Carbon mineralization of leaves from four Ethiopian agroforestry species under laboratory and field conditions

Abstract

Green leaves of Albizia gummifera G.F. Gmel, Milletia ferruginea (Hochst.) Baker, Cordia africana Lam., and Croton macrostachyus Del. were collected from trees growing in fields in southern Ethiopia, and used in laboratory and field experiments. The aim was to investigate differences in C mineralization parameters related to differences in the leaf qualities of the respective species, and to examine effects of amending soil ( Mollic Andosols) with leaves plus N, P, or N + P on decomposition and microbial activity. Rates of carbon mineralization were determined by measuring CO 2 evolution using an automated respirometer (Respicond V) in the laboratory and an infrared gas analyser in the field studies. When no nutrients were added about 11–44% and 10–42%, on average, of the initial C applied as leaves was mineralized within a month in the laboratory and field respiration experiments, respectively. In both experiments, the rates of C mineralization were highest for C. macrostachyus leaves, followed by M. ferruginea then A. gummifera and lowest for C. africana leaves. Hence the results of our short-term laboratory study agreed well with those of the field experiment. Microbial activity (e.g. specific growth rate) was generally stimulated by supplemental nutrients. However, in most cases cumulative C mineralization was either slightly depressed or not significantly affected by supplemental N or N + P. Similarly, P addition caused either a reduction in C mineralization or had little effect. Hence the quality of the leaves was more influential than the nutrient additions. The absence of a pronounced respiratory response to the added N and/or P might be due to increased microbial C-use efficiency, to adequate amounts of these nutrients being available from the leaves and/or soil, or both. Further in-depth studies using soils of differing soil fertility are needed to test these hypotheses.