DOI: 10.6078/d1dd85
title: DayCent simulations for California annual grasslands: Monthly data outputs

description: Composted manure and green waste amendments have been shown to increase
 net carbon (C) sequestration in rangeland soils and have been proposed as
 a means to help lower atmospheric CO2 concentrations. However, the effect
 of climate change on soil organic C (SOC) stocks and greenhouse gas
 emissions in rangelands is not well understood, and the viability of
 climate change mitigation strategies under future conditions is even less
 certain. We used a process-based biogeochemical model (DayCent) at a daily
 timestep to explore the long-term effects of potential future climate
 changes on C and greenhouse gas dynamics in annual grassland ecosystems.
 We then used the model to explore how the same ecosystems might respond to
 climate change following compost amendments to soils and determined the
 long-term viability of net SOC sequestration under changing climates. We
 simulated net primary productivity (NPP), SOC, and greenhouse gas fluxes
 across seven California annual grasslands with and without compost
 amendments. We drove the DayCent simulations with field data and with
 site-specific daily climate data from two Earth system models (CanESM2 and
 HadGEM-ES) and two representative concentration pathways (RCP4.5 and
 RCP8.5) through 2100. Net primary productivity and SOC stocks in unamended
 and amended ecosystems were surprisingly insensitive to projected climate
 changes. A one-time amendment of compost to rangeland acted as a
 slow-release organic fertilizer and increased NPP by up to 390–814 kg C
 ha-1 y-1 across sites. The amendment effect on NPP was not sensitive to
 Earth system model or emissions scenario and endured through the end of
 the century. Net SOC sequestration amounted to 1.96 ± 0.02 Mg C ha-1
 relative to unamended soils at the maximum amendment effect. Averaged
 across sites and scenarios, SOC sequestration peaked 22 ± 1 years after
 amendment and declined but remained positive throughout the century. While
 compost stimulated nitrous oxide (N2O) emissions, the cumulative net
 emissions (in CO2 equivalents) due to compost were far less than the
 amount of SOC sequestered. Compost amendments resulted in a net climate
 benefit of 69.6 ± 0.5 Tg CO2e 20 ± 1 years after amendment if applied to
 similar ecosystems across the state, amounting to 39% of California’s
 rangeland. These results suggest that the biogeochemical benefits of a
 single amendment of compost to rangelands in California is insensitive to
 future climate change and could contribute to decadal-scale climate
 mitigation goals alongside emissions reductions.
description: The DayCent Biogeochemical Model (Parton et al. 1998) was used to
 simulate the effects of climate and management on C and greenhouse gas
 dynamics in each rangeland ecosystem. DayCent is a widely utilized and
 well-established complex process model, developed using ecological
 concepts of grassland soil C and N dynamics (Parton et al. 1994). The
 model is parameterized for initial conditions using site-specific
 historical climate data, annual net primary productivity (NPP), and
 depth-specific measured values for soil texture and bulk density. The
 calibrated model provides a baseline from which the model can calculate
 trends with time and differences under changing conditions (e.g., climate
 and compost amendments in this study). DayCent partitions existing and
 added C into discrete soil pools based on estimated C turnover time:
 active (&lt; 1 year), slow (decadal), and passive (millennial). Dead
 plant material is partitioned into active or slow cycling pools initially,
 depending upon tissue chemistry (e.g., lignin:N ratio), using first-order
 kinetics. Carbon can move among pools through decomposition and
 stabilization. The movement among pools mimics microbial activity and the
 mineral association of organic matter; it includes a separate pool for
 microbial biomass, but DayCent does not explicitly model specific
 mechanisms of microbial interactions or mineral stabilization (Parton et
 al. 1994). Modeled SOC flows and NPP are both strongly dependent on soil
 water availability in DayCent, which has been shown to be an important
 driver of ecosystem C dynamics in grasslands (Burke et al. 1997, Harpole
 et al. 2007). The nitrogen gas sub-model of DayCent uses a daily timestep
 to simulate N<sub>2</sub>O fluxes from nitrification and
 denitrification based on diffusivity parameters of soil (water-filled pore
 space, texture, bulk density, field capacity, temperature), pH, and soil
 NH<sub>4</sub><sup>+</sup> and
 NO<sub>3</sub><sup>-</sup> concentrations (Parton
 et al. 2001). The grassland CH<sub>4</sub> oxidation sub-model
 simulates methanotrophy at a daily timestep as a function of soil water
 content, field capacity, porosity, and temperature (Del Grosso et al.
 2000). DayCent also models soil respiration and microbial respiration of
 CO<sub>2</sub>; here we report on total soil respiration which
 is more comparable with field data. DayCent facilitates the simulation of
 explicit management practices including grazing and compost amendments,
 and was originally developed, and has been used extensively, for modeling
 managed grassland and cropland ecosystems (Kelly et al. 2000; Parton et
 al. 1993; Parton et al. 1998; Ryals et al. 2015).
 <strong>Biogeochemical model
 inputs</strong> Field observations of soil
 texture, total organic C, bulk density, and biomass production from
 pre-treatment plots were used for the initial parameterization of the
 model for each site (Table 1). Total organic C was measured on five
 replicate soil cores at four depths down to 1 meter or point of refusal.
 The point of refusal was below one meter except for a minority of cores in
 Mendocino, Marin, and Tulare, where the mean points of refusal were 95.7 ±
 2.7 cm, 92.2 ± 3.4 cm, and 99.5 ± 0.5 cm, respectively. Soil texture was
 measured on 3 samples from each transect (first, third, and fifth core)
 from the 0-10 cm depth at each site. Soil texture data for 10-100 cm soil
 depths were obtained from the SSURGO database (Soil Survey Staff et al.
 2017). Bulk density samples were taken using a 6.35 cm diameter metal
 corer at 10 cm depth increments to 1 m or point of refusal from two soil
 pits per site. Aboveground NPP was measured by clipping vegetation at peak
 biomass from eight replicate 200 cm<sup>2</sup> subplots for
 both amended and unamended plots, oven drying at 65 ºC, and weighing;
 belowground NPP was measured in Marin and Yuba only by Ryals et al (2013).
 Soil subsamples were analyzed in duplicate for total C concentration at
 U.C. Berkeley on a Carlo Erba Elantech elemental analyzer (Lakewood, NJ,
 USA) using atropine as a standard at a rate of one per ten samples.
 Samples were re-run in if duplicates varied by more than 10%. Soils were
 tested for carbonates using 2M HCl; as no carbonates were found, results
 reported reflect only organic C concentrations. Bulk density was
 determined by calculating the rock volume and determining the oven dry
 (105°C) mass of soil per unit volume. Soil organic C contents were
 calculated by multiplying the C concentrations (%) by the oven-dry mass of
 the fine fraction (&lt; 2 mm) and dividing by the bulk density and
 depth (Throop et al. 2012). Additional details can be found in Silver et
 al. (2018).  Livestock effects on biomass and biogeochemical cycling were
 represented using scheduled time- and intensity-specific grazing events.
 Grazing management was simulated to reflect site-specific historic and
 current practices (Appendix S1). Simulations of future
 conditions were driven by daily climate data from 2006 to 2100 extracted
 from the CanESM2 (Canadian Centre for Climate Modeling and Analysis,
 Canada) and HadGEM2-ES (Met Office Hadley Centre, UK) Earth System Models
 (ESMs). We chose not to simulate CO2 fertilization in order to isolate the
 role of climate. There remains debate as to which ESM most accurately
 represents future weather in California. We used CanESM2 and HadGEM2-ES
 because they yielded contrasting projections for future precipitation (see
 below). We used two Representative Concentration Pathway (RCP) scenarios:
 RCP4.5 that assumes some emissions reductions, and RCP8.5 that assumes
 business as usual societal behavior with minimal emissions reductions. We
 chose these two scenarios because California used RCP4.5 and RCP8.5 for
 emissions reduction targets in their 2018 assessment report (Franco et al.
 2018). Data were extracted for the site-specific (2.8°x 2.8°) geographical
 grid of CanESM2 and HadGEM2-ES.
description: Output units are described in the ReadMe.txt file.