Subsurface Drip Irrigation in Poplar Bioenergy Systems: Biomass Production and Economic Evaluation in Mediterranean Climate

Fast growing hybrid poplar trees can be cultivated in agricultural areas for rapid biomass production under coppice management systems as block plantations or strips alternated to herbaceous crops (alley cropping). In Mediterranean conditions, irrigation is a necessary practice in poplar plantations for bioenergy purposes. Its usefulness is to balance out the precipitation shortcomings and implementing the plantation management properly, thus ensuring adequate biomass production. The main aim of the study is to quantify the biomass productivity and the economic sustainability of subsurface drip irrigation (SDI) practice in a Mediterranean poplar coppice plantation located in southern Italy (Montenero di Bisaccia). SDI is an advanced and efficient irrigation system; however, its high installation cost may be unprofitable when used for bioenergy production. The study plantation was established by using the hybrid poplar clone "AF2" under single row design, with overall plantation density of 6,060 trees ha-1. We collected the experimental data within two sub-plantation plots, the first under growing conditions of S2R4 (i.e., stem aged 2 years and roots aged 3 years); and the second under S3R5 (i.e., stem aged 3 years and roots aged 5 years). Every sub-plantation plot was divided into two equal parts, the first was subjected to subsurface drip irrigation; the second was rainfed, without irrigation. Measurements were taken at the end of the second and third growing seasons. The total irrigation water volume was 4831 m3ha-1year-1. The results showed that the fresh biomass yield, in the irrigated plots, increased by 79% compared to the non-irrigated plot, in the S2R4, while this difference increased up to 118% in S3R5. Considering the economic cost of SDI in relation to the variation of the sale price of woodchip and the production increase, an analysis of the economic sustainability of SDI on poplar SRC for different growing seasons was made. A financial evaluation via Net Present Value method (NPV) was carried out on a 10-year cycle. The initial cost of SDI, with technical materials, machines and workers included, was 1,280 EUR ha-1. Considering the biomass production obtained in the two different plots and the actual selling price of woodchip, the economic analysis shows that the farmer obtains a little economic loss in relation to the S2R4 plantation, while, a little economic advantage is obtained in the S3R5 plantation. To recover the investment cost of SDI during a period of ten years, the increase in biomass production should reach at least +90% in the irrigated plot compared to the non-irrigated ones. Key