How Much Nitrogen from the Air Enters the Soil through Legumes?

Symbiotic nitrogen fixation by bacteria in the soil coexisting with legumes leads to reduced fertiliser requirement. It is not easy to measure this variable on farms, however. Now researchers from Agroscope have developed a method for estimating nitrogen input via symbiotic fixation at farm level.

Farm-gate balances are a tool that can be used to depict a farm’s nitrogen efficiency. Because most farms in Switzerland have legumes in their crop rotations and in grassland, the ability to accurately estimate nitrogen inputs from symbiotic fixation at farm level can help improve overall nitrogen-use efficiency. Agroscope researchers have therefore developed a method for estimating the amount of nitrogen fixed by legumes at farm level. This estimate is performed primarily with data already available on the farms.

In a study of the literature, two models were selected to estimate the symbiotically fixed nitrogen: one for grassland, and one for annual grain legumes. The selected models were modified so as to allow the depiction of Swiss production systems and the use of farm data.

Model for permanent and temporary grassland

The model for grassland is based on nine input parameters. For five parameters, standard values were derived from the literature. Three further parameters can be estimated with the aid of farm data on utilisation intensity (grassland yield, fertilisation level and nitrogen content of the legumes at harvest). The last input parameter – the percentage of legumes out of total harvest yield – has a major impact on the estimate of the quantity of fixed nitrogen, but is not available in the farm data. As a compromise between accuracy and the effort required to estimate the proportion of clover, we propose visually assigning the grassland to six different categories with varying proportions of legumes.

Model for grain legumes

The model for grain legumes requires six input parameters. For four parameters, crop-specific standard values were determined based on data from the literature. An important parameter is nitrogen harvest index (ratio of nitrogen content in the grain to nitrogen content in the entire shoot). In future, this parameter could be derived from Swiss variety testing, and would thus enable a variety-specific estimate of nitrogen fixation. With the final parameter – the harvest yield determined by the farmer – plant nitrogen uptake can be estimated.  

The new estimation method enables a more accurate assessment of symbiotic nitrogen fixation on farms when calculating the farm-gate balance. It is based for the most part on data that is already available; the proportion of legumes on the various grassland plots must also be determined.