Nitrogen is a major nutrient for all living organism on Earth and plays a central role in regulating the composition, structure, and function of ecosystems. Most nitrogen within terrestrial ecosystems is stored in soil organic matter. Total nitrogen in soils is mostly correlated with soil organic carbon content. The enrichment experiments of carbon dioxide further indicate that nitrogen limitation directly influences carbon sequestration by terrestrial ecosystems. Increased nitrogen availability normally increases productivity and biomass accumulation in ecosystems. However, few studies have been conducted to calculate soil nitrogen density and storage and to analyze its tempo-spatial variations in the Songnen Plain Maize Belt.
Based on the second national soil survey of Jilin Province conducted around 1980 and field measurements of nitrogen density taken in years from 2003 to 2006, this study was devoted to calculate the density and total storage of nitrogen in top soils (0-20 cm) of the Songnen Plain Maize Belt and investigate the spatial and temporal variations of nitrogen density and possible underlying factors during the period from 1980 to 2005, including land-use types, original nitrogen content, and soil organic carbon changes. The totals of soil nitrogen storage were (40.47 ± 1.67)×10⁸kg and (40.28 ± 1.35)×10⁸kg in 1980 and 2005, respectively. The spatial patterns of soil total nitrogen density were similar in 1980 and 2005, higher in the centre and lower in the edge parts. The average of soil total nitrogen density over the entire study area changed only marginally in 25 years, about 0.31 kg/m² in 1980 and 2005. However, the changes of soil total nitrogen density in 25 years varied with soils and land use types. Soil total nitrogen density increased in dark brown, paddy, and boggy soils. It did not change or even decreased in other soil types. Soil total nitrogen density changed a little in dry lands while it decreased obviously in paddy lands. The annual decrease rate of total nitrogen storage in top soils was 7.6×10⁵ kg/a. The change of soil total nitrogen density in 25 years was negatively correlated with its initial value in 1980, but positively correlated with the change of soil organic carbon density. Soil nitrogen sequestration potential was generally defined as the difference between the equilibrium value of soil nitrogen storage and its initial value. The new equilibrium value of soil total nitrogen density was 0.32 kg/m² in the study area. Without changes in land use and management after 1980, the soils of croplands in the study area has a potential of annually sequestrating nitrogen at the rate of 5.18×10⁶ kg/a. However, the total nitrogen storage in 2005 was 1.2×10⁸ kg lower than the potential of nitrogen sequestration. Actually, the application of nitrogenous fertilizer increased in the past 25 years according to the census data of Jilin Province. Therefore, reasonable fertilization should be practiced in this area. The management of croplands should be enhanced, especially for paddy lands converted from uplands.