Search for: All records

Abstract Plants adjust their allocation to different organs based on nutrient supply. In some plant species, symbioses with nitrogen‐fixing bacteria that live in root nodules provide an alternate pathway for nitrogen acquisition. Does access to nitrogen‐fixing bacteria modify plants' biomass allocation? We hypothesized that access to nitrogen‐fixing bacteria would have the same effect on allocation to aboveground versus belowground tissues as access to plentiful soil nitrogen. To test this hypothesis and related hypotheses about allocation to stems versus leaves and roots versus nodules, we conducted experiments with 15 species of nitrogen‐fixing plants in two separate greenhouses. In each, we grew seedlings with and without access to symbiotic bacteria across a wide gradient of soil nitrogen supply. As is common, uninoculated plants allocated relatively less biomass belowground when they had more soil nitrogen. As we hypothesized, nitrogen fixation had a similar effect as the highest level of fertilization on allocation aboveground versus belowground. Both nitrogen fixation and high fertilization led to ~10% less biomass allocated belowground (~10% more aboveground) than the uninoculated, lowest fertilization treatment. Fertilization reduced allocation to nodules relative to roots. The responses for allocation of aboveground tissues to leaves versus stems were not as consistent across greenhouses or species as the other allocation trends, though more nitrogen fixation consistently led to relatively more allocation to leaves when soil nitrogen supply was low. Synthesis: Our results suggest that symbiotic nitrogen fixation causes seedlings to allocate relatively less biomass belowground, with potential implications for competition and carbon storage in early forest development. 

ABSTRACT Anthropogenic nitrogen (N) deposition is unequally distributed across space and time, with inputs to terrestrial ecosystems impacted by industry regulations and variations in human activity. Soil carbon (C) content normally controls the fraction of mineralized N that is nitrified (ƒ_nitrified), affecting N bioavailability for plants and microbes. However, it is unknown whether N deposition has modified the relationships among soil C, net N mineralization, and net nitrification. To test whether N deposition alters the relationship between soil C and net N transformations, we collected soils from coniferous and deciduous forests, grasslands, and residential yards in 14 regions across the contiguous United States that vary in N deposition rates. We quantified rates of net nitrification and N mineralization, soil chemistry (soil C, N, and pH), and microbial biomass and function (as beta‐glucosidase (BG) andN‐acetylglucosaminidase (NAG) activity) across these regions. Following expectations, soil C was a driver ofƒ_nitrifiedacross regions, whereby increasing soil C resulted in a decline in net nitrification andƒ_nitrified. Theƒ_nitrifiedvalue increased with lower microbial enzymatic investment in N acquisition (increasing BG:NAG ratio) and lower active microbial biomass, providing some evidence that heterotrophic microbial N demand controls the ammonium pool for nitrifiers. However, higher total N deposition increasedƒ_nitrified, including for high soil C sites predicted to have lowƒ_nitrified, which decreased the role of soil C as a predictor ofƒ_nitrified. Notably, the drop in contemporary atmospheric N deposition rates during the 2020 COVID‐19 pandemic did not weaken the effect of N deposition on relationships between soil C andƒ_nitrified. Our results suggest that N deposition can disrupt the relationship between soil C and net N transformations, with this change potentially explained by weaker microbial competition for N. Therefore, past N inputs and soil C should be used together to predict N dynamics across terrestrial ecosystems. 

Abstract Biological nitrogen (N) fixation (BNF) provides the N needed to produce proteins and other biological building blocks, helping feed humanity and mitigate climate change. Due to its high energetic cost compared to other forms of N acquisition, biotic investment in BNF indicates N limitation. Globally gridded BNF flux data provide an opportunity to determine the energetic investment in BNF across ecosystems, which would help reconcile conflicting indicators of N limitation. Here, we use a new BNF synthesis to quantify the relative importance of BNF in different N‐fixing niches, in different biomes, and across the globe by calculating the fraction of net primary productivity (NPP) invested in BNF and the fraction of plant N acquisition provided by BNF. Larger fractions of non‐agricultural NPP were invested in BNF in less‐productive, higher‐latitude biomes. This pattern was driven by biocrusts and mosses. Similarly, non‐agricultural symbiotic N‐fixing plants invested relatively more of their own NPP in BNF in less‐productive, higher‐latitude biomes. This symbiotic plant pattern was driven by shrubs and herbs, overriding the opposite pattern in trees. Symbiotic plants also acquired a higher fraction of their N from BNF at higher latitudes and in less productive biomes. Investments in symbiotic BNF were 10× higher in agricultural (2.9% of NPP) versus natural (0.29%) biomes, providing 26% versus 3.1% of ecosystem‐scale plant N acquisition. These results support the paradigm of strong N limitation at higher latitudes, help understand the rarity of N‐fixing trees at higher latitudes, underscore the dominance of human activity, and inform terrestrial biosphere models. 

We use the Multiple Element Limitation (MEL) model to examine the responses of twelve ecosystems - from the arctic to the tropics and from grasslands to forests - to elevated carbon dioxide (CO2), warming, and 20% decreases or increases in annual precipitation. The ecosystems respond synergistically to elevated CO2, warming, and decreased precipitation combined because higher water use efficiency with elevated CO2 and higher fertility with warming compensate for the responses to drought. The response to elevated CO2, warming, and increased precipitation combined is additive. We analyze changes in ecosystem carbon (C) sequestration based on four nitrogen (N) and four phosphorus (P) attribution factors of the ecosystem: (1) changes in total N and P in the ecosystem, (2) changes in the distribution of N and P between vegetation and soil, (3) changes in vegetation C:N and C:P ratios, and (4) changes in soil C:N and C:P ratios. In the combined CO2 and climate change simulations, all ecosystems gain C. The relative contribution of changes in these four N and P attribution factors to changes in ecosystem C storage varies among ecosystems because of differences in the initial distributions of N and P between vegetation and soil and the openness of the ecosystem N and P cycles. The net transfer of N and P from soil (low C:N and C:P) to vegetation (high C:N and C:P) dominates the C response of forests. For tundra and grassland ecosystems, the C gain is also associated with an increase in soil C:N and C:P. In ecosystems with symbiotic N fixation, gains in C resulted from the accumulation of N and sometimes P. Because of differences in the openness of the N versus P cycles and the distribution of organic matter between vegetation and soil, changes in the N attribution factors do not always parallel changes in the P attribution factors. These findings highlight how differences among ecosystems in C-nutrient interactions and the amount of woody biomass interact to shape ecosystem C sequestration under simulated global change. By using a single model framework across multiple ecosystems, we suggest that a better understanding of the factors influencing the openness of the N and P cycles, controls on N and P distribution within ecosystems, and controls on ecosystem stoichiometry is needed to improve the representation of nutrient effects on C sequestration in ecosystems and their responses to elevated CO2 and climate change.