Nutrient dynamics during decomposition of the residues from a sown legume or ruderal plant cover in an olive oil orchard

Spanish olive oil groves are undergoing a marked change in the way that inter-row land is managed. The current recommendation encourages the use of plant cover to increase plant residue input to the soil to improve fertility and reduce erosion. However, there is no quantitative information on the temporal trend and magnitude of nutrient release during decomposition of plant cover residues after the annual topping of vegetation. Decomposition rates and nutrient dynamics (C, N, P and K) were examined for aboveground residues from two types of plant cover (a sown legume and ruderal plant species) and in fine roots. Litterbag experiments were designed to evaluate the effects of the type and location of plant residues (above- or belowground) by placing litterbags of aboveground plant residues on the soil surface or within the soil, which were sampled over a whole year. The highest decomposition rates for above- or belowground residues were found in spring, and were higher for buried plant residues, regardless of plant cover type. After one year, the remaining C, K and P in the soil was about 30%, 20% and 30% of that added, respectively and therefore plant cover could be a useful strategy to improve C sequestration and increase soil nutrient content in olive groves. Decomposition of plant residues left on the soil surface immobilised N, whereas this was not the case when they were buried. The remaining C, N, P and K content in belowground residues was similar to aboveground samples with around 21%, 27%, 23% and 15%, respectively. This study highlighted the importance of plant cover for retaining nutrients when tree demand was low, but releasing a significant proportion of the nutrients in early spring when tree demand was high, especially when residues were incorporated into the soil.