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

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Abstract

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.

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Acknowledgments

Thanks to R. Sinsabaugh, S. Hobbie, D. Milchunas, and Y. Lin for their helpful comments on earlier versions of the manuscript. We appreciate the thorough and thoughtful comments provided by three anonymous reviewers. This work was supported by the National Science Foundation (NSF DEB 0542935 and 0935984).

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Correspondence to Jennifer Y. King.

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Appendix: Meta-analysis methods, data and references

Appendix: Meta-analysis methods, data and references

We conducted an extensive keyword and citation search using the ISI Web of Science for the words “photodegradation,” “UV,” “UV-B,” “photolysis,” and “solar radiation” in combination with “litter,” “decomposition,” and “organic matter.” Within each reference, we collected information on the field site (latitude and mean annual precipitation, MAP), litter (species, initial lignin content, initial C/N, area/mass), treatment (supplementary lamp, filter, or shade cloth), experiment duration and the final mass loss (ML) in each treatment as a percentage of initial litter mass.

Many of the references we found contained research investigating photodegradation of different litter types, in different locations or by excluding or increasing different amounts/types of radiation, etc. Each of these was incorporated into the database as a single experiment. Thus, the database contained 16 references and 50 experiments (Table 3). For each experiment (N = 50) we calculated a metric that examined the effect of increasing litter UV exposure based on the log response ratio (LRR of enhancing solar radiation or LRRenh): LRRenh = ln(MLenh/MLred), where MLenh is ML in the treatment with more solar radiation exposure and MLred is ML in the treatment with less UV exposure. MLenh values >0 indicate that ML increases with increasing solar radiation exposure; values <0 indicate that ML decreases with increasing solar radiation exposure.

Table 3 Data from 16 references used in the meta-analysis of field photodegradation experiments

We examined LRRenh averaged across all experiments (N = 50) and by solar radiation treatment combinations. Across the 50 experiments, we examined three types of treatment combinations: (1) enhanced versus ambient; (2) enhanced versus reduced; and (3) ambient versus reduced. Enhanced solar radiation treatments were accomplished via supplementary UV lamps, while reduced treatments used filters or shade cloths to block UVR and/or total solar radiation. We examined average LRRenh for each manipulation method (filter, supplementary lamp, or shade). For each mean LRRenh, we calculated a 95 % confidence interval (CI; Student’s t distribution). Means were considered to be significantly different from zero if the 95 % CIs did not include zero. We also examined linear relationships between experiment LRRenh and latitude, MAP, initial litter lignin content, initial litter C/N and leaf litter area/mass.

Meta-analysis references:

  • Austin AT, Ballaré CL (2010) Dual role of lignin in plant litter decomposition in terrestrial ecosystems. Proc Natl Acad Sci USA 107 (10):4618–4622

  • Austin AT, Vivanco L (2006) Plant litter decomposition in a semi-arid ecosystem controlled by photodegradation. Nature 442 (7102):555–558

  • Brandt LA, King JY, Hobbie SE, Milchunas DG, Sinsabaugh RL (2010) The role of photodegradation in surface litter decomposition across a grassland ecosystem precipitation gradient. Ecosystems 13 (5):765–781

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  • Day TA, Zhang ET, Ruhland CT (2007) Exposure to solar UV-B radiation accelerates mass and lignin loss of Larrea tridentata litter in the Sonoran Desert. Plant Ecol 193 (2):185–194

  • Gallo ME, Porras-Alfaro A, Odenbach KJ, Sinsabaugh RL (2009) Photoacceleration of plant litter decomposition in an arid environment. Soil Biol Biochem 41 (7):1433–1441

  • Gehrke C, Johanson U, Callaghan TV, Chadwick D, Robinson CH (1995) The impact of enhanced ultraviolet-B radiation on litter quality and decomposition processes in Vaccinium leaves from the sub-Arctic. Oikos 72 (2):213–222

  • Henry HAL, Brizgys K, Field CB (2008) Litter decomposition in a California annual grassland: interactions between photodegradation and litter layer thickness. Ecosystems 11 (4):545–554

  • Köchy M, Wilson SD (1997) Litter decomposition and nitrogen dynamics in aspen forest and mixed-grass prairie. Ecology 78 (3):732–739

  • Moody SA, Paul ND, Bjorn LO, Callaghan TV, Lee JA, Manetas Y, Rozema J, Gwynn-Jones D, Johanson U, Kyparissis A, Oudejans AMC (2001) The direct effects of UV-B radiation on Betula pubescens litter decomposing at four European field sites. Plant Ecol 154 (1–2):27–36

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  • Pancotto VA, Sala OE, Cabello M, Lopez NI, Robson TM, Ballaré CL, Caldwell MM, Scopel AL (2003) Solar UV-B decreases decomposition in herbaceous plant litter in Tierra del Fuego, Argentina: potential role of an altered decomposer community. Glob Change Biol 9 (10):1465–1474

  • Pancotto VA, Sala OE, Robson TM, Caldwell MM, Scopel AL (2005) Direct and indirect effects of solar ultraviolet-B radiation on long-term decomposition. Glob Change Biol 11 (11):1982–1989

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Lignin method references:

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King, J.Y., Brandt, L.A. & Adair, E.C. Shedding light on plant litter decomposition: advances, implications and new directions in understanding the role of photodegradation. Biogeochemistry 111, 57–81 (2012). https://doi.org/10.1007/s10533-012-9737-9

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