A greenhouse experiment was conducted to estimate the influence of various application rates of eucalyptus-derived biochar combined with cricket frass on the soil properties and soil N transformation, and, in turn, affecting both shoot biomass yield and nitrate (NO3-) contents of Chinese kale (Brassica oleracea). Two consecutive kale crops were grown to investigate the temporal effect of the combined amendments of cricket frass and biochar. Six rates of biochar, 0%, 0.125%, 0.25%, 0.5%, 1%, and 2% w/w in combination with 0.55% w/w of cricket frass, were applied only once at the start of the experiment in sandy loam soil. Shoot biomass significantly increased under treatments of 0.125% to 0.5% w/w in the first kale crop and 0.125% to 0.25% w/w in the second crop compared to the cricket frass alone. However, the higher rates of 0.25% and 0.5% w/w within the first and second crops decreased shoot biomass relative to their lower rates in each crop. Tissue NO3- concentrations of the first kale crop significantly decreased under all biochar rates, whereas the opposite effect was observed in the second crop. These contrasting effects of biochar on tissue NO3- concentrations were attributed to nitrification inhibition in the first crop and nitrification stimulation in the second crop. The 0.125% w/w rate of eucalyptus-derived biochar was, therefore, recommended to be combined with cricket frass to improve yield and reduce tissue NO3- content in the production of Chinese kale.
The inhibitory effects of neem leaf extract on fertilizer nitrogen (N) transformation in soil have not been fully recognized. This study therefore aimed to evaluate the effects of fi ve nitrifi cation inhibitors on the status of soil inorganic N, urea hydrolysis, and nitrifi cation: i) no inhibitor (control); ii) nitrapyrin; and three rates of neem leaf extract based on the dry weight of the raw material: iii) 1 g kg -1 soil; iv) 2 g kg -1 soil; v) 4 g kg -1 soil. Neem leaf extract in all rates increased urea hydrolysis rate on days 5−15. In contrast, nitrapyrin decreased urea hydrolysis on days 5−10, compared to the control. As for nitrifi cation, neem leaf extract showed both stimulatory and inhibitory effects. The stimulation appeared on day 3, whereas inhibition occurred and peaked on days 5−15. Nitrapyrin showed inhibitory effects on days 10−15, reaching its peak on day 15. Increasing rates of neem leaf extract brought about increases in stimulation and inhibition of urea hydrolysis and nitrifi cation. The results conclusively stated that the neem leaf extract had generally faster, stronger, and longer nitrifi cation inhibition than nitrapyrin. The stimulation in the early phase and the inhibition in the later phase of the incubation of neem leaf extract were more pronounced with higher application rates.
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.
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