Plant-based biogas production for improved nutrient management of beetroot in stockless organic farming

Transition from a nutrient management system based on green manure (GrM system) to one based on biodigested plant material produced within the crop rotation (BG system) was investigated in crop sequences including clover-grass,beetroot and cereals. The overall hypothesis was that transition would improve nitrogen (N) availability. In field experiments on sandy soil, harvested clover-grassley had lower N content in clover and biomass produced than GrM-ley. The residual N effect of clover-grass ley harvested twice (2H) or three times (3H) was 42 and 74 kg N ha-1 less than that of GrM-ley considering uptake in beetroot and mineral N in soil at harvest. Expressed as inorganic fertiliser equivalents the reduction was 52 and 80 kg N ha-1, respectively. Net inorganic N equivalents (from effluent plus pre-crops) were simulated for three crop sequences: (A) green manure ley, beetroot, winter rye; (B) harvested ley, beetroot, winter rye; and (C) harvestedley, spring barley, beetroot, where B and C represented BG systems and A a GrM system. For three hectares with the entire crop sequence A, B and C, net inorganic N equivalents were 73, 74 and 128 kg N, respectively. Net inorganic N equivalents in BG systems with 2H- and 3H-ley did not differ significantly. When the whole increase in net inorganic N equivalents was used for beetroot following barley, marketable beetroot yield increased by 1.7 Mg ha-1 (12%) in the BG system with 2H-ley and by 5.8 Mg ha-1 (34%) with 3H-ley compared with beetroot grown without digestate fertilisation following a GrM-ley. Fertilisation with a moderate level of effluent of beetroot directly following harvested ley gave unexpectedly low yield responses. Compositional nutrient diagnosis (CND) using norms derived from aeroponic experiments with 22 treatments with dynamic nutrient supply and partial least squares (PLS) were synonymous in showing K as more growth-limiting than Nat early growth stages. Growth limitation was more severe in effluent-fertilised beetroot following harvested ley than following barley. Pot experiments showed an apparent net mineralisation of organically bound N in digestate of 12%. The overall conclusion was that a BG system can greatly improve N efficiency. However, as the nutrient buffering capacity in sandy soil is low, inappropriate use of the effluent, e.g. at an unsuitable point in the crop rotation, can negate the N efficiency benefits.