nitrogen cycle in the taiga
An aliquot was transferred to a 50-mL plastic tube and kept frozen at –18°C until analysis. 2) might be of greater magnitude than that of the year-to-year variation (Fig. As seen in Table 2, in the beginning of the growing season (June until the middle of July), the inorganic N content in the soil was low. Results are expressed in delta notation (δ13C or δ15N): (1) where Rsample and Rstandard denote the isotope ratio (13C/12C or 15N/14N) of the sample and the standard, and the standard is PDB (Pee Dee Belemnite) for 13C or atmospheric N2 for 15N. Areas outlined with dashed lines represent the water-extractable part of the inorganic N pool in the soil solution. 2010). Each observational period presented in Table 1 consists of 1 to 10 individual samplings, although one was a snow sample taken during the winter after the snow cover stabilized. An increase in ammonium deposition during the warm period and an increase in nitrate deposition during the winter are consistent with the observation made in central Yakutia (Makarov 2007). It has been reported that at our study site the evapotranspiration was 1.5 to 2.3 mm per day (Ohta et al. N transformation in soil is a very dynamic process with large seasonal variation: There was a rapid increase in the soil inorganic N pool in late summer followed by consumption by soil microbiota in the fall. Bulk C and N content and isotopic ratios were assessed using an elemental analyzer (Flash EA, Thermo Scientific, Billerica, MA) coupled with an isotope ratio mass-spectrometer (Delta V, Thermo Scientific). A similar seasonal pattern for production and consumption was observed in the soil in a temperate beech (Fagus sylvatica L.) forest and explained by the availability of dissolved C for microorganisms (Kaiser et al. 2004). According to Kielland et al. The active layer depth (defined as the soil layer that thaws during the warm period) is usually 1.2–1.4 m. Soil thaw starts from the surface layer at the end of April, and the thaw depth increases until October, whereas the surface soil layer starts to freeze in September and the soil stays frozen for seven months of the year. Bottles filled with soil were returned to the original position in the soil. Briefly, soil was placed in 30-mL loosely capped bottles to keep the plant roots out and at the same time to allow the exchange of water and gases. However, some researchers have suggested there is microbial activity in the winter. 3099067 Leaching and denitrification processes are negligible or not significant in our study area, located on the permafrost (Shugalei and Vedrova 2004, Koide et al. Estimated annual total N deposition was 48 mg N m−2year−1 (Table 1), which was similar to that reported in Makarov (2007) for central Yakutia (1–40 mg N m−2 year−1) and lower than that reported for the Asian part of Russia (50–2.95 × 103 mg N m−2 year−1) (Polischuk et al. 2010), and no study on N dynamics has yet been performed. On Day 26, branches were sampled from the middle part of the labeled trees, and oven dried at 60°C; the needles were then separated. Automatic measurements of soil moisture and temperature in the larch forest were initiated by GAME-Siberia [GEWEX (The Global Energy and Water Cycle Experiment) Asian Monsoon Experiment] (Ohta et al. This study is a pioneering study of N dynamics in this area. In the fall the rate of inorganic N consumption is expected to prevail, leading to an observed decrease in the size of inorganic N pool in the beginning of each growing season. In the experiment performed in June, the maximum change observed in δ15N was +5.5‰, whereas it was +5.0‰ in July (Fig. 60 at% of 15N) and 0.018 M of non-labeled alanine. The inorganic N soil pool demonstrated clear non-linear seasonality, and it increased with soil temperature cumulative degree days (Fig. Nitrite (less than 0.31 mg N m−2) was not significant (not shown). During summer observations, sampling containers were placed under the canopy in the forest so that dry particulate matter and rainwater were collected together (n = 9). 1e and f). 1e and f). 2010) and northeastern parts of Siberia (Schulze et al. Plants use this fixed nitrogen to build amino acids, nucleic acids (DNA, RNA), and chlorophyll. Characteristics of soil moisture in permafrost observed in East Siberian taiga with stable isotopes of water, Importance of permafrost as a source of water for plants in east Siberian taiga, Soil nitrogen dynamics in larch ecosystem. The amount of water during rain events ranged from 1 to 21 mm. 2008). This is because such processes as recovery of N from needles before litterfall, which accounted for a large amount of N at the scale of individual trees in this study, is not always taken into consideration in the methods based on net primary production. 5 Howick Place | London | SW1P 1WG. Figure 1 The change in larch (Larix cajanderi Mayr.) 60 at% of 15N) and another nine trees with sodium nitrate Na15NO3 (min. Similarly to soil extracts, the amount of collected total N varied greatly from 50 to 475 mg N m−2 period−1 (Table S2). Although microbial activity during the winter has not been proven, a large soil inorganic N pool observed in late summer was consumed until the early summer of the following year. During summer observations, sampling containers were placed under the canopy in the forest so that dry particulate matter and rainwater were collected together (n = 9). Dörr et al. The needle δ13C of individual trees showed no significant change in experiments for both control and labeled trees with alanine and ammonium (data not shown). N demand and mass flow estimation were explained in sections 4.2 and 4.3, respectively. Among the samples collected during the period from 25 May to 29 July 2011, the highest deposition rate of N (35.3 mg N m−2) was observed in the sample taken for 17 to 22 July 2011, which overlapped a state of emergency declared by the local government due to a forest fire. 1995; Shibuya et al. Larch trees could not uptake organic N directly from the soil. Amino acid treated trees received 100 mL of 0.018 M of 13C-15N-alanine (min. 3). To describe variations of pools and fluxes in the ecosystem, we used the average annual N throughfall shown in Table 1, the highest observed values of bulk N and the soil inorganic N pool presented in Table 2, and the inorganic N production observed in the first incubation experiment, which covered almost one year (Table 3). Since the ratio of 13C:15N of alanine was 3:1, if this alanine was assimilated directly, the increase of heavy isotopes in the labeled trees would follow the same proportion. As we describe in Section 4.2, microbial activities are expected to be low in the first half of the growing season. Using the minimum and maximum values described above, in total roughly 850 to 3.1 × 103 mg N m−2 year−1 is the range of demand for above- and belowground production annually by a larch stand only (requirements for needles, above-, and belowground production), which has to be covered by nutrient uptake. The ion-exchange resin bags method was reported to overcome the disadvantage of chemical extractions accounting for the kinetics of nutrient release and transport (Curtin et al. The maximum net mineralization rate (Table 3) shown here was calculated from the incubation that covered almost one year. However, considering the reallocation of N from senescing larch needles, which has been reported to be about 70% (Gower and Richards 1990; Matsuura and Hirobe 2010), the estimated annual loss of N with litterfall would be only 930 to 1.2 × 103 mg N m−2 year−1. Total (inorganic and organic) C content sharply decreases with depth. In our study, the soil inorganic N pool seemed to exhibit seasonality, depending on the soil temperature. On the other hand, δ15N of needles from 15N-ammonium treated trees gradually increased beginning 1 h after the experiment started. Russian forests are a global sink accounting for 0.13 Pg C per year (Goodale et al. For each experiment nine larch seedlings (about 40 cm total height) were chosen at the tracer experimental plot: Three were used as the control, three for the 15N-ammonium treatment and three for the 13C15N-alanine treatment. Needles were collected from each tree at intervals of 1, 2, 4, 8, 12, 24, and 48 h. Ammonium and nitrate concentrations in the throughfall and soil extracts were analyzed using a continuous flow spectrophotometer autoanalyzer (Bran & Luebbe, Norderstedt, Germany). The δ15N of needles from trees labeled with 15N-ammonium (127 ‰) was about 1.5 times larger than that of trees labeled with 15N-nitrate (84‰) on Day 26 after application of the tracer, although the difference was statistically insignificant. 2) might be of greater magnitude than that of the year-to-year variation (Fig. 2001). Ivanova et al. This work was financially supported by Integrated Field Environmental Science-Global Center of Excellence (IFES-GCOE) and Grant-in-Aid for Scientific Research (Kakenhi) 21403011 granted by Japan Society for the Promotion of Science. To obtain the initial condition, the same soil was used for extraction of inorganic N with 2 M KCl. Abstract The inorganic nitrogen (N) cycle and its dynamics in the soil were observed in the ecosystem at the Spasskaya Pad experimental forest near Yakutsk in northeastern Siberia in order to estimate the N availability for the larch (Larix cajanderi Mayr.)

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