Abstract
Nitrogen (N) is, after water, the most limiting resource in semiarid ecosystems. However, knowledge on the N cycling ability of semiarid woody plants is still very rudimentary. This study analyzed the seasonal change in the N concentrations and pools of the leaves and woody organs of two species of semiarid sub-shrubs with contrasting leaf habit. The ability of both species to uptake, remobilize and recycle N, plus the main storage organ for N during summer drought were evaluated. We combined an observational approach in the field with experimental 15N labelling of adult individuals grown in sand culture. Seasonal patterns of N concentrations were different between species and organs and foliar N concentrations of the summer deciduous Lepidium subulatum were almost double those of the evergreen Linum suffruticosum. L. subulatum up took ca. 60% more external N than the evergreen and it also had a higher N resorption efficiency and proficiency. Contrastingly, L. suffruticosum relied more on internal N remobilization for shoot growth. Differently to temperate species, the evergreen stored N preferentially in the main stem and old trunks, while the summer deciduous stored it in the foliage and young stems. The higher ability of L. subulatum to uptake external N can be related to its ability to perform opportunistic growth and exploit the sporadic pulses of N typical of semiarid ecosystems. Such ability may also explain its high foliar N concentrations and its preferential storage of N in leaves and young stems. Finally, L. suffruticosum had a lower ability to recycle N during leaf senescence. These strategies contrast with those of evergreen and deciduous species from temperate and boreal areas, highlighting the need of further studies on semiarid and arid plants.
Highlights
After water, nitrogen (N) is the most limiting resource for plant growth in arid and semiarid ecosystems [1,2]
Seasonal dynamics of N concentrations and pools The foliar N concentration of L. subulatum almost doubled that of L. suffruticosum (Table 2; Fig. 1)
Foliar N concentrations of L. suffruticosum peaked in autumn, values subsequently decreased throughout winter, peaking again in spring during shoot elongation (Fig. 1)
Summary
Nitrogen (N) is the most limiting resource for plant growth in arid and semiarid ecosystems [1,2]. N sources for plants can be both external and internal The former include mineralization of the soil organic matter, microbial fixation of atmospheric N2, organic N transfers from mycorrhizal symbionts to plant roots and, in some areas, atmospheric N deposition [3]. Internal N sources come from storage through remobilization and recycling [4] They determine the proportion of N lost from the plant and the ability of plants to survive in areas where N availability through external sources is low. Internal nutrient cycling could be relevant in arid and semiarid areas, where the low, variable and unpredictable precipitation results in less productivity, subsequently smaller organic matter inputs to the soil and slower mineralization rates [5,6,7]. Understanding how plants from semiarid and arid areas adjust their internal N cycling is crucial
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