Biochar- or straw-mediated alteration in rice paddy microbial community structure and its urea-C utilization are depended on irrigation regimes

Abstract

Urea, one of the most widely used nitrogen fertilizers and known for its high nitrogen content, also contains 20 % carbon, which is often overlooked. The fate of urea-derived nitrogen (urea-N) in agricultural ecosystems is well-documented. However, little is known about the fate of the urea-derived carbon (urea-C) in the soil ecosystem, especially its utilization by soil microorganisms. To address these knowledge gaps, an experiment was conducted using ¹³C-labeled urea combined with ¹³C-PLFA-SIP to investigate which microorganisms benefit most from urea-C and how its utilization is affected by irrigation regimes (water-saving and flooded irrigation) and organic amendments (straw and biochar). Our results showed that both soil microbial biomass and community structure were strongly influenced by irrigation regimes and organic amendments, with microbial biomass significantly increased by straw application and/or water-saving irrigation. In water-saving irrigated soils, microbial biomass was higher, but the incorporation of urea-¹³C into PLFA was much lower compared to flooded conditions, indicating a higher potential for the assimilation of urea-C by microbes in flooded paddy fields. Some patterns in the partitioning of urea-C by microbial groups were similar across treatments: General and G- bacteria were the dominant groups assimilating urea-C, followed by fungi, G+ bacteria, and actinomycetes. Notably, the shifts in the pattern of ¹³C incorporation into PLFA induced by straw amendment were more pronounced in water-saving irrigation than in flooded irrigation, while shifts induced by biochar amendment were more pronounced in flooded irrigation than in water-saving irrigation. Similar patterns were also observed in their effects on soil microbial community structure, indicating that the effects of straw or biochar amendments on soil microbial community structure and their urea-C utilization patterns differed between irrigation regimes. These results provide valuable insights into the roles of different microbial functional groups in the competition for and processing of urea-derived C, enhancing our understanding of soil microbial communities and microbial-mediated carbon cycling under varying irrigation and soil amendment conditions.