Effect of defatted oilseed meals applied as organic fertilizers on vegetable crop production and environmental impact.

Defatted oilseed meals from sunflower (Helianthus annuus L.), and some Brassica species have a wellknown economic value as feed for animals, whilst their value as organic nitrogen fertilizers has been notfully explored so far. Compared to sunflower, the seed meals of Brassica species are reported to have apotentially higher capacity of nutrient supply, due to their lower content of indigestible fiber. The highcontent of glucosinolates may conversely reduce the availability of nitrogen, because of their inhibitoryeffect on nitrification processes.In this lysimetric study we compared the effect of the application of three seed meals: partially defattedmeals of sunflower and of Ethiopian mustard (Brassica carinata A. Braun), and completed defatted meal ofsunflower on the yield and N utilization of three vegetable crops (lettuce, chard and spinach), comparedto an unfertilized control and one fertilized with ammonium nitrate. Furthermore, we also investigatedthe residual effect of fertility of oilseed meal application on winter barley and fallow. Both sunflowermeals revealed a fertilization effect comparable to that of the mineral fertilizer, whilst B. carinata mealreduced the yields of chard and spinach, due to low N availability as shown by the lower N use efficiencyindex. We argue that this lower efficacy of B. carinata meal was due to the technique of the application ofthe meal, that might have produced a high concentration of glucosinolates in the soil layer explored byroots, concurrently determining a significant impact on soil biota and direct phytotoxicity phenomena.A relevant part of the N applied with fertilizers was not absorbed by the three vegetables and henceaccumulated in the soil. Despite the high values of unabsorbed soil N in the plots fertilized with oilseedmeals, nitrate leaching was lower than with ammonium nitrate. The impact on global warming potentialof fertilization with defatted oilseed meals was assessed and shown to be lower than or comparable tothe impact of the synthetic fertilizer. In particular, defatted sunflower meals caused a lower impact perhectare and total biomass, and a similar impact per fresh marketable product.Overall, our findings confirmed the high value of oilseed meals as a sustainable alternative to mineralfertilizers. However, their efficacy strongly depends on the technique of application to the soil.