ORGANIC CARBON AND NITROGEN ASSOCIATED WITH SOIL AGGREGATES AND PARTICLE SIZES UNDER DIFFERENT LAND USES IN TIGRAY, NORTHERN ETHIOPIA

Soil aggregation serves as storage and cycling of soil organic carbon (SOC) and total nitrogen (TN), which are essential for the functioning of terrestrial ecosystems. Thus, aggregate and particle associated SOC and TN in 0- to 10-cm and 10- to 20-cm layers were estimated for five land uses: rainfed cultivation (RF), agroforestry (AF), open pasture (OP), silvopasture (SP), and irrigation (IR) each with five replications within a watershed in Ethiopia. OP had the highest WSA>2mm (887%) and SOC associated with macroaggregates (200g kg(-1)), which were significantly higher (P<00001; P=00017 for WSA and SOC, respectively) than that in other land uses in 0- to 10-cm depth. SOC associated with both macroaggregates 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 (26g kg(-1)) followed by that in SP (23g 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. OP had also significantly higher SOC (P=00001) and TN (P=00004) associated with sand particles than RF, AF, and IR. Sand-associated SOC and TN were the highest in uncultivated systems. Moreover, the higher SOC concentrations associated with clay particles in soils under OP, SP, and AF showed that grass-based and tree-based systems are rich in stable SOC as clay-associated SOC has higher residence time than that associated with sand or silt fractions. Copyright (c) 2013 John Wiley & Sons, Ltd.