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25Little is known of the earliest stages of soil ecosystem development on volcanic ash, and how 26 this process is affected by temperature. We studied the first three years of soil development in 27 a field-based mesocosm experiment, situated in different climates across Japan. Newly fallen, 28 sterilized volcanic ash from the Sakurajima volcano (Kyushu, Japan) was placed into pots 29 and positioned at six locations with mean annual temperatures ranging from -1.6 ℃ to 18.6 ℃. 30At 24 months into the experiment, C and N accumulation showed only a weak linear 31 correlation with temperature, but by 36 months there was a clear exponential relationship. 32This applied only to the top 2 cm of the developing soil, and was not apparent in the lower 33 part of the ash. We suggest that this acceleration in warmer climates relates to a positive 34 feedback involving bryophyte cover, which had become much denser by the third year in the 35 warmer sites. Surprisingly, the abundance of 16S rRNA gene copies of bacteria, fungi, 36 archaea -as well as ammonia oxidizersdid not increase from 12 months to 36 months, and 37 did not show any relationship to temperature, suggesting that input from plants is the major 38 factor in increasing C and N buildup in the soil. Overall it appears that temperature effects on 39 bryophyte cover buildup may be important in controlling the temperature relationship in soil 40 development on volcanic ash. 41 42 Weathering, Volcanic ash 44 Synnott, 1971). 70 It is also unclear what relationship, if any, the process of soil ecosystem development 71 shows to climate -especially temperature. Temperature is seen as a major controlling factor 72 in ecosystem establishment or recovery (Pastor and Post,1986; Vitousek and Farrington, 73 1997) as well as pedogenesis (Tsai et al., 2010), but it is unclear how early in the process it 74 becomes important. In this study we were interested in experimentally comparing the effect 75 of temperature on soil ecosystem development from volcanic ash, over the first three years of 76 primary succession. This study was a follow-on from Kerfahi et al. (2017), which discussed 77 the first two years of experimental soil ecosystem development. As we will report here, a 78 pattern was obtained when Year 3 was added to the dataset, with implications for 79 understanding the processes of ecosystem establishment on volcanic ash. 80 Furthermore, we report here on the inorganic nitrogen (N) dynamics at very early 81 stage of soil development. Nitrogen is not normally present in parent materials such as rock, 82 lava and volcanic ash in certain kinds of metasedimentary and metavolcanic rocks (Holloway 83 et al. , 1998). N deposition from the atmosphere is thought to be the primary source in the 84 earliest stages of microbial community development on volcanic ash, while microbial N 85 fixation eventually becomes more important. 86Our main working hypothesis here was that there would be a temperature effect on 87 the rate of soil ecosystem development (organic C, organic N, inorganic...

Key message: Understory dwarf bamboo mitigated soil N competition with co-existing canopy oak trees by foraging in deeper soils and increasing dependence on N forms that differ from those used by canopy trees.

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There is little knowledge about microbial functional community structures and the relationships between microbial communities and nitrogen transformation processes. Here, we investigated the relationships between soil microbial communities and nitrogen mineralisation potentials in a cool temperate forest throughout the growing season. Microbial communities were assessed by quantification of the total bacterial, archaeal, and fungal gene abundances and the bacterial and archaeal amoA gene abundances, functional predictions of bacteria and fungi, and analysis of the bacterial-fungal co-occurrence network. In mid-summer, ectomycorrhizal fungal abundance was significantly higher, whereas the total bacterial abundance was significantly lower. Bacterial and archaeal amoA gene abundances were also significantly higher in mid-summer. However, regardless of the seasonal fluctuation of microbial gene abundances, the net nitrification and nitrogen mineralisation potential did not show clear seasonality. In the network analysis, the microbial community was divided into 13 modules, which were subgroups assumed to have similar niches.Furthermore, two modules that mainly consisted of microbial species of Proteobacteria and Bacteroidetes were significantly and positively correlated with the net nitrification and mineralisation potentials. Our results indicated that microbial subgroups sharing similar niches, instead of total microbial abundances and functional gene abundances, could be important factors affecting the net nitrogen mineralisation potential.

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PurposePlant roots alter nutrient cycling within the soil surrounding them (rhizosphere). Recent studies have focused on nutrient uptake by plants in low-temperature seasons. This study aimed to reveal the nutrient dynamics in rhizosphere during low-temperature seasons in a northern hardwood forest in Japan.MethodsThe potential extracellular enzymatic activity, bacterial, fungal, and archaeal abundances, and soil chemical properties in the rhizosphere of canopy trees and understory vegetation and non-rhizosphere bulk soil were measured at the beginning of the dormant season (November), end of the dormant season (April and May), and middle of the growing season (August) in a northern hardwood forest in Japan.ResultsThe abundance of fungi and the activity of nitrogen- and phosphorus-degrading enzymes were higher in the rhizosphere than in non-rhizosphere bulk soil regardless of the season. The concentration of extractable organic and inorganic N was higher in the rhizosphere than in the non-rhizosphere bulk soil at the beginning and end of the dormant season, but this trend was not observed in the middle of the growing season. ConclusionSince the concentration of nutrients in the rhizosphere is determined by the balance between nutrient uptake by fine roots and root-induced acceleration of decomposition, our results suggest that plant roots would accelerate N and P cycles during the dormant season, even though the amount of nutrient uptake by plants was lower during the season.