Exudate dynamics and rhizosphere priming in wetland ecosystems

The release of organic substances from roots (exudates) to the soil system can induce changes in the mineralization rate of soil organic carbon (SOC) via so-called priming effects. Compared to other terrestrial ecosystems, mechanistic knowledge about priming effects in anoxic wetland soils is scarce, and few studies have investigated the composition and magnitude of root exudation in wetland plants. Given the disproportionate role of wetlands in the global soil carbon budget, this represents a critical knowledge gap in our understanding of terrestrial soil-climate feedbacks.

Here we present data from (1) a meta-analysis to summarize all quantitative and qualitative observations on wetland root exudation; and (2) exudate-surrogate incubation experiments testing for exudate effects on wetland SOC decomposition under anoxic conditions.

The meta-analysis shows that few comparable data on wetland exudation rates exist because extraction methods differ strongly, and only few species have been evaluated frequently. The data demonstrate that wetland plants not only release sugars, amino acids and organic acids into the rhizosphere, but also secondary compounds with a high allelochemical (e.g. gallic acid) or decomposition-hampering potential (e.g. phenolics). Their combined effect on the stability of soil carbon stocks is currently unpredictable on the ecosystem level. Our incubation experiments show that labile C inputs into an anoxic soil have a great potential to suppress SOC decomposition via negative priming. This finding contrasts to positive priming effects commonly found in oxic terrestrial soils and yields important implications for the stability of wetland SOC stocks in response to climate induced vegetation shifts.