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Drying and rewetting conditions differentially affect the mineralization of fresh plant litter and extant soil organic matter

Lopez-Sangil, Luis, Hartley, Iain P., Rovira, Pere, Casals, Pere, Sayer, Emma J.
Soil biology & biochemistry 2018 v.124 pp. 81-89
carbon, carbon dioxide, drought, drying, field experimentation, microbial biomass, mineralization, plant litter, soil microorganisms, soil organic matter, solutes, temperature, terrestrial ecosystems, topsoil, wheat straw
Drought is becoming more common globally and has the potential to alter patterns of soil carbon (C) storage in terrestrial ecosystems. After an extended dry period, a pulse of soil CO₂ release is commonly observed upon rewetting (the so-called ‘Birch effect’), the magnitude of which depends on soil rewetting frequency. But the source and implications of this CO₂ efflux are unclear. We used a mesocosm field experiment to subject agricultural topsoil to two distinct drying and rewetting frequencies, measuring Birch effects (as 3-day cumulative CO₂ efflux upon rewetting) and the overall CO₂ efflux over the entire drying-rewetting cycle. We used ¹⁴C-labelled wheat straw to determine the contribution of fresh (recently incorporated) plant litter or extant soil organic matter (SOM) to these fluxes, and assessed the extent to which the amount of soil microbial biomass + K₂SO₄-extractable organic C (fumigated-extracted C, FEC) before rewetting determined the magnitude of Birch effect CO₂ pulses. Our results showed a gradual increase in SOM-derived organic solutes within the FEC fraction, and a decrease in soil microbial biomass, under more extreme drying and rewetting conditions. But, contrary to our hypothesis, pre-wetting levels of FEC were not related to the magnitude of the Birch effects. In the longer term, rewetting frequency and temperature influenced the overall (31-day cumulative) amount of CO₂–C released from SOM upon rewetting, but the overall ¹⁴CO₂–C respired from fresh straw was only influenced by the rewetting frequency, with no effect of seasonal temperature differences of ∼15 °C. We conclude that the mineralization of fresh plant litter in soils is more sensitive to water limitations than extant SOM in soils under drying-rewetting conditions. Moreover, we found little evidence to support the hypothesis that the availability of microbial and soluble organic C before rewetting determined the magnitude of the Birch effects, and suggest that future work should investigate whether these short-term CO₂ pulses are predominantly derived from substrate-supply mechanisms resulting from the disruption of the soil organo-mineral matrix.