Effects on soil phosphorus dynamics of municipal solid waste compost addition to a burnt and unburnt forest soil

Science of the Total Environment 642 (2018) 374–382

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Effects on soil phosphorus dynamics of municipal solid waste compost addition to a burnt and unburnt forest soil María-Belén Turrión a,b,⁎, Teresa Bueis a,b, Francisco Lafuente a, Olga López a, Esther San José c, Alexandros Eleftheriadis d, Rafael Mulas a a

Area of Soil Science and Soil Chemistry, E.T.S. Ingenierías Agrarias, University of Valladolid, Avda. de Madrid 57, 34004 Palencia, Spain Sustainable Forest Management Research Institute, University of Valladolid-INIA, Avda, Madrid 44, 34071 Palencia, Spain Biota Tecnología Forestal, C/ Vega Sicilia, 2bis, Parque Alameda, 47008 Valladolid, Spain d Technological Educational Institute of Eastern Macedonia and Thrace, Department of Landscape Architecture, Drama 66100, Greece b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The highest dose of compost was the most effective one for rehabilitation purposes. • In studied soils most P forms presented higher concentrations in clay fractions. • Compost increased available Pi and Ca associated Pi available in the long term. • Compost addition increased total P concentrations in sand and silt fractions.

a r t i c l e

i n f o

Article history: Received 30 January 2018 Received in revised form 4 June 2018 Accepted 5 June 2018 Available online xxxx Editor: Charlotte Poschenrieder Keywords: Soil remediation Particle-size fractionation Organic amendment Phosphorus sequential fractionation Rehabilitation Incubation

a b s t r a c t The main aim of this research was to assess the effects of municipal solid waste compost (MSWC) addition to a burnt and unburnt calcareous soil, on the distribution of soil P forms in particle-size and extractable fractions. Three MSWC doses (1, 2 and 4% w/w) were added to burnt and unburnt soil samples and were incubated for 92 days at 29 °C and 75% of field capacity moisture. A particle-size fractionation followed by a sequential P extraction procedure was carried out. The burnt soil showed significantly lower concentrations of organic P forms (Porg) and significantly higher concentrations of stable P forms than the unburnt soil. Besides, in both burnt and unburnt soils, most P-forms presented higher concentrations in the clay fractions than in the sand and silt fractions, possibly due to the different proportions of microbial synthesized and plant-derived substances in the different particle-size fractions. Finer fractions of MSWC showed higher total P and Porg concentrations than coarser fractions. Our results showed that the highest dose of MSWC was the most effective one for the rehabilitation of the burnt soil. MSWC amendment also caused an increase in soil P availability in the unburnt soil which initially contained relatively low levels of P. During the incubation process, a high proportion of organic P contained in the MSWC was mineralized into inorganic P forms. These forms were precipitated with Ca cations which are very abundant in these calcareous soils, significantly increasing the P fraction extracted by HCl in both amended

Abbreviations: MSWC, municipal solid waste compost; Corg, organic carbon; AEM, anion-exchange membrane; Pi, inorganic phosphorus; Ptotal, total phosphorus; Porg, organic phosphorus; P_AEM, P extracted with anion exchange membranes; Pi_NaHCO3, inorganic P extracted with 0.5 M NaHCO3; Porg_NaHCO3, organic P extracted with 0.5 M NaHCO3; Pi_NaOH, inorganic P extracted with 0.1 M NaOH; Porg_NaOH, organic P extracted with 0.1 M NaOH; P_HCl1M, P extracted with 1 M HCl; P_HClconc, P extracted with concentrate HCl; P_HClO4, P extracted through digestion with concentrate HClO4 (230 °C). ⁎ Corresponding author at: Area of Soil Science and Soil Chemistry, E.T.S. Ingenierías Agrarias, University of Valladolid, Avda, de Madrid 57, 34004 Palencia, Spain. E-mail addresses: [email protected] (M.-B. Turrión), [email protected] (T. Bueis), [email protected] (F. Lafuente), [email protected] (O. López), [email protected] (R. Mulas).

https://doi.org/10.1016/j.scitotenv.2018.06.051 0048-9697/© 2017 Elsevier B.V. All rights reserved.

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soils. Hence, adding compost to the soil involved an increase in the available P reservoir in the long term. The combination of particle-size fractionation, chemical sequential extraction and incubation experiments can be a valuable tool for splitting soil phosphorus into different fractions regarding their availability in relation to short and long-term transformations in soil. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Forest fires are one of the greatest environmental problems in the Mediterranean area, and one of the leading causes of desertification. After forest fires, which often lead to ecological and economic catastrophes, burnt areas must be effectively restored. Disturbances caused by fire in forest ecosystems have immediate effects on soils by eliminating the organic cover, affecting the stability of soil aggregates, altering the microbial community and changing the structural conditions as well as physical, chemical and biological soil properties (Larchevêque et al., 2006). These effects result in a loss of long-term fertility. Phosphorus is one of the most important elements limiting primary productivity. Phosphorus deficiencies are due to the poor solubility of P salts, its slow diffusion, and high P fixation, that cause a low P availability in soils even though the total soil P concentrations are usually high (Verma and Marschner, 2013). One of the most useful and studied alternatives for the recuperation of burnt soils in Mediterranean ecosystems is the application of composts from different sources (Cellier et al., 2012; Larchevêque et al., 2006; Malik et al., 2012). An increase in available nutrients, mainly in organic soil fractions, is caused by compost amendment, which enhances physical, chemical and biological soil properties (Larchevêque et al., 2006). When compost is added to soil, the effect on soil P availability does not only depend on chemical properties of the added compost (Verma and Marschner, 2013), but also on the particle-size distribution of the compost (Gómez-Muñoz et al., 2011) and on the subsequent P redistribution in the soil (Peters et al., 2011). Doublet et al. (2010) and Fangueiro et al. (2008) have studied the mutual dealings between particle-size distribution of composted organic matter and C and N dynamics. However, little is known about the redistribution of added P among the different P pools and particle sizes of soils. The Tiessen and Moir (1993) sequential fractionation method has been widely used for characterising the organic and inorganic forms of P differing in their availability for plants and microorganisms (Turrión et al., 2000a, 2000b, 2007). Fractionation procedures allow differentiating between inorganic and organic forms of P which participate in short and long-term transformations in the soil. These procedures also help to perform a complete assessment of different forms of P and to evaluate the availability of organic P for plants (Zamuner et al., 2008). Such information is essential to understand soil P dynamics and to determine whether redistribution between inorganic and organic fractions occurs when P is added to the soil. The current study deals with the use of municipal solid waste compost (MSWC) on recovering burnt forest soils. It was conceived in the context of a reclamation project of a burnt forest area and it also included a field experience of soil recovery using MSWC (Turrión et al., 2012). Besides, it comprises a double benefit for sustainability purposes: the contribution to the recovery of an ecosystem damaged by a forest fire, and the revalorization of waste materials. Our study aims to contribute to achieve efficient alternatives and solutions for soil recovering that could be used as a reference in future restoration projects. In order to determine which soil properties affect the compost dynamic in the soil, we compared a soil, burnt and unburnt soil, sampled 18 months after the fire, amended with the same compost. Moreover, it is also useful to determine the distribution of the added P taking into account the particle size of the soils. In this context, the main

purpose of this research was to determine how the addition of MSWC under laboratory conditions (Pérez-Lomas et al., 2010) affects soil P dynamics in burnt and unburnt soil samples. The main objectives of this research were to assess the effects of three doses of MSWC added to a burnt and unburnt soil on a) soil phosphorus forms, and b) the distribution of soil phosphorus in particle-size fractions.

2. Material and methods 2.1. Location The study area is located next to Burgos city in Northwest Spain (region of Castilla y León) at 897 m above sea level. Mean annual precipitation and mean annual temperature are 564 mm and 10.5 °C, respectively. Soils are developed over calcareous bedrock and they can be classified as Leptic Cambisols (eutric) by the IUSS Working Group WRB (2006). The area, called Monte de la Abadesa (42°19′14″ N, 3°41′ 11″ W), was forested during the 60s with Pinus sylvestris Mill. and Pinus pinaster Ait., and suffered the effect of a forest fire in October 2004. In the burnt area, consumption of litter layer could be observed, and no visible alteration of the mineral soil surface was found. Hence, the fire severity can be considered as moderate following the Pausas et al. (2003) classification. The fire only affected a part of the forested area. At the time of sampling, unburnt and burnt forests occurred in adjacent plots.

2.2. Soil sampling and material characterization Soil sampling was carried out 18 months after the fire. Laboratory incubation assay was performed with soil samples from the burnt and unburnt areas (0–5 cm depth). Composite samples from each area were obtained by mixing five subsamples. The soil samples were taken from burnt and adjacent unburnt plots which were 100 m away from each other. Visible plant litter and root residues were carefully removed and soil samples were sieved (b2 mm). The compost used for the incubation assay was a MSWC from the city of Burgos (Spain), where a non-selective collection of waste is made up. This is the reason for the high content of carbonates and relatively low percentage of Corg in MSWC. MSWC also presents a moderate content of heavy metals (in mg kg−1: Cd 1.83; Cr 57.0; Cu 187; Ni 29.8; Pb 121 and Zn 294). For soil and MSWC characterization, pH, electric conductivity, total N (N), total C, carbonate and organic C (Corg) were determined. Electric conductivity and pH were determined in a 1/2.5 soil/water suspension. Total concentrations of soil C and N were determined in an automated combustion analyser (CHN-2000, Leco). Carbonates were determined by titration (FAO, 2007). Organic carbon was calculated as the difference between total C and carbonate C. Table 1 shows some properties of the MSWC and soils used for the incubation assay. The increase in carbonate concentration observed in the burnt soil treatment in comparison with the unburnt one could be attributed to the carbonates formation due to fire (Bodí et al., 2014; Quintana et al., 2007). The Corg/N ratios of the studied soils are high, but they are in the range of the ratios showed by Cools et al. (2014) under pine species.

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Table 1 Properties of the soil and compost used in the incubation assay. pH

UNBURNT BURNT MSWC

7.2 7.4 8.0

EC

Carbonates

Corg

[dS m−1]

[g CaCO3 kg−1]

[g kg−1]

0.305 0.380 15.4

17 54 192

108 53.8 198

N

Corg/N

3.1 2.9 21

35 19 9.5

Note: MSWC: municipal solid waste compost; EC: electric conductivity; Corg: soil organic carbon; N: total nitrogen.

2.3. Incubation assay Three MSWC doses were added to burnt and unburnt soil samples: D1, D2 and D3 with 1, 2, and 4% dry weight, respectively, and equivalent to 25, 50 and 100 Mg ha−1(for 20 cm soil depth). Controls (soils with no addition of MSWC, D0) and compost samples (100% MSWC, no soil) were introduced as well. The doses of compost that were chosen can be considered as usual organic amendment doses for restoration purposes (Pedra et al., 2007; Barral et al., 2009; Turrión et al., 2012). Soil samples, MSWC, and amended soils were wetted to 70–75% of their water holding capacity and incubated in 1 L jars at 28–29 °C for

92 days (Llorente and Turrión, 2010). Five replications were carried out for each treatment. The moisture of each sample was weekly controlled and kept constant by weighing. 2.4. Particle size fractionation After incubation, particle size fractionation was carried out on MSWC, on burnt and unburnt soil samples and on burnt and unburnt soil with the highest dose of MSWC. Particle size fractionation consisted of a three-step procedure: 1) sonication to disrupt aggregates into single particles, 2) manual wet sieving to segregate particles coarser than 0.020 mm, and 3) centrifugation to separate silt from clay particles. Briefly, a 20 g soil sample was dispersed ultrasonically in 100 cm3 of water with a probe-type disintegrator (Branson Sonifier W450) using the 20 mm probe tip. The temperature of the suspension was kept below 30 °C by immersing the beaker in a water jacket. The sonicator was calibrated by verifying calorimetrically the real power output as described by Roscoe et al. (2000) and Oorts et al. (2005). Sand-size fraction (2–0.02 mm) was separated after applying an energy input of 300 J cm−3 by wet sieving. Silt-size particles (0.02–0.002 mm) and clay-size particles (b0.002 mm) were separated by centrifuging (Poppe et al., 1988). The silt fraction was then re-suspended in 200 mL deionized water and re-centrifuged at

Fig. 1. Diagram of the sequential phosphorus extraction procedure, modified from Tiessen and Moir (1993). AEM: anion exchange membrane; Pi: inorganic P; Porg: organic P.

M.-B. Turrión et al. / Science of the Total Environment 642 (2018) 374–382 Table 2 Interpretation of different forms of phosphorus following the sequential extraction procedure proposed by Tiessen and Moir (1993). P form

Extracts

Available inorganic P Highly labile organic P Moderately labile inorganic P

P_AEM + Pi Readily plant-available P _NaHCO3 Porg_NaHCO3 Labile organic P, easily mineralizable Pi_NaOH

Moderately labile Porg_NaOH organic P Primary inorganic P P_HCl1M Stable P P_HClconc + P_HClO4

Interpretation

Less plant-available P, associated with amorphous and some crystalline Al, Fe phosphates More stable form of organic P, involved in medium to long term transformations Ca-associated P P derived from non alkali extractable organic debris; highly recalcitrant P

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2.6. Statistical analyses Normality and homoscedasticity of data were verified with the Kolmogorov-Smirnov test and Levene test, respectively. The effect of compost amendments was tested through an Analysis of Variance (ANOVA) and Tukey HSD test (STATISTICA 7.0. Software package). The factors considered in the analysis were soil (with two levels: burnt and unburnt), compost dose (with four levels: control, dose 1, dose 2 and dose 3) and their interaction (soil ∗ dose). 3. Results 3.1. Phosphorus forms in municipal solid waste compost and studied soils

Note: P_AEM: available P, extracted with anion exchange membranes; P_NaHCO3: P extracted by 0.5 M NaHCO3; P_NaOH: P extracted by 0.1 M NaOH; P_HCl1M: P extracted by 1 M HCl; P_HClconc: P removed by concentrate HCl; P_HClO4: P extracted after digestion with concentrate HClO4 (230 °C) Pi: inorganic forms of P; Porg: organic forms of P.

26 ×g for 5 min; this procedure was repeated 5 times. The final suspension was poured in several bottles and centrifuged at 1920 ×g for 35 min to collect the clay. The sand fraction was dried at 40 °C, and the silt and clay fractions were freeze-dried prior to chemical analyses. 2.5. Soil phosphorus fractionation A sequential extraction following the Tiessen and Moir (1993) procedure was applied to incubated whole soils and their particulate size fractions to differentiate among different soil P pools (Fig. 1, Table 2). Briefly, 0.50 g of air-dried soil or fraction were placed into a 50 mL centrifuge tube and sequentially extracted with a) 30 mL distilled water and an anion-exchange membrane (AEM) strip (BDH n° 55164-U) of 10 cm2 surface (Turrión et al., 1997), b) 30 mL 0.5 M NaHCO3 (pH 8.5), c) 30 mL 0.1 M NaOH, d) 30 mL 1 M HCl, and e) 30 mL concentrated HCl (heating to 90 °C, during 15 min), to remove membrane extractable P, highly labile P, moderately labile P, primary P, and stable P, respectively. After shaking for 16 h, except for the extraction with concentrated HCl (only 1 h), samples were centrifuged at 9280 ×g for 10 min at 0 °C, and inorganic P (Pi) was determined in the supernatant. In the last step, soil residue was digested with concentrated HClO4 (70%) at 230 °C for 30 min to determine residual P. Total P (Ptotal) was also quantified in NaHCO3 and NaOH extracts, after oxidizing the organic P (Porg) with ammonium persulfate and H2SO4 in an autoclave at 120 °C (Greenberg, 1980). Then, Porg concentration was estimated as the difference between Ptotal and Pi in these fractions. The Pi concentration in each extract was determined according to the molybdenum-blue method by Murphy and Riley (1962). The pH of the extracts was adjusted to the range 5.4–6.6 with H2SO4 or NaOH and using p-nitrophenol as an indicator. Analytical determinations were conducted in triplicate and P concentration was expressed in mg P kg−1 in oven-dried soil bases.

Particle size fractionation of MSWC showed that the coarser fractions predominated (64% of mass was sand-size, Table 3). Total P (Ptotal) concentration in MSWC was around 2.6 g P kg−1. As can be seen in Table 3, total N, Ptotal and Porg concentrations were significantly higher in the clay fraction than in the coarser fractions. Corg/N, Corg/Porg, and N/Porg ratios were significantly lower in the clay fraction than in the others but no significant differences were found between sand and silt fractions. Table 4 shows the concentrations of P forms in MSWC and its particle size fractions. A remarkably low P_AEM (0.1%) to total P ratio and a very high proportion of primary inorganic P (36%) have been observed. Extractable organic P fraction (Porg_NaHCO3 + Porg_NaOH) in MSWC represented around 31% of total P. The clay-sized particles in MSWC showed significantly higher concentrations of most of the studied P forms (Table 4). Only primary inorganic P and P_AEM showed significant lower concentration in the clay particle size than in the others (Table 4). In the sand particle size, primary inorganic P was the most abundant P form (44.5% of total P) followed by the sum of highly and moderately labile organic P (29.6%), however in the clay particle size, these fractions represented a 19.7%, and near to 45% of total P, respectively. No significant differences were observed in total P between the burnt and unburnt soils (Table 5). However, the distribution of P forms was significantly different (Table 5). The most remarkable change observed in P fractions in soil eighteen months after the fire, was a significant decrease of the percentages in the most labile P-fraction extracted with anion exchange membranes (5.1% in the unburnt soil and 2.6% in burnt soil). Regarding the percentage of the extractable organic fractions (Porg_NaHCO3 + Porg_NaOH) a significant decrease was observed: from 42.7% in the unburnt soil to 25.0% in the burnt one. In contrast, a significant increase was found in the most stable fractions, mainly those recalcitrant inorganic forms (stable P), after fire. In both soils, N, Ptotal and Porg concentrations were significantly higher in clay fraction than in the other fractions (Table 6). Corg concentration was significantly higher in sand fraction than in silt and clay fractions for both soils. Significant decreases in Corg/N, Corg/Ptotal, Corg/Porg y N/Ptotal ratios were observed with decreasing particle size, following the sequence sandNsiltNclay. The N to Porg ratio did not show significant differences between sand and silt particle-size fractions. If we compare the

Table 3 Mass percentage distribution of particle-size fractions in the municipal solid waste compost (MSWC), organic C (Corg), total N (N), total P (Ptotal) and organic P (Porg) concentrations, and their ratios in the whole sample and in its particle-size fractions.

MSWC MSWC_Sand-size MSWC_Silt-size MSWC_Clay-size ANOVA

Mass

Corg

[%]

[g kg−1]

63.4 28.4 7.7

198 169b 255a 241a (50.935)***

N

Ptotal

Porg

Corg/N

Corg/Porg

N/Porg

20.8 15.5c 25.3b 34.6a (425.18)***

2.62 2.20c 3.15b 3.98a (51.087)***

0.814 0.72c 1.04b 1.72a (64.082)***

9.5 11.0a 10.1a 7.0b (15.742)***

243 235a 246a 142b (22.520)***

26 22ab 24a 20b (8.576)*

Note:*: p b 0.05; ***: p b 0.001: F values in brackets; Different letters in the same column indicate significant differences among particle-size fractions using the Tukey HSD test at level p b 0.05. Sand-size fraction (2–0.02 mm); Silt-size fraction (0.02–0.002 mm) and Clay-size fraction (b0.002 mm).

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Table 4 Concentrations of P forms [mg P kg−1 of MSWC or particle-size fraction] in municipal solid waste compost (MSWC) and in its particle-size fractions. The percentage of each P fraction in relation to the total P is shown in brackets.

MSWC MSWC_Sand-size MSWC_Silt-size MSWC_Clay-size

P_AEM

Pi_NaHCO3

Porg_NaHCO3

Pi_NaOH

Porg_NaOH

P_HCl1M

Stable-P

1.4 (0.1) 3.6b (0.2A) 5.3a (0.2A) 1.3c (0.0B)

168 (6.4) 142c (6.5B) 445b (14.1A) 628a (15.8A)

419 (16.0) 338b (15.4B) 390b (12.4B) 886a (22.3A)

289 (11.0) 207b (9.4B) 422a (13.4A) 351a (8.8B)

395 (15.1) 312c (14.2B) 650b (20.7A) 899a (22.6A)

942 (36.0) 978a (44.5A) 903ab (28.7B) 785b (19.7C)

402 (15.4) 216c (9.8B) 332b (10.5A) 428a (10.8A)

Note: Stable-P: P_HClO4 + P_HClconc. Different letters in the same column indicate significant differences for each P-form concentration or percentage (lowercase and uppercase, respectively) between particle-size fractions using the Tukey HSD test at level p b 0.05. Sand-size fraction (2–0.02 mm); Silt-size fraction (0.02–0.002 mm) and Clay-size fraction (b0.002 mm).

Corg and N concentrations in the different fractions between burnt and unburnt samples (Table 6), we can see that the unburnt soil had significantly higher concentrations than the burnt one. However, for Ptotal and Porg concentrations this only occurs in the clay fraction. As can be seen in Table 7, the chemical fractionation carried out in each particle-size fraction, showed significantly higher concentrations of all P-forms in clay fractions than in sand and silt fractions, except for primary inorganic P, with higher concentrations in the silt fraction. In the burnt soil, concentrations of most P forms (available and moderately labile inorganic P, highly and moderately labile organic P, and stable-P) followed the sequence: clay N silt N sand (Table 7). In the unburnt soil, only the less extractable fractions showed significant differences between silt and sand fractions. In both soils, the primary inorganic P concentration followed the sequence silt N clay N sand (clay = sand in the unburnt soil). 3.2. Effect of MSWC addition to burnt and unburnt soils Concentrations of P forms in unburnt and burnt soils with different doses of MSWC added are shown in Table 8. In the burnt soil, the dose effect was significant for Ptotal and for all P forms except for the most recalcitrant P (P_HClO4) (Table 8), with the highest dose (4% of MSWC addition) showing significantly higher concentrations than the control and dose 1 (1% of MSWC addition). According to the fact that the significant effect of MSWC addition was shown by the higher dose in all cases, increments in concentrations of the different P fractions have been calculated only for the highest dose in both soils (burnt and unburnt) regarding to their control soils and the quantity of compost added, in order to prove additive effects. Thus, expected increment values can be found only if the added MWC does not interact with the soils. Table 9 shows the P enrichment after the highest dose of MSWC addition for each P-form, calculated as mg P-form kg−1 soil (obtained by chemical extraction), divided by the expected mg P-form kg−1 of soil if an additive effect would have occurred with the compost addition. Enrichment factors observed in the total P values were near 1 in both soils (Table 9), which suggests a good recovery of the added P throughout the extraction process performed. Only for P_AEM, the enrichment factors were significantly different between burnt and unburnt soils, with higher values for the burnt soil. As no significant differences were observed for the pattern followed by the burnt

and unburnt soils after the addition of MSWC for the other enrichment factors, mean values for the enrichment factor were calculated in each chemical fraction. The comparison among enrichment factors for each fraction showed a different behaviour between Pi (highly labile Pi and primary Pi) and organic P fractions. The available inorganic P fraction and the primary inorganic P suffered significantly higher increments in their concentrations after adding MSWC than the expected additive effect (Table 9). Moreover, highly and moderately labile organic fractions showed enrichment factors smaller than 1, hence they suffered a significantly lower increase than expected with the additive effect alone. No significant differences were observed in the most stable fractions in comparison to enrichment of Ptotal. Concentration of Ptotal in the sand and silt particle-size soil fractions in both soils (burnt and unburnt) showed a significant increase after MSWC addition (Fig. 2). However, no significant effect was observed in the clay fraction. Fig. 3 shows the concentration of P forms in particle soil fractions in unburnt and burnt soils with and without MSWC addition (Dose 3 and Dose 0). In both soils, primary inorganic P was the fraction with a higher increase in concentration in the sand and silt fractions with MSWC addition. 4. Discussion 4.1. Phosphorus forms in MSWC and its particle-size distribution Total P in MSWC was in the range of values obtained by Frossard et al. (2002) in Swiss MSWC, and by Mkhabela and Warman (2005) in Nova Scotia MSWC, but ten times lower than Ptotal in MSWC from Indian cities (Rawat et al., 2013). The P contribution of the MSWC added was as primary inorganic P (36%) and moderately labile P (26.1%). Phosphorus in the MSWC was mainly inorganic P (69%). Only a minor proportion of the Ptotal was in organic forms (31%). These results are in agreement with those reported by Eghball (2003) and Frossard et al. (2002) and confirm that most of the P content in the studied MSWC was in inorganic forms. A high concentration of highly labile inorganic and organic P forms was found (588 mg P kg−1). However, less than a third (168 mg P kg−1) was inorganic labile P. Values of inorganic labile P above 200 mg P kg−1 can be considered excessive (Moody and

Table 5 Total P concentrations [mg P kg−1 soil], percentages of each fraction respect to total P in burnt and unburnt soils and ANOVA results for the comparison between burnt and unburnt soils values. Soil

UNBURNT BURNT ANOVA

Ptotal

P_AEM

[mg P kg−1 soil]

[% respect to total]

249 257 (1.409)NS

5.1a 2.6b (59.554)***

Pi_NaHCO3

Porg_NaHCO3

Pi_NaOH

Porg_NaOH

P_HCl1M

Stable-P

4.9 5.3 (0.136)NS

10.2a 6.1b (12.499)**

5.7 7.1 (1.539)NS

32.5a 18.9b (98.003)***

10.6 12.5 (4.561)NS

30.9b 47.6a (146.341)***

Note: Stable-P: P_HClO4 + P_HClconc; **: p b 0.01; ***: p b 0.001; NS: not significant: F values in brackets. Different letters in the same column indicate significant differences between soils using the Tukey HSD test at level p b 0.05.

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Table 6 Elemental composition of particle-size fractions in burnt and unburnt soils. Soil

Unburnt

Burnt

Particle-size

Corg

Fraction

[g kg−1]

Sand Silt Clay Sand Silt Clay

141aA 70bA 64bA 65aB 43bB 34bB

N

Ptotal

Porg

Corg/N

Corg/Ptotal

Corg/Porg

N/Ptotal

N/Porg

4.3bA 3.8bA 6.0aA 2.8bB 2.9bB 3.7aB

0.15cA 0.22bA 0.53aA 0.11cA 0.23bA 0.46aB

0.06bA 0.05bA 0.21aA 0.02bA 0.03bA 0.12aB

32.5aA 18.6bA 10.7cA 23.3aB 14.6bB 9.2 cB

974aA 319bA 121cA 589aB 185bB 75cB

2512aA 1406bA 312cA 2725aA 1468bA 288cA

30aA 17bA 11cA 25aA 13bB 8.0cB

78aB 75aA 29bA 117aA 102aA 31bA

Note: Different lowercase letters in the same column indicate significant differences between particle-size fractions for each soil (unburnt or burnt), different uppercase letters in the same column indicate significant differences between burnt and unburnt soils for each particle-size fraction. Tukey HSD test at level p b 0.05.

Table 7 P contents [mg P kg−1 soil] in different particle size fractions in burnt and unburnt soils. Soil

Unburnt

Burnt

Particle-size

P_AEM

Fraction

[mg P kg−1 soil]

Sand Silt Clay Sand Silt Clay

10.9bA 6.8b 19.3aA 5.9B 5.9 6.7B

Pi_NaHCO3

Porg_NaHCO3

Pi_NaOH

Porg_NaOH

P_HCl1M

Stable-P

13.6bB 11.1bB 33.9aA 10.6cA 12.6bA 24.8aB

8.3bA 11.3b 47.0aA 0.9cB 9.9b 32.5aB

8.3b 13.2b 50.4aA 9.1c 12.9b 31.3aB

48.9bA 40.3bA 160.1aA 0.5cB 20.3bB 87.3aB

17.4b 34.2aB 17.2bB 19.1c 44.6aA 25.6bA

37.4c 103.7b 262.1a 42.0c 124.4b 251.6a

Note: Different lowercase letters in the same column indicate significant differences between particle-size fractions for each soil (unburnt or burnt), different uppercase letters in the same column indicate significant differences between burnt and unburnt soils for each particle-size fraction. Tukey HSD test at level p b 0.05.

Table 8 Concentrations of P fractions [mg P kg−1 soil] in incubated burnt and unburnt soils with and without MSWC addition and ANOVA results. Soil

Dose

P_AEM

Pi_NaHCO3

Porg_NaHCO3

Pi_NaOH

Porg_NaOH

P_HCl1M

P_HClconc

P_HClO4

Ptotal

12.4c 17.1b 21.2b 29.3a (36.094) *** 13.7c 14.1c 22.9b 30.3a (20.427) ***

25.6b 23.5b 26.5b 33.6a (3.4826) * 15.7b 21.8b 26.7ab 31.2a (6.510) **

14.2c 21.8bc 26.5b 34.0a (14.153) *** 18.2c 17.0c 23.0b 30.3a (13.534) ***

80.8 81.8 84.2 78.1 (0.780) NS 48.5b 65.2a 61.5a 66.3a (9.563) ***

26.4c 24.8c 52.0b 82.2a (55.268) *** 32.4d 44.2c 74.8b 86.1a (48.888) ***

42.7c 51.0b 54.7ab 59.4a (9.770) *** 67.0b 69.6b 84.0a 80.6a (14.077) ***

34.3z 31.8 45.0 30.9 (2.8176)NS

249c 266c 324b 361a (74.522) *** 257c 284c 350b 384a (51.135)***

[mg P kg−1 soil] Unburnt

ANOVA Burnt

ANOVA

D0 D1 D2 D3 F-value

12.7 14.5 13.6 14.4 (1.082)NS

D0 D1 D2 D3 F-value

6.0b 8.2b 8.8b 10.7a (9.780) ***

55.3 44.3 48.8 48.3 (1.3268)NS

Note: D0: Dose 0 or control; D1: Dose 1 (1% weight of MSWC). D2: Dose 2 (2% weight of MSWC). D3: Dose 3 (4% weight of MSWC). All of them are referred to dry weight. *: p b 0.05; **: p b 0.01; ***: p b 0.001; NS: not significant. Different letters in the same column indicate significant differences for each soil among doses using the Tukey HSD test at level p b 0.05.

Bolland, 1999) and those above 500 mg kg−1 are extraordinarily high (Haynes et al., 2015). These high values could induce eutrophication problems if the MSWC dose added to soil is too high, but it is not the case of the MSWC used in the present study. A 25.4% of total Pi in the MSWC was in highly and moderately labile Pi forms. Studies carried out by Frossard et al. (2002) have also shown relatively low proportions of these labile fractions and high proportions of primary inorganic P. Our results showed that a large amount of Pi in MSWC was present as calcium phosphates which are neither extractable with AEMs, nor soluble

in bicarbonate, nor in sodium hydroxide. The compost from municipal solid waste used in this study showed a high proportion of P associated to Ca, which can be explained by the high concentration of CaCO3 in the MSWC and its high pH (Table 2). Other researches have also shown the presence of calcium phosphates in alkaline composts (Traore et al., 1999). Physical fractionation is a useful tool in studies of SOM stabilization by physical protection and organo-mineral interaction. It is also useful to understand the turnover of nutrient associated with SOM

Table 9 Enrichment factors for each P form after the addition of the higher compost dose (4% MSWC) for unburnt and burnt soils and their mean. Soil

EP_AEM

EPi_NaHCO3

EPorg_NaHCO3

EPi_NaOH

EPorg_NaOH

EP_HCl1M

EP_HClconc

EP_HClO4

EPtotal

Unburnt Burnt Mean

1.19b 1.42a 1.31**

1.60 1.63 1.61***

0.83 0.90 0.87***

1.37 1.09 1.23

0.83 0.93 0.88***

1.30 1.23 1.27***

1.08 1.02 1.05

1.02 1.01 1.01

1.05 1.07 1.06

Note: Enrichment factors were calculated as: mg P-form kg−1 soil obtained by chemical extraction divided by mg P-form kg−1 soil expected considering an additive effect with compost addition. Different letters in the same column indicate significant differences between burnt and unburnt soils using the Tukey HSD test at level p b 0.05. Significant differences between the enrichment factor of each P-fraction and the enrichment factor of total P are indicated by asterisks, **: p b 0.01 and ***: p b 0.001.

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Fig. 2. Concentrations of total P in particle-size soil fractions in unburnt and burnt soils without compost addition (Dose 0) and with 4% MSWC addition (Dose 3) at the end of 92 days of incubation. Different letters in the same particle-size fraction and soil indicate significant differences between doses using the Tukey HSD test at level p b 0.001.

(Guggenberger et al., 1994). Its interpretation is based on the fact that fractions associated with different size classes differ on their stability, dynamics, and function. Therefore, they can play different roles in nutrient turnover. However, the interpretation of the results of particle-size fractionation of MSWC should not be similar to soil fractions due to differences in mineralogical composition. Depletion of Corg, N and Porg in sand-size separates and enrichment of silt- and clay-size particles were observed in MSWC size fractionation. In general, low values of the Corg/N and Corg/Porg ratios were observed in this MSWC and in its particle-size fractions, indicating that the MSWC used in the present study was very stabilized and induced low mineralization as described by Cellier et al. (2012). Previous studies under laboratory conditions carried out with this MSWC by Turrión et al. (2012) showed that most of the OM added as compost was not easily mineralizable in the short and medium terms. No significant differences were obtained in Corg/N and Corg/Porg ratios between sand- and silt-size particles, showing a similar composition of these two particlesize fractions in this MSWC. Finer fractions of MSWC showed a high proportion of extractable Porg (N45% of Ptotal in clay-size was in organic forms) and higher Ptotal concentrations than coarser fractions. The knowledge and the evaluation of the distribution according to

particle-size fractions and the dynamics of P added in the MSWC to soil could be a useful tool to avoid eutrophication risks in water bodies. 4.2. Phosphorus forms in burnt and unburnt soils and in their particle-size fractions Total P concentration in MSWC was ten times higher than those in the studied soils. Total P concentrations were similar in burnt and unburnt soils, and these concentrations can be considered low compared to the concentrations obtained in the burnt and unburnt surface horizon in Dystric Cambisols under Pinus pinaster forests, as Turrión et al. (2010) have stated. In the unburnt soil, total extractable organic P represented around 43% of total P, decreasing in the burnt soil to 25%, probably due to the fire-induced mineralization of organic P. Also, significantly higher concentrations and proportions were found for the P_AEM fraction in the unburnt soil. However, our results showed significantly higher concentrations of inorganic stable P for the burnt soil, but no significant differences were found in moderately labile inorganic P. Fire can transform organic P forms, which are a relatively non-labile form, to labile inorganic forms which are soluble or scarcely adsorbed P forms (Giardina et al., 2000; Sharpley et al., 2000). Galang et al. (2010) obtained a

Fig. 3. Concentrations of P forms in particle soil fractions in the unburnt (UB) and the burnt (B) soils without MSWC addition (Dose 0, D0) and with MSWC addition (Dose 3, D3) after incubation.

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significant increase in the concentration of available inorganic P fraction one week after burning, likely due to the ash incorporation to the soil. These authors hypothesized that mineralization of the litter caused by the fire would increase labile soil P along with Porg transformation to Pi in the A horizon. In our case, the time lapse from the fire to the sampling was 18 months, enough time for the more labile inorganic P forms to evolve. In the studied soils, the clay fraction was enriched with Ptotal and extractable Porg two times compared to the whole soil samples. This finding is in accordance with those of other researchers (Agbenin and Tiessen, 1995; Turrión et al., 2000b). The ratio Corg/Porg significantly decreased with decreasing particle size, indicating a relative enrichment of Porg in the organic matter associated with finer particle-size fractions. A similar pattern was observed for Corg/N and N/Ptotal ratios which also decreased significantly as particle size decreased. The different proportions of microbial synthesized and plant-derived substances in the different particle-size fractions could explain the P and N enrichment in the SOM associated with fine fractions (Makarov et al., 2004). Carbohydrates in sand fractions were mainly of plant origin whereas microbial derived sugars accounted for a large proportion in the clay-size fractions (Glaser et al., 2000; Guggenberger et al., 1994; Makarov et al., 2004). 4.3. Effect of MSWC addition to soil on phosphorus forms The incorporation of MSWC to the studied soils originated an increase in P availability. The same result was found by other authors, as reviewed by Hargreaves et al. (2008), and it is a common result of organic amendments, as crop residues application (Reddy et al., 2005) and goat manure (Gighangi et al., 2009). Haynes and Mokolobate (2001) and Pypers et al. (2005) indicated that the increase in P availability when an organic amendment is added to soil could be due to the improvement of the soil characteristics such as the reduction in P adsorption strength and the incorporation of the highly available P compounds in the compost. Our results showed that the MSWC addition increased the soil P availability. Organic P contained in the MSWC was mineralized to orthophosphate anion, which is associated with readily plant-available P. This mineralized P mainly increased the available inorganic P pool and the primary inorganic P, which is a P reservoir available in the long term as it is considered Ca-associated P (Table 2). The high amounts of carbonates present in the compost and in these soils favour P fixation due to the reaction of CaCO3 with orthophosphate anions to form a precipitate (Fuentes et al., 2006). The increase observed in the Ca-linked fraction was similar in both studied soils. However, the increases observed in the labile fractions were higher in the burnt soil, where the concentration before compost addition was lower than in the unburnt soil. Adler and Sikora (2003) indicated that when a soil is amended with immature compost containing high amounts of easily degradable organic compounds such as organic acids and high microbial biomass, an increase in the more labile P fraction can be observed. This increment can be explained by the competition or organic acids for the adsorption sites decreasing the amount of P linked to Fe and Al surface and releasing available P forms. The compost used in the present study is a mature compost and this last effect was not observed. Ca-associated P in sand and silt fractions increased its concentration when MSWC was added to the soil. The compost addition to soil does not only result in an enrichment of the P source, but also increases soil microbial biomass leading to the P mineralization into inorganic forms. This could be explained as a result of the mineralization process through which added organic P changes into easily available inorganic forms and part can precipitate with Ca, which is abundant in limestone soils. This situation is in contrast with the one reported by Malik et al. (2012), as they observed that the compost addition involved an increase of organic P forms. These two different behaviours may be due to the different dynamics of P inside

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limestone and non-limestone soils. Initial immobilisation or mineralization of soil P after the addition of composts was found to be determined by the Corg/Ptotal ratio of the organic material added (Prasad, 2013). Immobilisation is likely to occur when Corg/Ptotal ratio is higher than 200 and the addition of compost with lower Corg/Ptotal values can cause mineralization of stable organic matter (Prasad, 2013). However, for mineralization to occur in soil, the organic amendment must contain at least 2 g of Ptogal kg−1 of soil. Otherwise, net immobilisation will occur (Iyamuremye and Dick, 1996). Compost used in the current study showed Corg/Ptotal rates around 75 and 2.6 g of Ptotal kg−1 of soil, which could improve mineralization rather than stabilization. Observed increments in the most stable fractions – moderately labile Pi and stable-P- were similar to the expected only if the additive effect was considered. In fact, it is confirmed that the mentioned fractions have slow dynamics and the incubation period considered does not affect them. The observed increase of Ptotal in the sand fraction after MSWC addition can be explained by the high proportion of sand particle size in MSWC (64%). 5. Conclusions The doses of MSWC added caused significant increments in most of the soil P fractions. The higher dose (100 Mg ha−1) ensured positive effects on plant-available P in the short and long terms. Phosphorated components of MSWC were basically constituted by P forms linked to Ca, and for this reason, the compost added to soil caused increments in the available reservoir for plants in the long term. MSWC addition originated an increment in mineralization of the organic P forms, resulting in lower concentrations of organic forms and higher concentrations of labile inorganic forms and Ca-linked forms than expected. These effects were mainly observed in the sand and silt fractions. Particle-size fractionation of soil, combined with chemical extraction procedure and laboratory assays have allowed an ecological interpretation of defined P pools related to short and long-term transformations in the soil. Acknowledgements The authors are grateful to Dr. Valentín Pando from the Department of Statistic and Operations Research (University of Valladolid) for his advice in statistical analysis, and to Carmen Blanco and Juan Carlos Arranz for their assistance in laboratory analysis. Funding This work was financially supported by the Spanish Ministry of Education and Science [reference: CGL2006-13505-C03-03/BOS]. References Adler, P.R., Sikora, L.J., 2003. Changes in soil phosphorus availability with poultry compost age. Commun. Soil Sci. Plant Anal. 34:81–95. https://doi.org/10.1081/CSS-120017417. Agbenin, J.O., Tiessen, H., 1995. Phosphorus forms in particle-size fractions of a toposequence from Northeast Brazil. Soil Sci. Soc. Am. J. 59:1687–1693. https://doi. org/10.2136/sssaj1995.03615995005900060026x. Barral, M.T., Paradelo, R., Moldes, A.B., Domínguez, M., Díaz-Fierros, F., 2009. Utilization of MSW compost for organic matter conservation in agricultural soils of NW Spain. Resour. Conserv. Recycl. 53:529–534. https://doi.org/10.1016/j.resconrec.2009.04.001. Bodí, M.B., Martin, D.A., Balfour, V.N., Santin, C., Doerr, S.H., Pereira, P., Cerda, A., Mataix Solera, J., 2014. Wildland fire ash: production composition, and eco-hydrogeomorphic effects. Earth-Sci. Rev. 130:103–127. https://doi.org/10.1016/j. earscirev.2014.07.005. Cellier, A., Francou, C., Houot, S., Ballini, C., Gauquelin, T., Baldy, V., 2012. Use of urban composts for the regeneration of a burnt Mediterranean soil: a laboratory approach. J. Environ. Manag. 95:S238–S244. https://doi.org/10.1016/j.jenvman.2010.10.042. Cools, N., Vesterdal, L., De Vos, B., Vanguelova, E., Hansen, K., 2014. Tree species is the major factor explaining C:N ratios in European forest soils. For. Ecol. Manag. 311: 3–16. https://doi.org/10.1016/j.foreco.2013.06.047. Doublet, J., Francou, C., Pétraud, J.P., Dignac, M.F., Poitrenaud, M., Houot, S., 2010. Distribution of C and N mineralization of a sludge compost within particle-size fractions. Bioresour. Technol. 101 (4):1254–1262. https://doi.org/10.1016/j.biortech.2009.09.037.

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