Elevated CO2 interacts with nutrient inputs to restructure plant communities in phosphorus-limited grasslands.

Globally pervasive increases in atmospheric CO2 and nitrogen (N) deposition could have substantial effects on plant communities, either directly or mediated by their interactions with soil nutrient limitation. While the direct consequences of N enrichment on plant communities are well documented, potential interactions with rising CO2 and globally widespread phosphorus (P) limitation remain poorly understood. We investigated the consequences of simultaneous elevated CO2 (eCO2 ) and N and P additions on grassland biodiversity, community and functional composition in P-limited grasslands. We exposed soil-turf monoliths from limestone and acidic grasslands that have received >25 years of N additions (3.5 and 14 g m-2 year-1 ) and 11 (limestone) or 25 (acidic) years of P additions (3.5 g m-2 year-1 ) to eCO2 (600 ppm) for 3 years. Across both grasslands, eCO2 , N and P additions significantly changed community composition. Limestone communities were more responsive to eCO2 and saw significant functional shifts resulting from eCO2 -nutrient interactions. Here, legume cover tripled in response to combined eCO2 and P additions, and combined eCO2 and N treatments shifted functional dominance from grasses to sedges. We suggest that eCO2 may disproportionately benefit P acquisition by sedges by subsidising the carbon cost of locally intense root exudation at the expense of co-occurring grasses. In contrast, the functional composition of the acidic grassland was insensitive to eCO2 and its interactions with nutrient additions. Greater diversity of P-acquisition strategies in the limestone grassland, combined with a more functionally even and diverse community, may contribute to the stronger responses compared to the acidic grassland. Our work suggests we may see large changes in the composition and biodiversity of P-limited grasslands in response to eCO2 and its interactions with nutrient loading, particularly where these contain a high diversity of P-acquisition strategies or developmentally young soils with sufficient bioavailable mineral P.

Legume-microbiome interactions unlock mineral nutrients in regrowing tropical forests.

Legume trees form an abundant and functionally important component of tropical forests worldwide with N2-fixing symbioses linked to enhanced growth and recruitment in early secondary succession. However, it remains unclear how N2-fixers meet the high demands for inorganic nutrients imposed by rapid biomass accumulation on nutrient-poor tropical soils. Here, we show that N2-fixing trees in secondary Neotropical forests triggered twofold higher in situ weathering of fresh primary silicates compared to non-N2-fixing trees and induced locally enhanced nutrient cycling by the soil microbiome community. Shotgun metagenomic data from weathered minerals support the role of enhanced nitrogen and carbon cycling in increasing acidity and weathering. Metagenomic and marker gene analyses further revealed increased microbial potential beneath N2-fixers for anaerobic iron reduction, a process regulating the pool of phosphorus bound to iron-bearing soil minerals. We find that the Fe(III)-reducing gene pool in soil is dominated by acidophilic Acidobacteria, including a highly abundant genus of previously undescribed bacteria, Candidatus Acidoferrum, genus novus. The resulting dependence of the Fe-cycling gene pool to pH determines the high iron-reducing potential encoded in the metagenome of the more acidic soils of N2-fixers and their nonfixing neighbors. We infer that by promoting the activities of a specialized local microbiome through changes in soil pH and C:N ratios, N2-fixing trees can influence the wider biogeochemical functioning of tropical forest ecosystems in a manner that enhances their ability to assimilate and store atmospheric carbon.

Niche differentiation and plasticity in soil phosphorus acquisition among co-occurring plants.

How species coexist despite competing for the same resources that are in limited supply is central to our understanding of the controls on biodiversity1,2. Resource partitioning may facilitate coexistence, as co-occurring species use different sources of the same limiting resource3,4. In plant communities, however, direct evidence for partitioning of the commonly limiting nutrient, phosphorus (P), has remained scarce due to the challenges of quantifying P acquisition from its different chemical forms present in soil5. To address this, we used 33P to directly trace P uptake from DNA, orthophosphate and calcium phosphate into monocultures and mixed communities of plants growing in grassland soil. We show that co-occurring plants acquire P from these important organic and mineral sources in different proportions, and that differences in P source use are consistent with the species' root adaptations for P acquisition. Furthermore, the net benefit arising from niche plasticity (the gain in P uptake for a species in a mixed community compared to monoculture) correlates with species abundance in the wild, suggesting that niche plasticity for P is a driver of community structure. This evidence for P resource partitioning and niche plasticity may explain the high levels of biodiversity frequently found in P-limited ecosystems worldwide6,7.

Functional complementarity of ancient plant-fungal mutualisms: contrasting nitrogen, phosphorus and carbon exchanges between Mucoromycotina and Glomeromycotina fungal symbionts of liverworts.

Liverworts, which are amongst the earliest divergent plant lineages and important ecosystem pioneers, often form nutritional mutualisms with arbuscular mycorrhiza-forming Glomeromycotina and fine-root endophytic Mucoromycotina fungi, both of which coevolved with early land plants. Some liverworts, in common with many later divergent plants, harbour both fungal groups, suggesting these fungi may complementarily improve plant access to different soil nutrients. We tested this hypothesis by growing liverworts in single and dual fungal partnerships under a modern atmosphere and under 1500 ppm [CO2 ], as experienced by early land plants. Access to soil nutrients via fungal partners was investigated with 15 N-labelled algal necromass and 33 P orthophosphate. Photosynthate allocation to fungi was traced using 14 CO2 . Only Mucoromycotina fungal partners provided liverworts with substantial access to algal 15 N, irrespective of atmospheric CO2 concentration. Both symbionts increased 33 P uptake, but Glomeromycotina were often more effective. Dual partnerships showed complementarity of nutrient pool use and greatest photosynthate allocation to symbiotic fungi. We show there are important functional differences between the plant-fungal symbioses tested, providing new insights into the functional biology of Glomeromycotina and Mucoromycotina fungal groups that form symbioses with plants. This may explain the persistence of the two fungal lineages in symbioses across the evolution of land plants.

From mycoheterotrophy to mutualism: mycorrhizal specificity and functioning in Ophioglossum vulgatum sporophytes.

Mycorrhizal functioning in the fern Ophioglossum is complex and poorly understood. It is unknown whether mature O. vulgatum sporophytes form mutualistic associations with fungi of the Glomeromycota and with what specificity. Are green sporophytes able to 'repay' fungal carbon (C) invested in them by mycorrhizal partners during the initially heterotrophic gametophyte and early sporophyte stages of the lifecycle? We identified fungal partners of O. vulgatum sporophytes using molecular techniques and supplied them with (33) P-orthophosphate and O. vulgatum sporophytes with (14) CO2 . We traced the movement of fungal-acquired nutrients and plant-fixed C between symbionts and analysed natural abundance (13) C and (15) N isotope signatures to assess nutritional interactions. We found fungal specificity of O. vulgatum sporophytes towards a mycorrhizal fungus closely related to Glomus macrocarpum. Our radioisotope tracers revealed reciprocal C-for-phosphorus exchange between fern sporophytes and fungal partners, despite competition from surrounding vegetation. Monocultures of O. vulgatum were enriched in (13) C and (15) N, providing inconclusive evidence of mycoheterotrophy when experiencing competition from the surrounding plant community. We show mutualistic and specific symbiosis between a eusporangiate fern and fungi of the Glomeromycota. Our findings suggest a 'take now, pay later' strategy of mycorrhizal functioning through the lifecycle O. vulgatum, from mycoheterotrophic gametophyte to mutualistic aboveground sporophyte.

First evidence of mutualism between ancient plant lineages (Haplomitriopsida liverworts) and Mucoromycotina fungi and its response to simulated Palaeozoic changes in atmospheric CO2.

The discovery that Mucoromycotina, an ancient and partially saprotrophic fungal lineage, associates with the basal liverwort lineage Haplomitriopsida casts doubt on the widely held view that Glomeromycota formed the sole ancestral plant-fungus symbiosis. Whether this association is mutualistic, and how its functioning was affected by the fall in atmospheric CO2 concentration that followed plant terrestrialization in the Palaeozoic, remains unknown. We measured carbon-for-nutrient exchanges between Haplomitriopsida liverworts and Mucoromycotina fungi under simulated mid-Palaeozoic (1500 ppm) and near-contemporary (440 ppm) CO2 concentrations using isotope tracers, and analysed cytological differences in plant-fungal interactions. Concomitantly, we cultured both partners axenically, resynthesized the associations in vitro, and characterized their cytology. We demonstrate that liverwort-Mucoromycotina symbiosis is mutualistic and mycorrhiza-like, but differs from liverwort-Glomeromycota symbiosis in maintaining functional efficiency of carbon-for-nutrient exchange between partners across CO2 concentrations. Inoculation of axenic plants with Mucoromycotina caused major cytological changes affecting the anatomy of plant tissues, similar to that observed in wild-collected plants colonized by Mucoromycotina fungi. By demonstrating reciprocal exchange of carbon for nutrients between partners, our results provide support for Mucoromycotina establishing the earliest mutualistic symbiosis with land plants. As symbiotic functional efficiency was not compromised by reduced CO2 , we suggest that other factors led to the modern predominance of the Glomeromycota symbiosis.

Bryophyte physiological responses to, and recovery from, long-term nitrogen deposition and phosphorus fertilisation in acidic grassland.

Atmospheric nitrogen deposition can cause major declines in bryophyte abundance yet the physiological basis for such declines is not fully understood. Bryophyte physiological responses may also be sensitive bioindicators of both the impacts of, and recovery from, N deposition. Here, responses of tissue nutrients (nitrogen (N), phosphorus (P) and potassium (K): NPK), N and P metabolism enzymes (nitrate reductase and phosphomonoesterase), photosynthetic pigments, chlorophyll fluorescence, sclerophylly and percentage cover of two common bryophytes (Pseudoscleropodium purum and Rhytidiadelphus squarrosus) to long-term (11 yr) enhanced N deposition (+3.5 and +14 g N m(-2) yr(-1)) are reported in factorial combination with P addition. Recovery of responses 22 months after treatment cessation were also assessed. Enhanced N deposition caused up to 90% loss of bryophyte cover but no recovery was observed. Phosphomonoesterase activity and tissue N:P ratios increased up to threefold in response to N loading and showed clear recovery, particularly in P. purum. Smaller responses and recovery were also seen in all chlorophyll fluorescence measurements and altered photosynthetic pigment composition. The P limitation of growth appears to be a key mechanism driving bryophyte loss along with damage to photosystem II. Physiological measurements are more sensitive than measurements of abundance as bioindicators of N deposition impact and of recovery in particular.

Mycorrhizal acquisition of inorganic phosphorus by the green-leaved terrestrial orchid Goodyera repens.

BACKGROUND AND AIMS: Mycorrhizal fungi play a vital role in providing a carbon subsidy to support the germination and establishment of orchids from tiny seeds, but their roles in adult orchids have not been adequately characterized. Recent evidence that carbon is supplied by Goodyera repens to its fungal partner in return for nitrogen has established the mutualistic nature of the symbiosis in this orchid. In this paper the role of the fungus in the capture and transfer of inorganic phosphorus (P) to the orchid is unequivocally demonstrated for the first time. METHODS: Mycorrhiza-mediated uptake of phosphorus in G. repens was investigated using spatially separated, two-dimensional agar-based microcosms. RESULTS: External mycelium growing from this green orchid is shown to be effective in assimilating and transporting the radiotracer (33)P orthophosphate into the plant. After 7 d of exposure, over 10 % of the P supplied was transported over a diffusion barrier by the fungus and to the plants, more than half of this to the shoots. CONCLUSIONS: Goodyera repens can obtain significant amounts of P from its mycorrhizal partner. These results provide further support for the view that mycorrhizal associations in some adult green orchids are mutualistic.