Date Published: September 27, 2018
Publisher: Public Library of Science
Author(s): Shixing Zhou, Yuanbin Xiang, Liehua Tie, Bohan Han, Congde Huang, Dafeng Hui.
Soil respiration is the second largest terrestrial carbon (C) flux; the responses of soil respiration to nitrogen (N) deposition have far-reaching influences on the global C cycle. N deposition has been documented to significantly affect soil respiration, but the results are conflicting. The response of soil respiration to N deposition gradients remains unclear, especially in ecosystems receiving increasing ambient N depositions. A field experiment was conducted in a natural evergreen broadleaf forest in western China from November 2013 to November 2015 to understand the effects of increasing N deposition on soil respiration. Four levels of N deposition were investigated: control (Ctr, without N added), low N (L, 50 kg N ha−1·a−1), medium N (M, 150 kg N ha−1·a−1), and high N (H, 300 kg N ha−1·a−1). The results show that (1) the mean soil respiration rates in the L, M, and H treatments were 9.13%, 15.8% (P < 0.05) and 22.57% (P < 0.05) lower than that in the Ctr treatment (1.56 ± 0.13 μmol·m−2·s−1), respectively; (2) soil respiration rates showed significant positive exponential and linear relationships with soil temperature and moisture (P < 0.01), respectively. Soil temperature is more important than soil moisture in controlling the soil respiration rate; (3) the Ctr, L, M, and H treatments yielded Q10 values of 2.98, 2.78, 2.65, and 2.63, respectively. N deposition decreased the temperature sensitivity of soil respiration; (4) simulated N deposition also significantly decreased the microbial biomass C and N, fine root biomass, pH and extractable dissolved organic C (P < 0.05). Overall, the results suggest that soil respiration declines in response to N deposition. The decrease in soil respiration caused by simulated N deposition may occur through decreasing the microbial biomass C and N, fine root biomass, pH and extractable dissolved organic C. Ongoing N deposition may have significant impacts on C cycles and increase C sequestration with the increase in global temperature in evergreen broadleaf forests.
The amount of carbon dioxide (CO2) released through soil respiration can reach 68–100 Pg C each year , and slight changes in soil respiration can significantly alter CO2 concentrations in the atmosphere [2, 3]. It is well known that a complex array of biotic and environmental factors, such as temperature, precipitation, soil moisture, root biomass, oxygen, changes in microbial C use efficiency and substrate availability, affect soil respiration [4–7]. Global changes (such as global warming, elevated CO2 and nitrogen (N) deposition) may also substantially mediate soil respiration [7–9].
In our study, we found that soil respiration followed a clear seasonal pattern in this natural evergreen broadleaf subtropical forest, and it was higher in summer and lower in winter (Fig 3). The clear seasonal pattern of soil respiration can be reflected by similar seasonal changes in soil temperature (Fig 2A). Many previous studies found that soil temperature and soil moisture were two of the most important environmental parameters that controlled the temporal variations in soil respiration [3, 4, 26, 28, 40]. In our study, we found that soil temperature explained 72–82% of the monthly variations, while soil moisture explained 47–61% of the monthly variations in soil respiration; adding soil moisture to a model with only soil temperature increased the R2 values from 0.72–0.82 to 0.79–0.83 (Table 2). Soil temperature was found to be more important than soil moisture in controlling soil respiration in our study. Generally, in most forest ecosystems, soil temperature is the dominant factor in soil respiration [26, 43]. In our study, this finding may be due to the relatively high annual rainfall in this zone (1700 mm) and the strong water-holding capacity of the soil during the monitoring period (soil moisture ranged from 22.7 ± 0.8% to 33.1 ± 2.2%, Fig 2B). As a result, relatively high soil moisture was maintained, and the humidity conditions in the soil were suitable for plant root and microbial activities. However, soil temperature can directly affect plants and microbial activities and indirectly change soil moisture and the amount and quality of soil organic matter [44, 45]. Therefore, the soil temperature becomes the dominant factor that influences soil respiration in this natural evergreen broadleaf forest.
Our results highlight that simulated N deposition significantly affects soil respiration, and this was most pronounced in the M and H treatments despite the high rates of ambient N deposition. The negative impacts of N deposition on soil respiration may occur via negative impacts to plant and microbial activities (i.e., decreasing the MBC, MBN, fine root biomass, and DOC contents). Furthermore, N deposition also decreased the Q10 value. Ongoing N deposition may have a potentially significant impact on C cycles and could increase C sequestration with the increase in global temperature, thereby threatening the long-term persistence of evergreen broadleaf forest ecosystems in western China.