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Research article2024Peer reviewedOpen access

Variability in soil carbon-to-nitrogen ratios explained by environmental conditions in a boreal catchment

Larson, Johannes; Kuglerova, Lenka; Hogberg, Peter; Laudon, Hjalmar

Abstract

Forest soil functions are influenced by interactions among trees, other organisms, and environmental factors such as the soil parent material and climate. Tree productivity and potential for forest C sequestration are currently receiving considerable attention. In boreal forests, plant productivity is commonly limited by the supply of N. Trees and other plants integrate the C and N cycles to form plant organs, which provide organic material for soils and subsequently feed the soil biota. Thus, plant growth has profound impacts on soil and ecosystem biogeochemistry. In this context, the soil C/N ratio is a critical parameter, which can display large variation across forest landscapes. This variation is correlated with forest productivity and other ecosystem functions. The aim of this study was to explore how C/N ratios in boreal forest soils are related to topography, dominant tree species, parent material, and soil texture. This was done by using a spatially-intense dataset of soil C/N ratios which included three sampling depths from 391 forest plots located within a 68 km (2) boreal forest catchment. Hydrological conditions related to topography (i.e., Topographic Wetness Index) demonstrated a significant influence on organic layer C/N ratios (R (2) =0.11, p <0.001), which decreased with increaced soil wetness. The topographic control on C/N ratios was strongest in unsorted sediments (R- 2 =0.15, p <0.001), where topography is the main driver of the variation in soil moisture conditions. The topographic influence on C/N ratios in mineral soil decreased with depth and was found to be non-significant at the 10 -20 cm depth. Forests dominated by Scots pine showed higher C/N ratios in the surficial organic layer than forests with other dominant tree species (mainly Norway spruce) or a mixed forests. In contrast, dominant tree species was not found to influence the C/N ratio in mineral soil layers. For mineral soil samples representing sorted sediments, C/N ratios decreased with grain size distribution, while no significant differences in C/N rations were observed in unsorted sediments. The study highlights a large landscape variation in soil C/N ratios within a boreal landscape. It also demonstrates the challenges associated with explaining soil properties, as a sizeable proportion of unexplained variation is caused by the complex interactions between multiple environmental factors linked to the biogeochemistry of forest soils including tree-soil interactions.

Keywords

Boreal forest; Forest soils; C/N ratio; Topographic wetness index; Environmental factors

Published in

Forest Ecology and Management
2024, Volume: 568, article number: 122108Publisher: ELSEVIER