Retention and partitioning of 15N-labeled deposited N in a tropical plantation forest

• Published in: Biogeochemistry 2021-02, Vol.152 (2-3), p.237-251
• Main Authors: Gurmesa, Geshere Abdisa, Mo, Jiangming, Gundersen, Per, Mao, Qinggong, Fang, Yunting, Zhu, Feifei, Lu, Xiankai
• Format: Article
• Language: English
• Subjects: Acidic soils; Acidification; Agriculture; Biogeosciences; Coniferous forests; Deposition; Earth and Environmental Science; Earth Sciences; Ecosystem recovery; Ecosystems; Environmental Chemistry; Forest ecosystems; Forest floor; Forests; Freshwater; Freshwater pollution; Growth; Hydrology; Inland water environment; Leaching; Life Sciences; Nitrates; Nitrogen isotopes; Partitioning; Pine; Plantations; Retention; Soil; Soil acidification; Soil pollution; Soil solution; Soils; Temperate forests; Terrestrial ecosystems; Tracers; Tropical climate; Understory
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-020-00750-y

The effects of deposited nitrogen (N) on forest ecosystems largely depend on the amount of N retained in the ecosystems and its partitioning among ecosystem pools. However, our understanding of the capacity of tropical plantations to retain deposited N is limited. To evaluate the retention of deposited N in a human-disturbed pine plantation in southern China and compare the result with previous findings in an adjacent old-growth forest, we added 15 N-tracer monthly to the forest floor for one year and determined its recovery in ecosystem compartments four months after the last addition. We monitored 15 N recoveries in soil solution monthly to quantify leaching losses. The pine forest retained about 58 ± 5% of the 15 N-labeled deposited N, which is lower than that reported in the adjacent old-growth forest (72 ± 6%). Both forests experience chronic N deposition (recently measured at 51 kg N ha −1  yr −1 ) and we attribute the difference in retention to effects of previous disturbance mainly understory and litter harvesting in the pine plantation. Only 3 kg N ha −1  yr −1 (5% of the 15 N-labeled deposited N) out of the measured total leaching (54 kg N ha −1  yr −1 ) originated from deposited (and labeled) N from the measurement year, suggesting that N leaching is dominated by unlabeled N sources. Furthermore, results from our study and other similar 15 N labeling experiments together show similar patterns of total ecosystem retention of deposited N in tropical and temperate forests, but here we demonstrate a decreasing retention of N with increased N deposition in these forests. Our findings indicate that plantation forests that experience human-disturbance and chronic N deposition have lower N retention compared to old-growth forests, and thus elevated N inputs in such ecosystems can cause risk of hydrological N losses, soil acidification, and freshwater pollution.