Seasonal Dynamics of Fertilizer-Derived Hydrolysable NH3 in an Arable Soil of Northeast China

Fertilizer applications to soil are widely known to be the most important anthropogenic sources to influence soil N turnover in agricultural ecosystems. More information is required on the relationships between soil organic N (SON) forms in order to predict the maintenance, transformation and stability of soil N. Accordingly, N-15-labeled (NH4)(2)SO4 (totally 200 kg N/ha) was applied to a maize crop throughout the entire growing period to investigate the distribution and the dynamics of fertilizer-derived N in hydrolyzable-NH3 fraction by measuring the labeled N in them. The accumulation of N-15 in hydrolyzable-NH3 fraction was time-dependent although the total N concentration changed only slightly. The transformation of the residual fertilizer N to hydrolyzable-NH3-N-15 was maximal during the silking and grain filling stages, suggesting the fertilizer N was immobilized at an early stage during the growing period. The rapid decrease of N-15 in hydrolyzable-NH3 pool indicated that hydrolyzable-NH3-N-15 was a temporary pool for fertilizer N retention and was able to release fertilizer N for uptake by the current crop.