DOI: https://doi.org/10.5513/JCEA01/27.3.4952
Original scientific paper
Critical nitrogen loads of arable Fluvisols determined by the simple mass balance (SMB) method in southern Bulgaria
2026, 27 (3) p. 839-855
Tsetska SIMEONOVA, Iliyana GERASIMOVA, Milena KERCHEVA, Miladin NAZARKOV
Abstract
This study aimed to estimate the maximum nitrogen input, referred to as the critical load, that maintains soil acidification within stable limits while ensuring that plant growth is not constrained. The critical load was determined using a mass balance approach based on major nitrogen and base cation input–output fluxes. The assessment was based on data collected during a four-year field experiment (2020–2023), which evaluated changes in the principal nitrogen and base cation pools under different nitrogen inputs, including mineral fertilization, atmospheric deposition, and irrigation. The experiment was conducted on a Fluvisol near Plovdiv, southern Bulgaria, under a maize–wheat crop rotation. The experimental design comprised a control treatment (T0) and three nitrogen application rates: for wheat, T1 (100 kg N/ ha), T2 (140 kg N/ha), and T3 (180 kg N/ha); and for maize, T1 (120 kg N/ha), T2 (160 kg/N ha), and T3 (200 kg/ N/ha). The results showed that annual nitrogen inputs from atmospheric deposition ranged from 10 to 23 kg N/ha during the study period (2020–2023). Nitrogen inputs through irrigation were of a comparable magnitude, ranging from 19.8 to 24.1 kg N/ha. Atmospheric deposition of Ca²⁺ and Mg²⁺ exhibited a declining trend over the study period, with annual average inputs of 42 kg Ca/ha and 15.9 kg Mg/ha, respectively. In contrast, the input of base cations through irrigation was three to four times greater than that supplied by precipitation. The average annual nitrogen leaching loss from the control treatment was 4.5 kg N/ha, which was four to five times lower than the losses recorded in the fertilized treatments (T2 and T3). Annual Ca²⁺ leaching losses ranged from 10 to 52 kg/ha, depending on the nitrogen application rate. Soil pH declined by approximately one unit after maize cultivation under the highest mineral nitrogen application rate (200 kg N/ha) compared with the control treatment. The estimated critical load for acidifying nitrogen was relatively high for this Fluvisol under irrigated maize, ranging from 170 to 227 kg N/ha/yr. These high values are likely attributable to the substantial input of base cations through irrigation, which enhanced the soil's buffering capacity. In contrast, following wheat cultivation, when irrigation was not applied, the critical load for acidifying nitrogen decreased markedly to 31–82 kg N/ha/yr.
Keywords
critical load, nitrogen, base cations, input-output, acidification
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