Soil Aggregate Size Distribution Alters CO 2 but not N 2 O Emission Rates in Chinese Fir ( Cunninghamia lanceolata ) Plantations with N and P Additions

Understanding the impacts of atmospheric nutrient deposition is very important for forest soil greenhouse gas (GHG) emissions and predicting future trends. While nutrient deposition is known to affect soil properties and GHG emissions, the roles of soil aggregate sizes and their distributions in affecting GHG emissions remain largely unknown. Here, the effects of N (0, 50, or 100 kg N ha −1 year −1 ), P (0 or 50 kg N ha −1 year −1 ), and soil aggregate size (< 0.25 mm, 0.25–0.5 mm, 0.5–1 mm, 1–2 mm) on soil CO 2 and N 2 O emissions were studied through a 7-year continuous N and P addition experiment in a 12-year-old Chinese fir plantation. Soil CO 2 emission rates, N 2 O emission rates, net N mineralization rate, net nitrification rate, and soil properties were measured in the laboratory soil incubation experiments. Soil chemical properties varied with different aggregate size, and net soil N mineralization rate increased significantly with aggregate size ( F = 16.7, P < 0.0001). Smaller aggregates had higher peak ( F = 74.9, P < 0.0001) and cumulative CO 2 emissions. Soil N 2 O emissions increased with N addition ( F = 18.5, P < 0.0001) but not with aggregate size ( F = 0.1, P = 0.9843). Compared with control treatment, higher N addition treatment increased both peak and cumulative N 2 O emissions, while soil N 2 O emissions from soil with lower N addition treatment did not differ from control treatment. Our results show that soil aggregate size interacts with N and P addition to alter GHG emissions. N 2 O and CO 2 responded differently to N, P, and aggregate size. Higher N addition alone caused the largest cumulative emissions of N 2 O (8.90 ± 2.67 ng g −1 ), while cumulative CO 2 emissions were only affected by particle size (peak of 81.66 ± 3.02 μg g −1 with < 0.25 mm particle size). The effects of N, P, and related changes in aggregate size on GHG emissions should be considered in future studies on soil GHG budgets and mitigation.