Consistent Effects of Canopy vs. Understory Nitrogen Addition on Soil Respiration and Net Ecosystem Production in Moso Bamboo Forests

Abstract

Nitrogen (N) deposition has been well documented to cause substantial impacts on ecosystem carbon cycling. However, the majority studies of stimulating N deposition by direct N addition to forest floor have neglected some key ecological processes in forest canopy (e.g., N retention and absorption) and might not fully represent realistic atmospheric N deposition and its effects on ecosystem carbon cycling. In this study, we stimulated both canopy and understory N deposition (50 and 100 kg N ha−1 year−1) with a local atmospheric NHx:NOy ratio of 2.08:1, aiming to assess whether canopy and understory N deposition had similar effects on soil respiration (RS) and net ecosystem production (NEP) in Moso bamboo forests. Results showed that RS, soil autotrophic (RA), and heterotrophic respiration (RH) were 2971 ± 597, 1472 ± 579, and 1499 ± 56 g CO2 m−2 year−1 for sites without N deposition (CN0), respectively. Canopy and understory N deposition did not significantly affect RS, RA, and RH, and the effects of canopy and understory N deposition on these soil fluxes were similar. NEP was 1940 ± 826 g CO2 m−2 year−1 for CN0, which was a carbon sink, indicating that Moso bamboo forest the potential to play an important role alleviating global climate change. Meanwhile, the effects of canopy and understory N deposition on NEP were similar. These findings did not support the previous predictions postulating that understory N deposition would overestimate the effects of N deposition on carbon cycling. However, due to the limitation of short duration of N deposition, an increase in the duration of N deposition manipulation is urgent and essential to enhance our understanding of the role of canopy processes in ecosystem carbon fluxes in the future.

Differential Response of Soil Respiration and Total Belowground Carbon Allocation to Simulated Nitrogen and Phosphorus Deposition in Moso Bamboo Forests

“…To represent the local NHx and NOy in a realistic way, we used NH 4 Cl and NH 4 NO 3 as the N source with a ratio of NHx and NOy of 2.08:1, while KH 2 PO 4 was the P source. In each plot except NP0 plot, a mixed solution of NHx and NOy was added in a volume equivalent to 1 mm of precipitation, which accounted for 6 mm per year [26]. This water deposition amounted to less than 0.5% of the mean annual precipitation.…”

“…Soil CO 2 flux measurement started in July 2019, four months after trenching [26], and continued monthly for two years until June 2021 using a Li-Cor-8100 (Li-Cor Inc., Lincoln, NE, USA). The measurement was conducted between 9:00 and 12:00 am because the soil CO 2 fluxes during this period could represent the diurnal average [44][45][46].…”

“…A previous study already demonstrated that N forms have different impacts on RS [38]. Thus, N deposition with different N forms could potentially improve our understanding of realistic atmospheric N deposition on carbon cycling in forest ecosystems [26]. To our knowledge, no such study has been conducted to evaluate the effects of N and P depositions on TBCA in Moso bamboo forests.…”

“…Mitford cv. Pubescens) forest, an important and unique forest type in subtropical China, is well known for its amazingly rapid growth, which can achieve a maximum height of up to 20 m and a diameter at breast height (DBH) of up to 16 cm within about 60 days of the bamboo shoot emergence [25,26]. For ecological protection, natural forests are not allowed to be felled in China; thus, Moso bamboo is a valuable substitute wood resource and it can be harvested every year.…”

“…Consequently, the areas of Moso bamboo forests have increased from 3.87 million hectares during 2004-2008 to 4.68 million hectares during 2014-2018 [27,28], accounting for 15-36% of the net ecosystem production (NEP) of all forests across China [29]. Therefore, Moso bamboo forests play a critical role in regional, national, and even global carbon cycling [26,30].…”