Abstract

Depletion of soil organic matter (SOM) is a major component of soil degradation that threatens the sustainability of smallholder farming systems in Ethiopia. In this study, soil organic carbon (SOC) and total nitrogen (N) dynamics following deforestation and subsequent cultivation were evaluated using natural abundance of C-13 and N-15 along a farm field chronosequence cleared from tropical dry Afromontane forest in south central Ethiopia. Soil samples were collected from five closely located farm fields cultivated for 7, 10, 26, 34 and 53 years after clearance and from an adjacent natural forest soil. All the soils in the study were Mollic Andosols/Humic Haplustands. The delta(13)C values of the SOC of the farm fields in the 0-10 and 10-20 cm layers were significantly elevated compared to the natural forest soil. In the 0-10 cm layer, the lowest delta(13)C value of -23.17 parts per thousand +/- 0.36 was recorded from the natural forest soil and the highest value of -15.7 parts per thousand +/- 1.50 from the 53-year-old farm field. Fractionation of the SOC of the bulk soil based on the delta(13)C values showed that in the 0-10 cm layer the SOC of the forest origin declined by 74.6%, which is equivalent to 54.1 Mg C ha(-1) or 740 kg ha(-1) year(-1), where as SOC input from the agricultural crops was low (240 kg ha(-1) year 1). However, part of the SOC of the forest origin lost from the surface 0-10 cm soil layer was translocated to the 10-20 cm layer. The SOC derived natural forest approached steady state around 30 years after cultivation while the SOC derived from agricultural crops reached steady state already after 10 years. The results show that the remaining forest derived SOC is recalcitrant while the SOC derived from agricultural crops has a relatively short mean residence time. The soils of the farm fields also showed increasing delta(15)N values indicating a loss of N from the system. The increasing delta(15)N values were consistent with the loss of total N along the chronosequence. A continued loss of N combined with insufficient input of fertilizer will result in a decreased productivity of the studied farming systems. (c) 2005 Elsevier B.V. All rights reserved

Keywords

13C; 15N; C3 plant; C4 plant; Land-use; Chronosequence; Soil organic carbon; deforestation; Mollic Andosol; Humic haplustand

Published in

Agriculture, Ecosystems and Environment
2005, Volume: 109, number: 1-2, pages: 9-19
Publisher: ELSEVIER SCIENCE BV

SLU Authors

• Karltun, Erik

  • Department of Forest Soils, Swedish University of Agricultural Sciences
    

• Olsson, Mats

  • Department of Forest Soils, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Environmental Sciences related to Agriculture and Land-use


Publication identifier

DOI: https://doi.org/10.1016/j.agee.2005.02.015

Permanent link to this page (URI)

https://res.slu.se/id/publ/6396