Shedding light on plant litter decomposition: Advances, implications and new directions in understanding the role of photodegradation

Litter decomposition contributes to one of the largest fluxes of carbon (C) in the terrestrial biosphere and is a primary control on nutrient cycling. The inability of models using climate and litter chemistry to predict decomposition in dry environments has stimulated investigation of non-traditional drivers of decomposition, including photodegradation, the abiotic decomposition of organic matter via exposure to solar radiation. Recent work in this developing field shows that photodegradation may substantially influence terrestrial C fluxes, including abiotic production of carbon dioxide, carbon monoxide and methane, especially in arid and semi-arid regions. Research has also produced contradictory results regarding controls on photodegradation. Here we summarize the state of knowledge about the role of photodegradation in litter decomposition and C cycling and investigate drivers of photodegradation across experiments using a meta-analysis. Overall, increasing litter exposure to solar radiation increased mass loss by 23% with large variation in photodegradation rates among and within ecosystems. This variation was tied to both litter and environmental characteristics. Photodegradation increased with litter C to nitrogen (N) ratio, but not with lignin content, suggesting that we do not yet fully understand the underlying mechanisms. Photodegradation also increased with factors that increased solar radiation exposure (latitude and litter area to mass ratio) and decreased with mean annual precipitation. The impact of photodegradation on C (and potentially N) cycling fundamentally reshapes our thinking of decomposition as a solely biological process and requires that we define the mechanisms driving photodegradation before we can accurately represent photodegradation in global C and N models. © 2012 US Government

The emission of volatile compounds from leaf litter

Leaf litter is available at the Earth’s surface in large quantities. During the decomposition of leaf litter, volatile compounds can be released into the atmosphere, where they potentially influence local air quality, atmospheric chemistry or the global climate. In this thesis the focus was on the emission of C2-C5 hydrocarbons, molecular hydrogen (H2), carbon monoxide (CO) and methyl chloride (CH3Cl) from leaf litter and the factors that control the emissions were investigated. For different plant species, the emission rates of several C2-C5 hydrocarbons increased with temperature between 20 and 100°C according to the Arrhenius relation. When leaf litter was irradiated with UV, the emission increased linearly with the intensity of the UV. UVB radiation was more efficient in the generation of hydrocarbons from leaf litter than UVA. A simple upscaling showed that C2–C5 hydrocarbon emissions from leaf litter are likely insignificant for their global budgets, but may have a small influence on atmospheric chemistry on the local scale. Senescent and dead plant material releases carbon monoxide (CO), methane and larger hydrocarbons upon heating or irradiation with UV, but emissions of hydrogen (H2) have not been reported. In this study, H2 was released from leaf litter of Sequoiadendron giganteum in detectable amounts at temperatures above 45°C, whereas CO was also emitted at ambient temperature. Leaf litter has been identified as a potentially important source of CH3Cl. However, the factors controlling the emissions are unclear. Laboratory experiments have been performed in which CH3Cl emissions were measured from leaf litter of different plant species. For each investigated plant species, the CH3Cl emission rate strongly increased with temperature according to the Arrhenius relation. However, at constant temperature, large differences between different plants were observed. Therefore, CH3Cl emissions were measured from halophyte leaf litter with a varying chloride content, but no significant correlation between the CH3Cl emission rate and the chloride content of the plant material was observed. A limited set of field experiments was performed in which CH3Cl emissions were measured. Leaf litter emitted CH3Cl, but only in periods with fresh leaf litter fall. Outside these periods, the flux from leaf litter was zero or even slightly negative. The CH3Cl emission rate increased with temperature, but the temperature increase did not follow the Arrhenius relation as was observed in the laboratory experiments. The global importance of leaf litter as a source of CH3Cl was investigated using the global chemistry transport model TM5. Forward simulations with different emission scenarios indicated that at station Trinidad Head (mid-latitudes of North America), a substantial seasonal emission from leaf litter was required to match the measured CH3Cl mixing ratios at this station. Inversions performed with the TM4-4D-Var system indicated that the main CH3Cl sources were located in the Tropics, whereas the mid- and high latitudes were only a minor source. Sensitivity studies performed to investigate the robustness of the optimized emissions indicated that more than 90% of the global net emissions was located in the Tropics

Emissions of H2 and CO from leaf litter of Sequoiadendron giganteum, and their dependence on UV radiation and temperature

Senescent and dead plant material releases carbon monoxide (CO), methane and higher hydrocarbons upon heating or irradiation with UV, but emissions of hydrogen (H2) have not been reported. This study investigated whether leaf litter is able to emit H2 and which factors control the possible emissions. In addition, the emission of CO from leaf litter was measured and compared to previous studies. H2 was released from leaf litter of sequoia (Sequoiadendron giganteum) in detectable amounts at temperatures above 45 °C, whereas CO was also emitted at ambient temperature. The emission rates of both H2 and CO increased with temperature according to the Arrhenius relation. UV radiation can induce emissions of both H2 and CO. However, UV induced H2 was only emitted under anoxic conditions, while CO emissions were higher in synthetic air, but strongly reduced in absence of oxygen

Methyl chloride and C2–C5 hydrocarbon emissions from dry leaf litter and their dependence on temperature

Emissions of methyl chloride and several C2–C5 hydrocarbons from dry leaf litter at temperatures in the range 20–100 °C are reported for different plant species. The emission rates of ethane, ethene, propane, propene, n-pentane and methyl chloride increased with temperature. Hydrocarbon emission rates up to 0.88 ng gdw−1 h−1 were measured at 20 °C, while methyl chloride emission rates between 0.03 and 0.85 ng gdw−1 h−1 were observed at this temperature. At 70 °C emission rates increased up to 650 ng gdw−1 h−1 for C2–C5 hydrocarbons and up to 18 μg gdw−1 h−1 for methyl chloride. The Arrhenius relation can be used to describe the temperature dependence of methyl chloride emissions, while for hydrocarbon emissions deviations from this relation were observed. The emissions were not due to enzymatic activity, which was indicated by emission rates that continuously increased with increasing temperature, and activation energies higher than 50 kJ mol−1. At constant temperature, the emission rate of both methyl chloride and hydrocarbons from dry leaf litter decreased in time. At high temperatures (80–100 °C) this was noticeable on a timescale of hours, while at low temperatures (20–30 °C) the decrease was very slow and only visible on a timescale of months. Emission of methyl chloride from leaf litter might be significant for its global budget, while temperature induced hydrocarbon emissions from leaf litter are likely insignificant