Nitrogen transformations and ammonia volatilization losses from 15N-urea as affected by the co-application of composted pig manure

Abstract

Co-application of composted manure (compost) and urea is considered an environment-friendly fertilization practice; however, the high urease activity in compost may stimulate NH3 volatilization and cause N loss from co-applied urea. To test the above hypothesis, we investigated the fate of urea co-applied with compost in a loam-textured soil through two laboratory incubation experiments. Urea (150 mg N kg-1) was co-applied with 0, 4.9, 9.8, and 14.6 g of compost (oven-dry basis) kg-1 of soil, designated as treatments UC0, UC1, UC2, and UC3, respectively. Co-application of compost and urea enhanced urea hydrolysis and increased the 1st order rate constant of urea hydrolysis from 0.047 h-1 in the UC0 to 0.139 h-1 in the UC3 treatments. Soil pH increased from 7.0 for UC0 to 7.6 for UC3, leading to greater NH3 volatilization (up to two times more) in the soils receiving 9.8 g kg-1 or more of compost. Compost co-application also increased the immobilization of urea-derived N, probably because the organic matter added in compost stimulated microbial growth or NH4+ fixation. Between 15 and 17% of urea-N was not recovered at the end of the incubation, but there was no difference in N loss among the treatments resulting from the contrasting effects (NH3 volatilization vs. NH4+ immobilization) of compost on N losses. Our results clearly show that application of compost with high urease activity increases NH3 volatilization loss of N from the co-applied urea, but the total amount of N lost is also affected by immobilization of NH4+ by the organic matter added to the soil through the applied compost. Key words: Ammonia volatilization, nitrogen dynamics, nitrogen-15 recovery, organic carbon, urease activity