Opposite roles of plant quality and soil exoenzymes in regulation of litter carbon transfer to fungal and bacterial necromass

Abstract

Plant litter is a primary source of soil organic matter (SOM) through microbial transformation, with its quality, especially the hemicellulose and lignin content, strongly influencing litter decomposition and carbon (C) cycling. However, how the hemicellulose:lignin ratio regulates litter decomposition, transformation, and contribution to SOM remains unclear. Here, ¹³C-labelled litter (hemicellulose:lignin ratios of 0.42–1.02) from six maize plant parts was used to trace ¹³C incorporation into dissolved organic ¹³C (DO¹³C), microbial biomass ¹³C (MB¹³C), particulate organic ¹³C (PO¹³C), mineral-associated organic ¹³C (MAO¹³C), and ¹³C-microbial necromass during an 84-day incubation. After 84 days, 11–18 % of litter decomposed to CO₂, with fast decomposition at high hemicellulose:lignin ratios. Most litter C (9.1–16 %) was incorporated into MAO¹³C, followed by MB¹³C, PO¹³C, and DO¹³C. High hemicellulose:lignin ratios reduced microbial C use efficiency (CUE), indicating microbes prioritized energy utilization from hemicellulose-rich, labile substrates. Lower CUE led to greater MAOC accumulation within 84-day incubation. Increased ¹³C-fungal necromass at a higher hemicellulose:lignin ratio suggests that labile C-rich litter supports new C sequestration, as fungal necromass is more stable than bacterial. ¹³C-microbial necromass accounted for 22–38 % of MAO¹³C, suggesting that a large fraction of litter C was directly incorporated into MAOC without microbial transformation during the 84-day incubation period. Bacterial necromass formation was regulated by C-degradation enzyme activity, while fungal necromass was governed by litter quality. These findings highlight the role of litter quality and C-degradation enzyme activities in forming newly sequestered C by regulating C incorporation into microbial necromass, emphasizing labile litter component importance in soil C sequestration.