Nitrogen economy and water productivity of lowland rice under water-saving irrigation

Water saving in irrigated lowland rice production is increasingly needed to cope with a decreasing availability of fresh water. We investigated the effect of irrigation regimes on grain yield and nitrogen (N) uptake and recovery, and the effect of N management on water productivity (grain yield/evapotranspiration (ET)). Four field experiments were carried out-three summer seasons at Tuanlin (2000-2002), China, and one dry season at Munoz (200 1), Philippines-using a hybrid for Tuanlin and an inbred cultivar for Munoz. Several water-saving regimes were compared with continuous submergence. N fertilizer was applied at 180 kg ha(-1) at Tuanlin and at 90 and 180 kg ha(-1) at Munoz and compared with a 0-N application. Grain yield ranged from 4.1 t ha(-1) at Munoz in 0-N plots to 9.5 t ha(-1) at Tuanlin in 2001 with 180 kg N ha(-1). Alternately submerged-non-submerged regimes showed a 4-6% higher yield than continuous submergence. Other water-saving regimes led to yield reduction. fit all seasons, N application significantly increased grain yield largely through an increased biomass and grain number. Water productivity was significantly increased by N application in three out of four seasons and under limited water stress ranged from 0.70 to 1.17 in 0-N plots and from 1.27 to 1.66 kg m(3) at 180 kg N ha(-1). Water-saving regimes also increased water productivity under non-water-stressed conditions compared with continuous submergence. A synthesis of the data of three seasons at Tuanlin showed that biomass and apparent N recovery declined linearly with the duration of the crop growth without submergence. We concluded that the absence of an effect of water-saving regimes was caused by shallow groundwater tables of < 40-cm depth in 2000-2001 at Tuanlin and at Munoz, whereas at Tuanlin in 2002 there was water deficit in all treatments caused by a deeper drainage. In irrigation systems with a relatively shallow water table, optimal N management is as important as water-saving irrigation to enhance water productivity. (c) 2004 Elsevier B.V. All rights reserved.