Foliar nitrogen dynamics of an invasive legume compared to native non-legumes in fynbos riparian zones varying in water availability

Abstract

The legume Acacia mearnsii invades South Africa’s fynbos riparian zones and may alter the nitrogen (N) dynamics and supply in these areas that typically support few native N fixers. Nitrogen uptake by A. mearnsii may also be influenced by water availability, potentially affecting riparian-specific performance and impact estimations. We expected to find functional differences between the invasive legume and the two co-occurring but non-leguminous native species Brabejum stellatifolium and Metrosideros angustifolia. We also wanted to examine whether in-situ water availability affected N source or uptake in the invasive species. We found A. mearnsii was indeed functioning differently from non-N-fixing native species, and had considerably higher foliar %N. Interestingly, ¹⁵N abundance and uptake were associated with site hydrology, meaning water availability should be scrutinised when assuming N-fixing in A. mearnsii using δ¹⁵N. Nonetheless, higher water availability to A. mearnsii in fynbos riparian ecosystems did increase foliar N uptake. This has implications for prioritizing clearing of sites with increased nutrient deposition, such as dense stands in relatively moist riparian zones.

Keywords: δ¹⁵N, Acacia mearnsii, Black Wattle, South Africa, stable isotopes, water stress