Critical N:P values:: Predicting nutrient deficiencies in desert shrublands

Many terrestrial ecosystems are nutrient limited. Koerselman and Meuleman (1996) proposed critical foliar N: P values that could predict wetland nutrient deficiencies ( N: P < 14, N limitation; N: P > 16, P limitation). Although critical N: P values have potential as ecological and diagnostic tools, species differences in N and P requirements must be tested. The objectives of our experiments were 1) to determine if two desert species share critical N: P values, and 2) to assess the N: P tool's applicability in a non-wetland system. We studied two common, co-occurring North American desert shrubs, Chrysothamnus nauseosus spp. consimilis and Sarcobatus vermiculatus. Based on leaf N and P concentrations, effective mycorrhizal status, and leaf N: P, we predicted Chrysothamnus would be N limited and Sarcobatus would be P limited. During the 2000 and 2001 growing seasons, juvenile shrubs received N, P, N+P, or control treatments. To assess interaction with water limitation, other shrubs were irrigated and fertilized in 2001 (N+IR, P+IR, N+P+IR, control IR). Contrary to the predicted N limitation in Chrysothamnus, stem growth was 1.7-fold greater in P+IR plants relative to IR controls, although without irrigation Chrysothamnus did not respond to P addition. Also, contrary to the predicted P limitation in Sarcobatus, stem growth was 2.7-fold and 1.5-fold greater in N and N+IR plants, respectively, relative to their controls. Leaf N was significantly higher in N-treated Sarcobatus, both with and without irrigation. Our study suggests species-specific critical N: P values and that the N: P tool does not effectively predict desert shrub nutrient limitations. Species adapted to low nutrient conditions may not respond to increased nutrient availability due to water and nutrient co-limitation, lack of plasticity, or lower tissue nutrient requirements.