Plasticity in nitrogen conservation strategy under C gain variation in annual and perennial Physaria (Brassicaceae)

Perennial plants have a greater reliance on internal cycling of nitrogen and are less influenced by external fluctuations of soil fertility than annuals. Despite their importance, the major mechanisms in which annuals and perennials differ in the way they couple nitrogen conservation strategies to imbalances in carbon economy are not fully understood. We characterized the functional mechanism by which Physaria (Brassicaceae) species differing in life cycle adjust nitrogen use efficiency and productivity, in response to variations in carbon gain (CO2 assimilation rate). We set up a field experiment with a split-plot design with two factors: species (2 annuals and 2 perennials) and incident radiation IR (100 % of IR - control, and 35 % of IR). Reduced photosynthesis (brought about by artificial shading) resulted in lower total biomass accumulation in all species. This reduction was linked to a differential plasticity in leaf traits (specific leaf area - SLA - and nitrogen pool) between annuals and perennials, and had different consequences for nitrogen conservation. Annuals showed a decrease in Nitrogen Use Efficiency (NUEleaf), and nitrogen conservation was dependent on carbon economy. Annuals also had higher nitrogen loss due to higher proportion of senescent leaves, and a decrease in resorption efficiency and proficiency, relative to control plants. Perennials kept NUEleaf stable, but the strategy was species-specific: on one hand shaded P. mendocina plants had conservative leaves (non-plastic and low SLA), lower leaf senescence and higher residence time in relation to control. On the other hand, Physaria pinetorum, a short-lived perennial, combined both strategies: greater nitrogen loss due to lower resorption and proficiency, low SLA, and lower leaf senescence under lower IR. These differences in quantity and quality of leaf litter may have consequences beyond the plant's C and N economy, providing differences in functions such as soil N cycling and thus in ecosystem services.