Impacts of reducing fallow periods on indicators of soil function in subtropical dryland farming systems

Abstract

Seasonal fallows – periods with no living plant cover – are frequently implemented in dryland cropping systems to allow recharge of soil moisture for the subsequent cash crop. Over time, repeated fallow periods deplete soil organic matter, a foundation of soil health that affects the long-term functional capacity of soils. Reducing fallow periods by growing cover crops (non-harvested crops terminated prior to reproductive development), double cropping (increasing the frequency of harvested crops), or rotating to ley pasture for several years, were investigated for their potential to improve or maintain soil functional attributes. These ‘reduced fallow’ systems were compared against a conventional cereal-based production system with seasonal fallows as well as against a permanent fallow. Soil in the reduced fallow systems had higher concentrations of total nitrogen and mineral-associated nitrogen compared with the conventional system and permanent fallow. Reduced fallow systems also had higher concentrations of total organic carbon and mineral-associated carbon than the permanent fallow, but not the conventional system. These changes were associated with greater soil microbial respiration and improved surface soil moisture storage at cash crop planting. The conventional, cover crop and ley pasture systems had similar soil structure with greater abundance of mega-aggregates. In contrast, the double crop system had greater abundance of micro- and macro-aggregates as did the ley pasture systems that were subject to tillage after three years in preparation for a return to cropping. The ley pasture also had substantially greater numbers of root-lesion nematodes (Pratylenchus thornei), surpassing the 2000 nematodes kg-1 threshold for economic damage. The results demonstrate that different methods for reducing fallow periods can generate a range of outcomes that are not uniformly desirable in terms of soil functional attributes. Across the indicators assessed, the cover crop system generated the widest range of improvements in soil fertility and function compared with the conventional system. This suggests cover cropping may be a valuable tool for promoting soil health in subtropical dryland farming systems.