Plant-Soil Interactions and Phosphorus Nutrition in Tropical Forests

The Nutrient Biogeochemisty Task of NGEE-Tropics

Objective: Improve understanding and model representation of the factors that control nutrient availability and nutrient controls over forest productivity, plant allocation and turnover, and post disturbance recovery rates, with a focus on model-measurement interaction and linkage with root traits.

Our approach started with insights from models and model uncertainty and be guided by the need to inform and improve nutrient interactions in models.

One of the key uncertainties identified by the model is the biochemical mineralization of soil organic P by phosphatase enzymes produced by fine roots, mycorrhizae, and microbes. Our field work in the Luquillo Experimental Forest in Puerto Rico has emphasized studies of fine-root production, root traits, and phosphorus biogeochemistry. Figure modified from Yang et al. 2019.

Our team in the field

We collaborated with the TRACE experiment team at Sabana Research Station to install minirhizotron tubes in February 2015. Daniela measured root production in the plots in response to warming and then in response to Hurricanes Irma and Maria.

The TRACE experiment site was also a good location to advance NGEE-Tropics goals on tropical forest hydrology.

Research on phosphorus biogeochemistry focused on the influences of species, soil conditions, and root traits on phosphatase activity. Field sampling of fine-root clusters required careful excavation to trace the roots to the tree of identified species, followed by laboratory analysis.

Kristine found that root phosphatase activity varies with tree species and P availability. Both phosphomonoesterase (PME) and phosphodiesterase (PDE) were significantly different among tree species though marginally not by site. Furthermore, there was no significant interaction between site and tree species. The average PME in Icacos was 60.03 ± 15.17 μmol pNP groot-1hr-1, 36.54 ± 7.56 μmol pNP groot-1hr-1 in El Verde Ridge, and 24.98 ± 7.88 μmol pNP groot-1hr-1 in El Verde Valley. PDE followed a similar trend, and site-averaged values of phosphatase declined with increasing P availability (Cabugao et al. 2017).

Guided by the identification of key data for P model improvement, we identified fine-root traits and soil variables that predicted soil and root phosphatase activity vertically in the soil profile at sites in the Luquillo Experimental Forest with contrasting soil characteristics and species. Kristine and Daniela determined that both fine-root traits and soil phosphorus data are important to understanding and predicting soil and root phosphatase activity throughout the soil profile. These findings relate a function (phosphatase activity) to existing fine-root biomass and soil P structures in ecosystem models used to estimate biochemical mineralization (Cabugao et al. 2021).

Daniela’s research on tropical root traits will inform a new collaborative Tropical Root Trait Initiative. She assembled 1,091 records from 46 studies since the 1940’s covering root measurements in the forests of Puerto Rico. The data were evaluated according to categories of root traits in the Fine Root Ecology Database (FRED). Root depth distribution in the wet forest is shallower (above 20 cm) than observed in other tropical studies. The dry forest has slightly deeper distribution than the wet forest with less root biomass. There is a positive correlation between fine-root biomass and soil N concentration (Yaffar & Norby, 2020).

We participate in the Tropical Root Traits Initiative — TropiRoots

Campaign in Panama, 2012-2013
Sometimes we look up! Our objective was to assess relationships between photosynthetic parameters and foliar nutrient concentrations from the tropical forests to inform model improvements. A-Ci curves were measured at Panama crane sites along with associated foliar SLA, [N], and [P].

We found significant relationships between Vcmax and foliar N & P
  •Stronger relationships with area-based nutrients
  •Stronger with P than with N
  •No difference between sites, but species and site are completely confounded.
    (Norby et al. 2017)


Cabugao KG, Yaffar D, Stenson N, Childs J, Phillips J, Mayes MA, Yang X, Weston DJ, Norby RJ. 2021. Bringing function to structure: Root-soil interactions shaping phosphatase
activity throughout a soil profile in Puerto Rico. Ecology and Evolution 11: 1150-1164, doi: 10.1002/ece3.7036. PDF

Yaffar D, Norby RJ. 2020. A historical and comparative review of 50 years of root data collection in Puerto Rico. Biotropica 52: 563-576, doi:10.1111/btp.1277. PDF

Yang X, Ricciuto DM, Thornton PE, Shi X, Xu M, Hoffman F, Norby RJ. 2019. The effects of phosphorus cycle dynamics on carbon sources and sinks in the Amazon region: a modeling study using ELM v1. Journal of Geophysical Research: Biogeosciences 124. doi: 10.1029/2019JG005082. PDF

Cabugao KG, Timm CM, Carrell AA, Childs J, Lu TS, Pelletier DA, Weston DJ, Norby RJ. 2017.  Root and rhizosphere bacterial phosphatase activity varies with tree species and soil phosphorus availability in Puerto Rico tropical forest. Frontiers in Plant Science 8:1834. doi: 10.3389/fpls.2017.01834. PDF

Norby RJ, Gu L, Haworth IC, Jensen AM, Turner BL, Walker AP, Warren JM, Weston DJ, Xu C, Winter K. 2017. Informing models through empirical relationships between foliar phosphorus, nitrogen and photosynthesis across diverse woody species in Panama. New Phytologist 215:1425-1437.doi: 10.1111/nph.14319. PDF