Dr Zhenke Zhu1, Dr Jun Cui1, Dr Tida Ge1, Dr Jinshui Wu1
1Institute of Subtropical Agriculture, Chinese Academy of Sciences, CHANGSHA, China
The relationship between the amount of labile C input and the resulting priming effects (PEs) on soil organic matter (SOM) mineralization remains unclear, particularly under anoxic conditions and at high C input rates common in microbial hotspots. PE intensity and the relevant mechanisms were investigated by incubation of three flooded paddy soils over 60 days after 13C-labeled glucose addition ranging from 50 to 500% of microbial biomass C (MBC). CO2, CH4 and N2O emissions were continuously measured and partitioned for C sources based on 13C-label. PE peaked at moderate glucose addition rates (50-300% of MBC) but decreased, and even became negative, under higher rates. Nitrate production from SOM and N2O emission also peaked at moderate glucose loads in accordance with the N mining from SOM expected for PE. However, low or negative PE at high glucose loads above 300% of MBC posed a paradox of stronger N-acquisition accompanied by decreased SOM mineralization. Particularly at glucose input >3 g kg-1 (corresponding to 300-500% of MBC), strong N immobilization by microorganisms was confirmed by minimum levels of soil mineral N and negligible N2O emission. Concomitantly, microorganisms intensified N acquisition from microbial necromass by increased N-acetyl glucosaminidase and leucine aminopeptidase activities without accelerating SOM decomposition. Several peaks of glucose-derived CO2 and CH4 effluxes were observed between days 13 and 30, confirming decomposition of glucose-derived microbial necromass (contribution to corresponding total CO2: >80%). Contents of glucose-derived mineral-bound C, which originated from microbial biomass turnover was used as a proxy of net necromass accumulation, increased with glucose input levels over 60 days. Labile C input also accelerated the conversion of living microorganisms to necromass. Therefore, necromass recycling was a hypothesized mechanism to alleviate microbial N deficiency without priming SOM (hereafter referred to SOM components other than necromass). Compound-specific 13C-PLFA confirmed redistribution of glucose-derived PLFAs, i.e. 13C recycling, among microbial groups during the 60 days. Initially after labile C input, gram-negative bacteria (presumably r-strategists) rapidly incorporated glucose together with N and used them for growth. After glucose was exhausted and r-strategists died, their necromass became a substrate providing labile C and N for the gram-positive bacteria, actinomycetes and fungi, which were less responsive initially after glucose addition. We conclude that N sources for microorganisms depend on labile C input: to cover N demand under C excess, microorganisms switch from SOM-N mining to the N recycling from microbial necromass.
Zhenke Zhu, an associate professor of the institute of subtropical agriculture, Chinese academy of sciences. His field of study is the carbon and nitrogen cycle process and the microbial mechanism in the subtropical paddy field. The main focus now is the mechanism of C:N:P stoichiometry regulates the organic carbon cycle in the paddy soils.