Grassland responses to environmental change

As a post-doc in Dr. Sarah Hobbie's lab and as part of the Nutrient Network, I collaborated with researchers at the University of Minnesota, Oak Ridge National Laboratory, and Pacific Northwest National Laboratory to examine how 10 years of nitrogen (N) and phosphorus (P) fertilization (in full factorial) have affected plant productivity and soil C storage across nine distinct temperate grasslands spanning the continental United States (see map). In a separate cross-continental study, we also assessed how nutrient supply interacts with grazing to affect leaf litter decomposition patterns.

Our research aims to improve predictions of ecosystem functioning in the face of interacting global changes (e.g., nutrient and C pollution and land use changes) and guide sustainable land management strategies. While elevated nutrient supply often enhances aboveground plant productivity, nutrient effects on belowground plant inputs and soil carbon sequestration are poorly understood -- especially in grasslands, which make up nearly a third of non-agricultural land world-wide and a third of terrestrial primary production!

In two complementary studies, we found that while nutrient fertilization stimulated aboveground plant productivity, belowground plant responses to fertilization were weaker, ranging from positive to negative across sites. Meanwhile, atmospheric N deposition was an important predictor of root allocation and root responses to fertilization (Keller et al. in press). We similarly found variable nutrient effects on soil C and N pools, again ranging from positive to negative depending on the site. Soil C and N pool sizes varied by more than an order of magnitude across sites and were strongly positively related to aboveground plant biomass (but not root biomass or productivity, see figure). This work was recently published in Global Change Biology (Keller et al. 2021). 

Picture1.png

Our field data revealed a strong positive relationship between soil carbon pools and aboveground plant biomass (a) but not root production (b) across nine temperate grassland sites. Adapted from Keller et al. 2021 GCB.

Fig1_map.tif

Map of nine temperate grassland study sites where we are examining the effects of nutrient supply on plant and soil properties. Grasslands/herbaceous land cover from National Land Cover Database (2016) shown in green. Sites include: Bunchgrass, OR (BNCH), Cedar Creek LTER, MN (CDR), Cedar Point Biological Station, NE (CDPT), Kong LTER, KS (KZA), Lookout Ridge, OR (LOOK), Sevilleta LTER, NM (SEV), Shortgrass Steppe LTER, CO (SGS), Temple, TX (TMPL), and Trelease Prairie, IL (TREL). 

Picture1.png

Sevilleta LTER, New Mexico, USA (SEV) experimental field site. Photo credits: Chris Walter