Photodegradation accelerates ecosystem N cycling in a simulated California grassland

Abstract

Photodegradation accelerates litter decay in arid grasslands where plant growth and litter decay are strongly controlled by precipitation and evapotranspiration. However, the effects of photodegradation on ecosystem C and N dynamics are not well understood. We examined the effects using an ecosystem biogeochemical model DayCent-UV with photodegradation explicitly represented and validated. The model was parameterized for a California grassland where photodegradation was documented to release CO 2 from litter. The model was parameterized with an inverse modeling approach using an extensive data set of six years of daily observed carbon and water gas exchange (gross primary production, ecosystem respiration, and evapotranspiration), soil temperature, and soil moisture. DayCent-UV correctly simulated the seasonal patterns of the observed gas exchange and closely simulated the inter-annual variation in the gas exchange and biomass production rates. The simulations suggested that the inter-annual variation is driven more by actual evapotranspiration than by precipitation because a large portion of precipitation is lost as runoff during wet years. Photodegradation in DayCent-UV accelerated C and N cycling, decreasing system C and N by 9.2% and 9.5% and C and N residence times by 9.4% and 18.2%. Accelerated N cycling made a greater fraction of system N available for plants, increasing net N mineralization and plant production for a given amount of system N. Increased net N mineralization was due to decreased immobilization by microbes in the aboveground organic matter. Photodegradation did not alter the control on plant production by evapotranspiration. These results suggest that at the ecosystem level, the central effect of photodegradation is to suppress microbial activity. We conclude that photodegradation accelerates N cycling at the expense of microbes in this grassland, making it more efficient in supporting plant growth for a given amount of N in the system.

Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2020

“…Recent assessments have emphasised the important role played by solar radiation in driving carbon cycling through litter decomposition to CO 2 in dryland ecosystems [ 149 , 167 ]. Since our last assessment [ 1 ], additional studies have provided more details about the significance of photodegradation in dry systems including an arid grassland [ 295 ], the Sonoran Desert [ 296 ], a semi-arid scrub system [ 297 ], and a semi-arid woodland ecosystem [ 298 , 299 ] (contributing towards SDG targets 15.1, 15.3). Other studies since our last assessment have shown that photodegradation is also associated with plant litter (dead plant material) decomposition in wetter ecosystems located in temperate and boreal forests [ 300 – 302 ].…”

The contribution of photodegradation to litter decomposition in a temperate forest gap and understorey

“…Solar radiation has not been included in these models until now (except for a few studies in the dryland systems, i.e. Chen et al ., 2016; Adair et al ., 2017; Asao et al ., 2018), even though it directly breaks‐down organic matter and may affect decomposition processes in forests. Our results illustrate that when gaps open in the forest canopy, photodegradation overrides temperature and moisture as a driver of leaf litter decomposition.…”

“…Meanwhile, it increases C flow back to the atmosphere by up to 63% relative to a continuous forest understorey, because of the increased incident of solar irradiance on surface litter. This suggests that accounting for photodegradation would significantly improve predictions of C loss from mesic forest ecosystems, consistent with its recent inclusion in the equivalent calculations for dry grassland ecosystems (Adair et al ., 2017; Asao et al ., 2018). Our estimate is based only on tree litter.…”

Thermal abiotic emission of CO2 and CH4 from leaf litter and its significance in a photodegradation assessment

“…), as well as N cycling (Asao et al. ), in drylands. There is growing evidence that the contribution of photopriming, through promoting microbial degradation, may overshadow that of abiotic photolysis, in terms of accelerating mass or C loss under photodegradation (Wang et al.…”

“…Hence, photodegradation can be significant and provides one explanation for the faster than expected litter decay in drylands. Furthermore, the inclusion of photodegradation in decomposition models improves their ability to predict litter mass and C losses (Chen et al 2016, Adair et al 2017), as well as N cycling (Asao et al 2018), in drylands. There is growing evidence that the contribution of photopriming, through promoting microbial degradation, may overshadow that of abiotic photolysis, in terms of accelerating mass or C loss under photodegradation (Wang et al 2015, Austin et al 2016, Gliksman et al 2017, Day et al 2018, Lin et al 2018).…”