N cycling increase after savanna afforestation with Eucalyptus or Acacia is reflected in the growth of soil ammonia-oxidizing archaea and nematode bacterial-feeders

In tropical humid Africa, sandy soils under periodically burnt herbaceous savannas exhibit generally low carbon (C) content and nitrogen (N) availability. Savanna afforestation may overcome these limitations through changes in soil functioning, yet these processes still need to be explored. In this study, we investigated whether changes in the composition of soil micro-food web may explain soil C and N cycling increases following savanna afforestation. We conducted a 7-year experiment in Congo including Eucalyptus and N2-fixing Acacia monocultures and Eucalyptus-Acacia mixtures established on former herbaceous savannas. We assessed in each of these modalities the soil attributes: organic C and N, pHH2O, nitrate, ammonium, net C and N mineralization and nitrification rates, along with the abundances of bacteria, fungi, nematodes, ammonia-oxidizing archaea (AOA) and bacteria (AOB) in the top 10 cm layer. Afforestation of savannas with Eucalyptus for timber production increased soil C by 1.7 times, soil net N mineralization rates by 1.9 times and soil inorganic nitrogen by 2.5 times. Mixed Acacia- Eucalyptus and Acacia monoculture plantations further improved the rate of net nitrogen mineralization by a factor of 1.4 and soil inorganic N by a factor of 2.3 compared with Eucalyptus monocultures. These changes were associated with a gradual increase in AOA abundance from savanna to Eucalyptus monoculture, Eucalyptus-Acacia mixtures and finally Acacia monoculture. Savanna afforestation resulted in a significant increase in the absolute abundance of bacterial-feeding nematodes by 678 %, but to a decrease in the abundance of both fungal-feeders, and omnivores and predators. Increase in N cycling was positively associated with both AOA and nematode bacterial-feeder abundances, underlining the importance of monitoring micro-food web structure to understand better how land use changes affect soil biogeochemical cycling in the context of tropical afforestation.