Rieko Urakawa

Microbial denitrification dominates nitrate losses from forest ecosystems

Significance Nitrogen (N) losses from terrestrial ecosystems can occur as inert forms or heat-trapping greenhouse gases, and via nitrate (NO3−) leaching to drainage waters, which can contribute to eutrophication and anoxia in downstream ecosystems. Here, we use natural isotopes to demonstrate that microbial gaseous N production via denitrification is the dominant pathway of NO3− removal from forest ecosystems, with gaseous N losses that are up to ∼60-fold higher than those based on traditional techniques. Denitrification becomes less efficient compared with NO3− leaching in more N-polluted ecosystems, which has important implications for assessing the connections between terrestrial soils and downstream ecosystems under rising anthropogenic N deposition. Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N2) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO3−) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6–30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO3− leaching, pointing to widespread dominance of denitrification in removing NO3− from forest ecosystems across a range of conditions. Further, we report that the loss of NO3− to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.

Changes in nitrogen transformation in forest soil representing the climate gradient of the Japanese archipelago

Net nitrogen transformation was investigated under different climate conditions by soil transplantation and in situ incubation of forest surface soils using the resin-core method. Selected conditions were considered to reflect those of the natural climate gradient in the Japanese archipelago. Study sites were established in natural forests in northern Hokkaido (Uryu), northern Kanto (Kusaki), central Kinki (Kamigamo), and southern Kyushu (Takakuma), representing the northernmost to the southernmost island regions of Japan. Field experiments comparing soils incubated at “native” and “transplanted” sites were conducted from June 2008 to May 2009. Net production, accumulation, and leaching of soil ammonium (NH4+) and nitrate (NO3−) were measured at each of the sites during the growing season (June–October), the dormant season (November–May), and throughout the year. Net nitrate production was highest in Kusaki soil, especially during the growing season, whereas net ammonium production was highest in Uryu soil, the coldest site, especially during the dormant season. Net nitrate production increased significantly in soils transplanted to a warmer climate during the growing season. However, net ammonium production increased in soils transplanted to colder climates during the dormant season. These findings indicate that, with the exception of the infertile soil samples from Kamigamo, the range of natural climates in Japan has a significant effect on nitrogen availability in surface soil. In addition, the original characteristics of the nitrogen cycle of the surface soil from each native site were retained, even when marked changes in soil temperature (approximately 8°C) occurred after transplantation.

Seasonality of factors controlling N mineralization rates among slope positions and aspects in cool-temperate deciduous natural forests and larch plantations

This study aimed to evaluate the spatial patterns of soil nitrogen (N) transformations in relation to slope aspect and position, and to investigate the main factors controlling N transformation patterns during both the growing and dormant seasons in cool-temperate deciduous natural forests and larch plantations in eastern Hokkaido, northern Japan. Net rates of N mineralization (NRminN) and of nitrification (NRnit) in surface soils on north-facing and lower slopes were higher than those on south-facing and upper slopes, whereas the net rate of ammonium-N production (NRamm) on south-facing and upper slopes was higher than that on north-facing slopes in both the natural forests and larch plantations. Both NRminN and NRnit were higher in the growing than in the dormant season, whereas NRamm was higher in the dormant season. The soil C/N ratio, water content, soil pH and frequency of freeze–thaw cycles were important variables affecting N transformation patterns in any season. In relation to seasonality, the solar radiation index, daily temperature range and earthworm biomass were important controlling factors only during the growing season, and watershed area and soil N concentration only during the dormant season, suggesting that biological control accompanied with wet–dry events were important factors affecting N transformations during the growing season, but that run-off water and chemical controls were important determinants of spatial variation in N transformations during the dormant season.

Effects of soil origin and current microclimate conditions on nitrogen mineralization in forest soil on different slope aspects in Hokkaido, Japan

Climate change may alter the rate of soil N transformation. Therefore, it is important to investigate how climate conditions and soil properties affect soil N transformation. In the present study, soil transplantation experiments were performed using soils on a xeric south-facing slope and a mesic north-facing slope in cool-temperate broad-leaved natural forests. Soil N transformation rates and leaching between slopes were compared using the resin-core method to clarify whether soil history (soil origins) or current environmental condition (locations) is the most important factor affecting soil N dynamics. The annual N mineralization did not differ significantly among soil from different origins and locations. In both locations, the annual net ammonification in south-facing soils was higher than that in north-facing soils, whereas the annual nitrification of north-facing soil was higher than that of south-facing soil. N mineralization and nitrification in north-facing soil were significantly higher during the growing season. N mineralization in south-facing soil was not significantly different between seasons. The interaction effect among seasons, soil origin, and location on net ammonification was significant. Net ammonification was higher in south-facing than in north-facing soils, and on south-facing than on north-facing slopes during the dormant season, suggesting that environmental change during winter affected the ammonification of south-facing soil. During the dormant season, N mineralization and leaching were not enhanced in soil of either origin at the transplanted locations, compared with the original locations, suggesting that, in this region, snow regime changes might not enhance the risk of N loss from forest ecosystems.

Stream Runoff and Nitrate Recovery Times After Forest Disturbance in the USA and Japan

To understand mechanisms of long-term hydrological and biogeochemical recovery after forest disturbance, it is important to evaluate recovery times (i.e., time scales associated with the return to baseline or predisturbance conditions) of stream runoff and nitrate concentration. Previous studies have focused on either the response of runoff or nitrate concentration, and some have specifically addressed recovery times following disturbance. However, controlling factors have not yet been elucidated. Knowing these relationships will advance our understanding of each recovery process. The objectives of this study were to explore the relationship between runoff and nitrate recovery times and identify potential factors controlling each. We acquired long-term runoff and stream water nitrate concentration data from 20 sites in the USA and Japan. We then examined the relationship between runoff and nitrate recovery times at these multiple sites and use these relationships to discuss the ecosystem dynamics following forest disturbance. Nitrate response was detected at all study sites, while runoff responses were detected at all sites with disturbance intensities greater than 75% of the catchment area. The runoff recovery time was significantly correlated with the nitrate recovery time for catchments that had a runoff response. For these catchments, hydrological recovery times were slower than nitrate recovery times. The relationship between these two recovery times suggests that forest regeneration was a common control on both recovery times. However, the faster recovery time for nitrate suggests that nitrogen was less available or less mobile in these catchments than water.