Nutrient limitations play a critical role in regulating tree growth and net primary production (NPP). To understand how nutrient availability constrains ecosystem processes on highly weathered soils in tropical forests in Africa, we established a large-scale nutrient manipulation experiment (NME) in a highly diverse semi-deciduous tropical forest in northwestern Uganda. Specifically, our objective was to investigate the roles of nitrogen (N), phosphorus (P), and potassium (K) and their interactions have on ecosystem processes across a hierarchy of scales. The NME was set up using a factorial experimental design including eight nutrient addition treatments (N, P, K, and in combination as N+P, N+K, P+K, and N+P+K), each replicated four times. At each of the thirty-two 40 x 40 m plots, we measured the effects of the nutrient additions on above- and belowground NPP. Aboveground NPP was assessed by measuring (i) tree diameter growth increments using dendrometer bands (monthly) and census techniques (bi-yearly), and litterfall was collected bi-weekly in litter collectors. Belowground, was measured root biomass (yearly) and rooting productivity was estimated using ingrowth cores and sequential coring approaches. Measurements reported here were made in the first two years of the NME.

In this experiment, we determined that multiple nutrients (co)regulate different ecosystem processes. While there were no community-wide tree growth responses to nutrient addition, we found that tree growth (and their limitations) varied depending on (1) tree species, where a few dominant species grew significantly larger in N addition plots, and semi-deciduous trees responded positively to K addition during dry periods, and (2) tree size, where medium-sized trees (10-30 cm diameter at breast height (DBH)) grew significantly faster in all N addition treatments in the second year of the experiment. Next, although tree growth among poles and saplings (1‑10 cm DBH) was not influenced by any nutrient additions, P addition reduced tree mortality of saplings (1-5 cm DBH).

Belowground, the addition of nitrogen reduced fine root biomass (FRB) by 35 % in the first year of the experiment. This rapid and large reduction in FRB highlights that maintaining a large fine root network is an energy and resource-intensive process and that trees will scale back their root network when they have adequate nutrients (and other limiting resources) available.

The findings of this study provide further evidence that NPP in highly diverse tropical forests is constrained by different nutrients, dependent on tree species, tree size, and moisture conditions.