Inhibitory Effect of Neem Seed Extract on Soil Nitrogen Mineralization under Chemical and Organic Amendments

Abstract

Background: Improvement of nitrogen use efficiency (NUE) of chemical fertilizers and N-rich-organic amendments using natural mineralization inhibitors, like neem extract, was recommended to minimize nitrogen (N) loss. This study hence aimed to evaluate the effects of combined uses of neem seed extract with cricket feces on soil N mineralization and plant NUE. Methods: The treatment combinations included i) amending materials involving unamended (Un), chemical fertilizer at a recommended rate of 312.5 kg N ha-1, 100 kg P ha-1 and 100 kg K ha-1 (CF) and three rates of cricket feces (CrF) [3.125 (CrFlow), 6.25 (CrFmedium) and 12.5 (CrFhigh) Mg ha-1], ii) in combination without (-Nm) and with (+Nm) neem seed extract. Amaranth (Amaranthus tricolor) was employed to assay the plant’s NUE. Result: Neem seed extract inhibited the N mineralization under the Un (74%) and CF (84%) treatments, but not in any of the CrF treatments (-102%, -99% and -2% for CrFlow, CrFmedium and CrFhigh, respectively. In addition, the neem extract significantly decreased NUE parameters, including N recovery efficiency and agronomy NUE, only in CF treatment. Neem seed extract could only function as an N inhibitor in unamended and chemically fertilized soils.