Global warming has become an indisputable fact. Nitrous oxide (N₂O) not only contributes to global climate changes due to its greenhouse effect, but also participates in many photochemical reactions in the atmosphere and depletes stratospheric ozone. Therefore, more and more researchers attach a great importance to the study on N₂O emissions. Forest soils can function as either a source or a sink of atmospheric N₂O and perform a vital function in the processes regulating global climate change. Therefore, long-term observations of N₂O emissions from forests soil in subtropical China are needed.

Forests are a major component of the terrestrial ecosystem, covering approximately one third of the global land surface. Masson pine is a species of pine that is native to a wide area of central and southern China. It is considered the most important commercial species for fiber and solid wood products. The Masson pine is also a common tree grown for fast-growing plantation forestry in southern China. Compared to numerous observational data available for greenhouse gas emissions in temperate and tropical forests, limited information on N₂O emissions from the Masson pine-growing region has been reported in subtropical central China.

Using the static chamber-gas chromatographic method, a team of researchers from China Agricultural University, the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) spent three years to measure N₂O emissions from the Masson pine forest soil.

The researchers found that the N₂O fluxes over the three years varied from -36.0 to 296.7 μg N m-2 h-1, averaging 18.4 ± 5.6 μg N m-2 h-1 (n = 3). The average annual N₂O emissions were estimated to be 1.6 ± 0.3 kg N ha-1 year-1. The N₂O fluxes exhibited clear intra-annual (seasonal) variations as they were higher in summers and lower in winters. "Compared with other forest observations in the subtropics, N₂O emissions at our site were relatively high, possibly due to the high local dry/wet N deposition. In addition, N₂O emissions were mostly sensitive to variations in precipitation and soil ammonium N content. Consistent with other studies, we may speculate that subtropical Masson pine forest soils are a source of atmospheric N₂O, which are likely to be primarily attributed to the soil nitrification process.” said LI Yong, a professor in ISA.

The researchers also developed a multiple linear regression model to estimate the daily N₂O fluxes by using a group variable of "Season" and four other key N₂O-controlling variables. Such a model presented an acceptable accuracy in predicting daily N₂O fluxes (multiple R² = 0.40, Radj² = 0.34, RMSE = 0.81, P < 0.001) and explained over one third of the total variation in the log-transformed daily N₂O fluxes from Masson pine forest soils.

The research was supported by the National Basic Research Program of China (No. 2012CB417105), the International Partnership Program for Creative Research Team of Chinese Academy of Sciences/the State Administration of Foreign Experts Affairs of China (Nos. KZCX2-YW-T07 and 20100491005-8), and the Hundred Talent Program of Chinese Academy of Sciences.

The study entitled "Nitrous oxide emissions from a Masson pine forest soil in subtropical central China" has been published in Volume 25, issue 2, February 2015 of Pedosphere.