Free-living nitrogen fixation responds to elevated nutrient inputs in tropical montane forest floor and canopy soils of southern Ecuador

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 ·yr ) 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 ·yr ) 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 ·yr ); however, medium and high N addition (100-150 kg N·ha ·yr ) 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.

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“…, Matson et al. ), and importantly, indicates that N addition can stimulate N fixation in some substrates (mosses and foliage).…”

Section: Discussionmentioning

confidence: 96%

Stoichiometry controls asymbiotic nitrogen fixation and its response to nitrogen inputs in a nitrogen‐saturated forest

Zheng

Zhang

Luo

et al. 2018

Ecology

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“…Free‐living N fixation has also been found to occur in less well‐studied ecosystem compartments such as deep soil, canopy soils and canopy leaves (Reed et al ., ; Matson et al . ), which may lead to an underestimation of global rates. Furthermore, even where putative N‐fixing tree species are abundant in the tropics, symbiotic N fixation has been found to be less prevalent than previously estimated, making free‐living N fixation comparatively more important (Barron et al ., ; Sullivan et al ., ).…”

Section: Introductionmentioning

confidence: 99%

Nutrient limitation of terrestrial free‐living nitrogen fixation

2017

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Nitrogen (N) fixation by free-living bacteria is a primary N input pathway in many ecosystems and sustains global plant productivity. Uncertainty exists over the importance of N, phosphorus (P) and molybdenum (Mo) availability in controlling free-living N fixation rates. Here, we investigate the geographic occurrence and variability of nutrient constraints to free-living N fixation in the terrestrial biosphere. We compiled data from studies measuring free-living N fixation in response to N, P and Mo fertilizers. We used meta-analysis to quantitatively determine the extent to which N, P and Mo stimulate or suppress N fixation, and if environmental variables influence the degree of nutrient limitation of N fixation. Across our compiled dataset, free-living N fixation is suppressed by N fertilization and stimulated by Mo fertilization. Additionally, free-living N fixation is stimulated by P additions in tropical forests. These findings suggest that nutrient limitation is an intrinsic property of the biochemical demands of N fixation, constraining free-living N fixation in the terrestrial biosphere. These findings have implications for understanding the causes and consequences of N limitation in coupled nutrient cycles, as well as modeling and forecasting nutrient controls over carbon-climate feedbacks.

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“…Additionally, lowered ANF by N addition may be attributed to decreased abundance of free‐living diazotrophs and altered taxonomic community structure (Wang, Liu, & Bai, ; Wang, Wang, et al, ; Zhou et al, ). Furthermore, the inhibited ANF under N addition may be associated to limited availability of other resources including carbon (C; Chen, Li, Zhao, et al, ; Vitousek & Hobbie, ) and phosphorus (P; Matson et al, ; Reed et al, , ; Zheng, Li, et al, ), or to altered C:N:P stoichiometry (Zheng, Zhang, Lou, Li, et al, ). However, N addition may not inhibit ANF if the activities of free‐living diazotrophs are limited by N availability (Reed et al, ) or if free‐living diazotrophs have been exposed to elevated atmospheric N deposition over long time and have adapted to high N status (Zheng et al, ).…”

Section: Introductionmentioning

confidence: 99%

Topography Modulates Effects of Nitrogen Deposition on Asymbiotic N₂ Fixation in Soil but not Litter or Moss in a Secondary Karst Forest

Wang

Zheng

et al. 2019

JGR Biogeosciences

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Evidence shows that nitrogen (N) addition may suppress or have no significant effect on asymbiotic N2 fixation (ANF) in soil, litter, or moss. However, the mechanisms underlying the differential responses of ANF to N addition are not well understood. The main objectives of the current study were to assess how ANF in soil, litter, and moss responds to N addition and whether topography modulates the responses of ANF in soil, litter, or moss to N addition. We conducted an N addition experiment at two topographic positions, that is, valley and slope of a secondary karst forest in southwest China. Three N addition treatments, that is, control (0 kg N·ha−1·year−1), moderate N addition (N50, 50 kg N·ha−1·year−1), and high N addition (N100, 100 kg N·ha−1·year−1) were included. Nitrogen addition had no significant effect on moss ANF at both topographic positions. Soil ANF was lower by 17.1% in the N100 plots relative to the control in the valley but was not significantly altered by N addition on the slope. In contrast, litter ANF was suppressed by N addition at both topographic positions by 77.9% to 87.4%. Our findings suggest that topography may modulate the responses of ANF to N addition, but the modulation effects likely differ among different ecosystem compartments.

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Free-living nitrogen fixation responds to elevated nutrient inputs in tropical montane forest floor and canopy soils of southern Ecuador

Cited by 55 publications

References 62 publications

Stoichiometry controls asymbiotic nitrogen fixation and its response to nitrogen inputs in a nitrogen‐saturated forest

Stoichiometry controls asymbiotic nitrogen fixation and its response to nitrogen inputs in a nitrogen‐saturated forest

Nutrient limitation of terrestrial free‐living nitrogen fixation

Topography Modulates Effects of Nitrogen Deposition on Asymbiotic N₂ Fixation in Soil but not Litter or Moss in a Secondary Karst Forest

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Free-living nitrogen fixation responds to elevated nutrient inputs in tropical montane forest floor and canopy soils of southern Ecuador

Cited by 55 publications

References 62 publications

Stoichiometry controls asymbiotic nitrogen fixation and its response to nitrogen inputs in a nitrogen‐saturated forest

Stoichiometry controls asymbiotic nitrogen fixation and its response to nitrogen inputs in a nitrogen‐saturated forest

Nutrient limitation of terrestrial free‐living nitrogen fixation

Topography Modulates Effects of Nitrogen Deposition on Asymbiotic N2 Fixation in Soil but not Litter or Moss in a Secondary Karst Forest

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Product

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Topography Modulates Effects of Nitrogen Deposition on Asymbiotic N₂ Fixation in Soil but not Litter or Moss in a Secondary Karst Forest