Biogeochemical consequences of grassland degradation on linked soil, stream, and lake ecosystems in watersheds: A short review

Grasslands are among one of the largest terrestrial biomes in the world, suffering severe threat from ongoing degradation. Grassland degradation not only alter terrestrial biogeochemical processes but also have strong potentials to affect nutrient stoichiometry and limitation in aquatic ecosystems. Moreover, the impacts of grassland degradation are likely to differentially affect the cycling of carbon (C), nitrogen (N), and phosphorus (P) in the linked terrestrial-aquatic ecosystems in watersheds, leading to multiple possible scenarios of nutrient stoichiometry and limitation in soil, stream, and lake ecosystems. However, we lack of integrated studies to consider biogeochemical consequences of grassland degradation transferring from soil to stream, and finally to the lake ecosystems. An integrated case study was conducted in the Qinghai Lake Watershed on the Qinghai-Tibet Plateau to reveal the influences of grassland degradation on soil, stream, and lake ecosystems from the perspectives of C:N:P stoichiometry , nutrient limitation, and bacterial communities. This case study demonstrated a clear cascading influence from terrestrial to aquatic ecosystem, that grassland degradation differentially influenced N and P in soil with soil N decreased but P did not change (N:P decreased in soil), influencing the nutrients delivered to streams with streamwater N decreased but P increased (N:P decreased in stream), alleviating P limitation of stream biofilms which are sensitive to P variation, and ultimately stimulating phytoplankton growth in the P-limited Qinghai Lake. This case study supported one scenario of biogeochemical consequences of grassland degradation, and other scenarios might be also possible in watersheds that have different geochemistry background, different landcover, and/or different lake trophic status. Integrated studies considering the coupled terrestrial-aquatic ecosystems can provide insights for protection and management of grassland watersheds in the rapidly changing world.