Exploring short-term leaf-litter decomposition dynamics in a Mediterranean ecosystem: dependence on litter type and site conditions María Almagro & María Martínez-Mena
Plant and Soil An International Journal on Plant-Soil Relationships ISSN 0032-079X Volume 358 Combined 1-2 Plant Soil (2012) 358:323-335 DOI 10.1007/s11104-012-1187-6
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Author's personal copy Plant Soil (2012) 358:323–335 DOI 10.1007/s11104-012-1187-6
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Exploring short-term leaf-litter decomposition dynamics in a Mediterranean ecosystem: dependence on litter type and site conditions María Almagro & María Martínez-Mena
Received: 13 May 2011 / Accepted: 15 February 2012 / Published online: 3 March 2012 # Springer Science+Business Media B.V. 2012
Abstract Aims Plant litter decomposition plays an important role in the storage of soil organic matter in terrestrial ecosystems. Conversion of native vegetation to agricultural lands and subsequent land abandonment can lead to shifts in canopy structure, and consequently influence decomposition dynamics by alterations in soil temperature and moisture conditions, solar radiation exposure, and soil erosion patterns. This study was conducted to assess which parameters were more closely related to short-term decomposition dynamics of two predominant Mediterranean leaf litter types. Methods Using the litterbag technique, we incubated leaf litter of Pinus halepensis and Rosmarinus officinalis in two Mediterranean land-uses with different degree of vegetation cover (open forest, abandoned agricultural field). Results Fresh local litter lost between 20 and 55% of its initial mass throughout the 20-month incubation period. Rosemary litter decomposed faster than pine litter, showing net N immobilization in the early stages of decomposition, in contrast to the net N release
Responsible Editor: Katja Klumpp. M. Almagro (*) : M. Martínez-Mena Departamento de Conservación de Suelos y Aguas, Centro de Edafología y Biología Aplicada del Segura- Consejo Superior de Investigaciones Científicas (CEBAS-CSIC), Campus Universitario de Espinardo, 30100 Murcia, Spain e-mail:
[email protected]
exhibited by pine litter. Parameters related to litter quality (N content or C:N) or land-use/site conditions (ash content, an index of soil deposition on litter) were found to explain the cross-site variability in mass loss rates for rosemary and Aleppo pine litter, respectively. Conclusions The results from this study suggest that decomposition drivers may differ depending on litter type in this Mediterranean ecosystem. While rosemary litter was degraded mainly by microbial activity, decomposition of pine litter was likely driven primarily by abiotic processes like soil erosion. Keywords Carbon cycle . Litter decomposition . Mediterranean ecosystem . N immobilization . Soil erosion . Vegetation structure
Introduction Plant litter decomposition is a key component in the terrestrial ecosystem carbon (C) and nitrogen (N) cycles, and it provides the primary source of nutrients for plants, and of both nutrients and energy for soil microorganisms (Currie 2003). As a result of ongoing global climate change, and the increasingly acknowledged importance of the roles of litter and soil organic matter as potential C sinks, much effort is being devoted to improving our understanding of the mechanisms driving litter decomposition dynamics and subsequent soil C storage.
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Understanding the factors that control short-term litter decomposition dynamics is particularly important in heterogeneous water-limited ecosystems, because of the small amount of soil organic matter and low availability of mineral nutrients (Carreira et al. 1996; Rashid and Ryan 2004) compared with other ecosystems. Additionally, the high mineralization rates (Martínez-Mena et al. 2002), and the episodic nature of precipitation events (Austin et al. 2004; Huxman et al. 2004) and associated soil water erosion processes (Martínez-Mena et al. 2001) might constrain the C sequestration potential of these ecosystems, making them highly susceptible to global change and prone to degradation. Extensive research has focused on predicting litter decay rates based on climate, litter quality and soil biotic interactions (Aerts 1997; Berg et al. 1993, 2010; Coûteaux et al. 1995; Kurz-Besson et al. 2006). However, models from mesic ecosystems are unsuited to water-limited ecosystems (Collins et al. 2008; Parton et al. 2007; Whitford et al. 1981). The differences observed regarding decomposition drivers between mesic and dry ecosystems may be partly explained by the patchy distribution of vegetation and resources in the latter. Unlike mesic ecosystems where vegetation cover is continuous and decomposition processes are predominantly biological (Moorhead and Sinsabaugh 2006), plant cover patterns in water-limited ecosystems may influence decomposition rates through physicochemical processes such as photodegradation (Austin and Vivanco 2006; Dirks et al. 2010), fragmentation by raindrop splash (Whitford 2002), or soil erosion and runoff (Throop and Archer 2007; Berhe 2011), even in periods in which microbial activity is constrained. The present study was undertaken as part of a project aimed at quantifying C pools and fluxes (outputs and inputs), and assessing the factors controlling the main C fluxes within different representative Mediterranean land-use types. In a previous study (Almagro et al. 2010), we estimated annual litter accumulation rates based on the annual litterfall inputs and decay rates of the predominant litter types within each land-use/site. Interestingly, we detected that the effect of land-use/site on decay rates differed between litter types. From those results, a new research question arose concerning the factors controlling the decomposition dynamics of two predominant Mediterranean leaf litter types (Pinus halepensis Miller and Rosmarinus officinalis L). More specifically, the objectives of the current study were: (1) to compare the mass loss rates and changes in nitrogen
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content of these two predominant litter types and (2) to assess whether the parameters most closely related to short-term decomposition dynamics differ between litter types. Recent studies in other water-limited ecosystems addressing the roles of abiotic and biotic factors in the litter decomposition process have highlighted that species with low-quality litter are prone to greater mass loss by abiotic mechanisms (Austin and Ballaré 2010; Brandt et al. 2007; Day et al. 2007; Martínez-Yrízar et al. 2007; Uselman et al. 2011). Aleppo pine litter has higher concentrations of lignin, suberins, resins, fats and waxes than rosemary litter (Berg et al. 2010; Boddi et al. 2002; Kaloustian et al. 2000; Traversa et al. 2011; Vokou and Liotiri 1999), thus making the former more chemically recalcitrant and potentially resistant to biodegradation (Minderman 1968; Rovira and Vallejo 2002). Thus, we hypothesized that decomposition of rosemary litter (more labile and nutrient-rich) would be more closely related to biotic factors such as microbial processes, while decomposition of Aleppo pine litter (more recalcitrant and nutrient-poor) would be predominantly linked to abiotic processes.
Materials and methods Study area The study area was located in Cehegín in the northwest of the province of Murcia in S.E. Spain. The climate is dry mesomediterranean with an average annual precipitation of 370 mm, which occurs mostly in autumn and spring. The mean annual temperature is relatively high, 15.5°C, and mean potential evapotranspiration (calculated by the Thornthwaite method; Thornthwaite 1948) is 800 mm yr−1, so the mean annual water deficit is 430 mm. July and August are the driest months. Two sites were selected to carry out the study: 1) a circa 150-yr-old mixed Aleppo pinekermes oak open forest, and 2) an abandoned agricultural field, which was cultivated with cereal for several years until abandonment in 1980, when establishment of typical Mediterranean vegetation started. Both sites are covered with a typical Mediterranean shrubland (Rosmarinus officinalis L., Quercus coccifera L., and Juniperus oxycedrus L.) with scattered Aleppo pines (Pinus halepensis Miller). Although both sites show the same dominant plant species, the open forest
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presents greater plant cover and aboveground biomass than the abandoned agricultural field site (see Table 1), thus resulting in a more developed vegetation structure in the former. The soils in the study area, with a loamy texture, derived from limestone colluvia (open forest) and Triasic marl colluvia (abandoned agricultural field), are classified as Petric calcisol (open forest) and Calcaric regosol (abandoned agricultural field), according to soil taxonomy (FAO 2006). The two sites are located on a glacis hillslope with a mean height of 640 m.a.s.l., and a mean slope of 10–12%, good drainage and a high percentage of surface stones.
Canopy structure characterization The above mentioned study sites were divided into the following two microsites: patches covered by Pinus halepensis, and open intershrub patches which were sparsely covered with Rosmarinus officinalis shrubs. Replicate (n04) plots (5×5 m) representative of each microsite were established within each land-use type, and every shrub and tree stem basal diameter was measured at the end of the growing season (2007) in order to estimate aboveground biomass from species-specific allometric relationships (Baeza et al. 2006, 2011; Barberá et al. unpublished data). Mean vegetation cover (%), stem density and litter layer thickness were also determined.
Precipitation, soil temperature and water content Precipitation depth, intensity and duration of each rainfall event were obtained from a recording rain gauge installed in the experimental area. Soil temperature at 2 cm depth was measured hourly using temperature probes connected to data-loggers (Hobbo, Onset Computer Corporation, Bourne, MA, USA). Each land-use type had at least three temperature probes. Soil volumetric water content (SWC) in the 0–10 cm depth interval was measured every 2 weeks at different locations within each land-use using a time domain reflectometry device (TDR).
Soil sampling and analysis Prior to litterbag deployment, soil within the above mentioned microsites was sampled. In December 2006 and April 2007, one soil core (10 cm diameter, 15 cm depth) was extracted within each microsite x land-use combination (n04). Each soil core was sealed in a plastic bag and stored in a freezer (4°C) until processed for microbial biomass estimates, and C and N analyses. Soil samples were air-dried, and sieved through a 2 mm sieve. The soil C and N content in the mineral soil
Table 1 Main characteristics of the experimental area. Numerical values are means (± standard errors) Open forest
Abandoned agricultural field
10
8
Site characteristics Species richness
R. officinalis
P. halepensis
R. officinalis
P. halepensis
Plant cover (%)
42
16
15
11
Relative abundance (%)
64
24
42
31
Standing biomass (g m−2)*
6862±4318a (n016)
735±103b (n08)
2.76±0.57a (n016)
1.9±0.57b (n08)
Stem density (stems m−2)* Mean tree stem diameter (cm)
*
Mean shrub stem diameter (cm) Plant cover (%)
*
Aboveground litterfall (g m−2 yr−1)*
a
8.37±0.4 (n030) *
a
1.72±0.07 (n0993) a
65±3.9 (n031) a
267.5±17.85 (n010)
3.97±0.48b (n021) 1.86±0.09a (n0510) 35±3.5b (n028) 184.1±17.8b (n010)
*Within each row, different lowercase superscript letters (a–b) indicate statistically significant differences between land uses/sites (P
