Potential effects of sowed, biodiverse pastures in the understory of an LTER Montado system: 15N and 13C in Cork Oak leaves

Background/Question/Methods: Improved pastures have been introduced in many areas of the world as means to increase fodder productivity and sustain heavier grazing pressure. In Portugal, a unique version of improved pastures – The Sown Biodiverse Permanent Pastures, Rich in Legumes (SBPPRL). A particularity of improved pastures in Portugal is their implementation not only in grassland/pasture fields on itself but in multidimensional agro-silvo-pastoral systems such as the characteristic Montado - a cork oak/holm oak savannah-type ecosystem traditionally managed for many centuries. The introduction of these new pasture types has been done under the claim of increased, long-term carbon sequestration. However, many factors can contribute to the overall ecosystem carbon balance. The interaction of carbon cycling with other biogeochemical cycles (water, nutrients), especially at the tree level, could be of great importance in these ecosystems.

Results/Conclusions: Here we present the results from a short-term study of foliar ¹⁵δN and ¹³δC values from 20 cork oak trees in the context of other long-term variables monitored at the site. ¹³δC ranged from-30.21 to -27.36, with an average value of -28.74 (± 0.12) and no significant differences found between pasture types (natural vs. improved) or time (spring vs. summer). Foliar ¹⁵δN on the other hand showed statistically significant differences between cork oaks in different pasture types (-2.96±0.09 natural vs. -2.21±0.17 improved pastures, t-test, p ≤ 0.05), but no differences across time points. The difference between pasture types reflects a ¹⁵δN signature closer to zero, consistent with a higher percentage of legumes (and N₂ fixation) in the system. Despite the clear signature influence of legume-fixed N in oak trees growing on the improved pasture plot, this difference was not translated into foliar N content (overall average of 1.43 ±0.02) suggesting that trees may not be taking advantage of higher levels of N present in the soil.