Enzyme activity and organic carbon content in agricultural spodosol

Polyphenol oxidases (PPO) and peroxidases (PO) are soil extracellular enzymes that can be found in free forms and can also be bound in organo-mineral-enzyme complexes. PPO are involved in the catalysis of the humic substances synthesis, while PO are involved in the catalysis of their decomposition. The accumulation of soil humus is associated with the activity of PPO and PO. In this work, the influence of PPO and PO activity on the accumulation of organic carbon in a sandy Spodosol and its clay fraction was studied for the first time. The purpose of the work was to evaluate the seasonal dynamics of the enzymatic activity of PPO and PO as well as its effect on the stabilization of organic carbon in a sandy Spodosol. Field study was carried out during the growing seasons of 2020-2021 at the experimental station of the Agrophysical Research Institute (Menkovo village, Gatchina District, Leningrad Province) on a plot of sandy Spodosol soil without application of mineral fertilizers. A plot of 2×2 m in 2020-2021 was a fallow land while in 2019 a vetch-oat mixture ( Vicia sativa L., Avena sativa L.) was grown there. Soil samples were collected with an Endelman soil drill from 0-10-cm layer using the envelope method, every month (from May to August). The content of total organic carbon (Corg) and carbon associated with the soil clay fraction (Cclay) was determined by the Tyurin method. Soil moisture was measured gravimetrically, temperature by electronic sensors iButton DS 1921 (Dallas Semiconductor, USA), installed at a depth of 10 cm. The soil carbon accumulation was calculated using the carbon enrichment coefficient of clay soil fractions (Esoc). The activity of PO and PPO was assessed using the Galstyan photocolorimetric method at λ = 440 and λ = 590 nm. The activity of oxidases in 2021 was, on average, 5 times higher for PPO and 3 times higher for PO compared to 2020. The activity of the PPO was, on average, 1.7 times higher in 2020 and 2.6 times higher in 2021 than the activity of the PO, which was reflected in the value of the humification coefficient (Kgum), which is calculated by the ratio of the PPO to the PO. In 2021, when the hydrothermal conditions in the soil were favorable for processes of humus synthesis (average soil temperature -20 °C, 16.4 % humidity), Kgum. was 47 % higher than in 2020. With a decrease in soil temperature, the statistically significant (p clay content over the same period increased by 4.78 % (p org and Cclay was stronger ( r = 0.66 and r = 0.90, respectively, p org and Cclay ( r = -0.43 and r = -0.73? p function show_eabstract() { $('#eabstract1').hide(); $('#eabstract2').show(); $('#eabstract_expand').hide(); }