«Abstract Recently, researchers have drawn their attention to industrial hemp ( Canabis sativa L.) and stinging nettle ( Urtica dioica L.), as ...»
All the parameters of the stinging nettle presented in Table 2.2 were significantly ( P 0.01) impacted by Yr. Nettle treatment (T; at P 0.01) and T × Yr interaction (at P 0.05 mostly) were weighty factors for DMY both of biomass and stems as well as for the respective CY. However, C in both biomass and stems was T and T × Yr independent parameter. Biomass DMY of wild nettle was more than twice as low as that of the clone of fibre nettle (on average 3945 kg ha− 1 vs 9194 and 9629 kg ha− 1). Such large distinction between clones of fibre nettle and wild ecotype was observed in each year of the study and not only in DMY of biomass, but in DMY of stems as well as in CY from biomass and stems. Differences in both DM and CY between treatments of fibre nettle grown in plots of diverse density were inappreciable with a trend of higher values in the sparser plots (60 × 100 cm). Biomass and stems of nettle plants had high C (528 and 546 g kg− 1 DM, respectively) and CY (4125 and 3719 kg ha− 1); although these values were lower in the respective fractions of hemp. C stock of clones of fibre nettle amounted to 4882–5389 kg ha− 1 in biomass and to 4579–4810 kg ha− 1 in stems.
averaging 7589 kg ha− 1 per trial and 9412 kg ha− 1 per clones of fibre nettle (Table 2.2). As reported by Vogl and Hartl  as well as Harwood and Edom  stinging nettle can be cultivated for 4 and more years. It is likely that in our study a sharp decrease in DMY and other related parameters in 2011 occurred due to the fact that after harvesting in 2010 before winter (November 15) the aftermath was cut. On the other hand, yielding potential of plants could fall with the harvesting year: 2011 was a third harvesting year of nettle.
Although, according to the most discussed features, stinging nettle conceded to industrial hemp, our results showed nevertheless that the annual C production per above-ground biomass and stems was distinctly higher for clones of fibre stinging nettle (5135 and 4695 C ha− 1 yr− 1 on average) than that of the mature forests .
Gower et al.  reported that the above-ground net primary C production of the mature forests in Canada ranged from 3490–3520 kg C ha− 1 yr− 1 for aspen stands to 1170–1220 kg C ha− 1 yr− 1 for jack pine stands. Consequently C stocks in the biomass and stems of wild nettle (2103 and 1767 C ha− 1 yr− 1) were lower than annual C 24 B. Butkutė et al.
accumulated per ha for aspen stands; however exceeded those for jack pine stands.
Our results also showed that C production per above-ground biomass and stems of hemp (5866 and 5149 kg C ha− 1 yr− 1 on average) noticeably exceeded that of mature forests. To accumulate these C quantities in above-ground biomasses, plants of hemp and clones of fibre nettle had utilised from atmosphere on average 21,509 and 18,828 kg CO2 ha− 1 yr− 1, respectively, (CO2 content needed for stubble and roots biomass not included). So theoretical calculation shows, that fixation of CO2 into biomasses of industrial hemp and clone of fibre nettle might contribute towards reducing its accumulation in the atmosphere. Our results support the conclusion of Finnan and Styles  that hemp could considerably reduce greenhouse gas emission. The same is true for stinging nettle. This chapter discusses the C stock in a part of the annual yield of nettle above-ground biomass and stems only, that is, in biomass accumulated during the short summer period of approximately 3 months.
Therefore in order to calculate the total annual C stock, one should sum the above described C yield with C stock in biomass, cut at the end of May–beginning of June and aftermath for stinging nettle as food ingredient, as well in stubbles and roots. It is noteworthy that, first, nettle is a perennial crop with root biomass exceeding that of annuals  and second, nettle fields could be expected to remain productive for several years with low labour costs which will positively influence the economic viability of nettle biomass production .
Three-way analysis of variance (ANOVA), applied to reveal the significance of factors for shives percentage from stems, shives DMY, C and CY in shives was carried out according to the following scheme: A factor R, B factor V or nettle treatment (T), C factor harvest year (Yr; Table 2.3). ANOVA was used for hemp and stinging nettle separately. The R was significant at P 0.01 for shives output from stems and C concentration in nettle shives only. Yr and V or nettle treatment were factors that impacted output, DMY, CY of shives both of hemp and stinging nettle. All the tested factors and Yr interaction with both R and T were significant ( P 0.01) for nettle shives output. Average per trial output of shives from nettle stems (60.5 %) was markedly higher than that from hemp stems (45.7 %). Water-retted stems provided lower percentage of shives (45.7 and 57.0 %, respectively for hemp and nettle), than dew-retted stems (46.2 and 64.0 %, respectively). Output of shives had a direct impact on DMY and CY in shives: DMY and CY for nettle shives were higher than for hemp shives, likewise DMY and CY for dew-retted shives were higher than for water-retted shives. The contribution of DMY of stems in combination with shives output for DMY and C stock in shives it cannot be disregarded.
As for C stock, it should be noted that C accumulated in shives of clones of fibre nettle was equal to that of the mature forests for aspen stands, CY in shives of wild nettle was close to annual C production per jack pine stands and CY in shives of hemp varieties took up an intermediate position (Table 2.3) and . So, for C stock that was found in the discussed agricultural waste, the target plants consumed large quantities of atmospheric CO2: 9068 and 9739 kg ha− 1 on an average for hemp and stinging nettle shives, respectively.
Average HHV, concentration of components, which are important to characterise hemp and nettle biomass, stems and shives as a source for solid biofuel are presented in Table 2.4.
A problem that herbaceous energy crops pose during combustion is the ash content: high ash amounts can cause slagging. High concentration of N in combusting biomass can promote greenhouse gas NOx emissions, and lignin is associated with HHV . All fractions of hemp and nettle showed good results of HHV: from 8.1–
18.2 MJ kg− 1 DM of whole above-ground plant part biomass to 19.1–19.5 MJ kg− 1 DM of shives. Ash, N contents in both hemp and nettle declined, whereas HHV and lignin increased in the following order: above-ground plant part—stems—shives.
From the data presented in Table 2.4, one can see that combustion properties (lower ash and N contents and higher HHV) of hemp fractions seem to be more valuable for solid biofuel than those of nettle. Therefore, our results corroborate an argument of Finnan and Styles  that hemp is a sustainable annual crop for climate and energy policy.
Figure 2.1 shows the linear relationships between averaged for fractions HHV and C as well as lignin concentration values.
Both parameters of biomass quality positively at P 0.01 correlated with the HHV. Demirbaş  showed also that the HHV of lignocellulosic fuels is highly correlated with lignin content.
2.4 Conclusions Hemp and clones of fibre stinging nettle could be promising candidates for bioenergy production. Annual C production per above-ground biomass and stems was distinctly higher for hemp (5866 and 5149 kg C ha− 1 yr− 1 on an average) and clones of fibre stinging nettle (5135 and 4695 kg C ha− 1 yr− 1 on an average) compared with mature forests. According to HHV, C, lignin concentration and other solid biofuel-related parameters shives were revealed to be the most valuable fraction of both crops—hemp and stinging nettle. When comparing the two crops, hemp fractions showed better properties for solid biofuel purpose than nettle. The CO2 content fixed into biomass of studied crops might contribute towards reduction of climate warming.
Acknowledgements The article presents research findings obtained through the long-term programme “Biopotential and Quality of Plants for Multifunctional Use” implemented by Lithuanian Research Centre for Agriculture and Forestry.
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