Soil organic carbon and total nitrogen stocks under different land uses in a semi-arid watershed in Tigray, Northern Ethiopia

In Ethiopia, massive deforestation of natural forests and extensive use of agricultural lands have resulted in soil degradation. Soil organic carbon (SOC) quantity and quality are crucial to soil quality. However, knowledge on the effects of land use change on soil carbon storage in semi-arid northern Ethiopia is very limited. To address this problem, a study was undertaken within a semi-arid watershed in eastern Tigray, Northern Ethiopia, to estimate SOC and total nitrogen (TN) concentrations and stocks in 0–5, 5–10, 10–20 and 20–30 cm soil layers for five land uses: rainfed crop production (RF), agroforestry based crop production (AF), open communal pasture (OP), silvopasture (SP) and irrigation based fruit production (IR) each with five replications. Generally, both magnitude and difference in SOC and TN concentrations showed a decreasing trend with depth within and among most land uses. SOC and TN concentrations were highly correlated in all 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 compared with 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 baseline and 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−1 yr−1 in comparison with 0.065, 0.038, and 0.022 Mg N ha−1 yr−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−1 yr−1 in the 0–30 cm layer and in comparison with the RF land use system on an average of 15 years. The results of this study revealed that conversion of croplands to grasslands or integration of appropriate agroforestry trees in cropping fields in the region has large technical potential of SOC and TN sequestrations.