Sinks for Inorganic Nitrogen Deposition in Forest Ecosystems with Low and High Nitrogen Deposition in China

Abstract

We added the stable isotope 15N in the form of (15NH4)2SO4 and K15NO3 to forest ecosystems in eastern China under two different N deposition levels to study the fate of the different forms of deposited N. Prior to the addition of the 15N tracers, the natural 15N abundance ranging from −3.4‰ to +10.9‰ in the forest under heavy N deposition at Dinghushan (DHS), and from −3.92‰ to +7.25‰ in the forest under light N deposition at Daxinganling (DXAL). Four months after the tracer application, the total 15N recovery from the major ecosystem compartments ranged from 55.3% to 90.5%. The total 15N recoveries were similar under the (15NH4)2SO4 tracer treatment in both two forest ecosystems, whereas the total 15N recovery was significantly lower in the subtropical forest ecosystem at DHS than in the boreal forest ecosystem at DXAL under the K15NO3 tracer treatment. The 15N assimilated into the tree biomass represented only 8.8% to 33.7% of the 15N added to the forest ecosystems. In both of the tracer application treatments, more 15N was recovered from the tree biomass in the subtropical forest ecosystem at DHS than the boreal forest ecosystem at DXAL. The amount of 15N assimilated into tree biomass was greater under the K15NO3 tracer treatment than that of the (15NH4)2SO4 treatment in both forest ecosystems. This study suggests that, although less N was immobilized in the forest ecosystems under more intensive N deposition conditions, forest ecosystems in China strongly retain N deposition, even in areas under heavy N deposition intensity or in ecosystems undergoing spring freezing and thawing melts. Compared to ammonium deposition, deposited nitrate is released from the forest ecosystem more easily. However, nitrate deposition could be retained mostly in the plant N pool, which might lead to more C sequestration in these ecosystems.

A lagged variable model for characterizing temporally dynamic export of legacy anthropogenic nitrogen from watersheds to rivers

“…2c) in the upper Jiaojiang watershed, wood product export, and forest biomass storage may account for a considerable proportion of the N imbalance. Assuming that~15 % of NANI (or 33 % of atmospheric N deposition) was exported by wood products and forest biomass storage as observed in eastern China (Sheng et al 2014), denitrification would by difference account for the fate of~51 % of NANI in the upper Jiaojiang watershed. This denitrification percentage is similar to the sum of agricultural land denitrification (i.e., 36-4 % of total N applied) (Yan et al 2011;Ti and Yan 2013) and in-stream denitrification (i.e., 10-35 % of total N input to rivers) (Yan et al 2011;Chen et al 2012) in the surrounding region.…”

Contrasting effects of ammonium and nitrate inputs on soil CO2 emission in a subtropical coniferous plantation of southern China

“…A positive correlation between soil CO 2 flux and soil NO 3 − -N content is generally observed at large spatial scales (Koba et al 2003;Inselsbacher et al 2010). Based on a 15 N tracer experiment in the subtropical forests of southern China, Sheng et al (2014) also reported that more 15 NH 4 + was recovered from organic and mineral soils, while a large proportion of 15 NO 3 − was recovered from plant roots. In the early stage of N saturation, most of the deposited N is sequestered within the vegetation biomass (Bowden et al 2004), and the increased C allocation to root systems could increase autotrophic respiration.…”

The contrasting effects of deposited NH4+ and NO3− on soil CO2, CH4 and N2O fluxes in a subtropical plantation, southern China

“…On the contrary, deposited NO 3 − is very mobile and is absorbed by plants and soil microbes (Inselsbacher et al, 2010). Based on a 15 N tracer experiment in the subtropical forests of southern China, Sheng et al (2014) also reported that more 15 NH 4 + was recovered from organic and mineral soils, while a large proportion of 15 NO 3 − was recovered from plant roots. In the early stage of N saturation, most of the deposited N is sequestered within the vegetation biomass (Bowden et al, 2004), and the increased C allocation to root systems could increase autotrophic respiration (Kou et al, 2015).…”

“…The subtropical plantation forests are vulnerable to increased N deposition due to single community structure and barren soil fertility. Previous studies show that simulated NH 4 NO 3 inputs to the subtropical forests significantly inhibit litter decomposition and soil CO 2 emission (Fang et al, 2007;Mo et al, 2008), decrease CH 4 uptake (Zhang et al, 2008b(Zhang et al, , 2012b, and increase gaseous N emission (N 2 O, NO, and N 2 ) (Zhang et al, 2008a(Zhang et al, , 2009 (Sheng et al, 2014). The contrasting effects of N cumulation on root autotropical respiration and microbial heterotropical respiration dominate the reponses of soil CO 2 flux to N addtion .…”