Shoot and root production in mixed grass ley under daily fertilization and irrigation: validating the N productivity concept under field conditions

Abstract

The literature suggests a linear relationship between the N supply and relative growth rate (RGR) of plants. Our objective was to evaluate this concept for a typical mixed grass-ley in Sweden under field conditions, subjected to various combinations of irrigation and N fertilization, and to assess belowground C and N allocation patterns. For that purpose, we measured aboveground biomass 30 times through two consecutive years. Environmental variables, such as soil temperature and radiation, were continuously monitored and soil mineral N concentrations were measured to 0.9 m depth at 10 occasions. During the second year, belowground biomass to 1 m depth was measured. The unfertilized and non-irrigated control treatment showed the lowest aboveground production. In this experiment, both fertilization and irrigation had significant effects on production. Growth dynamics for all growth periods and treatments was reflected by a logistic function (R2 = 0.98). RGR significantly increased with N concentration in aboveground plant tissues (R2 = 0.81), validating the nitrogen productivity concept for a forage crop under field conditions. Living root biomass (247–330 g m−2) was mainly (83–92%) located in the upper 0.30 m under all treatments. Generally, the shoot-to-root ratios (0.58–2.67) and N concentrations in living roots increased with irrigation and N fertilization. Soil profiles of mineral N gave no clear indications of environmental risks. Supplementary materials provide the detailed dataset useful for crop growth modelling purposes.