If the observed reduction in total nematode density as well as the decreased densities of most free-living nematode taxa (eight out of ten studied taxa) one day after biofumigation are decrements may be related to a combination of mechanical stress (tillage) and or the release of compounds other than ITC from the plant material. Little is known about sensitivities of individual nematode genera or families to the mechanical stress of tillage, as most studies fo-
e.g. Fu et al.,
2000, Timper et al., 2012, Treonis et al., 2010). However, Fiscus and Neher (2002) demonstrated -
instance, the genus Achromadora
(Fig. 2A), while Monhystera (Monhysteridae), which is placed into the same tillage sensitivity
category as Achromadora -
tode communities, data are analysed at trophic group level, while the original data, at genus level, are not included (e.g.
Next to ITCs, non-glucosinolate derived, sulphur-containing compounds are released during the incorporation of Brassica plant material in topsoil. In B. juncea,
less toxic than ITCs but are present in higher concentrations (Bending and Lincoln, 1999). Wang
in the degree of nematode decrease between B. juncea
assumption for our data. We conclude that the observed responses of the free-living nematode taxa one day after biofumigation are mainly due to mechanical disturbance.
Indian mustard into the topsoil are supposedly unrelated to the release of ITCs. All densities of free-living nematode taxa had increased, while the numbers of plant-parasitic taxa declined.
nematode density before biofumigation and 6 weeks after. As also mentioned in their study, the increase in the density of free-living nematode taxa is most likely related to the incorporation of plant biomass, i.e. green manuring. Our results suggest that all free-living nematode taxa
they used various Brassica
Brassica
observed.
We ascribe the decrease of plant-parasitic nematodes after biofumigation (three weeks and two months) to the mere absence of host plants. This option is also considered in the study by Valdes et al. (2012), in which a decrease in potato cyst nematodes (PCNs) was observed 6 weeks after biofumigation with yellow mustard. However, encysted PCNs can survive for months if related biotic suppression. Stirling et al. (2001) showed that the density of Tylenchorhynchus was very low in bare fallow soil and when planted with sugarcane could return to very high numbers. This indirectly suggests that a fraction of the population can survive without a host for a long period, and due to its short reproduction cycle it can re-establish high densities whenever a host is available again.
Conclusions
Our results demonstrate that ‘biofumigation’, the release of isothiocyanates due to incorpo- ration of mulched plant material from four distinct B. juncea
plant-parasitic or free-living nematode populations. The observed short-term (day 1 after incorporation, and therefore unrelated to B. juncea-derived GSLs. Also, direct application of relatively high concentrations of 2-propenyl ITC (up to around 480 mol ha-1 resulting in 195 nmol g-1 dry soil) did not result in shifts distinct from those observed in untreated wheat plots for the plant-parasitic and free-living nematode taxa under investigation. We therefore community were mainly attributable to a combination of tillage and green manuring, and not
B. juncea-produced ITCs per se.
Acknowledgements
The authors would like to thank Hubert Große Lengerich, Mechthild Neuhaus and Andreas Wilhauck from the Julius Kühn Institute (Münster, Germany) and Hanny van Megen from the experiment (HGL, MH and AW), counting of the plant-parasitic nematodes (MH) and primary
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