Soil Organic Matter Stabilization and Land-use Change in Tropical Ecosystems.

Replenishment of soil organic carbon (SOC) and nutrient stocks is dependent on the biophysical characteristics of the soil, quantity and quality of applied organic matter (OM) and the level of ecosystem degradation. A study was established in Western Kenya to investigate how status of pre-existing SOM, quality of added OM and soil texture influence stabilization of added OM. In SOC-poor soils, biochar (low quality organic) and tithonia green biomass (high quality organic) application increased whole SOC by 45 and 27% respectively above fully fertilized (NPK) control treatment. Biochar application, increased organic C content by 269.2 mg g^-1, 232.6 mg g^-1 and 3.7 mg g^-1 in free light (FLF), intra-aggregate (IAGF) and in the organo-mineral (ORGM) fractions respectively while tithonia increased FLF C by 49 mg g^-1; IAGF C by 17 mg g^-1 and 8 mg g^-1 above control in ORGM.  Rate of CO₂-C loss reduced by 27% with biochar application while tithonia increased C losses by 20% in the SOC-poor soils. Nature and content of the various functional groups in the IAGF OM distinctively changed with time of continuous soil use. C=C aromatic-C increased by 13% within 10 - 20 years of continuous soil tillage. In SOC-poor soils, biochar application enriched aromatic-C, carboxyl-C and traces of ketons and esters mainly in FLF and IAGF. Additions of tithonia biomass enriched conjugated carbonyl-C such as ketons and quinones as well as CH deformations of aliphatic-C mainly in the IAGF. Biochar application had greater impact on IAGF (64% increase) in the clayey soil compared with sandy soil (≈19% increase) above control soil. In this study we demonstrate that highly weathered tropical soils possess great potential for C increase and greater SOC increases could be attained with low quality organic resources whose stability in the soil is greater compared with high quality organics.