Grödans kontra odlingssystemets effekt på frilevande nematoder

Abstract: Nematodes are perhaps the most common animals in the world. Nematode populations can reach densities up to several millions per square meter in the soil. There are many factors which affect nematode communities in arable soil with respect to diversity and population size. Factors like tillage, crop, fertilization, nutrient status and moisture in the soil all have an impact on soil biology and in this way also have effects on the nematode community. This study is based on three different parts, soil samples taken in March 2012 and March 2013, soil samples taken monthly during the growth season of 2013 in bare fallow plots and a pot experiment. All soil samples and the soil in the pot experiment was taken from a long-term field trial in Önnestad which is located nearby Kristianstad, Sweden. The aim of this study was to investigate if cropping system or crop had the greatest effect on the abundance and diversity of nematode communities in arable soil. The hypothesis tested was that crop had a main impact on plant-feeding nematodes, especially when a non-host crop was grown or a bare fallow was establish. The field trial in Önnestad was established in 1987 and includes 5 cropping systems (2 conventional and 3 organic) with six year crop rotations. In March 2012, soil samples were taken in the Önnestad field trial and high numbers of plant-parasitic nematodes were observed. One year later, soil samples were again taken. The samples from 2012 and 2013 differed greatly in nematode abundances, and the two years were analyzed statistically to find out if cropping system had some effect on the differences between the 2012 and 2013 samplings. There were differences between the years for some nematode families and those differences were all connected with the fact that ley was present in the crop rotation. This indicates that ley has a positive effect on some free-living nematode families. Before the growing season of 2013, the crop rotations were reorganized because of the high numbers of plant-parasitic nematodes, especially root-knot nematodes. This resulted in plots with bare fallow in three of the crop rotations in 2013. In those bare fallows plots, soil samples were taken monthly and examined to determine how free-living nematodes were affected by this management. In this way, the free-living nematode populations were studied based on how they fluctuate during the season under bare fallow. The pot experiment that was preformed had four different crops; red clover, white clover, ryegrass and bare fallow and the soil in the pots where taken from the plots with bare fallow in the Önnestad field trial. Free-living nematodes were counted and classified after what they eat and in this way the population density of different free-living nematodes could be determined in relation to cropping system and crop. The results and conclusions of this study are that crop rotations including ley have a positive effect on the abundance of free-living herbivorous nematodes and the nematode population density in general. This means that cropping systems have a role in the complexity and density of free-living nematode populations. The study also showed that legumes may have a positive effect on population density of free-living nematodes regardless what they eat. There is also a crop effect on the community of free-living nematodes in the pot experiment. Herbivores had a lower abundance in pots with bare fallow in contrast to pots with a crop. This clear result didn’t show in the analysis of the season sampling. It’s possible that bare fallow need a longer time to give this effect in practice.