Mineral Fertilizers have Advantages over Slurry and Manure

Merike Kissa, agronomist from the Department of Field Crop and Grassland Husbandry, is taking leached water out from the mini-lysimeters with a vacuum pump at the Estonian University of Life Sciences’ Eerika experimental station. Author: Karin Kauer.
Merike Kissa, agronomist from the Department of Field Crop and Grassland Husbandry, is taking leached water out from the mini-lysimeters with a vacuum pump at the Estonian University of Life Sciences’ Eerika experimental station. Author: Karin Kauer.

Fertilization is often associated with different environmental problems. It is generally assumed that manure and slurry are more environmentally friendly compared to mineral fertilizers, but a doctoral thesis recently defended at the Estonian University of Life Sciences shows that it is not always true.

Not all nutrients from slurry are taken up by plants and if precipitation exceeds evaporation considerably, like in Estonia, leaching takes place according to the author of the thesis, Mailiis Tampere. Leaching is the movement of nutrients down the soil profile with percolating water. “Those nutrients can reach ground and surface water, cause eutrophication and spoil the quality of drinking water,” she explained.

The second problem related to fertilization that Tampere and her colleagues studied was the effect of slurry use on antibiotic resistance genes in grassland soil. A large part of antibiotics and their residues are excreted by animals, which means that they will inevitably get into the environment with slurry after fertilization. “There is a danger that antibiotics may stimulate the spread of antibiotic resistance in the soil and also increase the resistance problem in the human population. It is a very serious issue, as the spread of antibiotic resistance makes both human and animal treatment very difficult,” Tampere noted.

Nitrogen leaching depends on the magnitude of yield

The study showed that cattle slurry is less effective in grasslands compared to mineral fertilizer; nitrogen leaching was higher with slurry. Mineral fertilizer increased sward yield more and as plants used more water for a higher yield, water percolation together with nutrient leaching was reduced.

Tampere found surprising that nitrogen leaching in grasslands was similar without fertilization and in the case of using cattle slurry. This shows that there must be a substantial amount of available nitrogen in the soil capable of leaching. The reason why soil nitrogen was not taken up by plants and leached may have been that it leached in the form of dissolved organic nitrogen, which cannot be used by plants and microbes.

It also became clear that nitrogen leaching was similar with nitrogen rates of 60, 120 and 180 kilograms per hectare. “Therefore, it can be stated that increasing nitrogen rates does not increase leaching when it is efficiently used by growing vegetation,” Tampere said.

The second previously mentioned problem was studied as well—cattle slurry and its digestate were a considerable source of antibiotic resistance genes. When applied to the soil, the slurry and its digestate increased the abundance of resistance genes, Tampere said. However, it was discovered that there were antibiotic resistance genes present in soil fertilized with mineral fertilizer and in case of no fertilization as well. “The persistence of different resistance genes in the soil environment was different. Genes that can persist in the environment longer may also pose a greater threat in relation to the spread of antibiotic resistance,” she emphasised.

Biochar can keep Water and Nutrients in the Soil

A substance called biochar has been proposed for the alleviation of the impact of climate change and nutrient leaching. It is formed during the heating of organic material in an oxygen-limited environment. When applied to the soil, its estimated persistence there lasts hundreds to thousands of years. Tampere and her colleagues tested the effect of this substance on nutrient leaching in Estonian environmental conditions.

“We found that biochar reduces nitrate leaching and this effect emerged both with the use of organic and mineral fertilizers, but also when no fertilizers were used. Biochar reduced water percolation and increased the nitrogen uptake of plants,” Tampere noted.

Further Questions

Based on the results of her thesis Tampere recommended for the reduction of leaching to keep grasslands’ sward yield high, as high-yielding plants take up more water and nutrients. “It appeared that sward yield can be increased by including legumes like clover into the seed mixture. Legumes fix atmospheric nitrogen and thereby increase the sward’s nitrogen supply and increase the yield,” she said.

In addition to interesting results, several new questions that certainly need further investigation arose during the research. For example, it was discovered that cattle slurry efficiency was lower compared to mineral fertilizers but it was not clear why this was. “Our theory is that nitrogen leaching from cattle slurry can also occur in organic form but it needs to be controlled,” she said.

What is more, due to the fact that the properties of biochar are very varying it also has a very diverse effect on nutrient leaching. Therefore, Tampere considers it important to study the mechanism owing to which biochar reduces leaching before it would be possible to use this substance for leaching reduction in agricultural fields.

Tampere also mentioned that there has been very little research about the presence and abundance of antibiotic resistance genes in Estonian agricultural soils and it needs to be studied much more thoroughly. “For example, it is important to determine bacterial species carrying resistance genes and the genes’ effect on soil processes. We need to study the extent of antibiotic resistance genes in the leachate and on the growing vegetation,” she concluded.

This article was funded by the European Regional Development Fund through Estonian Research Council.