Effect of crop rotation diversity and windrow burning of residue on soil chemical composition under long-term no-tillage

Abstract The key components of conservation agriculture are diverse crop rotation, no-tillage and full residue retention. Contrary to this, rotations in Western Australia’s Wheatbelt are dominated by cereals and over 50 % of growers tackle herbicide resistant weed species with narrow windrow burning. This a form of harvest weed seed control, which reduces residue amounts. Therefore, it is important to understand the long term effects of current no-tillage practices on key soil constituents. This 12 year study examined the effect of crop rotation diversity and residue management through windrow burning on soil organic carbon, total nitrogen, phosphorus, potassium, sulphur, zinc and copper. Five rotations/treatments were compared: ‘cereal’ (cereal–cereal–cereal), ‘diverse’ (cereal–legume–canola), ‘farmer’ (cereal–cereal–break), ‘mono’ (monoculture wheat) and ‘pasture’. Crop residue comparisons included residue spread behind the harvester (retained) or windrowed at harvest and burnt before seeding (windrow burnt). After 2013, light tillage was applied to half the farmer rotation. By the end of the experiment, soil organic carbon stocks (0–30 cm) were significantly lower in the farmer rotation compared with the cereal and diverse rotations. There was no significant difference between the cereal and diverse rotations, therefore, the first hypothesis of higher organic carbon with greater diversity was not supported. This was likely because sufficient fertiliser was applied and biomass produced. There were no significant differences between rotations for soil total nitrogen by the end of the experiment. Overall, rotational diversity had little effect on soil organic carbon and total nitrogen on these relatively fertile soils, except where fallow was included in the farmer rotation. The annual burning of windrows for weed seed control was found to reduce soil organic carbon stocks and it would be prudent to only windrow burn when a build-up of weeds is observed, or use methods of harvest weed seed control that do not involve burning. Light tillage in the farmer rotation had no effect on soil organic carbon or total nitrogen. Soil potassium concentration was significantly higher in the pasture and diverse rotations compared with the farmer rotation and wheat monoculture, which declined in the last few years. The higher concentrations were thought to be due to the redistribution of potassium from deeper in the profile to the surface layers, particularly by legume crops and canola.