Microbial turnover of above and belowground litter components in shrublands

Shrublands cover a large proportion of the world’s land surface, yet they remain poorly studied in comparison to other ecosystems. Within shrublands, soil organic matter (SOM) is replenished from inputs of both above- and below-ground plant litter, however, their relative importance depends on their respective turnover rates. To critically address this, we measured the biodegradation rates of the soluble and insoluble components of 14C-labelled above- and below-ground plant litter in soil. During the 150 day incubation, the amount of plant-derived soluble-C lost as 14CO2 was similar for the different plant parts being 64.7 ± 2.3% for roots, 72.1 ± 7.4% for stems, and 72.4 ± 1.8% for leaves. In comparison, the turnover of the insoluble fraction was much slower. However, again little difference in mineralisation was seen for the different plant parts with the total losses being 21.1 ± 0.9% for roots, 19.5 ± 1.6% for stems, and 19.6 ± 1% for leaves. A double exponential first order kinetic model fitted well to the experimental data. It also allowed the partitioning of C between microbial anabolic and catabolic processes for the soluble C component. Using this model, we deduced that the soluble fraction turns over ca. 40 times annually, whereas it takes ca. 2.5 years to turnover the insoluble fraction. For the soluble plant component, the overall microbial carbon use efficiency (CUE) was estimated to be greater for root-derived C in comparison to that derived from aboveground (no difference was observed for the insoluble component). From this, we tentatively suggest that C sourced from belowground plant components may persist longer in soil than C derived from aboveground plant components.