Investigating the Double-Impact of Soil Health Promoting Practices on Water Quality

Issue

This project seeks to answer the question: Does soil health relate to the amount of nitrogen (N) a farmer needs to add to a field?  Recent studies offer some evidence that the answer to the question is a tentative yes, showing that with greater CO₂ Burst, a measure of soil microbial activity that approximates the N-supplying power of a soil, less N needs to be applied for optimal corn growth (Fig. 1). These findings imply that soil health promoting practices (SHPPs) have potential for a “double-impact” on improving water quality. This double impact would be: 1) reduction in nitrate N loss from SHPP implementation, and 2) reduction in N fertilization over time, due to an increase in soil N-supplying power.

Objective

The goal of this project is to derive a better prediction of the economically optimum nitrogen rate, or EONR, using a ‘combination’ soil health test that uses CO₂ Burst plus additional soil measurements. These data are critical to better understand the role of soil health in improving Iowa’s water quality.

To achieve this understanding, the project will:

• determine relationships between soil health promoting practices and nitrogen-supplying power of soils; and
• elucidate factors causing variation in CO₂ Burst in Iowa soils.

Approach

Researchers will take a three-pronged approach to compare the N-supplying power of conventional practices and SHPPs in three ISU long-term experiments (Table 1). This three-pronged approach is needed due to dynamic nature of N mineralization/immobilization, high variation in the field and inconsistencies among methodologies. Using three methods of measuring N-supplying power across three SHPPs (Table 1) can take into account a variety of ways to assess the N-supplying power of a soil.

Table 1. Long-term Experiments we will be testing the Double-Impact of Soil Health Promoting Practices

Researchers will measure actual net nitrogen mineralization (N supplying power of the soil) by using in situ soil core method and comparing this field-based method to a more common laboratory-based method. 

Researchers will also collect other data like plant-extractable nutrients and iron, which have been shown to affect decomposition of soil organic matter, and thus likely could control the CO_2­ Burst vs. EONR relationship.  We will use Machine Learning to determine which variables might be interacting to explain these residuals (or deviation from the predicted trend). 

Information and findings from this project will be incorporated into outreach and extension activities and developed for peer-reviewed presentations and publications. Researchers also aim to develop:

• a new “combination” soil health test that better predicts EONR compared to CO2 Burst alone
• an economic tool that reflects the Double-Impact of soil health on water quality and farm profits