Abstract

Conversion of native forests to cultivation is usually accompanied by a decline in soil organic carbon and nutrients, and deterioration of soil structure. Our knowledge of this process must increase if we are to optimise the physical regeneration of degraded soils in semiarid areas. In this work we aim to (i) determine the relationship between aggregate stability and soil and particulate organic carbon (ii) to study the evolution of this relationship with land use. Samples were collected from a cropland cultivated and an adjacent natural forest and pasture from tree toposequence in North Tunisia. Change in land use from forest soil to pasture and cultivation induced significant losses of soil and particulate organic carbon. The soil organic carbon loss from forest soils to agriculture soils reached 83 and 74% from forest soils to pasture lands. However, the average of particulate organic carbon in forest ranged from 15 to 27 g kg^-1 and 3 to 14 g kg^-1 in agriculture soils. Across 13 sites, the average of mean weight diameter significantly decreased from (2.79 mm) natural forest soil to (2.10 mm) to pasture soil, being more significant in the cultivation soil (1.70 mm). Fast wetting treatment, causing aggregate slaking, was the process that decreased aggregate stability the most. Significant correlation was found between aggregate stability and soil organic carbon fraction. However, the greater association was observed between aggregate stability and particulate organic carbon. Soil where the organic matter was the principal aggregation agent (soil with sandy texture and zero carbonate calcium content) showed greater degradation sensitivity to soil organic carbon loss. Soil aggregate stability and soil organic carbon fraction could be used as indicators to apply the most appropriate management practices to increase soil sustainability or productivity.  

Key words: Land use, soil organic carbon, particulate organic carbon, aggregate stability, North Tunisia.