Ultraviolet photodegradation facilitates microbial litter decomposition in a Mediterranean climate

Abstract

Abstract. Rates of litter decomposition in dryland ecosystems are consistently underestimated by decomposition models driven by temperature, moisture, and litter chemistry. The most common explanation for this pattern is that ultraviolet radiation (UV) increases decomposition through photodegradation of the litter lignin fraction. Alternatively, UV could increase decomposition through effects on microbial activity. To assess the mechanisms underlying UV photodegradation in a semiarid climate, we exposed high-and low-lignin litter to ambient and blocked UV over 15 months in a Mediterranean ecosystem. We hypothesized that UV would increase litter mass loss, that UV would preferentially increase mass loss of the lignin fraction, and that UV would have a negative effect on microbial activity. Consistent with our first hypothesis, we found that UV-blocking reduced litter mass loss from 16% to 1% in high-lignin litter and from 29% to 17% in low-lignin litter. Contrary to our second hypothesis, UV treatment did not have a significant effect on lignin content in either litter type. Instead, UV-blocking significantly reduced cellulose and hemicellulose mass loss in both litter types. Contrary to our third hypothesis, we observed a positive effect of UV on both fungal abundance and the potential activities of several assayed extracellular enzymes. Additionally, under ambient UV only, we found significant correlations between potential activities of cellulase and oxidase enzymes and both the concentrations and degradation rates of their target compounds. Our results indicate that UV is a significant driver of litter mass loss in Mediterranean ecosystems, but not solely because UV directly degrades carbon compounds such as lignin. Rather, UV facilitates microbial degradation of litter compounds, such as cellulose and hemicellulose. Thus, unexpectedly high rates of litter decomposition previously attributed directly to UV in dryland ecosystems may actually derive from a synergistic interaction between UV and microbes.

Photodegradation alleviates the lignin bottleneck for carbon turnover in terrestrial ecosystems

“…Losses of volatile carbon compounds, including CO 2 , CO, and methane (CH 4 ), from recently senesced plant material exposed to natural or artificial radiation have been documented (16)(17)(18) and have been linked to photochemical degradation of cellulose (16) and partial or complete degradation of lignin (19)(20)(21). Recent studies have demonstrated stimulation of litter decomposition and respiration with prior exposure to ultraviolet (UV) (22,23) or full solar radiation (24) in dry Mediterranean ecosystems. Nevertheless, the quantitative significance of photodegradation in terrestrial ecosystems that support greater microbial activity than arid-land ecosystems is generally considered to be minimal (25).…”

“…2 C and D) provides an important element for conceptual and empirical models aimed to understand the role of photodegradation in litter decomposition in different ecosystems. In addition, the contrasting response of woody and herbaceous species to UV exposure may help to resolve the contradictory results reported for UV effects on photodegradation in terrestrial ecosystems (23,27,(31)(32)(33)(34).…”

OMDY: a new model of organic matter decomposition based on biomolecular content as assessed by 13C-CPMAS-NMR

“…Traditional approaches have been based on correlating simple litter characteristics with decay rates for predictive purposes (Meentemeyer 1978;Melillo et al 1982;Coq et al 2010). For example, lignin content, the most abundant plant biopolymer after cellulose, is often negatively correlated with decay rate, especially when root and woody plant tissues are included in analysed samples (Taylor et al 1989), but not in the case of litter photodegradation in dryland systems (Austin and Vivanco 2006;Baker and Allison 2015). Consistent negative correlations with litter decay rates have been also reported for lignin to nitrogen (Lignin/N) (Melillo et al 1982) and carbon to nitrogen (C/N) ratios (Taylor et al 1989), with the latter being more predictive of decay rates in case of substrates with limited lignin content, such as understory herbs (Taylor et al 1989).…”

Uso de la tierra y ambiente local de descomposición en el Chaco Semiárido de Córdoba, Argentina

“…Si bien en el presente trabajo no se analizó el efecto de la fotodegradación de manera explícita, varios trabajos muestran que este proceso puede alcanzar valores entre 33 y 60% de pérdida de peso en zonas semiáridas y destacan su importancia en el proceso de descomposición Baker and Allison 2015;Day et al 2015). Estos autores proponen que la pérdida de carbono a través de la fotodegradación en estos ecosistemas implica que una fracción importante del C fijado en la biomasa de las plantas se pierde directamente a la atmósfera sin pasar antes a formar parte del C del suelo .…”