Biotic degradation at night, abiotic degradation at day: positive feedbacks on litter decomposition in drylands

Abstract

The arid and semi-arid drylands of the world are increasingly recognized for their role in the terrestrial net carbon dioxide (CO ) uptake, which depends largely on plant litter decomposition and the subsequent release of CO back to the atmosphere. Observed decomposition rates in drylands are higher than predictions by biogeochemical models, which are traditionally based on microbial (biotic) degradation enabled by precipitation as the main mechanism of litter decomposition. Consequently, recent research in drylands has focused on abiotic mechanisms, mainly photochemical and thermal degradation, but they only partly explain litter decomposition under dry conditions, suggesting the operation of an additional mechanism. Here we show that in the absence of precipitation, absorption of dew and water vapor by litter in the field enables microbial degradation at night. By experimentally manipulating solar irradiance and nighttime air humidity, we estimated that most of the litter CO efflux and decay occurring in the dry season was due to nighttime microbial degradation, with considerable additional contributions from photochemical and thermal degradation during the daytime. In a complementary study, at three sites across the Mediterranean Basin, litter CO efflux was largely explained by litter moisture driving microbial degradation and ultraviolet radiation driving photodegradation. We further observed mutual enhancement of microbial activity and photodegradation at a daily scale. Identifying the interplay of decay mechanisms enhances our understanding of carbon turnover in drylands, which should improve the predictions of the long-term trend of global carbon sequestration.

Wood decomposition is more rapid on than off termite mounds in an African savanna

“…In addition, moisture is an important requirement for decomposition in dry environments (Gliksmanet al. ). The strong sun and dry conditions in our open sites likely impede the microbial process that would facilitate decomposition.…”

“…We expected decomposition rates to be higher in the shaded microhabitat because of enhanced moisture availability (Gliksman et al. ). (4) Does wood density influence decomposition?…”

Modelling the diurnal and seasonal dynamics of soil CO<sub>2</sub> exchange in a semiarid ecosystem with high plant–interspace heterogeneity

“…Hence, the contribution of photodegradation to soil C balance could be greater than our estimation at the ecosystem level (see, e.g. Gliksman et al, 2016). Although the contribution of surface exchanges was only marginal as compared to the annual CO 2 efflux, removing the biocrust processes would substantially reduce the model validity.…”

“…Photodegradation is likely to dominate the mineralization during dry daytime periods, when radiation is strong and microbial activities are prohibited by low moisture content and high temperatures (e.g. Gliksman et al, 2016). On an annual basis, photodegradation could consume more than 10 % of soil organic matter (SOM) at the surface (e.g.…”

The Response of Soil CO2 Efflux to Water Limitation Is Not Merely a Climatic Issue: The Role of Substrate Availability

“…During summer in SDS, the Cistus plant community, unable to access deep water, moved to a dormant status in order to maintain turgor. This caused a reduction in photosynthates and substrate availability; however, the observed summer depression of SR was not due to a modification of SR h but to the suppression of autotrophic respiration, as already similarly observed in other ecosystem types under drought [13,58,59], and SR h accounted for almost 100% of SR. Concurrently, a significant increase in WEOC was observed, likely because of the high root mortality occurring in summer in Mediterranean ecosystems [60,61] and the simultaneous photodegradation of surface litter at high irradiance [62,63], which both release high amounts of C into the soil. At the same time, a change in the microbial community assemblage could be detected.…”