Experimental evidence of nutrient limitations on primary productivity in Afrotropical forests is rare and globally underrepresented yet are crucial for understanding constraints to terrestrial carbon uptake. In an ecosystem‐scale nutrient manipulation experiment, we assessed the early responses of tree growth rates among different tree sizes, taxonomic species, and at a community level in a humid tropical forest in Uganda. Following a full factorial design, we established 32 (eight treatments × four replicates) experimental plots of 40 × 40 m each. We added nitrogen (N), phosphorus (P), potassium (K), their combinations (NP, NK, PK, and NPK), and control at the rates of 125 kg N ha−1 year−1, 50 kg P ha−1 year−1 and 50 kg K ha−1 year−1, split into four equal applications, and measured stem growth of more than 15,000 trees with diameter at breast height (dbh) ≥1 cm. After 2 years, the response of tree stem growth to nutrient additions was dependent on tree sizes, species and leaf habit but not community wide. First, tree stem growth increased under N additions, primarily among medium‐sized trees (10–30 cm dbh), and in trees of Lasiodiscus mildbraedii in the second year of the experiment. Second, K limitation was evident in semi‐deciduous trees, which increased stem growth by 46% in +K than –K treatments, following a strong, prolonged dry season during the first year of the experiment. This highlights the key role of K in stomatal regulation and maintenance of water balance in trees, particularly under water‐stressed conditions. Third, the role of P in promoting tree growth and carbon accumulation rates in this forest on highly weathered soils was rather not pronounced; nonetheless, mortality among saplings (1–5 cm dbh) was reduced by 30% in +P than in –P treatments. Although stem growth responses to nutrient interaction effects were positive or negative (likely depending on nutrient combinations and climate variability), our results underscore the fact that, in a highly diverse forest ecosystem, multiple nutrients and not one single nutrient regulate tree growth and aboveground carbon uptake due to varying nutrient requirements and acquisition strategies of different tree sizes, species, and leaf habits.

94.0 m 3 ha -1 removed . This resulted in a higher abundance of mid-stage succession tree species (e.g. Funtumia elastica). Logging was mainly on trees of the Meliaceae Bulk density (g/cm 3 ) 1.23 ± 0.03 1.53 ± 0.03 1.38 ± 0.03 Soil pH (1:2.5 H2O) 6.43 ± 0.04 6.08 ± 0.09 5.90 ± 0.10 Total soil nitrogen (kg/m 2 ) 0.42 ± 0.01 0.57 ± 0.02 0.56 ± 0.02 Total organic carbon (kg/m 2 ) 4.02 ± 0.13 4.59 ± 0.20 4.09 ± 0.19 Soil C:N ratio 9.54 ± 0.10 7.99 ± 0.08 7.22 ± 0.11 15 N natural abundance (‰) 7.79 ± 0.06 9.22 ± 0.13 9.51 ± 0.16 Bray II phosphorus (g/m 2 ) † 1.80 ± 0.24 0.85 ± 0.08 0.65 ± 0.06 Effective cation exchange capacity (mmol + /kg) 149.

<p>Fine roots represent a small but important part of belowground plant biomass, however, field-based evidence of how nutrient availability control fine root production in species-rich tropical forests is scarce yet remain imperative to our understanding of ecosystem biogeochemistry.</p><p>To evaluate the responses of fine root production and plant-available soil nutrients to N, P and K fertilization thereby identifying which (if any) nutrients limit plant growth and microbial processes, we conducted a large-scale, full factorial nutrient manipulation experiment (8 treatments × 4 replicates: 32 plots of 40 × 40 m each) in a humid tropical forest in Uganda. We added nitrogen (N), phosphorus (P), potassium (K), their combinations (NP, NK, PK, and NPK) and control at the rates of 125 kg N ha<sup>−2</sup> yr<sup>−1</sup>, 50 kg P ha<sup>−2</sup> yr<sup>−1</sup> and 50 kg K ha<sup>−2</sup> yr<sup>−1</sup>, divided into four equal applications. We quantified fine root biomass (0−10 cm soil depth) at the end of the first and second years of the experiment by excavating soil monoliths (20 cm × 20 cm) at six random locations within each plot. Fine root production in the top 30 cm soil depth was estimated using the sequential coring technique in the second year of the experiment.</p><p>It was determined that the addition of N reduced fine root biomass (FRB) by 35% after the first year of the experiment and did not change in the second year whereas K addition was associated with reduced fine root production, suggestive of an alleviated ecosystem-scale N and K limitation. This rapid reduction in fine root biomass and production highlight that maintaining a large fine root network is an energy and resource-intensive process, therefore, trees will scale back their root network when they have adequate resources available. Next, a strong positive relationship was evident between FRB and NH<sub>4</sub>:NO<sub>3</sub> ratio and highlights how FRB decreases dramatically when NO<sub>3</sub> concentrations surpass NH<sub>4</sub> concentrations (NH<sub>4</sub>:NO<sub>3</sub> < 1). Additionally, nutrient additions resulted in a cascade of biochemical responses in soil nutrient availability. Specifically, (1) the interaction effects of all three nutrients (N, P and K) enhanced net N mineralization and nitrification rates. This highlights the complementary roles of these nutrients in regulating soil processes related to N-cycling in this ecosystem. (2)  Microbial biomass C increased with P additions but was dependent on the season. Lastly, P additions increased plant-available P by 80%. This large increase could indicate that the demand for P was not very high. Our data show that N and K are particularly important in regulating fine root growth in this ecosystem. </p>

<p>Nutrient availability in tropical forest ecosystems plays a critical role in sustaining forest growth and productivity. Observational evidence for nutrient limitations on net primary productivity (NPP) in the tropics is rare yet crucial for predicting the impacts of human-induced changes on tropical forests, particularly for underrepresented tropical regions in Africa. In an ecosystem-scale nutrient manipulation experiment, we assessed the response of different components of above-ground net primary production (ANPP) to nutrient addition of nitrogen (N), phosphorus (P), potassium (K) and all possible combinations (NP, NK, PK, and NPK) at rates of 125 kg N ha<sup>-1</sup>yr<sup>-1</sup>, 50 kg P ha<sup>-1</sup> yr<sup>-1</sup> and 50 kg K ha<sup>-1</sup>yr<sup>-1</sup>.</p><p>We established 32 (8 treatments × 4 replicates) experimental plots of 40 × 40 m<sup>2</sup> each and measured stem growth of over 15,000 trees with diameter at breast height (dbh) ≥ 1 cm as well as litter production and above-ground woody biomass production (AWBP), of a lower-montane tropical forest (1100 m a.s.l.) in northwestern Uganda.</p><p>After 18 months of nutrient addition, we found that different aspects of ANPP, including litter production and AWBP are controlled by multiple soil nutrients. Specifically, we measured higher total fine-litter production in the N (13.6 ± 1.4 Mg ha<sup>-1 </sup>yr<sup>-1</sup>) and K (13.3 ± 1.8 Mg ha<sup>-1 </sup>yr<sup>-1</sup>) addition plots than the control (11.1 ± 0.6 Mg ha<sup>-1 </sup>yr<sup>-1</sup>) plots. Both reproductive litter (flowers and fruits; 10% of total fine-litter fall) and leaf litter (62% of total fine-litter fall) significantly increased with K addition. In general, fine-litter production in our plots is higher than what has been reported so far for lower-montane tropical forests. Increased AWBP is associated with N addition plots. The response of trees to nutrient addition however, varied with tree sizes. Trees with dbh between 10 – 30 cm increased significantly in AWBP under PK addition. There was no effect of nutrient addition associated with either smaller (1 – 10 cm dbh) or larger trees (dbh > 30 cm). The medium-sized trees which may have experienced resource competition but have now transitioned into the canopy layer (exposed to sunlight) are able to use additional nutrient for active growth. In contrast, bigger trees may allocate extra nutrient for reproduction and leaf-vitality, while smaller trees remain shaded, co-limited by sunlight and therefore unable to utilize increased available nutrients for stem diameter growth. ANPP increased by 39% with N addition and marginally by 23% with K additions relative to the control. In conclusion, our experiment provides evidence of N and potentially K limitation of ANPP in this lower-montane tropical forest, and highlights that, in a highly diverse ecosystem different components of ANPP may be regulated by multiple nutrients. </p>