Application of organic carbon affects mineral nitrogen uptake by winter wheat and leaching in subsoil: Proximal sensing as a tool for agronomic practice.

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.

Consequence of altered nitrogen cycles in the coupled human and ecological system under changing climate: The need for long-term and site-based research.

Anthropogenically derived nitrogen (N) has a central role in global environmental changes, including climate change, biodiversity loss, air pollution, greenhouse gas emission, water pollution, as well as food production and human health. Current understanding of the biogeochemical processes that govern the N cycle in coupled human-ecological systems around the globe is drawn largely from the long-term ecological monitoring and experimental studies. Here, we review spatial and temporal patterns and trends in reactive N emissions, and the interactions between N and other important elements that dictate their delivery from terrestrial to aquatic ecosystems, and the impacts of N on biodiversity and human society. Integrated international and long-term collaborative studies covering research gaps will reduce uncertainties and promote further understanding of the nitrogen cycle in various ecosystems.