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

Diversification of the crops used in wheat production systems provides alternative sources of income and can interrupt wheat pathogen lifecycles. Two important alternative crops in Western Australia are canola and lupins, which may both improve growth of following wheat. Improved growth of wheat following canola may be the consequence of biofumigation or increased root penetration 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 soil fauna. Each crop in the rotation was sampled in June, August, and October 1998. Nematodes were sorted into functional groups and arthropods were sorted to order level. Prostigmatid mites were the dominant arthropod group and they were sorted to morphospecies. An active and abundant faunal community was present under all crops, demonstrating that the canola variety in this study, Pinnacle TT, did not eliminate the free-living fauna. The structure of the mite communities changed throughout the year and the changes were different under the 3 crops. The soil arthropod communities were distinctly different under lupins compared with the other crops at the end of the growing 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 numbers of a tydeid and a tarsonemid mite species. Second, the tarsonemid species was always the second most abundant species under lupins but was infrequently the second ranked species under the other 2 crops. The soil arthropod communities were also different at the start of the growing season when the 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 following wheat crop.