Adding ligneous litter to cultivated organic soil changes the soil micro-food web and alters soil nitrogen availability

Organic soils are major carbon (C) sinks that when drained for cultivation, become large sources of CO2-C to the atmosphere and are susceptible to high rates of wind erosion. Mulching organic soils with ligneous litter is one strategy for reducing erosion, however, this could shift a bacteria-dominated soil to a fungal-dominated soil, affecting the microbivorous nematode populations and C and nitrogen (N) cycling. We conducted two, six-week greenhouse experiments, with soil sourced in 2020 and then in 2021, to determine how the micro-food web responds to different ligneous litter inputs and placement. We also examined the coinciding effects on bioavailable N and gene abundances encoding for decomposition enzymes to assess how changes in the micro-food web potentially alter C and N dynamics. We compared four different litters (larch, miscanthus, ash, or willow) applied to soil separately and ranging in lignin:N between 65 and 339. To test the effect of litter placement, each litter type was left on top or incorporated into soil planted with lettuce. Starting soil nematode, fungal and bacterial community compositions differed between 2020 and in 2021. Litter additions changed the nematode and fungal community compositions, but the bacteria community was unaffected by litter inputs. We found that litter additions generally increased fungal and fungivore abundances, along with Cephalobidae (cp-2 nematodes) that are typical of low-resource availability. However, this effect was highly dependent on litter type, placement, and year. For instance, the increase in fungal abundance relative to no litter additions only occurred when litter was incorporated into the soil, and this was especially true for our mid-range lignin:N litters, miscanthus and ash (p < 0.001). Regardless of litter type, we saw little evidence for litter inputs reducing bioavailable N compared to the no-litter control. Incorporated miscanthus and ash litter also increased root biomass (but not lettuce biomass) relative to no litter inputs (p < 0.05). These litters had the lowest (miscanthus) and highest (ash) amount of lignin and were also the only litter to increase in decomposition gene abundance, suggesting that microbial function is sensitive to lignin content, but not lignin:N. We suspect that the higher abundance of nematodes with ligneous additions is elevating microbivory, helping to maintain soil bioavailable N. However, the variability we see between 2020 and 2021 suggests that the micro-food web response to litter additions depends on the starting field soil and its biological community. Our results show that ligneous litter amendments select for a fungal micro-food web, but do not necessarily lead to reduced bioavailable N within a 6-week period, despite the high lignin and C:N of the litters. Thus, it appears that soil mineral N levels are maintained with litter incorporation and that this is associated with changes to the micro-food web, particularly increases in fungal and nematode abundances.