Changes in Organic Carbon in Mineral Topsoil of a Formerly Cultivated Arenosol under Different Land Uses in Lithuania

Abstract

This study aims to determine the differences in the organic carbon, humic acid (HA), and fulvic acid (FA) concentrations in the A and AB horizons, depending on land use, in order to determine the influence of the land use on the soil organic carbon (SOC) sequestration rate in the A horizon, and to assess the impact of the land use on the quality of the humic substances (HS) (the humification rate (HR) and the HA/FA ratio). On the basis of the data of 1995–2018, it would be expedient to convert cropland (CL) areas to fertilized managed grassland (MGfert) in order to increase the SOC accumulation (28%) in the Arenosol. In the unfertilized managed grassland (MGunfer) areas, the SOC accumulation in the A horizon was similar to that in the MGfert (p > 0.05); however, significantly less (−45.0%) HAs were formed, the HR decreased 2.8%, and the HA/FA ratio was 1.12%. This means that less stable humic substances were formed in the MGunfer soil. In the Arenosol, the fastest SOC sequestration took place in the AL and PP areas, the annual SOC stocks increased by 393 and 504 kg ha−1 year−1, respectively, and the HR increased to 19.1–21.3% (CLfert: 11.9%). However, these types of land use produce more FAs (14.5 and 32.5% more, respectively, compared to the MGfert, and 36.3 and 57.7% more, respectively, compared to the CLfert), which can lead to soil acidification and can accelerate eluvial processes. Because of the faster leaching of the FAs from the upper layers of the A horizon to the AB horizon, the humus type changes from humate–fulvate in the A horizon, to fulvate–humate in the AB horizon.