Unexpected high retention of¹⁵N‐labeled nitrogen in a tropical legume forest under long‐term nitrogen enrichment

Abstract

The responses of forests to nitrogen (N) deposition largely depend on the fates of deposited N within the ecosystem. Nitrogen‐fixing legume trees widely occur in terrestrial forests, but the fates of deposited N in legume‐dominated forests remain unclear, which limit a global evaluation of N deposition impacts and feedbacks on carbon sequestration. Here, we performed the first ecosystem‐scale 15N labeling experiment in a typical legume‐dominated forest as well as in a nearby non‐legume forest to determine the fates of N deposition between two different forest types and to explore their underlying mechanisms. The 15N was sprayed bimonthly for 1 year to the forest floor in control and N addition (50 kg N ha−1 year−1 for 10 years) plots in both forests. We unexpectedly found a strong capacity of the legume forest to retain deposited N, with 75 ± 5% labeled N recovered in plants and soils, which was higher than that in the non‐legume forest (56 ± 4%). The higher 15N recovery in legume forest was mainly driven by uptake by the legume trees, in which 15N recovery was approximately 15% more than that in the nearby non‐legume trees. This indicates higher N‐demand by the legume than non‐legume trees. Mineral soil was the major sink for deposited N, with 39 ± 4% and 34 ± 3% labeled N retained in the legume and non‐legume forests, respectively. Moreover, N addition did not significantly change the 15N recovery patterns of both forests. Overall, these findings indicate that legume‐dominated forests act as a strong sink for deposited N regardless of high soil N availability under long‐term atmospheric N deposition, which suggest a necessity to incorporate legume‐dominated forests into N‐cycling models of Earth systems to improve the understanding and prediction of terrestrial N budgets and the global N deposition effects.

The Stimulation Effect of Nitrogen‐Addition on Soil Nitrogen Leaching Weakens Over Time in an Alpine Forest

“…The fluxes in our study site were comparable to those that were observed in temperate forests at lower altitudes (Geng et al., 2020; Michalzik et al., 2001; Neff & Asner, 2001). Both low and high N addition tended to increase TDN leaching from both organic and mineral soil layers; which is consistent with previous findings (Fang et al., 2009; Magill et al., 2000; Mao et al., 2022; Song et al., 2017, 2019). TDN leaching from the organic layer under high N addition was significantly greater than that under low N addition, supporting the first hypothesis that a higher dosage of N addition had a greater effect on TDN leaching.…”

“…We noted that the method of zero‐tension lysimeters has its limitations in the TDN fluxes calculation. However, this is a relatively simple and highly effective method for collecting soil solution and examining the nutrient losses, as evidenced by its extensive use in many studies (Fang et al., 2009; Mao et al., 2022; McDowell et al., 2004).…”

Patterns and drivers of atmospheric nitrogen deposition retention in global forests

“…Additionally, understory vegetation plays an important role in the absorption and cycling of 15 N fertilizers in forest ecosystems (Chang & Preston, 2000). However, there are relatively few measurements of 15 N retention in understory vegetation (Buchmann et al., 1996; Li et al., 2019; Liu, Yu, et al., 2017; Mao et al., 2022; Wang, Chen, et al., 2021), which poses a risk of underestimating 15 N retention in forest ecosystems.…”

“…Among these, the 15 N tracer method integrates the isotopic composition of external N inputs, the processes of N transformation, assimilation and loss, as well as the isotopic fractionation during internal migration, aiding in determining the availability of different N forms and the competitive absorption of N by plants and soils (van Cleve & White, 1980). Therefore, the 15 N tracer method has been widely utilized to investigate N retention and allocation in forest ecosystems (Koopmans et al., 1996; Mao et al., 2022; Perakis et al., 2005).…”

Multi‐ecosystem services differently affected by over‐canopy and understory nitrogen additions in a typical subtropical forest

“…The effects of elevated N deposition on N retention capacity in subtropical and tropical forests were hot topic but still under debates. Soil and plant play dominant roles in retaining added N according to the results of 15 N addition experiment, but the N retention capacity, indicated by 15 N recovery percentage, varies greatly from 16% to 42% in soil and from 6% to 60% in plants among different studies (Gurmesa et al, 2016(Gurmesa et al, , 2021Liu et al, 2017;Mao et al, 2022;Sheng et al, 2014;Wang et al, 2018;Wang, Chen, et al, 2021). The divergent retention capacities of N in soil and plants are mainly associated with N role in providing plant nutrient retention service and is sensitive to atmospheric N deposition.…”