Abstract
AbstractPhotodegradation 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 CO2 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.
Highlights
Plant litter decay strongly affects ecosystem carbon and nitrogen cycling, but its effects are underestimated by microbial decomposition models in arid grasslands where photodegradation may play a large role (Parton et al 2007a, Austin 2011)
We examine the effects of photodegradation on ecosystem C and N cycling in California grasslands, by extrapolating the results of Rutledge et al (2010) to the ecosystem scale using DayCent-UV that explicitly represents photodegradation
Observed actual evapotranspiration (AET) the simulated ecosystem variables had consistently higher correlation with annual rainfall and mostly lower correlations with AET. These results suggest that the annual variation in ecosystem dynamics of this grassland is strongly controlled by AET, but much less so by annual rainfall, and that the model is able to simulate the dynamics reasonably well
Summary
Plant litter decay strongly affects ecosystem carbon and nitrogen cycling, but its effects are underestimated by microbial decomposition models in arid grasslands where photodegradation may play a large role (Parton et al 2007a, Austin 2011). Photodegradation accounted for 60% of carbon lost from aboveground litter in a semiarid steppe of Patagonia (Austin and Vivanco 2006) and contributed more than half of dry season CO2 loss in a California annual grassland (Rutledge et al 2010) In such places, the effect of photodegradation on litter decay likely extends to ecosystem dynamics in ways not captured by microbial decomposition models. Photodegradation may increase litter decay in dry periods when microbial activity is low, decoupling N dynamics from precipitation (Yahdjian et al 2006), decreasing N uptake by microbes (Brandt et al 2007), and increasing N released from aboveground litter that can either be taken up by grasses or be lost from the system (Parton et al 2007a) These effects may be best examined with ecosystem models, and incorporating photodegradation into the models improves our ability to examine and predict the ecosystem dynamics of dry grasslands
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