Influence of weed cover on leaf-level CO2 and H2O fluxes in an olive grove

The management of olive groves has a direct impact on the environment in the Mediterranean region since it is one of the most representative crops in this area. In order to prevent erosion and improve the physical-chemical conditions of the soil in these crops, the maintenance of weed cover in the alleys is an increasingly common practice. It increases the organic carbon content in the soil, improves biodiversity indices and enhances various ecosystem services such as pollination and infiltration. Now, the role of vegetation cover in olive groves on biogeochemical cycles is being studied. Although previous studies have quantified the combined effect of weed cover and olive trees on carbon and water at ecosystem level, the role of this conservation practice at the leaf level has not yet been explored.

The aim of this study is to quantify the effect of weed cover on the net CO₂ assimilation (A_n) and transpiration (T) rates in an irrigated olive grove. To do this, two plots of olive trees with irrigation (Olea europea L. "Arbequina") in southeast Spain were sampled. In the weed-cover one (WC), spontaneous vegetation is maintained until it is mechanically mowed and left in place. In the weed-free (WF) a glyphosate-based herbicide is applied. The data were taken with a portable gas analyzer (LI-6800, Li-Cor) controlling the following environmental variables on olive leaves: atmospheric CO₂, relative humidity, photosynthetic active radiation and temperature. One campaign per month was carried out (from January-2018 to January-2019) where 10 random trees were analysed in each treatment. In addition, an eddy covariance tower provided CO₂ and H₂O fluxes at ecosystem level and they were compared with the fluxes obtained from leaf-level campaigns.

The results shown significant differences for T only in the period after mowing with T_wc= 2.0 ± 0.7 mmol H₂O m^-2s^-1 vs T_wf = 2.5 ± 1.0 mmol H₂O m^-2s^-1. However, in this period ET is equal in both treatments, which suggests that the alleys with mowed weed has more ET than bare soil in the other treatment. On the other hand, there are significant differences for A_net only in the period before mowing with A_net-wc = 5.5 ± 3.1 μmol CO₂ m^-2s^-1 vs A_net-wf = 8.0 ± 3.6 μmol CO₂ m^-2s^-1. When the weeds are mowed, A_net is matched in both treatments. However, higher values of NEE_wc than NEE_wf  are observed in the period before mowing. This suggest that the weed-cover olive groves at ecosystem level take up more carbon when the weed-cover is established although the leaves of olive trees are capturing less CO₂.