Residual Effect of Synthetic Nitrogen Fertilizers and Impact on Soil Nitrifiers

The residual effect of nitrogen (N) fertilizers on subsequent crops might affect N efficiency and soil microorganisms, but little is known about the underlying reasons. Our goal was to elucidate the origin of the residual effect of synthetic N fertilizers, with and without a nitrification inhibitor, in three field experiments located in different regions of Spain that shared a common design in a maize (Zea mays L.)/wheat (Triticum aestivum L.) rotation. Maize was fertilized with ammonium sulphate nitrate (ASN) or with ASN blended with 3,4 dimethylpyrazole phosphate (DMPP) (ASN + DMPP) or was not fertilized with N. Wheat after maize received the recommended N dose, a low N dose or no N application. Yield and N content were determined for each crop and treatment and soil samples were taken to determine non-exchangeable NH4+ as well as C and N in the microbial biomass. DNA extraction was conducted in soil samples at different times. In the two locations where a residual effect occurred as a result of fertilizer application on the previous crop, either as ASN or ASN + DMPP, the C retained in the microbial biomass increased. The abundance of bacterial and archaeal communities was related to the soil's C biomass, and the quantitative PCR approach was a more sensitive biomarker of the microbial activity than C and N in the microbial biomass. The residual effect was associated with a temporal increase of soil microorganisms caused by fertilizer application. Optical sensors identified the residual effect, opening the opportunity for adjusting the fertilizer rate to actual crop requirements.