Jukuri, open repository of the Natural Resources Institute Finland (Luke) All material supplied via Jukuri is protected by copyright and other intellectual property rights. Duplication or sale, in electronic or print form, of any part of the repository collections is prohibited. Making electronic or print copies of the material is permitted only for your own personal use or for educational purposes. For other purposes, this article may be used in accordance with the publisher’s terms. There may be differences between this version and the publisher’s version. You are advised to cite the publisher’s version. This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Qing-Wei Wang, Marta Pieristè, Titta K. Kotilainen, Estelle Forey, Matthieu Chauvat, Hiroko Kurokawa, T. Matthew Robson & Alan G. Jones Title: The crucial role of blue light as a driver of litter photodegradation in terrestrial ecosystems Year: 2022 Version: Published version Copyright: The Author(s) 2022 Rights: CC BY 4.0 Rights url: http://creativecommons.org/licenses/by/4.0/ Please cite the original version: Wang, QW., Pieristè, M., Kotilainen, T.K. et al. The crucial role of blue light as a driver of litter photodegradation in terrestrial ecosystems. Plant Soil (2022). https://doi.org/10.1007/s11104- 022-05596-x Vol.: (0123456789) 1 3 Plant Soil https://doi.org/10.1007/s11104-022-05596-x RESEARCH ARTICLE The crucial role of blue light as a driver of litter photodegradation in terrestrial ecosystems Qing‑Wei Wang   · Marta Pieristè · Titta K. Kotilainen · Estelle Forey · Matthieu Chauvat · Hiroko Kurokawa · T. Matthew Robson · Alan G. Jones Received: 24 February 2022 / Accepted: 4 July 2022 © The Author(s) 2022 region across biomes and plant communities remains uncertain. Methods  We performed a systematic meta-analysis of studies that assessed photodegradation through spectrally selective attenuation of solar radiation, by synthesizing 30 published studies using field incuba- tions of leaf litter from 110 plant species under ambi- ent sunlight. Results  Globally, the full spectrum of sunlight sig- nificantly increased litter mass loss by 15.3% ± 1% across all studies compared to darkness. Blue light alone was responsible for most of this increase in mass loss (13.8% ± 1%), whereas neither UV Abstract  Background and aim  Wherever sunlight reaches lit- ter, there is potential for photodegradation to contrib- ute to decomposition. Although recent studies have weighed the contribution of short wavelength visible and ultraviolet (UV) radiation as drivers of photo- degradation, the relative importance of each spectral Responsible Editor: Katharina Maria Keiblinger. Supplementary Information  The online version contains supplementary material available at https://​doi.​ org/​10.​1007/​s11104-​022-​05596-x. Q.-W. Wang (*)  CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China e-mail: wangqingwei@iae.ac.cn; wangqw08@gmail.com Q.-W. Wang  Jilin Changbai Mountain West Slope National Research Station of Forest Ecosystem, Shenyang 110016, China Q.-W. Wang · H. Kurokawa  Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki 305‑8687, Japan e-mail: hirokokurokawa@gmail.com M. Pieristè (*) · T. M. Robson  Organismal and Evolutionary Biology (OEB), Viikki Plant Science Centre (ViPS), University of Helsinki, P.O. Box 65 (Viikinkaari1), 00014 Helsinki, Finland e-mail: marta.pieriste@libero.it T. M. Robson  e-mail: matthew.robson@helsinki.fi M. Pieristè · E. Forey · M. Chauvat  UNIROUEN, IRSTEA, ECODIV, FR Scale CNRS 3730, Normandie Université, Rouen, France e-mail: estelle.forey@univ-rouen.fr M. Chauvat  e-mail: matthieu.chauvat@univ-rouen.fr T. K. Kotilainen  Natural Resources Institute Finland, Itäinen Pitkäkatu 4a, 20520 Turku, Finland e-mail: titta.kotilainen@luke.fi T. M. Robson  National Forestry School, University of Cumbria, Ambleside, UK http://orcid.org/0000-0002-5169-9881 http://crossmark.crossref.org/dialog/?doi=10.1007/s11104-022-05596-x&domain=pdf https://doi.org/10.1007/s11104-022-05596-x https://doi.org/10.1007/s11104-022-05596-x Plant Soil 1 3 Vol:. (1234567890) radiation nor its individual constituents UV-B and UV-A radiation had significant effects at the global scale, being only important in specific environments. These waveband-dependent effects were modulated by climate and ecosystem type. Among initial litter traits, carbon content, lignin content, lignin to nitro- gen ratio and SLA positively correlated with the rate of photodegradation. Global coverage of biomes and spectral regions was uneven across the meta-analysis potentially biasing the results, but also indicating where research in lacking. Conclusions  Across studies attenuating spectral regions of sunlight, our meta-analysis confirms that photodegradation is a significant driver of decompo- sition, but this effect is highly dependent on the spec- tral region considered. Blue light was the predomi- nant driver of photodegradation across biomes rather than UV radiation. Keywords  Biogeochemical cycling · Carbon flux · Decomposition · Litter traits · Spectral composition · Photodegradation · Meta-analysis Introduction The capability of sunlight to impact litter decomposi- tion in terrestrial ecosystems through the process of photodegradation is by now well established (Bais et al. 2018). Photodegradation involves three main mecha- nisms: photochemical mineralization, consisting of the direct breakdown of organic matter (Gallo et al. 2006), photofacilitation, meaning the facilitation of micro- bial decomposition following the photochemical min- eralization of complex polymers (Baker and Allison 2015), and photoinhibition, referring to the inhibition of microbial decomposition (Barnes et al. 2015). Which of these processes is dominant depends not only on the spectral region considered, but also on other environ- mental factors, such as temperature and precipitation, interacting with photodegradation (King et al. 2012). In some cases, the positive (photochemical mineralization and consequent photofacilitation) and negative (pho- toinhibition) effects offset each other (Bais et al. 2018). Since the 1990s, research into litter photodegra- dation in terrestrial ecosystems has largely focused on the effects of UV radiation (280–400  nm), and more specifically UV-B radiation (280–315  nm), due to concern about their impact on litter decom- position after the formation of the stratospheric ozone hole (Caldwell and Flint 1994; Zepp et  al. 1995). Only subsequently were the contributions of other spectral regions of sunlight to photodegrada- tion considered (reviewed by King et al. 2012). This research has revealed that short-wavelength regions of the visible spectrum, blue (400–490 nm) and green (500–570  nm) light, are also drivers of photodegra- dation (Austin and Ballaré 2010) due to their ability to photochemically degrade lignin (Austin and Bal- laré 2010; Austin et al. 2016). This process activates decomposer organisms by releasing breakdown prod- ucts, potentially releasing a bottleneck in microbial decomposition (Austin et  al. 2016). An additional step forward in our understanding of contribution of spectral regions to litter photodegradation was pro- vided by Day and Bliss 2019 who devised a poly- chromatic spectral weighting function for carbon dioxide emission in sunlight from the litter of species from the Sonoran Desert, Arizona. Visible light was found to have 30% effectiveness and UV-A radiation (315–400 nm) to be 61% effective, making these two spectral regions much more important in photodeg- radation compared to UV-B radiation (9%) (Day and Bliss 2019). Photodegradation has a role in litter decomposition in terrestrial ecosystems, not only in arid and semi- arid environments at low latitudes (Almagro et  al. 2015; Day et  al. 2007), as originally thought, but also at higher latitudes (Jones et al. 2016; Zaller et al. 2009) and in mesic environments (Brandt et al. 2010). Recently, forests have been added to the list of eco- systems where photodegradation affects biogeochem- ical cycling, extending the reach of this process to dynamic radiation environments where gap opening and forest management practices, as well as seasonal phenology, cause large fluctuations in received solar radiation (Méndez et  al. 2019; Pieristè et  al. 2019, 2020a, b; Wang et al. 2021). By identifying global trends in the importance and drivers of photodegradation, we can aim to incorpo- rate this knowledge into Earth System Models of the global carbon cycle. Currently, such models handle decomposition based on climatic factors, principally A. G. Jones  Forest Systems, Scion. 49 Sala Street, Private Bag 3020, Rotorua 3046, New Zealand e-mail: alan.jones@scionresearch.com Plant Soil 1 3 Vol.: (0123456789) precipitation and temperature and initial litter quality which drive soil organism activities (García-Palacios et  al. 2013). However, most studies have produced inconsistent and highly variable results across differ- ent environments (Parton et al. 2007). This could be explained by the interaction of photodegradation with other abiotic factors, such as temperature, precipita- tion and soil moisture, as the relative importance of photodegradation is reported to be enhanced in dryer conditions (Almagro et al. 2017; Brandt et al. 2007, 2010). Moreover, photodegradation rate increases with those factors that change the exposure of lit- ter to sunlight, such as season, canopy structure and phenological stage, litter layer thickness or litter posi- tion (Almagro et al. 2015; Bravo-Oviedo et al. 2017; Henry et  al. 2008; Mao et  al. 2018; Moody et  al. 2001; Rutledge et al. 2010). Additionally, the incident irradiance and spectral composition of solar radia- tion change with latitude, elevation and sun angle, meaning that underlying patterns of photodegradation should vary consistently across the globe (Aphalo 2018; Aphalo et al. 2012; Gallo et al. 2009). Several litter traits were suggested to be good pre- dictors of the photodegradation rate, such as initial lignin content (Austin and Ballaré 2010), initial hemi- cellulose and cellulose (Day et  al. 2018; King et  al. 2012; Pan et al. 2015). However, there are inconsist- encies among studies in the identity of effect traits mediating photodegradation and their hierarchy of importance. This suggests that, while we understand the underlying mechanisms of photodegradation, we are not yet able to account for how it is moderated by plant morphological and biochemical traits, and inter- actions with biotic and abiotic environmental factors. A quantitative assessment of the literature is required to test whether general trends in photodegra- dation globally, and the relative importance of differ- ent spectral regions, are consistent with expectations gleaned from the recent mechanistic advances iden- tifying the processes underpinning photodegradation. Previously, the effects of UV-B-driven photodegrada- tion were assessed in a meta-analysis by Song et  al. 2013 finding UV-B radiation to have no significant, direct or indirect, effects on litter decomposition at the global scale. King et al. 2012 reviewed the effects of UV radiation and visible light below 450  nm, finding that exposure to these spectral regions can increase litter mass loss. However, these two studies (Song et  al. 2013) and (King et  al. 2012) included both experiments employing supplemental radia- tion treatments, were produced prior to the majority of studies into visible light, and did not analyse the effect of the separate spectral regions (e.g. UV-B, UV-A, blue light). To date the results from studies on the effects of photodegradation driven by differ- ent spectral regions under ambient sunlight, have not been comprehensively synthesised at the global scale. Knowledge of the impact of waveband-dependent photodegradation on litter mass loss across differ- ent biomes and plant communities could represent the first step towards quantifying the impact of sun- light on decomposition on a global scale. These esti- mates will be important because photodegradation is responsible for the release of greenhouse gases, such as methane (CH4), carbon dioxide (CO2) and carbon monoxide (CO), into the atmosphere (Brandt et  al. 2009; Day et al. 2019; Schade et al. 1999). Our objective was to synthesize published stud- ies on the effect of photodegradation driven by UV radiation, its constituent UV-B and UV-A radiation, and blue light on mass loss from litter at the global scale. This would enable us to assess whether the relative importance of these spectral regions globally is consistent with the mechanistic advancements in our understanding of these processes. Moreover, we assess whether photodegradation rates are modulated by climate, ecosystem type, length of the experimen- tal period and litter habit (evergreen or deciduous), as well as litter traits. We expect blue light- and UV-A radiation-driven photodegradation to enhance litter mass loss, due to the relatively great ability of these spectral regions to degrade lignin (Austin and Bal- laré 2010). Moreover, we expect photodegradation to be more relevant (1) in arid than mesic conditions, where precipitation is likely to be the main driver of the decomposition process (Bais et al. 2018), as well as (2) in ecosystems with low canopy cover which allow most of the incident solar radiation to penetrate to the litter layer. Material and methods Data collection Data for the meta-analysis were extracted from lit- erature published between 1980 and January 2021, collected from Web of Science, Google Scholar, and Plant Soil 1 3 Vol:. (1234567890) Scopus database. Details of the keywords used are shown in Online Resource 1. We selected only stud- ies that spectrally selectively attenuated solar radia- tion to measure the photodegradation of surface leaf litter in terrestrial ecosystems. We excluded any studies that did not separate single wavebands or that did not allow the effect of single wavebands to be calculated due to the lack of a control treatment. Since one of our aims was to understand the effects of spectral composition on mass loss under ambient sunlight, all studies employing supplemental radia- tion were excluded. Moreover, as we aimed to exam- ine the correlation between photodegradation rate and litter traits, we retained only studies employing leaf litter from a single species, while we excluded stud- ies using litter mixtures. More details about study selection are found Online Resource 1. We consid- ered dark treatments to be only treatments blocking more than 95% of the solar spectrum. We extracted data concerning litter mass loss and initial litter traits. Where data were not presented in tables, we extracted them directly from the figures using WebPlotDigitizer 4.2 (Rohatgi 2019). We retained a total of 30 articles which produced a total of 325 datapoints. The list of retained studies is shown in Online Resource 2. Sev- eral papers included comparisons of multiple plant species (A list of the 114 species included is shown in Online Resource 3), field sites and spectral treat- ments. The effects of five spectral regions were calcu- lated: 1) UV radiation (280–400 nm); 2) UV-B radia- tion (280–315 nm); 3) UV-A radiation (315–400 nm); 4) blue light (400–490 nm) and 5) the full spectrum of visible light and UV radiation. The effect of each spectral region was obtained by comparison of pairs of spectral treatments applied in the original studies: the effects of excluding UV radiation, UV-B radiation and the full-spectrum were obtained by comparison of the control treatment with the no-UV, no-UVB and dark respectively; while the effect of UV-A radiation was obtained by comparison between the no-UV and the no-UVB treatment and the effect of blue light by contrasting the no-UV/blue and no-UV treatments as conducted in a study by Wang et al. (2020). There were too few studies to be able to test the effects of green light. Additionally, we extracted complementary informa- tion from each study: ecosystem (grassland, shrubland, woodland and open area); length of the decay period (the duration of the experiment in months); habit (evergreen or deciduous); litter form (herbaceous; shrub, tree); latitude. Details about the categorisation of these data and complementary information are shown in Online Resource 4 and in the dataset (Pieristè et al. 2021). The climate at each study site was defined according to the updated Koppen-Geiger climate classification using the map provided by Beck et  al. 2018, which divides the globe into five major climatic zones further separated into subdivisions based on temperature and precipitation. Details of the climate classification are shown in Online Resource 5. In order to estimate global-scale quantities of C released from surface litter by photodegradation, we extracted data from the Soil Respiration Data Base (SRDB) (Bond-Lamberty and Thomson 2010) on the annual litter carbon flux from each of the biomes cor- responding to the location of studies in the meta-anal- ysis. These data allowed us to roughly estimate the carbon flux in each of these biomes attributable to lit- ter mass loss due to photodegradation. Identification of the biomes was based on the World Wildlife Fund (WWF) biomes classification (Olson et al. 2001). Statistical analysis The effect sizes, expressed as log response ratio (lnRR) of mass loss, were computed with the func- tion ‘escalc’ from the package ‘metafor’ (ver. 2.1–0) (Viechtbauer 2019), which uses sample sizes, stand- ard deviations and means of the original studies and presents bias correction for small sampling. For each study, we selected only the final collection date to avoid the potential issue of time-dependent effect sizes. We used a three-level mixed effect model using study ID and effect size ID as random factors as described in (Assink and Wibbelink 2016), with cat- egorical variables “Ecosystem”, “Decay”, “Climate”, “Habit”, “Life form” and “Latitude” as fixed factors. The use of multilevel modelling in meta-analyses is a robust method for dealing with the problem of dependent effect sizes (Assink and Wibbelink 2016; Cheung 2014; Noortgate et  al. 2013). We used this method to test the overall effect of exclusion of each spectral region and the effect of the categorical vari- ables with the function rma.mv() from the package ‘metafor’ (ver. 2.1–0) (Viechtbauer 2019), employing the Knapp and Hartung correction method for random meta-analyses (Assink and Wibbelink 2016; Knapp and Hartung 2003). From these models we obtained Plant Soil 1 3 Vol.: (0123456789) the estimated average lnRR which we used to calcu- late the percentage change to better interpret the mag- nitude effect with the formula from Pustejovsky 2018. Following the same multi-level approach, we ana- lysed the correlation between the rate of photodegra- dation (effect size = lnRR) and the initial litter traits, climatic variables during the study period and abso- lute latitude, using them as continuous moderators in the model. The litter traits considered were those initial traits reported in each study: carbon content (C); nitrogen content (N); carbon to nitrogen ratio (C:N); lignin content; lignin to nitrogen ratio (Lig:N) and specific leaf area (SLA). The climatic variables used were average (Tmean), minimum (Tmin) and maximum temperature (Tmax), and cumulative pre- cipitation (PP), over the study period. These data were obtained from the NASA Langley Research Center POWER Project funded through the NASA Earth Science Directorate Applied Science Program (Sparks 2018) using the “nasapower” R package ver- sion 3.0.1 (Sparks 2020). The paucity of published studies from certain cli- mates, ecosystems, latitude, etc. has the potential to introduce bias into the meta-analysis. To assess the risk of bias, we explored the dataset of retained stud- ies to identify over- and under-represented categories. To evaluate literature bias we employed an Egger’s test (Egger et al. 1997) which uses the variance of the effect size as a moderator of a multi-level meta-analy- sis (Viechtbauer 2010), this allowed us to account for dependency among the effect sizes. Results Bias analysis and bias exploration We did not find bias in any of the datasets used to cal- culate the effect of each spectral region: full-spectrum (F1,74 = 0.265, p-value = 0.608); blue light (F1,49 = 0.336, p-value = 0.565); UV-A radiation (F1,32 = 0.226, p-value = 0.638); UV-B radiation (F1,43 = 0.03, p-value = 0.857) and UV radiation (F1,116 = 0.345, p-value = 0.558). UV radiation was the most studied spectral region (20 studies, n = 118), while blue light (5 studies, n = 51) and UV-A radiation (5 studies, n = 34) were under-represented in our dataset (Fig.  1a). Most studies were carried out at latitudes between 30° and 50°North and South, while data from high latitudes were lacking (Fig. 1b). Grassland and shrubland ecosys- tems were more studied than woodlands and open areas (Fig. 1c). Dry climates were the most studied, while polar and tropical climates were the least studied (Fig. 1d). In terms of the decay period, the first 12 months of decom- position were the most studied (Fig.  1e). The studies were located in seven biomes: “boreal forests/taiga”, “deserts and xeric shrublands”, “Mediterranean for- ests, woodlands and scrub”, “montane grasslands and shrublands”, “temperate broadleaf and mixed forests”, “temperate grasslands, savannas and shrublands” and “tropical and subtropical moist broadleaf forests” (Fig. 2; Online Resource 6). Effect of full‑spectrum‑driven photodegradation on litter mass loss The full-spectrum of sunlight compared to a control in darkness significantly increased litter mass loss over- all (+ 15.3% ± 1%, p = 0.040, n = 76, Fig.  3a, Table  1), however, this effect varied significantly depending on climate (p = 0.001, Table 2), ecosystem type (p < 0.001, Table 2), decay period (p < 0.001, Table 2) and life form of the litter (p = 0.020, Table 2). Specifically, only in dry (+ 36.3%, p < 0.001, Fig.  3a) and temperate climates (+ 18.6%, p = 0.026, Fig. 3a) did the full spectrum sig- nificantly increase mass loss. In terms of ecosystem type, the full-spectrum of sunlight increased mass loss only in open areas (+ 40.8%, p = 0.026, Fig. 3a) and shrublands (+ 36.3%, p < 0.001, Fig. 3a), while it had no significant effect in grasslands (p = 0.191, Fig.  3a) or woodlands (p = 0.131, Fig.  3a). Furthermore, the full spectrum of sunlight significantly increased litter mass loss in stud- ies that lasted six to twelve months (+ 34%, p < 0.001, Fig. 3a), but it had no significant effect across studies that lasted less than six months (p = 0.219, Fig. 3a) nor more than twelve months (p = 0.420, Fig. 3a). In terms of life form, the full-spectrum of sunlight increased mass loss only of shrub litter (+ 20.1%, p = 0.014, Fig. 3a). Effect of blue light‑driven photodegradation on litter mass loss Blue light caused an increase in mass loss overall (+ 13.8% ± 1%, p = 0.035, n = 51, Fig.  3b, Table  1) and this effect was dependent on climate (p = 0.013, Table  2) and ecosystem type (p < 0.001, Table  2). Blue light significantly increased litter mass loss only in dry climates (+ 9%, p < 0.001, Fig.  3b) Plant Soil 1 3 Vol:. (1234567890) but had a marginally non-significant effect on lit- ter mass loss in temperate climates (p = 0.052, Fig.  3b) and no significant effect in continental climates (p = 0.782, Fig.  3b). Moreover, blue light significantly increased litter mass loss in open areas (+ 64%, p < 0.001, Fig.  3b) and shrublands (+ 9%, Plant Soil 1 3 Vol.: (0123456789) p < 0.001, Fig. 3b), but not in woodlands (p = 0.254, Fig. 3b). Effect of UV‑driven photodegradation on litter mass loss UV radiation had no significant effect on mass loss overall (p = 0.397, n = 118, Fig.  3e, Table  1). How- ever, there was a significant interactive effect of UV radiation modulated by the decay period (p = 0.031, Table 2), whereby in studies shorter than 3 months and longer than 24  months UV radiation increased mass loss by 43.7% (p < 0.001, Fig.  3c) and 33.2% respec- tively (p = 0.031, Fig.  3c). The UV-B spectral region within UV radiation, likewise did not have a signifi- cant overall effect on litter mass loss (p = 0.770, n = 45, Fig. 3d, Table 1). However, the effect of UV-B radia- tion changed according to climate (p < 0.001, Table 2) and habit (p = 0.044, Table 2). UV-B radiation signifi- cantly increased mass loss in dry (+ 13.1%, p = 0.007, Fig.  3d) and temperate climates (+ 6.4%, p = 0.006, Fig. 3d), while it reduced mass loss in polar climates (-20.9%, p = 0.020, Fig. 3d). Moreover, UV-B radiation increased mass loss from the litter of both deciduous (+ 6.3%, p = 0.018, Fig.  3d) and evergreen (+ 20.6%, p = 0.012, Fig. 3d) shrubs and trees. We did not find a significant effect of UV-A radiation on mass loss over- all (p = 0.606, n = 34, Fig. 3e, Table 1). The relationship between photodegradation and abiotic factors Photodegradation driven by the full-spectrum of sun- light was moderated by Tmax (0.011; t(74) = -2.524, p = 0.014, Table  3). Photodegradation attributable to blue light was significantly moderated by: pre- cipitation (0.001; t(51) = 2.887, p = 0.006, Table  3) and Tmin (0.014; t(51) = 3.392, p = 0.001, Table  3). On the other hand, photodegradation attributable to the UV-B radiation was significantly moderated by: Tmean (0.017; t(44) = 4.461, p < 0.001, Table 3), Tmax (0.013; t(44) = 4.582, p < 0.001, Table 3) and absolute latitude (-0.006; t(44) = -2.313, p = 0.006, Table 3). The relationship between Initial litter traits and photodegradation Photodegradation driven by the full-spectrum of sunlight was moderated by initial C content (0.025; t(67) = 3.964, p < 0.001, Table  3). The same was true for photodegradation driven by UV-B radia- tion (0.017; t(36) = 3.527, p = 0.001, Table 3) and UV radiation (0.017; t(76) = 2.386, p = 0.020, Table  3). In addition, photodegradation driven by UV-B was moderated by Lig:N (0.008; t(29) = 2.156, p = 0.040, Table  3). Photodegradation attributable to blue light was significantly moderated by: initial lignin con- tent (0.017; t(36) = 2.455, p = 0.019, Table  3); Lig:N (0.016; t(36) = 2.666, p = 0.012, Table  3) and SLA (0.001; t(51) = 2.726, p = 0.009, Table 3). Discussion The relative importance of blue light and UV radiation in global photodegradation Exposure to the full-spectrum of sunlight increased litter mass loss by 15.3% ± 1% overall (Table  1, Fig. 3a), confirming that sunlight is among the suite of abiotic factors driving decomposition across the globe. This result is in agreement with previous findings analysing the effect of the full-spectrum of sunlight on litter mass loss (Day et  al. 2015; Ma et  al. 2017; Pan et  al. 2015). However, the mag- nitude of the effect is smaller than that found in an earlier meta-analysis (King et  al. 2012), which cal- culated an increase in mass loss of 23% due to sun- light. Our meta-analysis includes studies that were carried out in temperate and hemi-boreal forest envi- ronments (Pieristè et al. 2019, 2020a, b; Wang et al. 2021); ecosystems that were not represented in the meta-analysis by King et  al. 2012. In temperate and Fig. 1   Bias representation: number of studies and replicates by a) each spectral region, b) absolute latitude of the field sites of the studies, c) ecosystem type; d) climatic zone (see ESM Appendix-5 for more details about the climate classifi- cation); e) decay period (months), f) litter habit, g) litter form and h) biome type. The climate are: Tropical climate (Tropi.); Dry climate; Temperate climate (Tempe.); Continental cli- mate (Conti.); Polar climate. The biomes are: Boreal forests / Taiga (BF); Deserts and xeric shrublands (DXS); Mediterra- nean Forests, Woodlands and Scrub (MF); Montane grasslands and shrublands (MG); Temperate broadleaf and mixed forests (TB); Temperate grasslands, savannas and shrublands (TG); Tropical and subtropical moist broadleaf forests (TSB). The repilcates are not repeated measures, but represent the number of independent treatments (e.g. field sites) of one species ◂ Plant Soil 1 3 Vol:. (1234567890) Fig. 2   Locations of the experimental sites of the studies considered in the meta-analysis divided according to the World Wildlife Fund (WWF) biome classification (see Online Resource 6) Fig. 3   Effects of exclusion of a) the full spectrum, b) blue light, c) UV-A radiation, d) UV-B radiation and e) UV radia- tion on litter mass loss according to categories of climate, eco- system, decay period, habit and litter form. Average effect size (log response ratio) and 95% CI are shown. Numbers in paren- thesis represent the number of replicates Plant Soil 1 3 Vol.: (0123456789) boreal forests, sunlight tends to have the opposite net effect on photodegradation compared with forests at lower latitudes (Ma et  al. 2017), actually decreasing litter mass loss in some litter species (Pieristè et  al. 2019, 2020a, b). Hence, the inclusion of studies from these biomes may explain the lower net contribution of photodegradation to decomposition on the global scale that we report. Comparing spectral regions, blue light explained most of the mass loss attributable to solar radiation globally (a 13.8% ± 1% increase in decomposition due to blue light; Table 1, Fig. 3b); while UV, UV-A and UV-B radiation had no significant effect on lit- ter mass loss globally. This is in agreement with a previous meta-analysis showing no overall effect of UV-B radiation (Song et  al. 2013). The high ener- getic capacity of UV radiation to cause oxidative stress in living organisms has the potential to slow down microbial decomposition, as reported in sev- eral studies (Moody et al. 1999, 2001; Verhoef et al. 2000), although this photoinhibition may sometimes be offset by direct photochemical mineralization (Gallo et al. 2009). These two antagonistic processes can lead the effects of UV and UV-B radiation to differ across biomes with climate according to the importance of microbial decomposition: for exam- ple, decomposition is increased by UV and UV-B radiation in arid and semiarid climates but this effect does not extend to temperate and continental cli- mates (Gallo et  al. 2006, 2009; Pieristè et  al. 2019, 2020a, b). On the other hand, blue light is effective in causing photochemical mineralization, but appears not to produce photoinhibition (Austin et  al. 2016); this is likely to be the reason why the global positive effect of blue light on litter decomposition is distinct from the inconsistent effect of UV radiation glob- ally. Although the composition of spectral irradi- ance changes with latitude, elevation, canopy cover and structure (Wang et al. 2022), the energetic con- tribution of blue light always remains greater than that of UV radiation (Aphalo et al. 2012). The recent spectral weighting function for the emission of CO2 through photodegradation illustrates the action of sunlight on decomposing litter (Day and Bliss 2019) and its consistency with the results of this global meta-analysis for sunlight and blue light supports the use of this action spectrum when up-scaling across ecosystems. However, unlike Day and Bliss (2019), Table 1   Overall estimated log response ratio (lnRR) of mass loss, 95% confidence interval and p-value for each spectral region. Values in bold indicate statistical- significance. n indicates the number of replicates Spectral region n Estimate 95% CI p-value % change Full-spectrum 76 0.142 0.007 0.2768 0.040 15.26 Blue light 51 0.129 0.009 0.249 0.035 13.77 UV-A radiation 34 - 0.014 -0.070 0.041 0.606 - 1.39 UV-B radiation 44 0.012 -0.071 0.096 0.770 1.21 UV radiation 118 0.069 - 0.091 0.228 0.397 7.14 Table 2   Heterogeneity between groups (Qb) and p-values of the moderators for each spectral region. Values in bold indicate statistical-significance Spectral region Variable Qb p-value Full-spectrum Climate 5.82 0.001 Decay period 8.72  < 0.001 Ecosystem 9.70  < 0.001 Habit 0.71 0.405 Life form 4.14 0.020 Blue Climate 4.73 0.013 Decay period 0.54 0.586 Ecosystem 38.51  < 0.001 Habit 0.06 0.801 Life form 0.34 0.711 UV- A Climate 1.05 0.312 Decay period 1.25 0.301 Ecosystem 1.44 0.252 Habit 0.96 0.338 Life form 0.13 0.879 UV- B Climate 8.77  < 0.001 Decay period 2.19 0.124 Ecosystem 2.19 0.103 Habit 4.43 0.044 Life form 2.03 0.145 UV Climate 0.02 0.979 Decay period 2.76 0.031 Ecosystem 2.31 0.080 Habit 0.44 0.510 Life form 1.87 0.158 Plant Soil 1 3 Vol:. (1234567890) we did not identify UV-A radiation as the most effec- tive spectral region driving carbon emission through photodegradation across all studies in our meta-anal- ysis. This could be due to the fact that most studies testing the effect of UV-A radiation were located in cool moist temperate broadleaf forest biomes at high latitudes characterised by low UV-A radiation (Aph- alo et al. 2012; Grifoni et al. 2008). We estimated annual carbon flux from litter attrib- utable to photodegradation driven by different spec- tral regions, applying the percentage contributed by photodegradation to the gross annual carbon flux lost from litter in each biome obtained from the SRDB dataset (Bond-Lamberty and Thomson 2010). This produced an estimate of photodegradation driven by the full spectrum of sunlight of up to 5–61 g C m−2 per year according to biome type (Table  4), while blue light would potentially be responsible for 4–55 g C m−2 per year according to biome type (Table  4). Table 3   Number of replicates (n), regression coefficient (β), t-value and p-value obtained from the three-level meta-anal- ysis including initial litter traits, climatic variables and abso- lute latitude as continuous moderators. Initial litter traits are: carbon content (C), nitrogen content (N), carbon to nitrogen ratio (C:N), lignin content, lignin to nitrogen ratio (Lig:N) and specific leaf area (SLA). Climatic variables are: cumulative precipitation in mm (PP), average temperature in °C (Tmean), minimum temperature in °C (Tmin) and maximum tempera- ture in °C (Tmax). The latitude represents absolute latitude. Values in bold indicate statistical-significance Spectral region Variable n β t-value p-value Full-spec- trum C 67 0.025 ± 0.006 3.964  < 0.001 N 67 - 0.020 ± 0.078 - 0.251 0.803 C:N 67 0.004 ± 0.003 1.179 0.243 Lignin 47 0.007 ± 0.005 1.527 0.134 Lig:N 47 0.008 ± 0.006 1.298 0.201 SLA 67 - 0.001 ± 0.001 - 0.659 0.512 Latitude 76 0.003 ± 0.008 0.443 0.659 PP 76 - 0.001 ± 0.000 - 1.139 0.258 Tmean 76 0.001 ± 0.010 0.109 0.914 Tmin 76 - 0.011 ± 0.007 1.686 0.096 Tmax 76 0.011 ± 0.004 2.524 0.014 Blue C 43 0.023 ± 0.013 1.835 0.074 N 43 - 0.097 ± 0.056 - 1.650 0.107 C:N 43 0.004 ± 0.004 1.714 0.094 Lignin 36 0.017 ± 0.007 2.455 0.019 Lig:N 36 0.016 ± 0.006 2.666 0.012 SLA 51 0.001 ± 0.000 2.726 0.009 Latitude 51 - 0.007 ± 0.004 - 1.656 0.104 PP 51 0.001 ± 0.000 2.887 0.006 Tmean 51 0.009 ± 0.005 1.746 0.087 Tmin 51 0.014 ± 0.004 3.392 0.001 Tmax 51 0.004 ± 0.004 0.849 0.400 UV-A C 33 - 0.001 ± 0.010 - 0.118 0.907 N 33 - 0.053 ± 0.059 - 0.901 0.374 C:N 33 0.002 ± 0.002 1.352 0.186 Lignin 26 - 0.007 ± 0.008 - 0.788 0.439 Lig:N 26 0.003 ± 0.006 0.546 0.590 SLA 31 0.001 ± 0.001 0.372 0.713 Latitude 34 0.002 ± 0.005 0.376 0.709 PP 34 - 0.000 ± 0.000 - 0.060 0.953 Tmean 34 0.002 ± 0.009 0.159 0.875 Tmin 34 0.004 ± 0.006 0.677 0.503 Tmax 34 - 0.007 ± 0.009 - 0.755 0.456 Table 3   (continued) Spectral region Variable n β t-value p-value UV-B C 36 0.017 ± 0.005 3.527 0.001 N 36 - 0.035 ± 0.041 - 0.847 0.403 C:N 40 0.002 ± 0.001 1.663 0.105 Lignin 31 0.006 ± 0.004 1.695 0.101 Lig:N 29 0.008 ± 0.004 2.156 0.040 SLA 31 - 0.000 ± 0.000 0.610 0.547 Latitude 44 - 0.006 ± 0.003 - 2.313 0.026 PP 44 0.000 ± 0.000 1.759 0.086 Tmean 44 0.017 ± 0.004 4.461  < 0.001 Tmin 44 0.002 ± 0.007 0.259 0.797 Tmax 44 0.013 ± 0.003 4.582  < 0.001 UV C 76 0.017 ± 0.007 2.386 0.020 N 78 - 0.048 ± 0.047 - 1.021 0.311 C:N 78 0.001 ± 0.001 1.087 0.281 Lignin 84 - 0.002 ± 0.005 - 0.375 0.709 Lig:N 61 0.001 ± 0.003 0.344 0.732 SLA 82 - 0.001 ± 0.000 - 1.906 0.060 Latitude 118 0.003 ± 0.008 0.356 0.723 PP 118 0.001 ± 0.000 1.035 0.303 Tmean 118 - 0.010 ± 0.012 - 0.874 0.384 Tmin 118 - 0.007 ± 0.005 -1.231 0.221 Tmax 118 0.016 ± 0.008 1.946 0.054 Plant Soil 1 3 Vol.: (0123456789) Scaling up these estimates to a global scale, photo- degradation due to the full spectrum of sunlight would contribute 1.95 Pg to the annual global terrestrial car- bon flux over the seven biomes studied, with each biome responsible for carbon emissions of between 0.02 – 0.92 Pg. We would like to remind the reader that this estimate is indicative of the magnitude of the potential impact of photodegradation at the global scale based on upscaling the carbon flux data from studies of those biomes included in our meta-analysis and does not constitute a comprehensive global esti- mate. These estimates are greater than those from an existing modelling study (Foereid et  al. 2011) which found photodegradation not to have a signifi- cant impact on the global C budget. However, at the time of that modelling study, no data were available for high latitudes and forest ecosystems. There is the need for updated global modelling studies that incor- porate the recent conceptual advances in our knowl- edge of photodegradation at the mechanistic level and cover a broader diversity of environments. Climate moderated photodegradation driven by blue light, UV-B radiation and the full-spectrum of sunlight, with the highest photodegradation rates occurring in dry climates. These results support the hypothesis that dry climatic conditions tend to pro- mote photodegradation, where it is often the most important driver of decomposition when microbial activity is strongly reduced (Brandt et al. 2007; Gallo et al. 2006). On the contrary, in temperate and conti- nental climates decomposition is likely to be driven by factors promoting biotic processes, such as pre- cipitation and temperature cycles (Adair et al. 2008; Aerts 1997; Meentemeyer 1978). Nevertheless, we did not find a global correlation between cumula- tive precipitation and photodegradation driven by the full-spectrum of sunlight and UV radiation. This could be due to the fact that precipitation does not include other forms of moisture (e.g. fog and dew) known to be involved in photofacilitation (Gliksman et al. 2017). On the other hand, the precipitation was positively correlated with blue light photodegradation globally. This may suggest that blue light is involved in facilitating microbial decomposition (photofacilita- tion) in moist ecosystems, as previously proposed by Gliksman et al. (2017) and Pieristè et al. (2020a, b). In addition, we found that full-spectrum and UV-B photodegradation, were positively correlated with both the maximum and average temperature. This is in agreement with the trend reported for a Mediter- ranean grassland (Almagro et al. 2015). This relation- ship suggests that under warmer conditions, which increase evaporative demand and may consequently reduce in litter moisture (Maestre et  al. 2013), the relative importance of photodegradation may increase due to slower microbial decomposition (Almagro et al. 2015, 2017; Bais et al. 2018). Initial litter traits as predictors of photodegradation rate at the global scale In our meta-analysis, initial lignin content and initial Lig:N positively correlated with the rates of blue-light and UV-B photodegradation (Table  3). This result reaffirms the primary role of lignin in the process of photodegradation as a primary target of photochemi- cal mineralization due to its capacity to absorb blue light and UV radiation (Austin and Ballaré 2010). The importance of this process has been well established for dry climates, but the meta-analysis extends this pattern to temperate and continental climates as well as ecosystems characterised by high canopy cover, where litter typically receives low irradiance depleted in blue light (Pieristè 2020). Moreover, these results Table 4   Average carbon flux in g C m−2 and corresponding standard error (SE) attributable to photodegradation in each biome divided according to spectral regions. Contribution to carbon emission ( +) and retention (-). However, data for the tropical and subtropical biome were not available (“na”) Biome Full-spectrum Blue light Average SE Average SE Boreal forests / Taiga 61.05 23.52 55.09 21.23 Deserts and xeric shrublands 14.13 3.92 12.75 3.54 Mediterranean forests, woodlands and scrub 7.32 0.00 6.61 0.00 Montane grasslands and shrublands 20.20 10.94 18.23 9.87 Temperate broadleaf and mixed forests 35.07 2.39 31.65 2.15 Temperate grasslands, savannas and shrublands 5.46 3.36 4.93 3.03 Tropical and subtropical moist broadleaf forests na na na na Plant Soil 1 3 Vol:. (1234567890) highlight the greater importance of photodegradation in the decomposition of recalcitrant litter compared to labile litter (King et al. 2012; Pieristè et al. 2019). Recalcitrant litter is characterised by high Lig:N and high lignin content. This complex carbon macromol- ecule is not directly available to microbial decom- posers before photofacilitation (Austin et  al. 2016). Initial hemicellulose and cellulose content have also been proposed as potential targets of photodegrada- tion (Day et al. 2018; Lin et al. 2015). Unfortunately, few studies have measured these traits, so we could not test this hypothesis in our meta-analysis. Potential bias and further considerations Every meta-analysis is subjected to bias, for this rea- son results must be interpreted with care. Exploring the literature published about photodegradation under ambient sunlight, we identified some over- and under- represented categories that could potentially affect our results. For instance, UV-driven photodegrada- tion is the most studied, while relatively little atten- tion has focused on blue and green light, as their importance in driving photodegradation was revealed relatively recently (Austin et  al. 2016). We might expect that as more studies focus on these under-rep- resented spectral regions, our results would change. Moreover, studies of photodegradation were mainly located at latitudes between 30° and 50° North and South (Fig.  1b), with equatorial and high latitudes being under-represented. As photodegradation has even proved relevant even under relatively low irra- diances (Pieristè et  al. 2019, 2020a, b), the study of photodegradation in biomes at high latitudes and with a dynamic vegetation structure will be neces- sary to understand the real impact of photodegrada- tion at the global scale. Furthermore, woodlands are by far less studied than shrublands and grasslands and these studies are located at higher latitudes in temper- ate and continental climates, while grasslands have mainly been studied in arid and semiarid climates at lower latitudes. This segregation might partially explain the higher importance attributed to photodeg- radation in arid conditions. A particularly contentious subject in photobi- ology is how best to manipulate the solar spec- trum (Online Resource 7). In photodegradation studies, there is no standard method of filtering solar radiation and this makes it hard to compare multiple studies using different methods which create different micro-environments and exclude different classes of decomposers from reaching the litter, consequently altering the decomposition rates (King et al. 2012). Agreement on a standard method for the manipulation of solar radiation in photodegradation studies would allow a better comparison between them. Of course, the employ- ment of attenuating filters to selectively exclude spectral wavebands and shading treatments also cause a difference in the microclimate to which lit- ter is exposed. Unfortunately, filters almost inevi- tably modify moisture and temperature, and affect diurnal environmental fluctuations, which are even harder to control in field experiments than in labo- ratory conditions. Changes in moisture may lead to photofacilitation effects on litter decomposi- tion diurnally (Gliksman et al. 2017) or seasonally (Berenstecheret al. 2020). Thus, attention should be paid to influence of moisture in photodegrada- tion studies, especially those in mesic ecosystems where there is a strong interaction between solar radiation and moisture affecting litter decompo- sition. A recent assessment found C loss through thermal emission to be a relatively minor loss pathway compared to photolysis (Day et al. 2019), although the interaction of temperature with biotic processes may still significantly impact our results. In our analysis, we did not consider potential interactive effects of wavebands combinations, as our aim was to evaluate the impact of specific wavebands on decomposition across different eco- systems and climates. However, when interpreting the results of this meta-analysis, the reader should keep in mind that in a natural environment there is not a clear separation between spectral regions and interactive effects can occur. In addition, the dos- age of solar radiation could be more explicative of the results than the length of the decay period, since the amount of solar radiation incident on the litter greatly depends on the timing of the experi- ment, the weather conditions, and vegetation cover during the study period. Nevertheless, there are impediments to collecting the cumulative irradi- ance of specific spectral regions because many studies did not measure or give the data. The spec- tral measurement of experimental locations in the Plant Soil 1 3 Vol.: (0123456789) future studies will be essential to estimate the sig- nificance of photodegradation globally. Conclusion The present study confirmed the importance of sunlight as an abiotic driver of litter decomposi- tion through the process of photodegradation at the global scale. The full spectrum of sunlight increased litter mass loss by 15.3% ± 1% at the global scale. This implies that photodegradation is an important contributor to the global terrestrial carbon flux. Our meta-analysis scales-up findings from dry and Medi- terranean ecosystems that describe the mechanism of photodegradation, to affirm the important role of blue light in litter decomposition globally. This spec- tral region alone is responsible for an increase in mass loss of 13.8% ± 1%. On the other hand, UV radiation, and its constituents UV-B and UV-A radiation, had no significant effects overall only at a local scale: i.e., these waveband-dependent effects were modulated by climate and ecosystem type. Of covarying abiotic factors, average and maximum temperature positively correlated to photodegradation rate. Among initial litter traits, carbon content, lignin content, lignin to nitrogen ratio and SLA all positively correlated with the rate of photodegradation. However, we did not find one common trait that correlated with pho- todegradation across all the wavebands considered. The role of photodegradation at high latitudes and under tree canopies is at present understudied; more research in these areas will allow us to better define the role of photodegradation across the globe and would represent progress towards estimating its con- tribution to the global carbon budget. Author contributions  QWW and MP formulated the ini- tial idea and designed the study, MP collected the data, QWW and MP analyzed the data. MP wrote the draft of the manu- script and the remaining co-authors revised the manuscript. QWW formatted the manuscript materials and prepared the submission. Funding  Open Access funding provided by University of Helsinki including Helsinki University Central Hospital. This research was funded by the National Natural Science Founda- tion of China (32122059), the Chinese Academy of Sciences Young Talents Program, National Key R&D Program of China (2021YFD2200402), and LiaoNing Revitalization Talents Program (XLYC2007016) to QWW, by Chinese Academy of Sciences President’s International Fellowship Initiative (2022VCA0010) and the Japan Society for the Promotion of Science (KAKENHI, 17F17403) to QWW and HK, by Acad- emy of Finland decisions #266523, #304519 and #324555 to TMR, personal EF project and a grant from the Region "Haute- Normandie" through the GRR-TERA SCALE (UFOSE Project) to MP. The authors declare that no funds, grants, or other sup- port were received during the preparation of this manuscript. Data availability  The datasets generated during and/or ana- lysed during the current study are available from the corre- sponding authors on reasonable request. Declarations  Competing interests  The authors have no relevant financial or non-financial interests to disclose. Open Access  This article is licensed under a Creative Commons Attribution 4.0 International License, which per- mits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Crea- tive Commons licence and your intended use is not permit- ted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://​creat​iveco​ mmons.​org/​licen​ses/​by/4.​0/. 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