Net proton generation and consumption in a soil can be calculated according to proton budget theory (Van Breemen et al.,
1984). Representative processes and equations are presented in Table 1. Net proton generation (NPG) resulting from nitrification (NPGNtr), dissociation of organic acids (NPGOrg), dissociation of carbonic acid (NPGCar), and net proton influx from the overlying horizon were calculated based on the input-output budget of ions in precipitation, throughfall, and soil solution [Table 1(a), (c)–(e)]. Net proton generation associated with excess uptake of cations over anions by plants (NPGBio) consists of product (stem) removal in the SC field, while it consists of wood increment and product (fruit) removal in the MG plantation [Table 1(b)]. Because litterfall can be regarded as the circulating fraction in the MG plantation, cation uptake for litter production was not counted in NPGBio.