Nitrogen inputs to terrestrial ecosystems is increasing globally. The results of fertilization studies suggest that this should result in a global increase in above-ground net primary productivity (ANPP) of ~29%. However, the mechanisms driving this increase, such as the relative influence of leaf- versus plant-level processes, are not well understood. Nitrogen-dependent leaf-level processes, such as photosynthetic rate (A), leaf N content (% mass), specific leaf area (SLA) and Vcmax are hypothesized to increase with fertilization, but the magnitude of this effect is not well quantified. As such, it is difficult to assess responses reported in global-scale terrestrial ecosystem models. Here, we conducted a meta-analysis examining leaf-level traits. Species were further grouped by plant functional type to test how different groups are affected by nitrogen fertilization. We then compared our leaf-level results to ANPP estimates previously reported. We also compared our results to nitrogen fertilization responses from three versions of the Community Land Model (CLM4.0, CLM4.5, and CLM5.0). The three CLM versions contain different hypotheses for how fertilization effects plant processes with CLM4.0 being the most sensitive to soil N and each subsequent versions becoming less sensitive.
Results/Conclusions
Of the four, leaf-level traits analyzed as influencers of ANPP (leaf N, SLA, A, and Vcmax), only Vcmax significantly increased in response to fertilization (p<0.01, 38% increase). When analyzed by plant functional type, this difference was driven by conifers and herbaceous plants (p=0.015 and p=0.027, 77% and 41% increase) while deciduous woody plants did not have different of Vcmax rates under fertilization compared to the control (p=0.438). These changes were generally less than those reported for ANPP (22-35% increase by Lebauer and Tresder, 2008), suggesting that leaf processes are not the primary driver of ANPP responses to fertilization. When we compared our results to the output of the CLM, the N-sensitive CLM4.0 tended to overestimate ANPP responses to fertilization (40% increase), while the less N-sensitive CLM4.5 and CLM5.0 tended to underestimate ANPP responses to fertilization (17% and 13% increase). The leaf-level responses varied slightly by model version, but were in the range of the data confidence. These results suggest that whole-plant, rather than leaf-level processes drive ANPP responses to fertilization and should be the focus for model improvement. Basic research focusing on whole-plant response to fertilization needs to be conducted to improve model predictions of ANPP under variable N scenarios.