RESUMO
We tested the hypothesis that application of stable forms of organic carbon (C) into the soil reduces leaching of nitrogen (N). We also examined the potential to estimate N leaching employing N-sensitive spectral reflectance indices. During three growing seasons 2013-2015, field experiment at two experimental sites combining application of distinct N doses (0 (N0), 35 (N35), 70 (N70), and 140 (N140) kg N ha-1) and two stable forms of organic C (lignohumate and compost) was established to measure N uptake by winter wheat and its leaching to subsoil layers. The spectral reflectance at canopy level was measured simultaneously with N content in leaf dry matter at the beginning of the grain filling phase. At full maturity, the above-ground biomass, grain yield, and grain protein content were evaluated. That data was used to calculate N uptake in grain. The N140 dose led to increased N uptake by grain of 64% and 73% in the wetter years 2013 and 2014, respectively, and even by 118% in the drier year 2015 in comparison with the N0 treatment. N leaching to subsoil increased substantially with higher N dose, but only in wetter years 2013 (by 74%) and 2014 (by 87%). By contrast, no effect of N dose on leached N was found in the dry year 2015. The application of organic C along with the N140 dose substantially reduced N leaching by 26% and 29% in 2014 and 2015, respectively. Moreover, we demonstrated that normalized red-edge spectral reflectance index (NRERI) is able to predict N uptake by wheat and it can serve as an indicator of N leaching in heavy-rainfall years. Our results thus point towards possible agronomic practices and use of remote-sensing techniques to reduce groundwater contamination by N-based fertilizers.
