Effect of organic matter quality, soil temperature and moisture on soil organic matter decomposition of a forest -cropland chronosequence

Msc-Thesis

Temperature sensitivity of soil carbon decomposition is a key factor in determining the response of the terrestrial carbon balance to climate change. However, the effect of substrate on temperature sensitivity of decomposition has not been incorporated into the carbon cycle models because the differences between recalcitrant and labile carbon pools have not been demonstrated. The objective of the study was to evaluate the interaction of organic matter quality, soil temperature and water content on soil organic matter (SaM) decomposition. To obtain organic matter with varying quality attributes, a chronosequence approach was used to sample soils from a forest-cropland chronosequence in Kakamega, Western Kenya. The conversion periods were young conversion (12 years old), mid conversion (42 years old) and old conversion (77 years old). A paired design was used to pair a forest and cultivated field in the same conversion time. From each conversion time, soil was sampled to a 10 em depth from the paired sites in three replicates in January 2009 and taken to ICRAF laboratory where they were incubated for carbon dioxide (C02) evolution at different temperatures (10°C, 25°C and 33°C) and soil water contents (0%, 50% and 100% water holding capacity - WHC). The bulk soil sample was also analyzed for total carbon (C) and nitrogen (N) using dry combustion method while SaM quality characterized by Mid-Infrared spectroscopy. XVI Total C and N loss for the past 77 years of cultivation was found to be 60% with recalcitrant carbon observed in cultivated soils. Decomposition rates increased linearly (R2 = 0.99; P<O.OOl)with temperature for both forest and cultivated soils with higher emissions coming from forest soils. The effect of water content on the rate of CO2 emission had a parabolic fit with lowest emission recorded at 0% WHC and highest at around 50% WHC while the rates started declining as was content approached 100% WHC. Both labile and recalcitrant organic matter pools were found to influence (P<0.05) temperature sensitivity of SOM decomposition at different incubation periods subject to water content levels. This study clearly indicates that soil organic matter decomposition is significantly affected by soil temperature, water content and soil organic matter quality which is influenced by the land-use change and the cultivation period. Incorporation of the three factors; temperature, water content and organic matter quality into carbon cycle models will help in predicting the effect of climate change on SOC storage. It will also help in managing SOC content in various landscapes through improved land management that will also reclaim C released to the atmosphere due to land use change.

University of Nairobi