Dynamics and plant uptake of nitrogen and phosphorus in soil amended with sewage sludge

Abstract Sewage sludge (SS) represents an inexpensive source of plant nutrients, but the fertilizer value can vary considerably. A field study was carried out, in which anaerobically digested sewage sludge (SS1) and activated sewage sludge (SS2) from the same treatment plant were applied to a crop of oat ( Avena sativa L.). Application rates with SS1 were around 120 kg ha −1 P, 20 kg ha −1 NH 4 + –N and 130 kg ha −1 total N. With SS2, inputs of P and NH 4 + –N were similar to SS1, but total N input was 50% higher. Soil inorganic N and resin-extractable inorganic phosphate (P i ), microbial biomass N (MBN), microbial biomass P (MBP) and microbial biomass C (MBC), as well as crop uptake of N and P, were monitored between mid April and late June. Five weeks after sludge application, soil inorganic N had increased from 25 to >60 kg ha −1 in both treatments due to mineralization of sludge constituents. The soil microbial biomass N and biomass P increased temporarily with SS2, but by the end of June biomass N had decreased by 28–34 kg N ha −1 relative to the pre-amendment level. An increase in biomass P was observed after SS2 amendment, but in general P dynamics were obscured by a high background content of inorganic P in the soil. Both crop growth and crop nutrient uptake were increased with application of SS2 compared to SS1. Microbial biomass N decreased and biomass C:N increased during the period of crop N uptake, indicating competition for N between plants and microbes. The N fertilizer value of SS1 corresponded to 32%, and that of SS2 to 53% of total N in the sludge.