The sustainability of bioenergy coppice plantations is strongly affected by the Nitrogen (N) balance, whose removal is very high due to the frequent harvest of large quantities of biomass composed of small-sized shoots. Poplar bioenergy coppice plantations could have a Nitrogen removal comparable to herbaceous crops. In this study, five hybrid poplar genotypes (“AF2”, “AF6”, “Monviso”, “83.148.041”, “I214”) were compared for tree morphological traits related to yield, N removal in the harvested biomass and Nitrogen wood concentration (N%) after two biennial coppice rotations in two experimental plantations located in northern Italy. N removal was primarily influenced by biomass production, and linear positive relationships between biomass yield and N removal were established. N removal also varied greatly among genotypes due to clonal differences in yield and in N%, in relation to significant differences among clones for their branching and sprouting habits. In the first rotation, branchiness was positively correlated to N% with a significant coefficient of determination (R2=0.813), while at the end of the second rotation it was also significantly correlated to the shoots per stool ratio (R2=0.804). “Monviso” and “83.148.041” were the clones showing the highest yield, but also a high N% associated to an high level of branchiness and shoots per stool ratio. Our results highlight that poplar genotype selection for sustainable N management should be aimed at genotypes with low wood N concentration, coupling high yield with low branching and sprouting habits as in the case of the clone “AF2”.
Tree bioenergy plantations have the potential to produce large quantities of biomass to partially replace fossil fuels, as well as to mitigate the global warming (
SRC plantations are most frequently established on agricultural arable soils, and their cultivation cycle is fully mechanized, from establishing trees to harvesting the biomass. Despite this high level of mechanization, their energy balance is generally very positive (
The importance of N fertilization depends on two main factors. Firstly, similar to other plant nutrient elements, N is largely removed with the frequent and complete cyclical harvest of the aboveground biomass. Nutrient removal in SRCs is far greater than in conventional forestry (
A third element highlighting the importance of nitrogen fertilization for bioenergy sustainability lies in its environmental impact (
Nevertheless, little is known about the nutrient use and efficiency of poplar SRCs, and how they vary according to factors such as site conditions, biomass yield and genetic origin of the planting material. Significant variations in N uptake and N use efficiency (NUE) have been found among clones of important fast-growing tree species such as poplars (
The objective of this work was to examine the effects of coppicing cycles and hybrid poplar clones on stool morphology yield traits, wood N concentration (N%), NUE and N removal, with the aim of assessing clone suitability for bioenergy production and helping to refine the criteria of N fertilization for the woody crop in temperate plantation forestry.
In 2003, a network of three experimental plantations on arable soils was established in Italy in order to test new clones of poplar hybrids for SRC, by studying their biomass productivity and growth in relation to site characteristics and genetic background. Data on the first three biennial rotations were published in
Our aims were: (1) to investigate the effect of coppicing on N removal in the harvested biomass; (2) to test whether the studied poplar clones differed in their N use; and (3) to determine whether the morphological parameters of trees could be determinants in affecting wood N% at the clonal level.
This study was conducted in two experimental plantations established in spring 2003 aimed at comparing hybrid poplar clones under SRC conditions. Both plantations, Bigarello and Vinovo, are located in the Po valley, northern Italy, and their main characteristics are summarized in
The experimental design consisted in a randomized block with three-four replications per clone. Each replication was made up of homogenous plots with 60 trees organized in 5 rows (inter-row distance 280 cm), each with 12 trees (intra-row spacing 60 cm), with an inner sampling area of one row with 12 trees (20.16 m2).
No fertilization was supplied at any point along the cultural cycle of plantations, either before planting or during cultivation. Biomass harvesting, in biennial cycles, was performed mechanically in winter after leaf shedding.
Sampling and measurements were carried out at both plantations soon before mechanical harvesting in winter 2004/2005 and 2006/2007, at the end of the first and second biennial rotation cycle, respectively. Measurements were aimed at determining the main growth traits of the clones,
The field sampling for nitrogen content was organized as follows. In each experimental site, the diameter at 100 cm above the ground (D) of all individual shoots was measured, and the shoot population of each clone recorded into diameter classes (diametric classes 1 cm wide; the numeric value of each class represented the central value,
For each sample shoot, D and total height (H) were recorded with a caliper (precision 0.1 mm) and tape (precision 0.1 cm), respectively, and the fresh weight of both stem and branches was directly measured in the field with an electronic portable hanging scale (HBC 20K 50, KERN, Germany - precision 50 g).
The stem discs were collected at three different heights above the ground (
Percentage branchiness (branch %) of each sample shoot was calculated as the branch weight to total shoot weight ratio.
Immediately after sampling, the radius of each disc was recorded by cross-measurement with a digital caliper (precision 0.1 mm). Each disc was then placed into a plastic bag for minimizing moisture loss.
One week after field collection, sample disc volume over bark was estimated by the water displacement method, thereby obtaining individual density estimates. Each disc was then split into two equal parts, one used for dry weight determinations (at 105 °C, to constant weight), and the other for N determinations.
Discs for N determinations were dried at 40 °C to constant weight, and then ground by a laboratory mill (M10 Basic Microfine Grinder, IKA, Staufen, Germany), obtaining a homogeneous dust which was dried again overnight at 40 °C, and subsequently analyzed by CHN gas chromatography (Carlo Erba NA 1500, Italy) to determine the relative percentage N concentration (N%).
Stem volume over bark was estimated for each sample shoot by summing the volumes of the 2 stem sections and the top section, using the formulas for the basal cylinder (
Regarding the N content of three 3 stem sections, N concentrations were averaged over the discs at the basal (D20), intermediate (DH/2) and top (D2) sections, respectively. For the remaining branches, the branch N concentration was multiplied for the branch weight. The N content for each sample shoot was obtained by summing the N amounts of the three stem sections, including the branches wood nitrogen. The N uptake was then calculated per weight of each sample shoot according to its diameter class, and used for the estimation of total N uptake for each clone and site (N uptake, kg ha-1). NUE (kg dm kg N-1) was finally obtained dividing the above-ground woody biomass for each clone by its N uptake.
In each site, analysis of variance (ANOVA) was used to analyze the main effects of clone and cycle along with their interaction on growth parameters (D; H; Shoots/stool ratio; branch %) and N use parameters (N%; N uptake, NUE). This was carried out using a randomized complete block design with site, clone, cycle and clone × cycle as fixed factors and block as a random factor. Plot means for D, H, N shoots/stool ratio, N%, N uptake and NUE were the units of replication. Individual branchiness of test shoots after angular transformation (α = arcsin√%), was the unit of replication for branchiness. For significant ANOVA effects,
Results of the ANOVA carried out on growth parameters are shown in
At both sites, clone and cycle effects on branchiness were generally highly significant. Coppicing strongly reduced plant branchiness in both sites,
At both sites, the shoots/stool ratio was affected by the rotation cycle, strongly increasing from the first to the second rotation as a consequence of coppicing. After the first rotation, no significant differences among clones were found for this parameter at both sites. After the second rotation, the clone effect was very significant in Bigarello. Clones “Monviso” and “83.148.041” had the highest values of the shoots/stool ratio in both sites, while clone “AF2” had the lowest.
Results of the ANOVA carried out on N use parameters are shown in
NUE was significantly affected by clone and cycle effects at both sites, with a significant decrease from the first to the second rotation.
N% and NUE were never correlated with the shoot dimensions (
Yield data as aboveground woody biomass of the studied poplar clones are reported in
Bioenergy plantations of hybrid poplars, under short rotation coppicing (SRC) management are intensive systems of forestry plantation, requiring optimal site conditions in terms of soil quality, nutrient availability and soil moisture balance, as well intensive cultural practices such as weed control, irrigation and fertilization (
The two plantations considered in this study were established on alluvial arable soils with contrasting soil qualities within a larger network of plantations aimed at testing SRC poplar clones across Italy (
Using the data collected in these experiments, we established a relationship between the annual yield of aboveground biomass and N removal for poplar SRC in Italy (
Three main general observations regarding tailoring N fertilization in poplar SRC plantations may be highlighted here. Firstly, N uptake values in the aboveground woody biomass of poplar SRCs are comparable to those of many temperate agricultural crops (
We found an extremely high variability in average annual removal of N amongst the studied clones (
Our study highlighted the importance of productive determinants in Salicaceae clones such as branching and the shoots/stool ratio in affecting N% in the woody biomass produced by SRC plantations (
Two ideotypes of hybrid poplar clones with successful yield performance, but with opposite N% have been identified by this study. Clones like “AF2” combined low N% with low values of branch% and shoots/stool ratio during the first and second cycles, respectively. In contrast, clones like “Monviso” and “83.148.041” presented high N% with high values of the aforementioned morphological traits.
Nitrogen balance is a factor of considerable importance for the sustainability of bioenergy crops, in terms of their energy balance, greenhouse gas emission, environmental contamination, and biofuel quality (
The N removal by harvesting was studied in two poplar coppice bioenergy plantations during the first two rotation cycles of new hybrid clones, specifically selected for intensive management with high planting density (about 6000 plants ha-1) and a very short harvesting cycle (2 years). These kinds of agroforestry systems are intensive forms of bioenergy plantations of increasing importance in many countries with temperate climatic conditions (
N removal was found to be primarily influenced by biomass production. A significant relationship between biomass yield and N removal was detected. Such relationship can be used to refine N fertilization balance for the woody crop, both using chemical fertilizers and organic waste for disposal in bioenergy plantations (phytoremediation -
Our results demonstrate the feasibility of cultivar selection in poplars for SRC aimed at improving the NUE of genotypes, along with their biomass yield, the low branchiness and a low number of shoots per stool, as in the case of clone “AF2”.
This study was funded by the
Schematic layout of wood discs sampled from the stem (D20; h/2, D2) and branches of sample shoots and the three main sections for the stem volumetric determination (basal, median and apical sections). (
Linear regressions between the percentage woody biomass allocated to the branches (branchiness, in %) as the independent variable, and the percentage N concentration in the wood (N%) as the dependent variable, for the five clones in the two study sites (northern Italy), in relation to first and second biennial rotations. Each point represents the mean value of 3 replications. (*): P ≤ 0.05; (^): P ≤ 0.1. See
Linear regressions between the shoots/stool ratio (the number of shoots on the stool) as the independent variable, and the percentage N concentration in the wood (N%), as the dependent variable, for the five clones in the two study sites (northern Italy), in relation to first and second biennial rotations. Each point represents the mean value of 3 replications. (*): P ≤ 0.05; (^): P ≤ 0.1. See
Linear regressions between annual yield in dry matter (d.m.) woody biomass as independent variable and Nitrogen (N) uptake in the harvestable biomass as dependent variable for the 5 clones in the two study sites in northern Italy, in relation to first and second biennial rotations. Each point represents a single replication (plot) value. (**): P ≤ 0.01.
Main characteristics of the Vinovo and Bigarello study sites with experimental plots of hybrid poplar clones under short rotation coppicing management in northern Italy. (a): Köppen & Greiger classification (
Characteristics | Vinovo | Bigarello |
---|---|---|
Long /Lat | 7° 38′ 20.40″ E44° 57′ 25.03″ N | 10° 53’ 11″ E45° 11′ 26.93″ N |
Climate a | Cfb (Warm temperate, fully humid with warm summer) | Cfa (Warm temperate, fully humid with hot summer) |
May to August Prec. (mm) | 60 mm in 2003; 143 mm in 2004; 233 in 2005; 198 in 2006 | 67 mm in 2003; 216 mm in 2004; 292 in 2005; 181 in 2006 |
Soil (0-20 cm depth) | (14.4% clay, 19.9% sand); pHwater 7; tot N soluble 10.1 mg N kg-1 | (43.3% clay, 16.8% sand); pHwater 8.34; tot N soluble 10.6 mg N kg-1 |
Cultivation model | Planting date: April 2003; planting density 5900 cuttings ha-1; biennial harvestings in Jan. 2005 and Dec. 2006; No fertilization | Planting date: March-April 2003; planting density 5900 cuttings ha-1; biennial harvestings in Dec. 2004 and Jan. 2007; No fertilization |
List of hybrid poplar clones studied in northern Italy. (a): denotes where and by whom the crosses were made; (M) male; (F) female; (Yr): year of cross.
Genotype | Sex | Parentage | Yr | Breedera | Selection for |
---|---|---|---|---|---|
AF2 | M |
|
1992 | F. Alasia, Savigliano (Italy) | biomass |
AF6 | F |
|
1993 | F. Alasia, Savigliano (Italy) | biomass |
Monviso | F |
|
1991 | F. Alasia, Savigliano (Italy) | biomass |
83.148.041 | M |
|
1983 | CRA-PLF, Casale M. (Italy) | biomass |
I-214 | F |
|
1929 | G. Jacometti, Villafranca P. (Italy) | plywood |
ANOVA F and p-values for morphological growth traits and Nitrogen (N) status of shoots of hybrid poplar clones after two rotations in the study sites (northern Italy). (1): Total shoot height; (2): Shoot diameter, measured at 100 cm above the ground; (3): percentage allocation ratios between stem and branch woody biomass;. (4): number of shoots on the stool; (5): percentage concentration of N in the above ground woody biomass, including wood and bark; (6): N content in the harvestable above ground woody biomass; (7): Nitrogen Use Efficiency.
Site | Source ofvariation | H1 | D2 | Branchiness 3 | Shoots/stool Ratio4 | N%5 | N Uptake6 | NUE7 | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
F | p | F | p | F | p | F | p | F | p | F | p | F | p | ||
Vinovo | Clone | 16.4 | <0.0001 | 5.3 | <0.01 | 4.6 | <0.01 | 0.6 | 0.696 | 313.2 | <0.0001 | 16.5 | <0.0001 | 208.3 | <0.001 |
Cycle | 429 | <0.0001 | 396 | <0.0001 | 64.2 | <0.0001 | 360 | <0.0001 | 954.6 | <0.0001 | 34.4 | <0.0001 | 638.2 | <0.001 | |
Site × Clone | 1.1 | 0.383 | 0.7 | 0.596 | 0.8 | 0.512 | 1 | 0.414 | 107 | <0.0001 | 8.1 | <0.01 | 48.4 | <0.0001 | |
Bigarello | Clone | 27 | <0.0001 | 6.8 | <0.01 | 6.1 | <0.01 | 6.9 | <0.01 | 45.8 | <0.0001 | 16.5 | <0.0001 | 33.1 | <0.0001 |
Cycle | 258 | <0.0001 | 350 | <0.0001 | 129.3 | <0.0001 | 846.3 | <0.0001 | 38.2 | <0.0001 | 34.4 | <0.0001 | 40 | <0.0001 | |
Site × Clone | 12.3 | <0.01 | 0.7 | 0.602 | 5.5 | <0.01 | 7.3 | <0.01 | 8.2 | <0.01 | 8.1 | <0.01 | 9.9 | <0.0001 |
Means and standard errors (in parenthesis) of total shoot height (H) and diameter (D), shoots/stool ratio, and percentage branchiness of hybrid poplar clones after two biennial rotations in the study sites in northern Italy. (§): Shoot diameter measured at 100 cm above the ground; (†): number of shoots on the stool; (‡): percentage allocation ratios between stem and branch woody biomass. Values within each column followed by different letters are statistically different after Duncan’s MR Test (P ≤ 0.05). (ns): not significant; (*): P ≤ 0.05,(**): P ≤ 0.01 ; (***): P ≤ 0.001, levels of significance of the differences between the average value of the first and second rotation cycle of each parameter in each site.
Site | Clone | H (cm) | D (cm) § | Shoots/stool ratio † | Branchiness (%) ‡ | ||||
---|---|---|---|---|---|---|---|---|---|
1st rotation | 2nd rotation | 1st rotation | 2nd rotation | 1st rotation | 2nd rotation | 1st rotation | 2nd rotation | ||
Vinovo | AF2 | 844.5 (17.3) a | 628.0 (10.3) a | 6.3 (0.3) a | 3.5 (0.1) a | 1.2 (0.04) ns | 3.9 (0.29) ns | 15.5 (1.1) b | 8.5 (1.4) |
AF6 | 764.2 (22.6) bc | 587.5 (2.7) ab | 5.4 (0.3) b | 3.1 (0.03) ab | 1.5 (0.06) ns | 4.2 (0.24) ns | 16.9 (1.3) b | 10.6 (1.3) | |
Monviso | 731.0 (11.5) c | 510.7 (15.6) c | 5.4 (0.2) b | 2.9 (0.2) b | 1.3 (0.07) ns | 4.3 (0.34) ns | 24.1 (1.2) a | 13.2 (1.9) | |
83.148.041 | 816.5 (14.1) ab | 589.7 (17.4) bc | 5.2 (0.2) b | 3.1 (0.1) b | 1.1 (0.03) ns | 4.6 (0.31) ns | 19.6 (1.5) b | 9.8 (1.8) | |
I-214 | 742.5 (3.6) bc | 559.2 (10.5) b | 5.5 (0.1) b | 2.9 (0.1) b | 1.4 (0.1) ns | 4.2 (0.41) ns | 17.5 (1.8) b | 10.9 (1.8) | |
mean | 780.0 (11.9)*** | 575.0 (10.3) | 5.6 (0.1) *** | 3.1 (0.1) | 1.3 (0.05) *** | 4.2 (0.15) ns | 18.7 (0.7) *** | 10.6 (0.8) | |
Bigarello | AF2 | 536.5 (13.8) a | 434.0 (9.1) c | 4.4 (0.2) b | 2.4 (0.2) a | 1.0 (-) d | 4.1 (0.09) c | 20.7 (1.3) b | 11.3 (1.5) e |
AF6 | 476.2 (10.7) b | 319.2 (1.4) d | 3.8 (0.1) ab | 2.0 (0.1) b | 1.3 (0.06) d | 4.6 (0.33) c | 24.0 (2.3) bc | 19.3 (1.4) bcd | |
Monviso | 504.8 (5.4) ab | 259.5 (10.1) e | 4.2 (0.1) a | 2.1 (0.1) ab | 1.2 (0.06) d | 5.3 (0.3) b | 30.2 (1.4) a | 14.9 (1.6) de | |
83.148.041 | 512.0 (7.8) ab | 420.5 (17.5) c | 3.8 (0.1) ab | 2.0 (0.1) b | 1.0 (0.02) d | 6.1 (0.22) a | 31.3 (1.7) a | 11.5 (1.2) e | |
I-214 | 425.5 (18.1) c | 338.5 (11.9) d | 3.4 (0.3) b | 1.8 (0.3) b | 1.1 (0.04) d | 4.6 (0.25) c | 31.0 (1.7) a | 17.3 (2.4) cd | |
mean | 491.0 (10.1)*** | 354.3 (15.4) | 3.9 (0.1) *** | 2.0 (0.1) | 1.1 (0.03) *** | 4.9 (0.18) | 27.5 (1) *** | 14.7 (0.8) |
Means and standard errors (in parenthesis) of Nitrogen (N) status in the shoots of hybrid poplar clones after two biennial rotations in the study sites (northern Italy). (§): percentage concentration of N in the above ground woody biomass, including wood and bark; (†): N content in the harvestable above ground woody biomass; (‡): Nitrogen Use Efficiency. Values within each column followed by different letters are statistically different after Duncan’s MR Test (P ≤ 0.05). (*): P ≤ 0.05; (**): P ≤ 0.01; (***): P ≤ 0.001, levels of significance of the differences between the average value of the first and second rotation cycle of each parameter in each site.
Site | Clone | N % § | N uptake (Kg ha-1) † | NUE ‡ | |||
---|---|---|---|---|---|---|---|
1st rotation | 2ndrotation | 1st rotation | 2nd rotation | 1st rotation | 2nd rotation | ||
Vinovo | AF2 | 0.59 (0.016) f | 0.80 (0.003) d | 165.7 (4.4) de | 210.5 (16.1) cd | 168.8 (43) a | 125.2 (0.4) d |
AF6 | 0.75 (0.013) c | 1.05 (0.003) b | 225.9 (3.7) c | 286.0 (15.8) b | 133.2 (2.2) bc | 96.3 (0.3) f | |
Monviso | 0.96 (0.020) e | 0.96 (0.060) c | 245.5 (5.1) bc | 250.3 (16.7) bc | 104.2 (2.2) e | 103.8 (0.6) e | |
83.148.041 | 0.73 (0.016) e | 1.15 (0.002) a | 164.5 (3.6) de | 340.8 (18.3) a | 137.8 (3.0) b | 87.0 (0.2) g | |
I-214 | 0.60 (0.003) f | 0.78 (0.005) d | 132.2 (0.7) e | 172.2 (10.6) de | 165.4 (0.9) a | 127.5 (0.8) cd | |
mean | 0.73 (0.040) *** | 0.95 (0.030) | 186.8 (11) *** | 252.0 (15) | 141.9 (6.2) *** | 108.0 (3.6) | |
Bigarello | AF2 | 0.65 (0.024) f | 0.86 (-) cd | 73.1 (2.7) bc | 129.3 (14.9) bc | 155.4 (5.61) a | 116.6 (0.02) cd |
AF6 | 0.73 (0.036) e | 0.89 (0.001) bc | 84.2 (4.2) b | 102.6 (10.5) bc | 138.0 (7.23) b | 112.7 (0.08) cd | |
Monviso | 0.91 (0.031) bc | 0.93 (0.015) b | 123.1 (4.2) a | 175.1 (29.8) ab | 110.7 (3.91) de | 107.4 (1.89) de | |
83.148.041 | 1.00 (0.004) a | 1.00 (0.019) a | 111.3 (0.4) a | 222.6 (42.5) a | 100.2 (0.41) e | 99.8 (2.18) e | |
I-214 | 0.75 (0.002) e | 0.82 (0.012) a | 54.4 (0.8) c | 69.8 (14.0) c | 133.3 (1.85) b | 121.9 (2.13) c | |
mean | 0.81 (0.030) *** | 0.90 (0.020) | 89.2 (7.4) ** | 139.9 (16.5) | 127.5 (5.5) *** | 111.7 (1.8) |
Pearson’s correlation coefficients (R2) between Nitrogen use parameters and morphological yield traits of hybrid poplar clones after two biennial rotations in the study sites (northern Italy). (1): Shoot diameter, measured at 100 cm above the ground; (2): total shoot height; (3): percentage allocation ratios between stem and branch woody biomass;. (4): number of shoots on the stool; (5): percentage concentration of N in the above ground woody biomass, including wood and bark; (6): N content in the harvestable above ground woody biomass; (7): Nitrogen Use Efficiency.
Variable | Vinovo | Bigarello | ||||||
---|---|---|---|---|---|---|---|---|
1st rotation | 2nd rotation | 1st rotation | 2nd rotation | |||||
N%6 | NUE7 | N% | NUE | N% | NUE | N% | NUE | |
D1 | 0.298 | 0.385 | -0.055 | 0.065 | 0.040 | -0.076 | 0.004 | 0.001 |
H2 | -0.299 | 0.271 | -0.006 | 0.018 | 0.014 | 0.0003 | 0.012 | -0.006 |
Branchiness (%)3 | 0.813* | -0.747** | 0.017 | -0.039 | 0.622 | -0.701** | -0.205 | 0.192 |
Shoots/stool ratio4 | 0.041 | 0.045 | 0.558 | -0.505 | -0.012 | 0.001 | 0.804* | -0.761* |
Biennial Yield5 | 0.014 | -0.018 | 0.708^ | -0.694 | 0.097 | -0.074 | 0.839* | -0.844* |
Means and standard errors (in parenthesis) of dry matter (d.m.) biennial above ground woody biomass yield (Mg dm ha-1) of hybrid poplar clones after two biennial rotations in the study sites in northern Italy. Readapted from
Clone | Vinovo | Bigarello | ||
---|---|---|---|---|
1st rotation | 2nd rotation | 1st rotation | 2nd rotation | |
AF2 | 27.93 (3.81) ab | 26.37 (2.40) a | 11.31 (1.78) ab | 15.07 (2.01) ab |
AF6 | 30.07 (2.50) a | 27.56 (1.86) a | 11.52 (1.58) ab | 11.57 (1.36) ab |
Monviso | 25.53 (3.26) ab | 26.02 (2.13) a | 13.58 (1.73) a | 18.62 (3.26) a |
83.148.041 | 22.64 (1.63) b | 29.66 (1.86) a | 11.16 (0.50) ab | 22.24 (8.87) a |
I214 | 21.86 (1.58) b | 21.93 (1.43) b | 7.25 (1.69) b | 8.40 (1.87) b |
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