Methane (CH4) is the second most important greenhouse gas in terms of global warming potential. Methane is produced by methanogenic archaea in anaerobic conditions and consumed by methanotrophic bacteria in well aerated soil, mainly in forests and grasslands. Methane accumulation in the atmosphere has slowed and become erratic in the last two decades after several decades of exponential increase. A bottom-up approach, of the root causes of methane production and consumption on local and ecosystem scales, is needed to better predict future trends.
The study here presented is the first comprehensive attempt to understand the methane cycle of a Pine forest. Eighteen plots were established at each of two sites, located 40m apart, with different drainage regimes and understory vegetation, but continuous overstory tree composition. Nitrogen was added incrementally across the growing season in two different concentrations of NH4NO3. The sites were monitored for soil and air temperature, soil moisture, precipitation and soil oxygen throughout the growing season. CH4 flux was measured, and soil was extracted for NH4 and NO2+NO3 concentration measurements, on the day before fertilization, the day of fertilization and finally two days after fertilization. Each CH4 flux measurement was weighted for statistical analysis by 100 times the inverse of the standard error of d[CH4]/dt, based on four concentration readings by GC-FID at each of three time points.
Results/Conclusions
A weighted, full factorial ANOVA was performed on the CH4 flux measurements including site, month, block and whether the measurements were taken on the day of fertilization or two days later. It was found that drainage conditions and overall soil moisture played the greatest role in controlling methane flux, but that there was also an interaction between the site (drainage) and the nitrogen treatment added. When a weighted ANCOVA was performed on CH4 flux with the same variables as above as well as the covariates: precipitation, soil moisture, soil temperature, [NH4], and [NO2+NO3]. Soil temperature was the only covariate to significantly impact the variation in the model, and indeed even took over the variation explained by site and site by nitrogen treatment. It was concluded that soil moisture is the only factor that explains CH4 flux in a Pine forest soil. Therefore, CH4 flux can more accurately be predicted by including predictors of soil moisture (vegetation, drainage and precipitation regimes) into local, regional and global climate models.
