A field experiment was carried out in an organic certified field of the Experimental Station of the Lithuanian University of Agriculture in 2003–2007. The soil type was Calc(ar)i Epihypogleyic Luvisol (LVg-p-w-cc). Nitrogen balance was evaluated after four years of experiment. Four 7-year crop rotations with a different ratio of nitrogen fixing crops were compared: I – 43% (grass‑clover; grass-clover; winter wheat; spring barley; peas; winter wheat; spring barley); II – 43% (grass-clover; winter wheat; peas; spring barley; grass-clover; winter wheat; spring barley); III – 29% (grass-clover; potato; oat; spring barley; peas; winter wheat; spring barley); IV – 14% (grass-clover; winter wheat; potato; spring barley; winter rape; winter wheat; spring barley). The catch crop was (1) included, (2) not included in rotations. The objective of our study was to evaluate the influence of crop rotations with a different nitrogen fixing crop percentage and catch crop (for green manure) on nitrogen balance in organic farming.

The least soil nitrogen loss (1.87 to 4.58 kg ha^–1) due to leaching to deeper soil horizons was established in crop rotations II and IV, in grass-clover and winter rape crop. The biggest nitrogen loss (40.7 to 74.8 kg ha^–1) was found in the soil of crop rotation I in peas and in the soil of crop rotation III in peas without a catch crop. Catch crop for green manure reduced nitrogen leaching by 94.9% in the soil of crop rotation III in comparison with the same crop rotation (III) without a catch crop. The amount of nitrogen leached to the deeper soil layers depended on nitrogen content in the soil.

During the experiment, a negative nitrogen balance was established in the soil of all crop rotations (–46.69 to –145.13 kg ha^–1). The nitrogen balance was influenced by nitrogen uptake from soil (r = –0.77, P < 0.05). The obtained data show that biological nitrogen suppliers are very important in organic farming.

Keywords: nitrogen balance, crop rotation, catch crop, organic farming