Assessment of degradation and generation of humus in a coffee soil affected by weed cover by means of a stable carbon isotopic ratio

Abstract

We evaluated the effect of soil conservation by weeds on the degradation and generation of humic acids, fulvic acids, and water-soluble non-humic substances (WS-NHS) in a red-acid soil (Vertic Dystrudept) (Indonesia) from the changes in humus composition and stable carbon isotopic ratio (δ13C). Three plots, a weeded plot (T-1; the common practice), a plot covered with Paspalum conjugatum Berg., a C4 plant (T-2), and a plot in which native weeds were allowed to grow (T-3), were prepared. An incubation experiment determined the δ13C values of the humus fractions generated from Paspalum in soil. Based on the increase in δ13C value, the proportion of total C that originated from Paspalum C after 4 years under coffee was 16 ± 4% in the T-2 topsoil (0-10 cm). Humic and fulvic acids in the T-1 topsoil decreased to 46 and 84%, respectively, whilst both increased or remained constant in the T-2 and T-3 soils. The WS-NHS content varied little and was independent of land management. The preferential loss of the humic acids with a smaller degree of humification as assessed by their darkness in colour was shown in T-1. The decrease in the degree of humification suggested the accumulation of the weed-derived humic acids in T-2 and T-3. In the T-2 topsoil, 36 ± 2%, 13 ± 3% and 15 ± 2% of C in the humic acids, fulvic acids and WS-NHS, respectively, were estimated to be Paspalum-derived after 4 years. The estimated initial C loss during the same period was 17 ± 3%, 14 ± 2% and 7 ± 2%, respectively, for those fractions, which suggests the fastest turnover rate for the humic acids and significant retardation of their degradation in soil colonized by weeds. © 2007 British Society of Soil Science.