Preceding crops may reduce denitrification potential and enhance ammonium assimilation pathways.

Abstract

Background: Soil microorganisms are pivotal to nitrogen (N) cycling in croplands, yet how preceding crops modulate their functional profiles remains unclear.

Objective: This field study aimed to quantify the effects of barley (BT) and rapeseed (RT) preceding crops (vs. no preceding crop, CK) on soil microbial functions and N-metabolic pathways in tobacco fields.

Results: High-throughput metagenomics revealed that BT and RT significantly increased soil microbial richness (Chao1 index) compared to CK. At the genus level, CK contained 64% and 24% fewer unique taxa than BT and RT, respectively. While the top five KEGG functional pathways (e.g., Metabolic pathways, Biosynthesis of secondary metabolites) were conserved across treatments, their relative abundances differed. Critically, preceding crops reduced soil denitrification rates and increased glutamine dehydrogenase activity. Redundancy analysis confirmed that ammonium-N concentration was the key edaphic factor strongly correlated with microbial community structure and function (P < 0.01).

Conclusion: Our findings demonstrate that barley and rapeseed preceding crops enhance microbial richness and activity, thereby inhibiting denitrification and promoting N fixation via altered ammonium-N dynamics.