Effects of management on plant litter traits and consequences for litter mass loss and Collembola functional diversity in a Mediterranean agro-forest system

Highlights

• Management practices alter litter quality but not litter mass loss.

• Decay rates were higher in grassy (Agrostis) than in cork-oak (Quercus) litters.

• A higher proportion of eu-edaphic species was found in Agrostis treatments.

• Mixed litter increased Collembola richness, fungal biomass, P and phenolic concentrations.

• Mass loss was strongly related to the initial litter P and phenolic concentrations.

Abstract

Plant litter decomposition depends on the nutrient content and the amount of recalcitrant materials such as lignin and phenolic compounds. These traits are inherent to the plant species providing the litter. In agro-ecosystems, the type of land management may also shape litter traits and then influence the litter decomposition process. However, the effects of management practice/intensity on litter traits have not been addressed in previous studies, particularly in Mediterranean systems. Our aim was to test the effect of management practices on litter traits associated to nutritional quality and recalcitrance and the resulting effects on fungal biomass, Collembola communities and decomposition parameters in cork-oak agro-forest systems. A litterbag experiment was conducted using litter of dominant plant species, Quercus suber L. and Agrostis pouretii L., collected in an organic agro-forest system and in its neighbor conventional farm. We also tested the effects of litter mixtures using these two contrasting litter types, and the subsequent effects on Collembola communities and decomposition parameters. Land management influenced initial litter traits, mostly in terms of phenolics concentration, higher on the conventional site, and P concentrations, that were higher on the organic farm. After the decomposition experiment, fungal biomass was significantly higher on Quercus litters from the conventional management, while %P and %phenolics were significantly higher in litters from organic compared to conventional management. Collembola richness was significantly higher in Quercus treatments and in litter mixtures compared to single Agrostis litters. These recorded more epigeous species and collembolans with bigger body size, which apparently found more suitable habitat or resource availability in mixtures than in single Agrostis litters. Mixed litters also favored higher fungal biomass, and P and phenolics concentrations, in relation to single litters. Yet, land management and litter mixtures did not explain litter mass loss. Decay rates were higher in Agrostis than in Quercus litter, due to the differences in the initial litter traits (lignin, phenolics and P contents). Particularly, litter mass loss was positively related to P concentration and negatively associated to the concentration of phenolics.

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:

• 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;

• 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;

• 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.