Phosphorous is a key nutrient promoting plant growth, and a standard ingredient in many fertilizer blends.  But if too much phosphorous ends up in the region’s watershed, it can wreck havoc on wetlands, streams, and lakes,  causing harmful algae blooms (like the extreme cyanobacteria algae blooms in Lake Erie last year) and depletion of dissolved oxygen, leading to sometimes-massive die-offs of fish and other aquatic life.

Thus, phosphorus levels in agricultural watersheds are often carefully regulated.  But, as a team of Canadian scientists reports in a brand-new paper in the peer-reviewed journal Frontiers in Ecology and the Environment, the impact of rising glyphosate use on phosphorous loading needs to be taken into consideration, especially where Roundup Ready crops account for a high percent of cropland use.

Glyphosate acid contains 18.3% phosphorous (P) by mass, so farmers are applying additional P to their fields every time they spray a glyphosate-based herbicide (GBH).  But, glyphosate has an additional impact on P cycling, because it is chemically very similar to the phosphate ions that are the primary form of P found in nature.  So, glyphosate and phosphate ions compete for the same soil binding sites.

Once all binding sites are “full,” any additional P — from fertilizer, animal manure, or herbicide — tends to flow off fields in surface-water runoff.

The result is that in areas of heavy glyphosate use, additional phosphorous is ending up in surface water resources.

In other words, “[glyphosate] application inevitably leads to greater anthropogenic P inputs in the agricultural landscapes where it is used, and potentially to greater P export from soils to water bodies via diverse pathways” (Hébert et al., 2019).

The authors made some calculations to determine the trends in P inputs from glyphosate use at both a US-wide and global scale, using publicly-available datasets (i.e.  from the FAO, USDA and USGS).  They also took a closer look at data for 3,055 counties in the U.S., where they factored in changes in agricultural land cover to control for any potential impact from expansion of farmed acres.

Their analysis showed that from 1994 to 2014, global P inputs from glyphosate use increased from 10,300 to 151,300 metric tons per year, a 15-fold increase.  In the U.S. alone, P from glyphosate increased 8-fold from 2,900 to 23,000 metric tons over the same 20-year period.

The study authors make the case that while still just a fraction of the phosphorous load coming from fertilizers and manure, these numbers show that the P input from GBHs is not insignificant, and in fact “P inputs from glyphosate use have now reached levels comparable to those from sources for which P regulations were initiated in the past” (Hébert et al., 2019).

Thus, they argue, P inputs from glyphosate use should be included in management decisions in areas characterized by heavy GBH use.  They also recommend adjusting the maximum, allowed glyphosate application rate on any given farm based on local soil conditions, since such rates will determine how much P the herbicide is likely to shed into the system.  Plus, farmers could apply glyphosate and phosphorous-containing fertilizers at different times to minimize the amount of P that ends up in surface waters.

Source:

Hébert M-P, Fugère V, Gonzalez A.” The overlooked impact of rising glyphosate use on phosphorus loading in agricultural watersheds,” Frontiers in Ecology and the Environment, 2019, 17:48-56. doi:10.1002/fee.1985.