Abstract 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 season in the area. Therefore, the major causes of typically low summer productivity of temperate-type tall fescue growing in hydro-halomorphic soils of the Flooding Pampa are the intrinsic effects of low N and low water availability, with only uncommonly extreme water stress episodes reducing yield indirectly through an aggravation of crop N status.

中文翻译：

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水分胁迫的强度决定了温带型多年生牧草的氮素状况是否受干旱影响

摘要 温带型、夏季活跃的凉季多年生草在暖季由于大气需求高和土壤快速干燥而经常暴露于干旱期。由于水分供应对于草场氮 (N) 的吸收至关重要，并且大多数矿物质 N 位于表土中，如果水分胁迫减少 N 供应，则可能会阻碍草场 N 营养。在水盐型土壤中，夏季土壤氮供应和草场氮需求之间的不平衡可能会进一步加剧，首先是表层土壤的持水能力有限，其次是来自深层、贫氮土壤层的潜在可用水量由浅水位保持湿润。这项工作的目的是量化氮和水的可用性对在泛滥潘帕的水盐质土壤中生长的温带型多年生草丛的氮状况和生产力的影响。在连续两年的时间里，对温带老林（> 9 年）的夏季中期再生过程进行了长达 6 周的监测，跟踪了枝条和根系动态，以及辐射捕获、氮营养指数 (NNI)、土壤水分可用性和叶水势。 - 型高羊茅 [Lolium arundinaceum (Schreb.) Darbysh.] 播种在阿根廷巴尔卡塞的典型 Natraqualf 土壤上。草丛要么用雨养，要么用 40 或 200 kg de N ha-1 进行灌溉和施肥。根系很大（5.8–9.3 Mg DM 根 ha−1），但主要集中在 0.2 m 表土（根生物量或长度的 66%–75%）。尽管始终低于 0 的高可用水。在两个实验期间均为 4 m，每当上层土壤变干时，雨养草场都会面临短暂的水分胁迫。在夏季，高大的羊茅草受到水和 N 的强烈共同限制，对 N 添加和灌溉表现出相当的、非常高的响应。此外，当草丛同时接受补充水和氮时，牧草产量显示出协同增加。这主要是由于辐射利用效率和叶片寿命的一致协同反应。Sward NNI 仅因历史上严重的干旱而受到抑制，但当缺水强度较低时并未改变，接近该地区当季的平均水平。所以，