Abstract
Soil respiration (Rs) plays a critical role in the global carbon (C) balance, especially in the context of globally increasing nitrogen (N) deposition. However, how N-addition influences C cycle remains unclear. Here, we applied seven levels of N application (0 (N0), 54 (N1), 90 (N2), 126 (N3), 144 (N4), 180 (N5) and 216 kg N ha−1 yr−1 (N6)) to quantify their impacts on Rs and its components (autotrophic respiration (Ra) and heterotrophic respiration (Rh)) and C and N storage in vegetation and soil in alpine meadow on the northeast margin of the Qinghai-Tibetan Plateau. We used a structural equation model (SEM) to explore the relative contributions of C and N storage, soil temperature and soil moisture and their direct and indirect pathways in regulating soil respiration. Our results revealed that the Rs, Ra and Rh, C and N storage in plant, root and soil (0–10 cm and 10–20 cm) all showed initial increases and then tended to decrease at the threshold level of 180 kg N ha−1 yr−1. The SEM results indicated that soil temperature had a greater impact on Rs than did volumetric soil moisture. Moreover, SEM also showed that C storage (in root, 0–10 and 10–20 cm soil layers) was the most important factor driving Rs. Furthermore, multiple linear regression model showed that the combined root C storage, 0–10 cm and 10–20 cm soil layer C storage explained 97.4–97.6% variations in Rs; explained 94.5–96% variations in Ra; and explained 96.3–98.1% in Rh. Therefore, the growing season soil respiration and its components can be well predicted by the organic C storage in root and topsoil in alpine meadow of the north-eastern Qinghai-Tibetan Plateau. Our study reveals the importance of topsoil and root C storage in driving growing season Rs in alpine meadow on the northeast margin of Qinghai-Tibetan Plateau.
Highlights
Human activities such as agricultural expansion, industrial development and deforestation have enormously altered the rate of N deposition[1,2]
In 2014 and 2015, the soil temperature at 5 cm depth (ST5), soil moisture content at 10 cm depth and (SM10) soil microbial biomass carbon (SMBC), soil organic carbon (SOC) and soil total nitrogen (TN) contents were significantly increased in the N3, N4, N5 and N6 treatments relative to N0 treatment
Relative to the N5 level, the highest level of N addition (216 kg N ha−1 yr−1) decreased the vegetation, soil and ecosystem C and N storage. These results indicate that C and N storage appears to saturate at a high rate of N addition (216 kg N ha−1 yr−1) in excess of the N saturation point (180 kg N ha−1 yr−1)
Summary
Human activities such as agricultural expansion, industrial development and deforestation have enormously altered the rate of N deposition[1,2]. Eberwein et al.[16] observed that N addition increased Rs when C was abundant, while decrease Rs when C was limited The inconsistency of these results might reflect study differences in ecosystem type, nutrient limitation and the rate of N addition[17,18]. Soil temperature, soil moisture content[21,22], and C and N pools[23,24] are important factors that regulate Rs by regulating respiratory enzyme activity and substrate supplies. It is still unclear how these variables directly or indirectly control autotrophic and heterotrophic respiration. The ultimate goal of this research was to explore the underlying responses of ecosystem C storage and CO2 fluxes to N addition
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