Biogas residues as fertilisers - Effects on wheat growth and soil microbial activities

Residues from the biogas process are a relatively new type of organic fertiliser with composition varying with origin of the raw material fed into the biogas process. The performance of four different biogas residues (BRs) was evaluated and compared with that of pig slurry (PS) and mineral fertiliser (NPK) in pots filled with sandy soil and fertilised at rates corresponding to 35,70 and 140 kg NH4+-N ha(-1). After sowing with spring wheat, the pots were incubated in a growth chamber. Total biomass yield and relative yields of ear, straw and root fractions were measured at harvest. Nitrogen mineralisation capacity (NMC) and potential ammonium oxidation rate (PAO) in soil were assessed. The fertilisation based on NH4+-N led to high addition of Tot C, Org N, Tot P, Tot K, Tot Mg and Zn at application of PS compared with BR. Biomass yields generally increased with increasing fertiliser rate but the response varied between the fertilisers. The BR yields compared well with NPK, but PS resulted in significantly higher yields. PS gave lower relative biomass of ears and higher relative biomass of roots compared with BR and NPK. The BR treatment yielding the lowest overall biomass also produced plants with the lowest relative root biomass, but partly compensated for this by increasing the relative ear biomass with increasing fertiliser rate. The NMC and PAO in soil had increased by the end of the experiment in almost all treatments. The high NMC in the PS treatment resulted in calculated mineralised N corresponding to 50-82 kg ha(-1) during the experimental period. In conclusion, fertilisation with biogas residues gave similar biomass yields but increased NMC and PAO in soil compared with NPK. Pig slurry gave the overall highest yields and NMC, but a PAO comparable to most BR. (C) 2012 Elsevier Ltd. All rights reserved.