Purifying effect: ditches and small streams at the edge of fields make a significant contribution to reducing pollutants in the water from agriculture, as a study confirms. The bacteria that settle in the small streams primarily promote the breakdown of nitrate and thus have a significant influence on the nitrogen content in rivers and lakes, but also in the groundwater.
In intensive agriculture, large amounts of nitrate-containing fertilizers are usually used. Although the plants consume part of the nitrate, what is not decomposed in the soil through denitrification ends up unfiltered in the groundwater and the surrounding rivers and lakes. The excess nitrate thus also pollutes the drinking water.
Cleaning effect underestimated
In addition to the amount of fertilizer applied, the plants and the soil conditions, there could be other factors that influence how much nitrate enters the groundwater. One of them are the small ditches and streams on the edge of some fields. Because they function as drainage systems that collect the water from agricultural land and allow it to flow away again, and thus represent a link in the water cycle. These man-made or redesigned bodies of water on the edge of fields can collect up to 70 percent of the water in agricultural catchment areas.
But how do the watercourses near the fields influence the water quality? Zhe Wang from the University of Bayreuth and his colleagues have addressed this question. To do this, they analyzed the sediment of a stream that is influenced by agricultural land. On the one hand, they determined the points at which water flowed in or out, and on the other hand, they identified which characteristic types of bacteria accumulated in the stream bed and how this influenced the nitrate content.
Found nitrate “eaters” of various species
The balance of the small bodies of water turned out to be a lot more positive than expected: “The part that these streams play in cleaning the water has been significantly underestimated by research,” reports senior author Tillmann Lüders from the University of Bayreuth. As it turned out, huge amounts of nitrate-degrading microorganisms colonize the stream bed of the water bodies examined. “They reduce environmentally harmful nitrate to gaseous nitrogen,” explains Lüders. During this process, also known as nitrate respiration, the nitrogen can escape into the atmosphere and no longer pollute the soil and groundwater. According to the research team, bacteria that break down nitrate in this classic way were mainly found in the stream where most of the water drained into the environment.
At the inflow points, on the other hand, the researchers found an increased number of so-called chemolithoautotrophic nitrate degraders of the genera Sulfuricurvum and Thiobacillus. These sulfur bacteria are able to oxidize sulfur. For this, however, they do not use atmospheric oxygen, as is usually the case, but nitrate. In this reaction, too, nitrate is broken down and can escape into the atmosphere as molecular nitrogen. The diversity of species of nitrate-degrading bacteria found in the field streams indicates that the circulation processes of nitrogen and sulfur in the stream bed of an agriculturally used watercourse are coupled, Wang and his colleagues suspect. In any case, one thing is certain: the creeks near farmland play an important role in breaking down excess nitrate.
The groundwater also benefits
The colonization of the ditches by organisms that break down harmful nitrate is closely related to another phenomenon that the researchers have now been able to show: the small streams not only absorb water from the landscape, but also give water back to the surrounding groundwater at the same time away. Conversely, this groundwater can also flow back into the stream downstream. As a result, more than 80 percent of the water flowing in the ditch can be exchanged over a flow distance of a few 100 meters. “These ditches and streams play a part in the self-cleaning powers of an entire water catchment area that should not be underestimated,” reports Wang.
The cleaning effect of the nitrate-degrading microorganisms is therefore not limited to the water bodies directly next to the farmland, but also influences the groundwater and adjacent rivers. The studies thus offer important starting points for a more sustainable design of agricultural landscapes: In future, the peripheral areas of agricultural land can be designed in a targeted manner so that nitrate pollution from agriculture can be reduced more efficiently. This would benefit the adjacent rivers and also the ground and drinking water. “We’ve encountered a previously unknown intertwining of hydrology and microbiology that ecological landscape design should pay more attention to in the future,” says Wang.
Source: University of Bayreuth; Specialist article: Water Research, doi: 10.1016/j.watres.2022.118334