Nodularia spumigena is a nitrogen-fixing cyanobacterium that forms toxic blooms in the Baltic Sea each summer and the availability of phosphorous is an important factor limiting the formation of these blooms. Bioinformatic analysis identified a phosphonate degrading (phn) gene cluster in the genome of N. spumigena suggesting that this bacterium may use phosphonates as a phosphorus source. Our results show that strains of N. spumigena could grow in medium containing methylphosphonic acid (MPn) as the sole source of phosphorous and released methane when growing in medium containing MPn. We analyzed the total transcriptomes of N. spumigena UHCC 0039 grown using MPn and compared them with cultures growing in Pi-replete medium. The phnJ, phosphonate lyase gene, was upregulated when MPn was the sole source of phosphorus, suggesting that the expression of this gene could be used to indicate the presence of bioavailable phosphonates. Otherwise, growth on MPn resulted in only a minor reconstruction of the transcriptome and enabled good growth. However, N. spumigena strains were not able to utilize any of the anthropogenic phosphonates tested. The phosphonate utilizing pathway may offer N. spumigena a competitive advantage in the Pi-limited cyanobacterial blooms of the Baltic Sea.

<p>It has been assumed for a long time that stable soil organic carbon (SOC) results from selective preservation of plant residues. Yet, a new paradigm points to a more active role of microorganisms in building SOC storage. In this context, even labile C, such as sugars, may persist in soil for a long time due to their incorporation into microbial biomass and ultimately necromass. The latter is considered as a relatively stable pool. However, little is known about the cycling of labile C through the microbial biomass and the turnover time of its residues. Unraveling the mechanisms and regulating factors would be critical for understanding SOC stabilization in soil.</p><p>We assume that the fate of labile C is mainly driven by microbial nitrogen (N) demand and supply. Specifically, we hypothesize that (1) high N demand forces microbes to decompose N-rich substances (“microbial N mining”), such as amino sugars, leading to a rapid turnover of microbial necromass, and that (2) labile C is stabilized in microbial necromass when N demand is met.</p><p>To investigate these hypotheses, we set up a greenhouse pot experiment including four treatments: (1) bare soil, (2) bare soil+N, (3) tree, and (4) tree+N. The soil is a sandy and nutrient poor forest soil from southern Finland. Trees are 1 m high pines (Pinus Sylvestris), which are supposed to induce microbial N deficiency by exuding easily degradable C compounds and by competing with microbes for mineral N. In order to follow to fate of labile C, we added trace amounts of <sup>13</sup>C labeled glucose to the soil (4 replicates per treatment). As a control to account for background variations in <sup>13</sup>C, we added <sup>12</sup>C glucose to another set of pots (4 replicates per treatment). Up to now, we sampled the soil 1 day, 3 days, 8 days, 1 month, 3 months, 6 months, 9 months, and 1 year after glucose addition. Measurements of the <sup>13</sup>C recovery in soil, microbial biomass, water extractable C, PLFA, amino sugars, and DNA are in progress.</p><p>First results indicate that the largest loss of <sup>13</sup>C tracer occurred in the unfertilized tree treatment, i.e., where N demand was high but N supply was low. Here, only 22% of the <sup>13</sup>C glucose remained after 3 month, whereas 40% remained in the fertilized tree treatment. Only small proportions of the recovered <sup>13</sup>C were present in the pool of water extractable C (<1%) and in living microbial biomass (8±3%, 3 days after glucose addition). As protection by clay minerals and aggregates is likely not a relevant process in this sandy soil, we suspect the remaining <sup>13</sup>C to be stabilized in microbial residues, but depending on N demand. We assume that microbial necromass accounts for a considerable proportion to total SOC storage, especially under conditions of adequate nitrogen supply.</p>

<p>Soil organic matter (SOM) is any material produced by living organisms at various stages of decomposition. SOM enhances soil fertility and quality and influences soil’s ability to fight against soil-borne diseases. Atmospheric CO<sub>2</sub> sequestration into SOM through improved agricultural management practices has been suggested to be a cost effective way to mitigate climate change.</p><p>The build-up of SOM is largely regulated by soil microbial activity. Soil microbes use most plant-derived C and either produce CO<sub>2</sub> or incorporate C into their biomass and after death microbial necromass may contribute to stable SOM. Arbuscular mycorrhizal (AM) fungi are one of the root colonizing soil microbes important in nutrient cycling, plant nutrition, growth and composition and maybe soil aggregation. The benefits of microbes including AM fungi should be thus utilized for climate friendly agriculture by magnifying their benefits via better agricultural management.</p><p>Cover crops use is one of the climate friendly agricultural practices. Cover crops if managed right, can provide several benefits e.g. enhanced soil C sequestration, reduced emissions from fertilizer production, weed suppression, better soil moisture retention and microbial activity. Moreover, use of diverse cover crops may favor higher soil biodiversity leading to high SOM content. In this project, plant diversity impacts on soil and root fungal community composition and microbial activity related to soil C sequestration were studied in a field experiment. In addition, special attention was given to AM fungi.</p><p>The field experiment was started in May, 2019 in Viikki Research farm, University of Helsinki. The experiment consists of seven treatments comparing four different levels of biodiversity to conventional monoculture treatments and bare fallow. Eight different species of cover crops representing four functional traits were sown under barley: 1) nitrogen (N<sub>2</sub>)-fixing + shallow rooting , 2) deep rooting, 3) N<sub>2</sub>-fixing +deep rooting and 4) no N<sub>2</sub>-fixing and shallow rooting. Barley and cover crop root samples and soil samples were collected from two growing seasons 2019 and 2020. Root samples were analyzed for AM fungal colonization %. Soil samples were analyzed for soil microbial biomass and microbial respiration in different seasons. Preliminary results showed no significant cover crop diversity effect on AM fungal colonization % in barley root in 2019. Soil microbial biomass and soil microbial respiration showed seasonal variations but not significant cover crop diversity effect. Therefore, fungal communities in soil and root will be examined using Illumina (MiSeq) sequencing targeting the fungal internal transcribed spacer (ITS) region. Soil enzyme activities and carbon use efficiency will be performed to gain insight into microbial activity. Obtained results will show if microbial community and activity is affected by either plant family composition or plant diversity.</p>

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<p>Soil C sequestration through improved agricultural management practices has been suggested to be a cost-efficient tool to mitigate climate change as increased soil C storage removes CO<sub>2</sub> from the atmosphere. In addition, improved soil organic carbon (SOC) content has positive impacts on farming though better soil structure and resilience against climate extremes through e.g. better water holding capacity. In some parts of the world, low SOC content is highly critical problem for overall cultivability of soils because under certain threshold levels of SOC, soil loses its ability to maintain essential ecosystem services for plant production. Soil organic amendments may increase soil C stocks, improve soil structure and boost soil microbial activities with potential benefits in plant growth and soil C sequestration. Additional organic substrates may stimulate microbial diversity that has been connected to higher SOC content and healthy soils.</p><p>We performed a two-year field experiment where the aim was to investigate whether different organic soil amendments have an impact on soil microbial parameters, soil structure and C sequestration.</p><p>The experiment was performed in Parainen in southern Finland on a clay field where oat (Avena sativa) was the cultivated crop. Four different organic soil amendments were used (two wood-based fiber products that were leftover side streams of pulp and paper industry; and two different wood-based biochars). Soil amendments were applied in 2016. Soil C/N analysis was performed in the autumns 2016-2018 and soil aggregate in the summer and autumn 2018, as well as measures to estimate soil microbial activity: microbial biomass, soil respiration, enzymatic assays, microbial community analysis with Biolog ®  EcoPlates and litter bag decomposition experiment. The relative share of bacteria and fungi was determined using qPCR from soil samples taken in the autumns 2016, 2017 and 2018.</p><p>Data on how the studied organic soil amendments influence soil structure and C content, as well as soil microbial parameters will be presented and discussed.</p>