CO emissions from degrading plant matter (II). Estimate of a global sourcestrength

From relationships between integrated daily CO emissions and received solar radiation obtained for different standing dead grasses in field experiment s in a savanna region in South Africa, and making use of ecosystem and sola r irradiation databases, we derive estimates on global CO production and se asonality from photochemical decay of dry grasses and litter. The photochem ical CO source strength from standing dead plant material and litter in var ious grassland ecosystems and deciduous forests ranges from 20 to 65 Tg CO per year (1 Tg = 10(12) g). Accounting for potentially CO emitting ecosyste ms not included in the data set, we estimate that 60 +/- 30 Tg of CO are an nually emitted by photochemical degradation of decaying plant matter, mostl y 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 meas ured 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 tota l global source of CO by these mechanisms may thus be in the range 100(-50) (+70) Tg CO per year. Although the estimated CO source strength is a relati vely small contribution to the global CO budget (2-8%), CO emissions may si gnificantly compensate for CO deposition on soils in the tropics during cer tain times of the year. Currently, modeling studies mostly impose a constan t CO deposition velocity from the atmosphere to the soil surface, based gen erally on measurements on bare soil. Future modeling efforts may need to in clude geographical and photochemical factors which play a role in CO exchan ge in tropical ecosystems.