Inhibitory effect of neem seed extract on N mineralization occurring only in soils without cricket feces amendment
Soil mineral N concentrations and net N mineralization rates under Un and CF treatments significantly decreased in +Nm treatments relative to -Nm (Table 2). However, the opposite results were found in all cricket feces treatments. Soil mineral N concentrations and net N mineralization rates in -Nm and +Nm increased significantly with greater rates of cricket feces. Meanwhile, N mineralization inhibition occurred in Un and CF treatments under +Nm, but
vice versa in the cricket feces treatments.
The inhibitory effect on N mineralization under Un+Nm and CF+Nm was attributed to the neem active ingredients: azadirachtin, salanin, 14-epoxiazadiradione, meliantrol, melianone, gedunin, nimboline, nimbin, deacetilasalanin, azadiractol, azadirone, vilosinin and meliacarpine
(Choudhury et al., 2016). These ingredients have been reported to inhibit the N mineralization sub-processes,
i.
e., urea hydrolysis
(Mohanty et al., 2008) and nitrification
(Alves et al., 2009). Among these ingredients, azadirachtin was reported to serve as an essential mineralization inhibitor
(Sarawaneeyaruk, Krajangsang and Pringsulaka, 2015). The current study’s azadirachtin content of neem seed extract was 12.6 mg mL
-1.
Instead of inhibition, combinations of neem seed extract with cricket feces stimulated N mineralization, as elucidated in the negative values of N mineralization inhibition (Table 2). The denature of neem ingredients might be a consequence of malfunction of the inhibitory property.
Sundaram et al. (1995) reported that azadirachtin was denatured via hydrolytic and microbial degradations. The higher degree of azadirachtin hydrolytic degradation occurred in the higher pH values in the following orders: pH 10 >>> pH 7 > pH 4. In the current study, pH values of soils under +Nm combined with CrF
low, CrF
medium and CrF
high were 5.90, 5.93 and 6.29, respectively (Table 2). Meanwhile, pHs of Un+Nm and CF+Nm were 5.90 and 5.82, respectively. Therefore, lower pH of the unamended and chemical fertilizer treatments than cricket feces amended soils (CrF
medium and CrF
high) implied that less hydrolysis degradation of azadirachtin existed in the former soils.
Another malfunction of inhibitory property of neem seed extract in the cricket feces amended soils might be a consequence of microbial degradation of azadirachtin. Microorganisms were reported to play a crucial role in decomposing azadirachtin
(Stark and Walter, 1995). The current study showed significantly higher microbial activity, indicated by fluorescein released, in cricket feces amended treatments than the Un and CF treatments (Table 2). In addition, the higher rates of cricket feces brought about higher degrees of microbial activity, as shown by significantly higher fluorescein released in CrF
high than CrF
low and CrF
medium. Cricket feces was a vital source of energy and nutrients for soil microorganisms, as evidenced by the high contents of organic C and mineral N (Table 1). Our results agreed with
Agyarko et al. (2006), who determined that higher degrees of azadirachtin degradation were rendered by increased amounts of poultry and cattle manures.
A significant increase in N mineralization inhibition under CrF
high relative to its lower cricket feces rates (Table 2) might be manifested by the decomposing products derived from the feces, such as humic substances and organic molecules. Humic acid was reported as a nitrification inhibiting compound
(Benckiser, Schartel and Weiske, 2015). In addition, several organic molecules derived from the decomposition,
e.
g., quinines, catechols, ethylene, acetylene, gallic acid and tartaric acid, were reported to bring about microbial toxicity
(Kaal et al., 2012). Even though organic molecule concentration was not determined in this paper, higher rates of cricket feces were assumed to increase the content of soil organic molecules. This assertion was in line with
Nair et al. (2015) who stated that the greater input of farm yard manure brought about higher soil organic content.
Inhibition of N mineralization diminishing nitrogen use efficiencies of a vegetable amaranth
Significant decreases in N use efficiencies, involving NRE and AgrNUE, under CF+Nm compared to CF-Nm (Table 3) resulted from N mineralization inhibition. This effect was corroborated by the PCA results (Fig 1). The NRE and AgrNUE existed in the opposite PCA-quadrant to N mineralization inhibition but in the same quadrant with mineral N and net N mineralization rate (Fig 1A). In addition, CF+Nm (Fig 1B) was deposited on a similar quadrant with N mineralization inhibition (Fig 1B). This result was not the case for CF-Nm. The results indicated that CF+Nm decreased NRE and AgrNUE. Nitrification inhibition might stimulate N loss via gases such as NH3 volatilization.
Soares et al. (2012) demonstrated increased NH3 volatilization by using dicyandiamide as a nitrification inhibitor in an acidic soil. Loss of N from the soil system in this study was verified, in part, by significantly lower mineral N in CF+Nm than CF-Nm (Table 3).