The effect of roots and easily available carbon on the decomposition of soil organic matter fractions in boreal forest soil

Abstract

The priming effect induced by carbon (C) that is easily available to microbes has been shown to increase the mineralization of soil organic matter (SOM) that is resistant to decomposition, but the combined effects of easily available carbon and the living root system have rarely been studied. The aim of this research was to study the decomposition of SOM fractions of different solubility in water and their 13C, 14C and 15N abundance with and without the presence of a living root system and easily available carbohydrate in the form of glucose. The SOM collected from the organic horizon of a boreal forest soil in Hyytiälä, southern Finland (61°51′N, 24°17′E), and exposed to laboratory incubations with and without the presence of Pinus sylvestris L. seedlings and glucose, was separated into three chemical fractions with accelerated solvent (ASE) and pressurized hot water extractions (PHWE). Changes in the natural abundance of 13C, 14C and 15N, spectral properties assessed by Fourier transform infrared spectroscopy (FTIR) and the C and N pools of SOM fractions were studied after incubation for 6 months. The extractions separated SOM into fractions with distinctive isotopic composition. The most easily soluble SOM fraction showed the largest abundance of 15N and 14C, and the living root system induced changes in the abundance of 15N and FTIR spectra. Our research suggests that plant roots may induce SOM degradation and N uptake from soluble SOM fractions, but 13C, 14C, 15N or FTIR alone cannot be used to describe the recalcitrance of SOM and its accessibility to microorganisms. It is better to use several methods in parallel to study the decomposability of SOM. Highlights We studied the effects of tree roots and addition of glucose on the solubility of soil organic matter (SOM). SOM can be separated into pools with distinctive isotopic concentrations with pressurized hot water. Roots may induce organic matter degradation and N uptake from soluble SOM fractions. Larger 14C abundance in soluble SOM indicated it contained older C than the recently assimilated C.

Bacterial microbiomes of individual ectomycorrhizal Pinus sylvestris roots are shaped by soil horizon and differentially sensitive to nitrogen addition

“…Collection of the soil took place 3–4 weeks before the microcosms were set up and this disturbance, coupled with subsequent homogenization, may have resulted in mineralization of the easily available organic matter fraction (Pumpanen et al ., ) and destruction of mycorrhizal mycelium deriving C from intact connections with tree hosts, resulting in changes in fungal community structure (Lindahl et al ., ). However, mycorrhizal colonization of roots occurred rapidly (within five days), suggesting that the inoculum potential of the soil was still high, and reproducible differences in community structure were found.…”

Forest fires in Canadian permafrost region: the combined effects of fire and permafrost dynamics on soil organic matter quality

“…Previous studies have also noted that burning of the depleted litter is not necessarily the most dominating reason for enrichment with heavier isotopes. Instead, the enrichment may be attributed to volatilisation processes, which discriminate against heavier isotopes (Cook 2001;Boeckx et al 2005) and root activity (Pumpanen et al 2017). Forest fires generally also increase N leaching (Lamontagne et al 2000), especially the leaching of 15 N depleted NO 3- (Pardo et al 2002), which may have contributed to 15 N enrichment in the FIRE 3 area.…”

Roots and rhizospheres in forest ecosystems: Recent advances and future challenges

“…One important source of uncertainty regarding priming effects is which components of SOM are primed. Jussi Heinonsalo (University of Helsinki, Finland) analyzed the influence of pine roots on the decomposition of SOM originating from the organic horizon of a boreal forest (Pumpanen et al, 2017). They showed that roots changed C and N pools in the soluble SOM fraction.…”