Dynamics of carbon dioxide evolution from decomposing wheat straw
Increasing concentration of CO2 in the atmosphere is a concern because of the potential for global warming. Carbon exchange between soil and the atmosphere has an important role in the global C cycle, but partitioning soil CO2 emissions by source (soil organic matter mineralization, crop residue decomposition, root respiration) is difficult. Our objective was to determine the contribution of decomposing wheat straw to CO2 emissions from a Swinton silt loan under controlled conditions (constant 20°C). Two types of straw (i.e., fresh straw collected shortly after harvest and standing stubble that had ‘weathered’ in the field for a year) were either incorporated or placed on the surface of soil (contained in polythene-lined, wooden boxes 10 cm deep with area of 1363 cm2) at a rate equivalent to 2,800 kg ha-‘. One set of soils was watered every two or three days to 90% of field capacity and a second set was allowed to dry (from 90% field capacity) to permanent wilting point before watering. Emissions of CO2 were measured every two or three days using a vented chamber connected to a portable CO2 analyzer. Within 2 d, incorporation of straw increased CO2 flux from 0.3 to about 1.5 umol CO2 mm2 s-1. Surface straw significantly (P < 0.05) increased fluxes, but the effect was small compared with incorporated straw. Straw type had little effect on emissions. Total CO2-C emitted from continuously moist soil in 77 d was 3.3 g box-’ (no straw added), 5.5 g box-’ when straw was placed on soil surface and 9.6 g box-’ when straw was incorporated. In all, 37% of incorporated straw C and 13% of surface straw C was emitted as CO2. Moisture limitation in soil subjected to moist/dry cycles reduced emissions to 34-59% of those observed under continuously moist conditions. Fluxes from soil with incorporated straw were least sensitive to moisture availability. In the absence of straw, CO2-C emitted during the experiment was about equal to the amount by which light fraction C decreased, suggesting that this labile fraction of organic matter was the main source of respired C.
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