Variations of soil δ13C and δ15N across a precipitation gradient in a savanna ecosystem: Implications of climate change on the carbon cycle

Abstract:

Savannas are the third largest terrestrial carbon pool after only tropical and boreal
forests. They are highly productive ecosystems and contribute about 30% of the global
terrestrial net primary productivity and potentially contain 20% of the world's soil
organic carbon. Global circulation models have predicted that many savannas will
become warmer and drier during the twenty-first century. The impacts of the projected
climatic trend on the productivity and biogeochemical cycles of savannas are not fully
understood. Here, we assessed the abundance of stable carbon (δ¹³C) and nitrogen (δ¹⁵N)
isotopes in soil profiles at four sites along a 1000 km transect with a strong south-north
precipitation gradient in southern Africa. The south receives about 180 mm of rainfall per
year and dominated by grass species (C₄) whereas the north receives 540 mm·yr-1 and
dominated by woody plants (C₃). Soil surface δ¹³C showed that woody vegetation contributed
more than 75% of soil carbon input in the wet sites whereas grasses contributed about
65% of soil carbon input in the dry sites. The soil profile δ¹³C indicated that intermediate
sites have shifted from grass dominated to woody-shrub-dominated states
during recent past. The dry sites had relatively higher δ¹⁵N (~10‰) compared to the wet
sites (~5‰) indicating significantly greater N₂ fixation in the wetter sites or high rates of
NH₃ volatilization in the drier sites. Our results suggest that as savannas become warmer
and drier due to climate change, woody shrubs are likely to be the dominant form of
vegetation structure, a process that could alter biogeochemical processes and results in
savannas becoming net carbon sink or source.