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

We investigated total primary production and natural abundance of C-13 in s oil and plants in the landscape of tiger bush, Niger. Tiger bush is viewed as a natural cyclic succession of several types of vegetation (grasses, liv ing trees and senescent vegetation) occurring over very small areas, on soi ls with similar chemical and physical characteristics. Under the pioneer fr ont, production was 130 g m(-2) year(-1) of which 23% came from C-4 plants; under the thicket of mature trees,grass production was 190 g m(-2) year(-1 ) (all C-3 grasses) and under senescent vegetation, 40 g m(-2) year(-1) of which 1.5% came from C-4 plants. Total above- and belowground primary produ ction was estimated to be 890-4880 g m(-2) year(-1) of which 0.4-0.5% was c ontributed by C-4 plants. From 29 to 45% of the soil organic carbon origina ted from C-4 plants even though the contribution of C-4 grasses to total pr imary production did not exceed 0.5%. We suggest that the order in which th e different sources of organic matter entered the soil could lead to the ov erlabelling of soil organic matter with a C-4 print. Because all C-4 plants are grasses located in the pioneer front of tiger bush bands, their C-4 or ganic matter enters the soil first and fixes onto clays. The C-3 organic ma tter 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 activ ity and quickly decomposes. We postulate that the repetition of this patter n over many decades (incorporation of a pure C-4 material to soil, followed by the incorporation of a C-3-dominated material), leads to the overaccumu lation of C-4 compounds on the most protective sites.