<p>Arbuscular mycorrhizal fungi (AMF) form mutualistic associations with roughly 70% of vascular plant species, supporting the nutrient acquisition of their host plants and deriving carbon in return. AMF and plant communities are linked to each other by host-specificity and the ecological selection of favorable nutrient and carbon trading strategies. Changing soil nutrient availabilities can affect both plant and AMF communities directly and also indirectly via the response of their partners.  We aimed to elucidate the combined response of AMF (belowground) and plant (aboveground) community compositions to changing soil nutrient availabilities.</p> <p>We sampled soil and roots from a long-term nutrient deficiency experimental grassland in Admont (Styria, Austria). The grassland plots have been fertilized with different combinations of nitrogen (N), phosphorus (P), and potassium (K) over 70 years. Aboveground biomass cuts were removed three times each year, leading to long-term deficiencies of nutrients not replaced by fertilizers. Soil and root AMF community compositions were measured by DNA and RNA amplicon sequencing of the 18S rRNA gene. In addition, we assessed the plant community composition of the sampled roots by amplicon sequencing of the chloroplast rbcL (RuBisCo large subunit) gene region, and visually recorded the plant community composition on each investigated plot.</p> <p>Our results demonstrate that N and P deficiencies influenced soil AMF community composition, whereas K deficiency had a major impact on root AMF community composition. Interestingly, the plant community composition was affected by N and P, similar to the soil AMF community composition. Both, soil and root AMF community compositions were significantly correlated to plant community composition across all treatments, the correlation was however stronger for soil AMF communities (R2 = 0.55, p< 0.001). By using bipartite network analysis, we identified several fungus-plant pairs that responded consistently to treatments.</p> <p>Our results indicate that the response of grasslands to nutrient deficiencies is potentially driven by strong feedbacks between plant and belowground AMF community compositions. We here demonstrate that the known interactions between grassland plants and AMF - which are often investigated from a single plant or monoculture perspective - are major drivers of how diverse plant community compositions will respond to environmental change, such as fertilization. In conclusion, considering the ecology of the subsurface AMF communities may strongly benefit our understanding of plant communities in a future environment.</p>

<p>Arbuscular mycorrhiza (AM) fungi are associated with almost all land plants and provide soil nutrients and other benefits to their plant hosts in exchange for photosynthetic products. While fertilization regimes in managed grasslands or agricultural systems are tailored for increasing plant biomass, their potential effects on AM fungi are rarely taken into account. Nutrient-driven changes in abundance and community composition of AM fungi, however, may feedback on ecosystem performance in the long term. Therefore, it is necessary to get a better understanding on how AM fungal communities respond to changes and imbalances in soil nutrient availabilities.</p><p>Here, we evaluated how long-term nutrient deficiency of phosphorus (P), nitrogen (N) and potassium (K) affects the abundance and community composition of AM fungi in a mountainous grassland. In addition, we investigated how the responses of AM fungi to those deficiencies were modulated by liming and the type of fertilizer addition (inorganic versus organic).</p><p>Our study was carried out on a long-term nutrient deficiency experimental grassland site in Admont (Styria, Austria), established in 1946. Different fertilization treatments were applied for more than 70 years in a randomized block design, including numerous combinations of inorganic (P, N, K with/without lime) and organic (solid manure and liquid slurry) fertilizers. The hay meadow at the site is cut three times per year and biomass is not returned to the system. Therefore, biomass and nutrients have been continuously removed for decades, leading to different types of soil nutrient deficiency. In this study, we collected both root and soil samples in July 2019 and quantified AM fungi and other microbial groups by measuring neutral fatty acid (NLFA) and phospholipid fatty acid (PLFA) biomarkers, respectively. Additionally, we applied DNA and RNA-based amplicon sequencing of the 18S rRNA gene to identify AM fungal community composition.</p><p>Our data shows that deficiencies of one or more elements had a major impact on both AM fungal biomass and community composition. AM fungal biomass was higher in plots that received no fertilizers compared to inorganically fertilized plots, but lower in plots which were deficient only in certain single or multiple elements, specifically in plots fertilized with inorganic N only (i.e., deficient in P and K). Conversely, liming and organic fertilizer amendments increased AM fungal biomass compared to plots containing inorganic fertilizers without lime. Across all treatments, AM fungal biomass was positively correlated with pH and soil water content, and negatively with dissolved N compounds, indicating indirect effects via responses of other soil parameters to nutrient deficiency. Long-term nutrient deficiency also altered plant community composition, which may also have indirectly affected AM fungal communities.</p><p>We conclude that long-term nutrient deficiency, and in particular the stoichiometry of available nutrients, strongly affects the abundance and community composition of AM fungi in grassland soil. This response may be linked to changes in plant community composition or soil chemistry both as a result and as a cause, emphasizing the complexity of feedbacks determining the response of grassland ecosystems to changing nutrient conditions.</p>

<p>Soil harbours a huge diversity of soil microbes.  One reason for this is thought to be its small-scale chemical and physical heterogeneity, which offers innumerable different parallel habitats and thus niches for the development of microbial communities. However, while microbial diversity and soil microbial composition are usually assessed using gram-sized homogenized soil samples, knowledge of how microbes are actually spatially organized at smaller scales is lacking.  This not only hampers our ability to link microbial communities to their chemical environment or to functions they may mediate, but also limits our understanding of potential scale-dependent drivers of soil organic matter turnover.</p> <p>Here, we investigated fungal and bacterial community composition together with selected biogeochemical parameters across different spatial scales in a Beech forest soil. We picked around 200 individual two mm-sized soil aggregates, i.e. small soil clumps that sticked together and could be collected with a tweezer, from fourty 35 ml-sized soil cores (i.e. 4-5 aggregates per core) in two different soil depths across a 800 m<sup>2</sup> forest area. The remaining soil cores were homogenized. We carried out amplicon sequencing of the 16S rRNA gene (for bacteria/archea) and the ITS region (for fungi) from individual soil aggregates and the homogenized soil cores, and additionally measured C, N, δ<sup>15</sup>N and δ<sup>13</sup>C from both sample types.</p> <p>Microbial community compositions of individual aggregates were highly distinct from each other, and from their composite ‘parent’ soil core, exemplifying a high spatial hetereogeneity of microbial communities at that small scale. Delta<sup>13</sup>C values were constantly higher in individual aggregates compared to their composite ‘parent’  core, indicating that they were relatively enriched in C that already underwent microbial recycling. We found a striking correlation between C concentration and δ<sup>13</sup>C values across all individual aggregates, suggesting that the aggregates remain intact long enough to allow for continued degradation and recycling, and hence <sup>13</sup>C enrichment of the encapsulated C compounds. The variations of C concentration and δ<sup>13</sup>C across individual aggregates  are partially explained by variations in community composition at this small scale.</p> <p>Overall our data shows that mm-sized soil aggregates host distinct microbial communities which may be linked to a certain recycling state of C.  This suggests that aggregates are pieces of soil that stick together for a significant amount of time, allowing them to act as functional units of soil C degradation, and possibly as ‘evolutionary incubators’, as was previously hypothesized. We conclude that investigating microbial community structure and distribution at these small scales in the soil offer a promising way forward to better understand possible links between microbial community composition and soil functions.</p>

<p>Fertilization experiments provide insights into elemental imbalances in soil microbial communities and their consequences for soil nutrient cycling. By addition of selected nutrients, other nutrients become deficient and limiting for soil microorganisms as well as for plants. In this study we focused on microbial nitrogen (N) cycling in a long-term nutrient manipulation experiment. In many soils, the rate-limiting step in N cycling is depolymerization of high-molecular-weight nitrogen compounds (e.g., proteins) to oligomers (e.g., peptides) and monomers (e.g., amino acids) rather than the subsequent steps of mineralization (ammonification) and nitrification. The aim of our study was to determine whether nutrient deficiency directly or indirectly – via changes in plant carbon (C) inputs - affects soil microbial N processing.</p><p>We collected soil samples from a fertilization experiment, established in 1946 on a hay meadow close to Admont (Styria, Austria). The field experiment consisted of a full factorial combination of inorganic N, P, and K fertilization and a control with no fertilizers. Furthermore, liming (Ca-addition) and organic fertilizer application treatments (solid manure and liquid slurry) were established. In the experiment, plant biomass is harvested three times per year, inducing strong nutrient limitation in plots that have not received nutrient additions (fully deficient or deficient in a single element). We determined gross rates of microbial protein depolymerization, N-mineralization and nitrification via isotope pool dilution assays with <sup>15</sup>N-labeled amino acids, NH<sub>4</sub><sup>+</sup>, and NO<sub>3</sub><sup>-</sup>. We hypothesized that N deficiency (lack of N fertilization) would stimulate microbial N mining (depolymerization), and reduce subsequent N mineralization and nitrification. In contrast, we expected that organic fertilization would alleviate microbial C and N limitations, reducing N depolymerization rates and increasing mineralization and nitrification.</p><p>Our results show that organically fertilized and limed soils have significantly lower gross protein depolymerization rates than plots receiving inorganic N. No significant differences were found comparing gross N-mineralization and gross nitrification rates across the different treatments. Given the higher rates of protein depolymerization in inorganically fertilized soils as compared to organically fertilized and limed soils, microbial N processes seem to be controlled by plant C input and/or soil pH rather than by direct soil nutrient availability. However, depolymerization of macromolecular N does not only supply N to the soil microbial community but also organic C. Thus, the reduced plant C input compared to fully fertilized soils may have caused microorganisms to increase their mining for a C-containing energy source, thereby increasing protein depolymerization rates. In summary, this study suggests that long term nutrient deficiency or nutrient imbalances may affect soil nutrient cycling indirectly by changing plant C inputs (via reduced primary production) and/or changing soil pH, rather than directly, by nutrient availability. This further indicates that soil microbial communities are rather C than nutrient limited.</p><p> </p>