Phenolic matter from deadwood can impact forest soil properties

Highlights

• Deadwood of Fagus sylvatica influenced soil more extensively than Abies alba.

• Soil properties correlated strongly with phenolic content for F. sylvatica.

• Effects on soil depended on composition of phenolic matter and thus decay processes.

• Decay by brown-rot and white-rot provide compelling yet untested explanation.

Abstract

Deadwood is a key factor in forest ecosystems, yet how it influences forest soil properties is uncertain. We hypothesized that changes in soil properties induced by deadwood mainly depend on the amount of released phenolic matter. Consequently we expected softwood- and hardwood-related deadwood effects on soil to be explained by unequal enrichment of phenolic substances. We measured differences in the quantity and composition of soil organic matter (SOM), pH, nutrient concentrations, and enzymatic activity between paired control and treatment points influenced by deadwood of silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.), and checked for correlations with total C and phenolic matter; the latter was quantified as aromaticity of water-extractable organic C through specific UV absorbance at 280 nm. Near fir deadwood, aromaticity and effective cation exchange capacity (CEC) increased while pH decreased. In comparison, concentrations of water-extractable organic C, effective CEC, exchangeable Ca²⁺ and Mg²⁺, base saturation, and available molybdenum-reactive P increased near beech deadwood while exchangeable Al³⁺ decreased. For fir deadwood, soil properties strongly correlated almost exclusively with total C. For beech deadwood, numerous strong correlations with aromaticity indicated that extractable phenolics influenced soil properties. These differences in correlations imply that deadwood affects soil through the composition of added phenolic matter, which would stem from differing decay processes and organisms. Decayed, particulate lignin from brown-rot in fir deadwood as opposed to oxidized, dissolved lignin from white-rot in beech deadwood would account for our observations.

Introduction

Deadwood, primarily coarse woody debris (>10 cm; CWD), is a key factor in forest ecosystems (Harmon et al., 1986, Lachat et al., 2014). Specifically deadwood can influence soil organic matter (SOM) composition, exchangeable cations, nutrient concentrations, and pH (Bade et al., 2015, Goldin and Hutchinson, 2013, Kappes et al., 2007, Kayahara et al., 1996, Krzyszowska-Waitkus et al., 2006, Spears and Lajtha, 2004, Zalamea et al., 2007). Effects on soil associated with deadwood are unsurprising given the widely recognized role of SOM in soil chemical, biological, and physical properties.

However, deadwood strongly differs compositionally from bulk SOM, and how deadwood generally influences soil properties is unclear due to contradicting observations between softwood and hardwood species (woody gymnosperms and angiosperms, respectively). The two taxa release different quantities of and compounds within dissolved organic matter (Bantle et al., 2014). As such, several studies found that coniferous deadwood acidified soil (Krzyszowska-Waitkus et al., 2006, Spears and Lajtha, 2004). In comparison, soil samples near European beech (Fagus sylvatica L.) and oak (Quercus spp.) deadwood had higher pH values compared to nearby reference samples (Kappes et al., 2007). Similarly the effects of deadwood on enzymatic activity are inconclusive (Gonzalez-Polo et al., 2013, Spears et al., 2003).

A possible mechanistic explanation for changes induced by deadwood is the enrichment of phenolic compounds from deadwood in soils. Forest soils have more phenols (in absolute terms) than soils of other land uses (Buondonno et al., 2014). More specifically within forests, soil phenolic matter can spatially relate to single trees via leaf and root detritus (Spielvogel et al., 2016). They can also alter soil chemistry to such an extent that unique forest ecosystems such as Northern California's coastal pygmy forests are perpetually sustained (Northup et al., 1998).

Crucially, deadwood is a large source of phenolic compounds, and thus traits related to the phenolic composition of wood may explain how deadwood affects soils. Softwoods and hardwoods have unequal phenolic concentrations and compositions—primarily in terms of guaiacyl- and syringyl-type lignin. As lignin with higher percentages of guaiacyl units decay more slowly than lignin with less guaiacyl units (Talbot et al., 2012; softwood and hardwood lignin, respectively), softwoods typically decompose—and thus influence soil—more slowly than hardwoods (Strukelji et al., 2013, Weedon et al., 2009).

The central question of our study is whether deadwood influences soil properties through the input of phenolic substances from deadwood. We hypothesize that changes in soil properties depend on the quantity of phenolic matter released from deadwood. Consequently we predict softwood/hardwood deadwood effects on soil to be explained by differences in the enrichment of phenolic substances. We tested our hypothesis by quantifying differences in the amount and quality—i.e., composition and extractability—of SOM, pH, nutrients, and enzymatic activity between samples influenced by CWD of silver fir (Abies alba Mill., softwood) and European beech (hardwood), and samples from paired control points without current influence of CWD. We compared results by species and checked whether properties correlated with total C or phenolic content.