CO emissions from degrading plant matter (II).

Abstract

From relationships between integrated daily CO emissions and received solar radiation obtained for different standing dead grasses in field experiments in a savanna region in South Africa, and making use of ecosystem and solar irradiation databases, we derive estimates on global CO production and seasonality from photochemical decay of dry grasses and litter. The photochemical CO source strength from standing dead plant material and litter in various grassland ecosystems and deciduous forests ranges from 20 to 65 Tg CO per year (1 Tg = 10¹² g). Accounting for potentially CO emitting ecosystems not included in the data set, we estimate that 60 ± 30 Tg of CO are annually emitted by photochemical degradation of decaying plant matter, mostly in the tropics. We further estimate thermal CO production from the global topsoil non-woody litter pool on the basis of global climate data and measured Arrhenius parameters to add another 40 Tg CO per year, much depending on the chosen parameters, and probably uncertain by a factor of 2. The total global source of CO by these mechanisms may thus be in the range 100 ⁺⁷⁰_-50 Tg CO per year. Although the estimated CO source strength is a relatively small contribution to the global CO budget (2–8%), CO emissions may significantly compensate for CO deposition on soils in the tropics during certain times of the year. Currently, modelling studies mostly impose a constant CO deposition velocity from the atmosphere to the soil surface, based generally on measurements on bare soil. Future modelling efforts may need to include geographical and photochemical factors which play a role in CO exchange in tropical ecosystems.