Effects of activated charcoal and quebracho tannin amendments on soil properties in irrigated organic vegetable production under arid subtropical conditions

In many arid and semi-arid regions, irrigated vegetable production leads to major carbon (C) and nitrogen (N) losses owing to high turnover rates. The goal of this experiment was therefore to test two amendments, activated charcoal and quebracho tannins, in their ability to stabilize soil organic carbon (SOC) from goat manure application in order to enhance nutrient and water retention. To this end, a 2-year field experiment was conducted on a sandy alluvial soil in Northern Oman investigating the effects of the two amendments either by mixing them with goat manure in the soil (MC mix and MT mix ) or by applying manure from goats fed 2.5 % charcoal (MC fed ) or 3.6 % tannin (MT fed ) of their daily diet. Mineral fertilizer (NPK) and pure goat manure (M) served as controls. Application rates amounted to 335 kg N ha −1 year −1 and 6.4–8.2 t C ha −1 year −1 (depending on C and N concentrations) and 2.0 and 1.4 t activated charcoal ha −1 year −1 and 2.8 and 1.7 t quebracho tannin ha −1 year −1 (in 2010 and 2011, respectively). Goat manure applications, in general, increased SOC, total N, and basal respiration compared with mineral fertilizer. Mineral fertilizer reduced SOC by −25.5 % and total N by −20 %, whereas organic treatments increased SOC by up to 21 % and total N by 19 to 48 %. Basal respiration ranged between 4.1 and 8.5 μg CO 2 -C day −1 g −1 and was only affected by mineral fertilizer with an average reduction of 25 and 40 % in 2010 and 2011, respectively, compared with organic treatments. The metabolic quotient was not significantly altered by any of the treatments. Charcoal amendments increased SOC by 10.6 % when charcoal was fed to goats and by 21.3 % when charcoal was mixed with manure and reduced net C losses, whereas pure goat manure did not change it significantly. Tannins mixed with manure did not affect soil parameters, whereas tannins fed to goats showed opposite effects to the other goat manure treatments on pH and SOC.