Subsoil Denitrification Experiment at KBS MSU.

Denitrification plays two important roles in the global nitrogen cycle: returning active nitrogen to inert dinitrogen form and producing potent greenhouse gas nitrous oxide as a byproduct. Effects of denitrification in the deeper layers of soil on total soil denitrification are poorly understood. The experiment will be conducted at KBS to gain a better understanding dependency of rate of subsoil denitrification and molar ratio of denitrified N₂O to N₂ on depth in the profile and management practice applied. Experimental setup consists of 4 soil profiles (2 tilled and 2 no-till) enclosed in stainless steel boxes with open tops providing access to the soil profile for nondestructive measurements of soil temperature, soil moisture, soil atmosphere, and soil water (6 levels of measurements). Water discharged at the bottom of the profile (~2 m) is sampled as well as gas flux from the surface of the soil to the atmosphere. Inert tracer (hexafluoride) is introduced in the profile to estimate the diffusion rates. Profiles are planted to corn fertilized at 14.5 g/m² with 40% ¹⁵N-Ammonium Nitrate to improve accuracy of measurement and calculate a complete nitrogen balance. Preliminary results show high concentrations of nitrous oxide in the subsoil layers (up to 6 ppm) which suggest high potential contribution of subsoil denitrification to total soil flux of nitrous oxide. Simplified setup consists of gas measurements at two depths in the soil profile (7 and 70 cm) and static chamber at the top. It will be installed in duplicates at conventional tillage, no-till, reduced input, organic, and early successional treatments of Long-Term Ecological Research Site at KBS to expand the scope of findings made with more complex system. Further validation and scaling of the results is possible in terms of integrated semi-empirical models. Predictive equations developed in the study will be used together with other parts of SALUS (System Approach to Land Use Sustainability) model.