Nutrient limitation of fine roots and fertilization effects on soil nutrients in a moist tropical forest 

Abstract

<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>