| Development and utilization of bioenergy, one of renewable energy, is necessary to deal with the energy depletion and environmental pollution crisis. However, large-scale development of bioenergy needs large amout of bioenergy crops cultivation, which will compete resources with production of food, and affect economic, social and ecological environment. In order to realize the sustainable development of bioenergy industry, evaluation of large-scale production of biofuel feedstock on marginal lands need to be conducted, in arid and semi-arid areas of northern China.We conducted field experiment with sweet sorghum (Sorghum bicolor (L.) Moench) and switchgrass (Panicum virgatum L.), which was established in 2008 and 2009, in northern China. The biomass, moisture content and nutrient (N, P and K) accumulation dynamics of aboveground portion of sweet sorghum and switchgrass were examined from 40 to 160 days after sowing (sweet sorghum, DAS) or regrowth (switchgrass, DAR) in 2012 and 2013. The soil organic carbon (SOC) content at depths of 0-15 and 0-30 cm was quantified at sweet sorghum, switchgrass and native grassland sites, respectively. Meanwhile, we evaluated the water resource potential and allowed acreage of sweet sorghum cultivation through water requirements of sweet sorghum and the available water resources in Inner Mongolia. Given the uneven distribution of natural rainfall and water resources in this region, Tongliao, Ordos and Ulanqab were selected as the study areas, which located in the eastern, central and western part of Inner Mongolia, respectively. The water and land requirements of biofuel produced by sweet sorghum was evaluated through the water footprint concept; the water requirement of sweet sorghum cultivation was estimated by the actual crop evapotranspiration (ETc), with reference crop evapotranspiration (ET0) and crop coefficient (Kc). The available water resource was evaluated by the water pressure level, annual water withdrawals and total water resources. We also predict the maximum area allowed for cultivation of sweet sorghum by 2030, using the water red line released by the Ministry of Water Resources. In addition, according to the requirements of international sustainable development criteria, suggestions about cultivation management of large-scale production of sweet sorghum were proposed in Inner Mongolia. The main results are as follows:The aboveground biomass yield of sweet sorghum averaged 13.91 ha-1 (14.2 and 13.61 ha-1 in 2012 and 2013, respectively), while the switchgrass yield had a higher variability (14.8 versus 8.1 t ha- in 2012 and 2013, respectively) and averaged 11.51 ha-1. The removals of N, P and K averaged 56,10 and 160 kg ha-1, respectively, for sweet sorghum and 41,11 and 89 kg ha-1, respectively, for switchgrass. As nutrient concentration in leaf was higher than stalk, we suggested that leaf should be removed before processing and left in the field, which would significantly (p<0.05) reduce nutrient removals by sweet sorghum.There was no significant differences (p<0.05) in sweet sorghum yield, nutrient concentrations and moisture content between 120 and 160 DAS; but for switchgrass, biomass was nearly twice as high at 160 DAR than at 120 DAR, while the nutrient concentrations and moisture content were significantly lower at 160 DAR than at 120 DAR (p<0.05). To attain maximum biomass yield with minimum nutrient removal and the lowest transportation costs (due to the lower moisture content), switchgrass harvest should be delayed until approximately 160 DAR in this region, whereas sweet sorghum, which matures earlier, can be harvested as early as 120 DAS.Under sweet sorghum, the SOC content was reduced, although it did not differ from that of the native grassland at a significance level of p<0.05.The SOC content within the 0-15 cm layer averaged 15.8 and 11.6 g C kg-1 before and 3 years after the establishment of the switchgrass culture, respectively, and the SOC increased with the culture age. In contrast, the SOC content of the 15-30 cm layer did not differ between the adjacent native grassland and the switchgrass plantation.The amount of water averaged 559 and 608 mm need to meet the growth of early-maturing and late-maturing sweet sorghum, respectively. In growing seasons of the two varieties, natural precipitation was about 311-320 mm, which could not meet the demand of sweet sorghum, thus of about 248-288 mm of irrigation was necessary. To ensure the water pressure level be not severe, under ideal conditions, about 395000-430000 hectares of sweet sorghum allowing for cultivated in Inner Mongolia. According to the water red line released by the Ministry of Water Resources, we predict that the maximal allowed area for cultivation of sweet sorghum will be 675000-885000 hectares in Inner Mongolia by 2030. It’s obviously that the local water resources cannot support culture of sweet sorghum on all of the marginal lands (about 8 million hectares) in Inner Mongolia. The majority of water demand of sweet sorghum coincided with the period during which a large amount of annual rainfall occurs, especially in the central and eastern regions of Inner Mongolia, where cultivation of sweet sorghum could only rely on natural precipitation.According to the requirements of sustainable development, we suggested that large-scale cultivation of sweet sorghum, should mainly be located in mid-eastern regions of Inner Mongolia. Organic farming technology should be chose, reduced and no-tillage with mulch of crop residues, to reduce the negative impact of cultivation on soil quality; when rainfall can not meet the water demand of sweet sorghum, the high efficiency water-saving irrigation methods is adviced; leaves should be left on the field and incorporated into soil, to alleviate the nutrient removal and increase the sustainability of cropping system. |
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