Good soil structural stability reduces the risk of soil erosion, crust formation and loss of particle-associated pollutants. Previous studies have shown that the soil organic matter content plays an important role in the formation and stabilisation of soil structure, but only a few studies have investigated how quickly changes in the soil physical properties occur after conversion to restoration or degradation managements. Therefore, researchers from the Department of Agroecology at Aarhus University have investigated how quickly the soil organic carbon content, structural stability and soil structure change when grassland is converted to either arable or bare fallow management and when arable and bare fallow soil are converted to grassland.

Rate of change

The researchers have been using soil from the long-term field experiment, Highfield, at Rothamsted Research in England, one of the world's oldest agricultural research centres. The field trial was initiated in 1949. 

"In 2008, the long-term treatments were converted to the contradictions, that is, they tried to restore the soil physical properties by converting arable and bare fallow soil to grassland," says Postdoc Johannes L. Jensen from the Department of Agroecology.

Conversely, permanent grassland was converted to arable or bare fallow soil in order to quantify how quickly and how the soil physical properties degrade. Some parts of the fields were not converted, but instead serve as reference points for the researchers' measurements.

“It is unique to be able to measure both soil restoring and degrading measures at the same site, with the same managements and within the exact same time period, since they all were converted in 2008. We sampled soil six years after conversion, and it has given me the opportunity to calculate the rate of change,” says Johannes L. Jensen.

Six years after converting parts of the original areas in Highfield, measurements revealed to what extent the soil structural stability and pore size distribution in the area that was converted to grassland were on the structural stability and the pore size distribution in the original areas and vice versa. 

“It makes it possible to calculate the rate of change after six years, which means that we can compare short-term effects on soil physical properties with the long-term effect of the same management. It is unique as it requires the long-term treatments to be in equilibrium, which means that the soil organic carbon content does not change further, and this has been the case for the original bare fallow, arable and grassland treatments,” explains Johannes L. Jensen.

Soil organic carbon content, structural stability and pore size distribution

The researchers have measured changes in the soil organic carbon content, structural stability of aggregates and the pore size distribution of the soil. From the latter, it is possible to calculate the plant-available water capacity by looking at how many small pores there are in the soil.

"In addition, we have also calculated the structural void ratio, which relates to the large pores that are important in terms of root development and gas exchange," says Johannes L. Jensen.

The results show that after conversion from arable or bare fallow management to grassland, there is a rapid change in structural stability, it becomes better, whereas the plant-available water capacity and structural void ratio remain largely unchanged.

“Nothing happens with the distribution of pores, but it is because the soil is no longer tilled as previously and therefore the soil density may increase. We can see that when converting to grassland the topsoil may experience a period where the soil is more compact, especially if the starting point is a soil in poor condition such as bare fallow soil,” says Johannes L. Jensen.

Faster to degrade soil organic carbon, but slower to degrade structural stability

Contrary to expectations, the soil organic carbon content did not increase significantly when introducing grassland in bare fallow and arable soils, but according to the researchers, this does not mean that the conversion to grassland does not have a major impact on structural stability. On the other hand, conversion of grassland to arable or bare fallow management reduced the soil organic carbon content and soil physical properties.

"In other words, it is faster to lose soil organic carbon and degrade a complex soil structure than to gain soil organic carbon and develop a complex soil structure, however, it is faster to restore structural stability than to degrade it," says Johannes L. Jensen.

The researchers' results thus show that the rate of change after converting grassland to arable and vice versa from arable to grassland depends on which soil property one focuses on.

"It is important to remember that this experiment is located in England, so even though the climate is not that different from the Danish, the measurements are made on a soil with a very high clay content, which means that the results cannot be directly transferred to Danish conditions, but this is the best indication we have,” says Johannes L. Jensen.

Behind the research