We are now in the second year of a project investigating how to estimate nitrogen (N) mineralization in mineral and organic soils. Nitrogen mineralization is the conversion of organic forms of N, which are not plant-available, to inorganic forms of N which are plant-available, like ammonium and nitrate. Mineralization occurs as a result of microbial decomposition of nutrients. Understanding N mineralization is important because it can help us apply N fertilizers more efficiently, by accounting for soil-available nutrients. Due to high groundwater nitrate concentrations, California growers are facing increasing pressure to improve N use efficiency in an effort to reduce nitrate leaching. To maintain competitive yields, however, growers need accurate estimates of soil-available N so that they can adjust fertilizer application rates with confidence.

In Spring 2016, the project team collected soil samples from 30 fields from Tulelake to Fresno County, including five sites in the Delta (having organic soils) and four other sites in San Joaquin County (having mineral soils). All of the fields were in annual crop rotations and had no recent legume cover crops or manure applications. The Delta soils had organic matter that ranged from about 6 to 23 percent; whereas, the soils from other areas of San Joaquin County had about 1.5 to 2 percent soil organic matter (SOM). The bulk density (i.e. the mass divided by volume) of the Delta soils averaged 0.9 g/cm³ compared to the mineral soils, which averaged 1.2 g/cm³. The reason it is important to measure the bulk density is because when soil nutrients are measured, they need to be converted from concentration to lbs/acre per foot of soil depth using a conversion factor. That factor will change depending on the bulk density.

In general, N mineralization was higher in the organic soils than the mineral soils, but it also varied more across the organic soils (Figure 1). When N mineralization is expressed as a percent of total soil N, however, mineral soils were more variable. This is likely due to the fact that the SOM is more stable in Delta soils than in mineral soils, where SOM is largely derived from recent crop residues. In other words, crop residues influence N mineralization more in mineral soils than in organic soils. 

Figure 1. Net N mineralization rates of the 30 soils included in the study in 2016.

Preliminary results also show that soil temperature and other soil properties have a strong effect on N mineralization. The soil temperature effect has been modeled to show that as temperature increases, N mineralization increases exponentially. The soil properties which most influenced N mineralization included total soil N, total soil carbon (C), particulate organic C (a measure of the availability of organic matter to microbial decomposition), and pH. These soil variables are more predictive of N mineralization in organic soils than in mineral soils; more work is needed to determine which soil variables best help to predict N mineralization in mineral soils. Soil moisture likely also plays a role in N mineralization, and it will be studied in the future.

We are continuing the study again in 2017 and hope that the results will contribute to a better understanding of N mineralization in both organic and mineral soils, with the ultimate goal of developing an online decision-support tool for growers to help in estimating field-specific N mineralization rates.