Nutrients and hypoxia

Adapted from Crop and Soils Magazine, September-October 2022, By Tom Bruulsema, Plant Nutrition Canada, and Dr. Matt Helmers, Iowa State University by Dusty Sonnenberg, CCA, Ohio Field Leader, a project of the Ohio Soybean Council and soybean check-off

What is the role of plant nutrition in the Gulf of Mexico hypoxia 2022? 

The discussion of water quality issues in Ohio often revolves around what is going on with the algal bloom in Lake Erie. Ironically, a majority of Ohio’s 88 counties drain south to the Ohio River and the run-off eventually ends up in the Gulf of Mexico. That rainwater, carrying nutrients, drains to the Ohio River, then Mississippi River, and finally through the delta region into the Gulf of Mexico. In the hypoxia zone in the Gulf of Mexico, the concentration of oxygen in the bottom waters falls too low to support fish and other marine life.

What are the concerns and economic impacts of hypoxia?

The low oxygen levels in the bottom waters of the hypoxia zone cause most marine life, such as fish, shrimp and crabs to swim away to other areas. Animals unable to swim away often are stressed or killed. The northern part of the Gulf of Mexico contains almost half of the nation’s coastal wetlands and the commercial and recreational fisheries generate more than $2.8 billion annually. This area has undergone major changes due to nitrogen and phosphorus loading from run-off from the Mississippi River. The increased fertility increases plant life, which when it dies, sinks and depletes the oxygen levels in the bottom waters. Fish and shrimp that and are caught in this hypoxia zone are often smaller and there are fewer high value large ones. Since the large ones are more valuable, and there are fewer available, the overall economic return is much lower.

What are the drivers of hypoxia?

One of the main drivers determining the size of the hypoxia zone is nutrient load from the Mississippi River. Nitrogen (N) loading in May most strongly relates to the hypoxia development. The dissolved N and total N are used in forecasting. The size of the hypoxia zone per unit N load increases over time.

What is the agricultural contribution to the nutrient load?

Data from 2016 shows that agriculture land in the total watershed received 72% of the nation’s fertilizer for both N and phosphorus (P). A tool known as SPARROW, which stands for Spatially Referenced Regressions On Watershed Attributes, is used to estimate sources of nutrient load. In the 2012 SPARROW model, fertilizers accounted for 26% or the N load, and 38% of the P load. While the SPARROW model provides an estimate of the amounts and proportions of the nutrient load, it provides little in the way of assessment of the impact of management practices, particularly timing and placement, on load reduction.

What are the trends in agricultural nutrient balance on cropland?

Nitrogen inputs to cropland exceeded crop removal over the period from 1987 to 2016, but the surplus did not increase, despite rising amounts of fertilizer applied. During the same period, the deficit of phosphorus inputs relative to crop removal grew considerably. Projections indicate that these trends continue. Since 2016, crop yields and nutrient removals continue to increase more than inputs.

What management practices can contribute to mitigation?

A USDA Conservation Effects Assessment Project report noted a number of trends affecting the loss of applied nutrients. The area of cropland on which variable rate technology (VRT) was used to apply nutrients increased from 12.7 to 52.2 million acres over a 10-year period. This trend was consistent with ag retailers offering VRT services. Variable rate application can increase crop uptake of applied nutrients by better matching the rates applied to the time of plant uptake, which reduces nutrient losses.

The use of soil testing increased modestly from 56% to 60% of the cropland area. A greater increase was seen in an increase in sampling intensity of areas already soil testing. A considerable land area could still benefit from soil testing.

Reduced tillage has shown an impact on the potential run-off of nutrients. The proportion of applied nitrogen that was incorporated in the soil declined from 70% to 60% nationally. This is due to the increase in no-till and conservation tillage practices. These practices make it more difficult to incorporate fertilizers. The amounts of phosphorus applied without incorporation also increased. There is concern that the phosphorus fertilizer left on the soil surface is more likely to contribute to increased loads of dissolved reactive P in runoff if applied during partis of the year with substantial runoff risk.

Improved practices for 4R management of applied fertilizers (and livestock manure) needs to be a whole-farm systems approach. This includes improved technologies to reduce nutrient loss from manure between excretion and land application and better capture of those nutrients in forms that can be transported longer distances and readily applied as fertilizer. Soil conservation practices can also be improved by changing crop rotations to extend green cover on the soil with tillage systems that maintain soil residue cover and soil health. The starting point of nutrient loss is when the nutrients are applied. 4R Nutrient Stewardship practices can help manage this.