Bioindicators and nutrient availability through whole soil profile under orange groves after long-term different organic fertilizations

We investigated long-term (18 years) effects of three organic (cow manure (CM), poultry manure (PM), compost from agro-industry orange wastes (OW)) and one inorganic fertilization (IF) on various soil biological indicators (microbial biomass C, soil respiration, total phospholipid fatty acids (PLFAs), total bacteria, Gram-positive and Gram-negative bacteria and fungi) and nutrient contents (total and extractable organic C, total and mineral N, available P and K) along the profile of a Typic Haplustept under orange Mediterranean orchards. All fertilizers were added on the same N content basis (190 kg N ha−1 per year). Variables related to carbon cycling gradually worsened with depth, regardless of treatment, but at any soil depth they improved according to IF < PM < OW < CM. Similarly, mineral nutrient availability decreased with increasing soil depth, according to CM > OW > PM > IF. Organic manures favoured Gram-negative bacteria, especially between 35 and 55 cm depth, whereas Gram-positive bacteria flourished better under inorganic fertilization up to 55 cm. Organic fertilizers increased microbial biomass C, compared with IF, with CM being the most efficient, i.e. inducing the greatest C use efficiency, in turn coupled with a greater amount of fungi compared to bacteria. This is crucial from the environmental viewpoint, since fungi better contribute to soil C storage. PLFAs signatures of microbial communities reflected the differences in resources availability determined by soil depth and fertilizer type; indeed, fungi and Gram-negative bacteria responded more promptly to nutrient contents than other microbial groups. Oligotrophic conditions in deeper soil horizons increased the metabolic quotient, an indicator of stress conditions.