| Forest ecosystem carbon accumulation mainly depends on the balance between the total ecosystem productivity and total ecosystem respiration and is typically an order of magnitude smaller than respiration or photosynthesis. Therefore, quantifying forest ecosystem respiration components and their relations to environmental controls is essential for building forest ecosystem carbon model and accurately assess the role of forest ecosystem in the global carbon cycle. Ecosystem respiration is composed of autotrophic and heterotrophic components whose contributions to total respiration vary in space and time. Soil respiration and stem respiration are the main components of forest ecosystem respiration. With the increased global atmospheric nitrogen deposition levels, exploring how soil respiration and stem respiration affected by nitrogen addition in the form of deposition or fertilization is very important for a deep understanding forest ecosystem carbon balance and tree carbon balance. In general, net primary productivity in terrestrial ecosystems is often nitrogen (N) limited. Therefore, increased nitrogen deposition has the potential to alter the balance between primary productivity and ecosystem respiration. However, the effects of N addition on soil respiration and stem respiration are still poorly understood. The aim of this study is to investigate how the two main components of forest ecosystem respiration response to nitrogen addition and the potential mechanisms. In this study, response of simulated nitrogen deposition on soil and stem respiration in a40-year-old Larix gmelinii plantation in Northeastern China was studied. Nitrogen deposition experiments were initiated in April2011. Four nitrogen deposition treatments (in three replicates) were established:CK (without N added), Low-N (5g N·m-2a-1), Medium-N (10g N·m-2a-1), High-N (15g N·m-2a-1). Twelve plots, each with10m×20m dimensions were set up, surrounded by an approximately15m wide buffer strip. During each application, fertilizer was weighed, mixed with50L of water, and applied to each plot below the canopy using a backpack sprayer. Two passes were made across each plot in order to ensure an even distribution of fertilizer. The Control plot received50L water without N. We used an infrared gas exchange analyzer (LI-COR6400) to quantify the effects of simulated nitrogen deposition on soil surface CO2flux and stem surface CO2flux during the growing seasons from2011to2013. Our results showed that:(1) The total amount of atmospheric nitrogen deposition from precipitation was19.16kg N hm-2in2011.(2) During the early simulated nitrogen deposition experiment in2011, simulated N deposition significantly promoted soil respiration rates and cumulative soil surface CO2flux during the growing seasons (P<0.05). Average of soil respiration rates was estimated to be 2.35,2.43,2.57and2.70μ mol·m·s-1in the CK, Low-N, Medium-N and High-N stands, respectively. Cumulative soil surface CO2flux during the growing seasons was estimated to be365.1,370.8,381.5and397.6g C·m-2in the CK, Low-N, Medium-N and High-N stands, respectively. Simulated N deposition also significantly promoted Q10,and Q10was estimated to be3.10,3.22,3.25and3.32in the CK, Low-N, Medium-N and High-N stands, respectively.(3) During the late simulated nitrogen deposition experiment in2012and2013, simulated N deposition significantly decreased soil respiration rates, cumulative soil surface CO2flux and Q10during the growing seasons (P<0.05). Average of soil respiration rates during2012and2013were estimated to be2.67and2.90u. mol·m-2·s-1,2.83and2.65μ mol·m-2·sS-1,2.39μ mo·m2·s-1and2.50μ mol·m-2s-11,2.43and2.21· mol·m·s-11in the CK, Low-N, Medium-N and High-N stands, respectively. Cumulative soil surface CO2flux during2012and2013were estimated to be395.3and378.5g C·m-2,405.5and362.6g C·m-2,375.2and346.7g C·m-2,362.4and325.8g C·m-2in the CK, Low-N, Medium-N and High-N stands, respectively. Simulated N deposition also significantly reduced Q10, and Q10during2012and2013were estimated to be3.53and3.49,3.16and3.25,3.35and3.07,3.10and2.92in the CK, Low-N, Medium-N and High-N stands, respectively. Respiration rates (per gram of soil dry weight) of laboratory-incubated, root-free soils suggested that N deposition significantly reduced soil microbial respiration rates, soil microbial respiration at0-10cm soil depth was10.8,8.6,8.2and7.0mg C kg·soil-1h-1, respectively. Soil microbial biomass carbon explained an average of77%variations of soil respiration rates.(4) Greater differences were observed in estimating the fine root production and turnover rate by minimum-maximum calculation, balancing transfers method and the ingrowth core method. The results from balancing transfers method and ingrowth core method showed that N deposition significantly reduced fine root production and turnover rates (P<0.05), but the results from balancing transfers method and ingrowth core method showed that N deposition did not significantly fine root production and turnover rates (P>0.05). Moreover, fine root biomass (<2mm) explained an average of82%variations of soil respiration rates.(5) During the growing seasons from2011to2013, our results showed that simulated N deposition significantly promoted stem respiration rates and cumulative stem surface CO2flux during the growing seasons (P0.05), but it may vary depending on the rate of N deposition. Cumulative stem surface CO2flux during2011was estimated to be63.7,60.2,72.5and79.0g C·m-2in the CK, Low-N, Medium-N and High-N stands, respectively. Cumulative stem surface CO2flux during2012was estimated to be67.8,72.3,78.1and86.5g C·m-2in the CK, Low-N, Medium-N and High-N stands, respectively. Cumulative stem surface CO2flux during2013was estimated to be73.6,81.1,82.9and95.2g C·m-2in the CK, Low-N, Medium-N and High-N stands, respectively. Simulated N deposition also significantly promoted Q10(P<0.05), and Q10during2011was estimated to be1.97,1.95,1.97and2.05in the CK, Low-N, Medium-N and High-N stands, respectively. Q10during2012was estimated to be1.67,1.80,2.01and2.54in the CK, Low-N, Medium-N and High-N stands, respectively. Q10during2013was estimated to be1.72,1.54,1.88and2.20in the CK, Low-N, Medium-N and High-N stands, respectively. Furthermore, simulated N deposition also significantly promoted stem nitrogen concentration (P<0.05), stem nitrogen concentration was estimated to be1.00,1.32,2.02and2.35g kg-1in the CK, Low-N, Medium-N and High-N stands, respectively. And stem nitrogen concentration explained an average of38.3%variations of stem respiration rates.With the increased global atmospheric nitrogen deposition levels, our results suggested that N addition may decrease cumulative soil surface CO2flux during the growing seasons, and promote stem surface CO2flux during the growing seasons in Larix gmelinii plantations in Northeastern China. Moreover, our results emphasized that the duration of simulated N deposition experiments should be considered, when quantifying the response of soil respiration to simulated N addition. Therefore, this research has important implication for a deep understanding temperate forest ecosystem carbon balance and tree carbon balance. |
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