Changes in invertebrates population over the growing cycle of a N-fertilised wheat crop in rotations with cotton in a grey Vertosol

Sowing other crops in rotation with cotton ('Gossypium hirsutum' L.) has proven to be a successful strategy employed by farmers to reverse yield decline and maintain soil quality. The objectives of this study were to quantify soil macro-invertebrate abundance and composition with and without N fertiliser application during the wheat-cropping phase of a cotton-based rotational system. Comparison with invertebrate abundance was also made with an adjacent area of native vegetation. The experiment was first established in 1993, but invertebrate sampling was only conducted between June and December 1997, during the winter wheat ('Triticum aestivum' L.) phase at 6, 15 and 23 weeks after planting (WAP) the crop and in nearby native vegetation at the same time. Ants were identified to genera and all other invertebrates to order. Changes in crop cover and soil temperature were also evaluated at 6, 15 and 23 WAP. Ant abundance was higher in the treatments where N fertiliser was not applied, but only later in the season (summer), compared to the treatments where N fertiliser was applied. Other invertebrates showed a similar pattern of recovery to ants, with higher levels of abundance in summer compared with earlier in the wheat growing cycle. The use of ants as bioindicators for monitoring changes in soil health within cotton-based rotational systems, especially in the wheat-cropping phase, is complicated by the confounding factor of seasonality, although their fluctuations in abundance mimic other invertebrates.