Search

Asymbiotic nitrogen (N) fixation (ANF) is an important source of N in pristine forests and is predicted to decrease with N deposition. Previous studies revealing N fixation in response to N deposition have mostly applied understory N addition approaches, neglecting the key processes (for example, N retention and uptake) occurring in forest canopy. This study evaluated the effects of N deposition on N fixation in the soil, forest floor, mosses, and canopy leaves in a temperate forest (in central China) and a tropical forest (in southern China) with different treatments: control, understory N addition, and canopy N addition. Results showed that total ANF rates were higher in the temperate forest (2.57 ± 0.19 mg N m−2 d−1) than in the tropical forest (1.34 ± 0.09 mg N m−2 d−1). N addition inhibited the soil, forest floor, moss, and foliar N fixation in the temperate forest, whereas it inhibited only the soil N fixation in the tropical forest. Compared to canopy N addition, understory N addition overestimated the inhibitory effects of N deposition on total ANF slightly in the tropical forest (by 35%) but severely in the temperate forest (by 375–472%) due to neglecting canopy retention of N. In summary, our findings indicate that ANF has different rates and sensitivities to N addition between tropical and temperate forests and that understory N addition overestimates the N deposition effects on ANF in forests, particularly in the temperate forest. These findings are important for our accurate understanding and estimate of terrestrial N fixation under N deposition scenarios. [ABSTRACT FROM AUTHOR]

Abstract: Lowland tropical forests with chronic nitrogen (N) deposition and/or abundant N‐fixing organisms are commonly rich in N relative to other nutrients. The tropical N richness introduces a paradoxical relationship in which many tropical forests sustain high rates of asymbiotic N fixation despite the soil N richness and the higher energy cost of N fixation than of soil N uptake. However, the mechanism underlying this phenomenon remains unclear. Our study aims to test this phenomenon and examine potential mechanisms of nutrient concentrations vs. substrate stoichiometry in regulating N fixation using multiple linear regression models. We hypothesized that the rates of asymbiotic N fixation would be low in an N‐rich forest under N deposition and substrate stoichiometry would explain the variation in N fixation better than nutrient concentrations. We conducted a chronic N‐addition experiment in an N‐saturated tropical forest in southern China and measured the N fixation rates, carbon (C), N, and phosphorus (P) concentrations, and stoichiometry in different substrates (soil, forest floor, mosses, and canopy leaves). Total N fixation rates were high (10.35–12.43 kg N·ha−1·yr−1) in this N‐saturated forest because of the high substrate C:N and N:P stoichiometry (which explained 13–52% of the variation in N fixation, P < 0.037) rather than substrate nutrient concentrations (P > 0.05). Atmospheric N deposition (34–50 kg N·ha−1·yr−1) failed to down‐regulate asymbiotic N fixation in this forest possibly because the N deposition rate was insufficient to inhibit N fixation or N deposition maintained high N fixation rates by increasing C sequestration in the substrates. Our N‐addition experiment showed the insensitivity of N fixation in all the tested substrates to low N addition (50 kg N·ha−1·yr−1); however, medium and high N addition (100–150 kg N·ha−1·yr−1) stimulated the moss and foliar N fixation because of the increases in substrate C:N stoichiometry (which explained 30–34% of the variation in N fixation, P < 0.001). Overall, our results emphasize the importance of substrate (particularly mosses and foliage) stoichiometry as a driver of asymbiotic N fixation and sustained N richness in lowland tropical forests. [ABSTRACT FROM AUTHOR]

Asymbiotic nitrogen (N) fixation is an important source of new N in ecosystems, and is sensitive to atmospheric N deposition. However, there is limited understanding of asymbiotic N fixation and its response to N deposition in the context of forest rehabilitation. In this study, we measured N fixation rates (acetylene reduction) in different ecosystem compartments (i.e. soil, forest floor, moss Syrrhopodon armatus , and canopy leaves) in a disturbed and a rehabilitated subtropical forest in southern China, under 12 years of N treatments: control, low N addition (50 kg N ha − 1 yr − 1 ), and medium N addition (100 kg N ha − 1 yr − 1 ). The rehabilitated forest had higher nutrient (e.g. N) availability than the disturbed forest. In control plots, N fixation rates in forest floor were higher in the rehabilitated forest than in the disturbed forest, but N fixation rates in other compartments (soil, S . armatus , and canopy leaves) were comparable between the forests. Nitrogen addition significantly suppressed N fixation in soil, forest floor, S . armatus , and canopy leaves in the disturbed forest, but had no significant effect on those compartments in the rehabilitated forest. The main reasons for the negative effects of N addition on N fixation in the disturbed forest were NH 4 + inhibition (soil), the P and C limitation (forest floor), and the reduced N dependence on canopy N-fixers ( S . armatus and canopy leaves). We conclude that asymbiotic N fixation does not decline with increasing N availability after rehabilitation in the study forests. The inhibitory effects of N addition on asymbiotic N fixation occurred in the disturbed forest but not in the rehabilitated forest, indicating that forest rehabilitation may change the response of ecosystem function (i.e. N fixation) to N deposition, which merits further study in other tropical and subtropical regions. [ABSTRACT FROM AUTHOR]