Rend. Fis. Acc. Lincei DOI 10.1007/s12210-014-0376-z
COASTAL FOREST ECOSYSTEM NEAR ROME
Chemical characterization of soil organic profiles for assessing the European morphogenetic reference base of humus forms in Mediterranean environments Valentina Baratella • Marco Renzaglia Alessandra Trinchera
•
Received: 1 July 2014 / Accepted: 23 December 2014 Ó Accademia Nazionale dei Lincei 2015
Abstract In the Castelporziano Reserved Area (Tyrrhenian coast near Rome, Italy), the organic horizons of 14 forest sites were chemically characterized to assess the European reference base of humus classification in Mediterranean ecosystems and evaluate the impacts of typical ecological disturbances on humification processes. Soils were sampled in three diagnostic horizons OL (litter), OF (fulvic) and OH (humic), and the biological type of humus were investigated on a morpho-functional basis and assigned to the taxonomic levels Mull, Amphi and Moder. For each diagnostic horizon, the total organic carbon (TOC%), the C/N ratio, the extractable carbon (TEC%), the humic and fulvic acids carbon (CHA%, CFA%), the degree (DH%) and the index of humification were determined. All data were statistically analyzed (ANOVA, post hoc HSD Tukey’s). In the OF horizon of some OL/OF sites, the CHA values were found very high, indicating the presence of a not-detected OH horizon. Conversely, part of the sites sampled with a complete humus profile, did not show differences between OF and OH, which should thus be grouped into a single diagnostic horizon. The humification parameters showed a fairly good correlation with the humus forms, limited to the first taxonomic level and the OF horizon. Overall, several discrepancies were found between humus classification and the chemical fingerprint
This peer-reviewed article is a result of the multidisciplinary project coordinated by the ‘‘Accademia Nazionale delle Scienze detta dei XL’’, Rome, Italy, in the area of the Presidential Estate of Castelporziano near Rome. V. Baratella (&) M. Renzaglia A. Trinchera Consiglio per la Ricerca e la Sperimentazione in Agricoltura, Centro per lo Studio delle Relazioni tra Pianta e Suolo (CRA-RPS), via della Navicella 2-4, 00184 Rome, Italy e-mail:
[email protected];
[email protected]
of the humic profiles. In this regard, the use of screening analytical methods, i.e., the chemical characterization of OM, may provide a powerful support in modeling the morpho-functional references of humus forms on the peculiar features of Mediterranean forest environments. Keywords Humus form Morpho-functional classification system Organic matter Mediterranean ecosystem Humification parameters
1 Introduction The process of litter degradation, resynthesis and transformation of the organic matter (OM), constitute the main element of soil fertility (Stevenson 1994). The humified soil organic matter (SOM), called humus in chemical sense (Kumada 1988; Johnston et al. 2004), appears and segregates from mineral matter along soil profiles: the soil morphologists designate the humus forms on the basis of the different association of organic and mineral matter, in diagnostic O and A horizons (Ponge 2013). The humus form, content and characteristics result from a sequence of evolutionary processes affected by several external factors, e.g., the litter type (Page 1968, 1974; Bernier and Ponge 1994; Pinzari 1999; Eskelinen et al. 2009; McLaren and Turkington 2011), climate changes (De Deyn et al. 2008; Wall et al. 2008; Ponge et al. 2011, 2014), anthropogenic impacts (Scott et al. 1999; Strandberg et al. 2005; Tan and Lal 2005; Descheemaeker et al. 2009; Lagomarsino 2011). In this regard, previous studies on ecological indicators emphasize the key role of some soil parameters in the detection of ecological disturbances in forest systems, e.g., the C/N ratio, which seems to be strongly related to the content and features of the humic component of organic
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matter (De Nicola et al. 2007; Zanella et al. 2008, 2011; Testi et al. 2010). The humus type is considered indicator of ecosystem state and forest soil quality, especially of soil nutrient regime and, to a greater extent, of soil organic carbon status (Klinka et al. 1990; Wilson et al. 2001; Ponge and Chevalier 2006; Andreetta 2011; De Nicola et al. 2013). The study of humus forms and OM turnover of forest sites can provide important knowledge on the tendency of the specific ecosystem to maintain the C-sink in the long-term as humic pool, being the humification process functional in biological OM stabilization. Ecological systems that are in equilibrium show high capacity to ‘‘store’’ OM through humification processes, increasing over time the C-sink of the soil (conservative systems) (Pinzari et al. 1999). On the other hand, ecosystem disturbances result in a higher degradation of OM (C-source) and a poor capacity of humic re-synthesis (little preservatives systems). Among the few works focused on the humus of forest Mediterranean ecosystems (Brethes et al. 1995; Peltier 2001; Sadaka and Ponge 2003; Serra 2008; Andreetta 2011), the recent study of De Nicola et al. (2013) (Castelporziano Working Group ‘‘Vegetation’’) investigates the SOM of the Reserved Area of Castelporziano, one of the last pieces of natural Mediterranean lowland forests in Italy, not only through classical criteria, such as water and nutritional regimes (Ellenberg 1988; Ellenberg et al. 1992; Wilson et al. 2001; Sicuriello et al. 2014), but also performing for the first time the morphological and functional analysis of humus profiles, defined at European level (Zanella et al. 2011; Jabiol et al. 2013; IUSS Working Group WRB 2007). In the framework of the study of the WG Vegetation, the purpose of the present work (Castelporziano Working Group ‘‘Soil’’) was to investigate the relationship between forest humus forms and SOM, by means of a chemical analysis of the different fractions of organic matter along the organic horizons. Specifically, the OM humic and fulvic components are separated on the basis of their different acidity and molecular size: while humic acids are characterized by high molecular weights (MW [10,000 Da) and low acidity (pI 4.0–5.5), fulvic acids have typically molecular weight of 10,000 Da and isoelectric point greater than 3.0–4.5 (Ciavatta et al. 1990; Shinozuka et al. 2004; Trinchera et al. 2007). These different properties of the humic substances are closely related to their ability to withstand the mineralization processes, from the degradation of litter to the synthesis of fulvic acids first, and then to humic acids. In this regard, the chemical characterization of humus profiles gives clear indication not only on soil fertility, influenced by both natural and anthropogenic impacts, but also it may be considered a robust assessment method of the humus classification system, which relies on
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morphological features to infer rate and effectiveness of organic matter processing (Andreetta et al. 2011).
2 Materials and methods This research was conducted on n. 14 soil sites of the Castelporziano Reserved Area—Italy (xeric region, inferior Mediterranean thermotype). The study area, 4,800 ha approximately, 30 m.a.s.l., fits a Mediterranean climate with an average rainfall between 123.8 mm (October and November) and 12.2 mm (July). The average annual temperature ranges from 4 °C (January and February) to 30 °C (July and August). The soils are mainly sandy and alluvial, with both ancient and recent dune formations (Guidotti et al. 2010). The Reserve, as one of the last pieces of natural coastal environment, represents a key-hotspot of biodiversity, with the presence of about 900 plants species and 45 plant communities, ranging from mature holm oak forest to hydrophilous back-dune relict forest. The great number of plant associations is representative of—at least—three different biomes, coexisting at very small distance through several ecotones: Mediterranean evergreen forest, deciduous oak forest and deciduous Lauro-Carpinetum forest, residual nucleus of Laurisilva (Pignatti et al. 2001). The WG ‘‘Vegetation’’ (De Nicola et al. 2013) selected 14 forest sites as representative of the Reserve biomes, to be analysed in the present work (two sites per each woodland type): 1. 2. 3.
4. 5. 6. 7.
Turkey oak forests Echinopo-Quercetum frainetto, Erica arborea silvofacies on old dune, Ecotonal Turkey oak forests Echinopo-Quercetum frainetto, Erica arborea silvofacies, Semi-humid Turkey oak forest Echinopo-Quercetum frainetto, Carpinus orientalis silvofacies and Carpinus betulus silvofacies on old dune, Inter-dunal ash forests with Fraxinus oxycarpa, Holm oak forests Viburno-Quercetum ilicis on recent dunes, Laurel hornbeam forests, Cork oak forest Viburno-Quercetum ilicis suberetosum, Stipa bromoides xeric facies on old dune (Table 1).
Some of the sites have been undergoing long-term anthropogenic or natural disturbances, i.e., past agro-forestry practices (coppicing) and intensive wild boars rooting and wildlife grazing.The humus form of each soil was classified by the Working Group Vegetation (De Nicola et al. 2013), according to the European humus forms reference base, a system of classification based on morphogenetic descriptions and diagnostic horizons (Jabiol et al. 2013; Zanella et al. 2008, 2011). The organic horizons of each soil site were sampled in three diagnostic horizons—
Quercus ilex 80d, Erica arborea 20s, Phillyrea latifolia 20s, Arbutus unedo 10s
Ponte guidone (coppice)
Quercus suber 60d ? 10dd ? 5s, Quercus ilex 10dd, Pistacia lentiscus 40s, Cistus salvifolius 70s
Sughereta naturale grasceto
Fraxinus oxycarpa 40d ? 20dd, Alnus glutinosa 20d, Laurus nobilis 10dd, Ficus carica 5dd, Ulmus minor 5dd
Alneto
Quercus cerris 30d, Quercus frainetto 30d, Quercus ilex 30d ? 20dd, Fraxinus ornus 25dd ? 5s, Phillyrea latifolia 60s, Erica arborea 10s, Juniperus spp. 10s
Farneta 131/3
Quercus cerris 80d, Phillyrea latifolia 15s, Myrtus communis 10s
Quercus cerris 80d, Quercus frainetto 10d, Quercus ilex 30s, Phillyrea latifolia 30s, Myrtus communis 30s
Farneta 134/3
Strada per Valle Renaro
Ecotonal Turkey oak forest
Quercus cerris 70d, Quercus frainetto 30d, Erica arborea 70s, Phillyrea latifolia 10s, Arbutus unedo 5s
Farneta 134/2
Turkey oak forest, silvofacies Erica arborea
Fraxinus oxycarpa 60d, Ulmus minor 10dd
Frassineto als
Ash forest
Quercus suber 70d, Phillyrea latifolia 60d ? 25dd
Spagnoletta
Cork oak forest
Quercus ilex 80d ? 5dd
Lecceta
Holm oak forest
Association and woody species (coverage %)
Ruscus aculeatus 10, Smilax aspera 5
Juncus spp. 70, Rosa sempervirens 5, Ruscus aculeatus 5
Ruscus aculeatus 30
Carex flacca 40, Juncus depauper 10, Ruscus aculeatus 10
–
Carex spp. 30
Asphodelus spp. 20, Stipa bromoides 20, Oryzopsis miliacea 5
Stipa bromoides 50, Brachypodium sylvaticum 10
–
–
Herbaceous species (coverage [5 %)
dys/eumoder
MULL dysmull
Trees 30 % Understory 30 % (shrubs 90 %, grass 15 %)
Grass 80 %
MODER eu/dysmoder
Shrubs 20 %
eumesoamphi
Understory 45 % (shrubs 80 %, grass 40 %)
Trees 80 %
AMPHI
Trees 90 %
Grass 50 %
MODER Shrubs 80 %
oligomull
Understory 35 %
Trees 90 %
MULL
Trees 60 %
meso-eumull
OF
OL
OH
OF
OL
OH
OF
OL
OH
OF
OL
OF
OL
OF
OL
pachiamphi
Understory 20 % (shrubs 30 %, lower shrubs 89 %, grass 50 %)
MULL
OF
AMPHI
Trees 60 %
Trees 60 %
OL
hemimoder
Understory 10 % (grass 30 %)
OF
MODER
OL
OH
OF
OL
Understory 60 % (shrubs 30 %, grass 60%)
hemimoder
Understory 70 % (necromass 90 %)
OH
OF
OL
Diagnostic horizons
Trees 70%
MODER
eumesoamphi
Trees 80 %
AMPHI
Trees 90 %
Humus form
Understory 5 %
Structure (coverage %)
Table 1 Forest sites and classification of humus forms, data provided by WG Vegetation, geolocation available (De Nicola et al. 2013)
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The coverage of woody species refers to: d dominant specie, dd dominated specie, s shrub
OF eumull Understory 40 %
OL MULL Trees 70 % Carpinus betulus 70d, Laurus nobilis 35dd Valle Renaro3
Brachypodium sylvaticum 5
oligomull
OF
MULL
Understory 65 % (grass 10 %)
Brachypodium sylvaticum 10 Acer campestre 55d, Quercus frainetto 10d, Quercus ilex 30dd, Phillyrea latifolia 30dd, Laurus nobilis 5dd Valle Renaro1
Laurel-hornbeam forest
Quercus cerris 2d, Acer campestre 5dd, Carpinus orientalis 3dd
Trees 65 %
OL
OH
OF oligomull
OL MULL Trees 30 %
Understory 60 % (grass 15 %)
OH
Capocotta 2
–
lepto- eumacroamphi
OF
AMPHI Trees 75 %
Understory 30 % (shrubs 30 %, grass 5 %)
– Quercus cerris 50d, Quercus frainetto 20d, Quercus robur 5d, Carpinus orientalis 10dd, Carpinus betulus 20dd, Crataegus monogyna 20s, Phillyrea latifolia 10s Capocotta QdF (old coppice)
Semi-humid Turkey oak forest
Association and woody species (coverage %)
Table 1 continued
Herbaceous species (coverage [5 %)
Structure (coverage %)
Humus form
OL
Diagnostic horizons
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approximately, in correspondence with the US. Soil Taxonomy OL = Oi; OF = Oe; OH = Oa—basing on a visual estimation of the volume of their humic component: less than 10 % of the total volume for the litter horizon (OL), from 10 to 70 % for the fulvic horizon (OF), more than 70 % for the humic horizon, when present (OH). The biological type of the humus forms were classified on a morpho-functional basis, according to the thicknesses of OL, OF, OH and A horizons and the presence of macro-, meso- and microaggregates (Zanella et al. 2011), into the three first taxonomic levels mull, moder and amphi, up to the second taxonomic level (De Nicola et al. 2013; Zanella et al. 2011) (Table 1). The humus mull (subordinate levels eumull, oligomull, amphimull and dysmull) is characterized by a nutrient-rich organo-mineral horizon, so that plant growth is rapid, typically in multi-layered forests, and the site productivity is high. On the other hand, moders (eumoder and dysmoder) have a less rapid transformation of litter with accumulation of organic humus (stabilized OM). From the ecophysiological point of view, mulls have the higher functionality and are generally associated with early developmental stages of forest stands, while moders, with lower functionality, occur in phases of intense growth of trees up to maturity (Bernier and Ponge 1994; Salmon et al. 2006; Ponge et al. 2014). The humus form amphi, previously named amphimull, is a combination of moder (accumulated humus in organic horizons) and mull features (organo-mineral horizons), typically found in strongly seasonal climates, i.e., Mediterranean environments. On the basis of this groundwork, we performed a chemical investigation on the soil organic profiles in relation to the assigned humus forms and to the different vegetal coverage, with the aim to assess the morphofunctional classification system of the humus form, for the first time applied to Mediterranean environments. With regard to the soil quality, the level of biological stabilization of OM, thus the humus type, is considered indicator of ecosystem state in terms of C-sink preservation as humic pool: a higher degradation of OM (C-source) and a poor capacity of humic re-synthesis are typical features of not-stable systems or the result of ecological disturbances (Pinzari et al. 1999). The chemical characterization of the organic matter provides a ‘‘snapshot view’’ of the state of the soil, through parameters and indicators particularly suitable for detecting changes in the long-term, thus strongly related to the ecological functionality of the site. Many studies (Edmonds and Thomas 1995; Hobbie 1996; ˚ gren and Bosatta 1996) found that the Cortez et al. 1996; A proportions of extractable, acid-soluble, and acid insoluble C obtained from a conventional chemical fractionation could be used to assess the quality of forest litter, and its consequent relationship with decomposition. In particular,
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the chemical quali-quantitative determination of the humified fractions constitutes a set of fundamental diagnostic elements to investigate the evolution of organic matter along soil profiles and to identify the state of soil fertility in relation to natural or anthropogenic impacts on ˚ gren and Bosatta 1996; Trinchera humification process (A et al. 1998; Pinzari et al. 1999). The total organic C (TOC%), the C/N ratio, the extractable carbon in alkaline solution (TEC%), the humic (CHA%) and the fulvic acids carbon (CFA%) were determined for each diagnostic soil horizon in three replicates. Besides, the degree of humification (DH%), the index of humification (HI), the CHA/CFA ratio and the related indexes of humic acids formation were calculated. The total organic carbon content (Corg%) and the extractable organic C (TEC%) were obtained by applying the method of Springer and Klee (1954), the total N content by Kjeldahl digestion (Bremner and Mulvaney 1982). The characterization of the humic fractions of each diagnostic horizon was carried out by the extraction of organic C in alkaline environment (Ciavatta et al. 1990; Ciavatta and Govi 1993). The humic substances obtained by humic acid plus fulvic acid fractions were determined by extraction using a solution of NaOH 0.1 M and Na4P2O7 0.1 M incubated at 65 °C for 24 h (Stevenson 1994), the content in organic carbon of humic and fulvic acids (CHA and CFA%) was determined by mineralization (Springer and Klee 1954). The humic fraction was separated from fulvic fraction by solid chromatography using polyvinylpyrrolidone (PVP) resin according to the method described by Ciavatta et al. (1990). The humification parameters CHA/CFA ratio, the degree of humification DH % (Ciavatta et al. 1990), the humification index HI (Sequi et al. 1986) were calculated for
type with the lowest organic C content, whereas the more humid Turkey oak forests, laurel hornbeam forests and ash forests show the highest organic matter supply. This appears particularly when considering the OF horizon (Table 2). Even if it is confirmed a good correlation between forest type and Carbon input, higher amounts of organic C does not ensure per se a corresponding ability to stabilize the OM through humification, and consequently does not express the C-sink attitude of the soil. In fact, in ecotonal Turkey oak forests (Farneta 134/3 and Strada per Valle Renaro) and in semi-humid Turkey oak forest (Capocotta 2 and Capocotta QdF) the C content decreases noticeably from OL to OH, indicating a scarce ability to conserve the organic matter by means of biochemical/microbiological stabilization (i.e., humification) (Fig. 1). On the opposite, in ash forests (Alneto, oligomull and Frassineto als, mesoeumull) and in holm oak forests (Lecceta, eumesoamphi and Ponte Guidone, hemimoder), the content in organic C does not change in the deeper horizons (p [ 0.05). In the case of holm oak forests, this trend most probably results from a natural disturbance, particularly the impact of wild boars grubbing and rooting, which subvert and mix the natural succession of horizons along the humic profile. Differently, the organic C trend of ash forests, classified with an OL/OF profile, suggests a probable lack of identification of a distinctive OH horizon, as confirmed by their humification parameters (data following). 3.2 C/N ratio
3 Results
The highest C/N values were found in the oldest and most humid sites: laurel-honbeam forests, and Turkey oak forests with Carpinus spp. periodically flooded (Table 2). The humus forms of these forests are exclusively (laurel-hornbeam) or mostly (Turkey oak forests) mull. It must be considered that the decomposition of plant litter and organic materials can be hindered in anaerobic conditions, leading to much higher C/N ratios. Also ecotonal Turkey oak and cork oak forest types show high values of C/N ratio, especially in the sites Sughereta naturale (pachiamphi) and Strada per Valle Renaro (dysmull). On general terms, the C/N tends to decrease going from OL to OH horizons of the considered sites (the deepest horizons are the most humified ones). However, an opposite trend is noticed in Lecceta (holm oak forests, eumesoamphi), Ponte Guidone (holm oak forests, hemimoder) and in Farneta 131/3 (Turkey oak forest, Erica arborea silvofacies, eumesoamphi) (Fig. 2).
3.1 Organic carbon
3.3 Total extractable carbon, humic and fulvic acids
As expected, the soil organic C appears to be influenced by the different types of litter, being the holm oak forests the forest
The quantification of humic and fulvic acids and the determination of humification parameters along the organic
each site and horizon (Trinchera et al. 1998): DH% ¼ ðCHAþFA Þ TEC
HAþFA Þ 100 (Ciavatta et al. 1990), HI ¼ ðTECC ðCHAþFA Þ
(Sequi et al. 1986). The chemical parameters DH% and HI have been demonstrated to be effective in quantitatively evaluating the humic substances formation and the quality of OM and its level of stabilization (Sequi et al. 1986; Ciavatta et al. 1990; Ciavatta and Govi 1993; Roletto et al. 1985; Dell’Abate et al. 1998). All data were statistically analyzed by ANOVA analysis of variance with post hoc Tukey’s HSD test (SPSS software package, IBM Corp., Armonk, NY, USA).
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Rend. Fis. Acc. Lincei Table 2 Organic C (C org%), C/N ratio, extractable organic C (TEC%), humic acids C (CHA%), fulvic acids C (CFA%) and CHA/CFA Site name and humus form
Horizon
Corg (%)
r
C/N
TEC (%)
r
CHA (%)
r
CFA (%)
r
CHA/ CFA
DH (%)
r
HI
r
0.06
Holm oak forests Lecceta, eumesoamphi Ponte guidone, hemimoder
OL
9.1a
0.02
6.9
21.0
0.85
12.0
1.24
3.45
0.06
3.5
73.5a
3.21
0.36a
OF
9.1a
0.02
13.2
10.9
1.54
4.9
0.10
2.99
0.01
1.7
73.6a
11.42
0.38a
0.21
OH
7.6
0.02
14.9
6.2
0.05
2.6
0.08
1.88
0.04
1.4
72.6a
1.37
0.38a
0.03
OL
6.5a
0.02
7.1
18.1
0.10
13.8
0.11
1.82a
0.01
7.6
86.5
1.12
0.16
0.01
OF
3.8
0.01
4.2
12.3
0.01
9.6
0.28
1.83a
0.11
5.3
92.7
1.36
0.08
0.02
OH
6.5a
0.02
22.4
6.5
0.01
3.3
0.02
1.42
0.03
2.4
72.6
0.04
0.38
0.00
Cork oak forest Spagnoletta, hemimoder
OL
41.9
0.12
33.9
26.6
0.00
19.3
0.35
2.89
0.02
6.7
83.4
1.24
0.20
0.02
OF
25.5
0.09
30.7
15.0
0.00
11.6
0.03
2.64
0.04
4.4
94.9
0.51
0.05
0.01
Sughereta, pachiamphi
OL
39.1
0.07
61.1
24.8
0.01
17.7
0.13
3.69
0.03
4.8
86.2
0.67
0.16
0.01
OF
6.2
0.03
12.9
19.9
0.23
15.0
0.14
4.52
0.02
3.3
97.9
1.92
0.02
0.02
Frassineto als, meso-eumull
OL
37.9
0.12
23.2
28.1
0.35
18.4
0.35
3.80
0.01
4.9
79.0a
2.21
0.27a
0.04
OF
23.9
0.08
27.5
23.0
0.26
15.4
0.90
2.22
0.01
7.0
76.8a
3.08
0.30a
0.05
Alneto, oligomull
OL
40.1
0.10
18.3
28.0
0.31
21.3
0.26
2.44
0.02
8.8
84.9
0.09
0.18
0.00
OF
42.1
0.08
23.1
19.0
0.20
11.1
0.10
1.63
0.09
6.8
67.1
0.66
0.49
0.01
OL
50.7
0.14
25.1
28.2
1.66
15.9
1.11
7.70
0.04
2.1
83.8a
1.17
0.19a
0.02
OF
47.7
0.15
30.5
24.1a
1.33
13.5a
0.19
4.89
0.18
2.8
76.5a
5.76
0.31a
0.10
Ash forest
Turkey oak forest ericetosum Farneta 134/2, dys/eumoder Farneta 131/3, eumesoamphi
OH
38.7
0.09
17.8
23.6a
0.27
13.7a
0.07
4.33
0.04
3.2
76.4a
0.95
0.31a
0.02
OL OF
10.9 11.0
0.04 0.03
7.5 18.6
12.1 10.3
0.07 0.02
8.2 5.2
0.10 0.20
1.59 1.49a
0.03 0.03
5.2 3.5
81.0 64.5
0.60 2.04
0.23 0.55
0.01 0.05
OH
12.8
0.04
25.1
8.8
0.02
4.7
0.05
1.49a
0.00
3.2
70.9
0.41
0.41
0.01
OL
44.0
0.08
29.9
19.4
0.32
11.5
0.12
2.35
0.08
4.9
71.6
0.15
0.40
0.00
OF
16.5
0.04
28.5
11.5
0.11
7.5
0.02
1.92
0.02
3.9
82.5
1.14
0.21
0.02
Ecotonal Turkey oak forest Farneta 134/3, eu/dysmoder Strada per Valle Renaro, dysmull
OH
4.8
0.02
18.3
7.1
0.09
3.7
0.05
1.04
0.09
3.6
66.3
1.41
0.51
0.03
OL
43.1
0.13
58.7
23.6
0.15
15.1
0.05
4.05
0.00
3.7
81.0
0.71
0.23
0.01
OF
10.1
0.02
11.2
17.7
0.55
8.8
0.55
3.35
0.06
2.6
68.9
4.92
0.46
0.10
OL
48.9
0.14
28.6
21.0
0.02
16.0a
0.07
1.93
0.11
8.3
85.6a
0.92
0.17a
0.01
OF
17.0
0.03
32.6
20.1
0.04
15.8a
1.04
2.85
0.00
5.6
92.6a
4.94
0.08a
0.06
Semi-humid Turkey oak forest Capocotta QdF, lepto-eu macroamphi Capocotta 2, oligomull Laurel-hornbeam forest Valle Renaro1, oligomull Valle Renaro3, eumull
OH
4.6
0.01
12.1
4.2
0.05
1.9
0.07
1.12
0.02
1.7
72.0
2.17
0.39
0.04
OL
44.7
0.13
47.5
24.4
0.10
15.7
0.01
3.56
0.03
4.4
79.0
0.52
0.26
0.01
OF
16.9
0.03
33.1
16.6
0.11
10.2
0.04
3.80
0.06
2.7
84.6
0.69
0.18
0.01
OH
10.0
0.03
27.0
8.4
0.07
4.0
0.02
1.86
0.02
2.2
69.0
0.58
0.45
0.01
OL
44.7
0.09
70.9
12.1a
0.00
6.1
0.05
1.97
0.04
3.1
66.8
0.13
0.50
0.00
OF
32.7
0.08
37.9
12.2a
0.11
6.9
0.15
1.68
0.02
4.1
70.2
2.02
0.43
0.04
OL
38.0
0.11
46.9
15.5
0.02
8.6
0.17
1.91
0.01
4.5
67.7
1.08
0.48
0.02
OF
13.6
0.05
17.6
10.0
0.05
5.6
0.15
1.81
0.02
3.1
74.2
2.08
0.35
0.04
Data are reported as mean ± standard deviation r per each horizon Same letters represent not-significant differences across the horizons (ANOVA p [ 0.05, HSD Tukey’s)
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Fig. 1 Organic C content (TOC%) in plant associations showing different C-sink attitudes. Different letters represent significant differences across the horizons (ANOVA p \ 0.05, HSD Tukey’s)
hornbeam forest sites (humus type mull) show the lowest values of extractable C, CHA and CFA in the litter. It must be highlighted that, in the case of Farneta 134/2 (dys/eumoder), the values of TEC (*24 %), CHA (*13.6 %) and CFA (*4.6 %) in the OF and OH horizons are practically identical (p \ 0.05), and therefore chemically indistinguishable: this indicates the need to assimilate both OF and OH horizons to a single horizon, with a profile OL/OF/A. In addition, we note that the values of CHA found in the OF horizons of cork oak forest sites Sughereta (pachiamphi, CHA *15 %) and Spagnoletta (hemimoder, CHA *11.6 %), as well as in the ash forests Frassineto (meso-eumull, CHA/CFA *7.0 %) and Alneto (oligomull, CHA/CFA *6.8), to which has not been assigned a definite humic horizon OH, are very high: in these sites, the prevalence of humic acids in the OF horizon could lead to a reconsideration of the horizon with a distinction between OF and OH. 3.4 Humification parameters
Fig. 2 C/N ratio trends in holm oak forests (Lecceta and Ponte guidone) and Farneta 131/3 (Turkey oak forest, ericetosum). Different letters represent significant differences across the horizons (ANOVA p \ 0.05, HSD Tukey’s)
profile is applied to follow in detail the evolution of the humification processes and attest SOM quality. In the ‘‘OL’’ horizon, represented by the forest litter, there is interference in the analytical data because of humo-like substances, which may be erroneously regarded as already humified: humic-like substances (based on complexes of lignin) are the molecules which lead to the subsequent synthesis of the humic substances themselves. So, higher values of TEC, CHA and CFA% in the OL horizons can be explained by the presence of lignin and lignic-cellulosic components in the litter, with acidity values close to those of humic or fulvic acids, respectively. In general, in all sites, data indicate a good supply of humic-like substances in the litter, especially in site Farneta 134/2 (Turkey oak forest silvofacies E.arborea, dys/eumoder), in ash forests (humus form mull) and in cork oak forests Spagnoletta (hemimoder) and Sughereta naturale (pachiamphi) (Table 2). On the other hand, the Turkey oak forest ericetosum Farneta 131/3 (eumesoamphi), and both the laurel-
The humification parameters CHA/CFA ratio, the degree of humification DH%, i.e., the percentage of humic substances in the total extractable fraction (Ciavatta et al. 1990), and the humification index HI (Sequi et al. 1986) were calculated for each site and horizon (Table 3). The HI index, the ratio between not-humified and humified compounds, ranges from 0 to 1 and can be considered an index of soil humification activity (low values correspond to higher humification) as well as of availability of nothumified labile fractions. The degree of humification DH%, in other words the extractable humified OM, tends to increase as organic matter stabilization proceeds; is a parameter functional to highlight the evolutionary processes of a forest soil, very sensitive for evaluating the OF/ OH horizons interrelationship, where the humic matter evolution takes place (Wilson et al. 2001). The DH% is found lower in the horizons OF of Farneta 131/3 (Turkey oak forest silvofacies E. arborea, eumesoamphi), which undergo intensive wild boars rooting disturbance, Alneto (ash forest, oligomull), Strada per Valle Renaro (ecotonal Turkey oak forest, dysmull) (at around 64–69 %), compared to very high values in the same horizon of the cork forest sites Spagnoletta and Sughereta (hemimoder and pachiamphi, respectively), whose values are close to 95–98 % (Table 3). When the DH% value is close to 100 %, the extractable organic matter is quantitatively comparable to humic and fulvic acids. Inversely, in the ash forest Alneto and in the ecotonal Turkey oak forest Strada per Valle Renaro, both classified as mull humus form, the very low DH% values indicates a loss of organic
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profile, and thus a depletion of carbon stocks especially on long term. The DH% values for the OH horizons are actually much closer to each other, going from a low of 66.3 % in Farneta 134/3 (ecotonal Turkey oak forest, eu/dysmoder) to 76.4 % in Farneta 134/2 (Turkey oak forest silvofacies E. arborea, dys/eumoder). Also in the holm oak forest of Ponte Guidone (hemimoder), despite the low inputs in organic C in the upper horizons and the low contents of TEC%, CHA and CFA, it is found a good degree of humification, both in OF (92.8 %) and OH (72.6 %) horizon. The semi-humid Turkey oak forests Capocotta QdF (eumacroamphi) and Capocotta 2 (oligomull) show good DH% values in the OF horizon (92.7 and 84.6 %, respectively), but the former, an old coppice, undergoes a greater depletion of the pool of humic substances moving deeper along the humus profile (DHOH = 72 %). The same trends can be noticed in the case of Farneta 134/3 (eu/dysmoder). Overall, in the OF horizons, the DH% values show a fairly good correlation with the various forms of humus at the first taxonomic level, while neither correlation nor trends are found at the second level of classification. The humus forms mull, associated with early developmental stages of forest stands and characterized by more degraded organic matter, tend to lower values of DH% (76.6 %) compared to amphi and moder. Amphi humus forms, a combination of moder and mull morphological features, displays intermediate percentages of humic substances in the total extractable fraction (82.2 %), not significantly different from the other humus forms. Moder, the humus type of the mature forest stages, shows the higher DH% values (86.7) (Fig. 3). In the OH horizon though, it is not possible to detect any significant differences in the mean values of DH% among the classified humus forms, neither at the first nor at the second level of classification. Same results are obtained if analyzing the index of humification HI (Table 3). Reasonably, much more consistency between humification parameters and type of humus can be achieved, reconsidering the humus classification in light of the results obtained. For example, basing on the obtained results for the site Farneta 134/3, it can be assumed its wrong attribution to the humus form moder: the chemical features of the organic horizon suggest the mull form being more suitable. In confirmation of this, it should be noted that the correlation between the humification parameters (index and degree of humification) and the humus classification in the humic horizon OH, which were not found relevant, became statistically significant when analyzing Farneta 134/3 as a mull form site. Besides, it became possible notice also a
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significant trend (p \ 0.05) of the humification parameters, in relation to the second taxonomic level of the humus form: the index (HI) and the degree of humification (DH%, Fig. 4) start following the gradient of productivity of the humus forms.
4 Discussion and conclusions The humus forms were classified into the three higher taxonomic levels of mull, mostly associated with mesohygrophilous woodlands, moder, mainly present in the deciduous oak woodands, and amphi in the evergreen sclerophyllous woodlands (De Nicola et al. 2013). In forest soils, the humic matter evolution takes place within the organic OF and OH horizons. Not all soil profiles have the same organic horizons: in the humus form ‘‘mull’’ the horizon ‘‘OH’’, more specifically humic and therefore more stabilized, is typically not found and a deeper fulvic horizon ‘‘OF’’ evolves directly to the mineral horizon A/B. Basing on the European morphogenetic system of humus form classification, the OH horizon was found on 7 of the 14 Mediterranean forest sites analysed. However, the chemical characterization of the humus profile of cork oak forests (moder and amphi humus form) and ash forests (mull), classified as OL/OF sites, underlined a prevalence of humic acids in the OF horizons, indicating the need to reconsider the morpho-functional references of classification in consideration of the presence of a definite organic horizon OH for these sites, with a possible new attribution of the humus form. Inversely, in the case of the Turkey oak forest Farneta 134/2, the OF and OH horizons were chemically indistinguishable, thus assimilable to a single horizon OF. The degree of humification of these sites indicates that in Farneta 134/2 (dys-eumoder humus form) and in cork oak forests Spagnoletta and Sughereta (moder and amphi) the degradation of litter generates organic compounds that can be transformed effectively into humus. The differences in soil input—different forestry ecotypes and litters—and the different mode/rate of degradation, i.e., the microbial activity, affect in a synergistic way the humification processes (Hobbie 1996; Cortez et al. ˚ gren and Bosatta 1996). The reduced mineralizing 1996; A activity of these sites improves the preservation of the humified carbon pool, more resistant to microbial degradation (also confirmed by the high values of DH% in the OH horizon of Farneta 134/2). By comparing the pools of organic C, a loss of organic profile was evidenced in ecotonal and in semi-humid Turkey oak forests that mean a scarce ability to conserve the organic matter by means of biochemical/microbiological stabilization (i.e., humification) and a potential depletion of carbon stocks on long term. The humus forms
Rend. Fis. Acc. Lincei Table 3 Humification parameters of the ecological association analysed Holm oak forest
Cork forest
Ash forest
39.00ab ± 1.6
Turkey oak forest ericetosum
Ecotonal Turkey oak forest
Semi-humid Turkey oak forest
Laurelhornbeam forest
TOC% OL
7.81c ± 1.8
40.46ab ± 2.0
OF OH
6.45c ± 3.8 7.08b ± 0.8
15.87ab ± 13.7
33.01a ± 12.9
30.80b ± 28.1
43.52ab ± 0.6
46.76a ± 3.0
41.34ab ± 4.7
29.31ab ± 25.8 25.72a ± 18.3
13.33bc ± 4.5 4.78b ± 0.0
16.95ab ± 0.0 7.30b ± 3.8
23.16ab ± 13.5
C/N OL
7.06d ± 0.2
47.13ab ± 19.7
20.82ab ± 3.4
16.33d ± 12.4
44.05ab ± 20.0
38.04bc ± 13.4
58.92a ± 17.0
OF
8.65b ± 6.4
21.67ab ± 12.3
25.31ab ± 3.1
24.60a ± 8.3
19.88ab ± 12.2
32.91a ± 0.4
27.85a ± 14.3
OH
18.74a ± 5.4
21.45a ± 5.2
18.38a ± 0.1
19.57a ± 10.5
TEC% OL
19.56bc ± 2.0
25.75ab ± 1.3
28.08ab ± 0.1
20.18bc ± 11.4
21.53ab ± 3.0
22.74ab ± 2.5
13.86c ± 2.5
OF
11.65b ± 1.0
17.49ab ± 3.5
21.04ab ± 2.8
17.28ab ± 9.8
14.62ab ± 4.4
18.40a ± 2.5
11.17b ± 1.6
OH
6.42b ± 0.2
16.22a ± 10.5
7.18ab ± 0.1
6.36b ± 3.0
13.33cd ± 2.5
15.93bc ± 0.2
7.39e ± 1.8 6.30b ± 0.9
CHA% OL
12.93cd ± 1.3
18.55ab ± 1.1
19.90ab ± 2.1
12.10d ± 5.5
OF
7.30b ± 3.3
13.33a ± 2.4
13.32a ± 3.1
9.38ab ± 5.9
8.21b ± 0.9
13.06a ± 4.0
OH
3.01b ± 0.5
9.25a ± 6.3
3.72b ± 0.1
2.97b ± 1.5
4.64a ± 4.3 3.19ab ± 2.4
3.20ab ± 1.2 2.64ab ± 1.0
2.75ab ± 1.2 3.32ab ± 0.7
2.91a ± 2.0
1.04b ± 0.1
1.49ab ± 0.5
CFA% OL OF
2.63ab ± 1.1 2.41ab ± 0.8
OH
1.65ab ± 0.3
3.29ab ± 0.6 3.58a ± 1.3
3.12ab ± 1.0 1.93ab ± 0.4
1.94b ± 0.0 1.74b ± 0.1
DH% OL
80.03ab ± 9.2
84.87a ± 2.0
82.01ab ± 4.1
82.46ab ± 2.0
76.31b ± 6.7
82.37ab ± 4.7
67.28c ± 0.6
OF
83.22bc ± 13.5
96.43a ± 2.1
71.98c ± 6.9
70.53c ± 8.5
75.73bc ± 9.6
88.63ab ± 5.7
72.20c ± 2.8
OH
72.61a ± 0.0
73.70a ± 3.8
66.35b ± 1.4
70.55ab ± 2.1
HI OL
0.26bc ± 0.1
0.18c ± 0.0
0.22bc ± 0.1
0.21bc ± 0.0
0.32b ± 0.1
0.22bc ± 0.1
0.49a ± 0.0
OF
0.23abc ± 0.2
0.04c ± 0.0
0.40ab ± 0.1
0.43a ± 0.2
0.33ab ± 0.2
0.13c ± 0.1
0.39ab ± 0.1
OH
0.38b ± 0.0
0.36b ± 0.1
0.51a ± 0.0
0.42b ± 0.0
Data are reported as mean ± standard deviation r per each horizon Different letters represent significant differences of a same horizon across the forest types (ANOVA p \ 0.05, HSD Tukey’s)
of these sites (mull humus forms) are in accordance with the chemical characterization obtained, apart from Farneta 134/3 (ecotonal Turkey oak forests), classified as eu/dysmoder but presenting the typical features of mulls: a rapid transformation of litter with a fairly sustained mineralizing activity and an organo-mineral horizon riches in labile nutrients. The holm oak forests and Farneta 131/3 (Turkey oak forest, Erica arborea silvofacies, eumesoamphi) clearly showed the impact of wild boars rooting, which subvert and mix the natural succession of horizons along the humic profile, so that significant differences in organic C among the organic horizons were not found. Besides, data showed a significant reduction in the C/N values in the upper horizons, the lowest values of extractable C, humic and
Fig. 3 Degree of humification of forest sites (DH%) in relation to the humus form at the first level of classification. Different letters represent significant differences (ANOVA p \ 0.05, HSD Tukey’s)
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Rend. Fis. Acc. Lincei Fig. 4 Degree of humification (DH%) in relation to the humus form at the second level of classification, obtained analysing as ‘‘mull’’ the humus of the site Farneta 134/3. Different letters represent significant differences (ANOVA p \ 0.05, HSD Tukey’s)
fulvic acids and lower DH%, indicating the boosting of mineralization triggered by the animal disturbances. As exception, in the holm oak forest of Ponte Guidone it is observed a higher degree of humification, which increases along with the deepening of the profile. The understory of this forest site, silviculturally managed as coppice, is constituted by the 90 % of necromass, because of the lack of woodland management: shoots and suckers have crowded the stools and have died competing. In this specific case, the effect of past silvicultural practices (coppicing) that ultimately led to an increase in decaying organic material on the ground, probably overrides the impact of grazing/rooting disturbances at the greater depths. The relationship among forest coverage, quality of litter materials and decomposition depend on the different microbial associations living in the soil as well as on residue characteristics as N concentration and C/N ratios, lignin and/or polyphenol concentration (Hobbie 1996; Cortez et al. 1996). In the oldest and most humid sites laurel-honbeam forests, and semi-humid Turkey oak forests carpinetosum. periodically flooded, it were found higher C/N values. It must be considered that the decomposition of plant litter and organic materials can be hindered in anaerobic conditions, leading to much higher C/N ratios. The semihumid Turkey oak forests (Capocotta QdF e Capocotta 2) showed also good contents in humified compounds and a very good degree of humification, but with a substantial difference: the site Capocotta 2 (oligomull) undergoes a greater depletion of the pool of humic substances, moving deeper along the humus profile. The humus forms of laurel-hornbeam forests have been classified exclusively as mulls, and data substantially confirmed this attribution: these sites showed the lowest values of extractable C, humic and fulvic acids, indicating an attitude to organic matter mineralization and a reduced allocation into more stable and humified compounds, as typical of the humus form mull. The very high values of C/N ratio in the litter of the sites Sughereta naturale (pachiamphi) and Strada per Valle Renaro (dysmull), comforts the hypothesis of an high
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allocation of lignocellulosic material of the litter, prevalently characterized by aliphatic and aromatic chains deriving from lignin and lignocellulosic compounds, very poor in nitrogen. In Strada per Valle Renaro the C content, strongly decreasing from OL to OH, and the very low values of the humification parameters, indicate a scarce ability to conserve the organic matter by means of biochemical/microbiological stabilization (i.e., humification). In this regard, it must be highlighted that the C/N ratio, usually found strongly related to the content and features of the humic component of OM (De Nicola et al. 2007; Zanella et al. 2008, 2011; Testi et al. 2010), does not always work properly as indicator of ecological functionality for the Mediterranean forest sites analysed in the our work. Sclerophyllous woodlands or mesophylous forests with ingression of thermophilic species, i.e., cork oak forests and ecotonal Turkey oak forests with low functionality, may show C/N values comparable to those of high-functional forests as laurel-hornbeam, ash forests, semi-humid Turkey oak forests, especially in the upper horizons. The obtained results related to the DH% of OF and OH horizons show that the degree of humification is a parameter functional to highlight the evolutionary processes of forest organic profiles, more sensitive for evaluating the OF/OH horizons interrelationship. With regard to the correlation between the humic classification and the chemical fingerprint of the humus form, the DH% values show a fairly good correspondence with the investigated forms of humus, limited to the first taxonomic level and the OF horizon: mulls, being characterized by more degraded organic matter, gave lower DH% respect to amphi and moder forms, which showed not decomposed or only partially decomposed forest litter in the deepest organic horizon. The same results are obtained if analyzing the index of humification HI. Overall, the morpho-genetic system of humus classification analysed in the present work reported a fairly good correspondence with the chemical identity of the humic profiles, expressed by humification parameters. Nonetheless, several discrepancies have been found. This lack of consistency with the chemical features of the organic profile, which determines the ecological functionality of
Rend. Fis. Acc. Lincei
the humus form, clearly indicate the need to further model the morpho-functional references of the humus form on the peculiar features of the Mediterranean forest environments. In this regard, the use of screening analytical methods, such as the chemical characterization of humic profiles, may provide a powerful support in diagnosing the morphological and functional references for Mediterranean humus forms, being the humus proved to be correlated with soil physico-chemical variables and stand properties (Ponge and Chevalier 2006; Lalanne et al. 2008; Andreetta et al. 2011). Acknowledgments The authors thank the General Secretariat of the Republic Presidency, the management of the Estate of Castelporziano, Ing. Aleandro Tinelli, the Technical-Scientific Committee of Castelporziano and especially Prof. Ervedo Giordano and the Multidisciplinary Center for the Study of Mediterranean Coastal Ecosystems for allowing the course of these studies and the publication of data. This work is part of the collaboration between the WG ‘‘Vegetation’’, Department of Plant Biology of the University ‘‘La Sapienza’’ of Rome (Italy) and the ‘‘Centro di ricerca per lo studio delle Relazioni tra Pianta e Suolo’’ of ‘‘Consiglio per la Ricerca e la sperimentazione in Agricoltura’’ (WG ‘‘Soil’’).
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