Australian agriculture is increasingly tasked with managing the soil resource base to potentially offset greenhouse gas emissions. In dryland cropping systems, inputs of carbon are primarily derived from CO2 fixed by the cash crop, with fallow periods often extending several months over the dry summer period. Cover crops, grown primarily for their presumed positive impacts on many soil processes—including increasing soil carbon stocks—present one opportunity to potentially increase the amount of carbon fixed without displacing a cash crop. Whilst increasingly adopted in higher rainfall areas of American and European agriculture, scepticism remains about their suitability in Australian dryland cropping systems, particularly where water availability for the cash crop is a major limit to productive capacity.
In the present study, 19 replicated field experiments were conducted across the southern Australian grain-growing region, with annual rainfalls of 270-1250 mm. Treatments were single species, mixed species (inclusive of the single species), and a business-as-usual control. These ran for four years, and were conducted in a systems context; that is that cover cropping was only conducted in summer seasons where the local growers managing each on-farm experiment considered that enough summer rainfall was likely to enable cover crop establishment. In some dryer sites, a winter cover crop was grown in lieu of other more traditional break crops. Soils were collected before the final cash crop season in 2021, having been exposed to between 1-3 seasons of cover crops within the experimental period. Samples were analysed for a number of biogeochemical properties including soil carbon and its chemistry, microbial turnover of carbon and nutrients, and microbial community structure by 16S and ITS amplicon sequencing.
Across the 19 sites, the effect of cover crops could be classified as negative, neutral, or positive, and this roughly aligned with rainfall. Whilst bacterial community structure was mostly unaffected by the cover crop treatments, fungal community structure significantly shifted at eight of the 19 sites (Pperm ≤0.05). Interestingly, these treatment effects on fungal community structure occurred across the wide rainfall gradient, including the driest (Waikerie, SA) and wettest (Sisters Creek, Tas) sites. Given the wide range of results that were dependent upon climate x soil type context, we conclude that whilst summer cover cropping is most likely to be beneficial in higher rainfall zones, it can also impart positive influence on soil processes when adopted judiciously in lower rainfall areas.