Nitrogen alters microbial enzyme dynamics but not lignin chemistry during maize decomposition

Increases in nitrogen (N) availability reduce decay rates of highly lignified plant litter. Although microbial responses to N addition are well documented, the chemical mechanisms that may give rise to this inhibitory effect remain unclear. Here, we ask: Why does increased N availability inhibit lignin decomposition? We hypothesized that either (1) decomposers degrade lignin to obtain N and stop producing lignin-degrading enzymes if mineral N is available, or (2) chemical reactions between lignin and mineral N decrease the quality of lignin and limit the ability of decomposers to break it down. In order to test these hypotheses, we tracked changes in carbon (C) mineralization, microbial biomass and enzyme activities, litter chemistry, and lignin monomer concentrations over a 478-day laboratory incubation of three maize genotypes differing in lignin quality and quantity (F292bm3 (high lignin) < F2 (medium lignin) < F2bm1 (low lignin)). Maize stem internodes of each genotype were mixed with either an acidic or neutral pH sandy soil, both with and without added N. Nitrogen addition reduced C mineralization, microbial biomass, and lignin-degrading enzyme activities across most treatments. These dynamics may be due to suppressed fungal growth and reduced microbial acquisition of lignin-shielded proteins in soils receiving N. However, N addition alone did not significantly alter the quantity or quality of lignin monomers in any treatment. Our results suggest that abiotic interactions between N and phenolic compounds did not influence lignin chemistry, but mineral N does alter microbial enzyme and biomass dynamics, with potential longer-term effects on soil C dynamics.