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Nitrogen status of functionally different forage species explains resistance to severe drought and post-drought overcompensation

Hofer, D., M. Suter, N. Buchmann and A. Lüscher,. 2016.

Abstract

Forage species of intensively managed temperate grassland differ substantially in their drought responses. We investigated whether differences in resistance and resilience, based on biomass yield, are related to species nitrogen (N) acquisition and drought-induced N deficiency. A three-factorial field experiment was established with monocultures of four species (first factor) that differed in functional traits regarding N acquisition and rooting depth: Lolium perenne L. (shallow-rooted non-legume), Cichorium intybus L. (deep-rooted non-legume), Trifolium repens L. (shallow-rooted legume), and Trifolium pratense L. (deep-rooted legume). A ten-week summer drought was simulated (second factor) and compared to a rainfed control during two regrowths under drought and one regrowth during a subsequent six-week post-drought period. The distribution of applied fertiliser N (200 kg ha-1 year-1 in total) was manipulated (third factor) with plots receiving no N during drought or 60 kg N ha-1. Soil water availability during drought became increasingly restricted over time. Plant-available soil N was reduced up to 4- and 12-fold during the first and second regrowths under drought, respectively, but was increased up to 4-fold during the post-drought regrowth, compared to rainfed control conditions. Legumes were consistently less N-limited than non-legumes (P < 0.001). Nitrogen derived from the atmosphere (Ndfa) in the legume T. repens was 72% under severe drought (first regrowth under drought). Here, legumes were rather drought-resistant (biomass yield under drought was -22 % compared to the rainfed control), while non-legumes were not (- 41%). Further, N fertilisation mitigated the negative drought effect on biomass yield of non-legumes from - 41% (no N under drought) to 23% (N under drought). Under extreme drought (second regrowth under drought), all species were strongly impaired, irrespective of N fertilisation (- 75% on average); yet, Ndfa in T. repens was still 56%. During the post-drought regrowth, former drought-stressed non-legumes overcompensated and revealed 53% higher yield than the control. The interspecific differences in plant species responses to drought suggest a shift from N limitation under severe drought to water limitation under extreme drought. Because legumes were able to compensate for drought-induced restrictions in yield through symbiotic N2 fixation, and non-legumes overcompensated during post-drought, cropping selected legumes in mixtures with non-legumes could improve resistance and resilience of forage swards against severe drought events.

Key Words

Drought stress Intensively managed grassland Nitrogen nutrition index Post-drought recovery Root growth Symbiotic dinitrogen fixation