Experimental investigation of the importance of litterfall in lowland semi-evergreen tropical forest nutrient cycling

P>1. The cycling of nutrients in litterfall is considered a key mechanism in the maintenance of tropical forest fertility but its importance has rarely been quantified experimentally. 2. We carried out a long-term (5 years), large-scale litter manipulation experiment in lowland semi-evergreen tropical forest to determine how changes in litterfall affect forest nutrient cycling. We hypothesized that: (i) long-term litter removal would decrease the forest's nutrient supply; (ii) litter addition would increase the forest's nutrient supply; (iii) soil and foliar nutrient concentrations would change in response to litter manipulation and would eventually affect above-ground productivity. 3. To test our hypotheses, we measured trunk growth, litterfall, and nutrient concentrations in live leaves, litter and soil in plots where litter was removed once a month (L-), litter was added once a month (L+) and controls (CT). 4. After 5 years, the concentration of nitrate in the soil and soil stocks of inorganic nitrogen were higher in the L+ plots and lower in the L- plots compared to the controls. Ammonium concentrations in the soil were also lower in the L- plots. Nitrogen in leaves and litter and the annual nitrogen return by litter were higher in the L+ plots, while potassium return was lower in the L- plots. Surprisingly, our treatments had little effect on phosphorus in soil, leaves or litter, even though lowland tropical forests are generally thought to be largely phosphorus limited. 5. Trunk growth of large trees was not affected by litter manipulation but rainy season litterfall from 2003 to 2008 was 13% higher in the L+ plots compared to the controls. 6. Synthesis. Litter removal affected forest nutrient cycling and productivity less than expected, probably because the soil at our site is moderately fertile. However, litter addition increased litterfall indicating that some limitation of forest production was removed by litter addition. We expected strong effects of litter manipulation on phosphorus cycling; instead, we found a stronger effect on nitrogen cycling. Our results suggest that litter is an important source of nutrients, in particular nitrogen, to trees in this lowland semi-evergreen tropical forest.