This experiment is one implementation of a globally distributed experiment, known as the Nutrient Network. At Cedar Creek, as in over 170 other sites in grasslands around the world, the experiment aims to describe impacts of increased nutrients (nitrogen, phosphorus, potassium, sulfur and other metals) and decreased herbivory (removal of mammals by fencing).
Two overarching questions are being explored with these manipulations:
1. To what extent are plant production and diversity co-limited by multiple nutrients in herbaceous-dominated communities?
2. Under what conditions do grazers or fertilization control plant biomass, diversity, and composition?
By utilizing identical protocols at diverse grassland sites around the world, NutNet aims to uncover both the generalities in ecosystem functioning, and the contingencies or differences which can obscure those common mechanisms.
In addition to the standard NutNet protocol, E247 includes an additional low Nitrogen gradient (1 gram Nitrogen per meter squared per year and 5 grams Nitrogen per meter squared per year in addition to the standard 10 grams Nitrogen per meter squared per year).
We have also established a second NutNet experiment close to the LTER headquarters, for use as part of the Cedar Creek outreach and education programs and is referred to at the NutNEd site (Link to NutNet Education Site).
White tail deer grazing outside a Nutrient Network fenced plot at Cedar Creek.
Location of NutNet grassland research sites.
from: Borer, E. T., W. S. Harpole, P. B. Adler, E. M. Lind, J. L. Orrock, E. W. Seabloom, and M. D. Smith. 2014. Finding generality in ecology: a model for globally distributed experiments. Methods in Ecology and Evolution 5:65-+.
Nutrient Network Experiment design and protocols
The standard NutNet study is a completely randomized block (environmental gradient) design with three blocks and 10 plots per block (N = 30 total units/site), however at Cedar Creek there are 5 blocks and two additional nitrogen addition treatments (see below). Each experimental unit is a 5 by 5 m plot that is separated by at least 1-m walkways. Each plot is divided into four equal-sized subplots: one dedicated to core sampling, one to additional site-specific or subnetwork studies and the last two for future network-level research. Plots at Cedar Creek were established in 2007.
The experimental treatments are applied at the scale of the 5 by 5 m plots, as follows:
Three nutrient treatments (N, P and K plus micronutrients), each with two levels (control, added), are crossed in a factorial design, for a total of eight treatment combinations per block, to test multiple nutrient limitation on plant composition and ecosystem function. Nutrient addition rates and sources are: 10 g N m-2 year-1 as timed-release urea [(NH2)2CO], 10 g P m-2 year-1 as triple-super phosphate [Ca(H2PO4)2], 10 g K m-2 year-1 as potassium sulfate [K2SO4] and 100 g m-2 of a micronutrient mix of Fe (15%), S (14%), Mg (1.5%), Mn (2.5%), Cu (1%), Zn (1%), B (0.2%) and Mo (0.05%). N, P and K are applied annually; micronutrients are applied once at the start of the experiment to avoid toxicity.
The Cedar Creek implementation of this experiment has an additional two treatments beyond the standard NutNet design: 1 g N m-2 yr-1 and 5 g N m-2 yr-1 . This creates a gradient of four levels of N addition: 0, 1, 5, and 10 g N m-2 yr-1.
A fencing treatment is crossed with the control and NPK treatments to assess the interactive effects of fertilization and food web manipulation on plant composition and ecosystem function. The 230-cm-tall fences restrict access by mid-to-large-sized above-ground mammalian herbivores (>50 g). The lower 90 cm is surrounded by 1-cm woven wire mesh (hardware cloth) with a 30-cm outward-facing flange stapled to the ground to exclude digging animals (e.g. rabbits, voles), although not fully subterranean ones (e.g. gophers, moles). The upper fence is composed of four strands of tensioned wire strung at equal vertical intervals.
In each plot, the core sampling 2.5 by 2.5 m subplot is divided into four 1 by 1 m permanent subplots, surrounded by a 0.25-m buffer. Within the core-sampling subplot, one 1-m2 subplot is permanently marked for annual plant composition sampling; the other three are used for destructive biomass sampling. Core annual sampling includes clipping of total above-ground biomass of all plants rooted within two 0.1-m2 strips (10 by 100 cm) for a total of 0.2 m2. These are sorted to live and dead (or further, e.g. forb, grass, moss at many sites), dried at 60 degrees C to constant mass and weighed to the nearest 0.01 g. Leaves and current years woody growth are collected from shrubs and subshrubs.
Light availability above and at ground level below the canopy is measured in the core subplot using a linear 1-m bar (e.g. Apogee Instruments, Inc., Logan, UT, USA). Areal cover is estimated to the nearest 1% for each species rooted in the core subplot; cover estimates include woody overstorey, litter, bare soil, rock and animal activity (e.g. digging).
All core measurements are collected from all plots, annually at peak biomass. Two 2.5 cm diameter by 10 cm depth soil cores, free of litter and vegetation, are collected from each plot prior to initiation of the experiment (Y0) and 3 years after treatment initiation (Y3).
Soils from each plot are composited, homogenized, air-dried and shipped to a single laboratory for analysis and long-term storage. Samples are assayed for % total C and % total N, extractable soil phosphorus, potassium and micronutrients, soil pH, soil organic matter and soil texture.
The Nutrient Network full Protocols for establishing a new site can be found on the Nutrient Network Website
Globally consistent response of plant microbiome diversity across hosts and continents to soil nutrients and herbivores
EW Seabloom, MC Caldeira, KF Davies, L Kinkel, JMH Knops, ...
2023 Nature Communications 14 (1), 3516
Realistic rates of nitrogen addition increase carbon flux rates but do not change soil carbon stocks in a temperate grassland
ME Wilcots, KM Schroeder, LC DeLancey, SJ Kjaer, SE Hobbie, ...
2022 Global change biology 28 (16), 4819-4831
Grassland productivity limited by multiple nutrients
PA Fay, SM Prober, WS Harpole, JMH Knops, JD Bakker, ET Borer, ...
2015 Nature plants 1 (7), 1-5
Herbivores and nutrients control grassland plant diversity via light limitation
ET Borer, EW Seabloom, DS Gruner, WS Harpole, H Hillebrand, EM Lind, ...
2014 Nature 508 (7497), 517-520