Dynamics of particulate organic matter following biomass addition from fallow-improvement species in southern Mali

Knowledge about the nature and turnover rates of soil organic matter (SOM) is a key to soil fertility maintenance, especially in traditional farming systems. Particulate organic matter (POM), rather than total SOM content, is now considered a better measure of the soil's fertility. Fractionation of SOM, based on particle-size distribution after dispersion, was carried out on soil samples collected from a field experiment at N'Tarla, Mali (12.25degrees N latitude, 5.42degrees W longitude, Typic Plinthustalf). Biomass of off-site grown Gliricidia sepium (Jacq.) Walp. (gliricidia), Pterocarpus erinaceus Poir.(pterocarpus), and Stylosanthes hamata (L.) Taub. (stylosanthes) was applied to a maize (Zea mays L.) crop for two consecutive years. A significant decrease was noted in the soil's POM and POM-N contents after harvest of maize in both years as compared with the initial value measured at the beginning of the experiment. Small POM fraction (0.053 to 0.250 mm, POM53) contributed more to total POM than large POM fraction (0.250 to 2.000 mm, POM250). The POM N content was better correlated with maize grain yield than POM total weight, and the N content of POM53 was better related to maize grain yield (R-2 = 0.55) than that of POM250 (R-2 = 0.20). Total SOM was poorly correlated with maize grain yield (R-2 = 0.14). The results indicate that the quality of the applied biomass is of greater value than its quantity in sustaining maize grain yield, and POM53 might be a better indicator of soil fertility status than POM250 and total POM.