Effects of nitrogen addition on the branch CO₂ efflux of Larix principis-rupprechtii

Objective Branch CO₂ efflux is one of the important components of stand carbon release. Studying the change of branch CO₂ efflux of Larix principis-rupprechtii under simulated nitrogen deposition could provide a theoretical basis for the management of carbon sequestration and sink increase of L. principis-rupprechtii forest under the background of nitrogen deposition.

Method 25-year-old and 32-year-old plantations of L. principis-rupprechtii were selected. Four nitrogen addition treatments, i.e. control (CK, 0 kg/(ha·year)), low nitrogen (N1, 75 kg/(ha·year)), medium nitrogen (N2, 150 kg/(ha·year)) and high nitrogen (N3, 225 kg/(ha·year)) were set. From June to October in 2021, the branch CO₂ efflux was monitored in situ using LI-8100A, and the branch samples were collected to determine the nitrogen content.

Result (1) The CO₂ efflux and air temperature of L. principis-rupprechtii branches basically showed a “single-peak” monthly change, and the peak appeared from June to August. The air temperature could explain the changes of branch CO₂ efflux of two stands by 37%−82% and 40%−70%, respectively.(2) The average branch CO₂ efflux of L. principis-rupprechtii at 25-year-old and 32-year-old from June to October showed an increasing trend with the increase of N addition intensity, but only differed significantly under N3 treatment (P < 0.05). The CO₂ efflux of CK, N1 and N2 treatments at 25-year-old was significantly higher than that at 32-year-old (P < 0.05). The temperature sensitivity (Q₁₀) of branch CO₂ efflux was decreased by N addition except for 32-year-old plantations under N1 treatment. (3) Nitrogen addition significantly increased the 25-year-old branch nitrogen content; there was no significant change in shoot nitrogen content in 32-year-old branch nitrogen content (P > 0.05). There was a significantly negative linear relationship between the branch CO₂ efflux of L. principis-rupprechtii and the branch nitrogen content at both ages (P < 0.01). The nitrogen content can explain 16% (25-year-old) and 32% (32-year-old) variation of branch CO₂ efflux.

Conclusion The branch CO₂ efflux is affected by air temperature, nitrogen addition and forest age. All three factors should be considered when constructing a tree carbon release model of L. principis-rupprechtii.