May humic acids or mineral fertilisation mitigate arsenic mobility and availability to carrot plants (Daucus carota L.) in a volcanic soil polluted by As from irrigation water?

Carrot (Daucus carota L.) is a widely consumed root vegetable, whose growth and safety might be threatened by growing-medium arsenic (As) contamination. By this work, we evaluated the effects of humic acids from Leonardite and NPK mineral fertilisation on As mobility and availability to carrot plants grown for 60 days in a volcanic soil irrigated with As-contaminated water - representing the most common scenario occurring in As-affected Italian areas. As expected, the irrigation with As-contaminated water caused a serious toxic effect on plant growth and photosynthetic rate; the highest rate of As also inhibited soil enzymatic activity. In contrast, the organic and mineral fertilisation alleviated, at least partially, the toxicity of As, essentially by stimulating plant growth and promoting nutrient uptake. The mobility of As in the volcanic soil and thus its phytoavailability were differently affected by the organic and mineral fertilisers; the application of humic acids mitigated the availability of the contaminant, likely by its partial immobilisation on humic acid sorption sites - thus raising up the intrinsic anionic sorption capacity of the volcanic soil; the mineral fertilisation enhanced the mobility of As in soil, probably due to competition of P for the anionic sorption sites of the soil variable-charge minerals, very affine to available P. These findings hence suggest that a proper soil management of As-polluted volcanic soils and amendment by stable organic matter might mitigate the environmental risk of these soils, thus minimising the availability of As to biota.

Trichoderma spp. alleviate phytotoxicity in lettuce plants (Lactuca sativa L.) irrigated with arsenic-contaminated water.

The influence of two strains of Trichoderma (T. harzianum strain T22 and T. atroviride strain P1) on the growth of lettuce plants (Lactuca sativa L.) irrigated with As-contaminated water, and their effect on the uptake and accumulation of the contaminant in the plant roots and leaves, were studied. Accumulation of this non-essential element occurred mainly into the root system and reduced both biomass development and net photosynthesis rate (while altering the plant P status). Plant growth-promoting fungi (PGPF) of both Trichoderma species alleviated, at least in part, the phytotoxicity of As, essentially by decreasing its accumulation in the tissues and enhancing plant growth, P status and net photosynthesis rate. Our results indicate that inoculation of lettuce with selected Trichoderma strains may be helpful, beside the classical biocontrol application, in alleviating abiotic stresses such as that caused by irrigation with As-contaminated water, and in reducing the concentration of this metalloid in the edible part of the plant.