Temperature sensitivity of nitrogen mineralization in agricultural soils

As concerns continue to mount over nitrogen (N) use efficiency and the fate of excess N lost to the environment, the need for accurate assessments of the contribution of non-fertilizer sources is increasing. Nitrogen mineralization from soil organic matter (SOM) can supply large quantities of plant-available N through a growing season and is affected by numerous factors including soil temperature and soil properties. The objective of this study was to determine the temperature sensitivity of N mineralization in soils under annual crops in California and to determine whether growing region and soil properties affect the temperature sensitivity of N mineralization. Nitrogen mineralization was assessed for 55 soils from different regions in central and northern California by incubating undisturbed soil cores at 5, 15, and 25 A degrees C for 10 weeks. Modeling was done by fitting the data to both an S-shaped curve and a Q (10) function. Q (10) values averaged 2.87 and ranged from 2.67 to 3.15 in the different regions. While the magnitude of N mineralized varied greatly, the temperature response did not differ significantly across regions and was not affected by SOM content and quality, texture, electrical conductivity, or pH. Coupling the temperature response curve with daily soil temperature data measured in the field highlighted the differences in regional and seasonal mineralization rates and underscored the importance of taking soil temperature into account when determining N mineralization rates for N budgets. This is especially important in regions where crops are grown year-round.