19 September 2014 - Disputas Aweke Mulualem Gelaw (IMV)

Norwegian title of the thesis:
Jordkvalitet og karbonfotspor av forskjellige bruksområder på småbruk i Etiopia

Prescribed subject of the trial lecture:
How can Climate Smart Agriculture (CSA) be implemented in Ethiopia – how can soil science contribute?

Time and place for the trial lecture and the public defence:
12:15 at SKP

Professor Bal Ram Singh
Professor Rattan Lal

Evaluation committee:
Professor Thomas Kätterer, Swedish University of Agricultural Sciences
Dr. Arne Grønlund, Bioforsk
Professor Trine Sogn, NMBU


The doctoral thesis is available for public review at the UMB library.
Thesis number 2014:70, ISSN 1894-6402, ISBN 978-82-575-1232-3



In Ethiopia, deforestation of natural forests and extensive use of agricultural lands have resulted in soil degradation. Despite recent massive restoration measures implemented on degraded landscapes, nowhere in the country theproblem is more manifest than in Tigray, Northern Ethiopia. Most soils in this part of the country are already exhausted by several decades of over exploitation and mismanagement. There are different types of land uses in the region butquantitative information is lacking about the impacts of these land uses on soil organic carbon (SOC) and total nitrogen(TN) storage capacities and on soil quality. Therefore, this study assessed effects of different land uses on soil organic carbon (SOC) and total nitrogen (TN) stocks, on associations of SOC and TN with soil aggregates and primary particles, and on soil quality. Data for papers I, II, III and V have been collected from the following five land uses within Mandae watershed in eastern Tigray: (1) tree-less rainfed cultivation (RF) (2) Faidherbia albida based agroforestry (AF), (3) open pasture (OP), (4) irrigation based Guava fruit production (IR) and (5) Faidherbia albida based silvopasture (SP). The objectives of this study include: (i) measuring SOC and TN stocks and concentrations in soils under the five land uses (AF, RF, OP, IR and SP) and four depths (0-5, 5-10, 10-20 and 20-30 cm) (Paper I), and soils under four land uses (AF, RF, OP and SP) and three depths (0-15, 15-30 and 30-50 cm) (Paper V), (ii) determining magnitudes of SOC and TN associated with soil aggregates and primary particles under the five land uses and two depths (0-10 and 10-20 cm) (Paper II), and (iii) compare the effects of only three agricultural land uses (AF, RF and IR) in 0-15 cm depth on selected physical, chemical and biological soil quality indicators, and on an overall integrated soil quality index (SQI) of the soils under these land uses (Paper III). In addition, C-use efficiency, and C-sustainability index of the smallholder crop-livestock mixed production systems in the whole country, Ethiopia, was assessed using C footprint analysis on data obtained from the abstracts of the central statistics agency (CSA) of Ethiopia and FAO databases (Paper IV).

Soil OC and TN concentrations differed significantly among different land uses and across depths. Both SOC and TN were higher in OP and SP than in other land uses. The highest SOC concentration in 0-5 cm was measured in OP (25.4 g kg-1) followed by that in SP (16.0 g kg-1), and the lowest was in RF (2.29 g kg-1). In 5- 10 and 10-20 cm depths, SOC concentration followed the same trend except that the amount of SOC in OP and SP land use systems decreased by about 50% compared with that in the top 0-5 cm depth. Total N concentration followed similar trends. Further, SOC and TN concentrations were highly correlated among land uses and depths. Total stocks in 0-30 cm layer were 25.8, 16.1, 52.6, 24.4 and 39.1 Mg ha-1 for SOC, and 2.7, 1.6, 4.9, 1.9 and 3.5 Mg ha-1 for TN in AF, RF, OP, IR and SP land uses, respectively. With RF as the baseline and taking the duration of 50 years since land use conversion, the average rate of accumulation was 0.73, 0.46, and 0.19 Mg C ha-1yr-1 and 0.065, 0.038, and 0.022 Mg N ha-1yr-1 for OP, SP and AF, respectively. Soils under IR also accumulated 0.56 Mg C ha-1 yr-1 and 0.019 Mg TN ha-1yr-1 in the 0- 30 cm layer and in comparison with the RF land use system on an average of 15 years. Similar trends were also observed for both SOC and TN stocks in 0-50 cm depth soils under OP, SP and AF land uses in comparison with RF.

Open pasture had the highest WSA >2 mm (88.7 %) and SOC associated with macroaggregates (20.0 g kg-1)

which were significantly higher (P < 0.0001; P < 0.01 for WSA and SOC, respectively) than that in other land uses in 0-10 cm depth. SOC associated with both macro- and microaggregates decreased with depth. Macroaggregates contained higher SOC than microaggregates in both layers under all land uses. AF had the highest SOC associated with microaggregates (2.6 g kg-1) followed by that in SP (2.3 g kg-1), indicating its potential to stabilize SOC more than other land uses. TN associated with macroaggregates followed a trend similar to that of SOC. Similarly, OP had significantly higher SOC (P <0.001) and TN (P <0.001) associated with sand particles than RF, AF and IR. Sandassociated SOC and TN were the highest in uncultivated systems. Moreover, the higher SOC associated with clay particles in soils under OP, SP and AF showed that grass and tree based systems are rich in stable SOC as clayassociated SOC has higher residence time than that associated with sand or silt fractions. Among the three agricultural land uses, AF had significantly higher values (P <0.05) than RF for all soil functions except for soil’s resistance against degradation (RD). For the overall SQI, the values for the three land uses were in the order: 0.58 (AF) >0.51 (IR) >0.47 (RF). Thus, AF scored significantly higher SQI (P <0.01) than that of RF. Major driving soil properties for the integrated SQI were soil organic carbon (26.4 %), water stable aggregation (20.0 %), total porosity (16.0 %), total nitrogen (11.2 %), microbial biomass carbon (6.4 %) and cation exchange capacity (6.4 %). These six parameters together contributed more than 80 % of the overall SQI.

Carbon-based inputs increased 2-fold from the lowest (0.32 Tg Ceq y-1) in 1994 to the highest (0.62 Tg Ceq y-

1) in 2010. Similarly, total C-output increased linearly from the lowest (5 Tg Ceq y-1) in 1994 to the highest (17 Tg Ceq y-1) in 2011. Further, the average rate of increase in C-output from 1994 to 1999 was marginal at 0.3 Tg Ceq y-1, but the 11 years average rate of increase from 2000 to 2011 was relatively higher at 0.8 Tg Ceq y-1. The relationship between annual total C-based input and output was strong (R2 =0.86; P <0.001). The CSI of the smallholder  agricultural production systems in Ethiopia was comparable with other more intensive systems in other regions of the world with the 18-year average value of ~22.

In conclusion, results from the case study in Tigray showed significant decline in SOC and TN contents and

their association with aggregates and primary particles by land use change from grazing lands and silvopastures to agricultural lands. Agroforestry and irrigation land uses also showed improvements in many soil quality indicators than that of the control, RF. On the other hand, the study on C footprint analysis for the whole country, Ethiopia, showed a recent nationwide significant expansion in area of cultivated land encroaching to the remaining grazing lands and forest areas and this trend raises questions about the sustainability of the process. Therefore, improvement of crop yields via intensification on land already under cultivation and conservation of the remaining grazing lands and forests should be prominent among a portfolio of agricultural development strategies both at regional and national levels.


Keywords: Land use; C-sequestration; Soil Organic Carbon or Total Nitrogen; Soil quality; C-footprint;

Sustainability; Ethiopia

Published 31. oktober 2016 - 17:34 - Updated 23. mai 2017 - 19:28