Effect of neem leaf inclusion rates on compost physico-chemical, thermal and spectroscopic stability

Neem leaves possess antimicrobial, insecticidal and nitrification inhibitory biochemicals that may influence compost stability. However, if neem-based compost achieves stability, it can prove useful by providing fertilizer-cum-pesticide properties. Therefore, conventional (physico-chemical), thermogravimetric (TGA) and spectroscopic analyses (FTIR and ¹³C SS NMR) were used to evaluate the effects of neem leaf content on stability. Treatments included 0, 10, 20, 30, 40 and 50% neem leaves by volume combined with complimentary amounts of corn stover to form 50% of the substrate formulation (SF). Cow manure constituted the additional 50%. Despite all treatments reaching ambient temperature (32 °C ± 1 °C) by the 40th day, Solvita® results showed high CO₂ respiration, thereby classifying the compost treatments as active, whereas decreased C:N ratio, NH₄⁺: NO₃^- ratio and NH₄⁺ values among treatments indicated stability. Furthermore, TGA, FTIR and ¹³C NMR revealed degradation of labile organic matter and showed that complex aromatic and lignin compounds were also degraded, particularly when neem leaves were added to the mixture, suggesting that aromatisation does not always indicate stability in compost. Spearman's rank correlation showed that physico-chemical methods were poorly correlated to respirometric, thermal and spectroscopic methods. It also suggests that these respirometric and advanced methods are important in understanding the mechanisms affecting neem compost stability.