Estimating soil nitrogen mineralization for fertilizer adjustments
Due to high groundwater nitrate concentrations, California growers are facing increasing pressure to improve nitrogen use efficiency in crop production to reduce nitrate leaching. To maintain high yield levels, growers need accurate estimates of nitrogen availability that doesn’t come from fertilizer so that they can adjust fertilizer application rates with confidence. This project determined nitrogen mineralization rates in the field and lab, and data was used to develop an online tool that allows growers and crop advisers to estimate field-specific nitrogen mineralization rates. The tool has the potential to increase nitrogen use efficiency in crop production, resulting in lower risks of nitrate leaching to groundwater.
Nutrient Management Specialist, Dept of Land, Air, and Water Resources
University of California, Davis
California is a highly productive agricultural region. Its nutrient intensive production, however, has led to increased nitrate concentrations in the groundwater with a large proportion of this nitrate likely originating from fertilizer use in agriculture. California growers are now facing increasing pressure to improve nitrogen use efficiency in crop production to reduce nitrate leaching. At the same time, growers need to maintain high yield levels to remain competitive. This is only possible when non-fertilizer nitrogen inputs can be quantified accurately, so that fertilizer application rates can be adjusted with confidence. Besides residual nitrate and nitrate in irrigation water, nitrogen mineralized from organic material during the growing season is a major source of crop available non-fertilizer nitrogen.
Many biological and chemical soil tests estimating net nitrogen mineralization from organic material have been proposed over the years, but none of them has gained widespread adoption, as they are often weakly correlated with nitrogen mineralization rates measured in the field outside the area they were developed for. This is mainly due to the fact that soil microbial activity is affected by a number of climatic, soil-specific, and management related factors. One common weakness of these methods is that they do not take into account soil temperature. Soil temperature is likely an important factor determining nitrogen mineralization rates in California, as crops are planted and grown at different times during the year in regions with a wide range of climatic conditions.
The overall objective of this project was to determine nitrogen mineralization rates in the field and laboratory, as well as the temperature response of nitrogen mineralization in soils from California under annual crops. Field sites were selected in collaboration with local farm advisors in different regions of the state with widely different soils and climatic conditions. The temperature response of nitrogen mineralization were determined by incubating undisturbed soil cores from each field at different temperatures. In addition, soil samples were taken in each field twice in spring, with the two sampling events approximately 6-8 weeks apart. The increase in mineral nitrogen between the two sampling events reflects the net nitrogen mineralization rate. Data from the nearest CIMIS station was used to determine the average soil temperature between sampling events. The data from the field sampling and laboratory incubation was used to develop an online tool that allows growers and crop advisers estimating field-specific nitrogen mineralization rates during the cropping season based on the observed nitrogen mineralization rate in spring, soil properties, and climate data. The tool has the potential to increase nitrogen use efficiency in crop production, resulting in lower risks of nitrate leaching to the groundwater.