Litter decomposition rates of green manure as affected by soil erosion, transport and deposition processes, and the implications for the soil carbon balance of a rainfed olive grove under a dry Mediterranean climate
Introduction
Soils are the largest carbon (C) reservoir of the terrestrial C budget (Batjes and Sombroek, 1997). Therefore, even a relatively small increase or decrease in soil C content due to changes in land use or management practices may result in a significant net exchange of C between the soil reservoir and the atmosphere (Houghton, 2003). Under natural conditions, soil systems are considered to be in equilibrium, as losses through erosion and decomposition of litter and soil organic matter are compensated by inputs from aboveground and belowground net primary production on both short- and long-term scales (Giardina and Ryan, 2002). However, conversion from native to agricultural ecosystems has increased C losses to the atmosphere without enhancing C inputs into soil, and therefore the global C cycle has been drastically altered (DeFries et al., 1999, IPPC, 2007). These C losses, given by previous native vegetation and subsequent annual crop removal, tillage operations and enhancement of soil erosion rates (Lal, 2003, Quinton et al., 2010, Van Oost et al., 2005, Van Oost et al., 2012), may be compensated by sustainable land management (SLM) practices. In this regard, the incorporation of cover crops such as leguminous species (i.e., green manure) into the soil of a given cropping system is considered a promising sustainable management practice to reduce soil erosion risk (Almagro et al., 2013a, Alliaume et al., 2014, De Baets et al., 2011, Francia Martínez et al., 2006, Gómez et al., 2009) while compensating soil C losses derived from land-use change and tillage in agricultural fields (Gómez-Muñoz et al., 2014, Lal, 2013, Milgroom et al., 2007, Ramos et al., 2010). This is an important issue in Southern Spain, where subsidy policies have induced the expansion and intensification of olive and almond orchards in hilly environments while overlooking erosion-derived problems, and furthermore, an increase in the area suitable for olive production is expected with climate warming (García-Ruiz, 2010, Olesen and Bindi, 2002).
Soil erosion (detachment, transport and deposition) affects not only the distribution of sediments and associated C within the hillslope (Ritchie and McCarty, 2003, Nadeu et al., 2012, Zhang et al., 2006) but also litter decomposition dynamics through several abiotic and biotic mechanisms (Barnes et al., 2012, Berhe, 2012, Hewins et al., 2013, Lee et al., 2014, Throop and Archer, 2009). There are several mechanisms involved in litter decomposition during the process of soil erosion by water along hillslopes. During detachment and transport, soil deposition onto litter (or burial of litter) may not occur but collision with soil particles may be an important mechanism driving litter fragmentation, increasing the surface area available for microbial decomposers transported within the sediments (Barnes et al., 2012, Berhe, 2012, Throop and Archer, 2009). During deposition, besides physical fragmentation by soil abrasion, burial of litter may accelerate decay rates by promoting soil-litter mixing and improving soil moisture and temperature conditions for microbial decomposition (microclimate buffering) (Barnes et al., 2012, Throop and Archer, 2009, Whitford, 2002). Therefore, to understand erosional effects on litter decomposition dynamics the consideration of all three phases (soil erosion, transport and deposition) is necessary.
As a result of ongoing global climate change, and the increasingly acknowledged importance of the role of abiotic factors in comparison to biotic factors in litter decomposition dynamics in dry ecosystems (King et al., 2012, Throop and Archer, 2009), much effort is being devoted to improving our understanding of the abiotic factors driving litter decomposition dynamics and subsequent soil C storage. Although the importance of photodegradation as an abiotic mechanism in litter decomposition has been highlighted recently (Austin and Vivanco, 2006), accounting for the liberation of 1–4 g C m−2 yr−1 (Brandt et al., 2009), the role of soil erosion has been overlooked in most litter decomposition studies. Most of these studies have been conducted on flat landscapes and little is known about how litter decomposition rates vary along dynamic hillslope environments experiencing lateral soil distribution with soil erosion, despite the fact that these hilly environments account for more than 60% of the terrestrial Earth surface (Staub and Rosenzweig, 1992). Although recent studies have shown that soil erosion can be a significant driver of litter decomposition in arid, semiarid and Mediterranean ecosystems (Almagro and Martínez-Mena, 2012, Berhe, 2012, Hewins et al., 2013, Throop and Archer, 2007), the magnitude, direction and mechanisms (physical fragmentation or microbial decomposition facilitation) behind litter decomposition by soil water erosion remain largely unexplored. To the best of our knowledge, this is the first study to examine the roles of the different phases of soil erosion in litter decomposition rates (but see Berhe, 2012) and their implications for the soil organic carbon (SOC) balance along a dynamic hilly Mediterranean agroecosystem.
The main purpose of this study was to assess whether the process of soil erosion plays a significant role in the soil C balance of this dry Mediterranean agroecosystem, when its interaction with litter decomposition is considered. More specifically, the objectives were: (1) to understand the role of the different phases (erosion, transport and deposition) of soil erosion in the short-term litter decomposition of two contrasting species, common oat (Avena sativa L.) and vetch (Vicia sativa L.); (2) to identify the factors controlling litter decomposition rates during the different phases of the soil erosion process; and (3) to assess the contribution of green manure incorporation to the soil C balance of a rainfed olive grove under dry Mediterranean climate in Southeast Spain. We hypothesised that soil erosion is an important driver of litter decomposition and that, therefore, litter decomposition would increase along the hillslope driven by a complex interplay of different factors across landform positions. We also hypothesised that green manure incorporation would account for a significant C gain within the soil C balance of this rainfed olive grove, significantly improving its belowground C sequestration capacity.
For the purposes of our study, we define erosional effects on litter decomposition as the direct effect of physical fragmentation by soil abrasion during the detachment and transport of soil particles and any indirect effect derived from the breakdown and burial of litter (such as an increase in litter surface availability, soil-litter mixing and microclimate buffering) that facilitates microbial decomposition.
Section snippets
Study area
This study was conducted in an agricultural field situated in Cehegín in the Northwest of the province of Murcia, in Southeast Spain (38°3′N, 1°46′W). It was carried out in a 100-year-old organic rainfed olive grove without terraces, which was regularly ploughed along the contour lines and planted with a 10 × 10 m spacing (107 olive trees ha−1). The olive grove (170 m long by 70 m wide) is located on a convex–concave hillslope facing Southeast, with a mean slope of 10–12%, altitudes ranging from 600
Erosion/deposition patterns and soil characteristics along the hillslope
During the experiment (from 28th July to 15th December 2010), 23 events produced a total rainfall of 137 mm, of which only three rainfall events produced soil water erosion in the experimental area. Two rainfall events of 20 and 45 mm, respectively, and moderate- to high-intensity (maximum intensity in 30 min of 23 and 20, respectively), which occurred during the same week in August 2010, accounted for 99% of total soil loss in the experimental area, leading to the burial of most of the litterbags
Discussion
Despite the short-term nature of our experiment, our soil erosion rates were within the range of the annual erosion rates previously reported in longer-term studies conducted in similar dynamic hillslope Mediterranean environments (García-Ruiz, 2010, Taguas et al., 2010, Yoo et al., 2005). This highlights the important role of high-intensity rainfall events in the annual soil erosion rates in Mediterranean environments, where two or three rainfall events can be responsible for the main part of
Conclusions
The results of this study stress the importance of soil erosion in litter decomposition rates, and emphasise the need to incorporate the effect of the different phases of soil erosion in litter decomposition models in dry and disturbed ecosystems. This is a necessary step towards understanding the relative importance of soil erosion with respect to other controlling factors, as well as its implications for the SOC balance of hilly agricultural fields under Mediterranean climate conditions,
Acknowledgements
This research was supported with funds from the Murcia regional government (Project: SENECA 03027/PI/05; 08757/PI/08), the Spanish CICYT projects PROBASE (CGL2006-11619HID) and IMPASEC (AGL201125069) and the Spanish Ministerio de Medio Ambiente (RESEL project). We thank Javier Melgares, the owner of the experimental area, and the members of the Soil and Water Conservation Group (Eloísa García, Antonio Zamora and Inmaculada Montoya) who helped us in the laboratory and field work. The State
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