The aim of this study was to describe the processes that control humic carbon sequestration in soil. Three experimental sites differing in terms of management system and climate were selected: (i) Abanilla-Spain, soil treated with municipal solid wastes in Mediterranean semiarid climate; (ii) Puch-Germany, soil under intensive tillage and conventional agriculture in continental climate; and (iii) Alberese-Italy, soil under organic and conventional agriculture in Mediterranean subarid climate. The chemical-structural and biochemical soil properties at the initial sampling time and one year later were evaluated. The soils under organic (Alberese, soil cultivated with
Soil systems are exposed to a variety of environmental stresses, of a natural and anthropogenic origin, which can potentially affect soil functioning. For this reason, there is growing recognition for the need to develop sensitive indicators of soil quality that reflect the effects of land management on soil and assist land managers in promoting long-term sustainability of terrestrial ecosystems [
The SOM consists of chemical components differing in biological degradability: (i) rapid and medium turnover fractions and (ii) more recalcitrant forms that turn over slowly. The former provide immediate and short-term sources of carbon substrate for the soil biota and contribute more to nutrient cycling. The latter, on the other hand, represent long-term reservoirs of energy that serve to sustain the system in the longer term and they improve soil structure.
In order to understand the temporal dynamics of SOM in managed systems, it is therefore vital to characterize soil organic carbon quantity and quality.
In particular, by providing nutrients and physical protection for enzymes and microorganisms, soil humic carbon content has widely been recognized as an important fraction of SOM that can be used to study soil quality in ecosystems influenced by agricultural practices or adverse climate conditions. Humic substances are able to bind extracellular enzymes (humic-enzyme complexes) and preserve them from proteolysis and chemical degradation. As suggested by other studies [
Humic-
Agricultural management systems affect organic carbon turnover and can modify the structural composition of SOM [
Characterization of SOM quality in soil can be obtained by using various analytical methodologies, such as infrared, ultraviolet-visible, nuclear magnetic resonance spectroscopy, oxidative reductive polymeric degradation, and gel column filtration. Several researchers have used pyrolysis-gas chromatography (Py-GC) as a reproducible and relatively rapid technique for studying qualitative changes in the structure of SOM under different agronomic uses [
Based on chemical composition, the following group of compounds can be identified: (i) aliphatics, fatty acids and sterols, (ii) carbohydrates, (iii) lignin, (iv) aromatic compounds and polycyclic aromatic hydrocarbons (PAHs), and (v) N-containing compounds.
On the other hand, other soil easily measurable descriptors can be used to study the processes related to the active labile carbon pool in soil. For instance, dehydrogenase activity, indicating the status of soil microbial activity, gives information on soil metabolism. This enzyme activity has been proposed as a valid indicator of soil quality under different agronomic practices and climatic conditions [
Also total
Due to the complex interactions and dynamics of these soil properties, many researchers have emphasised the need to develop indices of soil quality through a combination of variables which reflect a range of soil functions, such as humification and mineralization processes, metabolism, and nutrient cycling [
The aim of this study was to (i) describe properties and processes that control soil organic carbon accumulation and decrease turnover rate and (ii) illustrate the importance of conservation practices and management systems that reverse the trend to degradation and facilitate carbon sequestration in soil.
These objectives may be achieved by analysing chemical, chemicophysical, and biochemical properties in order to define the most important indicators that describe organic carbon dynamics in relation to the management practices adopted in the different pedoclimatic conditions.
The site is located in Abanilla (38°12′N, 01°02′W) in open scrubland not used for agricultural purposes. The climate is Mediterranean semiarid. The mean annual rainfall is 300 mm y−1 and the mean annual temperature is 18°C. The studied soil is poorly developed with an ochric epipedon as the diagnostic horizon and is classified as a Haplic Calcisol (World Reference Base classification). The Abanilla site has a sandy clay loam soil (USDA classification) and it is characterized by a TOC and TIC content of 0.5% and 9%, respectively, and a pH of 6.5.
In this site, six fields of 85 m2 each, three treated with the organic fraction of a municipal solid waste (S-WOF treatment) and three untreated fields (S-C, control), were set up. The waste organic fraction addition was made, 16 years before soil sampling, in such a dose as to increase the SOM by 1.5%. This fraction was incorporated into the top 15 cm of the soil using a rotovator. In the S-WOF, plant cover developed spontaneously (60–70% plant coverage), while very scant vegetation grew in the control soil (20–30% plant coverage). The vegetation of the area is the typical of Mediterranean semiarid lowlands:
The site is located in Alberese (42°40′N, 11°06′E), characterized by a Mediterranean semiarid climate. The soils were taken at two agricultural areas: an organic area (I-BA) and a conventional area (I-CA). Both areas had durum wheat (
The Alberese site has a sandy clay loam soil (USDA classification) and it is characterized by a TOC and TIC content of 0.15% and 2.1%, respectively and a pH of 7.8. The soil is an Chromic Cambisol (World Reference Base classification). The main vegetation of the area is
The fields in Puch are located about 40 km north-west of Munich (48°10′N, 11°13′E). In this site, plant cover has been intentionally modified during the last 50 years in a long term experiment. Three plots under intensive tillage (P-IT) have been kept without plants since 1953 by ploughing twice a year and by repeated grubbing; these soils are not fertilized and are ploughed whenever vegetation appears. As a result, there is no input from plants and the SOM is constantly exposed to aeration. Three plots under conventional agriculture (P-CA) were cultivated with wheat (
The monitoring of each soil ecosystem consisted in samplings carried out once a year. In this paper, the results of the initial sampling (T1) and one year later (T2) are reported. The T1 and T2 sampling were done at the same time for the different experimental sites, even if the treatments started in different periods for the different sites.
Each soil sample was a composite of nine bulk soil subsamples randomly collected from the top layer (15 cm; 150 cm3 soil cores) of an homogenous area. Three composite soil samples per each replicate treatment were taken, air-dried, sieved (<2 mm), and stored at room temperature prior determining chemical, physical, and biochemical properties.
Total organic carbon (TOC) and the total inorganic carbon (TIC) contents were measured with a LECO, U.S.A. RC-412 Multiphase Carbon/Hydrogen/Moisture Determinator. Total Nitrogen (TN) content was determined by a LECO, U.S.A. FP-528 Protein/Nitrogen Determinator. Water Soluble Carbon (WSC) was extracted using the method reported by [
Total (TG) and extracellular (EG)
The Py-GC is based on a rapid decomposition of organic matter under a controlled high flash of temperature, in an inert atmosphere of gaseous N2 carrier. A gas chromatograph is used for the separation and quantification of pyrolytic fragments. Fifty micrograms of an air-dried and ground (<100 mesh) soil sample and 300
Pyrolysis was carried out at 800°C for 10 s, with a heating rate of 10°C ms−1 (nominal conditions). The probe was coupled directly to a Carlo Erba 6000 gas chromatograph with a flame ionization detector (FID). Chromatographic conditions were as follows: a 3 m × 6 mm, 80–100 mesh, SA 1422 (Supelco Inc.) poropak Q packed column; the temperature program was 60°C, increasing to 240°C by 8°C min−1. Pyrograms were interpreted by quantification of seven peaks corresponding to the major volatile pyrolytic fragments [
Some ratios between relative abundances of some of the peaks were determined [
(ii) B/E3: humification index. The higher the ratio, the higher the humification of organic matter, because benzene is derived mostly from pyrolytic degradation of condensed aromatic structures, while toluene comes from aromatic uncondensed rings with aliphatic chains [
The Statistica 7.0 software (StatSoft Inc., Tulsa, Oklahoma, USA) was used for the statistical analysis. All results are the means of three field replicates. Differences among treatments within each site were tested by analysis of variance (one way ANOVA). The means were compared by using least significant differences calculated at
Finally, a correlation matrix of the data was also calculated in order to determine the relationship between the indicators. The significant levels reported (
The chemical, physical (Table
Chemical and physical properties at T1 and T2 sampling times. For each site, different letters indicate statistically different values among the treatments (
TOC | THC | AHC | TN | WSC | Porosity | ||
---|---|---|---|---|---|---|---|
g kg−1 | g kg−1 | g kg−1 | g kg−1 | g kg−1 | mm3 g−1 | ||
T1 | |||||||
Abanilla-Spain | S-C | 5.1b | 0.81b | 0.58b | 0.72b | 82a | 192b |
S-WOF | 27.6a | 2.82a | 1.96a | 3.28a | 225b | 203a | |
Alberese-Italy | I-CA | 12.6b | 1.35b | 0.89b | 1.14b | 63b | 131b |
I-BA | 18.7a | 1.93a | 1.35a | 1.38a | 70a | 147a | |
Puch-Germany | P-CA | 20.0a | 3.09a | 1.59a | 1.79a | 128a | 204a |
P-IT | 10.3b | 2.05c | 1.13c | 0.80c | 98b | 161b | |
P-C | 11.5b | 2.45b | 1.34b | 0.98b | 96b | 160b | |
|
|||||||
T2 | |||||||
Abanilla-Spain | S-C | 5.9b | 0.91b | 0.52b | 0.75b | 76b | 198b |
S-WOF | 24.0a | 2.63a | 1.49a | 2.52a | 154a | 217a | |
Alberese-Italy | I-CA | 13.1b | 1.38a | 0.69b | 0.81b | 46b | 142b |
I-BA | 18.2a | 1.41a | 0.95a | 1.64a | 53a | 154a | |
Puch-Germany | P-CA | 17.3a | 2.56a | 1.58a | 1.53a | 75a | 210a |
P-IT | 8.0c | 2.19c | 1.08c | 0.57c | 65b | 174b | |
P-C | 11.4b | 2.40b | 1.51a | 1.18b | 58c | 167b |
TOC, total organic carbon; THC, total humic carbon; AHC, active humic carbon; TN, total nitrogen; WSC, water soluble carbon.
C, control; WOF, waste organic fraction added; CA, conventional agriculture; BA, organic agriculture; IT, intensive tillage. T1, initial sampling time; T2, one year later.
Biochemical properties at T1 and T2 sampling times. For each site, different letters indicate statistically different values among the treatments (
EG | TG | DH-ase | ||
---|---|---|---|---|
mg PNP kg−1 h−1 | mg PNP kg−1 h−1 | mg INTF kg−1 h−1 | ||
T1 | ||||
Abanilla-Spain | S-C | 3.3b | 117a | 1.08b |
S-WOF | 44.1a | 405b | 3.21a | |
Alberese-Italy | I-CA | 6.7a | 511b | 2.10b |
I-BA | 6.6a | 883a | 2.52a | |
Puch-Germany | P-CA | 16.1b | 95b | 4.67a |
P-IT | 4.4c | 82c | 1.38c | |
P-C | 44.7a | 182a | 2.26b | |
|
||||
T2 | ||||
Abanilla-Spain | S-C | 3.6b | 75b | 1.78b |
S-WOF | 31.3a | 438a | 5.06a | |
Alberese-Italy | I-CA | 6.3a | 639b | 2.98b |
I-BA | 6.4a | 823a | 3.58a | |
Puch-Germany | P-CA | 15.8b | 145b | 4.31a |
P-IT | 4.3c | 50c | 1.22c | |
P-C | 40.4a | 190a | 2.86b |
EG, extracellular
C, control; WOF, waste organic fraction addition; CA, conventional agriculture; BA, organic agriculture; IT, intensive tillage. T1, initial sampling time; T2, one year later.
Total organic carbon (TOC) was closely related to soil type and management systems. There were, in fact, significant differences (
The dehydrogenase activity, especially when referred to the energetic and immediately available C substrate, gives an idea of the metabolic potentiality of soil rehabilitation. This metabolic potential, calculated as the ratio between the activity of the viable microbial community (dehydrogenase activity) and the sources of energy for microorganisms (water soluble carbon concentration), was higher in the S-WOF with respect to the control soil.
Moreover, the higher total humic carbon (THC) and enzymatically-active humic carbon (AHC) observed in the S-WOF with respect to the control soil indicated the positive impact of organic matter addition on the maintenance of the stable carbon pool. The higher AHC also suggested the higher capacity of this stable humic fraction >104 molecular weight to preserve the extracellular enzymes in an active form, as confirmed by the significantly (
Significant differences in chemical and biochemical indicators related to the carbon cycle were also observed between organic (I-BA) and conventional (I-CA) agricultural soils in the Alberese site. The organic management stimulated soil metabolic potential, expressed by the ratio between the dehydrogenase activity and water soluble carbon [
The Puch soils showed a decrease in the amount of TOC in the intensively tilled soil (P-IT), with respect to the control (P-C) and conventionally cropped (P-CA) soils. In particular, P-IT showed a lower content of active humic carbon fraction (AHC) and specific extracellular
The carbon turnover may be assessed also through the chemicostructural composition of SOM as determined by the pyrolytic technique. The ratio between benzene (B) and toluene (E3) pyrolytic fragments, which has been considered as a “humification index,” and the ratio between furfural (N) and pyrrole (O), which has been interpreted as a “mineralization index” [
Pyrolytic indices of mineralization (N/O) and humification (B/E3). For each site, different letters indicate statistically different values among the treatments (
Whole soil | AHC extract | ||||
---|---|---|---|---|---|
N/O | B/E3 | N/O | B/E3 | ||
Abanilla-Spain | S-C | 1.49a | 0.547b | 1.21a | 0.581b |
S-WOF | 1.23b | 0.827a | 1.16a | 0.732a | |
Alberese-Italy | I-CA | 0.85b | 0.645b | 0.93b | 0.796b |
I-BA | 0.96a | 0.738a | 1.13a | 0.977a | |
Puch-Germany | P-CA | 1.39a | 0.930a | 1.28a | 0973a |
P-IT | 1.04c | 0.768b | 1.13b | 0.910b | |
P-C | 1.25b | 0.765b | 1.31a | 0.911b |
N/O, furfural/pyrrole; B/E3, benzene/toluene; AHC, active humic carbon.
C, control; WOF, waste organic fraction addition; CA, conventional agriculture; BA, organic agriculture; IT, intensive tillage. Data reported as mean values of T1 (initial sampling time) and T2 (one year later), coefficient of variation of the two sampling times ranging from 2 to 10%.
In the Abanilla site, B/E3 resulted higher in the organically treated soil (S-WOF) than in the control soil (S-C), suggesting the activation of humification by organic amendment; root exudates, as previously described for WSC, seem to be responsible for the low value of the N/O index found in the whole soil of the S-WOF treatment (Table
Also in the Alberese site, the increase of B/E3 in the organic with respect to the conventional agriculture system confirmed the prevalence of the humification process over mineralization. According to the humification index, the furfural/pyrrole (N/O) ratio showed higher values in the I-BA-treated soil, thus indicating the presence of more evolved (less mineralizable) humic matter in both the whole soil and soil extract (Table
In the Puch soils, significative differences were found (
By considering the three management systems which were expected to negatively affect soil properties, that is, Abanilla control soil S-C, Alberese conventional agriculture I-CA, and Puch control soils P-C or P-IT, one can, on the basis of all the indicators measured, rank the soils in a decreasing order of degradation: Abanilla ≫ Alberese > Puch. In addition, being the climate one of the most important factors affecting SOM turnover, the soil degradation reflected the geographical distribution of the three selected sites, from driest to more humid places.
Therefore, Abanilla could be expected to show a slower metabolism than the other soils, which may be reflected in a different carbon turnover, and this was actually found. The management with ameliorating practices, instead, undoubtedly slows down, arrests, or even reverses soil degradation. In order to explain more clearly the factors (TOC, THC, AHC, TN, total and extracellular
Soil properties can be summarized in three independent PCs, which explained 83% of the total variance (Table
Principal components (PC) and component loadings related to physical, chemical, and biochemical properties determined in the different soils.
PC1 | PC2 | PC3 | |
---|---|---|---|
TOC |
|
0.518 | 0.355 |
THC |
|
−0.175 | 0.184 |
AHC |
|
0.007 | 0.281 |
TN | 0.585 | 0.496 | 0.611 |
EG |
|
−0.005 | 0.402 |
TG | 0.196 |
|
−0.111 |
WSC | 0.288 | −0.091 |
|
DH-ase |
|
0.419 | 0.261 |
B/E3s |
|
0.164 | −0.169 |
N/Os | 0.101 |
|
0.233 |
porosity | 0.057 | −0.331 |
|
Var. Sp. | 4.498 | 2.196 | 2.382 |
Prp. Tot. | 0.409 | 0.200 | 0.217 |
TOC: total organic carbon; THC: total humic carbon; AHC: active humic carbon; TN: total nitrogen; EG: extracellular
Although cause-effect relations are difficult to establish given the collinearity of variables, the positive loading of WSC and porosity on the third PC suggested a relationship between decomposable organic matter inputs and soil porosity improvement.
Figure
Biplot of factor scores and loadings at each sampling time (T1 and T2), in each treatment. TOC: total organic carbon; THC: total extractable carbon; AHC: extractable carbon fraction >10.000 Da; TN: total nitrogen; EG: extracellular
Similarly, the different Puch treatments were spread on the PC1. In particular, the P-IT treatment was shifted with respect to P-C and P-CA along negative values indicating the establishment of mineralization processes.
The adoption of organic (Alberese site, I-BA) and/or nonintensive management (Puch site, P-CA) practices in comparison with conventional agriculture (Alberese site, I-CA) or plough-based tillage methods (Puch site, P-IT) provoked a considerable stimulation of metabolic potential (dehydrogenase activity/water soluble carbon) and an increase of humic carbon and humic-associated enzymes.
In Abanilla site, the application of municipal solid wastes (S-WOF) stimulated the specific
The PCA analysis was able to assess the evolution of the carbon cycle and the shift of metabolic processes towards humification or mineralization pathways in the different soil ecosystems.
The AHC showed a positive dependence on TOC and microbial activity, indicating an active metabolism sustained by the decomposable SOM, which promoted the synthesis of persistent
The authors declare that there is no conflict of interests regarding the publication of this paper.
The study was carried out within the framework of the EU project “Indicators and thresholds for desertification, soil quality, and remediation” INDEX (STREP Contract no. 505450 2004/2006).