Plant lignin and nitrogen contents control carbon dioxide production and nitrogen mineralization in soils incubated with Bt and non-Bt corn residues

Bt (Bacillus thuringiensis) corn is reported to produce lignin-rich residues, compared to non-Bt (NBt) corn, suggesting it is more resistant to decomposition. As the Bt gene is expressed selectively in stem and leaf tissue, it could affect lignin distribution in corn, which naturally has greater lignin content in roots than in stems and leaves. Our objective was to evaluate the effects of corn plant components, the Bt gene and elevated-lignin inputs on decomposition. Roots, stems and leaves from Bt corn and NBt corn isolines enriched with 13C and 15N were finely ground and mixed separately with soil, then incubated at 20 °C for 36 weeks. The effect of elevated lignin on decomposition was tested by adding a commercial lignin source (indulin lignin) to half of the samples. In addition to weekly CO2 analysis and regular measurement of N mineralization, the degree of lignin degradation was evaluated at 1 and 36 weeks from the acid to aldehyde ratio (Ad/Al) of vanillyl and syringyl lignin-derived phenols. The CO2 production and N mineralization was lower in root-amended soils than stem- and leaf-amended soils. The Bt genetic modification increased CO2 production from stem-amended soils (P < 0.05) and decreased N mineralization in root-amended soils. The 13C and 15N results also showed more residue-C and -N retained in soils mixed with NBt stem residues. After 36 weeks leaf- and stem-amended soils with indulin lignin had a lower Ad/Al ratio and were less degraded than soils without exogenous lignin. In conclusion, plant lignin and nitrogen contents were good predictors of CO2 production and N mineralization potential. Corn roots decomposed more slowly than aboveground components emphasizing the importance of recalcitrant root residues in sustaining the organic matter content of soil.

Research highlights► CO2 production inversely related to lignin content and lignin:N ratio of tissue. ► Corn roots decompose slower than leaves and stems. ► Bt and NBt leaves and roots decompose at similar rates. ► Bt stems decomposed faster than NBt stems due to smaller C:N ratio of Bt stems. ► Phenylpropanoid units are more susceptible to biodegradation in NBt than Bt tissue.