Use of nutrient balances for environmental impact calculations on experimental field scale

Abstract

Nitrogen (N) leaching was analyzed in relation to nutrient use efficiency, N balance, and mineral N content in the soil in different crop rotations. In spring in the field occupied by sugar beet mineral nitrogen content was 87.8±10.9 kg N ha −1 per year, and after harvest there was a decrease to 46.0±15.7 kg N ha −1 per year. In the fields under winter and spring grain crops, these values were 102.6±54 and 80.5±32.8 kg ha −1 per year, respectively in spring, and 76.0±46.1 and 79.6±34.9 kg N ha −1, respectively, after harvest. The highest leaching from the fields where sugar beet and grain crops were grown was from 7.3 to 39 kg N ha −1 per year, compared to 2–17.1 kg N ha −1 per year from fields with perennial grasses in the rotation. The leaching from sugar beet fields was attributed to mineralization of large amounts of organic residues (tops and small roots) left in the field after harvesting. The quantity of organic matter left after harvesting sugar beet was 2480–7270 kg dry matter (DM) ha −1, compared to barley harvest (490–1150 kg DM ha −1) or winter wheat harvest (1890–2270 kg DM ha −1). The most negative N balance during the 5-year period was in the field with perennial grasses with a total of −426 kg N ha −1. The highest positive balance was for the field with sugar beet in the rotation (+174 kg N ha −1). In the field where cereals dominated the surplus of N was +74 kg N ha −1. Agronomic efficiency (AE) calculated as the ratio between DM yield and amount of fertilizer applied was calculated for main crop groups as well as for the investigated crop rotations with different crop structure. For whole rotations the highest AE was for sugar beet (23.4 kg DM (kg N) −1) followed by winter cereals (8 kg DM (kg N) −1), while the spring cereal yield response to N fertilization was equal to zero. The highest yield response (151 kg DM (kg N) −1) was observed in the rotation where sugar beet comprised 40%, grain crops 40%, and grasses 20% of the rotation period. AE was 42.5 kg DM (kg N) −1 in the rotation with 60% sugar beet and cereal plus leguminous grain crops 40% of the rotation and negative in the 100% grain crop rotation. In summary, the lowest environmental impact in terms of N leaching was in the rotation with grasses. AE was highest in the rotations where sugar beet was included and the area of spring cereals was reduced.