Organic matter dynamics and microbial activity during decomposition of forest floor under two native neotropical oak species in a temperate deciduous forest in Mexico

Abstract

Litter decomposition, which represents the main pathway of nutrient return to soil, affects the fertility of forest fragments. The biochemical composition of litter and the way that it decomposes are determined by the physiological characteristics of the plant species from which the material is derived. The main physiological process affecting nutrient concentrations in litter is the resorption of foliar nutrients. In this study, we determined organic matter composition, nutrient dynamics and microbial activity in samples of forest floor litter collected over a two year period under two native oak species, Quercus castanea (Qc) and Quercus deserticola (Qd), growing in a semi-natural deciduous forest in central Mexico. These coexisting oak species differ in relation to nutrient resorption efficiency, and their spatial distribution is vulnerable to climate change. The Nuclear Magnetic Resonance (13C-NMR) spectra revealed a higher O-alkyl:aromatic C ratio and more decomposable lignin structure (based on lignin subunit ratios) in the Qd litterfall than in the Qc litterfall (3.7 and 3.6, respectively), in concordance with results obtained by Fourier Transform Infrared (FTIR) Spectroscopy and Differential Scanning Calorimetry (DSC). Consequently, the Qd litter had a higher proportion of thermolabile compounds. In the same way, the nutrient concentrations were very different in each species, as a result of the different patterns of foliar nutrient resorption: both nutrient concentrations and microbial activity were higher in the Qd litter than in the Qc litter. Moreover, the specific enzymatic activity (SEA) of the microbial community was higher in the Qc litter, suggesting that these microorganisms required investment of more energy to increase nutrient acquisition rather than to increase microbial growth. The results suggest that the chemical composition and chemical quality of litterfall, resulting from foliar resorption, strongly affect forest floor microbial activity. Consequently, the physiological footprint of both tree species is a key factor in decomposition processes and soil fertility.