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Soil Biology & Biochemistry 43 (2011) 1383e1386

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Soil Biology & Biochemistry

journal homepage: www.elsevier .com/locate/soi lb io

Short Communication

Modelling photodegradation in the global carbon cycle

Bente Foereid a,*, Maria J. Rivero b, Oscar Primo b, Inmaculada Ortiz b

a Institute of Biological and Environmental Sciences, University of Aberdeen, Aberdeen AB24 3UU, Scotland, UKbChemical Engineering Department, University of Cantabria, ETSIIT, Av. Los Castros s/n, 39005 Santander, Spain

a r t i c l e i n f o

Article history:Received 28 September 2010Received in revised form3 March 2011Accepted 4 March 2011Available online 21 March 2011

Keywords:PhotodegradationPlant litter decompositionModelGlobal carbon cycle

* Corresponding author. Cornell University, DepSciences, 915 Bradfield Hall, Ithaca, NY 14850, USA. T607 255 2644.

E-mail address: bf228@cornell.edu (B. Foereid).

0038-0717/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.soilbio.2011.03.004

a b s t r a c t

Photodegradation has been shown to play a role in plant litter degradation in some ecosystems, andtherefore potentially in the global carbon cycle. To introduce photodegradation into models of carbonturnover we need an equation that relates mass loss to incident radiation. Based on experimental datafrom the literature we developed a linear equation for photodegradation as a function of light exposure.We also scaled up the results to global scale, based on global data on incident radiation and shading byplant cover. The results indicate that photodegradation plays a part in semi-arid environments andpotentially some arctic and alpine environment. However, the percentage of global plant productiondecomposed by photodegradation is estimated to be less than 1% (range 0.5e1.6). Photodegradation istherefore important locally, but probably not very important for the global carbon budget.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Several recent studies have indicated that photodegradationmay play an important role in the carbon cycle in semi-aridecosystems where other climatic factors appear less important(Austin and Vivanco, 2006; Parton et al., 2007; Tian et al., 2007)These environments are characterised by high radiation levelscoinciding with low vegetation cover for at least part of the year,meaning that the degrading litter on the soil surface will beexposed to high radiation. The importance of photodegradation inother ecosystems is probably less, but Brandt et al. (2007) foundthat it played some role in a prairie system. The importance ofphotodegradation for other systems has, to our knowledge, notbeen investigated.

Models of carbon turnover usually model decomposition asexponential decay of 2 or more pools with different turnover ratemodified with temperature and moisture, in some cases also otherfactors (Jenkinson et al., 1987; Parton et al., 1987). Carbon turnovermodels are increasingly being used to predict effects of, and feedback to global change (Cox et al., 2000; Betts et al., 2008; Crameret al., 2001; Jones et al., 2005). The largest uncertainty in global

artment of Crop and Soilel.: þ1 607 255 1730; fax: þ1

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land surface models is the soil response (Friedlingstein et al., 2006),so improving the soil part of carbon turnover model is a priority.

The effect of photodegradation has not yet been incorporatedinto any simulation model. In the present study we develop anequation for the effect of radiation on litter degradation. We alsoestimate the importance of photodegradation globally based onincident radiation and vegetation cover.

2. Model development

We reviewed the literature for journal papers were decompo-sition of plant material exposed to radiation had been measured asa time series. We excluded papers where we had reason to believethat significant vegetation cover may have shaded the litter mate-rial. The experiments we used are listed in Table 1. We used data onsolar radiation from NASA (http://eosweb.larc.nasa.gov/sse/).

Part of the degradation observed will be due to microbial andpossibly mechanical breakdown. This was accounted for bya conventional carbon turnover model, a simplified version of theRothamsted carbon model (Coleman and Jenkinson, 1999;Jenkinson et al., 1987). We used essentially the litter decomposi-tion part of the model, but we assumed that soil organic matterformed from litter decomposition does not degrade. Whilst this isclearly unrealistic in the long term, it gives good approximation tomeasured results in the short term (Fig. 1).

As we had some data from degradation under shaded conditions(Austin and Vivanco, 2006), we used those data to fit the parame-ters we did not take directly from RothC, the moisture modifier and

Table 1Details about the sites for the experimental data used in the model development.

Site Rio Mayo ExperimentalStation, Argentina

Sonoran Desert, Mexico Sevilleta National WildlifeRefuge, New Mexico

Lat Long (deg) Elevation (m) 45�410S, 70�160W, 500� 29�010N, 110�570W, 250 34�N, 107�W, 1510e1971Rainfall (mm/yr) 152 335 243e282Length of experiment (yr) 1.6 1 1.2Comment Litter was mixed, vegetation was

removed before the litter boxes wereput in. Shaded controls were used.

Compared litter types and sites.No shaded controls

Compared location, litter typesand years. No shaded controls.

Reference Austin and Vivanco, 2006 Martinez-Yrizar et al., 2007 Vanderbilt et al., 2008

Table 2

B. Foereid et al. / Soil Biology & Biochemistry 43 (2011) 1383e13861384

the fraction turned into soil organic matter. As our sites were verydry, we allowed low values of themoisturemodifier, below 0.2 as inRothC. The results are shown in Fig. 1. This model will be referred toas the model for microbial degradation. The calculated degradationin the dark was subtracted from total degradation before photo-degradation was calculated in each experiment. We fitted the %material degradation per unit cumulative incident radiation. As therelationship between cumulative radiation and mass loss in pho-todegradation appeared to be approximately linear, a simple modelwas chosen:

Cp ¼ 100� a$l (1)

where Cp (%) is the percentage carbon left, a (%/MJ) is a fittedparameter and l (MJ e mega joule) is the cumulative amount ofradiation received during the degradation period. The parametera expresses the percentage litter loss that can be expected per unitradiation received, and is expressed in %/MJ. It is assumed that theeffect of radiation is cumulative, and proportional to the totalamount of radiation received. a was fitted for each experiment(litter type and site) separately. Here we have expressed carbon lossas a percentage of initial carbon. The error was calculated as:

error ¼ %Cexp � %Cmod%Cexp

$100 (2)

where %Cexp is the experimental value of % carbon remaining and%Cmod is the model predicted value. This error was calculated foreach experimental point and averaged for each experiment. Someof the experiments fitted the model well, others less so. Aweighted

time (months)

0 5 10 15 20

Mas

s re

mai

ning

(%)

50

60

70

80

90

100

110

model dark model light data dark data light

Fig. 1. The models for degradation under shaded conditions and exposed conditions asa function of time for the data from Austin and Vivanco (2006). The function fordegradation in the dark was subtracted from the observed total degradation whenphotodegradation was calculated.

average of the a values was therefore calculated based on givinghigher weight to experiments where the error was low. The errorsof all the experiments were summed, and the fraction of the totalerror for each experiment was calculated. The weight was taken tobe 1 minus the fraction. The weighted average value (a ¼ 0.0027%/MJ) was used in the global simulation (Table 2). We tended tocalculate higher values of a in the experiments with many con-founding factors, because everything we have not accounted forwill be in a. However, these experiments also get the lowestweights. The value of a calculated from different litter types andsites was relatively constant (Table 2). However, we repeated thecalculation for the lowest and highest values of a in Table 2 asa sensitivity analysis. There was also no evidence in this data setthat the value of a depended on litter type in the same way asmicrobial degradation does, as also found by Brandt et al. (2009),although under more controlled conditions Austin and Ballare(2010) found that lignin promotes photodegradation.

3. Global simulation

To estimate the global significance of photodegradation wecalculated global potential photodegradation based on total radia-tion and attenuation by plant cover. We used leaf area index (LAI)data from Modis (http://edcdaac.usgs.gov/dataproducts.asp) andaverage monthly values of radiation from NASA. For LAI, monthlyaverages were calculated averaging over all years of measurements

Results of model fitting for each site and litter type.

Experiment reference Litter/site a Calculatedphotodegradationper unitradiation (%/MJ)

Error (%)

(Austin and Vivanco, 2006) Stipa speciosa 0.0019 4.05Poa lingularisStipa humilis

(Martinez-Yrizar et al., 2007) Encelia fabrinosaPlains

0.0044 317.54

Encelia fabrinosaHillside

0.0044 42.73

Encelia fabrinosaArroyos

0.0044 45.34

Olneya tesota 0.0044 149.06Olneya tesota 0.0026 16.63Olneya tesota 0.0026 18.39

(Vanderbilt et al., 2008) Juniperusmonosperma

0.0016 3.52

Larrea tridentata 0.0026 10.62Oryzoposishymenoides

0.0021 8.95

Boutelounaeriopoda

0.0016 5.60

Boutelouagracilis

0.0013 6.85

Fig. 2. Percentage of total NPP that can be degraded by photodegradation according to the model developed shown on a 0.5� resolution. Areas with no NPP are left blank (black) andtaken out of further calculation as no percentage could be calculated.

B. Foereid et al. / Soil Biology & Biochemistry 43 (2011) 1383e1386 1385

up to summer 2008. All GIS modelling was implanted in Idrisisoftware. Radiation reaching ground level (Rg) was calculated foreach pixel on a 0.5� resolution using:

Rg ¼ R*e�LAI$X (3)

where X is the extinction coefficient for vegetation (unitless), hereset at 0.45 (Jones, 1992) and R (MJ) is the radiation above thecanopy. This was summed to make a total yearly radiation atground level. The radiation was used to calculate percent photo-degradation for each month all over the globe on a 0.5� resolution.We then calculated the maximum amount that could be photo-degraded based on net primary productivity (NPP), also fromModis, and the percentage degradation calculated. 40% of totalyearly NPP was assumed to be below ground and thereforeunavailable for photodegradation. It should be seen as an upperbound or potential photodegradation as it assumes that litter isavailable for degradation each month. This is probably mostly thecase, but may not always be.

Fig. 2 shows the percentage of NPP that is decomposed byphotodegradation according to the model. Mostly the percentagewas low in forests and high in drylands and on the edges of desertsand to a lesser extent grassland/prairie areas, as also found exper-imentally (Austin and Vivanco, 2006; Brandt et al., 2007; Day et al.,2007; Henry et al., 2008; Vanderbilt et al., 2008). However, thepercentage was also surprisingly high in arctic Canada and toa lesser extent Siberia, due to long hours of radiation combinedwith sparse vegetation cover. Some part of mountain ranges alsohad high values, probably due to sparse vegetation combined withhigh radiation.

Photodegradation varied from 0 to more than 14% of total NPP(Fig. 2). The range of maximum values based on the range of a-values in Table 2 was 7e24%/MJ. This is in reasonable correspon-dence with the results of a recent field study by Ruthledge et al.(2010). Although they found that a very high percentage of thecarbon flux was from photodegradation during the dry season, thiswas a small part of the total yearly flux. Photodegradation wastherefore important in some areas, but never enough to degrade allthe organic matter produced in an area. On a global scale, about0.96% of total NPP was photodegraded (range 0.46e1.57%). Themain reason for the low value is that the ecosystems where pho-todegradation is important are not very productive. Global budgetsof CO2 production will therefore not be significantly affected byincluding photodegradation. However, photodegradation is locally

important in carbon turnover in relatively large areas. Thisconclusion is robust over the range of variability of a in Table 2.

4. Model limitations

The model presented here assumes that all wavelengths areequally important in photodegradation. There is evidence,however, that the wavelength distribution is important (Austin andBallare, 2010), and that UV-B radiation is particularly effective(Austin and Vivanco, 2006; Pancotto et al., 2005). Ambient wave-length distribution is fairly constant, but varies with solar height,cloud cover and altitude. We have not considered any differences inwavelength distribution in our study. However, as higherpercentage UV is expected at low latitudes, high altitudes and areaswith much clear weather, the effect is expected to be somewhatmore pronounced in those areas. We are only aware of data onphotodegradation in relatively low latitudes (45 and below,Table 1). Our equation is calibrated using those data. That maymean that it overestimates photodegradation in higher latitudes.Austin and Vivanco (2006) found that excluding UV-B decreasedlitter mass loss by 33%. Solar UV-B is reduced by about 80% fromlow to high latitudes and UV-A is reduced by about 60% (Lee-Tayloret al., 2010). UV-A is less effective in photodegradation than UV-B,but still more effective than visible light.We therefore estimate thatthe actual photodegradation in arctic latitudes may be about30e40% lower than our estimates.

Furthermore, we do not know to what extent photodegradationmay interact with other factors, such as mechanical degradation(wind, difference between day and night temperature etc.) ormicrobial degradation. Although Austin and Vivanco (2006) foundno effect of microbial degradation, photodegradation may interactwith microbial degradation (Foereid et al., 2010; Henry et al., 2008)and possibly water content and mechanical disturbance in lessextreme environments.

In the global simulation we have assumed that the extinctioncoefficient is constant, but in reality it varies with leaf orientation(Jones, 1992). Our model also assumes that all aboveground litterstays on the ground. In agricultural areas some of it may beploughed under, and so reducing photodegradation. Leaf area indexwill also be kept artificially low large parts of the year in agriculturalareas. This is taken into account in our global simulation, as we basethe leaf area index estimate on remotely sensed data. However,changes agricultural land use could change photodegradation

B. Foereid et al. / Soil Biology & Biochemistry 43 (2011) 1383e13861386

potential, as agricultural areas are relatively high productive andstill often have low leaf area index for much of the year.

Acknowledgements

This study was supported by a travel grant from the EuropeanScience Foundation Under the activity “Functional Dynamics inComplex Chemical and Biological Systems”. Three anonymousreviewers are acknowledged for thorough review and usefulsuggestions.

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