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An assessment of energy efficiency in reclamation to forest
e c o l o g i c a l e n g i n e e r i n g 3 0 ( 2 0 0 7 ) 341–348
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An assessment of energy efficiency in reclamation to forest Marcin Pietrzykowski ∗ , Wojciech Krzaklewski Department of Forest Ecology, Forest Faculty, Agricultural University of Cracow, Al. 29 Listopada 46, Pl. 31-425 Cracow, Poland
a r t i c l e
i n f o
a b s t r a c t
Article history:
The work presents an assessment of energy efficiency in reclamation to forest as illustrated
Received 2 October 2006
by an open-cast sand mine in southern Poland. A total of 20 plots of different age class (5, 17,
Received in revised form
20 and 25 years old) were set up on reclaimed areas of the open cast or in succession areas.
16 March 2007
Studies of initial soil conditions and plant community biomass were conducted to establish
Accepted 15 April 2007
carbon content and accumulation. This was the basis on which energy in the ecosystem was calculated using known conversion factors. It was found that a full reclamation treatment increased the amount of accumulated energy in the developing ecosystem by approximately
Keywords:
(at least) two-fold in comparison to ecosystems left to natural primary succession.
Reclamation to forest
© 2007 Elsevier B.V. All rights reserved.
Post-mining sites Biomass Organic carbon Trapped energy Energy consumption Energy efficiency
1.
Introduction and background
Post-mining landscapes following open cast mining are examples of large-scale land transformation. According to most international laws, all surface-mined lands must receive reclamation and a large part of these areas are reclaimed for forestry. From the economic point of view, reclamation is not limited to restoring the use value of degraded soil but also encompasses the restoration of the entire landscape. From an ecological point of view, reclamation is a process of restoring ¨ the ecosystem (Krzaklewski, 1993; Bradshaw and Huttl, 2001). The ecosystem is a basic ecological unit (Tansley, 1935; Golley, 1993), constituting an integrated system of biotic and abiotic elements in which all trophic levels contain a set of species ensuring circulation of matter and energy flow (Tansley, 1935). In the case of post-mining reclaimed sites the key question is when the restored biological systems actually become
∗
Corresponding author. Tel.: +48 12 6625302; fax: +48 12 4119715. E-mail address: [email protected] (M. Pietrzykowski). 0925-8574/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2007.04.003
ecosystems and what criteria are used to evaluate the effi¨ ciency of reclamation (Bell, 2001; Bradshaw and Huttl, 2001; Knoche et al., 2002). The forest ecosystem, like all other land ecosystems, consists of aboveground and underground parts and their impact on one another is crucial for circulation of matter and energy flow (Wardle et al., 2004). It is therefore important from the point of view of reclamation to forest to determine the soil formation rate, including the depth of organic horizons, nutrient accumulation rate, particularly of carbon and nitrogen (Bradshaw, 1983; Daniels et al., 1992; Li and Daniels, 1994; Bendfeldt et al., 2001; Schaaf, 2001). In case of natural succession, it is crucial to determine the number of species, biodiversity of communities and the proportion of species characteristic of forest and non-forest communities (Krzaklewski, 1993; Jochimsen, 1996; Pietch, 1996; Wiegleb and Felinks, 2001). A complex assessment of the reclamation processes should take into consideration both ecological and economic factors
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(Rodrigue et al., 2002). It would be a good idea to assess energy efficiency in reclamation on the basis of energy input and gain balance and the total amount of energy accumulated in the ecosystem and its individual components. Energy accumulation and carbon content in the biomass of various biocenosis components (a group of interacting organisms that live in an ecological community), make it possible to describe its organisation from an individual to a population. This allows reducing biocenosis to a single unit, i.e. joule (J) or gram (g) (Golley, 1961; Krebs, 1994). The quantity of biomass produced or of assimilated carbon corresponds to the level of energy trapped during photosynthesis, and its equivalent amounts to ca. 17–21 kJ/1 g dry mass, subject to differences connected with the chemical content of biomass, e.g. different lipid content. An equivalent of 20 kJ/1 g dry mass was assumed (Krebs, 1994; Weiner, 2004) for plant matter which contains very little protein but a high content of poly- and oligo-saccharides. Matter produced by plants largely consists of cellulose and other polysaccharides which are in turn mainly made up of glucose. The proportion of carbon in molecular mass determined by stachometry gives a coefficient of 2.5 to calculate carbon in dry mass organic matter expressed in grams and vice versa (Weiner, 2004). In case of soil organic matter (SOM) with a complex structure, it is preferable to determine the carbon content and assume 41 kJ/1 g of carbon as indicated in literature (Weiner, 2004). In the case of ecosystems restored on post-mining sites, the evaluation of energy accumulation may indicate the restoration phase of the reclamation process. The evaluation of energy efficiency in reclamation may in turn be based on the balance of energy consumption and on energy gain calculated on the basis of carbon accumulation in soil and the entire plant population (phytocenosis as defined by Krebs, 1994). The evaluation of biological and economic feasibility of forest restoration on reclaimed post-mining sites is possible by measuring carbon sequestration, wood production and multiple benefits and values provided by forests (Rodrigue et al., 2002). However, this balance is difficult to interpret due to the fact that the comparison of technical systems and self-sustaining biological systems is based on a simplification since in natural ecosystems primary efficiency is very low and technical systems are relatively efficient (Odum, 1971; Odum and Odum, 2003). Although transformity values in energy analysis or thermodynamic accounting of ecosystem contribution to economic sectors have been used to convert the flow of diverse resources into a consistent thermodynamic unit, they do not inherit any of the controversial aspects of Odum’s work (Ukidwe and Bakshi, 2004). Furthermore, it has already been mentioned that energy stored in the biomass and in organic matter is energy in the chemical sense and may only be roughly compared to energy used in the course of the reclamation process. Moreover, the quantity of carbon accumulated in the soil is subject to complex circulation cycles diversified in time (Wali, 1999) and so it is difficult to determine the amount of energy simply on the basis of carbon accumulated in the SOM. Therefore, in energy assessment of reclamation it is advisable to use community biomass for calculations since at least theoretically it may be treated as yield and converted into usable heat energy (Bungart et al., 2000). Also, in ecological research, vascular plant biomass, including that of trees which constitute the majority of forest biocenosis
biomass, does not change much during the vegetation season and may be determined with acceptable accuracy on the basis of dendrometric measurements and appropriate empir´ ical formulas (Sulinski, 1997; Orzeł et al., 2005).
2.
Materials studied and methods
The assessment of energy efficiency in reclamation to forest was performed on the ‘Szczakowa’ open-cast sand mine in the upper Silesia region in southern Poland (19◦ 26 E; 50◦ 16 N) in the Przemsza river valley (Gilewska, 1972). The deposits are fluvioglacial quaternary sediments of pre-quaternary morphological depression. They (mainly sands) were deposited approx. 80,000–240,000 years ago in the Biała Przemsza river valley (Kozioł, 1952). An open cast of an area of over 2700 ha and depth of 5–25 m appeared as a result of mining. Since the late 1950s it has been reclaimed and reforested. In the 1970s and the 1980s some parts of the open cast were abandoned only to be mined again some 10–20 years later. However, falling demand for sand filling meant that they were eventually abandoned. Vegetation appeared there spontaneously and initial soil formation started by natural primary succession. The study plots were located on sections of sand excavations which were either reforested or left to successional process. In general, the areas with spontaneous succession were occupied by bio-groups of trees with over 50% domination of Scotch pine (Pinus sylvestris L.) and common birch (Betula pendula ˛ Roth.) (Krzaklewski and Fraczek, 1999; Pietrzykowski, 2005). In the reclaimed sites the treatment included forming and levelling off the surface, humus addition (approx. 300 m3 ha−1 ). Humus (organic matter) used in reclamation was usually a mixture of forest litter and mineral horizons with an average organic carbon content of 0.3–1.0%, selectively collected from forest soils in areas to be mined (Strzyszcz, 2004), liming, a 2year cycle of NPK fertilization (total amount of 140 kg N ha−1 , 300 kg P2 O5 ha−1 , 180 kg K2 O ha−1 ), a 2-year cycle of lupine (Lupinus luteus L.) cultivation followed by digging in green manure. Next, the sites were reforested using mainly 1-yearold Scotch pine, common birch and some other deciduous trees. As the reclamation methods improved over the last 25 years, certain modifications were introduced: these included decreasing the quantity of humus and NPK mineral fertilisers. This modification followed an experience that addition of organic matter (forest humus) was not functional in case of this particular open cast as it mineralised very quickly and became hydrophobic on drying. In recent years, better results have been obtained by utilising plants which facilitate humus formation and digging them in as green manure (Skawina and ˛ Wachalewski, 1972). Bendfeldt et al. (2001) writing about postmining sites in Virginia (Wise County, USA) reported a similar beneficial effect of introducing plant communities on the soil formation processes, SOM accumulation and reclamation cost in comparison to the addition of organic matter at the onset of reclamation efforts. The most accurate calculation of energy consumption in reclamation was made on the basis of the best materials of the decade (according to information from the ‘Szczakowa’ sand mining). The pHH2 O of sandy deposits from the bottom of the open cast before the reclamation treatment was on average 6.7 and
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pHKCl was on average 6.1; in the less abundant muddy deposits pHH2 O was on average 6.8, whereas pHKCl was on average 5.7. In both types pH increased with depth and top soils acidified with age due to organic accumulation under plant communities. The organic horizons were reported to be very strongly acidic to strongly acidic: pHH2 O from 4.4 to 5.3; pHKCl from 3.8 to 4.7, whereas the organic-mineral horizons: pHH2 O from 5.1 to 5.4; pHKCl from 4.1 to 4.8. The pH of parent rock horizons, regardless of their age was similar to the pH of deposit samples from the bottom of the open cast before reclamation treatment. The sorption capacity (T) of soils in both categories was on average less than 3.5 cmol(+) kg−1 . Higher sorption (T above 6.6 cmol(+) kg−1 of soil) and higher total values of alkaline cations (S) (of up to around a dozen cmol(+) kg−1 soil) only occurred in horizons with more abundant muddy deposits. The content of nutrients (Mg, Ca, Na, K and P) was very low in both categories and there was no marked connection between soil age and properties of the sorption complex and nutrients (Pietrzykowski, 2006). A total of 20 research plots (400 m2 each) were chronosequentially arranged in age classes of 5, 17, 20 and 25 years. One plot was designated from each 5-year-old category (with herbaceous plant community dominated by grey hair-grass) and three plot replications were designated in case of older sites with trees communities. Each plot was sampled (6 points from 5-year-old plots and 12 points from older plots) with a soil drill and the depth of the top horizons was measured. The following soil horizons were determined based on their colour: the organic (litter and raw humus horizon) referred to as L/Of, the initial humus horizon referred to as Ai and the initial humus horizon with parent rock features referred to as AC. Total 36 replications from distinct age groups and categories in the Ai and AC horizons and 12 replications from the L/Of horizon were sampled. To calculate carbon accumulation the mass of the L/Of horizons (three replications for each plot of 1 m × 1 m) and the volumetric density of samples from the Ai and AC horizons (cylinders of 250 cm3 ) was determined (Pietrzykowski and Krzaklewski, 2006). The community biomass condition of herbaceous plants was evaluated according to the yield method and on the basis of tree ´ stand measurements and empirical formulas (Sulinski, 1997; Weiner, 2004). The tree community root biomass was assumed to constitute 20% of the aboveground woody biomass (only large timber, without branches, leaves and needles) (Lieth and Whittaker, 1975; Miller et al., 2006). In the case of herbaceous communities, the ratio of root mass to the aboveground part was found to be 1:1 according to literature data on postmining sites (Krzaklewski, 1999). Organic carbon content in the soil and in plant material was assayed on Leco CNS 2000, and then carbon accumulation was calculated on the basis of the assayed volumetric density, thickness of mineral organic horizons and community biomass. The results, i.e., the total carbon accumulation in soil, were statistically verified using Statistica 6.0 programme. Distribution conformity of the investigated features was compared to normal distribution using the Shapiro–Wilk test. The average values within an age class for both soil categories were compared using ANOVA preceded by Leven’s variance homogeneity test. To distinguish groups which substantially differed from one another, the Kruskal–Wallis significance test (p < 0.05) was
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used (Table 3). Calculations on the amount of energy consumption at different phases of reclamation were based on records of the projects (technical documentation from the ‘Szczakowa’ sand mining), and energy equivalents for materials (including mineral fertilisers, humus, seeds and fuels). The energy used during reclamation work was determined on the basis of catalogues and literature data applied also during energy efficiency analyses in agriculture and forestry (according to FAO by Wielicki, 1989; Informator, 1989/1990; Koradecka and Bugajska, 1998). To determine the energy consumption of the reclamation the following equation was applied: TEC (kJ ha−1 ) = TR (kJ ha−1 ) + BR (kJ ha−1 ) + R (kJ ha−1 ),
(1)
where TEC is the total energy consumption as part of reclamation treatment and reforestation (kJ ha−1 ); TR the energy consumption at the technical reclamation phase (excluding the construction of roads and infrastructure), including the biotope restoration (kJ ha−1 ); BR the energy consumption at the biological reclamation stage including agrotechnical treatment, NPK fertilising, phytomelioration and the introduction of plants facilitating the formation of humus (e.g. lupine) (kJ ha−1 ); and R is the energy consumption as part of reforestation together with forest plant cultivation in the course of the first year (kJ ha−1 ). To estimate the amount of energy trapped in the restored forest ecosystem in the reclaimed areas, the following equation was also applied: TEA (kJ ha−1 ) = SEACorg (kJ ha−1 ) + BEA (kJ ha−1 ) + REA (kJ ha−1 ),
(2)
where TEA is the energy trapped in the restored ecosystem in reclaimed sites (energy trapped in zoocenosis was not considered) (kJ ha−1 ), SEACorg the energy trapped in the soil calculated on the basis of organic carbon accumulation on the assumption that 1 g Corg in the soil corresponds to 41 kJ (kJ ha−1 ), BEA the energy trapped in the aboveground plant community biomass calculated on the basis of organic carbon accumulation on the assumption that 1 g of dry mass corresponds to 20 kJ (kJ ha−1 ), and REA is the energy trapped in the root biomass calculated on the assumption that 1 g of dry mass (biomass) corresponds approximately to 20 kJ (kJ ha−1 ). To estimate the energy efficiency of the reclamation process for sites undergoing complete reclamation, the following equation was applied: EER = BEA (kJ ha−1 ) : TEC (kJ ha−1 ).
(3)
In case of succession areas following biotope restoration (at technical reclamation phase) the following equation was applied: EER = BEA (kJ ha−1 ) : TR (kJ ha−1 ),
(4)
where EER is the reclamation energy efficiency coefficient (in absolute numbers), BEA the energy trapped in the aboveground plant community biomass (kJ ha−1 ) as in Eq. (2), TEC the total energy consumption in the course of reclamation
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Table 1 – Energy consumption during forest reclamation as illustrated by ‘Szczakowa’ sand mine excavation (South Poland) Balance variable
Energy consumption (kJ ha−1 ) × 106
TR (technical reclamation phase) Preparation and earth works Workers Machines Total Drainage of areas Workers Machines Total
Total TR
0.27 36.78 37.05
0.48 39.47 39.94
76.99
BR (biological reclamation phase) Agrotechnical maintenance, plant (lupine) cultivation, fertilization Workers 0.04 Machines 15.57 Materials 7.40 Total 23.00 R (reforestation) Reforestation (soil preparation, seedlings) Workers Machines Materials Total
0.61 0.54 0.30 1.44
Forest crop management (including fertilization) in first year Workers 0.10 Machines 0.85 Materials 0.66 Total 1.61
Total BR + R Total energy consumption (TEC) = TR + BR + R Variables not included in balance Technical road building Workers Machines Total
26.05 103.04
0.20 83.37 83.57
treatment and management (kJ ha−1 ) (as in Eq. (1)), and TR is the energy consumption at technical reclamation phase (kJ ha−1 ) (as in Eq. (1)).
3.
Results and discussion
3.1.
Energy consumption of the reclamation process
The total amount of energy input during reclamation (TEC, according to Eq. (1)) in the open-cast sand mine was estimated to be 103.04 × 106 kJ ha−1 (Table 1). The energy consumption of the technical reclamation phase (TR) during which biotope was restored (but excluding the construction of roads and infrastructure which used 83.57 × 106 kJ ha−1 ), amounted to a total of 76.99 × 106 kJ ha−1 i.e. 75% TEC, including prepara-
tory work and ground work which used up 37.05 × 106 kJ ha−1 , whereas draining and regulation of water conditions used up 39.94 × 106 kJ ha−1 (Table 1). From the ecological point of view, work such as the construction of farm roads and infrastructure on the reclaimed site should not be included in the energy consumption balance, as it has no immediate bearing on the initial energy status of the restored ecosystem. In case of natural succession sites, the energy balance should only include technical reclamation (TR) and biotope restoration for communities from succession whose energy consumption was found to be the same as in the case of reclaimed areas, i.e. 76.99 × 106 kJ ha−1 . According to the “direction of succession” method (Luken, 1990; Krzaklewski, 1993), suitable restoration of biotope, i.e. the technical reclamation phase, makes it possible to give ecological succession a desirable course. In the case of the investigated open cast sand mine, biotope restoration was not fully planned as these areas were at first scheduled to be mined again and so energy input at the technical reclamation stage is an approximated value. Energy consumption at the biological stage (BR) in the reclaimed areas (Eq. (1)), including a cycle of agrotechnical treatment, NPK mineral fertilization and phytomelioration of herbaceous plants (mostly with lupine) was 23.0 × 106 kJ ha−1 , and together with reforestation and forest plant cultivation (R) it was 26.05 × 106 kJ ha−1 (Table 1). This was only 25% of total energy consumption (TSE) in the course of the whole reclamation process. Low energy consumption by machinery as compared to other components of the balance including materials and labour at the reforestation and forest crop management phase (Table 1) was due to the fact that most work was done by hand (planting trees, NPK fertilising, weeding). At this stage vehicles were mostly used for transportation purposes unlike at the technical reclaim phase when they were use in heavy ground work (according to technical documentation from the ‘Szczakowa’ sand mining).
3.2. Carbon accumulation in the ecosystem and community biomass Total accumulation of organic carbon in the soil (in the organic and organic mineral horizons) was 394 kg ha−1 in case of the youngest age class of 5-year-old succession sites and increased significantly statistically to 4640 kg ha−1 in case of the oldest, 25-year-old sites. In the reclaimed sites, carbon accumulation in the soil was considerably higher (p < 0.05) and amounted to 3912 kg ha−1 in the youngest 5-year-old areas and 7402 kg ha−1 in case of the oldest 25-year-old sites. In the reclaimed area category, the total increase of carbon in soil in chronosequence from 5 to 25 years was not significant (Table 3). The accumulation of energy trapped in organic matter in the soil forming under 5-year-old succession communities was 16.2 × 106 kJ ha−1 and increased considerably with age of the area to 190.2 × 106 kJ ha−1 under 25-year-old communities (Table 2). In reclaimed areas, the accumulation of energy trapped in SOM was higher, however, the differences between them and succession areas decreased in the subsequent age groups. In the youngest reclaimed areas, the accumulation of energy trapped in SOM was 160.4 × 106 kJ ha−1 , i.e. 10-fold higher than in the same age group with succession communities.
Tree community root biomass assumed to be 0.2 of wood biomass (according to Lieth and Whittaker, 1975; Miller et al., 2006), for the youngest herbaceous communities the ratio of root mass to the aboveground part was estimated to be 1:1 (on post-mining areas according to Krzaklewski, 1999). Estimation: 1 g C → 2.5 g biomass. c Total wood and assimilatory organ biomass of trees with dbh > 7 cm. d Wood biomass: only large timber without branches, leaves and needles. e Abbreviations see Section 2. b
Explanations: 394 (156) mean (S.D.); n.d. not determined.
a
0.02 5.46 10.71 11.86 165.6 835.1 1568.6 1741.4 2.6 562.9 1104.2 1222.2 2.6 50.6 190.4 215.7 160.4 221.6 274.0 303.5 0.01 2.08 3.30 3.30 52 11,258 22,083 24,444 130 (80) 2532 (1986) 9521 (4247) 10,787 (7373) 130 28,145 55,208 61,111 n.d. 12,658 (9930) 47,607 (21,235) 53,937 (36,865) n.d. 28,010 (19,440) 55,156 (24,349) 61,070 (40,404) 130 (80) 135 (173) 52 (49) 41 (59) 3912 (2607) 5404 (2418) 6684 (3818) 7402 (4968) Reclaimed areas 5 17 20 25
0.04 3.08 5.08 4.95 21.08 372.0 570.0 637.0 2.8 237.5 391.8 381.0 2.8 50.6 35.1 70.7 16.2 99.4 115.6 190.2 0.14 1.96 2.78 1.64 56 4750 7836 7619 140 (27) 1754 (219) 3129 (405) 3298 (829) 140 11,876 19,589 19,048 n.d. 8768 (1095) 15,646 (2024) 16,489 (4147) n.d. 10,303 (1837) 18,912 (1190) 18,191 (4446)
Aboveground biomass (BEA) Roots (REA) Corg soil (SEA Corg )e Total Wood Trees (n = 3) Herbaceous and shrubs (n = 9)
140 (27) 1573 (1070) 677 (515) 857 (664) Areas left for succession 5 394 (156) 17 2425 (1247) 20 2820 (2393) 25 4640 (3601)
Total (TEA)
Energy efficiency of reclamation index (EER) Energy trapped with (kJ ha−1 ) × 106
C biomass/ Corg soil ratio Root Carbon in biomassa biomassb (kg ha−1 ) (kg ha−1 ) d
c
Aboveground biomass (dry biomass in kg ha−1 )
Organic carbon (Corg ) accumulation in soil (n = 36) (kg ha−1 ) Age of areas (years)
Table 2 – Energy trapped in soil organic carbon and community plant biomass and energy efficiency of reclamation as illustrated by ‘Szczakowa’ sand mine excavation (South Poland)
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It was 303.5 × 106 kJ ha−1 in case of the oldest 25-year-old areas, i.e. 1.5 higher than in case of the oldest succession areas (Table 2). Higher accumulation of energy in soils in the youngest reclaimed areas was connected with an initially higher content of organic matter in soil which had been added during the reclamation treatment, mainly due to the use of lupine as green manure. Distinct differences between categories of reclaimed sites and succession sites occurred in case of aboveground plants biomass. In this case, trees played a crucial role as their participation in the aboveground biomass increased intensively with the age of an area (Table 2). In the youngest areas, the aboveground biomass of herbaceous vegetation communities with relatively few tree seedlings and cuttings was similar and amounted to 140 kg ha−1 in sites with succession and 130 kg ha−1 in reclaimed areas. Examples quoted in literature referring to aboveground biomass size of pioneering succession communities (dominated by Corynephorus canescens) in inland dunes were considerably lower at 27 kg ha−1 (De Kovel et al., 2000). In the investigated reclaimed sites ranging in age from 17 to 25 years, the aboveground biomass of trees in communities rose two-fold from 28,010 to 61,070 kg ha−1 . These quantities were similar to the biomass of arborescent succession communities in 45-year-old inland dunes which amounted to 75,000 kg ha−1 (De Kovel et al., 2000) and forest communities formed on poor habitats with dry coniferous forests of the temperate climatic zone which amounted to approx. 60,000 kg ha−1 (Weiner, 2004). The aboveground biomass of forest habitats in the temperate climatic zone was much higher and amounted from approximately 300,000 to 350,000 kg ha−1 (Lieth and Whittaker, 1975). The biomass of mixed stands in southern Poland (Niepołomicka Forest) was assumed to be on average 158,500 kg ha−1 , however, these values visibly depend on the species composition of tree stands (Orzeł et al., 2005). In areas with succession, the aboveground tree biomass was on average more than two- or three-fold lower and increase with age was not so dynamic. By contrast, herbaceous plants and shrubs had a much larger share in the community biomass than in reclaimed areas where there was a marked crown density increase with age leading to a decrease in the amount of light for herbaceous vegetation (Pietrzykowski, 2005). In case of 17-year-old site, the aboveground community biomass was 19,589 kg ha−1 , which in comparison to biomass size in 25-year-old sites amounting to 19,048 kg ha−1 may indicate periodic stagnation in the biomass growth rate of succession communities. In studies on succession of inland dunes, a visible increase of biomass size at succession stage was found (De Kovel et al., 2000). The obtained results allow concluding that the conducted reclamation treatment had a significant beneficial effect on the size of the aboveground community biomass i.e. the productivity of habitats. If we assume that the biomass of communities from succession in areas which are not undergoing reclamation may be an indicator of potential habitat productivity, then reclamation brought a two- to three-fold increase. This clearly indicates that reclamation of such sites brings economic benefits. The estimated root biomass for arborescent communities in reclaimed areas was from 2532 to 10,787 kg ha−1 , and in areas with succession from 1754 to 3298 kg ha−1 (Table 2). In
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Table 3 – Statistical differences in carbon accumulation in soil depending on the age and categories of areas of ‘Szczakowa’ sand mine excavation (Kruskal–Wallis test) Age years/category of areas
Succession areas
Reclaimed areas
5
17
20
25
5
17
20
25
Succession areas 5 17 20 25
– 1.000 0.164 0.002*
1.000 – 0.334 0.000
0.164 0.334 – 0.478
0.002 0.000 0.478 –
1.000 1.000 1.000 1.000
0.000 0.000 0.018 1.000
0.000 0.000 0.000 0.963
0.000 0.000 0.000 0.105
Reclaimed areas 5 17 20 25
1.000 0.000 0.000 0.000
1.000 0.000 0.000 0.000
1.000 0.018 0.000 0.000
1.000 1.000 0.963 0.105
– 1.000 0.301 0.084
1.000 – 1.000 1.000
0.301 1.000 – 1.000
0.084 1.000 1.000 –
Explanations: *marked differences are significant at p < 0.05.
the youngest herbaceous communities, the estimated root mass was up to several dozen times smaller and differences between categories were not large. The forest community root biomass in the temperate zone was assumed to be 42,000 kg ha−1 (coniferous forests) to 44,000 kg ha−1 (deciduous forests), of which 50–60% mass is located in the upper 30 cm of the soil (Jackson et al., 1996; Helmisaari et al., 2002; Claus and George, 2005). The ratio of carbon in the aboveground community biomass to carbon in the soil (C biomass/Corg soil) differed depending on site category. In areas with succession, it was from 0.14 on 5-year-old sites to 2.78 on 17-year-old sites, and in reclaimed areas from 0.01 on the youngest 5-year-old sites and much more, even up to 2.08 on 17-year-old sites and 3.30 on 20- and 25-year-old sites. It indicates significant differences in relation to forest ecosystems of temperate climatic zone where the ratio of C biomass/Corg soil is on average 1.13; however, the ratio decreases for biomass in cooler climates or when soil abundance decreases (Lieth and Whittaker, 1975). In the first stages of soil formation, the quantity of organic matter increases, whereas in mature ecosystems the accumulation rate may sometimes drop to 0 or else remains consistently high (Weiner, 2004). In terrestrial ecosystems there is usually twice as much organic matter and carbon (including detritus) in soil as in biomass. It is only in equatorial rain forests and forests of the temperate zone with a quick decomposition rate and element flow that biomass accumulation is higher than soil organic matter and carbon (Lieth and Whittaker, 1975). In boreal forests, low organic matter decomposition rate means that only a fraction of carbon and nitrogen becomes accumulated in biomass (Krebs, 1994). In case of a restored ecosystem on an open cast, the reported high ratio of C biomass/Corg soil indicated high organic matter decomposition rate and quick carbon circulation in the vegetation soil system. However, this issue requires further investigation to determine organic matter decomposition rate.
3.3.
An assessment of energy efficiency in reclamation
Total energy accumulation in the ecosystem restored in reclaimed areas (according to Eq. (2)) was 165.6 × 106 kJ ha−1 in the youngest 5-year-old sites and 1741.4 × 106 kJ ha−1 in
the oldest 25-year-old sites (Table 2). This was a nearly 10-fold increase in the studied time interval. A comparison of the amount of energy accumulated in the restored ecosystem (TEA) in this category of sites following 25 years with the amount of energy used during the reclaim process (TEC) i.e. 103.04 × 106 kJ ha−1 showed a 15-fold difference. The amount of energy trapped in the aboveground community biomass (BEA) during photosynthesis was 2.8 × 106 kJ ha−1 on unreclaimed 5-year-old sites, 237.5 × 106 kJ ha−1 on 17-year-old sites, 391.8 × 106 kJ ha−1 on 20-year-old sites and 381.0 × 106 kJ ha−1 on 25-year-old sites. BEA was 2.6 × 106 kJ ha−1 in case of the youngest 5-year-old reclaimed sites, 562.9 × 106 kJ ha−1 on 17-yearold sites, 1104.2 × 106 kJ ha−1 on 20-year-old sites and 1222.2 × 106 kJ ha−1 on the oldest sites (Table 2). In case of sites with natural succession, the total energy trapped in the developing ecosystem (TEA) and in the aboveground community biomass (BEA) was approximately two- to three-fold lower in comparison to reclaimed sites. However, it should be acknowledged that energy input was also lower since it only involved the technical reclamation stage (TR) and was 76.99 × 106 kJ ha−1 , as opposed to full reclamation where the total energy consumption (TEC) was 103.04 × 106 kJ ha−1 (Table 2). The energy trapped in the aboveground succession community biomass was 237.5 × 106 kJ ha−1 on 17-year-old sites to 391.8 × 106 kJ ha−1 on 20-year-old sites and 381.0 × 106 kJ ha−1 on the oldest 25year-old sites, respectively. These values may be generally regarded as relatively low. For instance, the annual energy production trapped in the biomass in coniferous forests of the temperate zone (southern Poland) may be assumed to be over 140 × 106 kJ ha−1 year−1 , and in deciduous forests over ´ 1984). 220 × 106 kJ ha−1 year−1 (Weiner and Grodzinski, The calculated reclamation energy efficiency coefficient (EER) (according to Eq. (3)) increased in chronosequence in both types of surfaces. In case of the youngest reclaimed sites EER was 0.02 and 5.46 on 17-year-old sites, 10.71 on 20-year-old sites to 11.86 on 25-year-old sites. In case of succession sites after the biotope was developed, the EER coefficient was from 0.04 on 5-year-old sites, 3.08 on 17-year-old sites, 5.08 on 20year-old sites and 4.95 on the oldest 25-year-old sites. To sum up then, energy gain (determined using the EER coefficient)
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after 25 years was two-fold higher in reclaimed areas than in succession areas. This provided the basis for an assumption that the reclamation process significantly influences energy accumulation in the developing ecosystem.
4.
Conclusion
The amount of energy trapped in the biomass and soil organic matter was much higher in a forest ecosystem restored on post-mining areas of a sand cast during a full-scale reclamation treatment including technical restoration of biotope, biological reclamation and reforestation than in an ecosystem left for primary natural succession in parts of the sand cast formed in the course of technical reclamation. In fully reclaimed areas, the amount of energy accumulated in the aboveground community biomass after 25 years was approximately 12-fold higher than energy consumption in the course of reclamation processes and reforestation and in case of areas with succession about 5-fold higher from energy consumption during biotope restoration at the technical reclaim stage. The obtained results show that a full reclamation treatment including the restoration of biotope, phytomelioration, agrotechnical treatment and reforestation increases the amount of energy accumulated in the restored ecosystem by about two-fold. This clearly indicates that reclamation of such sites brings economic benefits and the amount of energy used to restore and reforest it is fully justifiable. However, this paper did not analyse other factors such as plant community features. Other publications on this post-mining site (Krzaklewski ˛ and Fraczek, 1999) showed that if left for natural succession to take place, the areas exhibited higher biodiversity than the reclaimed parts and that they became the habitat of rare species protected in Poland. This confirms the complexity of criteria selection for the assessment of reclamation although in this particular case, the economic benefits were obvious.
Acknowledgement The authors would also like to thank Jarosław Socha, PhD from Department of Forest Mensuration, Agricultural University of Krakow for his kind assistance during the preparation of statistical analysis.
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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ecoleng
An assessment of energy efficiency in reclamation to forest Marcin Pietrzykowski ∗ , Wojciech Krzaklewski Department of Forest Ecology, Forest Faculty, Agricultural University of Cracow, Al. 29 Listopada 46, Pl. 31-425 Cracow, Poland
a r t i c l e
i n f o
a b s t r a c t
Article history:
The work presents an assessment of energy efficiency in reclamation to forest as illustrated
Received 2 October 2006
by an open-cast sand mine in southern Poland. A total of 20 plots of different age class (5, 17,
Received in revised form
20 and 25 years old) were set up on reclaimed areas of the open cast or in succession areas.
16 March 2007
Studies of initial soil conditions and plant community biomass were conducted to establish
Accepted 15 April 2007
carbon content and accumulation. This was the basis on which energy in the ecosystem was calculated using known conversion factors. It was found that a full reclamation treatment increased the amount of accumulated energy in the developing ecosystem by approximately
Keywords:
(at least) two-fold in comparison to ecosystems left to natural primary succession.
Reclamation to forest
© 2007 Elsevier B.V. All rights reserved.
Post-mining sites Biomass Organic carbon Trapped energy Energy consumption Energy efficiency
1.
Introduction and background
Post-mining landscapes following open cast mining are examples of large-scale land transformation. According to most international laws, all surface-mined lands must receive reclamation and a large part of these areas are reclaimed for forestry. From the economic point of view, reclamation is not limited to restoring the use value of degraded soil but also encompasses the restoration of the entire landscape. From an ecological point of view, reclamation is a process of restoring ¨ the ecosystem (Krzaklewski, 1993; Bradshaw and Huttl, 2001). The ecosystem is a basic ecological unit (Tansley, 1935; Golley, 1993), constituting an integrated system of biotic and abiotic elements in which all trophic levels contain a set of species ensuring circulation of matter and energy flow (Tansley, 1935). In the case of post-mining reclaimed sites the key question is when the restored biological systems actually become
∗
Corresponding author. Tel.: +48 12 6625302; fax: +48 12 4119715. E-mail address: [email protected] (M. Pietrzykowski). 0925-8574/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2007.04.003
ecosystems and what criteria are used to evaluate the effi¨ ciency of reclamation (Bell, 2001; Bradshaw and Huttl, 2001; Knoche et al., 2002). The forest ecosystem, like all other land ecosystems, consists of aboveground and underground parts and their impact on one another is crucial for circulation of matter and energy flow (Wardle et al., 2004). It is therefore important from the point of view of reclamation to forest to determine the soil formation rate, including the depth of organic horizons, nutrient accumulation rate, particularly of carbon and nitrogen (Bradshaw, 1983; Daniels et al., 1992; Li and Daniels, 1994; Bendfeldt et al., 2001; Schaaf, 2001). In case of natural succession, it is crucial to determine the number of species, biodiversity of communities and the proportion of species characteristic of forest and non-forest communities (Krzaklewski, 1993; Jochimsen, 1996; Pietch, 1996; Wiegleb and Felinks, 2001). A complex assessment of the reclamation processes should take into consideration both ecological and economic factors
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(Rodrigue et al., 2002). It would be a good idea to assess energy efficiency in reclamation on the basis of energy input and gain balance and the total amount of energy accumulated in the ecosystem and its individual components. Energy accumulation and carbon content in the biomass of various biocenosis components (a group of interacting organisms that live in an ecological community), make it possible to describe its organisation from an individual to a population. This allows reducing biocenosis to a single unit, i.e. joule (J) or gram (g) (Golley, 1961; Krebs, 1994). The quantity of biomass produced or of assimilated carbon corresponds to the level of energy trapped during photosynthesis, and its equivalent amounts to ca. 17–21 kJ/1 g dry mass, subject to differences connected with the chemical content of biomass, e.g. different lipid content. An equivalent of 20 kJ/1 g dry mass was assumed (Krebs, 1994; Weiner, 2004) for plant matter which contains very little protein but a high content of poly- and oligo-saccharides. Matter produced by plants largely consists of cellulose and other polysaccharides which are in turn mainly made up of glucose. The proportion of carbon in molecular mass determined by stachometry gives a coefficient of 2.5 to calculate carbon in dry mass organic matter expressed in grams and vice versa (Weiner, 2004). In case of soil organic matter (SOM) with a complex structure, it is preferable to determine the carbon content and assume 41 kJ/1 g of carbon as indicated in literature (Weiner, 2004). In the case of ecosystems restored on post-mining sites, the evaluation of energy accumulation may indicate the restoration phase of the reclamation process. The evaluation of energy efficiency in reclamation may in turn be based on the balance of energy consumption and on energy gain calculated on the basis of carbon accumulation in soil and the entire plant population (phytocenosis as defined by Krebs, 1994). The evaluation of biological and economic feasibility of forest restoration on reclaimed post-mining sites is possible by measuring carbon sequestration, wood production and multiple benefits and values provided by forests (Rodrigue et al., 2002). However, this balance is difficult to interpret due to the fact that the comparison of technical systems and self-sustaining biological systems is based on a simplification since in natural ecosystems primary efficiency is very low and technical systems are relatively efficient (Odum, 1971; Odum and Odum, 2003). Although transformity values in energy analysis or thermodynamic accounting of ecosystem contribution to economic sectors have been used to convert the flow of diverse resources into a consistent thermodynamic unit, they do not inherit any of the controversial aspects of Odum’s work (Ukidwe and Bakshi, 2004). Furthermore, it has already been mentioned that energy stored in the biomass and in organic matter is energy in the chemical sense and may only be roughly compared to energy used in the course of the reclamation process. Moreover, the quantity of carbon accumulated in the soil is subject to complex circulation cycles diversified in time (Wali, 1999) and so it is difficult to determine the amount of energy simply on the basis of carbon accumulated in the SOM. Therefore, in energy assessment of reclamation it is advisable to use community biomass for calculations since at least theoretically it may be treated as yield and converted into usable heat energy (Bungart et al., 2000). Also, in ecological research, vascular plant biomass, including that of trees which constitute the majority of forest biocenosis
biomass, does not change much during the vegetation season and may be determined with acceptable accuracy on the basis of dendrometric measurements and appropriate empir´ ical formulas (Sulinski, 1997; Orzeł et al., 2005).
2.
Materials studied and methods
The assessment of energy efficiency in reclamation to forest was performed on the ‘Szczakowa’ open-cast sand mine in the upper Silesia region in southern Poland (19◦ 26 E; 50◦ 16 N) in the Przemsza river valley (Gilewska, 1972). The deposits are fluvioglacial quaternary sediments of pre-quaternary morphological depression. They (mainly sands) were deposited approx. 80,000–240,000 years ago in the Biała Przemsza river valley (Kozioł, 1952). An open cast of an area of over 2700 ha and depth of 5–25 m appeared as a result of mining. Since the late 1950s it has been reclaimed and reforested. In the 1970s and the 1980s some parts of the open cast were abandoned only to be mined again some 10–20 years later. However, falling demand for sand filling meant that they were eventually abandoned. Vegetation appeared there spontaneously and initial soil formation started by natural primary succession. The study plots were located on sections of sand excavations which were either reforested or left to successional process. In general, the areas with spontaneous succession were occupied by bio-groups of trees with over 50% domination of Scotch pine (Pinus sylvestris L.) and common birch (Betula pendula ˛ Roth.) (Krzaklewski and Fraczek, 1999; Pietrzykowski, 2005). In the reclaimed sites the treatment included forming and levelling off the surface, humus addition (approx. 300 m3 ha−1 ). Humus (organic matter) used in reclamation was usually a mixture of forest litter and mineral horizons with an average organic carbon content of 0.3–1.0%, selectively collected from forest soils in areas to be mined (Strzyszcz, 2004), liming, a 2year cycle of NPK fertilization (total amount of 140 kg N ha−1 , 300 kg P2 O5 ha−1 , 180 kg K2 O ha−1 ), a 2-year cycle of lupine (Lupinus luteus L.) cultivation followed by digging in green manure. Next, the sites were reforested using mainly 1-yearold Scotch pine, common birch and some other deciduous trees. As the reclamation methods improved over the last 25 years, certain modifications were introduced: these included decreasing the quantity of humus and NPK mineral fertilisers. This modification followed an experience that addition of organic matter (forest humus) was not functional in case of this particular open cast as it mineralised very quickly and became hydrophobic on drying. In recent years, better results have been obtained by utilising plants which facilitate humus formation and digging them in as green manure (Skawina and ˛ Wachalewski, 1972). Bendfeldt et al. (2001) writing about postmining sites in Virginia (Wise County, USA) reported a similar beneficial effect of introducing plant communities on the soil formation processes, SOM accumulation and reclamation cost in comparison to the addition of organic matter at the onset of reclamation efforts. The most accurate calculation of energy consumption in reclamation was made on the basis of the best materials of the decade (according to information from the ‘Szczakowa’ sand mining). The pHH2 O of sandy deposits from the bottom of the open cast before the reclamation treatment was on average 6.7 and
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pHKCl was on average 6.1; in the less abundant muddy deposits pHH2 O was on average 6.8, whereas pHKCl was on average 5.7. In both types pH increased with depth and top soils acidified with age due to organic accumulation under plant communities. The organic horizons were reported to be very strongly acidic to strongly acidic: pHH2 O from 4.4 to 5.3; pHKCl from 3.8 to 4.7, whereas the organic-mineral horizons: pHH2 O from 5.1 to 5.4; pHKCl from 4.1 to 4.8. The pH of parent rock horizons, regardless of their age was similar to the pH of deposit samples from the bottom of the open cast before reclamation treatment. The sorption capacity (T) of soils in both categories was on average less than 3.5 cmol(+) kg−1 . Higher sorption (T above 6.6 cmol(+) kg−1 of soil) and higher total values of alkaline cations (S) (of up to around a dozen cmol(+) kg−1 soil) only occurred in horizons with more abundant muddy deposits. The content of nutrients (Mg, Ca, Na, K and P) was very low in both categories and there was no marked connection between soil age and properties of the sorption complex and nutrients (Pietrzykowski, 2006). A total of 20 research plots (400 m2 each) were chronosequentially arranged in age classes of 5, 17, 20 and 25 years. One plot was designated from each 5-year-old category (with herbaceous plant community dominated by grey hair-grass) and three plot replications were designated in case of older sites with trees communities. Each plot was sampled (6 points from 5-year-old plots and 12 points from older plots) with a soil drill and the depth of the top horizons was measured. The following soil horizons were determined based on their colour: the organic (litter and raw humus horizon) referred to as L/Of, the initial humus horizon referred to as Ai and the initial humus horizon with parent rock features referred to as AC. Total 36 replications from distinct age groups and categories in the Ai and AC horizons and 12 replications from the L/Of horizon were sampled. To calculate carbon accumulation the mass of the L/Of horizons (three replications for each plot of 1 m × 1 m) and the volumetric density of samples from the Ai and AC horizons (cylinders of 250 cm3 ) was determined (Pietrzykowski and Krzaklewski, 2006). The community biomass condition of herbaceous plants was evaluated according to the yield method and on the basis of tree ´ stand measurements and empirical formulas (Sulinski, 1997; Weiner, 2004). The tree community root biomass was assumed to constitute 20% of the aboveground woody biomass (only large timber, without branches, leaves and needles) (Lieth and Whittaker, 1975; Miller et al., 2006). In the case of herbaceous communities, the ratio of root mass to the aboveground part was found to be 1:1 according to literature data on postmining sites (Krzaklewski, 1999). Organic carbon content in the soil and in plant material was assayed on Leco CNS 2000, and then carbon accumulation was calculated on the basis of the assayed volumetric density, thickness of mineral organic horizons and community biomass. The results, i.e., the total carbon accumulation in soil, were statistically verified using Statistica 6.0 programme. Distribution conformity of the investigated features was compared to normal distribution using the Shapiro–Wilk test. The average values within an age class for both soil categories were compared using ANOVA preceded by Leven’s variance homogeneity test. To distinguish groups which substantially differed from one another, the Kruskal–Wallis significance test (p < 0.05) was
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used (Table 3). Calculations on the amount of energy consumption at different phases of reclamation were based on records of the projects (technical documentation from the ‘Szczakowa’ sand mining), and energy equivalents for materials (including mineral fertilisers, humus, seeds and fuels). The energy used during reclamation work was determined on the basis of catalogues and literature data applied also during energy efficiency analyses in agriculture and forestry (according to FAO by Wielicki, 1989; Informator, 1989/1990; Koradecka and Bugajska, 1998). To determine the energy consumption of the reclamation the following equation was applied: TEC (kJ ha−1 ) = TR (kJ ha−1 ) + BR (kJ ha−1 ) + R (kJ ha−1 ),
(1)
where TEC is the total energy consumption as part of reclamation treatment and reforestation (kJ ha−1 ); TR the energy consumption at the technical reclamation phase (excluding the construction of roads and infrastructure), including the biotope restoration (kJ ha−1 ); BR the energy consumption at the biological reclamation stage including agrotechnical treatment, NPK fertilising, phytomelioration and the introduction of plants facilitating the formation of humus (e.g. lupine) (kJ ha−1 ); and R is the energy consumption as part of reforestation together with forest plant cultivation in the course of the first year (kJ ha−1 ). To estimate the amount of energy trapped in the restored forest ecosystem in the reclaimed areas, the following equation was also applied: TEA (kJ ha−1 ) = SEACorg (kJ ha−1 ) + BEA (kJ ha−1 ) + REA (kJ ha−1 ),
(2)
where TEA is the energy trapped in the restored ecosystem in reclaimed sites (energy trapped in zoocenosis was not considered) (kJ ha−1 ), SEACorg the energy trapped in the soil calculated on the basis of organic carbon accumulation on the assumption that 1 g Corg in the soil corresponds to 41 kJ (kJ ha−1 ), BEA the energy trapped in the aboveground plant community biomass calculated on the basis of organic carbon accumulation on the assumption that 1 g of dry mass corresponds to 20 kJ (kJ ha−1 ), and REA is the energy trapped in the root biomass calculated on the assumption that 1 g of dry mass (biomass) corresponds approximately to 20 kJ (kJ ha−1 ). To estimate the energy efficiency of the reclamation process for sites undergoing complete reclamation, the following equation was applied: EER = BEA (kJ ha−1 ) : TEC (kJ ha−1 ).
(3)
In case of succession areas following biotope restoration (at technical reclamation phase) the following equation was applied: EER = BEA (kJ ha−1 ) : TR (kJ ha−1 ),
(4)
where EER is the reclamation energy efficiency coefficient (in absolute numbers), BEA the energy trapped in the aboveground plant community biomass (kJ ha−1 ) as in Eq. (2), TEC the total energy consumption in the course of reclamation
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Table 1 – Energy consumption during forest reclamation as illustrated by ‘Szczakowa’ sand mine excavation (South Poland) Balance variable
Energy consumption (kJ ha−1 ) × 106
TR (technical reclamation phase) Preparation and earth works Workers Machines Total Drainage of areas Workers Machines Total
Total TR
0.27 36.78 37.05
0.48 39.47 39.94
76.99
BR (biological reclamation phase) Agrotechnical maintenance, plant (lupine) cultivation, fertilization Workers 0.04 Machines 15.57 Materials 7.40 Total 23.00 R (reforestation) Reforestation (soil preparation, seedlings) Workers Machines Materials Total
0.61 0.54 0.30 1.44
Forest crop management (including fertilization) in first year Workers 0.10 Machines 0.85 Materials 0.66 Total 1.61
Total BR + R Total energy consumption (TEC) = TR + BR + R Variables not included in balance Technical road building Workers Machines Total
26.05 103.04
0.20 83.37 83.57
treatment and management (kJ ha−1 ) (as in Eq. (1)), and TR is the energy consumption at technical reclamation phase (kJ ha−1 ) (as in Eq. (1)).
3.
Results and discussion
3.1.
Energy consumption of the reclamation process
The total amount of energy input during reclamation (TEC, according to Eq. (1)) in the open-cast sand mine was estimated to be 103.04 × 106 kJ ha−1 (Table 1). The energy consumption of the technical reclamation phase (TR) during which biotope was restored (but excluding the construction of roads and infrastructure which used 83.57 × 106 kJ ha−1 ), amounted to a total of 76.99 × 106 kJ ha−1 i.e. 75% TEC, including prepara-
tory work and ground work which used up 37.05 × 106 kJ ha−1 , whereas draining and regulation of water conditions used up 39.94 × 106 kJ ha−1 (Table 1). From the ecological point of view, work such as the construction of farm roads and infrastructure on the reclaimed site should not be included in the energy consumption balance, as it has no immediate bearing on the initial energy status of the restored ecosystem. In case of natural succession sites, the energy balance should only include technical reclamation (TR) and biotope restoration for communities from succession whose energy consumption was found to be the same as in the case of reclaimed areas, i.e. 76.99 × 106 kJ ha−1 . According to the “direction of succession” method (Luken, 1990; Krzaklewski, 1993), suitable restoration of biotope, i.e. the technical reclamation phase, makes it possible to give ecological succession a desirable course. In the case of the investigated open cast sand mine, biotope restoration was not fully planned as these areas were at first scheduled to be mined again and so energy input at the technical reclamation stage is an approximated value. Energy consumption at the biological stage (BR) in the reclaimed areas (Eq. (1)), including a cycle of agrotechnical treatment, NPK mineral fertilization and phytomelioration of herbaceous plants (mostly with lupine) was 23.0 × 106 kJ ha−1 , and together with reforestation and forest plant cultivation (R) it was 26.05 × 106 kJ ha−1 (Table 1). This was only 25% of total energy consumption (TSE) in the course of the whole reclamation process. Low energy consumption by machinery as compared to other components of the balance including materials and labour at the reforestation and forest crop management phase (Table 1) was due to the fact that most work was done by hand (planting trees, NPK fertilising, weeding). At this stage vehicles were mostly used for transportation purposes unlike at the technical reclaim phase when they were use in heavy ground work (according to technical documentation from the ‘Szczakowa’ sand mining).
3.2. Carbon accumulation in the ecosystem and community biomass Total accumulation of organic carbon in the soil (in the organic and organic mineral horizons) was 394 kg ha−1 in case of the youngest age class of 5-year-old succession sites and increased significantly statistically to 4640 kg ha−1 in case of the oldest, 25-year-old sites. In the reclaimed sites, carbon accumulation in the soil was considerably higher (p < 0.05) and amounted to 3912 kg ha−1 in the youngest 5-year-old areas and 7402 kg ha−1 in case of the oldest 25-year-old sites. In the reclaimed area category, the total increase of carbon in soil in chronosequence from 5 to 25 years was not significant (Table 3). The accumulation of energy trapped in organic matter in the soil forming under 5-year-old succession communities was 16.2 × 106 kJ ha−1 and increased considerably with age of the area to 190.2 × 106 kJ ha−1 under 25-year-old communities (Table 2). In reclaimed areas, the accumulation of energy trapped in SOM was higher, however, the differences between them and succession areas decreased in the subsequent age groups. In the youngest reclaimed areas, the accumulation of energy trapped in SOM was 160.4 × 106 kJ ha−1 , i.e. 10-fold higher than in the same age group with succession communities.
Tree community root biomass assumed to be 0.2 of wood biomass (according to Lieth and Whittaker, 1975; Miller et al., 2006), for the youngest herbaceous communities the ratio of root mass to the aboveground part was estimated to be 1:1 (on post-mining areas according to Krzaklewski, 1999). Estimation: 1 g C → 2.5 g biomass. c Total wood and assimilatory organ biomass of trees with dbh > 7 cm. d Wood biomass: only large timber without branches, leaves and needles. e Abbreviations see Section 2. b
Explanations: 394 (156) mean (S.D.); n.d. not determined.
a
0.02 5.46 10.71 11.86 165.6 835.1 1568.6 1741.4 2.6 562.9 1104.2 1222.2 2.6 50.6 190.4 215.7 160.4 221.6 274.0 303.5 0.01 2.08 3.30 3.30 52 11,258 22,083 24,444 130 (80) 2532 (1986) 9521 (4247) 10,787 (7373) 130 28,145 55,208 61,111 n.d. 12,658 (9930) 47,607 (21,235) 53,937 (36,865) n.d. 28,010 (19,440) 55,156 (24,349) 61,070 (40,404) 130 (80) 135 (173) 52 (49) 41 (59) 3912 (2607) 5404 (2418) 6684 (3818) 7402 (4968) Reclaimed areas 5 17 20 25
0.04 3.08 5.08 4.95 21.08 372.0 570.0 637.0 2.8 237.5 391.8 381.0 2.8 50.6 35.1 70.7 16.2 99.4 115.6 190.2 0.14 1.96 2.78 1.64 56 4750 7836 7619 140 (27) 1754 (219) 3129 (405) 3298 (829) 140 11,876 19,589 19,048 n.d. 8768 (1095) 15,646 (2024) 16,489 (4147) n.d. 10,303 (1837) 18,912 (1190) 18,191 (4446)
Aboveground biomass (BEA) Roots (REA) Corg soil (SEA Corg )e Total Wood Trees (n = 3) Herbaceous and shrubs (n = 9)
140 (27) 1573 (1070) 677 (515) 857 (664) Areas left for succession 5 394 (156) 17 2425 (1247) 20 2820 (2393) 25 4640 (3601)
Total (TEA)
Energy efficiency of reclamation index (EER) Energy trapped with (kJ ha−1 ) × 106
C biomass/ Corg soil ratio Root Carbon in biomassa biomassb (kg ha−1 ) (kg ha−1 ) d
c
Aboveground biomass (dry biomass in kg ha−1 )
Organic carbon (Corg ) accumulation in soil (n = 36) (kg ha−1 ) Age of areas (years)
Table 2 – Energy trapped in soil organic carbon and community plant biomass and energy efficiency of reclamation as illustrated by ‘Szczakowa’ sand mine excavation (South Poland)
e c o l o g i c a l e n g i n e e r i n g 3 0 ( 2 0 0 7 ) 341–348
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It was 303.5 × 106 kJ ha−1 in case of the oldest 25-year-old areas, i.e. 1.5 higher than in case of the oldest succession areas (Table 2). Higher accumulation of energy in soils in the youngest reclaimed areas was connected with an initially higher content of organic matter in soil which had been added during the reclamation treatment, mainly due to the use of lupine as green manure. Distinct differences between categories of reclaimed sites and succession sites occurred in case of aboveground plants biomass. In this case, trees played a crucial role as their participation in the aboveground biomass increased intensively with the age of an area (Table 2). In the youngest areas, the aboveground biomass of herbaceous vegetation communities with relatively few tree seedlings and cuttings was similar and amounted to 140 kg ha−1 in sites with succession and 130 kg ha−1 in reclaimed areas. Examples quoted in literature referring to aboveground biomass size of pioneering succession communities (dominated by Corynephorus canescens) in inland dunes were considerably lower at 27 kg ha−1 (De Kovel et al., 2000). In the investigated reclaimed sites ranging in age from 17 to 25 years, the aboveground biomass of trees in communities rose two-fold from 28,010 to 61,070 kg ha−1 . These quantities were similar to the biomass of arborescent succession communities in 45-year-old inland dunes which amounted to 75,000 kg ha−1 (De Kovel et al., 2000) and forest communities formed on poor habitats with dry coniferous forests of the temperate climatic zone which amounted to approx. 60,000 kg ha−1 (Weiner, 2004). The aboveground biomass of forest habitats in the temperate climatic zone was much higher and amounted from approximately 300,000 to 350,000 kg ha−1 (Lieth and Whittaker, 1975). The biomass of mixed stands in southern Poland (Niepołomicka Forest) was assumed to be on average 158,500 kg ha−1 , however, these values visibly depend on the species composition of tree stands (Orzeł et al., 2005). In areas with succession, the aboveground tree biomass was on average more than two- or three-fold lower and increase with age was not so dynamic. By contrast, herbaceous plants and shrubs had a much larger share in the community biomass than in reclaimed areas where there was a marked crown density increase with age leading to a decrease in the amount of light for herbaceous vegetation (Pietrzykowski, 2005). In case of 17-year-old site, the aboveground community biomass was 19,589 kg ha−1 , which in comparison to biomass size in 25-year-old sites amounting to 19,048 kg ha−1 may indicate periodic stagnation in the biomass growth rate of succession communities. In studies on succession of inland dunes, a visible increase of biomass size at succession stage was found (De Kovel et al., 2000). The obtained results allow concluding that the conducted reclamation treatment had a significant beneficial effect on the size of the aboveground community biomass i.e. the productivity of habitats. If we assume that the biomass of communities from succession in areas which are not undergoing reclamation may be an indicator of potential habitat productivity, then reclamation brought a two- to three-fold increase. This clearly indicates that reclamation of such sites brings economic benefits. The estimated root biomass for arborescent communities in reclaimed areas was from 2532 to 10,787 kg ha−1 , and in areas with succession from 1754 to 3298 kg ha−1 (Table 2). In
346
e c o l o g i c a l e n g i n e e r i n g 3 0 ( 2 0 0 7 ) 341–348
Table 3 – Statistical differences in carbon accumulation in soil depending on the age and categories of areas of ‘Szczakowa’ sand mine excavation (Kruskal–Wallis test) Age years/category of areas
Succession areas
Reclaimed areas
5
17
20
25
5
17
20
25
Succession areas 5 17 20 25
– 1.000 0.164 0.002*
1.000 – 0.334 0.000
0.164 0.334 – 0.478
0.002 0.000 0.478 –
1.000 1.000 1.000 1.000
0.000 0.000 0.018 1.000
0.000 0.000 0.000 0.963
0.000 0.000 0.000 0.105
Reclaimed areas 5 17 20 25
1.000 0.000 0.000 0.000
1.000 0.000 0.000 0.000
1.000 0.018 0.000 0.000
1.000 1.000 0.963 0.105
– 1.000 0.301 0.084
1.000 – 1.000 1.000
0.301 1.000 – 1.000
0.084 1.000 1.000 –
Explanations: *marked differences are significant at p < 0.05.
the youngest herbaceous communities, the estimated root mass was up to several dozen times smaller and differences between categories were not large. The forest community root biomass in the temperate zone was assumed to be 42,000 kg ha−1 (coniferous forests) to 44,000 kg ha−1 (deciduous forests), of which 50–60% mass is located in the upper 30 cm of the soil (Jackson et al., 1996; Helmisaari et al., 2002; Claus and George, 2005). The ratio of carbon in the aboveground community biomass to carbon in the soil (C biomass/Corg soil) differed depending on site category. In areas with succession, it was from 0.14 on 5-year-old sites to 2.78 on 17-year-old sites, and in reclaimed areas from 0.01 on the youngest 5-year-old sites and much more, even up to 2.08 on 17-year-old sites and 3.30 on 20- and 25-year-old sites. It indicates significant differences in relation to forest ecosystems of temperate climatic zone where the ratio of C biomass/Corg soil is on average 1.13; however, the ratio decreases for biomass in cooler climates or when soil abundance decreases (Lieth and Whittaker, 1975). In the first stages of soil formation, the quantity of organic matter increases, whereas in mature ecosystems the accumulation rate may sometimes drop to 0 or else remains consistently high (Weiner, 2004). In terrestrial ecosystems there is usually twice as much organic matter and carbon (including detritus) in soil as in biomass. It is only in equatorial rain forests and forests of the temperate zone with a quick decomposition rate and element flow that biomass accumulation is higher than soil organic matter and carbon (Lieth and Whittaker, 1975). In boreal forests, low organic matter decomposition rate means that only a fraction of carbon and nitrogen becomes accumulated in biomass (Krebs, 1994). In case of a restored ecosystem on an open cast, the reported high ratio of C biomass/Corg soil indicated high organic matter decomposition rate and quick carbon circulation in the vegetation soil system. However, this issue requires further investigation to determine organic matter decomposition rate.
3.3.
An assessment of energy efficiency in reclamation
Total energy accumulation in the ecosystem restored in reclaimed areas (according to Eq. (2)) was 165.6 × 106 kJ ha−1 in the youngest 5-year-old sites and 1741.4 × 106 kJ ha−1 in
the oldest 25-year-old sites (Table 2). This was a nearly 10-fold increase in the studied time interval. A comparison of the amount of energy accumulated in the restored ecosystem (TEA) in this category of sites following 25 years with the amount of energy used during the reclaim process (TEC) i.e. 103.04 × 106 kJ ha−1 showed a 15-fold difference. The amount of energy trapped in the aboveground community biomass (BEA) during photosynthesis was 2.8 × 106 kJ ha−1 on unreclaimed 5-year-old sites, 237.5 × 106 kJ ha−1 on 17-year-old sites, 391.8 × 106 kJ ha−1 on 20-year-old sites and 381.0 × 106 kJ ha−1 on 25-year-old sites. BEA was 2.6 × 106 kJ ha−1 in case of the youngest 5-year-old reclaimed sites, 562.9 × 106 kJ ha−1 on 17-yearold sites, 1104.2 × 106 kJ ha−1 on 20-year-old sites and 1222.2 × 106 kJ ha−1 on the oldest sites (Table 2). In case of sites with natural succession, the total energy trapped in the developing ecosystem (TEA) and in the aboveground community biomass (BEA) was approximately two- to three-fold lower in comparison to reclaimed sites. However, it should be acknowledged that energy input was also lower since it only involved the technical reclamation stage (TR) and was 76.99 × 106 kJ ha−1 , as opposed to full reclamation where the total energy consumption (TEC) was 103.04 × 106 kJ ha−1 (Table 2). The energy trapped in the aboveground succession community biomass was 237.5 × 106 kJ ha−1 on 17-year-old sites to 391.8 × 106 kJ ha−1 on 20-year-old sites and 381.0 × 106 kJ ha−1 on the oldest 25year-old sites, respectively. These values may be generally regarded as relatively low. For instance, the annual energy production trapped in the biomass in coniferous forests of the temperate zone (southern Poland) may be assumed to be over 140 × 106 kJ ha−1 year−1 , and in deciduous forests over ´ 1984). 220 × 106 kJ ha−1 year−1 (Weiner and Grodzinski, The calculated reclamation energy efficiency coefficient (EER) (according to Eq. (3)) increased in chronosequence in both types of surfaces. In case of the youngest reclaimed sites EER was 0.02 and 5.46 on 17-year-old sites, 10.71 on 20-year-old sites to 11.86 on 25-year-old sites. In case of succession sites after the biotope was developed, the EER coefficient was from 0.04 on 5-year-old sites, 3.08 on 17-year-old sites, 5.08 on 20year-old sites and 4.95 on the oldest 25-year-old sites. To sum up then, energy gain (determined using the EER coefficient)
e c o l o g i c a l e n g i n e e r i n g 3 0 ( 2 0 0 7 ) 341–348
after 25 years was two-fold higher in reclaimed areas than in succession areas. This provided the basis for an assumption that the reclamation process significantly influences energy accumulation in the developing ecosystem.
4.
Conclusion
The amount of energy trapped in the biomass and soil organic matter was much higher in a forest ecosystem restored on post-mining areas of a sand cast during a full-scale reclamation treatment including technical restoration of biotope, biological reclamation and reforestation than in an ecosystem left for primary natural succession in parts of the sand cast formed in the course of technical reclamation. In fully reclaimed areas, the amount of energy accumulated in the aboveground community biomass after 25 years was approximately 12-fold higher than energy consumption in the course of reclamation processes and reforestation and in case of areas with succession about 5-fold higher from energy consumption during biotope restoration at the technical reclaim stage. The obtained results show that a full reclamation treatment including the restoration of biotope, phytomelioration, agrotechnical treatment and reforestation increases the amount of energy accumulated in the restored ecosystem by about two-fold. This clearly indicates that reclamation of such sites brings economic benefits and the amount of energy used to restore and reforest it is fully justifiable. However, this paper did not analyse other factors such as plant community features. Other publications on this post-mining site (Krzaklewski ˛ and Fraczek, 1999) showed that if left for natural succession to take place, the areas exhibited higher biodiversity than the reclaimed parts and that they became the habitat of rare species protected in Poland. This confirms the complexity of criteria selection for the assessment of reclamation although in this particular case, the economic benefits were obvious.
Acknowledgement The authors would also like to thank Jarosław Socha, PhD from Department of Forest Mensuration, Agricultural University of Krakow for his kind assistance during the preparation of statistical analysis.
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