NPKD Network

Welcome to the NPKD Network: A global test of combined precipitation changes and nutrient addition on plant community dynamics and functioning

Rationale: Changes in precipitation patterns and nutrient enrichment are two major global change drivers with strong impacts on plant community dynamics and functioning. The Nutrient Network (NutNet) and Drought Network (Drought-Net) are invaluable initiatives for increasing our understanding of how these drivers act independently. However, in our increasingly human dominated planet, drought events and nutrient enrichment are co-occurring and likely interacting to affect the functioning of our ecosystems. For example, in grasslands, drought events not only limit water availability to plants but also reduces their uptake of soil nutrients leading to reduced ecosystem productivity even under nutrient enrichment. Understanding the generalities and contingencies of how these two drivers act in concert on grassland and heathland functioning requires a globally replicated experiment manipulating the two drivers using standardized protocols. This will further allow the identification of which climatic conditions restrict or enhance joint effects. Most studies on water-nutrient limitations in natural grasslands have increased water alongside soil nutrients, repeatedly revealing co-limitation. Thus, we identify the lack of drought-nutrient addition studies as a critical gap, and propose this new initiative, NPKD-Net.

Call for participation: Many participants in the NutNet and Drought-Net communities have already begun to combine the nutrient addition and drought treatments from the two networks. The first goal of this proposal is to centralize and standardize these efforts, in order to create a new coordinated distributed experiment. To this end, we have a short questionnaire attached to first ascertain how close we are to that goal at the onset. Our second goal is to attract NutNet and Drought-Net participants who have not crossed these treatments to begin doing so according to our proposed design. Finally, we encourage and welcome entirely new participants to join this initiative.

Steering committee: Yann Hautier (Co-PI, Utrecht University, The Netherlands), Anke Jentsch (Co-PI, University of Bayreuth, Germany), Qiang Yu (Co-PI, Chinese Academy of Agricultural Science, China),  Mohammed A.S. Arfin-Khan (Co-PI, Shahjalal University of Science and Technology, Bangladesh), Peter Wilfahrt (Co-PI, University of Bayreuth, Germany), Melinda Smith (Colorado State University), Kate Wilkins (Colorado State University), Elizabeth Borer (University of Minnesota), Ashley Asmus (University of Minnesota), Eric Seabloom (University of Minnesota)

Principle correspondents: y.hautier@uu.nl

Data collection: Submit your data to slett152@umn.edu

Data management: Data will be received and processed according to the NutNet protocol. This involves receiving data from NPKD participants, standardizing data format and plant taxonomic names using repeatable R scripts while leave the raw data intact, and documenting and versioning changes to the datasets.

Authorship: We will follow NutNet and Drought-Net guidelines for authorship.

Guidelines for Participation: There are two ways to get involved. If you have a Drought-Net site, there are two options depending on your shelter size. If you constructed the minimum 3 x 3 m shelters, you will likely need to construct additional shelters to accommodate the new plots. However, for managed grasslands that are mown (sub)-annually you can designate a single 1 x 1 m subplot under the existing shelters for NPK addition with cover surveys and biomass harvest (Fig. 4). If you constructed larger shelters in natural grasslands (i.e. no regular disturbance), then it is possible to designate at least two 1 x 1 m subplots under existing shelters – one for cover surveys and one to three for yearly biomass harvests. You should only pursue this option if you can leave a buffer zone of at least 0.5 meters between the nutrient addition and ambient soil nutrient plots. NPK treatments should similarly be integrated into the control plots. Any NPK additions in Drought-Net sites should NOT interfere with the core Drought-Net plots (i.e. minimum 0.5 m buffer between subplots). For those without rainout shelters or insufficient space, we require three replicates each of ambient resources (i.e. control), drought shelters, NPK addition, and NPK x drought shelters. We propose a split plot design to minimize the number of necessary drought shelter constructions. The split plot should have a minimum 0.5 m buffer between +NPK and unfertilized plots. Each nutrient class should have four subplots of size 1.25 x 1.25 m, following the NutNet design of a permanent cover subplot with a rotation of subplots for biomass harvest (Fig. 4). This construction of rainout shelters should follow the Drought-Net protocol, which you can find here.

Additional treatments (optional): In addition to the minimum design, we encourage participants to consider three additional treatments. 1) Addition of plots with +N in ambient precipitation and drought shelters (e.g. ambient soil resource supply, +N, +NPK) 2) A water addition treatment that is of the same severity as the drought reduction (e.g. -40% reduction, ambient, +40% addition) crossed with nutrient addition. 3) Crossing the herbivore exclosures from NutNet with the minimum design listed above.

Please do not hesitate to contact us (y.hautier@uu.nl) for any additional remarks or questions.