Barley undersown with red clover in organic and conventional systems: nitrogen aftereffect on legume growth

Nitrogen (N) deficiency is a major problem in organic and low-input farming systems. Growing spring cereals with undersown legumes has a positive effect on soil fertility, enriching it with nitrogen, through symbiosis of legumes with nodule bacteria. Two hypotheses were tested: 1) undersowing of red clover increases the protein content of barley grains and 2) mineral N has negative aftereffect on growth of legume plants. The aim of this study was to investigate the effect of organic (cattle manure, off-season cover crop) and mineral N in organic and conventional farming systems on (i) barley (Hordeum vulgare L.) grain yield and quality, (ii) biomass yield formation of undersown red clover (Trifolium pratense L.) and (iii) the aftereffect of mineral and organic N fertilisers on the red clover crop biomass yield in the following year. The experiment was established in 2008 at the Estonian University of Life Sciences (58 degrees 22'N, 26 degrees 40'E) on the soil described as Stagnic Albic Luvisol (LV ab-st) (WRB, 2014) with sandy loam surface texture, 1.38% C, 0.13% N, 133 mg kg(-1) P, 210 mg kg(-1) K and pH(KCl) 6.0. The crops were treated using different farming systems: three organic (Org0, OrgI and OrgII) and four conventional (N0, N40, N80 and N120); nitrogen sources during crop cycle period: N0 and Org0 = symbiotically fixed atmospheric N-2, N40 = N-2 + mineral N 40 kg ha(-1) N, N80 = N-2 + mineral N 80 kg ha(-1) N, N120 = N-2 + mineral N 120 kg ha(-1) N, OrgI = N-2 + N taken up by cover crops (N-CC) and OrgII = N-2 + N-CC + N applied with manure (N-m). Our study revealed that grain yield of barley was strongly affected by undersown red clover; because of competition for nutrients in cereal-legume mixture the grain yield of barley in organic system was 11. 61% lower than that in conventional system. The seed rate of undersown red clover (2.84 million viable seed per ha) was too high. The content of protein depended on the availability of mineralised nitrogen. The mean protein content of barley grains over the trial years and treatments was 99 +/- 1.6 g kg(-1) in the conventional system, which was 17% higher than that in the organic system. Undersowing of red clover had no significantly positive effect on the grain yield and protein content of barley. Mineral N fertiliser had no negative aftereffect on growth of the red clover crop the following year. Mean above ground biomass yield over the trial years for the red clover crop 2nd cut was 17% higher in the conventional system than in the organic system.