Dairy farmers often harvest grass to produce silage for winter cattle feed, using chemical fertilisers to maximise dry matter yield.
Silage harvesting removes mineral ions, including nitrate, from the ecosystem.
Give two examples of biological molecules containing nitrogen that are removed from the soil system when silage is harvested.
Agricultural scientists monitored changes in the application of nitrogen fertiliser in a silage-producing region between 1990 and 2020. They also monitored the fertiliser response ratio, which is the mass of silage dry matter produced per kilogram of nitrogen fertiliser applied.
The table below shows their findings:
| Year | Nitrogen fertiliser used (kg ha−1 yr−1\text{kg ha}^{-1}\text{ yr}^{-1}kg ha−1 yr−1) | Fertiliser response ratio (kg dry matter / kg fertiliser\text{kg dry matter / kg fertiliser}kg dry matter / kg fertiliser) |
|---|---|---|
| 1990 | 40 | 22.0 |
| 1995 | 60 | 18.5 |
| 2000 | 85 | 15.2 |
| 2005 | 110 | 12.4 |
| 2010 | 135 | 10.5 |
| 2015 | 155 | 9.2 |
| 2020 | 170 | 8.6 |
Use these data to calculate the difference in the mass of silage dry matter produced per hectare in 1990 compared with 2020. Show your working.
Use the data in the table to evaluate the use of nitrogen fertilisers for silage production in this region.