Highlights

• Nitrogen mineralization increased exponentially with increasing temperature
• N mineralization in soils with a high soil organic matter content could be well predicted
• Soil properties were less successful predicting N mineralization in soils from the Central and Salinas valleys

The Team

Ken Miller, Brenna Aegerter, Nick Clark, Michelle Leinflder-Miles, Gene Miyao, Richard Smith, Rob Wilson, Daniel Geisseler

Background

Due to high groundwater nitrate concentrations, California growers are facing increasing regulatory pressure to improve N use efficiency in crop production to reduce nitrate leaching. To maintain high yield levels, growers need accurate estimates of crop available N that does not come from fertilizer so that they can adjust fertilizer application rates with confidence. Soil N mineralization is affected by climatic factors, soil properties, and crop management. With all these factors affecting N mineralization, estimating site-specific mineralization rates is a challenging task. Currently, no widely applicable and reliable method is available to estimate N mineralization rates for cropping systems in California.

Objectives

The objectives of this study were to assess the magnitude and range of N mineralization rates from soil orgnaic matter in California soils under annual crop rotations and to relate N mineralization rates to measured soil properties and temperature in order to develop a predictive model.

Our Approach

In spring 2016 and 2017, we collected undisturbed soil cores from 57 fields under annual crops located in the northern half of California. The soil organic matter contents in these soils differed widely, ranging from 1-23%. The soil cores were incubated at optimal moisture content and different temperatures for 10 weeks. A number of analyses were performed to characterize the physical and chemical properties of the soil mineral and organic fractions. A greenhouse trial was also included in the study. Tall fescue was grown in pots and harvested several times during the 7-month trial.

Results

The amount of N mineralized in undisturbed soil cores incubated at 25 °C for 10 weeks varied considerably. Using multiple linear regression, a model based on total soil C and N, particulate organic C and N and sand content was best in predicting N mineralization rates in soils from the Sacramento-San Joaquin Delta and Tulelake basin with a high soil organic matter content. These variables explained 95% of the variability in N mineralization in these soils.

For soils from the Central and Salinas valleys with a low soil organic matter content, FDA hydrolysis, a measure for soil enzyme activity, and pyrophsphate extractable Fe were the best predictors. However, this model only explained about half of the variability in N mineralization. Total N was a poor predictor of N mineralization in these soils. Interactions with soil minerals and cropping history may strongly affect N mineralization rates in soils with a low soil organic matter content.

The N taken up by tall fescue in the greenhouse trial was closely related to the N mineralized in the incubation study. However, the laboratory study over-predicted the amount of N available for the plants.

Nitrogen mineralization increased exponentially within the investigated temperature range of 5-25 °C. The temperature response differed little across regions and was not affected by soil properties.