Marcin Pietrzykowski · Bart?omiej Wo? · Pawe? Tylek · Dariusz Kwa?niewski ·Tadeusz Juliszewski · Józef Walczyk · Justyna Likus-Cie?lik · Wojciech Ocha? ·Sylwester Tabor

Abstract This research estimates carbon sink and allocation in above- and below-ground biomass of a 12-year-old willow coppice plantation on fluvisol soil near the Vistula River (southern Poland). The plantation showed high C sink potential and sequestration rates. C sequestration by aboveground biomass was estimated at 10.8 Mg C ha ?1 a ?1 . Accumulation in coarse roots was estimated at 1.5 Mg C ha ?1 a ?1 and in fine roots at 1.2 Mg C ha ?1 a ?1 . Total C sequestered (above-ground biomass, coarse roots and fine roots)was estimated at 13.5 Mg C ha ?1 a ?1 . These results conf irm the potential of fast-growing plantations of willow to mitigate, over a short time span, the effects of high CO 2 concentrations.

Keywords Willow coppice · Biomass · Roots · Carbon sequestration

Introduction

A large share of annual anthropogenic greenhouse gas emissions, estimated at 12-25%, is attributed to global deforestation. However, increasing forested areas has a positive effect on carbon (C) sequestration. The mitigation of high atmospheric CO 2 concentrations could be brought about by fast-growing tree species such as willow (Grogan and Matthews 2002; Heller et al. 2003; Rytter 2012). Shortrotation coppice willow is of considerable interest, not only as a source of biomass for fuel but also as a carbon sink. Willows (Salix spp.) are cultivated in many countries of Europe, and its biomass is a substitute for conventional energy sources such as coal and natural gas (Grogan and Matthews 2002; Fischer et al. 2005; Rytter 2012). There is increasing interest in energy crops in Poland. The production and harvesting of the biomass of fast-growing species,following the global trend, is being promoted as a new direction for agricultural production (Ericsson et al. 2006; Stolarski et al. 2008; Igliński et al. 2011; Krasuska and Rosenqvist 2012). The most common energy crop in Poland is the basket willow (Salix viminalis L.). In 2009, the area of willow plantations was approximately 6160 ha (Gajewski 2010), while in 2014 it was more than 10,000 ha (Juliszewski et al. 2015). Willow plantations for biomass production have a rotation of 20-25 years (Szczukowski et al. 2004) in 3-4 year cycles of production. Cuttings are planted in two rows (0.5 m × 0.75 m) 1.25-1.5 m apart or 2.8 m (in the case of access paths), resulting in 18 to 20 thousand seedlings per hectare (Szczukowski et al. 2004).

As noted above, an additional positive aspect of coppice plantations is their potential to capture or sequester carbon.Although the amount of above-ground biomass of a willow plantation, its habitat as well as the dynamics of use are understood, recognition of the size and allocation of C sinks requires determination of below-ground biomass.This requires an estimation of the distribution of roots and size of root thickness fractions (coarse and fine roots). It is particularly important to determine the biomass of fine roots (diameter < 2.0 mm), considering the dynamics of the annual carbon sequestration cycle in the soil because the roots of this fraction die and grow every year (Norby et al.2004; Rytter 2012).

Determination of root biomass is difficult in terms of method and is very labour-intensive. Therefore, the research methods applied most often yield approximate results. Methods of root examination can generally be divided into direct and indirect ones (B?hm 1979). I Indirect methods, used in forestry for example, are based on relations between biomass of above-ground and below-ground parts of a tree. These relationships are represented by allometric equations and the R/S ratio (root to shoot ratio) requires measurements only of the aboveground parts (Mokany et al. 2006). The most appropriate method for assessment of the amount and allocation of carbon by a willow plantation under specific habitat conditions is the direct method, based on measurements of roots extracted directly from the soil. However, there is a lack of empirical data necessary to assess the overall potential of C sequestration by willow plantations (Heller et al.2003; Pacaldo et al. 2012).

The present study estimates the potential for the capture and allocation of carbon in above- and below-ground biomass of coppice willow. The new information presented in this paper estimates below-ground biomass using oflarge samples of extracted roots, and samples of fine roots biomass, neither of which has been studied in this region of Europe.

Materials and methods

Experimental site and sampling

A site with 12-year-old willow coppice was located in southern Poland on fluvisol soil according to the IUSS Working Group WRB (2014), near the Vistula River (FL) in Kaniów(Upper Silesia, 49°56′30.96″ N, 19°3′2.01″ E, approximately 240 m a.s.l.). According to the K?ppen-Geiger climate classification system, the region is cold without a dry season,and with warm summer (Dfb) (Peel et al. 2007). The study area has an average annual temperature of + 7.7 °C (?3.4 °C in January, + 17.6 °C in July). Average annual precipitation is 702 mm. The annual growing season lasts about 219 days.Snow cover occurs for 23 to 114 days (Wo? 1999).

The willow plantation was established in three hectares along the river bed and inside the levees on the floodplain.Previously there were arable fields of cereal crops, and to a less extent, maize and potatoes until the end of the 1970s.Only manure had been applied, and the fields were then turned into pasture. In 1997 after flooding, the pastures became wasteland. In the spring of 2003, willow cuttings(Salix viminalis) 25-cm long and 1-cm thick were planted in single rows for energy production. The plantation was cultivated extensively without mineral fertilization. Stem material was harvested at 3-year intervals, after which the plantation was left to produce coppice. The current density of willow coppice is 20 000 plants per ha (Tylek et al. 2017).

Field and laboratory studies

Soil study

To determine the basic soil properties of the willow coppice, samples were collected using an Ejikelkamp soil auger set for horizons 0-20 and 20-40 cm at 20 points evenly along a zigzag line in the plantation. Samples were pooled into four groups of five samples separately for the two horizons. In the laboratory, the samples were dried at 105 °C, passed through a 2-mm sieve, and the following determined: texture (sand 2-0.05 mm, silt 0.05-0.002 mm and clay < 0.002 mm content) using the Fritsch GmbH Laser Particle Sizer ANALYSETTE 22; the pH potentiometrically in 1 M KCl (1:2.5 soil-solution ratio); organic carbon content (Corg) using the LECO TruMac?CNS analyzer; and,basic exchangeable cations (Ca 2+ , Mg 2+ , K + , Na +) extracted in 1 M NH4Ac using the ICP-OES (iCAP? 6000 Series).To measure exchangeable acidity, 40-g samples were treated with 1 M Ca (CH3COO) 2 using the 1:2.5 soil/solution ratio.Suspensions were shaken for 1 h, filtered and titrated with 0.1 M NaOH to a pH of 8.2. Exchangeable acidity (Hh)was calculated from the amount of base used and expressed in cmol (+) kg ?1 . Total exchangeable bases (TEB) were calculated as a sum of base cations (Na + , K + , Ca 2+ , Mg 2+ )extracted in 1 M CH3COONH 4 (pH 7). Concentrations of Na + , K + , Ca 2+ , Mg 2+ were measured using atomic absorption spectrometry. Cation exchange capacity (CEC) was def ined as the equivalent sum of TEB and exchangeable acidity; base saturation (BS) was def ined as the sum of base cations as a percentage of CEC (Ostrowska et al. 1991).

Willow biomass study

Thirty willow bushes of 3-year-old above-ground shoots were cut and weighed in the field. From each bush, composite d samples were collected for analyses in the laboratory for moisture and C content in dry above-ground biomass.The 12-year-old root systems of each bush were lifted using a hydraulic head used for transplanting large trees. The depth of root system extraction was 1.0 m. Each extracted root ball was wrapped in a jute mat and steel mesh, and transported to a storage site. After cleaning with water, each root system was weighed using a Mensore WM150P2 electronic hook scales. Biomass fresh weight was determined; root morphology was described and isolated root systems photographed.The coarse root samples were taken for laboratory determination of moisture content by drying (65 °C) and C content.Carbon was determined using the infra-red absorption with the LECO TruMac?CNS analyzer.

Fine root biomass was measured with a steel cylinder with 2-l volume and a depth of 40 cm; n = 30 replications on plantation) at points distributed in a regular 10 × 10 m grid. The samples were transported to the laboratory and stored at 4 °C for a maximum of 2 weeks. Roots were rinsed and measured with a calliper with 0.1 mm accuracy, and divided into fine root fractions with diameters ≤ 2.0 mm and> 2.0 mm (B?hm 1979). The samples were dried at 105 °C and weighed to an accuracy of 0.01 g on a technical balance.Carbon analysis used a LECO TruMac?CNS analyzer. The weight was calculated per root biomass in a volume unit(g dm ?3) by soil layer.

Results

Basic soil characteristics

The soil of the coppice plantation had 71% in the 0-20 cm layer and 77% in the 20-40 cm layer silt content, 8% clay in the both layers. The pH was slightly or strongly acidic according to the soil horizon (Soil Survey Division Staff2017), i.e., 6.1 in the 0-20 cm layer and 5.5 in the 20-40 cmlayer. Organic carbon (Corg) was 2.2% and 1.4%, respectively for the 0-20 and 20-40 cm layers. Cation exchange capacity (CEC) was 13.4 to 16.3 cmol c kg ?1 , base saturation(BS) from 80.3% to 87.8%, respectively for the 0-20 and 20-40 cm layers (Table 1).

Table 1 Soil characteristics on study sites with willow coppice (mean values, n = 4)

Above- and below-ground biomass

The above-ground biomass was 69.0 Mg ha ?1 and the annual rate was 23.0 Mg ha ?1 a ?1 . Coarse root biomass (diameter > 2 mm) was 42.2 Mg ha ?1 , and fine root biomass (diameter < 2 mm) 2.7 Mg ha ?1 (Table 2).

Most of the root systems lacked a well-formed, strong taproot. The main mass of roots stretched offthe rootstock perpendicularly and was concentrated in the 30-cm subsurface layer (Fig. 1).

Carbon sink and allocation

The rate of accumulation of carbon in the above-ground biomass was approximately 32.3 Mg ha?1, in coarse roots 18.3 Mg ha ?1 and in fine roots 1.2 Mg ha ?1 (Table 3). The total annual C sequestered was estimated at 13.5 Mg ha ?1 a ?1 for below-ground and above-ground biomass, 10.8 Mg ha ?1 a ?1 for shoots, 1.5 Mg ha ?1 a ?1 for coarse roots and 1.2 Mg ha?1a ?1 for fine roots (Table 3).

Discussion

The productivity of a willow crop depends on many factors, particularly on the type of treatments applied to prepare the soil, the type and amount of mineral fertilizers and pesticides, planting density, and the length of the production cycle (Kopp et al. 1997; Mola-Yudego 2010; Wangand MacFarlane 2012; Schulz et al. 2016). The study plantation was cultivated extensively without the use of mineral fertilizers and plant protection products. According to the literature, willow biomass of 23.0 Mg ha ?1 a ?1 must be regarded as high. Such values have been achieved with plantations by intensive fertilization and irrigation(Ford-Robertson et al. 1993; Mikó et al. 2014). Under Canadian conditions, Labrecque and Teodorescu (2003)pointed to the possibility of achieving willow biomass production of more than 20 Mg ha ?1 a ?1 in a 3-year cycle with intensive fertilization. Research by Szczukowski et al.(2005) under conditions present in north-eastern Poland indicates that the amount of biomass of S. viminalis var.gigantea increased simultaneously with a prolonged production cycle-from 14.9 Mg ha ?1 a ?1 in a 1-year cycle to 21.5 Mg ha ?1 a ?1 in a 3-year cycle. Similar observations may be drawn from research by Stolarski et al. (2008). In research in northern Poland by Stolarski et al. (2008), productivity of seven clones of Salix viminalis ranged from 14.0 to 20.0 Mg ha ?1 a ?1 on plantations cut annually, and from 4.0 to 33.2 Mg ha?1a ?1 on plantations operated in a 4-year cycle.

Table 2 Above- and belowground biomass of willow coppice

Fig. 1 Willow root system (the scale is in cm)

With regards to the amount of belowground biomass,coarse roots (> 2 mm) biomass values are high compared with the literature. Lower values for coarse root biomass were obtained by Pacaldo et al. (2012) for the clone of Salix dasyclados Wimmer in New York State, USA. For plantations 5 to 19 years-of-age, biomass of coarse roots ranged from 2.9 to 5.7 Mg ha ?1 .

Biomass of fine roots (< 2 mm) may be regarded as average compared with vales in the literature. Values of fine root biomass ranging from 5.6 to 9.9 Mg ha ?1 were reported by Pacaldo et al. (2012) for Salix dasyclados in New York State, USA. Heinsoo et al. (2009) estimated fine root biomass in plantations of S. viminalis and S. dasyclados clones from 1.8 to 3.8 Mg ha ?1 . Fine root biomass decreased with increasing levels of mineral fertilization. In a Swedish study,average annual increase in biomass of fine roots of S. viminalis was estimated at 4.2 Mg ha ?1 a ?1 (Rytter 2012). In another study in Sweden, fine root biomass of S. viminalis was 0.9-7.2 Mg ha ?1 . These differences are the result of different methods to determine the biomass of fine roots (Rytter 1999).

The data conf irm the potential of willow plantations for carbon sequestration and for minimizing the greenhouse effect. However, the coarse roots of willows start to decompose and become carbon sources when the production cycle is completed and a new plantation is established. Gradually,most carbon sequestrated into the root systems is released back to the atmosphere. Due to the natural cycle of death and decay, fine roots supply significant amounts of organic matter to the soil and later release their carbon back to the atmosphere (Richards 1978). Although decomposition occurs later than carbon sequestration in fine roots, this,however, means that net carbon balance (biomass growth minus decomposition) of fine roots is zero over a longer time if the willow plantation is harvested. However, if the period production is 3 years, fine roots constantly decay and the estimated carbon in the roots is maintained. The aboveground biomass can have permanent climate change mitigation because it can replace fossil fuels (Kaul et al. 2010).

Table 3 Root and aboveground shoot biomass C stocks and estimated sequestration rates

The total carbon sequestered in belowground and aboveground biomass had high values compared to the literature.In aboveground biomass, it was 10.8 Mg ha?1a ?1 , higher than values of Grelle et al. (2007) for willow coppice in Sweden ofless than 5 Mg ha ?1 a ?1 . The rate of C sequestration reported by Grelle et al. (2007) of up to 3 Mg ha ?1 a ?1 for belowground biomass were with this study (Table 3).The potential for significant carbon sequestration by willow is often higher compared with other species used for energy biomass. Dewar and Cannel (1992) estimated that under UK conditions the rate of carbon sequestration by willow coppice was approximately 5.6 Mg ha ?1 , higher compared to Picea sitchensis (Bong.) Carrière, Nothofagus spp., Pinus sylvestris L. and Pinus contorta Dougl. ex Loud. Higher values of carbon sequestration compared to willow coppice,were attained only by poplar plantations (Dewar and Cannel 1992). Similarly, values of C sequestration by poplar plantations of 4.0 Mg ha ?1 a ?1 compared to willow of 3.5 Mg ha ?1 a ?1 were reported by Rytter (2012) for Sweden.

According to Gajewski (2010), 10,000 ha of abandoned arable land in Poland were planted with willow in short rotation cycle, and thus about 135,000 Mg carbon would be sequestered annually in aboveground biomass and in roots.These results conf irm the significant potential of fast-growing species on arable land in Poland to bring an advantage of mitigating high CO 2 concentrations over a short time span.

Conclusions

The results conf irm the potential for willow plantations for carbon sequestration in minimizing the greenhouse effect.The willow coppice showed a potential to be significant carbon sink. Average rates of sequestration of aboveground biomass were estimated at 10.8 Mg C ha ?1 a ?1 . Accumulation of carbon by coarse roots and fine roots was approximately 1.5 Mg C ha?1a ?1 and 1.2 Mg C ha ?1 a ?1 , respectively Total carbon sequestration by this willow coppice (aboveground and belowground) was approximately 13.5 Mg C ha ?1 a ?1 .These values conf irm the potential for fast-growing plantations of willow in Poland to mitigate the effect of high CO 2 concentrations.

AcknowledgementsThis study was conducted within the framework of the research project No. PBS2/A8/26/2014, “The development of new technology and the functional model of a machine for reclamation of fields after cultivation of willow”. The project is being implemented by a scientific and industrial consortium from the University of Agriculture in Krakow, Faculty of Forestry (Department of Forest Work Mechanisation and Department of Forest Ecology and Reclamation),Faculty of Production and Power Engineering (Institute of Machinery Management, Ergonomics and Production Processes, Institute of Agricultural Engineering and Informatics); Industrial Institute of Agricultural Engineering in Poznań, Research Laboratory of Agricultural Engineering, Team for the Research and Development of Devices for the Acquisition of Renewable Energy of Farm and Warehouse Work,Multibranch Company PROMAR Ltd. in Poznan.