Soil organic carbon change after 28 years of fertilisation in temperate conditions of Estonia

Substantial progress has been made in estimating and mapping soil organic carbon stocks, but the quantification of carbon stock change rates still needs exploration in numerous soil and management combinations. In this research, the soil organic carbon (SOC) changes and soil organic matter (SOM) composition in the long-term experiment (established in 1989, Tartu, Estonia) with sandy loam soil were studied. The experiment involves three-field crop rotation (potato-spring wheat-spring barley) with two fertilization regimes: (1) mineral fertilisation (N rates 0, 40, 80, 120 and 160 kg ha^-1) and (2) same mineral fertilisation treatments with farmyard manure amendment. During 1989–2017, every spring the soil samples were taken from the 0-20 cm soil layer, and SOC concentration was measured by dry combustion. In 2017, the size fractionation of soil samples was also performed. In 2022, the soil samples were also taken from the area surrounding the experimental site, which has not been cultivated since the experiment was established and has been as permanent grassland (GR). Soil mineral-associated organic matter (MAOM fraction;<63 µm) of permanent grasslands is considered to be saturated with C thus the GR was used to estimate the MAOM-C saturation potential in treatments.

The SOC stock increased (0.08–0.18 Mg ha^-1 y^-1 depending on N fertiliser rate) only in manure treatments. In GR treatment the SOC stock remained unchanged. Without manure, the SOC stock decreased in range from -0.15 to -0.26 Mg ha^-1 y^-1. N fertilization had a positive effect on SOC stock. Initially, 27% of total SOC stock is related to particulate organic matter (POM fraction, 63-2000 µm) and 73% to MAOM fraction. Without manure, the proportion of C related to POM fraction decreased during 28 years to 21%, while in treatments with manure, it remained stable (28%). POM fraction plays an important role in plant nutrient supply, thus a higher proportion of C related to POM fraction in treatments with manure indicates sustainable nutrition conditions for plants and all soil biota. The MAOM-C concentrations in treatments without manure varied from 17.8 to 19.4 mg g^-1 and were lower compared to treatments with manure (21.0–21.8 mg g^-1). The MAOM-C of GR was 20.7 mg g^-1 indicating that the soil is nearly saturated in manure treatments.  However, treatments without manure are far from saturation and have considerable potential for additional MAOM-C sequestration, varying between 3.0-6.8 Mg ha^-1 depending on N fertilization. Soil organic matter fractionation into POM and MAOM fractions allows us to assess the soil's properties for sustainable plant production, and actual C sequestration capabilities, and provide crucial recommendations for effective management strategies.