Impacts of long-term inorganic and organic fertilization on lignin in a Mollisol

Fertilization is an essential management method to maintain and increase soil organic carbon (SOC) content in agroecosystems. Fertilizer application is known to markedly influence the turnover of labile and refractory SOC components. However, the dynamics of lignin in response to different types of fertilization remains unclear. This study addresses the impact of long-term fertilization on lignin accumulation in an arable soil. Samples from a Mollisol soil were collected from 0 to 20 and 20 to 40 cm below the ground surface at a long-term experimental site with maize (Zea mays L.) monoculture. Experimental plots received one of six fertilization options: nitrogen (N) only; N and phosphorus (P); N, P, and potassium (K); pig manure at either of two application rates; or no fertilizer. Lignin levels were quantified by alkaline CuO oxidation of the samples to release lignin monomers, followed by gas chromatography. Lignin-phenol content was calculated as the sum of vanillyl (V), syringyl (S), and cinnamyl (C) type phenols. In addition, the acid-to-aldehyde ratios of V- and S-type monomers were used to assess the degree of lignin degradation. After a 29-year test period, inorganic fertilizer applications exhibited no significant effect on soil lignin content or the proportion of lignin-derived carbon in SOC for the 0-20-cm layer. However, inorganic fertilization markedly increased both measures in the 20-40-cm layer because of the high lignin content of crop residue as well as low microbial decomposition of lignin. In contrast, long-term organic fertilization resulted in significant lignin increases in bulk soil and SOC in both layers. This was mainly attributed to the higher lignin content of the organic fertilizer (pig manure). Lignin degradation in the 0-20-cm layer was evidently retarded after fertilization, while neither inorganic nor organic fertilizers exhibited any influence on the degree of oxidation of lignin phenols in the 20-40-cm layer. Selective accumulation of lignin was enhanced after long-term organic fertilization, which was coincident with SOC sequestration. The results suggest that the long-term organic fertilization facilitates lignin stabilization in both soil layers, while the long-term application of inorganic fertilizer can only be beneficial to lignin accumulation in subsoil (20-40-cm layer) of the Mollisol. This study elucidates effects of long-term fertilization on lignin dynamics and has significant implications for sustainable management of Mollisols under the maize cropping system in northeastern China.