Carbon dynamics of soil organic matter in bulk soil and aggregate fraction during secondary succession in a Mediterranean environment
Highlights
► Volcanic soils of semi-arid vine old fields may be C sinks. ► Soil organic carbon increases linearly with abandonment age. ► Soil organic carbon increases the most in the micro-aggregate fraction. ► With increasing time since abandonment, micro-aggregate mass increases. ► New C replaces old C in all soil aggregate fractions, except for the smallest one.
Introduction
Because of human activities, the concentration of atmospheric CO2 is increasing rapidly while the long-term storage capacity of terrestrial and ocean ecosystems is declining (Canadell et al., 2007). Understanding the role played by soil in global C dynamics requires estimation of soil carbon (C) stocks. Because more C is stored in the soil than in vegetation or the atmosphere (Eswaran et al., 2000), changes in soil C content could greatly affect the atmosphere (Lal, 2004).
The effects of the conversion of native vegetation to cropland or pasture on C storage are well known (Del Galdo et al., 2003, Desjardins et al., 2004, Romkens et al., 1999). In contrast, less is known about the dynamics of soil organic carbon (SOC) after agricultural abandonment, and this is especially true for Mediterranean areas (Alberti et al., 2011, La Mantia et al., 2007). Soil carbon dynamics after the abandonment of cultivated land is connected to the development of the natural vegetation through secondary succession processes (Kosmas et al., 2000, Martinez-Fernandez et al., 1995, Van Rompaey et al., 2001). There is some evidence that abandonment of agricultural land and the subsequent regeneration of forests through secondary succession may return C storage to the pre-agricultural levels, although the rate of recovery depends on the time one considers and whether the land was previously used for crops or pasture (Guo and Gifford, 2002, Post and Kwon, 2000).
A major factor affecting the dynamics of SOC after the abandonment of an agricultural land is climate (Alberti et al., 2011). Jinbo et al. (2007) found that abandonment led to an increase in SOC in a favourable (medium rainfall and high temperatures that supported high primary productivity) climate but a decrease in SOC in an unfavourable climate. This finding, however, was not confirmed by other experimental evidence. For example, abandonment under climatic conditions that limit primary production (as in Mediterranean climate) caused an increase in the soil C stock (Alberti et al., 2011). Similarly, a lack of soil disturbance in a semi-arid environment resulted in an increase of C in the soil surface (Alvaro-Fuentes et al., 2009).
Furthermore, the increase of SOC content is determined by the incorporation of new organic matter in the coarse fraction and the reduction of mineralisation processes in the finest ones (Barbera et al., 2012, Ouédraogo et al., 2005). While during the early stages of secondary succession the increase of carbon stock is mainly due to anthropic disturbance reduction, in the older stages of succession it is caused by an increasing plant productivity, which is generally found along secondary succession in mesic Mediterranean conditions, and which has been confirmed also during old field succession on Pantelleria Island (La Mantia et al., 2007, La Mantia et al., 2008). It is not clear whether SOC content can continue to increase as a function of time since agricultural abandonment or whether there is some limit. Whether SOC accumulates or decreases greatly depends on how interactions between climate and soil type affect SOC mineralisation rates and/or accumulation in soil.
SOC mineralisation can be reduced within soil aggregates in comparison to bulk soil, and the formation and turnover of soil aggregates are affected by agricultural abandonment and other changes in land management (Schimel, 1995). There is still lack of knowledge on how management conditions and their abandonment affect aggregate formation and the protection of SOC. Generally, micro-aggregates (< 250 μm) (Tisdall and Oades, 1982) are relatively stable and are bound by persistent polysaccharide-based glues produced by roots and microbes and by calcium bridges. On the other hand, micro-aggregates are bound into macro-aggregates (> 250 μm) by a network of roots and hyphae. Therefore, macro-aggregate stability is thought to respond more rapidly to changes in land use. Several authors have reported that the aggregate stability and SOC content of stable macro-aggregates were higher in native grassland than in cultivated fields (Cambardella and Elliot, 1992, Six et al., 2002). Others reported a higher mean residence time of SOC for reduced-tilled soils (Collins et al., 1999, Six et al., 2002), and the increased residence time was attributed to an increased physical protection of soil aggregates in the absence of disturbance.
As noted, the physical protection of SOC provided by aggregates favours SOC accumulation whereas cultivation tends to break aggregates apart and therefore increases SOC mineralisation. The inclusion of SOC in soil aggregates and the mineralisation of SOC in broken aggregates are accompanied by changes in the chemical structure of SOC. These processes have been studied with the aid of δ13C analysis (Buzek et al., 2009, Desjardins et al., 1994, Desjardins et al., 2004, Desjardins et al., 2006, Wookey et al., 2002).
After agricultural abandonment, old fields are spontaneously colonised by various plants, a gradual process during which different plant communities develop (secondary succession). In general, annual and perennial herb communities dominate early and are then partially or completely replaced by perennial grasses, shrubs, and/or trees. If during succession there is a switch between C3 and C4 photosynthetic pathways, the 13C/12C ratio (δ13C) of inputs to SOC is modified. Using the time since perturbation and the shift in belowground δ13C of SOC relative to the aboveground δ13C of the plant community, researchers can estimate the SOC turnover rate (Wolf et al., 1994). SOC turnover rates have been estimated when C4-dominated natural grasslands were converted to C3 annual crops and perennial pastures (Balesdent et al., 1990, Follet et al., 1997), or when C3-dominated natural forests and grasslands were converted to C4 annual crops and perennial pastures (Balesdent et al., 1987, Gregorich et al., 1995, Jastrow et al., 1996), but no data are available for secondary successions characterised by a C3–C4–C3 pathway. When separated on the basis of chemical composition, aggregate size, or particle density, SOC aggregate fractions have been shown to differ in age and thus in turnover rate (Balesdent et al., 1990, Jastrow et al., 1996).
The present study analyses the change in soil carbon stock along a secondary succession after agricultural abandonment describing differences in SOC turnover rates along succession for all soil aggregate fractions. The study of soil carbon dynamics linked to spontaneous secondary succession processes has been identified as a priority in policy-oriented research since data on natural post-abandonment soil evolution is lacking (Zanchi et al., 2007). Furthermore, the present study evaluates SOC turnover using changes in the natural abundance of δ13C. This evaluation contributes to the knowledge on aggregate formation and the protection of SOC under different management systems, and provides data on SOC turnover along a C3–C4–C3 succession pathway.
Section snippets
Study and sampling area
The study was carried out in cultivated and abandoned terraced vineyards on Pantelleria Island (Italy), which is situated in the rift of the Sicilian Channel (83 km2 surface area; 36°44′ N, 11°57′ E) (Fig. 1a). Pantelleria is of volcanic origin, its highest summit is Montagna Grande (836 m a.s.l.), and its surface rocks are mainly acidic, silicic vulcanites (pantellerites and trachytes). The most frequent soils are Lithosols, Regosols (mainly escalic), and Cambisols (mainly vitric). The island is
Total C and N
SOC content (g kg− 1) increased linearly (R2 = 0.89 and 0.73 for 0–15 and 15–30 cm soil depth) with the age of abandonment; it increased from 12 g kg− 1 in the cultivated vineyard (terrace 1) to 26 g kg− 1 in the vineyard abandoned 60 years earlier (terrace 7) (Table 2, Fig. 2a). There was no interaction between age and depth, and SOC content was greater near the surface (0–15 cm) than deeper (15–30 cm) in the soil (Fig. 2a); after 60 years of abandonment, SOC content was 28 g kg− 1 at 0–15 cm and 26 g kg− 1 at 15–30
Discussion
In the semi-arid Mediterranean environment, where climate is one of the major driving forces for determining both rate of vegetation community turnover within secondary succession, and the period necessary to complete conversion (new steady state), knowledge of C stock and the resilience of the soil must be increased. The results of the current study indicate that, following abandonment of vineyards on volcanic soil in a semi-arid Mediterranean area, the soil acts as a C sink. This finding is
Conclusions
In this study, we evaluate the SOC incorporation in bulk soil and aggregate fraction after vineyard abandonment and consequent natural vegetation encroachment, using an approach based on natural differences in δ13C of plants with C3 and C4 photosynthesis. Our findings have some implications for the understanding of carbon turnover and organic matter stabilisation in semi-arid Mediterranean environments. The study of a C3-C4–C3 succession pathway has enabled us to put into evidence the quantity
Acknowledgements
This work was financially supported by the Italian government through the PRIN project “The impacts of secondary succession processes on carbon storage in soil and biomass and on biodiversity and the role of dispersal centers and vectors for recolonisation processes”. We are grateful to Bruce Jaffee for revising the English version of the manuscript.
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