1. Introduction
The intensification of agricultural activities has led to serious environmental problems and one of the main issues is related to the maintenance of soil fertility at different levels [
1]. Generally, the use of organic amendments (OA) instead of chemical ones improves soil fertility by slowly releasing nutrients to support soil quality and productivity [
2]. The incorporation of OA also leads to an increase in carbon dioxide (CO
2) emissions from the soil as a result of rapid soil organic matter decomposition [
3].
In recent years, biosolids were proposed as a novel solution to mitigate greenhouse gases (GHGs), especially in agriculture (which is responsible for 70% of N
2O emissions) and forestry [
4,
5,
6]. N
2O is a part of the global nitrogen cycle and is bound to other forms of N (organic N, ammonia, nitrate). In the soil, N
2O is generated by the microbial transformation of organic and inorganic N and is enhanced under wet conditions [
7]. Nunes et al. [
8] showed that the addition of deinking sludge as an organic amendment with a very high C/N ratio in alkaline soil can cause N immobilization and reduce yield. Rashid et al. [
9] also showed that adding deinking paper sludge (DPS) on acidic soil for one-year results in total N immobilization. In this context, the use of pre-existing agricultural, forestry, or pulp waste (biosolids) is more interesting to mitigate climate change than using chemical fertilizers [
10]. Moreover, the use of biosolids such as DPS or pulp paper sludge as soil amendments instead of landfilling them considerably reduces GHG emissions. DPSs, however, contain organic matter (OM) that is mainly lignocellulosic (cellulose, hemicelluloses, and lignin) and are characterized by neutral to basic pH, high carbon and calcium contents and low nutrient (N, P, K) levels [
11,
12] compared to other sources of organic matter and can be used as a soil amendment or as a fertilizer residual (RFM). Based on the study by Beauchamp et al. [
13], DPSs contain relatively low levels of metals, and the risk of contamination, if any, is organic rather than metallic. However, the composition of DPS varies from one industry to another depending on the pulping and deinking process [
12], hence the importance of redoing the characterization and pre-treatment steps for each industry. The purpose of detailing the deinking process is to identify the sources of contamination and the composition of the deinking sludge. The deinking process is carried out in several stages [
13]: The first stage is pulping, during which the wastepaper is suspended, and ink particles are removed due to the addition of certain chemicals such as fatty acids and soda. The second stage is flotation, during which the fibers are recovered following a separation of the various constituents by injecting air bubbles into the flotation cell. The third stage is washing, during which there is a dispersion of ink particles made hydrophilic by the surfactants that contain clay. In the fourth step, clarification, a flocculation polymer agglomerates the dispersed ink particles. The final step is the removal of excess water under mechanical pressure to facilitate handling. The sludge comes mainly from the flotation, washing, and clarification stages. DPS accounts for about 95% of the volume of discharges from paper recycling plants [
14]. Characterization of the DPS and understanding the deinking process is a key step in determining whether trace elements and contaminant levels exceed the limits set for RFM. DPSs contain fatty and resinous acids from the wood or chemicals added during the deinking process and may contain polycyclic aromatic hydrocarbons (PAHs) and organochlorine compounds such as dioxins from the chlorine products used in pulp bleaching. Hébert et al. [
12] established limits for these contaminants and DPSs can be applied in practice depending on their composition.
DPS has a high carbon (C)-to-nitrogen (N) ratio (C: N = 70–150) [
9,
11], and N supplement was required to avoid N immobilization [
15,
16]. According to the Kyoto protocol, the reduction of GHGs through waste management and the use of residual fertilizing matter (RFM) can help to mitigate climate change [
12,
14].
Tian et al. [
17] describe that the use of de-inking biosolids significantly increases carbon sequestration in agricultural soils decreasing CO
2 emissions. Indeed, the concentration of soil organic carbon increases with the addition of biosolids, and several factors come into play: temperature, soil moisture, and initial composition of the introduced material [
18,
19,
20,
21]. For example, the use of lignocellulosic products in agricultural soils can constitute new carbon reservoirs other than forests references added [
17,
20]. To better understand the role of lignocellulosic material in carbon sequestration, it is essential to understand the processes of mineralization and humification. The fate of soil organic matter depends on its composition; the higher the lignin content, the slower it decomposes [
22,
23,
24]. According to the theory of Waksman and Hutchings [
25], humic substances, mainly humic acids, are derived from lignin. Some lignin-rich wood by-products such as de-inking biosolids are said to be recalcitrant and their residence time in the soil is important with a slow decomposition rate. The use of this material in the soil would have benefits comparable to those of no-till, which would promote carbon sequestration in the soil [
16,
17,
18]. This use of materials of lignocellulosic origin seems interesting and deserves to be further developed in new research work. Carbon storage in the soil depends on intrinsic soil factors such as pH, mineralogy, microbiology, aggregation, microclimate [
25], and extrinsic factors or quality of organic matter [
26]. Organic matter forms, together with soil minerals, a clay-humic complex, which confers stability and protection to the soil organic matter.
In Canada, DPS has been used as an RFM for crop fertilization and to increase yield [
14] in species such as alder and aspen [
15], as well as for soil restoration and land management [
17,
18] because of the various nutrients that DPS contains [
9,
18]. For example, in Quebec where the norms are considered to be the most strict, DPS was used according to the regulations of the
Bureau de normalisation du Québec (BNQ), BNQ 0419-090, and BNQ 0419-200 which covers liming residues [
11,
12].
In developing countries, especially those in North Africa, waste management is a major problem, as is the deterioration of soil properties, while organic matter (OM) sources are limited. In Tunisia, for example, soils have very low OM content (less than 2%) so OM is required to improve soil fertility [
21]. DPS could be used as an OM source for the restoration of calcareous agricultural soils [
22]. Agricultural areas in the semi-arid regions have silty clay soils, which are characterized by high CaCO
3 and low organic matter content [
3,
21]. After COP21 (Paris Climate Conference), Tunisia’s reduction target was estimated to be 40% of total emissions, whereas the recycling rate for paper residues such as DPS is around 0%; this lack of recycling represents a big loss of organic matter (OM) [
11]. However, DPS can be used as RFM.
The pulp industry generates a large amount of lignocellulosic waste that needs to be managed [
11,
23]. The use of DPS as an amendment may improve the biological, physical, and chemical properties of calcareous soils. However, most studies have focused on the effects of DPS (an alkaline agent) on the chemical and biological properties of acidic or slightly acidic soils. The work of Nunes et al. [
8] on secondary sludge showed that paper sludge is a good potential source of OM, nitrogen (N), phosphorus (P), and potassium (K), while it is a potential liming agent for acidic soils, especially when an appropriate fertilizer is also used. The absence of studies evaluating CO
2 emissions and the effects of DPS as a source of OM for low fertility calcareous soils in Tunisia led us to propose the present line of research. Much research is needed to provide data for determining the effects of DPS on soil properties under such conditions.
The objectives of this research were (1) to study the effects of increasing application rates of DPS (0, 30, and 60 Mg ha−1) on the change in the physical (permeability and structural stability), chemical (particularly soil pH), biological (microbial biomass and microbial metabolic quotient—qCO2) and CO2-C emissions of a calcareous soil following two successive annual applications and (2) to determine whether the addition of N fertilizer induces changes in these agricultural parameters.
4. Discussion
The DPS contained mainly lignocellulosic OM (cellulose, hemicellulose, and lignin) (
Figure 1) and formed a clay-humic complex after incorporation into the soil. The presence of the typical lignin peaks at 1600 to 1650 cm
−1 and at 1460 cm
−1 is proof of the presence of lignin-derivative products in DPS (
Figure 1) [
43]. The C:N ratio was very high (
Table 1), whereas the suitable C:N ratio had previously been established at between 20 and 30, and the C:P ratio had been established at between 40 and 50 [
8]. The Ca: Mg ratio is another measure of the suitability of organic residues as a nutrient source for plant growth [
8,
44,
45,
46]. This ratio was estimated at 60.13. Difficult mobilization of N, P, and Mg may be expected when DPS is added to the soil.
The addition of N fertilizer had a positive and highly significant effect (
p < 0.001) on EC in comparison to the control. This salt enrichment of the soil was due to the salts in the fertilizer. Comparing the two fertilized doses with the fertilized control, we noted a decrease in EC for the DPS60F treatment (247.8 to 231.6 µS cm
−1) and a significant increase in EC for DPS30F (from 247.8 to 260.3 µS cm
−1) (
Table 2). It should be noted that the effect of ammonium nitrate on soil EC was greater in DPS30F than in DPS60F. This decrease in EC for the 60-Mg ha
−1 treatment may be correlated with the effect of this rate on the improvement in soil permeability. To avoid an EC increase resulting from the salts in chemical fertilizer, an organic N source can be recommended.
The DPS amendment significantly (
p < 0.05) increased soil pH for the 30 and 60-Mg ha
−1 doses (
Table 2). However, this soil pH change, either with an acidic or a basic amendment, is temporary, and the soil returns to its original pH after a certain period. This principle is very important when calculating the dose needed to be incorporated into acidic soils to correct the pH. It can be concluded that the soil returned to its initial pH after a period longer than one year and that the pH remained unchanged until a new amendment was applied. These results are in accordance with other studies reporting a close relationship between paper sludge application rate and pH in Mediterranean soils [
8,
47]. Soils with high initial pHs are expected to be more strongly buffered than soils with low pHs [
8,
47]. Indeed, following a limestone amendment, the Ca
2+ ions replace the H
+ ions on the clay-humic complex and the acidity thus decreases. After a period, however, the soil buffering capacity plays a role in regulating the pH to reach values close to those before the amendment [
47,
48]. A slight decrease in pH (−0.11 units) was observed in the 60-Mg ha
−1 treatment without added ammonium nitrate in comparison with the same rate with fertilizer. This decrease highlights the acidifying effect of this fertilizer since it was characterized by an acidic pH. However, few studies have reported the effect of ammonium nitrate in combination with DPS on soil properties [
15]. Buffer capacity of calcareous soil is mainly attributed to calcium carbonate. Then the buffering capacity potential of DPS is not expected to be effective in such soils.
The decrease in OM content for the soil amended with ammonium nitrate in combination with DPS (
Table 2). is due to the acceleration of the mineralization process, which increased the availability of N to the plant; a positive effect on crop yield is expected [
15,
20]. Based on the results below, if the objective of adding DPS to the soil is to increase the soil organic C, the 60-Mg ha
−1 rate is recommended for a calcareous soil.
Studies have reported that amending soils with high C:N paper sludge caused a net immobilization of soil N [
5] and reduced plant growth [
8], whereas low C:N sludge increased plant- available N and biomass production in N-limited ecosystems [
44]. This effect can be explained by the rapid mineralization of organic N following the addition of mineral N in the form of chemical fertilizer [
5]. This suggests that the mineral N could have been immobilized during the degradation of labile C constituents in the DPS and that the organic N mineralized very slowly when high levels of sludge were applied to the FLc soil. The DPS amendment improved acidic soil properties, but at the same time induced N immobilization, which was the major cause of yield loss in a barley crop seeded shortly after DPS application [
44]. This result highlights the importance of integrating organic N with DPS (i.e., sewage sludge, poultry manure, or secondary sludge) in the future [
11]. Thus, the choice of N supplement is important; one could believe that an organic form of an N supplement would lead to slower OM mineralization compared to a mineral form because the OM creates humic complexes with soil clay [
2].
The results for available P (
Table 2) suggest that DPS with or without N fertilizer led to a better mobilization of available P. The use of DPS can constitute a valuable means for enhancing soil available P for crops [
20]. As for the effect of ammonium nitrate, the dose-effect persists in both cases, but P levels were lower in the soil amended with a combination of DPS and ammonium nitrate than in the soil without N supplementation, owing to OM mineralization. This result can be explained by the fixation of P on soil complexes via Ca
2+ cations or by the few positive charges on the edges of the clay particles.
The DPS combined with N fertilizer increased K
+ and Ca
2+ following two successive annual applications of DPS (
Table 2).
In calcareous soils, Ca
2+ cations are found in large quantities and there is thus the possibility of an antagonistic effect between P and Ca through the formation of insoluble compounds [
49]. This effect could explain the slight decrease in Ca
2+ content in the DPS60F treatment.
According to our previous study [
11], the composted DPS does not contain high concentrations of Na
+. No significant effect on soil Na
+ content was observed when the DPS dose increased (
p < 0.0001), whereas the study of [
50] noted that the addition of ammonium nitrate at a rate of 150 kg ha
−1 to the control or DPS-amended soil led to a significant decrease in Na
+. However, few studies have considered the effect of ammonium nitrate in combination with DPS and more research is therefore needed to confirm whether the Na
+ migrates to the plants and/or whether DPS can be used as an effective amendment to help desalinate soil. According to our previous study [
11], the composted DPS does not contain high concentrations of Na
+. No significant effect on soil Na
+ content was observed when the DPS dose increased (
p < 0.0001), whereas the study of [
51] noted that the addition of ammonium nitrate at a rate of 150 kg ha
−1 to the control or DPS-amended soil led to a significant decrease in Na
+. However, although many studies have described decreased salinity effect of organic amendments [
50,
51], few studies have considered the effect of ammonium nitrate in combination with DPS and more research is therefore needed to confirm whether the Na+ migrates to the plants and/or whether DPS can be used as an effective amendment to help desalinate soil.
As a lignocellulosic matter, DPS incorporated into the soil is expected to form a clay-humic complex, which could explain the positive dose effect on soil permeability as well as the better soil structure, confirming the results of [
52]. The addition of ammonium nitrate at 150 kg ha
−1 did not affect permeability in the control soil or the amended soil. However, there was a slight decrease in permeability, from 17.79 to 14.33 cm h
−1, for the DPS60F treatment in comparison with DPS60, but permeability was still very high. The 60-Mg ha
−1 dose seems to be the recommended dose for ensuring better soil permeability. The presence of clay in the amendment could play a role in modifying the permeability and structural stability of soil by enhancing particle adhesion. Physical soil tests may invalidate or confirm the hypothesis that the clay in the DPS modifies soil physical properties.
Cations near the negatively charged clay surfaces are subject to electrostatic attraction towards the surface as well as a tendency to diffuse into the bulk solution. As a result, the concentration of cations diminishes exponentially as the distance from the clay surfaces increases [
47]. Dispersion and flocculation phenomena are important factors determining the effects of liming on soil physical properties [
48].
The positive effects of direct liming on soil structure can be ascribed to the flocculating and cementing actions of calcium carbonate (CaCO
3) itself and of newly precipitated iron and aluminum oxides and hydroxides [
48,
49]. However, for the 60-Mg ha
−1 rate, structural stability was close to the adequate stability zone. Therefore, after two successive annual DPS applications, there was no significant effect in terms of improving soil stability. Applying a high dose of DPS (>60 Mg ha
−1) may not improve soil stability. The studies of Trépanier et al. and Nemati et al. [
51,
52] established that the addition of DPS did not significantly increase structural stability until two years later. Based on the results of the FTIR spectra (
Figure 1), which showed the richness in the clay of the DPS, there may be settling and compaction problems in the soil, as well as a decrease in soil stability. In this case, 60 Mg ha
−1 would be the recommended dose for ensuring better soil stability (
Figure 3).
A previous study also concluded that the application of high rates of DPS (≥45 Mg ha
−1) should be avoided if the land is to be used for grain or cereal crops immediately afterwards, because of the yield loss that will occur with such rates [
44].
The short-term effects of DPS on soil biological properties following two successive annual DPS amendments were a decrease in the soil respiration rate (CO
2 evolution) and significant increases in MBN and in the MBC:MBN ratio for the DPS60 treatment in comparison with the control and DPS30. A significant effect on MBC was observed. A previous study of Chan and Heenan [
53] found that the beneficial effect of liming on aggregate stability was evident only after two successive annual amendments, whereas other authors reported that an increase in MBC content was observed in the first year after application [
51]. Soil treatment with de-inking sludge for durum wheat cultivation showed that the higher the rate (data not shown, work in progress), the lower the yield in the absence of nitrogen fertilizer. The added DPS, due to its high carbon content, would not be sufficiently mineralized to be available and assimilated by the plant. Soil nitrogen would be consumed by the plant in the control treatment, whereas it would be used by soil microorganisms for DPS mineralization in the other treatments. Moreover, the addition of organic fertilizer (pig slurry) to the soil amended with paper sludge increased straw and ear yields [
5,
10,
15]. These results show that DPS amendment increases or decreases soil biological activity depending on the N content and form of the raw amendment and on the N soil content, which is a good index of the mineralization process. It depends whether the objective is soil sequestration or plant growth. The increase of soil respiration expresses less C sequestration but can traduce mineralization that leads to more nutrients for crops.
According to the literature, the short-term effects of liming, in terms of causing a flush of microbial activity, will affect soil aggregation by increasing aggregate stability. The microbial biomass produces gelatinous extracellular polysaccharides that act as binding agents in soils and fungal hyphae by forming a network that also promotes aggregation [
54].
Soil fumigation by chloroform vapors denatures the cell membranes of microorganisms and releases MBN and organic C. Thanks to its high cellulose content, DPS is an important energy source for microorganisms [
54]. The qCO
2 was used as an indicator of the ecosystem and microbial stresses [
55]. Therefore, for the 30 Mg ha
−1 rate, there was no microbial stress in comparison with the control, whereas for 60 Mg ha
−1, there was a significant difference. The kinetics C
o parameter values in the model used were much higher in the DPS30 than in DPS60 or the control. This could be explained by higher cumulative CO
2-C emissions and thus higher microbial activity after mineralization of the fresh OM. Generally, increasing the application rate led to increases in the C
o values [
52], whereas for DPS, there was a decrease (
Table 4). The values of total mineralization rate and qCO
2 (
Table 4 and
Table 5) showed a reduction in CO
2 emissions when the 60 Mg ha
−1 rate was used. In fact, the soil microbial community represents a sensitive indicator of the effect and changes produced by agricultural management practices on soil quality [
1]. Soils can act as a source of C during organic matter decomposition after the organic matter amendment or as a sink of C by enhancing carbon sequestration into the soil [
1]. Results showed that the application of 60 Mg ha
−1 of DPS to calcareous soil is an effective strategy to reduce atmospheric soil C losses. According to Verdi et al. [
1], soil CO
2 emissions represent the main cause of soil C losses and may be used as an early indicator for the estimation of soil organic C level in the short-term.
Further research is needed to establish the exact relationship between the short-and long-term effects of the DPS amendment on microbial activity and soil stability. Moreover, studying the effects of repeated DPS amendments on the bioavailability of metallic trace elements would make it possible to assess one of the essential environmental impacts of such amendments in calcareous soils.