The Shola-Grassland ecosystem of the Nilgiris, a part of the Western Ghats biodiversity hotspot, India, is a mosaic of cloud forests (called Shola) and grasslands. These grasslands have low soil Nitrogen (N), but are being invaded by woody N-fixing species. We found that invasion increases soil inorganic N availability, particularly the soil nitrate concentration (as compared to ammonium). Since nitrate is more mobile as compared to ammonium, it can be taken up more easily for plant growth (including by the invasives) and can flow out in great quantities during heavy rainfall. We also found that the activity of N-rich phosphatase enzymes was higher in invaded soils, potentially giving the invasives greater access to phosphorus too. Further, nitrate, phosphate, and sediment outflow in streamflow is higher in invaded catchments, particularly in the beginning of the monsoons and during extreme rainfall events, which are becoming more frequent in the Nilgiris. Finally, invasion has a negative impact on the native grassland vegetation diversity, with the understorey of the invasives being dominated by few fast-growing native and exotic grasses.

The Forest Department is attempting to clear the invasives. However, invasives are back in many of the cleared patches. We suggest that restoration efforts need to explore soil N management strategies (e.g. addition of carbon substrate), and re-introduction of native grass species in cleared areas. We also outline a framework that helps identify urgent research question that can help guide the restoration of the Nilgiri grasslands, as well as other forest-grassland mosaics around the world.