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- Materials and methods
1. In minesoil reclamation, the establishment of a sustainable plant cover requires the improvement of limiting conditions and the re-initiation of carbon (C) and nutrient cycling.
2. The approach used in this study for reclaiming an abandoned sandpit in Quebec, Canada, was based on a heavy organic amendment as an attempt to accelerate the reconstruction of a functional ecosystem.
3. The one-time intervention consisted of incorporating paper de-inking sludge into soil at two rates (0 and 105 dry t ha–1), supplemented with nitrogen (N) at three rates (3, 6 and 9 g kg–1 sludge) and phosphorus (P) at two rates (0·5 and 1·0 g kg–1 sludge) followed by seeding (mid-summer) of Agropyron elongatum (Host) Beauv. (tall wheatgrass).
4. Standing biomass increased in the presence of sludge after both the first and second full growing seasons. High N application rates further increased yield, more importantly in the second season. The high P rate improved grass establishment in all cases. Ground cover increased with time and doubled in the presence of sludge whereas it declined in the absence of sludge. Phosphorus and N uptake was improved consistently in the presence of sludge.
5. Sludge application resulted in improved water retention and cation exchange capacities, and an increase in pH and bulk density of sandpit minesoil, all of which may have accounted for the significant improvement in plant responses. Levels of soil C and N suggest that this reconstructed system is approaching sustainability.
6. Adequate N and P supplements will accentuate the positive influence of sludge on revegetation.
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- Materials and methods
Standing plant biomass was greater in the presence than in the absence of sludge. This increased accumulation of total biomass was observed for the first and second growing seasons (year 1 and year 2, respectively), but it was statistically significant for year 2 only (Fig. 1). In some cases, biomass accumulation was increased by a factor of four. Average standing dry mass for treatments without sludge declined from 96 g m–2 in year 1 to 79 g m–2 in year 2. In contrast, average dry mass for treatments with sludge increased from 173 g m–2 (year 1) to 268 g m–2 (year 2). High N rates (6 and 9 g kg–1 sludge) increased biomass production; this effect was more pronounced in year 2 especially in the presence of sludge, where the difference between the low and high N rates was about two-fold. The high P rate significantly stimulated biomass production in year 1 both in the presence and in the absence of sludge.
Figure 1. Total biomass of Agropyron elongatum for the first and second full growing seasons, as affected by sludge, N and P amendments. N3, N6 and N9 represent N treatments of 3, 6 and 9 g N kg–1 sludge, respectively. P treatments were 0·5 (□) and 1·0 g (▪) P kg–1 sludge. The error bars represent the Least Significant Difference (LSD). The summary results of the anova are presented.
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Ground cover was significantly larger in the presence than in the absence of sludge and the difference was about two-fold (Fig. 2). As observed for biomass production, mean ground cover of plots without sludge declined from 35% to 31%, while ground cover of plots with sludge still increased from 62% to 68% from year 1 to year 2. Nitrogen and P treatments did not significantly affect ground cover. However, the highest P rate tended to increase ground cover in year 1. Ground covered by volunteer species was significantly higher (P = 0·035 and 0·007 for year 1 and year 2, respectively) in the absence of sludge, covering about 10% of the ground (data not shown). These species generally covered less than 3% of the ground in the presence of sludge. Volunteer species were mostly Capsella bursa-pastoris L., Chrysanthemum leucanthemum L., Rumex acetosella L. and Solidago canadiensis L., all of which are known to perform well on low-fertility soils (Marie-Victorin 1995).
Figure 2. Percentage of ground covered by vegetation after the first and second full growing seasons, as affected by sludge, N and P amendments. N3, N6 and N9 represent N treatments of 3, 6 and 9 g N kg–1 sludge, respectively. P treatments were 0·5 (□) and 1·0 g (▪) P kg–1 sludge. The error bars represent the Least Significant Difference (LSD). The summary results of the anova are presented.
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Tissue N content was slightly lower in the presence than in the absence of sludge (Fig. 3). This difference was significant only in year 2 and is probably the effect of N dilution in a larger biomass. In the presence of sludge, the two highest N rates (6 and 9 g kg–1 sludge) resulted in tissue N content lower than for the low N rate (3 g kg–1 sludge), explaining the significant N-sludge interaction. The high P rate resulted in an increased tissue N content in the absence of sludge, but for year 1 only (significant P-sludge interaction).
Figure 3. Tissue N content of standing Agropyron elongatum at the end of the first and second full growing seasons, as affected by sludge, N and P amendments. N3, N6 and N9 represent N treatments of 3, 6 and 9 g N kg–1 sludge, respectively. P treatments were 0·5 (□) and 1·0 g (▪) P kg–1 sludge. Interactions are presented only if significant. The error bars represent the Least Significant Difference (LSD). The summary results of the anova are presented.
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Tissue P content was significantly higher in the presence than in the absence of sludge at both sampling dates (Fig. 4). In year 2, high N rates resulted in lower tissue P content than the lowest rate, especially in the presence of sludge as indicated by the significant N-sludge interaction. Tissue P content was diluted in an increased biomass resulting from high N rates, in the presence of sludge. Phosphorus rates did not influence tissue P content.
Figure 4. Tissue P content of standing Agropyron elongatum at the end of the first and second full growing seasons, as affected by sludge, N and P amendments. N3, N6 and N9 represent N treatments of 3, 6 and 9 g N kg–1 sludge, respectively. P treatments were 0·5 (□) and 1·0 g (▪) P kg–1 sludge. Interactions are presented only if significant. The error bars represent the Least Significant Difference (LSD). The summary results of the anova are presented.
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Water retention capacity (WRC) of minesoil was improved immediately after sludge application and the difference was maintained through to the end of the study (Fig. 5). For soil matric potentials in the range of plant available water (−33 to −1500 kPa), volumetric water content was, on average, doubled (increase of 42%, 27% and 72% on day 4, 359 and 743, respectively) when compared to treatments without sludge (P < 0·06 for the three dates). Neither N nor P treatment affected minesoil water retention.
Figure 5. Water desorption curves of sandpit minesoil as affected by de-inking sludge amendment after 4, 359 and 743 days of incorporation; N treatments were pooled (n = 8).
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The average pH of the minesoil was 4·7 initially. It increased from 4·7 to 6·8 four days after the incorporation of sludge and remained higher than 6 subsequently (Fig. 6). In the absence of sludge, pH temporarily increased by about half a unit, but it regained its pre-treatment level thereafter. Nitrogen rates did not significantly affect pH at any time.
Figure 6. pH (CaCl2) and cation exchange capacity (CEC, estimated as the sum of Mehlich-III-exchangeable cations) of sandpit minesoil as affected by de-inking sludge amendment 4, 359 and 743 days after incorporation; N treatments were pooled (n = 8). The error bars represent the Least Significant Difference (LSD). The summary results of the anova are presented.
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Cation exchange capacity (CEC) of minesoil, as estimated by the sum of Mehlich-III-exchangeable cations, was improved with the incorporation of sludge whereas it remained near pre-treatment level (i.e. 5·3 meq 100 g–1) in the absence of sludge, at all times (Fig. 6). The effect of sludge was apparent at day 359 but not at day 4; this difference became significant at day 743. Nitrogen or P treatments had no effect on CEC at any time.
At day 743, soil organic C was higher in the presence than in the absence of sludge by a factor of 10 (Table 2). In the absence of sludge, C content remained near the pre-treatment level. Less C remained in minesoil with the highest N rate compared to the lowest rate. Total N was higher in the presence than in the absence of sludge by about 50%. In the absence of sludge, N content of minesoil was similar to pretreatment content, in spite of N additions. In the presence of sludge, N content was higher (23%) with the highest than with the lowest N rate.
Table 2. Organic C and total N contents (mean ± SD) of the sandpit minesoil 743 days after incorporation of de-inking sludge. P treatments were pooled (n = 8)
|With sludge||Without sludge|
|C (mg g–1)||12·1 ± 5·1||19·3 ± 8·2||1·6 ± 0·6||1·4 ± 0·3|
|N (μg g–1)||298 ± 69||366 ± 83||190 ± 42||193 ± 52|
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- Materials and methods
Values obtained for standing biomass (i.e. average above-ground primary productivity) in the absence of sludge compared well with those reported for other unamended minesoils that have been revegetated over two growing seasons (Smith, DePuit & Schuman 1986; Corbett, Anderson & Rodgers 1996). In contrast, the high values obtained in the presence of sludge are similar to those reported when the unamended minesoil had been revegetated for more than 15 years (Corbett, Anderson & Rodgers 1996). In other words, when sludge had been applied, biomass production was equivalent to that of a self-sustaining vegetation cover that was believed to be steady. Standing plant biomass is an indicator of ecosystem function (Aber 1987; Bradshaw 1996), but over short periods of time (i.e. one growing season), it does not necessarily reflect its degree of establishment and stability (Elkins et al. 1984). In this study, standing biomass was enhanced in the presence of sludge and this response was far more apparent at the end of the second growing season than after only one season. Sludge application appeared to have been effective in accelerating restoration of ecosystem function, at least from a biomass production standpoint.
High N rates tended to accentuate the effect of sludge on biomass production. This response to N rates in the presence of sludge was strongest for the second growing season. This agrees with the fact that decomposing sludge had a higher N content with the two highest N rates than with the lowest rate; more N was therefore remineralized (about 32% more over the first 27 months; A. Fierro, unpublished data), supporting better growth. Also, this N remineralization may partially explain the improved N uptake (dry mass yield × N concentration) in the presence of sludge. In the absence of sludge, N treatments had no effect on vegetation, in spite of the severe N deficiency of the minesoil. This was probably due to early, heavy N leaching losses, as assessed by nitrate concentrations in the soil solution (Fierro, Angers & Beauchamp 1998) that were initially (first 3 months) 10-fold higher in the absence of sludge than in its presence.
The high P rate stimulated biomass production in year 1, and establishment of vegetation was therefore improved in both the presence and the absence of sludge. Competitive vigour in young stands of grass is enhanced with increased P availability (Sansen & Koedam 1996). In the presence of sludge, vegetation was primarily affected by P treatments in the first growing season, whereas in the second season it was affected by N treatments. There is evidence that P as well as N limits early primary succession, but the importance of P varies among substrate types (Raich et al. 1996).
Phosphorus nutrition was consistently improved with sludge application. In contrast, P rates did not affect tissue P content. This indicates that general improvement of soil conditions by sludge application probably had a greater impact on P nutrition than P supplements. Improved uptake during both growing seasons may be explained by the neutralizing effect of sludge on soil reaction, greater nutrient flow due to more available water in the root zone or increased root mass and density due to lower soil bulk density (discussed below). Also, it has been shown recently that organic amendments with high P content can ameliorate P availability in acids soils by both increasing pH and reducing P sorption capacity (Iyamuremye, Dick & Baham 1996). In addition, biological conditions were certainly improved in this initially C-deficient minesoil.
Tissue P and N contents were generally lower than those considered adequate for forage grasses (CPVQ 1986). This may reflect the poor nutrient conditions of the site (Raich et al. 1996). However, this interpretation should be considered with caution since Agropyron elongatum exhibits a relatively low demand for N (Smith, DePuit & Schuman 1986) which would be consistent with low tissue N content. Also, samples included all above-ground biomass; consequently, levels of N and P were expected to be lower than those specified for sufficiency tests which usually require sampling of selected parts of the plant at a specific growth stage.
One important goal of minesoil reclamation is to develop an erosion-resistant system. The accumulation and retention of C and nutrients greatly depend on this goal. It is generally agreed that a closed plant canopy is an effective way to stabilize and protect the soil. Therefore, ground cover is a critical parameter in the evaluation of revegetation studies. Application of sludge had its greatest effect on this parameter. About two times more ground was covered compared to treatments without sludge. This effect was apparent early, hence indicating that sludge rapidly promoted soil protection. In addition, sludge itself may provide a physical protection against erosion (Watson & Hoitink 1985). In the absence of sludge, ground cover was rather poor and probably insufficient for soil stabilization. Ground cover increased over time in the presence of sludge, whereas it declined in its absence. A revegetated pyritic minesoil that was amended with various organic wastes also had an increasing ground cover with time compared to an unamended control, which exhibited a decline (Pichtel, Dick & Sutton 1994).
Agropyron elongatum is a bunch grass and propagates mainly by seeds. In the presence of sludge, it was able to complete its growing cycle and produce abundant viable seeds that generated healthy seedlings. In contrast, no seedlings were observed at any time in the absence of sludge. Successional sustainability of vegetation depends in large part on changes in soil properties or the rate of soil development from skeletal spoil material (Elkins et al. 1984).
Water retention capacity (WRC) of the exposed layer in the sandpit is one of the most limiting factors impeding revegetation. Internal drainage is also extreme, and germination and establishment of seedlings may be impeded in spite of the wet local climatic conditions. In this study, however, the days following seeding of the grass were very rainy and emergence was uniform in all plots. Hence, differences observed in biomass production and ground cover were not due to deficient germination and emergence, but in part to the subsequent low survival rate and poor establishment of seedlings in the absence of sludge. Over time, the addition of sludge increased by 27–72% the amount of water available to plants. In bentonite mine spoils, soil water content was also increased by heavy additions of wood residue (90 and 135 t ha–1) (Smith et al. 1985). In the present study, establishment of second generation seedlings was certainly supported by increased WRC in sludge amended plots. Also, mass flow of nutrients to roots was probably facilitated: an increased volume of soil solution may imply better ion transport that will contribute to better plant nutrition. Indeed, mass flow of ions is the primary process in the uptake of N, Ca, Mg, S, Cu, B, Mn, and Mo (Gardner, Pearce & Mitchell 1985). The sodium content and electrical conductivity were measured initially and showed no salt problem.
Sludge application had a long-lasting neutralizing effect on minesoil acidity. The basic pH of sludge is due to the use of pulping and de-inking chemicals such as sodium hydroxide and sodium silicate (NCASI 1991). The increase in minesoil pH with sludge application probably had a positive impact on plant nutrition through increased nutrient availability. For example, a close positive correlation (not shown) was found between pH and tissue P content. Other nutrients not monitored in the current study may also have become more available (i.e. Ca, Mo and K). In a greenhouse experiment, sand–sludge mixtures resulted in poor plant P recovery efficiency (Fierro et al. 1997). However, in that case, sand had a basic pH and it was suggested that microbial immobilization impeded plant recovery of added P. In this study, increased P availability by the improvement of pH and other limiting conditions, may have compensated for P immobilization caused by sludge application.
As expected, CEC of the minesoil increased after sludge incorporation. Most of this rapid effect was probably due to the clay present in the sludge which is derived from paper fillers (NCASI 1991). Over time, this effect became more important, probably due to the increasing formation of organic colloids as sludge decomposition proceeded and to ionized phenolic compounds present in decaying lignin from wood fibre. Thus, the retention and supply of cationic nutrients was certainly improved with sludge application.
Bulk density of the unamended minesoil was ≈1·7 Mg m–3. Sludge application decreased it to 1·3 Mg m–3, and this value was maintained through the duration of the study, whereas it remained at pre-treatment level in the absence of sludge (data not presented). For various soil textures, bulk densities higher than 1·5 Mg m–3 can severely restrict root growth of many species (Hemsat & Mazurak 1974; Zeleznik & Skousen 1996). Thus, lowering minesoil density with sludge incorporation may also partially explain the differences observed for plant responses by facilitating a more vigorous and extended root system.
The cycling of most plant nutrients and in particular N, P and S, is closely related to the cycle of C through the processes of decomposition. When attempting to re-initiate nutrient cycles in the sandpit, pools of organic C must be restored. One heavy application of sludge had apparently restored such pools. Two years after sludge incorporation, organic C content of amended minesoil was similar to that of surrounding non-degraded soils. Most of this C originated from sludge and is considered to have stabilized in soil since it exhibited an annual decomposition rate of about 4% (for the second year; A. Fierro unpublished data) which is typical for soil organic matter decomposition in temperate regions (Bradshaw 1987).
Plant cover had the best responses overall when sludge was applied and supplemented with the two highest N rates. In temperate regions, a soil N capital of about 1000 kg ha–1 is usually considered necessary to support a self-sustaining ecosystem (Bradshaw 1996). In this study, the soil N capital necessary to maintain the same productivity as in the second year, for the highest N rate, can be estimated (Bradshaw 1987). Some assumptions have to be made: (i) most soil N is contained in the organic fraction derived from sludge and vegetation; (ii) net N mineralization occurs at the same rate as net decomposition (≈4% per year); (iii) below-ground biomass and tissue N content are equivalent to those above-ground (i.e. 3200 kg ha–1 year–1 and 0·44% for biomass and N content, respectively); and (iv) there are no more N losses (i.e. all N is recycled). Therefore, to maintain a productivity of about 3200 kg ha–1 year–1 (above-ground), which seems adequate for reclamation purposes of this system, a soil N capital of ≈700 kg ha–1 is required. At day 743 after the one-time intervention, the soil N capital was about 950 kg ha–1 (calculated from total N = 366 μg g–1, bulk density = 1·3 Mg m–3 and sampling depth = 0·2 m). Hence, this reconstructed system, if stable, may have approached sustainability at the end of the study.
Sandpits and other minesoils can be classified as profoundly disturbed sites (Aber 1987). Ecosystem reconstruction on such sites involves, in its earliest stages, the restoration of ecosystem function. In the limiting conditions of the sandpit, changes needed for successful revegetation and restoration of ecosystem function are likely to be autogenic rather than allogenic in origin. Organic matter and nutrient accumulation are obviously required in this system, and such demands may eventually be fulfilled after long-term vegetal succession. In the present study, we attempted to substitute, at least partially, these long-term changes by a single amendment that would supply organic C and macronutrients at levels allowing re-initiation of their cycles in the soil-plant-microbial ecosystem. The use of organic amendments has been considered as a high input and therefore, expensive approach for minesoil revegetation (Piha et al. 1995). However, if the organic amendment is actually a waste which is difficult or expensive to dispose, then its use as a revegetation tool is a less expensive and a more environmentally sound alternative than disposal (if, of course, contaminants are not introduced with the waste). The paper and wood industry is one example among many industries (i.e. processing industries of sugar, tequila, coffee, rice, canned foods, seafood, etc.) generating enormous amounts of organic wastes.
This study has shown that under the humid and cool conditions of eastern Canada, paper de-inking sludge can be a valuable tool for revegetation of degraded soils presenting limitations in water retention, acidity, nutrient retention and availability, and bulk density. Sludge application enhanced establishment and subsequent growth of plant cover by improving the limiting conditions prevailing in the sandpit. Phosphorus nutrition was improved considerably. Adequate N and P supplements will accentuate this positive influence of sludge on revegetation. Further research on sludge decomposition and dynamics of nutrient pools in the minesoil should assess the viability of the system. The longer the overall positive effect of sludge on plant productivity, the more organic matter from vegetation will accumulate. This organic matter will eventually substitute the decomposing sludge in providing the retention and supply of water and nutrients. Consequently, reconstruction of a functional and erosion resistant ecosystem may be greatly accelerated by a heavy organic amendment.