Introduction of legume cover crops practice in intensive grain corn crop system to mitigate soil threats in the Mediterranean region

In Portugal, grain corn is the main cereal produced, comprising 56% of total cereal yield. It is grown in intensive monoculture cropping systems that may have negative effects on soil quality, affecting long-term fertility and productivity, and therefore the sustainability of production. A promising management practice to mitigate soil degradation is to grow a cover crop during the usual fallow period. This study examined in which extend six species of legume cover crops (forage pea (pisum sativum L), yellow lupin (lupinus luteus), crimson clover (trifolium incarnatum), balansa clover (trifolium michelianum), persian clover (trifolium suaveolens), and arrowleaf clover (trifolium vesiculosum) are suitable to mitigate soil threats in grain corn systems specifically in the Mediterranean region. Specific objectives were to identify the effectiveness of the legume 6 species in improving soil fertility (i.e., soil organic matter content), mitigating nutrient leaching, nutrient recycling, and weed control. The study was performed in the lower Mondego valley in central Portugal. It covered two autumn to spring periods of cover crop cultivation, and assessed changes in soil fertility, dry biomass yield of legumes and weeds, and their associated nutrient content (total nitrogen-phosphorus-potassium).

In general, the six legume cover crops (LCC) species showed good adaptation to Mediterranean conditions, yielding large amounts of biomass (up to 8 ton/ha for clovers species). At the short term, LCC incorporation into the soil had no clear effect in soil organic matter content. The median uptake of NPK macronutrients for all species was high respectively 176-20-172 kg/ha, due to their generally high biomass production, highlighting their great potential to mitigate nutrient leaching. The capacity of the LCC to provide green manure services enabled a median reduction of 40% of N, 60% of P, and 100% of K supplied by mineral fertilizers necessary to attain a corn grain yield of 12t/ha. LCC showed a good effectiveness in weeds control, although only in the second year of the study. Three clover species (crimson, balansa, arrowleaf) performed best in terms of weed control maintaining weed production below 0.5 ton/ha, vs 3-4 ton/ha in control plots, due to early establishment and/or high biomass production in later growth stages, and avoiding the first application of herbicide in pre-emergent herbicide for grain corn cultivation.