Spatial distribution and chemical composition of soil organic matter fractions in rhizosphere and non-rhizosphere soil under European beech (Fagus sylvatica L.)

Abstract

Little is known about how trees and their roots may influence the spatial distribution and chemical composition of soil organic matter (SOM) in subsoils with subsequent effects on soil organic carbon (SOC) storage and turnover. The aim of this study was to assess the impact of individual trees and their root system on the spatial distribution and chemical composition of SOM fractions and the storage of SOC in subsoils.A Dystric Cambisol was sampled along three vertical replicate transects (3.15m in length, 2.00m in depth) in a regular grid (45cm horizontal spaces, 25cm vertical spaces) at increasing distance from three individual mature European beech trees (Fagus sylvatica L.). Soil OM fractions were obtained from rhizosphere soil and bulk soil samples taken at 10 and 85cm depth increments by a combined density and particle size fractionation. Carbon and nitrogen measurements were performed, and the chemical composition of the SOM fractions was further characterized by solid state cross polarization magic angle spinning 13C nuclear magnetic resonance spectroscopy.The distance from the individual trees had no influence on the SOC contents and stocks or the chemical composition of the SOM fractions. This was ascribed to the dense and even rooting at 0–40cm depth across all sampled distances. Instead, the SOC contents and stocks highly differed between 10cm depth (11.4gSOCkg−1), where particulate organic matter (POM) dominated, and 85cm depth (0.5gSOCkg−1), where clay associated SOC dominated. These differences seemed to be strongly influenced by the roots of the trees which were almost completely absent from depths ≥60cm. Elevated SOC contents in the rhizosphere soil (40.1gSOCkg−1) were ascribed to root exudates in the root's vicinity and a very high amount (109.3gkg−1) of fresh POM (alkyl/O/N alkyl C ratio of 0.8). The data revealed that, besides root exudates, also root derived POM contributed significant amounts of SOC to the soil.Although only low amounts of the clay fraction were found at 85cm depth (22.8gclaykg−1), it accounted for high amounts of SOC and played a crucial role for the storage of SOM. The relatively high SOC stocks at 40–200cm depth (1.4kgCm−2) compared to the SOC stocks at 0–40cm depth (3.8kgCm−2) indicate that also sandy forest subsoils with low SOC contents have to be considered in terrestrial carbon inventories.