Combined effects of microenvironment and land use on C fluxes in a Mediterranean agro-silvopastoral system
Graphical abstract
Introduction
Agroforestry practices, by integrating perennial woody plants and agricultural crops and/or pastures as understory on the same land base (Ehret et al., 2015), contribute significantly to carbon sequestration, ecosystem services and biodiversity, compared to conventional agriculture (Kay et al., 2019). Mediterranean agroforestry systems are dominated by oak species, such as Iberian dehesa and montado, and by a mix of oak species on silvopastoral farms of Sardinia (Moreno et al., 2018) where the oak surface represent 90 % of Italian cork area. A mosaic of microenvironments characterizes the existing Mediterranean agro-silvopastoral systems as a result of different tree densities, land uses, reared livestock, long-term human activities, and their more recent changes driven by external inputs (Moreno et al., 2018). The most frequent structure is a two-layered wood pasture, combining scattered large trees with an understorey of native grasses. However, tree density may vary from few to more than 100 trees ha−1. Trees supply soil organic matter (SOM) as fallen dead leaves and through rhizodeposition (Grayston and Campbell, 1996), affect light interception and soil water regime, and minimize soil erosion by reducing runoff and thus improving water conservation (Young, 1989). Moreover, wood plants modify the microclimate by moderating extremes in daily photosynthetically active radiation and soil temperatures (Feldhake, 2001) and by reducing evapotranspiration (Belesky, 2005; Kyriazopoulos et al., 2013). Compared to open fields, the altered microclimatic factors in shaded environments such as moderated air and soil temperatures, higher humidity and more soil moisture, and most importantly, reduced light quantity and altered quality may induce changes in biomass yield and quality of understorey (Callaway, 2007; Pang et al., 2019a, b).
Understorey and open grasslands are typically dominated by annual species, whose duration of growing season is strictly dependent of the rainfall amount and its seasonal distribution. Annual species germinate after late summer rains, reach peak growth rates in spring and finish life cycle as seed at the start of subsequent dry season. More recently, the cyclic sowing of annual self-reseeding legume species, which are able to persist for several years after their establishment, and/or legume-grass mixtures has been recognized as a key agronomic intervention to increase grassland productivity and SOM of agro-silvopastoral systems (Sulas, 2005; Crespo, 2010; Teixeira et al., 2011; Kyriazopoulos et al., 2013; Moreno et al., 2018). However, that intervention represents a substantial change in land use, which might interfere with carbon cycling and affect carbon dioxide (CO2) fluxes on time.
Soil respiration (Rs) is an ecosystem process that releases CO2 from soil via root respiration, microbial decomposition of litter and SOM, and fauna respiration (Yiqi and Zhou, 2010; Hu et al., 2018). The Rs represents one of the largest C fluxes between the soil and atmosphere, by which soil organic carbon (SOC) is emitted from terrestrial ecosystems (Raich et al., 2002; Trumbore, 2006).
Global climate change caused by rising levels of CO2 and other greenhouse gases (GHGs) is recognized as a serious environmental issue, requiring the increase of the carbon storage capacity of terrestrial ecosystems (Kumar and Nair, 2011). Additionally, climate projections show that the arid climate type is likely to expand into Euro-Mediterranean areas during the 21st century. However, changes are already being noticed particularly in the Mediterranean basin, experiencing an increase in the interannual variability in precipitation and temperature, as well as in the occurrence of extreme climatic events (Aguilera et al., 2020).
The European Union (Council Regulation 1698/2005; EU, 2013) has promoted the establishment of silvopastoral systems not only as a sustainable land use with proven benefits (complementary fodder resources, greater diversity of productions, sustainable soil use, prevention of fire risk, etc.) but also as a tool for increasing soil carbon sequestration (Sharrow and Ismail, 2004; Nair et al., 2010; Mosquera-Losada et al., 2018). As a mitigation strategy, carbon sequestration has considerable benefits in the improvement of physical, chemical, and biological soil properties and it is essential for sustaining both crop productivity and food security (Aguilera et al., 2013). According to Francaviglia et al. (2017), Sardinian pastures and cork oak forests had the highest SOC contents, compared to other existing land use in the study area, due to plant material residues, return of ruminant faeces, organic input by root and the lower soil disturbance. Therefore, farming systems based on agroforestry can substantially increase the carbon stocks. Unfortunately, very few information is available regarding the dynamic and variation of CO2 soil effluxes at microenvironment level in Mediterranean agro-silvopastoral systems. The main distinctive traits and related ecological interactions of different microenvironments, coupled with the concurrent effects of climate change, need to be carefully considered. We hypothesized that, within the same system, the combination of contrasting microenvironment conditions and the conversion from native understorey to improved pasture might affect soil-atmosphere CO2 effluxes for a time longer than the conversion year. In the framework of FP7 Agforward project, the study was started in 2015 in Sardinia (Italy) to investigate seasonal and annual values of CO2 soil effluxes and to measure their variations due to land use change and contrasting microenvironments within an evergreen cork oak agro-silvopastoral system. Concurrently, the forage yields were monitored at each microenvironment and land use.
Section snippets
Experimental site, treatments description and management
Field experiments were carried out across four consecutive years, from 2015 to 2019, on a private farm (Buddusò municipality, 40°37’99’’N, 9°15’33’’E, elevation 700 m a.s.l.) located in North East Sardinia (Italy). The climate is Mediterranean with hot dry summer. Long-term rainfall is 840 mm and average annual temperature is 12.7 °C. Termopluviometric trends markedly differed across the four years of the experiment (Fig. 1). Total rainfall was 726 mm in 2015−2016 (from September 2015 to August
Termopluviometric trends and climate changes
The highly variable annual weather trends experienced across the four-year research were more indicative of an ongoing climate change process than of its forecast. In the four consecutive years, annual rainfall represented 86, 70, 107, and 85 % of the climatic value, respectively, indicating – 15 % of rainfall during the four-year study, quite in accordance with climate projections that forecast a 20 % decline in precipitation in the Mediterranean basin (Giorgi, 2006). Notably, the negligible
Discussion
Few papers based on CO2 efflux measurements highlighted the potential contribution in carbon sequestration from Mediterranean agro-silvopastoral systems (Shvaleva et al., 2014; Uribe et al., 2015). Conversely, our study reports detailed four-year field measurements of Rs and Rh rates carried out at single microenvironment scale, each one subjected to land use change, within the same Mediterranean agro-silvopastoral system dominated by a high-density cork oak forest.
Conclusions
The four-year field measurements allowed to characterize the substantial diversity in the amounts of photosynthetically active radiation and microclimatic traits between the two contiguous microenvironments belonging to the same cork oak agro-silvopastoral system.
Seasonal dynamics of soil respiration were significantly affected by termopluviometric trends, microenvironment and land use.
The specific combination of microenvironment/land use resulting from the occasional conversion of the native
Funding
This research was partially funded by the European Union FP7 Agforward project (ID 613520).
CRediT authorship contribution statement
Leonardo Sulas, Giovanni Antonio Re and Giovanna Piluzza: wrote the manuscript and analyzed the data. Giovanni Antonio Re, Sanna Federico and Giuseppe Campesi: performed the experiment, and collected the data. Leonardo Sulas, Paola Deligios, Luigi Ledda: revised the manuscript. Giovanni Antonio Re, Sanna Federico, and Leonardo Sulas: conceived and designed the experiments and analysed the data. All authors read and approved the final manuscript.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
This paper is dedicated to the memory of the owner of the private farm, Mr. Nino Taras, who has passed away recently. The excellent assistance in field and laboratory of technicians Mr. Piero Saba, Ms. Maddalena Sassu, Mr. Daniele Dettori, Mr. Anton Pietro Stangoni, and Mr. Salvatore Nieddu is gratefully acknowledged.
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