The role of root foraging and mining strategies in response to nutrient manipulation in Amazon forests

Laynara F. Lugli and Carlos Alberto Quesada, Coordination of Environmental Dynamics, National Institute of Amazonian Research (INPA), Manaus, Brazil, Jessica Schmeisk Rosa, Raffaello Di Ponzio, Renata Vilar de Almeida, Maria Pires, Hellen Fernanda Viana Cunha, Nathielly Martins, Anna C. M. Moraes, Sheila T. Souza, José Luís C. Camargo, Bruno T. Portela and Karst J. Schaap, National Institute of Amazonian Research, Manaus, Brazil, Kelly M. Andersen, Nanyang Technological University, Singapore, Singapore, Amanda Cordeiro, Colorado State University, Fort Collins, CO, Rafael Leandro de Assis, University of Oslo, Oslo, Norway, Luiz Aragao, Instituto Nacional de Pesquisas Espaciais, Brazil, Lucia Fuchslueger, Department of Microbiology & Ecosystem Science, University of Vienna, Austria, Oscar J. Valverde-Barrantes, Department of Biological Sciences, Kent State University, Kent, OH, Patrick Meir, Research School of Biology, Australian National University, Canberra, Australia, Lina Mercado and Iain Hartley, College of Life and Environmental Sciences, University of Exeter, Exeter, United Kingdom

Background/Question/Methods Root functional traits usually represent a balance between maximising the acquisition of limiting resources and minimising root tissue construction and maintenance. Phosphorus (P) is hypothesised to be the main nutrient limiting tropical forest productivity, but more recent evidence suggests that multiple nutrients could regulate forest functioning. However, direct support for the P paradigm is limited, and nitrogen (N) and cations might also control tropical forests functioning. On low-fertility sites, plant economic theory predicts that if the supply of the limiting nutrient in soils is increased, plant investment in root biomass and nutrient uptake strategies should change. To test this, we used a large-scale experiment installed in 32 plots in Manaus, Brazil to determine how the factorial addition of N, P and cations (Ca, Mg and K) affected root traits in a primary rainforest growing on low-fertility soils in Central Amazonia after up to 2 years of fertilisation. We measured fine root stocks and productivity and traits related to nutrient acquisition strategies, such as morphological traits, phosphatase activity, arbuscular mycorrhizal (AM) colonisation and root nutrient contents.
Results/Conclusions Multiple root traits and productivity were affected by nutrient manipulation. Phosphorus additions increased annual root productivity, but no changes were detected for roots stocks after 2 years of nutrient addition, pointing to increased root turnover with P addition-only. Phosphorus addition also increased root diameter, but decreased root phosphatase activity, with no changes in AM colonisation. Cation additions increased root productivity at certain times of year, also increasing root diameter and mycorrhizal colonisation. Phosphorus and cation additions increased their element concentrations in root tissues. No responses were detected with N addition. Our results suggest that with P addition, fine roots are shifting their investments from mining organic P (phosphatases) to direct foraging for P via increased fine root productivity and turnover, relying still on AM colonisation. The significance of AM fungi to nutrient acquisition seems to increase with cations additions, suggesting that the symbiosis might not only be important regarding P uptake, as commonly discussed. Here we showed that rock-derived nutrients determined root functioning in low-fertility Amazonian soils, demonstrating not only the hypothesised importance of P, but also highlighting the role of cations. The changes in fine root traits and productivity indicate that even slow-growing tropical rainforests can respond rapidly to changes in resource availability.