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.

Responses of terrestrial ecosystems to climate change have been explored in many regions worldwide. While continued drying and warming may alter process rates and deteriorate the state and performance of ecosystems, it could also lead to more fundamental changes in the mechanisms governing ecosystem functioning. Here, we argue that climate change will induce unprecedented shifts in these mechanisms in historically wetter climatic zones, towards mechanisms currently prevalent in dry regions, which we refer to as "dryland mechanisms". We discuss twelve dryland mechanisms affecting multiple processes of ecosystem functioning, including vegetation development, water flow, energy budget, carbon and nutrient cycling, plant production and organic matter decomposition. We then examine mostly rare examples of the operation of these mechanisms in non-dryland regions where they have been considered insignificant at present. Current and future climate trends could force microclimatic conditions across thresholds and lead to the emergence of dryland mechanisms and their increasing control over ecosystem functioning in many biomes on Earth.

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Decomposition of organic matter in semi‐arid ecosystems is a key component of the terrestrial carbon (C) cycle. The well‐known inaccuracies in predicting litter decay in water‐limited regions were lessened by considering solar radiation as an abiotic decay driver of photodegradation. Moreover, exposure to high solar irradiance in dry periods often led to massive facilitation of litter decay in subsequent wet periods (“photoacceleration”), though in many studies this effect was absent. Recently, water vapour and dew were identified as modulators enabling substantial microbial degradation during rainless periods. Here, we investigated, (1) whether the activity of micro‐organisms modifies litter traits, such as litter quality and microbial community in dry periods, consequently altering the loss of litter mass and nitrogen (N) in wet periods, and (2) whether it can co‐occur with photoacceleration. By successively introducing litter to the field at the beginning and the end of the dry season, we found that microbial activity during the dry season affected litter mass and N loss during the wet season. Low microbial activity in the dry season led to inhibition of mass loss in the wet season, while high microbial activity led to facilitation of mass loss. Microbial activity during the dry season also caused strong inhibition of N loss from litter during the wet season, likely by enhancing the dry‐season N loss. A microclimate manipulation experiment using radiation filters showed that microbial activity and exposure to solar radiation jointly modified the litter during the dry season and affected subsequent decay in the wet season. Knowledge of biotic and abiotic modifications of litter during dry periods and their implication for wet periods enhances our understanding of litter decay in semi‐arid regions. Furthermore, it can improve biogeochemical model predictions of C and N cycling in drylands and in the many regions that are projected to experience a drier climate during the coming decades. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13018/suppinfo is available for this article.