Land use change affects on subsoil organic matter in a steppe soil

Climate change-induced extreme weather poses significant challenges. Enhancing soil organic matter (SOM) can help mitigate its effects by sequestering carbon and increasing soil resilience. Large carbon losses from land-use changes highlight the potential for carbon replenishment through restoration practices. Despite its lower concentration, subsoil carbon is more stable and has a larger pool than that in the topsoil, offering untapped opportunities against climate change. However, the dynamics of subsoil SOM after reducing or stopping cultivation are not yet well understood. This research aims to fill this gap. Three land-use types are studied on steppe soil: ancient grassland (AG), arable land abandoned 12 years ago (AAL), and arable land (AL). The changes in SOM are investigated across three soil horizons: A (0–30 cm), AC (30–60 cm), and C (> 60 cm). Two organic carbon (OC) pools are examined: i.) carbon bound to the fine fraction (stable pool) < 63 μm; ii.) carbon stored within aggregates (labile pool) > 63 μm. OC content is determined by the difference between total carbon (TC) and inorganic carbon (IC). TC and IC were measured by Dumas principle-based Vario Macrocube CHNS elemental analyzer and a TOC-L analyzer equipped with an SSM-5000A, respectively. The SOM composition was estimated by Fourier transform infrared (FTIR) spectroscopy (Vertex 70) in DRIFT mode with an RT-DLaTGS detector (Bruker, USA). Overall, due to tillage intensity reduction, SOM increases in the topsoil and decreases in the subsoil, and its composition varies significantly among horizons and pools. The OC content under AAL increased 12 years after abandonment in the A horizon, mainly via the labile pool. In the AC and C horizons, the higher OC content under AL indicates SOM transport to the subsoil, a phenomenon . Cultivation has caused OC increase in both pools of the AC horizon, with a greater degree for the stable pool. Following abandonment, the OC stored in the labile pool of the AC horizon decreased to levels comparable to AG, whereas the decline in OC stored in the stable pool has not yet reached AG levels. In the C horizon, the reverse is true. Cultivation reduced the variance of most investigated properties in all horizons. The highest C/N and variance were observed under AAL, suggesting spatially uneven decomposition due to fresh OM inputs and that a new equilibrium has not been established yet. The lowest C/N was under AL, indicating a higher role of necromass in SOM there. Aromaticity was the highest in the AC horizon across all land uses compared to the A and C horizons. In terms of land use, aromaticity was greatest under AAL.
The authors are grateful to the SediLab staff. The study was supported by the eköp-kdp-24 university excellence scholarship program cooperative doctoral program of the Ministry for Culture and Innovation from the source of the National Research, Development and Innovation Fund via ELTE/15573/1(2024).