with the most condensed soil where only A. caliginosa, A. chlorotica, and L. terrestris were present. The farmlands showed reduced earthworm activities as measured by cast production and macropore density with respect to the natural park site. Cast production in the farmlands appeared to be independent of soil compactness and relative humidity. Soil compactness above 1.7 Mg · m-2 hampered earthworm activity. The total cast weight was inversely correlated with soil pH. The two most alkalic sites had significantly lower total cast weights per m2 than the park site.
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