The intensity of water stress dictates whether the N status of temperate-type perennial grass swards is affected by drought

• Published in: Field crops research 2020-10, Vol.257, p.107928-18, Article 107928
• Main Authors: Errecart, Pedro Manuel, Marino, María Alejandra, Agnusdei, Mónica Graciela, Lattanzi, Fernando Alfredo, Durand, Jean-Louis
• Format: Article
• Language: English
• Subjects: Environmental Sciences; Forage yield; Nitrogen Nutrition Index; Resource co-limitation; Roots; Water-nitrogen interaction
• ISSN: 0378-4290; 1872-6852
• DOI: 10.1016/j.fcr.2020.107928

•Temperate perennial grass swards show a productivity slump during the warm season.•Root systems were markedly shallow, so swards faced drought when topsoil desiccated.•Intrinsic effects of drought and N deficiency were the major causes of low yields.•Sward N status was affected under extreme drought but not by moderate water stress. Temperate-type, summer-active cool-season perennial grasses are frequently exposed to dry spells during the warm season due to the high atmospheric demand and fast soil desiccation. Since water availability is essential for sward nitrogen (N) uptake and most mineral N is located in the topsoil, water stress may hinder sward N nutrition if it reduces N supply. In hydro-halomorphic soils, imbalances between soil N supply and sward N demand during summer could be further aggravated by, first, the limited water holding capacity of the topsoil, and second, potential availability of water from deep, N-poor soil horizons that are kept moist by a shallow water table. The aim of this work was to quantify the effect of N and water availability on N status and productivity of temperate-type perennial swards growing in a hydro-halomorphic soil of the Flooding Pampa. During two consecutive years, shoot and root dynamics, as well as radiation capture, N nutrition index (NNI), soil water availability and leaf water potential were followed over six weeks long mid-summer regrowths of old stands (>9 years) of temperate-type tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] sown on a typical Natraqualf soil in Balcarce, Argentina. Swards were either rainfed or irrigated and fertilized with either 40 or 200 kg de N ha−1. Root systems were large (5.8–9.3 Mg DM roots ha−1), but concentrated mostly in the 0.2 m topsoil (66%–75% of root biomass or length). Despite invariably high available water below 0.4 m throughout both experimental periods, rainfed swards faced transient episodes of water stress whenever the upper soil layer dried out. Tall fescue swards were strongly co-limited by water and N during summer, showing comparable, very high responses to both N addition and irrigation. Further, forage yield showed synergistic increases when swards received both supplemental water and N simultaneously. This was mainly due to consistent synergistic responses in radiation use efficiency and leaf lifespan. Sward NNI was depressed only by an historically severe drought but was not altered when water stress intensity was lower, close to average for the