Does the timing of litter inputs determine natural abundance of ¹³C in soil organic matter? Insights from an African tiger bush ecosystem

Abstract

We investigated total primary production and natural abundance of ¹³C in soil and plants in the landscape of tiger bush, Niger. Tiger bush is viewed as a natural cyclic succession of several types of vegetation (grasses, living trees and senescent vegetation) occurring over very small areas, on soils with similar chemical and physical characteristics. Under the pioneer front, production was 130 g m^–2 year^–1 of which 23% came from C₄ plants; under the thicket of mature trees, grass production was 190 g m^–2 year^–1 (all C₃ grasses) and under senescent vegetation, 40 g m^–2 year^–1 of which 1.5% came from C₄ plants. Total above- and belowground primary production was estimated to be 890–4880 g m^–2 year^–1 of which 0.4–0.5% was contributed by C₄ plants. From 29 to 45% of the soil organic carbon originated from C₄ plants even though the contribution of C₄ grasses to total primary production did not exceed 0.5%. We suggest that the order in which the different sources of organic matter entered the soil could lead to the overlabelling of soil organic matter with a C₄ print. Because all C₄ plants are grasses located in the pioneer front of tiger bush bands, their C₄ organic matter enters the soil first and fixes onto clays. The C₃ organic matter enters the soil several years later and is also fixed by the clays but in a lower proportion. Therefore it is less protected from microbial activity and quickly decomposes. We postulate that the repetition of this pattern over many decades (incorporation of a pure C₄ material to soil, followed by the incorporation of a C₃-dominated material), leads to the overaccumulation of C₄ compounds on the most protective sites.