Nitrate leaching from organic farming systems with rotational grass-clover and arable crops

Summary

Organic farming is considered an effective mean of reducing nitrogen losses compared with more intensive conventional farming systems. However, under certain conditions organic farming may also be susceptible to high nitrogen (N) losses. This is especially the case for organic dairy farms on sandy soils that use grazed grass-clover in rotation with cereals, which may lead to high rates of nitrate leaching.
A study was conducted on two commercial organic farms on sand and loamy sand soils. On each farm a three year old grass-clover field was selected. Half of the field was ploughed the first year and the other half was ploughed the next year. Spring barley (Hordeum vulgare L.) was sown after the spring ploughing. Measurements showed a low N leaching during the pasture period (9-64 kg N ha-1) but a high leaching the first (63-216 kg N ha-1) and second (61-235 kg N ha-1) year after ploughing. In addition, a high residual N effect of the pasture was observed on the barley yields in both years. There was thus a low response to manure application on the sandy soil both during the first and second year after ploughing.
To study the underlying processes affecting the residual effects of the pasture and the N leaching, the dynamic whole farm model FASSET was used to simulate all the treatments on both farms. The simulations agreed with the observed barley yields. However, for the sandy soil the simulation of nitrate leaching and mineral nitrogen in the soil deviated from the measurements. Three scenarios with changes in model parameters were constructed to investigate this discrepancy. These scenarios suggested that the organic matter turnover model should include a pool with a half-life of about 2-3 years. There may also be a need to include effects of soil disturbance (tillage) on soil organic matter turnover.