Soil microbial C and N turnover under Cupressus lusitanica and natural forests in southern Ethiopia assessed by decomposition of 13C- and 15 N-labelled litter under field conditions

Natural forests in Ethiopia are frequently replaced by Cupressus lusitanica plantations, but little is known about consequences of this land use change for soil C and N dynamics. The objectives of the study were: (i) quantification of microbial incorporation of litter-derived C and N under field conditions, (ii) identification of forest management effects on microbial incorporation of litter-derived C and N and (iii) elucidation of soil moisture effects on microbial utilization of litter-derived C and N. Natural litter in the Munessa forest was replaced by C-13 and (15) N labelled litter and its degradation was studied over 2 years. Microbial incorporation of litter-derived C and N was measured by chloroform fumigation extraction and stable isotope analysis. Most of the C-13 and (15) N tracer remained in the litter or was incorporated into bulk soil, whereas soil microbial biomass showed minor incorporation. Silvicultural management practices influenced microbial litter-derived C utilization with increased microbial incorporation under wet soil conditions under plantations. Thinning of Cupressus trees led to increased litter decomposition during dry soil conditions. Soil humidity is the main influencing factor for microbial turnover of litter-derived C in this ecosystem. Fast-growing tree plantations had no negative effects on microbial C and N turnover when compared to natural forests.