Among energy crops, short-rotation coppices (SRC) are recommended to provide renewable energy. Compared to annual crops, willows and poplars are regarded as plants with low requirements for nutrients, herbicides, pesticides, and soil maintenance. However, only little is known about N-fertilizer effects on SRC and the few studies are even inconsistent. Therefore, we studied the effects of N on yields of willows and poplars in a field experiment. The effects of N fertilization on nitrate leaching and nitrous oxide emissions from the loamy-sand soil were also measured. Cuttings of willows (Salix viminalis clone Inger) and poplars (Populus maximovizcii × P. nigra clone max 4) were planted on farmland in 2008. The experiment was arranged in a random block design with three levels of N fertilizer (0, 50, and 75 kg N ha -1 y -1 ). After 2 y, the trees were harvested for the first time. Fertilization did not affect the yields of willows or poplars. However, the application of 75 kg N ha -1 y -1 caused an average increase of N leaching in the willow and poplar plots of 25 kg N ha -1 y -1 and 40 kg N ha -1 y -1 , respectively. Emissions of N 2 O were increased by a maximum of only 0.2 kg N ha -1 y -1 . Further, the N fertilizer stimulated the growth of the weed biomass in case of the willow plots by 46% and of the weed N content by 52% (r = 0.53). In conclusion, in the first 2 y, SRC could be produced in a more effective and environmentally friendly manner without mineral fertilizer.
An assessment of the potential availability of selected residues from maize, cassava, millet, plantain, groundnuts, sorghum, oil palm, palm kernel, and cowpeas for possible conversion to renewable energy in Nigeria has been made. It is estimated that nearly 58 million tonnes of these residues were potentially available in the year 2004 with energy potential of about 20.8 million tonnes oil equivalents. The residue availability for 2010 is projected to be about 80 million tonnes. These residues, when converted to energetically usable forms, can substitute or complement the fossil energy sources in Nigeria by more than 80%.
Carbon (C) sequestration and soil emissions of nitrous oxide (N 2 O) affect the carbon dioxide (CO 2 ) advantage of energy crops. A long-term study has been performed to evaluate the environmental effects of energy crop cultivation on the loamy sand soil of the drier northeast region of Germany. The experimental field, established in 1994, consisted of columns (0.25 ha each) cultivated with short rotation coppice (SRC: Salix and Populus) and columns cultivated with annual crops. The columns were subdivided into four blocks, with each receiving different fertilization treatments. The soil C content was measured annually from 1994 until 1997, and then in 2006. Soil N 2 O levels were measured several times per week from 1999 to 2007. Water-filled pore space (WFPS) and soil nitrate measurements have been performed weekly since 2003. Increased C stocks were found in SRC columns, and C loss was observed in blocks with annual crops. The soil from fertilized blocks had higher levels of C than the soil from nonfertilized blocks. SRC cropping systems on dry, loamy sand soils are advantageous relative to annual cropping systems because of higher C sequestration, lower fertilized-induced N 2 O emissions, and reduced background N 2 O emissions in these soils. SRC cropping systems on dry, loamy sand soils have a CO 2 advantage (approximately 4 Mg CO 2 ha -1 year -1 ) relative to annual cropping systems.
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