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

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.