Yemadje Lionel

Effect of wetting-drying cycles on carbon mineralisation: the cotton area of North Cameroon

Thesis defended on 28 September 2015 - Montpellier University

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The soil is a major organic carbon storage compartment within the terrestrial ecosystem. It plays an important role in the regulation of climate and agricultural production. Any change in the flows of carbon between the atmosphere and the terrestrial ecosystem could have a major impact on the concentration of CO2 in the atmosphere as well as on the reduction in the amount of soil organic matter and thus soil fertility.

In North Cameroon, the soils are often exposed to long dry periods alternating with a wet season. The period of transition between these two periods is characterised by very irregular rainfall which can last for several weeks. These wetting-drying cycles can accentuate soil organic carbon mineralization and nutrient cycles. This study set out, therefore, to assess the impact of wetting-drying cycles on carbon mineralisation in sudano-sahelian conditions.

To take measurements in the field, it is necessary to study the change in soil respiration over 24 hours after wetting during a dry period. The methodology developed showed that soil respiration had a quadratic curve during the day which became nearly linear during the night. The soil temperature and humidity explained at least 73% of the changes over 24 hours. These observations were used to propose a method for estimating the mean diurnal and nocturnal respiration of the soil after wetting. The method proposed in this study has the advantage of being based on a small number of measurements and is, therefore, easier to implement for monitoring soil respiration over 24 hours after the first rains. An initial field experiment showed that re-wetting the soils and the straw management method increased the mineralisation of soil carbon. However, frequent soil wetting-drying cycles over 50 days did not increase the total soil carbon mineralisation.

In North Cameroon, the rapid mineralisation of straw makes it difficult to increase oil carbon stocks by keeping straw from previous crops on the soil surface. In a second laboratory experiment, the wetting-drying cycles did not increase the mineralisation of the soil organic carbon and nitrogen compared with soils that were kept moist. However, CO2 emissions increased when straw was added. Adding labelled straw increased the mineralisation of soil organic matter (priming effect). The mineralisation of the straw reduced with the wetting-drying cycles and the amount of straw remaining on the soil was 102 µg C g-1 soil for re-wetted soils compared with 48 µg C g-1 soil for soils kept moist. The lack of response of the mineralisation of soil C and N to the wetting-drying cycles may be related to a reduction in microbial activity during the drying periods and the absence of a sustained increase in the carbon mineralisation rate with subsequent wetting-drying cycles.







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