Changes in free living soil nematode and microarthropod communities under a canola-wheat-lupin rotation in Western Australia

Diversification of the crops used in wheat production systems provides alte rnative sources of income and can interrupt wheat pathogen lifecycles. Two important alternative crops in Western Australia are canola and lupins, whi ch may both improve growth of following wheat. Improved growth of wheat fol lowing canola may be the consequence of biofumigation or increased root pen etration by the wheat. Available nitrogen may be increased following lupins . We examined free-living soil fauna in a canola-wheat-lupin rotation near Moora, Western Australia, to determine the effects of these crops on the so il fauna. Each crop in the rotation was sampled in June, August, and Octobe r 1998. Nematodes were sorted into functional groups and arthropods were so rted to order level. Prostigmatid mites were the dominant arthropod group a nd they were sorted to morphospecies. An active and abundant faunal communi ty was present under all crops, demonstrating that the canola variety in th is study, Pinnacle TT, did not eliminate the free-living fauna. The structu re of the mite communities changed throughout the year and the changes were different under the 3 crops. The soil arthropod communities were distinctl y different under lupins compared with the other crops at the end of the gr owing season in 2 ways. First, 5 times more animals were present under the lupins than under wheat or canola, primarily due to an increase in the numb ers of a tydeid and a tarsonemid mite species. Second, the tarsonemid speci es was always the second most abundant species under lupins but was infrequ ently the second ranked species under the other 2 crops. The soil arthropod communities were also different at the start of the growing season when th e prostigmatid community under canola was dominated by a rhagidiid species, whilst under lupins and wheat a caligonellid and eupodid species dominated . The canola followed a lupin crop and therefore the difference in June may be attributed to the preceding lupins. Mite data from the lupin plots were consistent with a previously described succession from another environment . We hypothesise that if net nutrient mineralisation rates are greatest at the start of a succession then net mineralisation rates under lupins may be rapid at the end of the lupin crop and slow when the next crop is planted in the remaining lupin stubble. The difference between lupins and canola in their mite communities would then imply that net mineralisation rates are a factor creating differences between the effects of break crops on the fol lowing wheat crop.