Date Published: January 31, 2019
Publisher: Public Library of Science
Author(s): Jianbo Wang, Xiaoling Fu, Zhen Zhang, Maihe Li, Hongjie Cao, Xiaoliang Zhou, Hongwei Ni, Xiujun Wang.
This study was designed to test the hypothesis that nitrogen (N) addition leads to enhanced soil respiration (SR) in nitrogen deficient marsh. Here, we report the response of SR to simulated N deposition in a temperate marsh of northeastern China from June 2009 to September 2011. The experiment included three-levels of N treatment (control: no N addition, Low-N: 4g N m-2 y-1, and High-N: 8 g N m-2 y-1). Our study showed various responses of SR to level and duration of N addition. Yearly SR was increased by 11.8%-15.2% (P<0.05) under Low-N addition during the three years, while SR showed a strong increase by 27.5% (P<0.05) in the first year and then decreased by 4.4% (P>0.05) and 15.4% (P<0.05) in the next two years under High-N addition. Soil respiration was positively correlated with soil temperature and negatively correlated with soil water content. High-N treatment reduced soil pH value (P<0.05). The negative response of SR to High-N addition in the following two years may attribute to lower microbial activity, microbial biomass and alteration in the microbial community due to lower soil pH, which consequently leads to decreased SR. Meanwhile, we found root biomass were increased under High-N addition. This implies that the increase of autotrophic respiration was lower than the decline of heterotrophic respiration in the following two years. Our findings suggest complex interactions between N deposition and SR, which is needed to be further investigated in the future studies.
Human activities such as fossil fuel combustion and fertilizer production have been enhancing the N deposition [1,2]. It is estimated that 200 Tg N yr-1 will be emitted and then deposited to the Earth’s surface by 2050 . In Asia, reactive N deposition increased from 14 Tg N yr-1 in 1961 to 68 Tg N yr -1 in 2000 and is expected to reach 105 Tg N yr-1 in 2030 . Currently this leads to high atmospheric N deposition (NH4+ -N, NO3- -N), thus causing N saturation to an extent and influencing terrestrial ecosystems by altering the soil N availability. In general, soil N availability affects plant growth, net primary productivity in N-deficient terrestrial ecosystems. As a result, N deposition can influence root respirations [5,6], microbial composition and activities [7–9], the decomposition rate of soil organic matter (SOM) . It has been reported that N deposition can affect the CO2 exchange between the biosphere and atmosphere, thus potentially causing positive or negative feedbacks to future climate [11,12]. It is, therefore, significant to address the effects of N deposition on the fluxes of CO2.
The large seasonal variations in SR were similar in the control and N addition plots during the experiment. The strong seasonal pattern of SR has also been reported in many other studies, such as a temperate steppe, grassland [40–42] and forest [23,24,43]. We found positive exponential relationships with Tsoil in all treatment, as well as negative exponential relationships between SR and SWC, which was consistent with one previous study . In our study, Tsoil and SWC explained 50%-69% and 23%-34% variations of soil CO2 flux, respectively.
We find that soil respiration exhibited a strong seasonal pattern, with the highest rates observed during the summer (June-August) and the lowest rates during in the spring and autumn. Both soil temperature and soil water content were dominant factors on soil respiration in our study. Our results found that different responses of SR to level and duration of N addition: High-N addition significantly stimulated SR during the growing season in the first year of the experiment, and decreased SR in the following two years; however, Low-N addition continued enhancing SR rate during the three years. The negative effect in High-N plots on SR in the following two years may be attributed to lower microbial activity, lower microbial biomass and the alteration in the microbial community, which consequently inhibited heterotrophic respiration. This implies that the contribution of the autotrophic respiration was lower than the heterotrophic respiration in High-N addition treatment. Our results suggest that response of soil respiration to N deposition could be significantly influenced by the magnitude of N deposition in the temperate wetland. This requires further long-term experiments to fully quantify the consequences of nitrogen deposition on soil respiration, especially with a focus on the response to high nitrogen deposition.