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[MIS09-08] Long-term fate of atmospheric deposited nitrogen: 15N recovery 10 years after application of 15N tracer in a red pine forest
Keywords:15N tracer, red pine forest, δ15N, 15N recovery, soil
Elucidating the mechanisms of nitrogen retention in forests is essential for the evaluation of ecosystem functioning in forests under environmental changes (e.g., increasing nitrogen deposition). We measured δ15N and 15N recovery in each pool of vegetation and soil 10 years after the addition of 15N tracer in a red pine forest in the northern Kanto region exposed to high load of atmospheric N deposition to determine its fate. The long-term effect of atmospheric N deposition was evaluated by comparing 15N recovery 10 years and 1 year after its addition.
The experiment was conducted in a red pine forest at Akagi Testing Center of Central Research Institute of Electric Power Industry located in the Gunma prefecture. We set plots of 15N addition and control (20 m2) and sprayed ammonium chloride solution labelled with 15N (0.5 kgN ha-1) in the former plot. In January 2013, 10 years after the 15N addition, the aboveground parts of the red pine trees (discs of trunks, branches, leaves, and cones); O layer (mostly organic matter); and soil samples at 0–10, 10–20, and 20–40 cm depth along with fine roots were collected. The δ15N was analyzed using a mass spectrometer and its recovery was calculated for each pool by dividing the 15N amount in the pool, which was calculated from biomass or content, N concentration, and δ15N, by the added 15N amount.
The δ15N of each part of the trees was significantly higher in plots where 15N was added than in the control plot. The tree ring formed by separation of the xylem after every 5 years during the last 15 years did not affect δ15N. δ15N of other organs of trees was higher in plots with added 15N than in the control plot. The δ15N in each organ showed that fine roots registered the highest (9.7–10.0‰) values, whereas other organs had negative values. This was probably attributed to 15N dilution due to the larger nitrogen pool in the aboveground parts. On the other hand, δ15N of each tree organ was considerably lower than that collected 1 year after 15N addition, indicating that 15N was diluted and moved to the O layer pool due to the mortality of leaves and fine roots and gradually replaced natural nitrogen (14N). The δ15N of the O layer and soil was higher in plots with added 15N, and maximum value was observed at 0–10-cm depth. The δ15N of the O layer dramatically decreased compared to that collected 1 year after 15N addition. This might be attributed to rapid litter decomposition and lower δ15N in the newly supplied litter. The δ15N of the soil was similar to that collected 1 year after 15N addition. 15N recovery in the trees and O layers was 2% and 0.2%, respectively, suggesting that the deposited nitrogen did not accumulate in the vegetation and moved to the soil in the long-run. On the other hand, sum of 15N recovery in the soil was 32%, accounting for the majority of 15N recovery in the whole ecosystem (34%). There was no decrease in the 15N recovery in the soil compared to that 1 year after 15N addition. This study demonstrated that soil, rather than vegetation or the O layer, plays an important role in retaining N in the forest ecosystem from a long-term viewpoint. Furthermore, a decline in tree biomass due to pine wilt disease would accelerate the reduction in the function of nitrogen retention by vegetation via decreased uptake/transportation and decreased nitrogen pool. However, 15N recovery in the whole ecosystem decreased from 84% to 34% during 9 years from 1 year to 10 years after 15N addition, implying that a substantial amount of nitrogen was lost from the forest soil.
The experiment was conducted in a red pine forest at Akagi Testing Center of Central Research Institute of Electric Power Industry located in the Gunma prefecture. We set plots of 15N addition and control (20 m2) and sprayed ammonium chloride solution labelled with 15N (0.5 kgN ha-1) in the former plot. In January 2013, 10 years after the 15N addition, the aboveground parts of the red pine trees (discs of trunks, branches, leaves, and cones); O layer (mostly organic matter); and soil samples at 0–10, 10–20, and 20–40 cm depth along with fine roots were collected. The δ15N was analyzed using a mass spectrometer and its recovery was calculated for each pool by dividing the 15N amount in the pool, which was calculated from biomass or content, N concentration, and δ15N, by the added 15N amount.
The δ15N of each part of the trees was significantly higher in plots where 15N was added than in the control plot. The tree ring formed by separation of the xylem after every 5 years during the last 15 years did not affect δ15N. δ15N of other organs of trees was higher in plots with added 15N than in the control plot. The δ15N in each organ showed that fine roots registered the highest (9.7–10.0‰) values, whereas other organs had negative values. This was probably attributed to 15N dilution due to the larger nitrogen pool in the aboveground parts. On the other hand, δ15N of each tree organ was considerably lower than that collected 1 year after 15N addition, indicating that 15N was diluted and moved to the O layer pool due to the mortality of leaves and fine roots and gradually replaced natural nitrogen (14N). The δ15N of the O layer and soil was higher in plots with added 15N, and maximum value was observed at 0–10-cm depth. The δ15N of the O layer dramatically decreased compared to that collected 1 year after 15N addition. This might be attributed to rapid litter decomposition and lower δ15N in the newly supplied litter. The δ15N of the soil was similar to that collected 1 year after 15N addition. 15N recovery in the trees and O layers was 2% and 0.2%, respectively, suggesting that the deposited nitrogen did not accumulate in the vegetation and moved to the soil in the long-run. On the other hand, sum of 15N recovery in the soil was 32%, accounting for the majority of 15N recovery in the whole ecosystem (34%). There was no decrease in the 15N recovery in the soil compared to that 1 year after 15N addition. This study demonstrated that soil, rather than vegetation or the O layer, plays an important role in retaining N in the forest ecosystem from a long-term viewpoint. Furthermore, a decline in tree biomass due to pine wilt disease would accelerate the reduction in the function of nitrogen retention by vegetation via decreased uptake/transportation and decreased nitrogen pool. However, 15N recovery in the whole ecosystem decreased from 84% to 34% during 9 years from 1 year to 10 years after 15N addition, implying that a substantial amount of nitrogen was lost from the forest soil.