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Grasslands throughout the world are responding in diverse ways to changing climate and environmental conditions. In this study we analyze indicators o...
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Assessing Cumulative Effects of Climate Change Manipulations on Phosphorous Limitation in a Californian Grassland. Travis Mellett, Corinne Selvin, Delphine Defforey, Kathryn Roberts, Alanna Lecher, Kate J. Dennis, Jessica Gutknecht, Christopher Field, and Adina Paytan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04362 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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Environmental Science & Technology

Assessing Cumulative Effects of Climate Change Manipulations on Phosphorus Limitation in a Californian Grassland. Travis Mellett1^ , Corinne Selvin1, Delphine Defforey1, Kathryn Roberts2, Alanna L. Lecher3, Kate Dennis4, Jessica Gutknecht5, Christopher Field6, Adina Paytan2* 1

Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA, 95064

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Institute of Marine Sciences, University of California Santa Cruz, Santa Cruz, CA, 95064 3

Natural and Applied Sciences, Lynn University, Boca Raton, FL, 33431 4

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Picarro Inc., 3105 Patrick Henry Drive, Santa Clara, CA, 95054

Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN, 55108

Department of Global Ecology, Carnegie Institution of Washington, Stanford, CA 94305 * corresponding author: [email protected], phone 831-459-1437

^ present address: College of Marine Science, University of South Florida, Saint Petersburg, Fl, 33701

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KEYWORDS: Phosphate, Biogeochemistry, Grasslands, Climate Change

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ABSTRACT

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Grasslands throughout the world are responding in diverse ways to changing climate and

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environmental conditions. In this study we analyze indicators of phosphorus limitation including

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phosphorus concentrations, phosphorus to nitrogen and carbon ratios, oxygen isotope ratios of

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phosphate in vegetation, and phosphatase enzyme activity in soil to shed light on potential

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effects of climate change on phosphorus availability to grassland vegetation. The study was

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conducted at the Jasper Ridge Global Change Experiment, (JRGCE) California where

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manipulations mimicking increases in temperature, water, nitrogen and carbon-dioxide have

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been maintained for over 15 years. We compare our results to an earlier study conducted 3 years

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after the start of the experiment, in order to assess any change in the response of phosphorus over

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time. Our results suggest that a decade later the measured indicators show similar or only slightly

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stronger responses. Specifically, addition of nitrogen, the principle parameter controlling

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biomass growth, increased phosphorus demand but thresholds that suggest P limitation were not

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reached. A study documenting changes in net primary productivity (NPP) over time at the

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JRGCE also could not identify a progressive effect of the manipulations on NPP. Combined

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these results indicate that the vegetation in these grassland systems is not very sensitive to the

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range of climate parameters tested.

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1.0 Introduction

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Phosphorus (P) is an essential macronutrient for life and a major component of plants. It

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plays a critical role in the structure of DNA, RNA, and cell membranes as well as in transmission

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of energy in the form of ATP 1. The abundance and availability of P in soil has the capacity to

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limit the growth of plants, and thus to affect food supply, plant productivity, and ecosystem

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structure and function 2. In many ecosystems, the essential nutrients nitrogen (N) and/or P

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constrain growth 3-5. It has been suggested that the influence of anthropogenic global change and

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specifically the increase in atmospheric N deposition may push more systems towards P limited

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conditions

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productivity of many grassland systems worldwide

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primary productivity, including temperature, water availability, day length, along with other

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environmental parameters; and actual responses to each environmental variable or combination

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of variables are difficult to predict 6. This underscores the importance of experimental

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investigation in understanding the impacts global change will have on terrestrial environments,

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including their response to anthropogenic perturbations and specifically how P dynamics may be

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impacted by such changes.

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. Recent work has highlighted the role of N and P, and other nutrients in limiting 4, 5, 9

. However other factors can affect

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The JRGCE is a long-term study on the effects of expected climate and anthropogenic

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changes on California grasslands. The experiment is designed to test the effects of increased bio-

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available N, carbon dioxide (CO2), temperature, and precipitation and all combinations of these

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effects on the ecology and biogeochemical cycles in this ecosystem

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incorporates eight iterations of each of the four amendment combinations resulting in 16 unique

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treatments and 128 total treatment plots. A study on P biogeochemistry at the site by Menge and

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Field

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treatments where N was added plants exhibited signs of P limitation. We note that the term P

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10, 11

. The study site

which was conducted three years after the start of the experiment concluded that in

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limitation used in that study as in many other studies including this one does not necessarily

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imply that NPP is reduced but rather that the biologically available inorganic P is lower than the

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plants’ demand, resulting in physiological changes such as production of enzymes to acquire

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other P forms or changes in cellular P quotas. Six indices of P limitation were used to evaluate

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treatment impacts: soil phosphatase activity, P concentration and N:P ratios in green and in

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senescent leaves of the dominant grass genus, Avena and the total aboveground plant P content.

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They found that all indices indicated that N deposition, the only factor that stimulated NPP (by a

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factor of 23% relative to control

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temperature and water supply), did not stimulate NPP or an increase in P demand by the

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vegetation. A more recent analysis of changes in NPP response to the climate manipulations over

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the 15 years of the JRGCE suggested that given the interacting effects of predicted climate

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change conditions, future conditions will likely result in lower NPP (albeit with large interannual

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variability) 12. The current study reported here was also conducted 15 years after the start of the

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experiment and 12 years after the study of P indicators by Menge and Fields 7, in order to

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determine if any further progression or decrease in P limitation conditions has occurred and how

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this may inform other trends observed at the JRGCE.

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), increased P demand. The other three factors (CO2,

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We use some of the same indices employed by the previous study: total P concentrations

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and N:P ratios of the bulk aboveground biomass and of green A. barbata, and soil alkaline

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phosphatase activity to evaluate changes in these indices over time. We also use C and N

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concentrations, C:N and C:P ratios of A. barbata, and of the bulk homogenized aboveground

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biomass, as well as the soil enzyme activities of acid, alkaline, and neutral phosphatases, and

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β−Glucosidase to gain additional information of the nutrient dynamics in this ecosystem. It is

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expected that as P becomes less available P content the plants will decrease as less luxury uptake

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and storage will be possible. If N is not depleted then this will also result in higher N:P and C:P

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ratios in plant tissue. Under such condition plants (and bacteria) will produce phosphatase

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enzymes that hydrolyze organic P compounds to increase phosphate availability hence an

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increase in the activity of phosphatase enzymes suggests that there is not enough phosphate to

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satisfy growth. In addition to these measurements, the oxygen isotope ratio of phosphate (δ18OP)

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in A. barbata was measured to evaluate if P limitation affects this ratio in living plant leaves.

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δ18OP has not been previously tested as an indicator of P limitation; we hypothesized that when P

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is scarce cellular P will be extensively cycled resulting in isotopic equilibrium

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when available may be stored as polyphosphate which might not be cycled and equilibrated with

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plant cellular water. This suite of analyses was used to shed light on P cycling and allocation,

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and changes in the extent of P limitation (e.g. availability vs demand) since the previous study of

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Menge and Field 7 and how this relates to trends in NPP at JRGCE.

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while excess P

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2.0 Methods

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2.1 Study Site and Experimental Design

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Jasper Ridge Biological Preserve is located in the San Francisco Bay area (37°24’N,

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122°14’W). The site experiences a Mediterranean-type climate, with cool, wet winters and hot,

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dry summers. The soil at the site is a fine grain, mixed Typic Haploxeralf derived from either

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sandstone or greenstone from the Franciscan complex with a pH ranging from 6.5 and 7 14. The

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plant community at the site is dominated by introduced forbs (primarily Geranium dissectum and

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Erodium botrys) and grasses (Avena barbata, Avena fatua, Bromus hordeaceus, Lolium

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multiflorum, and Bromus diadrus) with few native species 11.

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The four parameters chosen for manipulation in this climate study are atmospheric CO2

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(CO2), temperature (H), precipitation (W), and nitrate deposition (N). Atmospheric CO2 is

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elevated by +275 µmol·mol-1. Temperature is increased by +80 to +250W·m-2 (corresponding to

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1-1.5 °C above ambient), precipitation is elevated to 150% ambient values, and N deposition is

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increased by adding 7g NO3- m-2 yr-1 (as a 2 g pulse as liquid Ca(NO3)2 with the first autumn

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rains and 5 g as a time-release pellet application (Osmocote) in January of each year) in addition

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to the background rate of