Fine root decomposition under grass and pine seedlings in controlled environmental conditions

Soil carbon (C) stocks generally decrease following land use conversion from pasture to conifer (pine) plantation. In exploring whether factors relating to fine root decomposition might be involved in that effect, a 1-year pot experiment in a controlled temperature glasshouse was designed to test the role of three factors on fine root decomposition. The factors were: fine root type (roots of grass-Themeda triandra, and pine-Pinus radiata), soil source (top soil from a native grass pasture and top soil from an adjacent pine plantation), and species planted and its treatment (pine, uncut grass, defoliated grass, and soil-only control). The dry mass loss in 1 year, averaged across all factors, was 39% of the initial mass and the average decomposition rate constant (k) was 0.51 year−1. Only the species planted had an effect on dry mass loss from decomposing roots, i.e. pine seedlings reduced the decomposition rate of fine roots. Carbon loss from decaying fine root was affected by the decomposing root type as well as the species planted in the soil. The C loss from pine root decomposition was slower than from grass root decomposition, and pine seedlings reduced the rate of C loss from decomposing root. All three factors affected N loss. Net N loss was found only from decaying grass roots, N being accumulated in decaying pine roots. Decaying pine fine root accumulated more N when in soil from the plantation (having lower N concentration) than when in soil from the native pasture (with higher N concentration). In conclusion, the findings have led us to a hypothesis for field testing that, in terms of carbon, pine roots decompose to soil humus slower than grass roots because of differences between ecosystems in both root litter quality and soil environment as determined by the plant species growing.