Molybdenum and Phosphorus Interact to Constrain Asymbiotic Nitrogen Fixation in Tropical Forests

Biological di-nitrogen fixation (N2) is the dominant natural source of new nitrogen to land ecosystems. Phosphorus (P) is thought to limit N2 fixation in many tropical soils, yet both molybdenum (Mo) and P are crucial for the nitrogenase reaction (which catalyzes N2 conversion to ammonia) and cell growth. We have limited understanding of how and when fixation is constrained by these nutrients in nature. Here we show in tropical forests of lowland Panama that the limiting element on asymbiotic N2 fixation shifts along a broad landscape gradient in soil P, where Mo limits fixation in P-rich soils while Mo and P co-limit in P-poor soils. In no circumstance did P alone limit fixation. We provide and experimentally test a mechanism that explains how Mo and P can interact to constrain asymbiotic N2 fixation. Fixation is uniformly favored in surface organic soil horizons - a niche characterized by exceedingly low levels of available Mo relative to P. We show that soil organic matter acts to reduce molybdate over phosphate bioavailability, which, in turn, promotes Mo limitation in sites where P is sufficient. Our findings show that asymbiotic N2 fixation is constrained by the relative availability and dynamics of Mo and P in soils. This conceptual framework can explain shifts in limitation status across broad landscape gradients in soil fertility and implies that fixation depends on Mo and P in ways that are more complex than previously thought

Acquisition of N By Plants from 15N-Labeled Root and Leaf Litter

Organic soils, particularly in northern climates, are often inhabited by ericoid mycorrhizal shrubs. Previous work, generally conducted under axenic conditions and/or with simple forms of organic N, suggests ericoid mycorrhizal fungi may confer a competitive advantage to their hosts through N uptake from organic sources. We hypothesized that ericoid mycorrhizal shrubs would acquire more N from 15N-enriched litter (pine and huckleberry roots and foliage) than would ectomycorrhizal pines. Plants were grown alone and in pairs in a pot experiment with 15N-enriched litter and N uptake was assessed after 1.5, 5, and 12 months. Chemical properties of the 15N-litter differed by species and by plant part. Lower concentrations of polyphenols and tannins, lower C/N ratio, and higher ADF-lignin/N ratio were characteristic of pine relative to huckleberry litter and of root relative to foliage litter. When grown alone, pines acquired more N from root litter than from foliage litter and both species recovered more N from pine litter than from huckleberry litter. Across all treatments, the greatest quantity of litter N was recovered by pines from pine root litter. When N recovery was compared between pines and huckleberries, it was evident that this difference increased in favor of ericoid mycorrhizal huckleberries as litter tannin concentration increased. Tannin concentration was lowest in pine root litter and highest in huckleberry foliage litter. Thus, when planted singly, each species had the greatest advantage in litter N recovery when grown in its own litter. When pines and huckleberries were planted in pairs, pines in interspecific pairs grew less and acquired less litter N than did pines in conspecific pairs. This effect was not observed in huckleberries and was independent of differences in litter quality, suggesting that, in addition to the production of recalcitrant litter, huckleberries possess other mechanisms to suppress competition by pines in organic soils

Acquisition of N By Plants from 15N-Labeled Root and Leaf Litter

Organic soils, particularly in northern climates, are often inhabited by ericoid mycorrhizal shrubs. Previous work, generally conducted under axenic conditions and/or with simple forms of organic N, suggests ericoid mycorrhizal fungi may confer a competitive advantage to their hosts through N uptake from organic sources. We hypothesized that ericoid mycorrhizal shrubs would acquire more N from 15N-enriched litter (pine and huckleberry roots and foliage) than would ectomycorrhizal pines. Plants were grown alone and in pairs in a pot experiment with 15N-enriched litter and N uptake was assessed after 1.5, 5, and 12 months. Chemical properties of the 15N-litter differed by species and by plant part. Lower concentrations of polyphenols and tannins, lower C/N ratio, and higher ADF-lignin/N ratio were characteristic of pine relative to huckleberry litter and of root relative to foliage litter. When grown alone, pines acquired more N from root litter than from foliage litter and both species recovered more N from pine litter than from huckleberry litter. Across all treatments, the greatest quantity of litter N was recovered by pines from pine root litter. When N recovery was compared between pines and huckleberries, it was evident that this difference increased in favor of ericoid mycorrhizal huckleberries as litter tannin concentration increased. Tannin concentration was lowest in pine root litter and highest in huckleberry foliage litter. Thus, when planted singly, each species had the greatest advantage in litter N recovery when grown in its own litter. When pines and huckleberries were planted in pairs, pines in interspecific pairs grew less and acquired less litter N than did pines in conspecific pairs. This effect was not observed in huckleberries and was independent of differences in litter quality, suggesting that, in addition to the production of recalcitrant litter, huckleberries possess other mechanisms to suppress competition by pines in organic soils

Supplement 1. The complete data set.

<h2>File List</h2><div> <p><a href="fertilization_treatments.csv">fertilization_treatments.csv</a> (MD5: 89bc45540bf3305cb2ef152cc68eaa0f)</p> <p><a href="root_abundance_and_morphological_traits.csv">root_abundance_and_morphological_traits.csv</a> (MD5: 31d7fb5a0a3858148df72683d227b914)</p> <p><a href="root_nutrients.csv">root_nutrients.csv</a> (MD5: 168940dbbfac94021d6be522d4978e9a)</p> <p><a href="mycorrhizal_colonization.csv">mycorrhizal_colonization.csv</a> (MD5: f83d44888e6ffd45b60544ae605873f1)</p> </div><h2>Description</h2><div> <p>fertilization_treatments.csv is a comma-separated text file containing the fertilization treatments of the Gigante experiment. Column definitions are:</p> <ol> <li>treatment is the fertilization treatment, where N = nitrogen, P = phosphorus and K = potassium. </li> <li>plot is the plot number associated with each treatment. </li> </ol> <p>root_abundance_and_morphological_traits.csv is a comma-separated text file containing the root abundance and morphological trait dataset. Column definitions are:</p> <ol> <li>plot is treatment plot. </li> <li>core is core sample number. </li> <li>size_class is root diameter size class, where 0 = 0–1 mm, 1 = 1–2 mm and 2 = 0–2 mm. </li> <li>biomass is root biomass in g/m2</li> <li>length is root length in m/m2</li> <li>t_den is root tissue density in g/cm3</li> <li>srl is specific root length in cm/g</li> <li>avg_diam is average root diameter in mm.</li> </ol> <p>root_nutrients.csv is a comma-separated text file containing the nutrient content of root tissue samples. Column definitions are:</p> <ol> <li>plot is treatment plot</li> <li>size_class is root diameter size class, where 0 = 0–1 mm, 1 = 1–2 mm and 2 = 0–2 mm. </li> <li>N is root tissue nitrogen concentration (%).</li> <li>C is root tissue carbon concentration (%).</li> <li>P is root tissue phosphorus concentration (ppm).</li> <li>K is root tissue potassium concentration (ppm).</li> </ol> <p>mycorrhizal_colonization.csv is a comma-separated text file containing mycorrhizal colonization dataset. Column definitions are:</p> <ol> <li>plot is treatment plot.</li> <li>arb_per is colonization of arbuscules expressed as a percent of root length</li> <li>ves_per is colonization of vesicles expressed as a percent of root length</li> <li>hyp_per is colonization of hyphae expressed as a percent of root length</li> <li>tot_per is total colonization expressed as a percent of root length</li> <li>arb_len is colonization of arbuscules expressed as root length (cm).</li> <li>ves_len is colonization of vesicles expressed as root length (cm).</li> <li>hyp_len is colonization of hyphae expressed as root length (cm).</li> <li>tot_len is total colonization expressed as root length (cm).</li> </ol> </div

Phosphorus and species regulate N2 fixation by herbaceous legumes in longleaf pine savannas

Longleaf pine savannas house a diverse community of herbaceous N2-fixing legume species that have the potential to replenish nitrogen (N) losses from fire. Whether legumes fill this role depends on the factors that regulate symbiotic fixation, including soil nutrients such as phosphorus (P) and molybdenum (Mo) and the growth and fixation strategies of different species. In greenhouse experiments, we determined how these factors influence fixation for seven species of legumes grown in pure field soil from two different regions of the southeastern US longleaf pine ecosystem. We first added P and Mo individually and in combination, and found that P alone constrained fixation. Phosphorus primarily influenced fixation by regulating legume growth. Second, we added N to plants and found that species either downregulated fixation (facultative strategy) or maintained fixation at a constant rate (obligate strategy). Species varied nearly fourfold in fixation rate, reflecting differences in growth rate, taxonomy and fixation strategy. However, fixation responded strongly to P addition across all species in our study, suggesting that the P cycle regulates N inputs by herbaceous legumes

Available Mo and P in organic soils and Mo recovery from tannic acid solutions.

<p>A) Fraction of available (resin-extractable) Mo and P in O horizon soils averaged across six forest sites. B) Recovery of Mo and P (resin-extractable) after addition and 20–25 hours of incubation of molybdate and phosphate to freshly sampled O horizon soils of six forest sites. C) Recovery of Mo (resin-extractable) from 0.1 mM solutions of Mo and increasing concentrations of tannic acid (<i>n</i> = 6) relative to tannic-acid free controls. Values represent means ± s.e.m.</p

Nitrogenase activity (Acetylene reduction activity in nmol C₂H₂ g⁻¹ h⁻¹) in response to additions of P to soils from six forest sites (mean (s.e.m.); <i>n</i> = 5.

<p>Nitrogenase activity (Acetylene reduction activity in nmol C₂H₂ g⁻¹ h⁻¹) in response to additions of P to soils from six forest sites (mean (s.e.m.); <i>n</i> = 5.</p

Plant species richness negatively affects root decomposition in grasslands

1. Plant diversity enhances many ecosystem functions, including root biomass production, which drives soil carbon input. Although root decomposition accounts for a large proportion of carbon input for soil, little is known about plant diversity effect on this process. Plant diversity may affect root decomposition in two non-exclusive ways: by providing roots of different substrate quality (e.g. root chemistry) and/or by altering the soil environment (e.g. microclimate).2. To disentangle these two pathways, we conducted three decomposition experiments using a litter-bag approach in a grassland biodiversity experiment. We hypothesized that: (i) plant species richness negatively affects substrate quality (indicated by increased C:N ratios), which we tested by decomposing roots collected from each experimental plot in one common plot; (ii) plant species richness positively affects soil environment (indicated by increased soil water content), which we tested by decomposing standardized roots in all experimental plots; (iii) the overall effect of plant species richness on root decomposition, due to the contrast between quality and environmental effects, is neutral, which we tested by decomposing community roots in their ‘home’ plots.3. Plant species richness negatively affected root decomposition in all three experiments. The negative effect of plant species richness on substrate quality was largely explained by increased root C:N ratios along the diversity gradient. Functional group presence explained more variance in substrate quality than species richness. Here, the presence of grasses negatively affected substrate quality and root C:N ratios, while the presence of legumes and small herbs had positive effects. Plant species richness had a negative effect on soil environment despite its positive effect on soil water content which is known to stimulate decomposition. We argue that – instead of soil water content – a combined effect of soil temperature and seasonality might drive environmental effect of plant diversity on decomposition in our plant communities, but this remains to be tested.4. Synthesis. Our results demonstrate that both substrate quality and soil environment contribute to the net negative effect of plant diversity on root decomposition. This study promotes our mechanistic understanding of increased soil carbon accumulation in more diverse grassland plant communities.<br/