Microbial carbon source utilization in rice rhizosphere and nonrhizosphere soils with short-term manure N input rate in paddy field

In the present study, the results indicated that soil MBC content in rhizosphere and non-rhizosphere soils were increased with application of organic manure practices, compared with without N fertilizer input practices. The reason maybe that soil available nutrients increased by application of organic materials, which provided carbon substrates and nutrients for the growth and reproduction of soil microorganisms, and then increased soil microbial growth rate and microbial biomass¹⁹, compared with without N fertilizer input practices. Meanwhile, the soil MBC content with organic manure treatments were higher than that with chemical fertilizer alone treatment, the reason maybe that decomposition of organic manure, soil C/N and slow-acting nutrients were increased under input organic manure conditions, which promote the growth of soil microorganisms²⁰. And the basal respiration of soil microorganism were increased under application of organic manure conditions, the reason maybe that decomposable organic carbon is the main carbon source for microbial utilization, and there are significant differences in the content of decomposable organic carbon under different manure N input fertilizer treatments²¹. On the other hand, rice root exudates can induce a positive stimulating effect of soil organic carbon decomposition, which in return increases soil microbial respiration²². And the amount of root exudates were affected by application of different fertilization practices, root biomass and root exudates were increased under application of organic manure conditions⁹. In this studies, the C_Growth with fertilizer treatments were higher (p < 0.05) than with without N fertilizer input treatment, the reason maybe that there are significant differences in soil organic carbon content among different fertilizer treatments, which may lead to significant differences in available carbon sources of soil microorganisms²³.

In this study, the results showed that CUE of soil microbial in rhizosphere soils were lower than that of non-rhizosphere soils, the reason maybe that value of C_Respiration were higher in rhizosphere soils, thus the distribution of respiration carbon by rhizosphere soils microorganisms were higher than that of growth carbon. On the other hand, carbon content in rhizosphere soils were increased in the process of root system absorbs nutrients, and the soil microbial CUE were decreases with the increase of C content⁹. There is no significant difference in CUE of non-rhizosphere soils among different fertilizer treatments, the reason may be that physiological environment of non-rhizosphere soils is relatively stable, which was consistent with previous research²⁴. In this study, the higher CUE of soil microbial with chemical fertilizer alone treatment (M0) than that of with organic manure treatments (M30, M50 and M100) consistent with our Hypothesis 1 that soil microbial CUE were increased under application chemical fertilizer conditions (Fig. 1), the main reason was that soil carbon nutrient ratio and carbon dioxide (CO₂) through overflow respiration were increased, soil carbon nutrient ratio was benefit to meet the nutritional needs of soil microorganisms¹⁰, and thus lower soil microbial CUE under application of organic manure conditions⁹. However, the oxidase involved in the degradation of aromatic compounds in N were inhibited, the energy demand of soil microorganisms were reduced with chemical NPK fertilizer input, therefore, the soil microbial CUE were increased under application of chemical fertilizer conditions⁹.

The characteristics of soil microbial communities on carbon metabolism can reflect the bioavailability and functional diversity of soil microorganisms⁷. The average CO₂ production rate was usually used as an effective indicator of soil microbial activity, and also reflects the ability of soil microbial communities to utilize carbon sources⁵. In the present study, the metabolic capacity of soil microorganisms to exogenous carbon sources with application of organic manure treatments were higher than with chemical fertilizer alone and without N fertilizer input treatments, the reason maybe that organic manure contains a large number of living microorganisms and active organic carbon sources, and thus the metabolism of microorganisms to carbon sources were promoted when application of organic manure. As a result, the higher metabolism of exogenous carbon sources with organic manure treatments than that of with chemical fertilizer alone and without N fertilizer input treatments consistent with our Hypothesis 2 that metabolic capacity of soil microorganisms to exogenous carbon sources would be higher with organic manure input conditions than that of with chemical fertilizer alone and without N fertilizer input conditions. And the metabolic capacity of soil microorganisms to exogenous carbon sources with chemical fertilizer alone treatment were higher than without N fertilizer input treatment, indicating that soil C/N were increased, soil microbial activity and decomposition rate were decreased, and thus soil microbial metabolic ability to carbon source were decreased under without N fertilizer input conditions¹⁹. Our results indicated that average utilization rate of carboxylic acids were higher than that of complex compounds, which suggested that carboxylic acid carbon sources account for only a small proportion of soil dissolved organic carbon, but they are important energy sources for the growth and metabolism of soil microorganisms²⁵. On the other hand, degradation of complex compounds requires the interaction of various extracellular enzymes, more carbon sources and energy were invested in the synthesis of extracellular enzymes during soil microorganisms growth, and thus the utilization rate of complex compounds were reduced^3,26. In this study, the RDA analysis results showed that there was obvious difference in metabolism of soil microorganisms to exogenous carbon sources between without N fertilizer input, chemical fertilizer alone and organic manure treatments, indicating that organic manure treatments has greatly changed the soil environment, heterogenous organic manure and microorganisms were introduced, and then significantly changed the utilization characteristics of soil microorganisms to exogenous carbon sources.

In the present study, the soil microbial growth rate were increased, but the soil microbial CUE were decreased under application of organic manure condition. These results were also confirmed in the previous research by using MicroResp^TM method, the soil microbial metabolism ability to exogenous carbon (release to CO₂) were significantly increased with organic manure practices²⁷. In addition, the metabolic capacity of soil microorganisms to exogenous carbon sources in non-rhizosphere soils were higher than that of rhizosphere soils (Fig. 2), which was inconsistent with the results of soil basic respiration based on ¹⁸O-H₂O method. The reason may be that MicroResp^TM method mainly monitors microbial decomposition of exogenous carbon, and it doesn’t differentiate between excitation effects, but ¹⁸O-H₂O method mainly monitors microbial decomposition of soil carbon. On the other hand, rhizosphere soils were adaptability to exogenous carbon source input (root exudates) conditions. Meanwhile, the positive stimulation effect of decomposition of original organic carbon in non-rhizosphere soils were induced with input of exogenous carbon sources, and thus microbial mineralization of organic carbon were enhanced²⁶.

Source: Ecology - nature.com