Influence of changes in land use and earthworm activities on carbon and nitrogen dynamics in a steepland ecosystem in Northern Vietnam

This manuscript focuses on the effects of land-use change and earthworm activities on the dynamics of dissolved nutrients (carbon and mineral nitrogen) in a steep slope ecosystem in Northern Vietnam. We investigated the properties of soil aggregates (casts and surrounding soils) sampled in different agrosystems: a plantation of Bracharia ruzziziensis (BRA), a fallow (FAL) and a plantation of Acacia mangium and Venicia montana (FOR), following a cultivation of cassava (CAS), and a fallow after a forest of Eucalyptus (EUC). Soil physical, chemical and biological properties were determined on the sampling date and dissolved organic C (DOC), and mineral N (N min) contents were followed during a 21-day incubation period. CAS, BRA and FOR were characterised by a high rate of N mineralisation, followed by a rapid loss of N min. Conversely, FAL and EUC were more interesting in terms of soil fertility because these systems had higher soil N min content that could become available to plants. Lost of C through DOC leaching was very low with values ever less than 0.5% of the total soil C content. The greatest lost of C through leaching was in FAL, EUC and FOR. The impacts of earthworms on the soil seemed site-specific. The protection of organic matter in earthworm casts varied with the initial substrate soil and agroecosystem management. Casts were characterised by greater enzymatic activities, except for alkaline phosphatase, than the surrounding soil. While SOM mineralisation was not affected in casts collected in FAL, EUC and FOR, the leaching of N min was increased over the surrounding soil. Conversely, mineralisation of SOM and nitrification activity were less in BRA-casts than in the surrounding soil, causing a greater retention of N min in soil. While the DOC leached from casts increased in BRA, it was reduced in FAL and was similar in the other sites. Our study indicates that while land-use change occurred only 3 years ago, the extent of leaching was mostly regulated by the type of agroecosystem and by earthworm activities.