Soil respiration and nitrogen recovery in maize after digested slurry application

Power generation through anaerobic digestion of residual and dedicated biomass releases a slurry poor in C and relatively-rich in N. In order to assess its efficiency as a nitrogen source for maize and its CO2 rellease to the atmosphere, a field trial was set up in 2009. 7 fertilizer treatments were carried out in a randomized block design: an unfertilized control (Unfert), plus the cross combination of two rates (100 and 200 kg N ha-1) and three N sources: Urea (U), the liquid (LF) and the solid (SF) fraction (d.m. = 75 and 286 mg g-1, respectively) of the slurry coming from a digester which processes “green” (maize silage and vegetable wastes) biomass. Fertilizers, applied as topdress, were incorporated in the top 0.1 m soil of the inter-row space, followed by soil respiration point measures and NH3 volatilisation continuous trapping. Crop growth was followed through allometric traits. At harvest, grain and total-biomass dry yield (GY and BY, respectively), plant N uptake (N-upt) and N apparent recovery fraction (ARF) were determined. Plant morphology was not significantly affected by fertilizer treatments during growth. Soil respiration was greatly enhanced especially by SF application. Subjected to the contrast analysis, data showed a higher overall respiration at: 200 vs. 100 kg N ha-1; LF+SF vs. U; SF vs. LF. It has to be noted, however, that SF supplied a 2.7-higher amount of carbon than LF, attaining 2 Mg C ha-1 in the 200SF treatment. In contrast to this, ammonia volatilization from the soil was barely noticeable, indicating a very strong restraint of NH3 losses with respect to a vast range of cases. Yield and nutrient-efficiency traits at harvest outlined a picture of modest crop response to added nitrogen. The complex of the six fertilizer treatments (Fert) involved a ~10% GY increase with respect to Unfert; no further effect was observed between N rates (100 vs. 200), nor among fertilizers (U, LF, SF). The supply of fertilizers as a whole (Fert) involved a significant although modest increase in plant N-upt with respect to Unfert, whereas no further difference was observed between N rates and among fertilizers. At last, ARF did not show significant variations, indicating a substantial equivalence between N rates and especially among fertilizers, despite differences in chemical-physical characteristics. The overall ARF (6.4%) indicates a condition of limited nutrient efficiency, which is consistent with a low fertilizer need to achieve optimum crop nutrition. On concluding, both fractions of digested slurry can successfully be used as N sources for maize. Further evidence is needed to prove their effectiveness in conditions of major nutrient stress, as well as to assess their carbon-sink potential, given the non-negligible amounts of supplied C.