Earthworms as indicators of sustainable production in dryland cropping in southern Australia

Crop-monitoring by farmer groups has been established to identify agronomic and soil factors limiting crop yields and to promote the adoption of sustainable farming practices in South Australia. The use of earthworms as a potential indicator of sustainability has been investigated with a survey of 95 paddocks sown to wheat, barley or peas, within an area of about 3500 km2. Mean annual rainfall ranged from less than 350 mm to more than 500 mm, and soils varied from coarse sands through lighter loams to heavy clays. The dryland cropping soils in South Australia have been colonized by four immigrant earthworm species, Aporrectodea rosea (Savigny), Aporrectodea trapezoides (Duges), Microscolex dubius (Fletcher) and Microscolex phosphoreus (Duges); populations are generally dominated by A. rosea. Population differences, such as the higher numbers of A. rosea recorded under barley than peas and more juveniles under peas than wheat, and the occasional abundance of other species in some paddocks may be used to indicate changes in soil conditions associated with management. Conservation tillage, retaining plant-residues and reducing cultivation, is being promoted as desirable in developing sustainable farming systems and the inverse relationship between earthworm abundance and intensity of tillage (r = -0.69...) provides support for earthworms as a potential indicator of sustainability. Earthworm abundance showed a small but significant correlation between earthworm activity and grain yields (r = 0.44...) and grain protein content (r = 0.52...). A positive correlation between nitrogen fertilizer levels and earthworm numbers (r = 0.48...) and biomass (r = 0.43...) may be related to increased soil organic matter, derived from increased plant growth. The distribution and abundance of earthworms is dependent not only on management related to crop production, but also on local soil and climatic factors. Density and biomass were significantly higher in paddocks with higher annual rainfall and inversely correlated with levels of coarse sand. The soils, crops, climate, management and history of a region of the geographic scale surveyed here is probably too diverse to provide simple relationships between earthworms and plant productivity. It is important to distinguish the broad concept of sustainable productivity on a regional basis from the reality of productivity and sustainability on the spatial and temporal scales of activity of the organisms studied and the production and management process involved. At an appropriate scale, ecological data on both the species composition and on functional groups within earthworm communities, may be useful indicators of crop production and its sustainability.