Calculated rates of soil acidification of intensively used grassland in the Netherlands

Abstract

The major processes involved in acidification of soils under intensively managed grassland are the transformation and subsequent leaching of applied nitrogen (N), assimilation of excess cations in herbage and acidic atmospheric deposition. Carbonates from fertilizers and excess cations in purchased concentrates are the most important proton (H⁺) neutralizing agents applied to grassland. In this study, the effects of grazing, cutting and N application on the net proton loading from each of the main processes were calculated, using a simple model.

On mown swards, simulated excess cation uptake by the sward released 4.5–9.3 kmol_c H⁺ ha⁻¹ yr⁻¹. The total proton loading on mown grassland decreased from about 8.0 to 5.3 kmol_c ha⁻¹ yr⁻¹ when fertilizer N input as CAN-27 increased from 0 to about 400 kg ha⁻¹ yr⁻¹. Contributions from atmospheric deposition ranged from 2.2 kmol_c ha⁻¹ yr⁻¹ when herbage yield exceeded 10 Mg ha⁻¹ yr⁻¹ to 3.0 kmol_c ha⁻¹ yr⁻¹ when herbage production was only 5.5 Mg ha⁻¹ yr⁻¹.

On grazed swards, transformation of organically bound N from urine and dung to nitrate (NO ^-₃ ) and the subsequent leaching of excess NO ^-₃ was the main source of protons. Application of 400 kg N ha⁻¹ yr⁻¹ to grazed swards increased the proton loading from transformed N from 3.9 to 16.9 kmol_c ha⁻¹ yr⁻¹. The total proton loading on grazed swards exceeded that of mown swards when the input of fertilizer N exceeded 150 kg ha⁻¹ yr⁻¹.

Underestimation of the amount of N immobilized in the soil biomass and lost by denitrification may have resulted in a slight overestimation of the amount of N lost by leaching and thereby also the simulated total proton loading.