Do biopores created by perennial fodder crops improve the growth of subsequent annual crops? A synthesis of multiple field experiments

The subsoil contains valuable nutrient and water resources for crop production, but root growth and therefore resource access can be impeded by compact soil layers. Large-sized biopores formed by perennial taprooted crop species with large root diameters and deep-rooting habit such as chicory and lucerne or anecic earthworms can provide pathways through compacted subsoil layers. Following annual crops may derive benefits from improved root growth, water and nutrient uptake. Our study aimed to explore the extent and mechanisms of growth and yield benefits to annual crops from perennial precrops in long-term crop sequences. In five field experiments over 14 years we compared the effect of three precrops, fibrous rooted fescue, taprooted chicory and taprooted legume lucerne, that were cultivated for one, two and three year phases, on soil conditions and crop growth of a sequence of subsequent annual crops. Rooting depth and density as well as growth and yield of subsequent annual crops were monitored for evidence of improved performance related to increased subsoil macroporosity. We found that taprooted chicory increased root growth only of following spring crops in the subsoil during dry years. This increased root growth after chicory only benefitted yields of subsequent spring crops in dry years. There were no significant benefits for yields during wet years or long-term effects that could be related to biopores. Lucerne precrops increased plant available nitrogen to depths of 1 m for two years and significantly increased yield, nitrogen, phosphorus, and potassium uptake for up to 3 years. Our findings highlight the benefits of perennial fodder legumes such as lucerne for a crop sequence, as they improve nitrogen availability in the soils and can significantly benefit yields for multiple years in a row. Benefits on yields of main crops grown in biopore rich conditions seem to be strongly limited to dry and compacted soils with high penetration resistance that are also dependent on climatic conditions and the main crops root architecture. Our findings highlight the potentials of adding perennial fodder legumes to a crop sequence and improve our understanding of precrop effects on crop sequences.