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

Abstract

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

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

“…Total 15 N recovery in plants in our studied non-legume forest was in line with plant N uptake fractions (ranged from 17% to 21%) observed in some other tracer experiments in forests in south China (Gurmesa et al, 2021;Wang et al, 2021). However, we found a significantly higher plant subtotal 15 N recovery in the legume forest than in the non-legume forest.…”

“…However, we found a significantly higher plant subtotal 15 N recovery in the legume forest than in the non-legume forest. High plant uptake fraction in the legume forest (28%) was comparable with the mean values from some N-limited forests (25%, Gurmesa et al, 2021) in which plant growth is limited by N. This contrasts with our hypothesis that plants in the legume-dominated forest would retain less 15 N tracer than that in F I G U R E 3 Percentage of 15 N retention rates in plants (tree, understory species, and fine roots), soil (litter layer and mineral soil layer), and ecosystem total in the legume and non-legume forests. Error bars represent standard error of means (n = 3).…”

“…However, we found a significantly higher plant subtotal 15 N recovery in the legume forest than in the non‐legume forest. High plant uptake fraction in the legume forest (28%) was comparable with the mean values from some N‐limited forests (25%, Gurmesa et al, 2021) in which plant growth is limited by N. This contrasts with our hypothesis that plants in the legume‐dominated forest would retain less 15 N tracer than that in the non‐legume forest (H1). The greater plant subtotal 15 N recovery was mainly attributed to canopy trees because 15 N retention of the understory species showed an increasing trend between the legume forest and the non‐legume forest.…”

“…It is likely that the leached N was mainly derived from soil N pool or deposition during previous years after recycling. Large loss of recycled old N was also observed in other N-rich tropical forests (Gurmesa et al, 2016(Gurmesa et al, , 2021. This may result from fast N turnover rates and high N-cycling fluxes in tropical forests (Gurmesa et al, 2016).…”

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.…”