Improving the Capacity to Detect Load Reductions

Aug 2016


Detection of nutrient load reductions is difficult. Yet, Iowans increasingly seek evidence of rapid water quality improvements. To meet this demand, new approaches must be developed to detect load reductions that result from management improvements on the landscape.


The overall goal of this project is to improve the ability to model watershed-scale nitrogen budgets so that Iowans can assess changes in nitrogen loads with greater accuracy and speed. This will include improving nitrogen (N) budgets by linking field measurements of corn grain N concentration, soybean grain N concentration, and soybean N fixation to the APSIM process-based cropping systems model. The Agricultural Production Systems sIMulator (APSIM) is internationally recognized as a highly advanced simulator of agricultural systems. The outputs will be used to develop statistical models of nitrate load and flow-weighted concentration.


Six watersheds will be selected based on a number of factors researchers believe contribute to relatively rapid measurement of trends in nitrate load and flow- weighted concentration. Such factors include size, duration of nitrate measurements, soil type, availability of farm management data, and level of implementation of the Iowa Nutrient Reduction Strategy. Within each watershed, researchers will measure grain N concentration and soybean N fixation on representative soil types across several farm management factors. Data will be used to calibrate and validate the APSIM cropping systems model. These model outputs will be input into statistical models that interpret variations in nitrate load and flow-weighted concentration.

Project Updates

October 2019


Soybean biological nitrogen fixation, soybean grain nitrogen concentration and corn grain nitrogen concentration were measured and modeled, based on experiments at Iowa State University research farms in Ames, Nashua and Crawfordsville. In addition to these sites where intensive measurements were taken, data was included from measurements of grain nitrogen concentration in late summer on several private farms from different watersheds. 

The results, including experiments and a literature survey, demonstrate that soybeans typically fix less nitrogen from the atmosphere than is harvested in grain. Hence, the nitrogen outputs from the soybean phase of Iowa cropping systems exceed the nitrogen inputs. Extra nitrogen required by soybeans can be derived from soil organic matter or residual nitrogen fertilizer from the previous corn crop.

Using the experiments and computer model simulations, we addressed several objectives: First, we compared three different methods to measure soybean nitrogen fixation:

  1. comparisons of two soybean varieties that are genetically similar except for the ability fix nitrogen;
  2. comparison of N uptake by soybean and a non-fixing plant growing nearby;  
  3. isotopic pool dilution.

The methods rely on a technique that measures the isotopic ratio of nitrogen in the plants (methods i and ii) and the soil (method iii) to determine if it is derived from the atmosphere or the soil. Methods i and iii measured across multiple locations and over two years agreed well with one another, providing similar estimates of nitrogen fixation. The consistency between these methods provides confidence that estimates of soybean nitrogen fixation are accurate. This allowed creation of a simple statistical model to predict soybean nitrogen fixation using aboveground biomass. The model is the linear equation y = 0.011x + 18.11 where y is kg N fixation per hectare and x is kg of dry aboveground biomass per ha. The model fit the data with r2 = 0.51 and p < 0.01. Because aboveground biomass can be accurately estimated with the harvest index (i.e., the ratio of grain to total aboveground biomass), which is highly consistent across soybean varieties and locations, the equation offers a simple yet relatively accurate way to estimate soybean nitrogen fixation from yield data. This is a major improvement upon conventional approaches that assume nitrogen fixation equals nitrogen harvest in grain. 

The new capacity to more accurately estimate soybean nitrogen fixation has been incorporated into nutrient budgets at the HUC 8 watershed scale and also improved the ability to estimate the corn and soybean grain nitrogen concentration in these budgets as well as manure inputs. The next steps will be to publish improved nitrogen budgets and assessments of nitrogen-use efficiency at the HUC 8 watershed scale.