Abstract
The effects of plant species (castor bean (Ricinus communis L.) versus sunflower (Helianthus annus L.)) and irrigation regime (freshwater versus secondary treated municipal wastewater) on soil properties and on seed and biodiesel yield were studied in a three year pot trial. Plant species were irrigated at rates according to their water requirements with either freshwater or wastewater effluent. Pots irrigated with freshwater received commercial fertilizer, containing N, P, and K, applied at the beginning of each irrigation period. The results obtained in this study showed that irrigation with effluent did not result in significant changes in soil pH, soil organic matter (SOM), total kjeldahl nitrogen (TKN), and dehydrogenase activity, whereas soil available P was found to increase in the upper soil layer. Soil salinity varied slightly throughout the experiment in effluent irrigated pots but no change was detected at the end of the experiment compared to the initial value, suggesting sufficient salt leaching. Pots irrigated with effluent had higher soil salinity, P, and dehydrogenase activity but lower SOM and TKN than freshwater irrigated pots. Sunflower showed greater SOM and TKN values than castor bean suggesting differences between plant species in the microorganisms carrying out C and N mineralization in the soil. Plant species irrigated with freshwater achieved higher seed yield compared to those irrigated with effluent probably reflecting the lower level of soil salinity in freshwater irrigated pots. Castor bean achieved greater seed yield than sunflower. Biodiesel production followed the pattern of seed yield. The findings of this study suggest that wastewater effluent can constitute an important source of irrigation water and nutrients for bioenergy crop cultivations with minor adverse impacts on soil properties and seed yield. Plant species play an important role with regard to the changes in soil properties and to the related factors of seed and biodiesel yields.
Highlights
There is a growing interest worldwide with regard to the use of renewable energy sources as a mean to both reduce the environmental pollution associated with energy produced by fossil fuels and to tackle the problem of the depletion of these fuels
Soil pH in the deeper soil layer continued to increase during the last period and as a result of this trend a significant effect was detected in that period, with the deepest soil layer showing higher pH values compared to those of the upper ones
Pots irrigated with effluent were found to have slightly higher soil pH compared to those irrigated with freshwater, an effect which appeared at the beginning of the second irrigation period and peaked at the end of that period
Summary
There is a growing interest worldwide with regard to the use of renewable energy sources as a mean to both reduce the environmental pollution associated with energy produced by fossil fuels and to tackle the problem of the depletion of these fuels. The most common biofuels are biodiesel and bioethanol, mainly produced from biomass or renewable energy sources, with biodiesel representing 82% of the total biofuel production in the EU [2]. A major drawback of these bioenergy crops is the fact that the intensive cultivation and large scale production of vegetable oils for biodiesel production leads to significant increases in oil prices and an imbalance in the food market, in developing countries. These crops may compete with traditional food crops for land and available water. Castor bean can be grown on marginal lands which are usually unsuitable for food crops [3,4,5]
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