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Figure 1. 

Isotope analysis diagram of NO₃⁻ sources in precipitation (modified from Song et al.^[7]).

Figure 2. 

Emission fluxes from major NO_x sources in the atmosphere and NO_y transportation and deposition fluxes (modified from Song et al.^[26]).

Figure 3. 

Main principles and key findings of atmospheric NH₃ and NH₄⁺ isotope analysis (modified from Chen et al.^[6]).

Figure 4. 

Proportion and flux of the primary exchange process of nitrate between precipitation and canopy (modified from Liu et al.^[32]).

Figure 5. 

Conceptual diagram of soil nitrogen transformation process model (modified from Xu et al.^[43]). D: Depolymerization; M: Mineralization; N: Nitrification; TN: Total nitrogen; SON: Soil organic nitrogen; EON: Extractable organic nitrogen; TEN: Total extractable nitrogen. IS: The initial substrate of the nitrogen conversion process; CP: The reaction products of the nitrogen conversion process; RS: Remaining substrates in the nitrogen conversion process. f: The proportion of reaction products in the initial substrate for each nitrogen conversion process. 1-f: The proportion of residual substrates in the initial substrate for each nitrogen conversion process.

Figure 6. 

Distribution of Δ¹⁷O and δ¹⁸O in nitrates of atmosphere-plant-soil systems (modified from Liu et al.^[8]).

Figure 7. 

Sources and processes of PUN in terrestrial ecosystems (modified from Hu et al.^[9]). PUN: plant-used N, SON: soil organic N, TEN: total extractable N, EON: extractable organic N, NH₄⁺: ammonium, NO₃⁻: nitrate, Dep: depolymerization, Min: mineralization, Nitri: nitrification.

Figure 8. 

Variations of soil N-source contributions to PUN with MAT (modified from Hu et al.^[9]). (a) $f_{\rm NO_3^-} $, (b) $f_{\rm NH_4^+} $, and (c) $f_{\rm EON} $ are fractional contributions of soil NO₃⁻, NH₄⁺, and EON to PUN, respectively. The 0.1° (latitude) × 0.1° (longitude) grid-based mean ± SD values are shown. The regression was analyzed by fitting effects with 95% confidence intervals.

Figure 9. 

Carbon consumption for nitrogen assimilation by global terrestrial plants and its response to climate warming (modified from Hu et al.^[61]). (G_(i) represents the gross C consumption associated with the assimilation of total N (TN), nitrate (NO₃⁻), ammonium (NH₄⁺), and extractable organic N (EON) by global terrestrial plants. ΔC_(TN) denotes the relative change in C consumption for TN assimilation under the 2.0 °C warming scenario compared with the present-day.

Figure 10. 

Mechanisms of plant nitrogen assimilation and its carbon costs in response to climate warming (modified from Hu et al.^[61]).