Improving the Capacity to Detect Load Reductions

Date: 
Aug 2016

Issue

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.

Objective

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.

Approach

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

FINAL REPORT: 

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.

December 2018

During this period, field trials were harvested that will allow measurement of soybean nitrogen fixation and grain nitrogen concentration at three locations in Iowa: Ames, Nashua and Crawfordsville. This is the second year of the study. At each location, three planting dates provide three different growing environments in which to assess N fixation and grain nitrogen concentration. Thus, the 2017 data represent three locations x three plantings to equal nine environments. This provides a range of data, which allows better prediction of fixation and grain nitrogen concentration. At present, plant samples from all field trials in 2017 were processed for nitrogen isotope analyses to estimate nitrogen fixation. The samples were then shipped to a service lab that performs analyses for isotope ratio mass spectrometry. The data has been received and nitrogen fixation and grain nitrogen harvest are being calculated for the 2017 growing season.

The 2018 plantings were harvested, ground and processed for nitrogen isotope analyses. Analyses of the 2017 data is being completed prior to submitting the 2018 samples for isotope analyses to ensure procedures are optimized.

All analyses and data interpretation are expected to be complete by June 2019. There is a delay between harvest and data analysis/interpretation because nitrogen isotope analyses require significant time for sample preparation and data interpretation. However, the data will support major advances in understanding of soybean nitrogen fixation.

Because the grant was awarded in July 2016-June 2018, it included two field seasons 2017 and 2018. A no-cost extension will allow completion of the 2018 season.

September 2018

This is the second year of the study. During this period, field trials were maintained to allow measurement of soybean nitrogen fixation and grain nitrogen concentration at three Iowa locations: Ames, Nashua and Crawfordsville. At each location, there were three planting dates, to  create three different growing environments. Thus, our 2017 data represent three locations x three plantings to equal nine environments. This range of data allows better predictions of fixation and grain nitrogen concentration. 

At present, plant samples from all field trials in 2017 were processed for nitrogen isotope analyses to estimate nitrogen fixation. The samples were then shipped to a service that lab that performs analyses for isotope ratio mass spectrometry. The data is being processed to calculate nitrogen fixation and grain nitrogen harvest for the 2017 growing season. The 2018 plantings are in good condition and soybeans were harvested (crops are harvested at physiological maturity rather than 13 percent moisture). 

Next, the samples are being prepared for nitrogen and isotope ratio mass spectrometry analyses and interpretation, expected to be complete by June 2019. There is a delay between harvest and data analysis/interpretation because nitrogen isotope analyses require significant sample preparation and time. The data is expected to allow major advances in understanding of soybean nitrogen fixation. 

Note: The grant was awarded in July 2016 - June 2018, which included two field seasons (2017 and 2018), so a no-cost extension was filed to allow completion of the 2018 season.

June 2018

Field trials were planted at three locations to allow researchers to measure soybean nitrogen fixation and grain nitrogen concentration. Three planting dates were used at each location, which creates three different growing environments in which to assess N fixation and grain nitrogen concentration. Plant samples from all field trials in 2017 were processed for nitrogen isotope analyses to estimate nitrogen fixation. The samples next were shipped to a service lab that performs analyses for isotope ratio mass spectrometry. Researchers are reviewing all 2017 data, calculating nitrogen fixation and grain nitrogen harvest.

March 2018

Plant samples from all field trials in 2017 were processed for N isotope analyses to estimate N fixation. The samples then were shipped to a service lab that will perform analyses for isotope ratio mass spectrometry. Once all data are received, the team will calculate N fixation by soybean for the 2017 growing season.

December 2017

Harvest of corn and soybean from the research farm and producer farm trials was completed this quarter. After yields were recorded, grain and stover samples were prepared for analysis of nitrogen concentrations (corn and soybeans) and isotope ratios (soybeans). These analyses will provide grain N concentrations to help predict grain N export, plus estimates of soybean nitrogen fixation. Data analysis is expected to be complete by April 2018.

 

September 2017

Soybean nitrogen fixation and grain nitrogen concentration/harvest were measured at three locations — Ames, Nashua and Crawfordsville. There were three planting dates at each location, creating three different growing environments in which to assess N fixation and grain nitrogen concentration. Plant samples from all field trials are being processed in the lab and prepared for N isotope analyses to estimate N fixation. 

March 2017

Three experiments will measure and model soybean biological nitrogen fixation, soybean grain nitrogen concentration and corn grain nitrogen concentration. The experiments are located on Iowa State University research farms in Ames, Nashua and Crawfordsville. In addition to these sites where intensive measurements will be taken, work is underway to identify farms that will permit extensive measurements of grain nitrogen concentration in late summer. 

December 2016

During this reporting period, the research team visited University of Iowa IIHR collaborators to identify monitored watersheds where improved nitrogen balance parameters would lead to improved load change modeling by the IIHR group. This discussion led to the identification of five potential watersheds: Clear Creek in Oxford; Otter Creek in Elgin; English River in Kalona; South Fork in New Providence; and Boone in Boone. The team began to identify farms in each watershed where measurements can begin in 2017 on soybean biological nitrogen fixation, soybean grain nitrogen concentration, and corn grain nitrogen concentration. In addition to measurement of these properties on farms in those watersheds, researchers will make more comprehensive measurements of these same properties on ISU research farms. 

 

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