In a field experiment in Palmira, Colombia, we studied mycorrhizal root colonization, phosphomonoesterase activities and P and N foliar content before and after N fertilization among different Brachiaria genotypes with demonstrated biological nitrification inhibition (BNI) capacity. Furthermore, we tested the potential nitrification rate (PNR) in soil in order to confirm the inhibition of nitrification of the selected genotypes and relate the BNI performance with P acquisition. We hypothesized that (i) genotypes will differ in key variables related to P acquisition, and that there will be a positive correlation between (ii) arbuscular mycorrhizal fungi (AMF) root colonization, P uptake, and BNI activity, and (iii) between the activity of acid and alkaline phosphomonoesterase and BNI performance. Higher N immobilization 1 week after application of synthetic fertilizer (ammonium sulfate) and low PNR of Brachiaria humidicola CIAT 679 and CIAT 16888 confirmed that these genotypes have high-BNI activity. Despite the relatively high soil P status, high affinity of Brachiaria grasses for AMF was observed at the study site: more than 60% of root length was colonized by AMF in high-BNI genotypes, versus 45% in low-BNI Brachiaria cv. Mulato. The N content of high-BNI genotypes was positively correlated with mycorrhizal root colonization suggesting the uptake of NH₄⁺ by AMF and its transfer to high-BNI genotypes and/or regulation of AMF colonization by P demand. Furthermore, increased activity of acid phosphomonoesterase (6.98 and 7.68 μmol g⁻¹ h⁻¹ in high-BNI versus 5.20 μmol g⁻¹ h⁻¹ in low-BNI genotypes) and the depletion of the most labile available P fractions in the rhizosphere of high-BNI genotypes (by 21–32%) suggest enhanced P uptake and P-use efficiency. To the best of our knowledge, this is the first study that explored relations between BNI and biotic factors affecting P acquisition. Our results highlight the importance of AMF in Brachiaria grasses even under high P availability and warrant further studies including a larger number of different BNI genotypes that can elucidate biotic plant-soil interactions affecting nutrient-use efficiencies in improved pastures under low and high P status.

Background and aims: The amount of nitrogen (N) derived from symbiotic N₂ fixation by legumes in grasslands might be affected by anthropogenic N and phosphorus (P) inputs, but the underlying mechanisms are not known. Methods: We evaluated symbiotic N₂ fixation in 17 natural and semi-natural grasslands on four continents that are subjected to the same full-factorial N and P addition experiment, using the ¹⁵N natural abundance method. Results: N as well as combined N and P (NP) addition reduced aboveground legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Addition of N and/or P had no significant effect on the symbiotic N₂ fixation per unit legume biomass. In consequence, the amount of N fixed annually per grassland area was less than half in the N addition treatments compared to control and P addition, irrespective of whether the dominant legumes were annuals or perennials. Conclusion: Our results reveal that N addition mainly impacts symbiotic N₂ fixation via reduced biomass of legumes rather than changes in N₂ fixation per unit legume biomass. The results show that soil N enrichment by anthropogenic activities significantly reduces N₂ fixation in grasslands, and these effects cannot be reversed by additional P amendment.

Background and aims: A synergistic response of aboveground plant biomass production to combined nitrogen (N) and phosphorus (P) addition has been observed in many ecosystems, but the underlying mechanisms and their relative importance are not well known. We aimed at evaluating several mechanisms that could potentially cause the synergistic growth response, such as changes in plant biomass allocation, increased N and P uptake by plants, and enhanced ecosystem nutrient retention. Methods: We studied five grasslands located in Europe and the USA that are subjected to an element addition experiment composed of four treatments: control (no element addition), N addition, P addition, combined NP addition. Results: Combined NP addition increased the total plant N stocks by 1.47 times compared to the N treatment, while total plant P stocks were 1.62 times higher in NP than in single P addition. Further, higher N uptake by plants in response to combined NP addition was associated with reduced N losses from the soil (evaluated based on soil δ¹⁵N) compared to N addition alone, indicating a higher ecosystem N retention. In contrast, the synergistic growth response was not associated with significant changes in plant resource allocation. Conclusions: Our results demonstrate that the commonly observed synergistic effect of NP addition on aboveground biomass production in grasslands is caused by enhanced N uptake compared to single N addition, and increased P uptake compared to single P addition, which is associated with a higher N and P retention in the ecosystem.