Organic nitrogen improves the water use of tropical tree seedlings cultivated for restoration plantings

Abstract

Societal Impact Statement: Restoring degraded tropical lands is important for biodiversity protection and human livelihoods. Newly planted tree seedlings are often challenged by drought brought about by climate change. Here, we explored how nitrogen sources used for plant growth affected the water use of tropical tree seedlings under water limitation. We found that the application of the amino acid arginine reduced water use in the studied seedlings under water limitation, compared with the conventional ammonium nitrate fertiliser. Alternative sources of nitrogen should be considered for tree seedling production in nurseries as this could enhance drought resilience traits and improve the survival of seedlings in restoration plantings. Summary: Restoration via tree planting is impacted by climate change-induced water scarcity. Nitrogen (N) supply modulates the morphology and physiology of plants and impacts water use. We compared the responses of rainforest tree seedlings Acacia mangium and Alphitonia petriei grown with inorganic N (Osmocote™, ammonium nitrate) or organic N (Argrow™, liquid arginine), hypothesising that organic N confers drought resilience by increasing water use efficiency (WUE). Seedlings were grown in a glasshouse for 12 weeks with organic or inorganic N in well-watered conditions, and then half the seedlings were subjected to water limitation for a further 4 weeks. A. mangium grew equally well on all N sources, but water limitation reduced biomass production. In contrast, N sources but not water regimes influenced biomass production in A. petriei. Under water limitation, arginine-supplied A. petriei had higher WUE and more depleted leaf δ13C than inorganic N-supplied plants. Our results suggest that organic N in the form of arginine can regulate stomatal conductance in A. petriei to convey drought resilience in seedlings. The generality of these findings should be explored to evaluate if organic N is a feasible source for generating drought-resilient seedlings for restoration plantings.