Fertilization management of paddy fields in Piedmont (NW Italy) and its effects on the soil and water quality

The fertilization management of the rice crop in Piedmont was analyzed at a regional scale, and the agronomic and environmental sustainability of the actual fertilization strategy of rice was evaluated through the analysis of its effect on the soils and waters quality. On average, a total amount of 127 kg ha⁻¹ of N, 67 kg ha⁻¹ of P₂O₅ and 161 kg ha⁻¹ of K₂O were supplied to the rice crop. In most cases N and P fertilization was rather well balanced with crop removal. The N balance was in the range ±50 kg for 77% of the surface. The low concentration of N in the groundwater reflected the small N surplus. P fertilization resulted to be smaller than removal for 53% of the surface. Nevertheless, the soil extractable P was very high, probably because of former higher P inputs. This resulted in a high concentration in water courses and aquifers. The K fertilization was excessive (surplus >100 kg ha⁻¹) for 53% of the surface, but most soils showed a low K content. K is probably contributing to nutrient leaching to a great extent. The average soil organic matter (SOM) content of paddy fields was higher than that of normally-cultivated soils in Piedmont, and the C/N was higher, owing to the low mineralization rate in waterlogged conditions. The SOM content was in relation with the management of the crop residues, as the tradition of burning straw after harvest was still widespread on 65% of the paddy surface.