A novel source of atmospheric H 2 : abiotic degradation of organic material

Introduction Conclusions References


Introduction
Atmospheric H 2 is one of the most abundant reduced gases in the atmosphere, with a seasonally varying dynamic equilibrium of approximately 530 ppb.The known sources of atmospheric H 2 are photochemical oxidation of methane and non-methane hydrocarbons (40 ± 16 Tg yr −1 ), biomass burning (16 ± 5 Tg yr −1 ), fossil fuel burning (15 ± 10 Tg yr −1 ), N fixation (3 ± 1 Tg yr −1 ), and ocean degassing (3 ± 2 Tg yr −1 ) (Novelli et al., 1999).Once emitted to the atmosphere, H 2 is either oxidized by OH (19 ± 5 Tg yr −1 ) or consumed through microbially-mediated soil uptake (56 ± 41 Tg yr −1 ), thus maintaining a seasonally dependant dynamic equilibrium in the troposphere (Novelli et al., 1999).H 2 is not considered a direct greenhouse gas species.However, it is considered an indirect greenhouse gas because its OH mediated oxidation reaction reduces the amount of OH available for reaction with CH 4 and oxidation of H 2 in the stratosphere produces H 2 O.Among the known source-sink dynamics at the soil-atmosphere interface, the dominant pathway for loss of H 2 from the atmosphere is via microbially-mediated soil uptake (Novelli et al., 1999;Ehhalt and Rohrer, 2009) although the magnitude of this loss is still regarded as highly uncertain (Constant et al., 2009;Ehhalt and Rohrer, 2009).
Recent studies suggest that photodegradation of plant litter and soil organic matter can be an important mechanism of decomposition in ecosystems with a high solar radiation load and low precipitation (Austin and Vivanco, 2006;Rutledge et al., 2010).Much of the mass loss from litter photodegradation appears to be from CO 2 release (Brandt et al., 2009;Lee et al., 2012;Rutledge et al., 2010), but release of various other gases has also been documented, including CO (Lee et al., 2012;Schade et al., 1999;Tarr et al., 1995;Derendorp et al., 2011c), CH 4 (Lee et al., 2012;Vigano et al., 2008;Bruhn et al., 2009;Keppler et al., 2006), CH 3 Cl (Derendorp et al., 2011b(Derendorp et al., , 2012;;Hamilton et al., 2003), C 2 -C 5 hydrocarbons (Derendorp et al., 2011a, b), and H 2 (Derendorp et al., 2011c).In addition, several studies have reported small but significant C, N, and H-based trace gas release from organic matter in the absence of solar radiation (Conrad and Seiler, 1985;Lee et al., 2012;McCalley and Sparks, 2009;Tarr et al., 1995;Vigano et al., 2008;Derendorp et al., 2011c), with strong positive correlations between the rate of gas release and temperature.These studies suggest that abiotic degradation of organic matter may occur not only from photodegradation, but also from thermal degradation processes at relatively low temperatures (< 100 • C) that are well below the ignition point.
The mechanisms driving abiotic trace gas production from organic matter are still poorly understood.Proposed mechanisms include photochemical oxidation of organic compounds (Armstrong et al., 1966;Miller and Zepp, 1995;Valentine and Zepp, 1993) and direct cleavage of chemical groups by radiative energy absorption (Keppler et al., 2008;Schade et al., 1999;Tarr et al., 1995;Vigano et al., 2008).Among these, CH 4 release via direct cleavage of methoxyl groups abundant in pectin and lignin from live and dead plant material was recently documented (Keppler et al., 2008;Vigano et al., 2008).Based on these observations, CO and CO 2 release during abiotic degradation of plant material have been proposed to result from direct cleavage of carbonyl and carboxyl groups, respectively (Lee et al., 2012;Tarr et al., 1995).Some support for this methoxyl groups as a source of H 2 is found in a recent wood burning study (Röckmann et al., 2010), where, relative to water, the isotopic composition of bulk biomass was slightly depleted while it was strongly depleted in both methoxyl groups and H 2 .This isotopic fractionation indicates that a small portion of H 2 may have originated from methoxyl groups.Given the nature of the chemical groups and bonds composing plant tissue, we speculated that H 2 would be produced during abiotic degradation of plant litter [e.g., 2C n H m + nO 2 + hν → 2nCO + mH 2 : as a byproduct (mH 2 ) of partial oxidation (nO 2 ) of methyl groups (2C n H m ) under radiation energy (hν) absorption].Such thermal and photo production of H 2 has recently been documented using a single plant species (Sequoiadendron giganteum) (Derendorp et al., 2011c).
In our study, we quantified the steady state production rate of H 2 from different plant materials in a factorial manipulation of solar radiation (+rad: solar radiation present, −rad: solar radiation absent) and temperature (15 to 55 • C).We used four plant litter types that varied in chemical and structural composition to investigate the range of abiotic H 2 release from plant derived organic material: two grass species (C 3 Indian ricegrass, Oryzopsis hymenoides and C 4 little bluestem grass, Schizachyrium scoparium), leaves of two woody species common in the desert in the southwest USA (velvet mesquite, Prosopis velutina and piñon pine, Pinus edulis), and proxies for pure cellulose (cellulosic filter paper, 92 % pure cellulose) and high lignin woody material (thin sheets of basswood, Tilia sp.).For the plant material with the highest H 2 production rate (basswood), we extended measurements to 80 • C. We also assessed the role of atmospheric O 2 on H 2 release (+O 2 : aerobic conditions and −O 2 : anaerobic conditions) with the high lignin woody material.
We hypothesized that (1) there would be detectable abiotic production of H 2 and the production rates would be positively correlated with temperature as potential energy and oxidative potential increase with temperature, (2) the rate of H 2 production would be close to zero in the absence of O 2 if abiotic production of H 2 is via partial oxidation of methyl groups, and (3) abiotic production of H 2 from plant litter would vary among plant materials due to species-and tissuespecific differences in chemical composition (e.g., the presence and abundance of methyl groups).
In this study, we define H 2 produced in the absence of solar radiation as thermal degradation (temperatures below 100 • C).The H 2 produced in the presence of solar radiation was considered total abiotic degradation, thereby, we define photodegradation as the difference in H 2 production between total abiotic and thermal degradation.It is important to note that both thermal degradation and photodegradation could be thermally enhanced (i.e., H 2 production increasing with temperature).

Materials and methods
To assess the patterns of H 2 production across a wide range of plant materials differing in chemical and structural composition, we used plant materials from four species collected from their native habitats in the southwestern US [dried leaflets of velvet mesquite (Prosopis velutina), culms and leaves of a C 3 grass (Indian ricegrass, Oryzopsis hymenoides) and a C 4 grass (Schizachyrium scoparium, little bluestem grass), and piñon pine needles (Pinus edulis)] and two proxies for cellulose and lignin end members [cellulosic filter paper (92 % pure cellulose, Whatman 42, GE Healthcare Inc., Piscataway, NJ, USA) and 1.6 mm thick sheets of wood from basswood (Tilia sp., high lignin content, National Balsa, Ware, MA, USA)].The materials were air dried at 35 • C for two days before the incubation.Filter paper and basswood sheets were precut to fit inside the chamber area; grass materials and piñon pine needles were cut to 1 cm lengths to facilitate distributing the litter in a non-overlapping monolayer.Mesquite leaflets were used without cutting.Due to differences in litter density, the mass of materials used to fill in the experimental surface varied (e.g., typical mass to fill in the chamber area was approximately 1 g cellulosic filter paper, 5 g basswood sheet, 3 g piñon pine needles, and 2 g mesquite and grasses).However, gas production was sensitive to area of solar radiation exposure rather than mass (e.g., exposing multiple layers of filter paper did not influence the rate of gas production during photodegradation; data not shown).
We did not pre-treat the litter materials to eliminate microbial activity, but experiments took place under conditions where materials were completely dry and were exposed to intense solar radiation, thus minimizing the possibility of microbial activity (Johnson, 2003).Pre-treating of the material to completely negate the possibility of any biological activity was not feasible due to the potential for chemical alteration during certain sterilization treatments (e.g., autoclaving).Chemical sterilization was also ruled out since these technics may add compounds that are vulnerable to breakdown under heat and/or UV radiation, thus confounding our H 2 production measurements.Consequently, we did not completely rule out the possibility of some microbial activity, although the abiotic conditions suggest that it would be either non-existent or extremely low.
The plant materials were exposed to a factorial manipulation of solar radiation (+rad and −rad) and temperatures (15, 25, 35, 45, and 55 • C) to quantify the rates and patterns of H 2 release during abiotic degradation of plant material.We used basswood sheets, which exhibited the highest rate of H 2 production in the presence of O 2 , to quantify the rate of H 2 production during abiotic degradation of plant material at a higher temperature (80 • C) and to quantify the rates and patterns of H 2 production in the presence and absence of O 2 in combination with solar radiation (+rad and −rad) and temperature (15, 35, and 55 • C).
The plant litter incubation experiments were conducted in a custom built quartz chamber (Blue Flame Technologies, McKinney, TX) that was transparent to over 85 % of the radiation generated from the solar radiation simulator across all wavebands.Solar radiation was simulated with a 300 W xenon lamp and lamp housing equipped with an atmospheric attenuation filter (Oriel Instruments, Newport Corp., Irvine, CA).The xenon lamp emitted a wavelength range of 0-2400 nm, but the atmospheric attenuation filter eliminated the shorter and longer wavelengths (< 290 and > 1600 nm), thus representing the range of radiation wavelengths reaching the surface of Earth.The intensity of radiation relative to natural sunlight varied with wavelength; UV-B (50 µW cm −2 ) was generated at a similar intensity as the solar radiation, but UV-A was much lower (1 mW cm −2 ) than natural solar radiation (e.g., measured UV radiation intensity of cloud-free solar noon during early August in Los Alamos, NM, USA was approximately 55 µW cm −2 for UV-B and 5 mW cm −2 for UV-A).Ozone production and accumulation in the chamber from the xenon lamp was likely minimal, as the atmospheric attenuation filter eliminated UV-C (100-280 nm) and the residence time of gases in the chamber was on the order of 10 s.
The area of solar radiation exposure was defined by a Viton-core O-ring (53.5 cm 2 and 10.5 ml), which was clamped between the top and bottom halves of the quartz chamber, making an airtight seal.The outflow of the chamber was attached to a CO-H 2 analyzer (Peak Performer 1 RCP, Peak Laboratories LLC, Mountain View, CA, USA).A controlled flow of headspace gas (zero air or N 2 at 50-70 ml min −1 ) flowed through the chamber and to the analytical system.The zero air we used contains little or no H 2 , so we cannot rule out the possibility of H 2 outgassing from the substrate in the zero air.However, emissions did not diminish through time as would be expected from outgassing.Instrument calibration was performed with a suite of natural air standards and mixtures (NOAA ESRL and Scott Marrin, Riverside, CA, USA) that were routinely tested for internal consistency in order to establish stability in H 2 concentrations.Data were corrected for instrument non-linearity and a reference gas was analyzed every 35 min during the course of the automated analytical sequence.Instrument precision was ± 5 % or better at H 2 concentrations < 100 ppb and ± 2 % or better at concentrations > 100 ppb.The detection limit for observed H 2 production was ± 0.01 nmol m −2 hr −1 .
The chamber temperature was controlled using a water bath integrated into the bottom of the chamber and connected to a chiller/heater (ThermoCube 200/300/400, Solid State Cooling Systems, Wappingers Falls, NY, USA).The chamber temperature was continuously monitored with a thermocouple (error range ± 0.5 • C) and radiation influences on chamber temperature (approximately 2 • C) were controlled with the water bath.A hotplate was used to assist the temperature increase to 80 • C. The exposed litter material was exchanged after one full set of temperature and radiation manipulations (temperature increase from 15 to 55 • C and ±rad), although reusing the material did not change the rate of gas production (one set of measurements was approximately equivalent to one afternoon of exposure of solar radiation).
The rate of H 2 production during thermal degradation of plant material for a given temperature was estimated by excluding incident radiation with an aluminum foil shroud over the chamber.We then subtracted the H 2 concentration measured from the empty chamber at the same temperature (empty chamber values were considered "blanks").To estimate the rate of H 2 production during photodegradation of plant material, we measured the H 2 concentrations at a given temperature in the presence of solar radiation and subtracted the H 2 production during thermal degradation at the same temperature.Again, H 2 production during photodegradation was corrected using blanks, e.g., the empty chamber exposed to the solar radiation simulator over the same range of temperature and light conditions as samples.In all cases, the blank values were < 5 % of the values measured with plant material in the chamber.Changes in H 2 concentrations in response to changes in temperature and solar radiation were essentially instantaneous, but we waited between 30 to 60 min after changing conditions before taking the mean H 2 (Fig. 1).The final 3 to 4 analyzes of a sequence were typically used to www.biogeosciences.net/9/4411/2012/Biogeosciences, 9, 4411-4419, 2012 estimate the steady state H 2 production rate.Each measurement set was replicated three times for each litter type.
To quantify the rate of H 2 production during anaerobic thermal degradation and photodegradation of basswood sheets, we followed the same basic procedure used for measurements in the presence of O 2 with ultra high purity N 2 substituted for zero air.Measurements were taken at 15, 35, and 55 • C.
The H 2 production rates during thermal degradation and photodegradation of plant litter were normalized in two ways: per area (nmol H 2 m −2 s −1 ) and per mass (nmol H 2 kg −1 s −1 ): however, we suggest that H 2 production rate by thermal degradation is most appropriate on a mass basis because temperature would affect litter biomass as a whole.In contrast, photodegradation is most appropriately reported per unit area, as the UV radiation effects would only apply to the area of litter exposed to UV.
Activation energy of H 2 production during thermal degradation and photodegradation of litter under temperature manipulation was calculated using the Arrhenius equation (Eq.1): where k is the reaction rate coefficient, A is the preexponential factor, E a is the activation energy, R is the gas constant, and T is the temperature.We also estimated Q 10 of the reaction according to Eq. ( 2) to better understand temperature sensitivity of the reaction.
Q 10 = (P n /P n−1 ) exp(10/(T n − T n−1 )) (2) where P n is H 2 production rate at time n and T n is chamber temperature at time n.Statistical analyzes including repeated measures analysis with mixed effects and regression analyzes were conducted using R 2.11.1 (R Development Core Team).
The differences in H 2 production rate within the plant litter types were analyzed using repeated measures analysis.

Results
Thermal degradation of plant litter typically led to a higher H 2 production rate (range = 0.00069 to 2.17 nmol m −2 s −1 across all materials and temperatures) than did photodegradation alone (Fig. 2 and Supplement 1; range = 0.0036 to 1.01 nmol m −2 s −1 ).This difference was particularly pronounced at temperatures higher than 45 • C (Table 2).Averaged across all materials, the molar ratio of thermal degradation to photodegradation rose from 0.52 to 4.07 as temperature increased from 25 to 55 • C, implying that production of H 2 is more sensitive to changes in temperature than solar radiation alone.When temperature was raised to 80 • C for the high lignin proxy (basswood), total H 2 production rate reached 7.01 nmol m −2 s −1 (4.58 nmol m −2 s −1 from thermal degradation and 2.43 nmol m −2 s −1 from photodegradation).
We identified measurable H 2 production during abiotic degradation of plant litter with behavior typical of reactions following the temperature-dependent Arrhenius equation.The reactive energy (E a ) for H 2 production during thermal degradation of plant litter ranged from 60 to 146 kJ mol −1 and those of photodegradation ranged from 40 to 88 kJ mol −1 among the 6 different litter types (Table 1).The Q 10 values for all materials during thermal degradation and photodegradation exhibited normal values ranging from 2 to 3 (Davidson and Janssens, 2006) with a few notable exceptions (Table 1).Both C 3 grass and mesquite litter exhibited high Q 10 values during thermal degradation (5.7 and 7.1, respectively).Even with the elimination of a potential outlier in the C 3 grass data at 25 • C, Q 10 remained high at 3.7.Both the cellulose proxy filter paper and C 4 grass exhibited low Q 10 of 1.3 and 1.1 during photodegradation over the full temperature range and again when potential outliers were excluded at 55 • C, Q 10 remained low at 1.6 and 1.1, respectively.
When the H 2 production rates were normalized by mass, the general temperature sensitivity patterns were similar with the exception of an increased relative rate of H 2 production for filter paper, the cellulose proxy (cf., Fig. 2a and b to c and  d, Supplement).On a per-mass basis, the rate of H 2 production during thermal degradation of plant litter ranged from 0.0055 to 1.77 nmol kg −1 s −1 across all materials within the temperature range of 15 to 55 • C. The H 2 production rate during photodegradation of plant litter ranged from 0.029 to 1.03 nmol kg −1 s −1 .
The rate of H 2 production varied among litter types representing different litter chemical compositions.Basswood sheets exhibited the highest H 2 production rates, followed by piñon pine needles.The remaining four materials (filter paper, mesquite, C 3 grass, and C 4 grass) showed H 2 production rates that were much lower than those of basswood or piñon needles (F 1,79 = 18.22,P < 0.0001) and were not observed to be statistically different from each other (F 1,49 = 1.57,P = 0.22).The H 2 production rates for basswood and piñon pine needles diverged from that of the other materials with increasing temperatures (Fig. 1).H 2 production was observed under anaerobic conditions, ranging from 0.060 to 1.05 nmol m −2 s −1 for basswood sheet within the temperature range of 15 to 55 • C.This production was on the order of half that produced under aerobic conditions at any given temperature and radiation combination (Fig. 3).

Discussion
Previous work on trace gas production during thermal degradation and photodegradation of plant litter showed that various C-and N-based trace gases are released during abiotic degradation of litter (Brandt et al., 2007;Conrad and Seiler, 1985;Schade et al., 1999;Vigano et al., 2008).A recent study suggested that this may be attributed to direct breakdown of various chemical compounds and chemical groups within the organic material (Lee et al., 2012) by absorbing the activation energy from the heat or solar radiation.In addition to various C-based gas species, measurable H 2 release was previously reported from S. giganteum, but thermal degradation was reported only at temperatures above 45 • C and photodegradation was reported only in anaerobic conditions (Derendorp et al., 2011c).In this study, we identified and quantified the rate of H 2 release during abiotic degradation of various plant litter types and showed that the rate of H 2 production increases exponentially with temperature (Fig. 2, Tables 1 and 3).Additionally, we identified release of H 2 from dry litter under intense UV radiation, high chamber temperatures (Fig. 2), and in both the presence and absence of O 2 (Fig. 3) and that the temporal response to changing temperature is essentially instantaneous (Fig. 1) suggesting that the mechanism involved in the release of H 2 is abiotic.
The positive exponential response of H 2 production to temperature (Fig. 2 and Table 1) suggests that H 2 release during abiotic degradation of plant litter will be particularly high in hot and dry environments where microbial activity is minimal.In our experiment, we did not completely exclude the possibilities of H 2 production from anaerobically fermenting bacteria and aerobically by fungi.However, we suggest that this would be minimal.In addition, an E a greater than 50 lends additional supports that the process involved in H 2 release in our observations is abiotic (Schonknecht et al., 2008).Substantial production of H 2 in the absence of O 2 (Fig. 3) contradicted our hypothesis that anaerobic H 2 production would be close to zero since oxidation of methyl groups would not occur in anaerobic conditions and indeed, our observation of anaerobic H 2 production confirmed the findings of Derendorp et al. (2011c).On the other hand, Derendorp et al. (2011c) observed no H 2 production in the presence of O 2 , whereas our results suggest roughly equal production for aerobic (total minus anaerobic) and anaerobic processes in the case of high lignin basswood for the www.biogeosciences.net/9/4411/2012/Biogeosciences, 9, 4411-4419, 2012  total abiotic H 2 production.It is unclear at this time why the experimental results differ, but it is possible that there is a difference in plant species response between S. giganteum and the basswood studied here or that there is a fundamental difference in experimental setup that is not readily obvious.
Regardless of this difference, our results suggest that multiple mechanisms drive the abiotic release of H 2 from plant litter.We speculated in previous experiments with C-based trace gases that both photochemical oxidation (via reaction with O 2 ) and direct breakdown of carboxyl, carbonyl, and methoxyl groups could lead to production of CO 2 , CO, and CH 4 (Lee et al., 2012).In our observations, typical H 2 :CO mole fraction ratios across all litter samples were 0.14-0.59for thermal degradation and 0.02-0.07during photodegradation.The variability shown in the E a and Q 10 values of H 2 production rate across the six different plant litter types (Table 1) suggests that different chemical composition of these plant litter types (Lee et al., 2012) have varying rates of reactivity (Davidson and Janssens, 2006).Similarly, the higher rate of H 2 production in the presence of O 2 may be due to partial photo-oxidation of organic compounds.Taken together, our results suggest that the plant litter pool is a previously unrecognized source of atmospheric H 2 .
Our findings have substantial implications with respect to the most important loss process for atmospheric H 2 .For instance, a previous field observation of soil H 2 uptake from different soil layers showed that the rate of H 2 uptake increased when the surface litter and organic matter layers were removed (Smith-Downey et al., 2008).The original interpretation of this observation was that litter and organic matter removals eliminated a diffusive barrier and increased access to atmospheric H 2 for soil microbes.Interpretation of these data in light of our results, however, would suggest that the litter removal eliminated a proximal source of H 2 that subsurface microbes had previously been consuming.We suggest that in this previous study, the observed uptake of H 2 , occurred even though H 2 was likely produced within the chambered system, but consumption was simply greater than production.
The range of H 2 production rates during thermal degradation of plant litter in vitro observed in our study (0.02-33.62 ng H 2 gdw −1 h −1 within a 15-80 • C temperature range across the six different materials) corresponds well with the single species from which H 2 release was previously reported (Derendorp et al., 2011c; approximately 0-60 ng gdw −1 h −1 within a 20-80 • C temperature range).While Deredorp et al. (2011c) reported measurable rates of H 2 from S. giganteum only over 45 • C, for most litter types we detected measureable H 2 release beginning at 35 • C. In contrast to previously published findings, we observed substantial release of H 2 during photodegradation of litter both with and without O 2 , whereas Derendorp et al. (2011c) only observed photo-induced H 2 release in anaerobic conditions.We expect that if H 2 release is due to direct breakdown of chemical groups within the organic material, H 2 release should be observed in both cases of thermal and photodegradation process unless the H 2 produced in Derendorp et al. (2011c) was somehow oxidized during the measurement.
Our observations of abiotic H 2 release suggest that this may be a substantial flux relative to the documented range of soil uptake of H 2 (0.2-12.0 nmol m −2 s −1 ; Conrad and Seiler, 1985;Gerst and Quay, 2001;King, 2003;Rahn et al., 2002;Smith-Downey et al., 2008;Yonemura et al., 2000).A detailed estimate of the global abiotic H 2 production would require up-scaling our results to account for global patterns of litter chemistry and pools, surface temperatures, and solar radiation reaching the soil surface.Geographically widespread arid and semi-arid lands comprise over 40 % of the global land surface (Bailey, 1996); these systems likely produce measureable amounts of H 2 due to the typically high surface temperatures, high radiative loads, and in many cases large amount of standing dead and surface litter.Although the majority of this H 2 is likely lost immediately to soil microbiota due to proximity to the most important atmospheric H 2 sink, in the case of standing dead biomass, H 2 produced might indeed yield H 2 directly to the atmosphere.Given differences in reported H 2 flux response to O 2 between our study and that of Derendorp et al. (2011c), we suggest future research assessing the role of O 2 on abiotic release of H 2 and the correlation between UV intensity and the rate of H 2 release from various organic materials.In addition, future field work measuring H 2 flux from litter isolated from soil substrates will be required to verify and quantify field flux rates.
In summary, we quantified direct abiotic production of H 2 from plant litter resulting from both thermal-and photoinduced processes and have shown that these processes exhibit classic temperature dependence evidenced by approximate doubling of gas production rate per every 10 • C (Table 1).These results indicate that abiotic degradation of plant material is a ubiquitous source of H 2 , especially under arid conditions.Because these processes occur at the soil-atmosphere interface, they provide a previously unrecognized proximal source of H 2 for microbial uptake and confound interpretation of direct, chamber based, measurements of atmospheric uptake that are important for constraining the global H 2 budget.An exploration of the varying UV intensity effects would be an important future step for understanding H 2 release through photodegradation.In addition, the importance of abiotic processes in the overall budget of land-atmospheric boundary layer H 2 is yet to be determined.Given projected increases in atmospheric temperatures and drought in some parts of the world (IPCC, 2007;Overpeck and Udall, 2010), and detailed investigation from field and large scale observations is warranted to further extrapolate the effects of abiotic processes on a global scale.

Fig. 1 .
Fig. 1.Example of an analytical sequence exhibiting increasing H 2 concentrations with increasing temperature and light conditions (dark and UV+visible light).The automated GC system ran on a five minute sequence with analysis of a reference gas every 35 min.Open symbols indicate stabilizing period and filled symbols indicate period used for averaging.Note the nearly instantaneous response of the plant material, in this case of Basswood, to changes in temperature.

Fig. 2 .
Fig. 2. The rate of H 2 production from abiotic degradation of plant litter across a range of temperatures.Abiotic degradation is divided into (a) and (c) thermal degradation (H 2 production in the absence of solar radiation minus blank) and (b) and (d) photodegradation (H 2 production in the presence of solar radiation minus thermal degradation).Six plant litter types were used.Means and standard error (n = 3) are depicted.

Fig. 3 .
Fig. 3.The rate of H 2 production during thermal degradation (dark colors; measured in the absence of solar radiation) and photodegradation (light colors; H 2 production in the presence of solar radiation minus thermal degradation) of high lignin proxy basswood sheet in the (a) absence of O 2 and (b) presence of O 2 .

Table 1 .
The activation energy (E a : kJ mol −1 ) and the Q 10 values of H 2 production during thermal degradation and photodegradation of litter for the six plant litter types calculated by Arrhenius equation over the 15 to 80 • C temperature range for E a and 25 to 55 • C temperature range for Q 10 .

Table 2 .
Mean (standard error: SE)molar ratios between H 2 production rate during thermal degradation and photodegradation of six plant litter types (n = 3).Only the lignin proxy (wood sheets of basswood) was used for the 80 • C exposure.