Nitrogen input ¹⁵N signatures are reflected in plant ¹⁵N natural abundances in subtropical forests in China

Natural abundance of ¹⁵N (<i>δ</i>¹⁵N) in plants and soils can provide time-integrated information related to nitrogen (N) cycling within ecosystems, but it has not been well tested in warm and humid subtropical forests. In this study, we used ecosystem <i>δ</i>¹⁵N to assess effects of increased N deposition on N cycling in an old-growth broad-leaved forest and a secondary pine forest in a high-N-deposition area in southern China. We measured <i>δ</i>¹⁵N of inorganic N in input and output fluxes under ambient N deposition, and we measured N concentration (%N) and <i>δ</i>¹⁵N of major ecosystem compartments under ambient deposition and after decadal N addition at 50 kg N ha⁻¹yr⁻¹, which has a <i>δ</i>¹⁵N of −0.7 ‰. Our results showed that the total inorganic N in deposition was ¹⁵N-depleted (−10 ‰) mainly due to high input of strongly ¹⁵N-depleted NH₄⁺-N. Plant leaves in both forests were also ¹⁵N-depleted (−4 to −6 ‰). The broad-leaved forest had higher plant and soil %N and was more ¹⁵N-enriched in most ecosystem compartments relative to the pine forest. Nitrogen addition did not significantly affect %N in the broad-leaved forest, indicating that the ecosystem pools are already N-rich. However, %N was marginally increased in pine leaves and significantly increased in understory vegetation in the pine forest. Soil <i>δ</i>¹⁵N was not changed significantly by the N addition in either forest. However, the N addition significantly increased the <i>δ</i>¹⁵N of plants toward the ¹⁵N signature of the added N, indicating incorporation of added N into plants. Thus, plant <i>δ</i>¹⁵N was more sensitive to ecosystem N input manipulation than %N in these subtropical forests. We interpret the depleted <i>δ</i>¹⁵N of plants as an imprint from the high and ¹⁵N-depleted N deposition that may dominate the effects of fractionation that are observed in most warm and humid forests. Fractionation during the steps of N cycling could explain the difference between negative <i>δ</i>¹⁵N in plants and positive <i>δ</i>¹⁵N in soils, and the increase in soil <i>δ</i>¹⁵N with depths. Nevertheless, interpretation of ecosystem <i>δ</i>¹⁵N from high-N-deposition regions needs to include data on the deposition ¹⁵N signal

Data from: Fate of atmospherically deposited NH4+ and NO3- in two temperate forests in China: temporal pattern and redistribution

The impacts of anthropogenic nitrogen (N) deposition on forest ecosystems depend in large part on its fate. However, our understanding of the fates of different forms of deposited N as well as the redistribution over time within different ecosystems is limited. In this study we used the 15N-tracer method to investigate both the short-term (1 week to 3 months) and long-term (1 to 3 years) fates of deposited NH4+ or NO3- by following the recovery of the 15N in different ecosystem compartments in a larch plantation forest and a mixed forest located in northeastern China. The results showed similar total ecosystem retention for deposited NH4+ and NO3-, but their distribution within the ecosystems (plants vs soil) differed distinctly particularly in the short-term, with higher 15NO3- recoveries in plants (while lower recoveries in organic layer) than found for 15NH4+. The different short-term fate was likely related to the higher mobility of 15NO3- than 15NH4+ in soils instead of plant uptake preferences for NO3- over NH4+. In the long-term, differences between N forms became less prevalent but higher recoveries in trees (particularly in the larch forest) of 15NO3- than 15NH4+ tracer persisted, suggesting that incoming NO3- may contribute more to plant biomass increment and forest carbon sequestration than incoming NH4+. Differences between the two forests in recoveries were largely driven by a higher 15N recovery in the organic layer (both N forms) and in trees (for 15NO3-) in the larch forest compared to the mixed forest. This was due to a more abundant organic layer and possibly higher tree N demand in the larch forest than in the mixed forest. Leachate 15N loss was minor (<1% of the added 15N) for both N forms and in both forests. Total 15N recovery averaged 78% in the short-term and decreased to 55% in the long-term but with increasing amount of 15N label (re)-redistributed into slow turn-over pools (e.g., trees and mineral soil). The different retention dynamics of deposited NH4+ and NO3- may have implications in environmental policy related to the anthropogenic emissions of the two N forms

Effects of nitrogen deposition on carbon cycle in terrestrial ecosystems of China:A meta-analysis

Nitrogen (N) deposition in China has increased greatly, but the general impact of elevated N deposition on carbon (C) dynamics in Chinese terrestrial ecosystems is not well documented. In this study we used a meta-analysis method to compile 88 studies on the effects of N deposition C cycling on Chinese terrestrial ecosystems. Our results showed that N addition did not change soil C pools but increased above-ground plant C pool. A large decrease in below-ground plant C pool was observed. Our result also showed that the impacts of N addition on ecosystem C dynamics depend on ecosystem type and rate of N addition. Overall, our findings suggest that 1) decreased below-ground plant C pool may limit long-term soil C sequestration; and 2) it is better to treat N-rich and N-limited ecosystems differently in modeling effects of N deposition on ecosystem C cycle. (C) 2015 Elsevier Ltd. All rights reserved