Combined application of N-fixing PGPB and rice straw mulch compensates N immobilization by straw, improving crop growth

Background Rice straw, often treated as waste, provides many benefits to crops when used as a mulch. However, straw degradation promotes nitrogen immobilisation due to its high C:N ratio, causing N competition between soil microorganisms and the crop. Currently, sustainable practices to remedy nitrogen immobilisation are hardly being implemented. In microcosm conditions we assessed whether the inoculation with N-fixers could offset the transient nitrogen deficiency caused by straw mulch, thereby harnessing the benefits of straw while mitigating its negative impact on nitrogen depletion and exerting a synergistic effect on crop growth. Results Inoculation with N-fixers increased the nitrogen content in the soil (the increase ranged from 14% up to 90% for NH 4 + and from 20% to 60% for NO 3 − ) and, in most cases, also the nitrogen content in the plant (ranging from 10% to 15% increase), compared to the non-inoculated control. Therefore, inoculation would compensate for the lack of nitrogen caused by nitrogen immobilisation, and this resulted in an increased biomass production by the crop compared with the uninoculated control (the increase ranged from 25% to 85%). In addition, inoculation with N-fixers did not lead to a permanent change in the bacterial community composition, whereas straw addition increased the biodiversity of the soil microbiome. Conclusions The results obtained in microcosm conditions are a first indication that complementing straw mulching with the inoculation of N-fixers could avoid the transient N immobilisation produced during straw degradation. Thus, the benefits of the combination would be a yield increase, while improving the biodiversity of the soil microbiome, stabilising soil temperatures and increasing water soil content. Graphical Abstract