Effects of water and nitrogen management on fibrous root distribution and turnover in sugar beet

Two field trials were carried out in two years in heavy soils of NE Italy, with the aim of studying the effects of water and nitrogen management on fibrous root distribution and dynamics in sugar beet (cv. Dorotea). In conditions of moderate water deficit (year 2002, Conselice, Ravenna, clay soil), two water regimes (irrigation to 100% of potential evapotranspiration, and rainfed) were factorially combined with three rates of nitrogen application (180, 90, 0 kg ha⁻¹). Irrigation increased volumetric root length density (RLD_v) without N application and at the medium N rate – a common amount in beet cultivation – but reduced it at the maximum N dose. The medium N rate increased RLD_v and shifted root distribution towards shallow layers, regardless of water regime.In the conditions of marked drought of 2003 (Legnaro, Padova, silty-loam soil), at a single rate of N supply (90 kg ha⁻¹) irrigation increased total production (length) of fibrous roots throughout the soil profile (1.8 m), except in the 0.5–1 m interval, and improved the length of standing living roots during the season. Although the maximum root depth at the end of the season was similar in the two water regimes (about 1.9 m), irrigated roots reached the saturated soil layers 10 days earlier than in rainfed plants. The main result was reduced root turnover in deep soil layers (>1 m) and an increase at the surface in the rainfed treatments in conditions of drought, a probable mechanism of adaptation to a more marked gradient of soil moisture compared with irrigation.