Effect of Biophysical Soil and Water Conservation Measures on Labile and Recalcitrant Soil Carbon and Nitrogen Pools in Tigray, Ethiopia

This study quantified how soil and water conservation (SWC) practices, slope position, and soil depth influence labile and recalcitrant carbon (LC, RC) and nitrogen (LN, RN) fractions in the Tigray highlands of northern Ethiopia. Ninety-six composite soil samples were collected from four SWC-treated subwatersheds, Mechahile (20.54 ha), Hararu (9.45 ha), Abdi-Ruba (10.37 ha), and Rva (11.30 ha), and four untreated ones—Endacherkos (12.06 ha), Lihay (8.95 ha), Bringede (14.40 ha), and Zban-Dera (19.25 ha). Two factorial experiments (management × slope × depth; distance × slope × depth) were established, each with 12 treatments across 48 plots. Data were analyzed in R (v4.2.3) using ANOVA and Tukey’s HSD at p<0.05. SWC practices and slope position significantly increased LN and RN (p<0.001), with LN rising from 0.00115 to 0.00155 mg g−1 and RN from 1.29 to 1.71 mg g−1 on treated lower slopes. Similarly, LC increased from 0.0269 to 0.0451 mg g−1 and RC from 33.6 to 56.3 mg g−1, indicating strong enrichment of both labile and stable pools. All fractions were higher at 4 m than at 8 m from SWC structures, showing nutrient accumulation near conservation features. Overall, SWC structures enhanced soil organic carbon and total nitrogen, particularly on lower slopes, by improving both fast-cycling and stable fractions. The findings highlight slope-specific management and optimized SWC spacing as critical strategies for maximizing soil C and N sequestration, fertility restoration, and sustainable land management in erosion-prone drylands.