Turnover and Microbial Utilization of Four Different Photosynthesized Carbon in Paddy Soil
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Soil C is mainly composed of old, indigenous soil organic C (SOC), but also includes exogenous C representing recent products of photosynthesis. Input of fresh organic C such as green manure, straw, rhizodeposits and microbial assimilated carbon to soil serve as substrates for CO2 and CH4 production. Input of low-molecular-weight organic C such as rhizodeposits and sugars could stimulate an opportunistic subset of the bacterial biomass, and the labile substrates can be directly metabolized or recycled for energy production and microbial growth, leading to a decline in microbial abundance, species richness, and diversity.Accordingly, differences in substrate quantity, quality and pathways of C input into paddy soil and the microbial utilization of this C are essential for soil C turnover and sequestration.

The turnover of exogenous carbon (C) plays a significant role in the soil organic C (SOC) cycle. However, there is limited information about the turnover of plant- and soil microbe-derived recently photosynthesized C in paddy soil.

Recently, researchers in the Institute of Subtropical Agriculture,Chinese Academy of Sciences(ISA) employed 13C labelling techniques and 13C-PLFA-SIP to investigate the turnover and microbial utilization of organic C from different sources by conducting a 300 days incubation study with four different 13C-labelled substrates: rice shoots (Shoot-C), rice roots (Root-C), rice rhizodeposits (Rhizo-C), and microbe-assimilated C (Micro-C).

The team found that Shoot- and Root-C were initially rapidly incorporated into the dissolved organic C (DOC) pool, while their recovery in into microbial biomass C (MBC) and SOC increased. With the exhaustion of the DOC pool after 50 d incubation, the ratios of Shoot- and Root-C incorporated into MBC and SOC declined. However, the pattern of Rhizo- and Micro-C incorporated into DOC, MBC, and SOC pools slowly decreased over time, and most of the Rhizo- and Micro-C were recovered in the MBC and SOC pools. The soil microbes that utilized the input C depended upon the substrate quantity and incubation time. Input of Shoot- and Root-C greatly increased 13C- phospholipid-derived fatty acid (13C- PLFA) concentrations in the initial 50- d incubation. The dominant ‘active’ microbes, gram-positive bacteria, then gradually decreased, while fungi and actinomycetes increased during the whole incubation period. The key microbial decomposers utilizing Rhizo- and Micro-C showed an inverse pattern to that of those using Shoot- and Root-C. In addition, the ratios of 13C incorporation into fungi to that into bacteria and into gram-positive to into gram-negative bacteria significantly decreased over time in the Shoot- and Root-C treatments, but not in the Rhizo- and Micro-C treatments. Principal component analysis showed that microbial community composition shifted obviously in the Shoot-C and Root-C treatments over time, but that composition changed little in the Rhizo-C and Micro-C treatments, which suggests that the rhizodeposited and microbial-assimilated C were mainly in the forms of microbial biomass and SOC, stable soil C pools that were metabolized via microbial internal cycling and that exhibited higher efficiency in soil C sequestration. Therefore, the amount and composition of input of C from plant residues into soil varied during the incubation period, which could significantly affect the active functional microbial groups and the soil autochthonous microbes.

The results suggested that he input C substrates drived soil microbial community structure and function with respect to carbon stabilization. Although rice plant residues were widely used and large amounts enter the soil as shoot and root litter, their contribution to SOC formation was inefficient. In contrast, rhizodeposits and microbial-assimilated C had lower input rates, but were better stabilized by microbial metabolism, microbial recycling, and associations with soil minerals, and thus were characterized by high-efficiency soil C sequestration

One referee said: “Such long-term incubations are very seldom, and surely provide good options to analyze medium-term utilization processes of the C added to soils. Further, the use of 13C tracing with subsequent tracing in the soil pools but especially in PLFA clearly showed the pathways of C fluxes after residue incorporation in soil”.

This research was supported by funding from the National Natural Science Foundation of China (41522107; 41501321), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB15020401), and the Recruitment Program of High-End Foreign Experts of the State Administration of Foreign Experts Affairs, awarded to Prof. Georg Guggenberger (GDT20164300013).

The study entitled “Fate of rice shoot and root residues, rhizodeposits, and microbial assimilated carbon in paddy soil - Part 2: turnover and microbial utilization” has been published in Plant and Soil, details could be found at
https://link.springer.com/article/10.1007/s11104-017-3210-4?wt_mc=Internal.Event.1.SEM.ArticleAuthorOnlineFirst 

Contact: GE Tida
        E-mail: gtd@isa.ac.cn
        Institute of Subtropical Agriculture, Chinese Academy of Sciences