Soil type and microclimatic conditions as drivers of urea transformation kinetics in maize plots

This study presents a multidisciplinary approach (hydrological, microbiological, agronomic) to determine the dynamics of transformation of synthetic urea applied as soil fertilizer during maize production. The approach consisted in field and laboratory experiments on urea hydrolysis, ammonia volatilisation and nitrification, in four soil types (named Silty-loam, Silty-clay, Peat and Sand) intensively fertilized with synthetic urea. The field plots were modelled with HYDRUS-1D to determine the fate and transport of N species in the top soils. The numerical models successfully captured the main N transformations throughout the simulated period. In addition to the field monitoring of nitrogen species, microbial C and N, and urease activity were screened in each soil. The field soil sampling highlighted that the kinetics of ammonification was consistent with the hydrolysis of urea except for the sandy soil; kinetics of ammonification decreased in the order: Silty-clay > Peat ≫ Silty-loam > Sand. The differences of urease activities and nitrification potential rates between soils supported the measured field nitrification rates, and were as follow in order of decreasing rate: Silty-clay > Silty-loam ≥ Peat > Sand. The lowest nitrification rates pertained to the inherently more vulnerable sandy soil due to the scarcity of nitrifiers. The performance of volatilization was as follow in order of decreasing rate: Sand ≫ Silty-loam ≥ Peat > Silty-clay; but in general very low volatilization rates were found. This was imputed to the concurrence of both elevated soil CEC that promoted ammonium sorption and to low wind speed in the monitored plots. The presented multidisciplinary approach should be employed in many other agricultural settings to obtain robust data for numerical models simulations on the fate and transport of reactive N species in agricultural lands.