Effects of management on plant litter traits and consequences for litter mass loss and Collembola functional diversity in a Mediterranean agro-forest system
Introduction
Plant litter decomposition is a key process underlying nutrient cycling in natural and managed systems (Swift et al., 1979). A main factor controlling this process is litter quality, which is characterized by chemical traits, including stoichiometry(Cornwell et al., 2008; Kurokawa et al., 2010; García-Palacios et al., 2013). For instance, higher nutrient content (namely nitrogen and phosphorus) underlies more labile litters, while higher lignin/N ratios and repellent secondary compounds (e.g. polyphenols) are associated to increased recalcitrance (Cadisch and Giller, 1997; Cornwell et al., 2008; Gessner et al., 2010).
Differences in litter quality are inherent to the different plant species providing the litter. As an example, leaf litters from different tree species may have considerable different traits in terms of lignin/N and C/N ratios (e.g. Hättenschwiler and Gasser, 2005; Gerlach et al., 2014). Yet, litters from the same plant/crop species probably differ in their nutrient and secondary metabolites contents depending on the management practice in a given agro-ecosystem (Quetier et al., 2007; Wickings and Grandy, 2013; Lamarque et al., 2014). For instance, nutrient content and stoichiometry of plant litters can be influenced by the levels of soil tillage and fertilizer inputs, which may lead to divergent plant litter traits in terms of chemical and physical properties (Quetier et al., 2007; Wickings and Grandy, 2013; Lamarque et al., 2014). Accordingly, previous studies have asserted that intensively managed agro-ecosystems support more labile (less recalcitrant) plant litters due to their exploitative nutrient economy, in contrast to the conservative nutrient economy of less disturbed systems (Garnier et al., 2004; Fortunel et al., 2009). Yet, still very little is known on the effects of management on litter traits.
Different management practices may also influence litter decomposition through changes in the composition of the plant litter mixed in the litter matrix (Eviner, 2004; Freschet et al., 2012). Previous studies have showed that litter mixtures may promote synergistic (or antagonistic) effects on the decomposition process (Salamanca et al., 1998; Hättenschwiler et al., 2005; Lummer et al., 2012). Both the indirect effect of management and the effect of litter mixtures on the decomposition process are particularly important in managed systems with poorer soils and lower levels of organic matter, which is the case of Mediterranean agro-forest systems (Pulido-Fernández et al., 2013). A specific limitation in Mediterranean soils concerns the depletion of key nutrients, namely phosphorus (Sardans and Rodà, 2004; Cubera et al., 2009). P fertilization is often done by applications of chemical fertilizers, although these are not a sustainable P resource (Reijnders, 2014; Faucon et al., 2015). Also, several studies have demonstrated that organic P plays a vital role in P cycling and plant nutrition in temperate soils (Damodar Reddy et al., 2000).
Mediterranean agro-forest systems (Montados) are usually dominated by a single tree species, cork-oaks (Quercus suber L.), and an understory of mainly herbaceous plants. Montados may be subjected to different management practices, from organic to conventional farming. Thus, in these systems the litter matrix is especially important to support the decomposition process and the subsequent nutrient uptake to plants, including cork-oak trees. However, knowledge on the influence of litter traits on the decomposition process is scarce in Mediterranean montados.
Besides litter quality, the decomposition process also depends on decomposer and detritivore communities (García-Palacios et al., 2013). Also, the influence of microbivores is crucial for the decomposition process, especially Collembola communites, which are dominant organisms in the litter layer and play a key role by stimulating fungal activity and growth (e.g. Hanlon and Anderson, 1979; Tordoff et al., 2008; Yang et al., 2012). Collembola present a range of functionally different species according to their body size and life-form traits, as a proxy of their vertical distribution in soils (Hopkin, 1997; Parisi et al., 2005). A consistent association has been found between life-form and feeding habits of most collembolan species (Rusek, 1998; Ponge, 2000; Addison et al., 2003). Yet, only few studies investigated the response of collembolan communities to litter traits (e.g. Yang and Chen, 2009; Wickings and Grandy, 2013) and hardly any addressed their influence on fungal biomass in Mediterranean montados.
Here we addressed the effect of management practices on plant litter traits, particularly in terms of phenolic and P contents, and the subsequent effects on litter mass loss, fungal biomass and Collembola communities in a Mediterranean agro-forest system. Our aim was also to test whether the mixture of two plant species with contrasting litter traits in terms of nutrient content (namely, differences in P concentrations) and recalcitrance (phenolic concentrations) could improve the decomposition process due to a synergistic effect. To tackle this question we selected leaf litters of the dominant tree (Q. suber) in a portuguese montado, as well as litter from the most common herbaceous plant found in this system (Agrostis pouretii L.), with poorer nutrient content (e.g. lower N content) but less recalcitrant (lower lignin and phenolic contents) than Quercus litters (Reis et al., 2018; Fortunel et al., 2009). This selection was done taking into account that the decomposition of recalcitrant litter may be primed by the presence of softer and more labile litter (Veen et al., 2015). Accordingly, we hypothesized that:
- 1)
Different management practices from two contiguous montado systems (conventional vs. organic) influence litter traits of Quercus and Agrostis and we predict that conventional management promotes less recalcitrant litter;
- 2)
Besides the effect of the litter type (Quercus vs. Agrostis), the more easily decomposable litters from conventional management will promote fungal biomass and influence Collembola community parameters, litter mass loss, phenolics and P dynamics during the decomposition process;
- 3)
Litter mixing of Quercus and Agrostis litters will influence fungal biomass, Collembola community parameters, litter mass loss, phenolics and P dynamics during the decomposition process.
Section snippets
Study area
The study area was a typical Portuguese cork-oak (Quercus suber L.) and holm-oak (Quercus ilex L.) agro-forest system (Montado) in the region of Alentejo and located at the Herdade do Freixo do Meio (440 ha, Montemor-o-Novo, district of Évora, coordinates: 38° 42′ N 8° 19′ W). This farm (ORG, hereafter) was adapted into an organic farming around 1997 and the management of the soil organic matter is based on the Holistic Resource Management and planned grazing (cattle and pigs) according to
Management effects on initial litter traits and decay rates
A clear management effect on litter traits was found for some parameters, namely the %P, recording the highest levels in litters from the organic site, as well as the C/P and phenolics/P ratios, which were significantly higher in litters from the conventional farm (Table 2). The effects of management, but also of the litter type (Quercus vs. Agrostis) were also detected for the N/P and lignin/P ratios. %Phenolics was significantly higher in litters of the conventional farm relative to litters
Effect of management on litter quality and mass loss
A clear management effect was found on Agrostis and Quercus initial litter quality (e.g., P, phenolics, lignin/N), supporting the first hypothesis of this study. This result was consistent with the assumption that changes in land management may affect decomposition indirectly via its effect on plant litter traits (Wickings and Grandy, 2013; Lamarque et al., 2014). During the decomposition experiment, higher levels of fungal biomass in the litterbags were associated to the conventional
Conclusions
Although land management did not significantly explain litter mass loss dynamics, its indirect influence in shaping litter traits was detected. The decomposer communities also responded to the farming type, as fungal biomass in Q. suber leaf litters depended whether this tree species was integrated in an agro-forest subjected to conventional or organic farming. Moreover, management practices can alter the proportion of Quercus and herbaceous litter types in the litter mosaic and therefore they
Acknowledgments
This work was funded by the ECO-SERVE project through the 2013–2014 BiodivERsA/FACCE‐JPI joint call for research proposals, with the national funders ANR, NWO, FCT (BiodivERsA/001/2014), MINECO, FORMAS, and SNSF. EN was supported by CNPq – Brazil (CNPq Fellowship Holder – Brazil). FR (fellowship reference SFRH/BD/133182/2017), PMS (SFRH/BPD/109511/2015) and ALG (SFRH/BPD/94820/2013) were supported by FCT, within the POCH - Human Capital Operating Programme. The authors would like to thank to
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Current affiliation: LIBRE – Laboratory for Integrative Biodiversity Research, Finnish Museum of Natural History, University of Helsinki, PO Box 17 (Pohjoinen Rautatiekatu 13), 00014 Helsinki, Finland.