Calculation of the critical loads for acidity and nitrogen levels on arable Fluvisol

This paper aims to estimate the maximum nutrient nitrogen input, called critical load that will balance the system in the stability limits regarding soil acidification while also not limiting plant growth as a result of mass balance calculation. The assessment was made on the basis of data obtained during a 4-years (2020-2023) field experiment and changes in the major input-output pools of nitrogen and base cations as a result of changing N loads. The study was carried out on Fluvisol, near Plovdiv in Southern Bulgaria under a cereal crop rotation. The experiment design included four N rates (T0-control, T1 N100, T2 N140, T3 N180) for wheat (Triticum aestivum L.) and (T1 N120, T2 N160, T3 N200) kg/ha for maize (Zea mays L.) respectively. Results show that average amount of N added by atmospheric deposition and irrigation water for the period 2020-2023 ranged from 10 to 24 kg.ha-1. Depositions of Ca2+ and Mg2+ via rainfall was reaching to 42 kg.ha-1 Ca2+ and 15.9 kg.ha-1 Mg. The average amount of N leached out of the control treatment (reaching 4.5 kg.ha-1 N) was four to five times lower in comparison with fertilized treatments T2 and T3. It was established that when using criteria - pH of the soil the critical load for acidifying N for this soil type was considerably high (170-227 kg.ha-1 per year) in case of maize. A significant reduction in the critical load for acidifying N was registered after wheat when irrigation was not applied (31-82 kg.ha-1 per year).