RESUMEN
The contribution of large molecular size organic nitrogen (N) to plant N uptake is unclear. Soils with and without maize, at three pH levels, were treated with (carbon-14 and -13 (14 C, 13 C), 15 N) triple-labelled > 100 kDa organic N. After 48 h, soil and maize were sampled for bulk and compound specific isotope analysis to study the turnover in soil and plant 13 C and 15 N uptake. Mineralization of > 100 kDa organic N increased with higher pH only in soil without maize. The > 100 kDa organic N disappeared rapidly in soils with and without maize, but surprisingly more > 100 kDa organic N derived amino acids remained in soil with than without maize - most likely in the microbial biomass. Total 15 N uptake in maize increased with higher soil pH. The organic N uptake was estimated to account for 20-30% of the total 15 N uptake. Organic N uptake was confirmed by the presence of 13 C-labelled amino acids in maize roots. The study suggests that the importance of plant organic N uptake increases when N is derived from complex molecules such as proteins compared to studies using single amino acids as N source, and that rhizosphere microorganisms increase anabolic utilization of organic N compared to microorganisms in the bulk soil.