DTA

Dottorato

VIRILI, ALESSANDRA

etd-09292022-161713

Investigating weed-crop interactions and potential weed-mediated agro-ecosystem services in horticulture

AGR/02

Istituto di Scienze della Vita - PHD IN AGROBIODIVERSITY

relatore Dott.ssa MOONEN, ANNA CAMILLA

• Agro-Ecosystem Services
• Brassica oleracea (L.)
• Functional Traits
• Lactuca sativa (L.)
• Solanum lycopersicum (L.)
• Weed-crop competition

11/11/2022;

parziale

Agricultural landscapes have been managed for decades to maximize productivity, but in recent years the priority has shifted to promote more sustainable agricultural practices, which focus on supporting other Agro-Ecosystem Services (AES) aside from providing food, feed and fiber. With respect to weed and pest management, modern agricultural systems are facing a higher threat from weeds compared to the start of the Green Revolution, because of the continuous use of few solution-based tools and cropping cycles which have selected for species-poor communities dominated by competitive weeds. On the other hand, complex agro-ecosystems are associated with diverse and more balanced weed communities which pose a lower threat to crop yield. Promoting non-crop biodiversity in agro-ecosystems serves important conservation purposes and can decrease reliance on external inputs. In fact, many weed species offer multiple AES in both annual and perennial cropping systems such as supporting beneficial arthropods and local fauna, or regulating carbon sequestration, soil erosion and nutrient leaching. Most of the literature investigating both crop-weed interactions and the effects of increased biodiversity in annual cropping systems has mainly focussed on cereal crops. It is thus necessary to investigate whether these findings are applicable in other agricultural sectors, such as vegetable cropping systems. It is common belief that vegetable crops tolerate poorly weed pressure and the ephemeral nature of these systems has led to a scarce research interest in investigating crop-weed interactions and agroecological practices in horticulture.
This PhD project aimed to provide more knowledge on weed-crop interactions in horticulture, by quantifying the trade-off related to increasing biodiversity in these systems. To achieve this goal, the trials focussed on three focal crops (lettuce, cabbage and tomato) and two weed-mediated AES (support of beneficial arthropods and reduction of nutrient leaching). This research focussed mainly on determining whether the effects associated with higher weed abundance were related to the species richness or to the functional composition of the weed community. The necessary weeding practices combined with the short growing cycle of many vegetable crops imply that positive species may not be able to express traits related to the AES of interest. Considerations are thus necessary regarding the most efficient way to increase biodiversity in vegetable cropping systems, stemming from the land-sharing vs land-sparing debate.
The research was carried out for three years, in the province of Pisa (Italy). The methodology was based on experimental trials, at both mesocosm and field level, including a case study in an organic farm in the province of Livorno. The thesis also uses a functional trait approach which allows to analyse the weed community based on the species’ characteristics linked to a specific (dis-)service, going beyond simple taxonomic classification.
The trade-off analysis concerning weed-mediated support of beneficial insects (pest antagonists, pollinators) and weed-crop competition was carried out through two trials, using lettuce (Lactuca sativa, var. Romana) as the focal crop. A greenhouse pot trial was conducted to investigate whether weed diversity could: 1) mitigate the competitiveness of individual weed species and 2) decrease successful landings and colonization of aphids on lettuce. In this case, lettuce was grown with three common model weeds which expressed common traits found in arable weed communities (Sinapis alba, Lolium multiflorum and Malva moschata). We were able to show that weed diversity was able to mitigate the effect of the most competitive weed (Sinapis alba). Despite this, no linear relationship was found between weed diversity and the competitive balance index of lettuce. The results indicate that, on one hand, the ability of single weed species to compete with other weeds and the crop determines the extent to which increased biodiversity mitigates yield loss. On the other hand the findings were promising as they partly confirmed results found in cereal crops. Unfortunately, the COVID-19 pandemic did not allow to test the hypothesis regarding appropriate landings.
The second trial investigated whether the weed community which developed after the critical weed-free period in open-field romaine lettuce could support BPC. We found that weeding was necessary for the first 20 days after crop transplanting to avoid unacceptable yield losses. From the perspective of BPC support, the weed community left in the field after this critical period could not guarantee the service. We concluded that field margins would be a more efficient option to deliver BPC. Thus, we observed the relationship between spontaneous field margins and arthropod group abundances in an organic tomato (Solanum lycopersicum, L.) field. In this case, the specific composition of the naturally regenerated field margins only supported pest abundance. Findings from the two lettuce trials and the tomato case study confirm that the capacity of weeds to support BPC is determined by community composition rather than diversity.
The second trade-off analysis investigated whether the weed community which developed after the critical weed-free period in cabbage (Brassica oleracea, var. capitata) could reduce nutrient leaching. To answer this question we set up a lysimeter trial in three fall and two spring cropping seasons. In this case, the service was expected to depend only on increased ground cover. Nitrate (NO3-), phosphorus and potassium leaching were sampled during the growing season and analyzed. Weeds left after the critical weed-free period did not significantly reduce nutrient leaching compared to the weed-free plots. A significant reduction of NO3- and K was registered in the drained waters from the weedy control plots compared to the weed-free ones, in one fall and one spring cropping season. Despite this, significant yield losses were registered in the weedy control plots and even in plots left weed-free for the first 20 days after crop transplanting in three out of five seasons.
Results from this research suggest that minimal necessary weed management in vegetable crops could not sustain the provision of the two weed-mediated AES of interest. Reduced weed management alone was not enough to allow for the development of a functional weed community to sustain beneficial insects during the time in which the crops were grown. Different time of the year should be considered to further investigate this topic. Other Integrated Weed Management (IWM) and more efficient input strategies should also be investigated in these systems to promote the development of functionally diverse weed communities. Results from this thesis favor a land sparing approach for biodiversity conservation in vegetable cropping systems. Despite this, relying on naturalized margins without knowledge on species composition can be detrimental, as proven by the farm case study that not all biodiversity is beneficial. In fact, margins dominated by grasses such as Sorghum spp., Avena spp., or Bromus spp. have been shown to be associated more to phytophagous insects, offering no support to beneficial arthropods.

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