Enhanced Nitrogen Release from Sugarcane Straw in a Sandy Soil Via Microbial Inoculants Efficient in Decomposing High C/N Ratio Organic Matter

Organic inputs with high carbon (C)/nitrogen (N) ratio reveal generally a slow decomposition rate along with increased N immobilization. It could be suggested that such N limitation inhibiting subsequent mineral N (Nmin) release could be compensated by inoculation of fungi and bacteria efficient in decomposition (C cycling) and proteolysis (N cycling), respectively. Our primary goal was to evaluate the effect of a microbial inoculum consisting of C-degrading fungi and Bacillus sp. on the decomposition of sugarcane straw. We investigated the resulting effects on Nmin levels and short-term soil organic carbon (SOC) dynamics. The incubation experiment (84 days) consisted of a completely randomized design (CRD) with three treatments: (1) CT, no inputs; (2) I, sugarcane straw incorporated at the rate of 0.83 g kg-1 DW equivalent to 6.25 tons ha-1; (3) I + MC, I treatment plus inoculation of a consortium of decomposing microorganisms comprising 1.9 x 106 (fungi) and 2.2 x 108 (Bacillus sp.) CFU g-1 (MC). Destructive soil samplings were performed at 7, 28 and 84 days after incubation (DAI) start to assess protease activity and abundance of the neutral metalloproteases (npr) gene encoding neutral metalloproteases of proteolytic microorganisms including Bacillus sp. Microbiological analyses were complemented with measurements of dissolved organic carbon (DOC), SOC, dissolved organic nitrogen (DON), and Nmin contents. Inoculation of C-degrading fungi and Bacillus sp. stimulates the decomposition of high C/N ratio organic inputs with low protein content, leading to increased Nmin (42%) and SOC (23%) accumulation compared to CT at 28 DAI. The increase of Nmin content was the result of stimulated protease activity accelerated by inoculation. The promotion of proteolysis and Nmin content was facilitated by the availability of sufficient substrate (e.g., DOC, DON), which was enabled by C-degrading fungi, benefitting short-term SOC accumulation in the early stages of decomposition.