Browsing by Author "Yano, Yuriko"

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Long-term snowpack manipulation promotes large loss of bioavailable nitrogen and phosphorus in a subalpine grassland
(2015-05) Yano, Yuriko; Brookshire, E. N. Jack; Holsinger, Jordan P.; Weaver, T.
Nutrient retention in ecosystems requires synchrony between the supply of bioavailable nutrients released via mineralization and nutrient uptake by plants. Though disturbance and chronic nutrient loading are known to alter nitrogen (N) and phosphorus (P) dynamics and induce nutrient export, whether long-term shifts in climate affect source-sink synchrony, and ultimately primary productivity, remains uncertain. This is particularly true for snow-dominated ecosystems, which are naturally subject to lags between nutrient inputs and uptake. To address how climate change may affect nutrient source-sink synchrony we examined the impacts of deepened snowpack on N and P losses in a subalpine grassland in the Northern Rocky Mountains, USA, where we have experimentally increased snowpack depths by two- and four-times ambient snow for 45 years. Long-term snow addition resulted in remarkably high levels of bioavailable-N leaching (up to 16 kg ha^-1 year^-1) that were 11-80 times higher than those under ambient snowpack. Estimated bioavailable-P losses also increased with snow addition, but to a lesser degree (up to 0.3 kg ha^-1 year^-1), indicating greater enhancement of N losses over P losses during snowmelt. Because these losses could not be explained by changes in nutrient inputs in snowpack or by changes in plant-soil turnover, our results suggest that high bioavailable-N leaching under deep snowpack originates not from a lack of N limitation of plant productivity, but rather from enhanced subnivean microbial processes followed by snowmelt leaching prior to the growing season. This is supported by reduced soil N pools in the snow treatments. Snow-dominated regions are projected to experience shifts in seasonal snowpack regime. These shifts may ultimately affect the stoichiometric balance between available N and P and future plant productivity.
Nitrate is an important nitrogen source for Arctic tundra plants
(2018-03) Liu, Xue-Yan; Koba, Keisuke; Koyama, Lina A.; Hobbie, Sarah E.; Weiss, Marissa S.; Inagaki, Yoshiyuki; Shaver, Gaius R.; Giblin, Anne E.; Hobara, Satoru; Nadelhoffer, Knute J.; Sommerkorn, Martin; Rastetter, Edward B.; Kling, George W.; Laundre, James A.; Yano, Yuriko; Makabe, Akiko; Yano, Midori; Liu, Cong-Qiang
Plant nitrogen (N) use is a key component of the N cycle in terrestrial ecosystems. The supply of N to plants affects community species composition and ecosystem processes such as photosynthesis and carbon (C) accumulation. However, the availabilities and relative importance of different N forms to plants are not well understood. While nitrate (NO3-) is a major N form used by plants worldwide, it is discounted as a N source for Arctic tundra plants because of extremely low NO3- concentrations in Arctic tundra soils, undetectable soil nitrification, and plant-tissue NO3- that is typically below detection limits. Here we reexamine NO3- use by tundra plants using a sensitive denitrifier method to analyze plant-tissue NO3- Soil-derived NO3- was detected in tundra plant tissues, and tundra plants took up soil NO3- at comparable rates to plants from relatively NO3--rich ecosystems in other biomes. Nitrate assimilation determined by 15N enrichments of leaf NO3- relative to soil NO3- accounted for 4 to 52% (as estimated by a Bayesian isotope-mixing model) of species-specific total leaf N of Alaskan tundra plants. Our finding that in situ soil NO3- availability for tundra plants is high has important implications for Arctic ecosystems, not only in determining species compositions, but also in determining the loss of N from soils via leaching and denitrification. Plant N uptake and soil N losses can strongly influence C uptake and accumulation in tundra soils. Accordingly, this evidence of NO3- availability in tundra soils is crucial for predicting C storage in tundra.
Nitrogen acquisition strategies of mature Douglas‐fir: a case study in the northern Rocky Mountains
(Wiley, 2021-01) Qubain, Claire A.; Yano, Yuriko; Hu, Jia
Nitrogen (N) limits plant growth in temperate ecosystems, yet many evergreens exhibit low photosynthetic N use efficiency, which can be explained in part by their tendency to store more N than to use it in photosynthesis. However, it remains uncertain to what extent mature conifers translocate internal N reserves or take up N from soils to support new growth. In this study, we explored N dynamics within mature Douglas-fir (Pseudotsuga menziesii var. glauca) trees by linking N uptake in field-grown trees with seasonal soil available N. We used a branch-level mass balance approach to infer seasonal changes in total N among multiple needle and stem cohorts and bole tissue, and used foliar d15 N to evaluate N translocation/uptake from soils. Soil resin-exchangeable N and net N transformation rates were measured to assess whether soils had sufficient N to support new needle growth. We estimated that after bud break, new needle biomass in Douglas-fir trees accumulated an average of 0.20 0.03 mg N/branch and 0.17 0.03 mg N/branch in 2016 and 2017, respectively. While we did find some evidence of translocation of N from older stems to buds prior to bud break, we did not detect a significant drawdown of N from previous years’ growth during needle expansion. This suggests that the majority of N used for new growth was not reallocated from aboveground storage, but originated from the soils. This finding was further supported by the d15 N data, which showed divergent d15 N patterns between older needles and buds prior to leaf flushing (indicative of translocation), but similar patterns of depletion and subsequent enrichment following leaf expansion (indicative of N originating from soils). Overall, in order to support new growth, our study trees obtained the majority of N from the soils, suggesting tight coupling between soil available N and N uptake in the ecosystem.
Snowpack influences spatial and temporal soil nitrogen dynamics in a western U.S. montane forested watershed
(2019-07-19) Yano, Yuriko; Qubain, Claire; Holyman, Zach; Kelsey, Jencso; Hu, Jia
Declines in winter snowpack have increased the severity of summer droughts in western U.S. forests, with the potential to also impact soil available nitrogen (N). To understand how snowpack controls spatiotemporal N availability, we examined seasonal N dynamics across elevation, aspect, and topographic position (hollow vs. slope) in a forested watershed in the northern Rocky Mountains. As expected, peak snow‐water equivalent (SWE) was generally greater at higher elevations and on north‐facing aspects. However, the effects of topographic position and snowdrift led to variability in snow accumulation at smaller spatial scales. Spatial patterns of the snowpack, in turn, influenced soil moisture and temperature, with greater SWE leading to generally higher soil moisture levels during the summer and smaller temperature fluctuations throughout the year. Wetter conditions in spring or fall generally supported greater inorganic N pools, but at the driest locations (low‐elevation slope), pulses of N mineralization in summer may have played important roles in overall N dynamics. More importantly, soil moisture during the summer appeared to be more influenced by antecedent snowpack from the previous year than by current‐year summer rain. Subsequently, N mineralization under snowpack may be strongly influenced by soil moisture and temperature conditions from the previous fall, before snowpack accumulation. Together, our results indicate that snowpack strongly influences N dynamics beyond the current growing season in western coniferous forests through mediation of soil moisture and temperature, and suggest that further decline in winter snowpack may affect these forests through constraints in both water and N availability.