Inoculation of cellulolytic and ligninolytic microorganisms accelerates decomposition of high C/N and cellulose rich sugarcane straw in tropical sandy soils

Due to the limited functional potential of the microbial decomposer community in tropical sandy soils, a significant proportion of organic inputs with a high C/N ratio and cellulose (CL) content may remain undecomposed. Moreover, high C/N ratio organic input led to N immobilization and suppressed succeeding crop yield. Inoculation of competent microbial decomposers to such soils could constitute a promising approach to overcome this problem, with beneficial effects on soil organic carbon (SOC) accumulation. Therefore, the objectives were (i) to assess the ability of selected cellulolytic and ligninolytic microbial inoculants to promote the decomposition of high C/N ratio and high cellulose inputs in the initial stages after application, using soil enzyme activity measurements, and (ii) to verify the decomposition efficiency of microbial inocula to enhance short-term soil organic carbon (SOC) quantity and quality, using mid-infrared diffuse reflectance Fourier transform spectroscopy (midDRIFTS)-based functional SOC group analysis, integrated with assayed soil enzyme activities. The experiments consisted of 4 treatments: 1) Control (C): no application of organic inputs; 2) Mulch (M): sugarcane straw mulched at a rate of 6.25 tons ha−1; 3) Incorporation (I): sugarcane straw incorporated to soil at a rate of 6.25 tons ha−1; and 4) Incorporation (treatment 3) + inoculation of an artificial microbial consortium (I + MC). Two experiments were conducted i.e. experiment I (field-based micro-plot and litter bag study (32 weeks)) using a randomized complete block design (RCBD) and experiment II (lab-based incubation study (84 days)) using a completely randomized block design (CRD). Sandy soils derived from a farmer's field located in the Khon Kean province (Northeast Thailand) were used. MC was composed of cellulolytic and ligninolytic bacteria (2.2 × 108 CFU g−1 soil) and fungi (1.9 × 106 CFU g−1 soil) with an application rate of 0.1 g kg−1 sugarcane straw. Our results indicated that treatment I + MC led to the highest dry weight loss of sugarcane straw (73%), highest cellulolytic and ligninolytic enzyme activities (β-glucosidase, cellulase, peroxidase, and phenoloxidase), and short-term SOC accumulation. I + MC also showed increased peroxidase and phenoloxidase activities in the early stages of decomposition. Integration of midDRIFTS and enzyme activity analyses demonstrated that in the initial stages of decomposition cellulolytic and ligninolytic enzyme activities enhanced labile SOC components, while in the later stage, cellulolytic and ligninolytic activities enhanced both labile and stable SOC components. In conclusion, farmers may benefit from this approach, but field validation would be needed prior envisioned large-scale application, including the development of formulation and field application processes, evaluated against environmental and economic indicators.