Effect of manipulating surface organic matter on C and N cycles in the soils of a Eucalyptus plantation in the Congo
Thesis defended on 16 November 2012 in Nancy
In recent years, rapid-growth tree plantations have been considered a means of meeting the increasing demand for ligneous forest products as an alternative to exploiting natural forests in tropical areas. A major challenge now is to ensure the sustainability of these plantations which are often on poor soil with significant export of biomass every 6-7 years. The organic matter (OM) on the surface of the soil can play an important role in the mineral and organic fertility of the soils which govern the productivity of these tropical ecosystems in the long and short term.
This study set out to understand the mechanisms through which the removal of all OM from, or the addition of OM to, the surface of the soil affect the development of Eucalyptus plantations during the first two years after planting. In parallel several experiments were carried out to measure the growth of the trees and determine the nitrogen (N) mineralization dynamics during the decomposition of harvesting residues. The flows of dissolved nutrients were compared with the initial stocks in various compartments of the ecosystem and to the mineral stocks in the trees at the end of the study. The N recovery from harvest residues was estimated using 15N at plantation scale when the stand was planted. The longer term effect of manipulating the surface OM on the soil OM stocks was also evaluated. Soil analyses (C and N) and complete partitioning of the source of CO2 emitted at the surface of the soil were also carried out. In particular, the quantitative and qualitative dynamics of the flows of dissolved OM were also studied.
The results showed that the rapid release of K and the more progressive release of the N contained in the surface OM explained most of the differences in growth observed between different surface OM management methods. Eucalyptus trees make full use of these nutrients released with very low losses from drainage given the depth of these soils and rapid root development both at depth and within the organic horizon. This study also showed the fundamental role of OM in the 0-15 cm horizon in producing nitrates and dissolved OM. Using a 15N tracer showed that N is initially retained within the organic horizon, probably by microbial immobilisation, transferred at the same time equally between the various litter components (leaves, bark, branches) and then probably transported in the form of particulate OM into the A horizon by gravitational water, thus supplying the pool of OM on which the fertility of these soils depends. The presence of surface OM maintains the stocks of soil OM after clear felling whereas removing the OM reduces the stocks of OM at the start of planting although this appears to be compensated subsequently by input from litter fall. Adding surface OM increases the dissolved OM in the top 15 cm of the soil which, however, is still negligible relative to the OM stocks in these soils and which does not interact to any great extent with the OM stocks. Despite the rapid reconstitution of the pool of soil OM, some nutrients may limit the productivity of these plantations after several rotations as a result of repeated export of biomass. Consequently, harvest residues should be retained as far as possible and the export of ligneous biomass should be compensated by the input of fertilisers, burning should be banned and wildfires avoided.