Quantifying nitrogen (N) dynamics from organic fertilizers at the field scale is difficult because decomposition, plant uptake, and N losses interact under fluctuating hydrological and thermal conditions. We conducted four years of field observations in paddy plots amended with rice bran or cow manure and paired them with unfertilized controls to directly quantify fertilizer-derived mineral N and plant N uptake without laboratory incubation. Field observations showed that 14–42% of the applied organic N was mineralized from application to harvest, and more than 88% of the mineralized N was taken up by rice plants. A simple first-order decay model driven only by observed soil temperature reproduced seasonal mineral N dynamics with good agreement, with rate constants increasing from 0.0029 day - ¹ at 10 °C to 0.078 day - ¹ at 30 °C. Introducing a first-order loss term representing denitrification during algal proliferation markedly improved model performance. When denitrification losses were estimated using a coupled heat–water–solute transport model, the apparent decomposition ratio increased to 38–71%, indicating that ignoring denitrification leads to substantial underestimation of organic matter decomposition under flooded conditions. Model results further showed that 87–93% of plant N uptake occurred through active NH 4 –N absorption, providing field-based quantitative support for rice as an ammonium-preferring crop. These results demonstrate that observation-driven first-order modeling, combined with physically based transport simulation, can provide quantitative and practical evaluation of organic fertilizer decomposition, N losses, and plant uptake under real paddy field conditions.
Eigen et al. (2026) studied this question.