Weed seedbank dynamics in Mediterranean organic horticulture

- Weed dynamics in organic cropping systems is crucial for the agronomic sustainability.
- The weed seed bank in open fields is very high but showed low emergence rate.
- In the greenhouse, the lower seed bank shows a high emergence rate due to agronomic and climatic reasons.
- The higher weed emergence rate of the less biodiverse weed community found in the greenhouse requires a more timely control.
- Weed seed bank analysis could become an useful agronomic tool in improving the agronomic sustainability of organic vegetable productions.

Managing weeds in organic cropping systems is notoriously difficult. We analyzed the seed bank dynamics in vegetable crop rotations carried out in both the typical Mediterranean scenarios of the open field and greenhouse. The lower seed bank detected in the greenhouse (about 53,000 seeds m−2) showed a higher emergence rate than the seed bank found in the open field, which was almost twice the size. This higher emergence rate was due both to: i) the more favorable temperature in the greenhouse, and ii) the less depth-mediated soil-imposed dormancy, due to the seed bank being at a lower depth as a result of minimum tillage. A strong infestation of Cyperus rotundus was found in the greenhouse again, due to the tuber arrangement in the shallowest soil layer (0-10 cm). The ubiquitous and abundant Amaranthus retroflexus, taken as a model to investigate the dormancy-status of exhumed seeds showed that the lower emergence rate in the open field, was not due to dormancy but exclusively to depth-mediated soil inhibition. In addition, the calculation of various indexes of the weed seed bank botanical composition highlighted that the more quantitatively abundant greenhouse weed community was formed by more graminoid species (poaceae and cyperaceae), perennial species, C4 photosynthetic pathway species and by lower biodiversity. Lastly agronomic strategies are proposed which could improve the weed control sustainability in organic cropping systems.