The climate change mitigation potential of annual grasslands under future climates

Abstract

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 time step 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. NPP 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 year−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. Though 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 are insensitive to climate change and could contribute to decadal‐scale climate change mitigation goals alongside emissions reductions.

Compost Amendment to a Grazed California Annual Grassland Increases Gross Primary Productivity Due To a Longer Growing Season

“…(2018). The compost amendment added C at a rate of 6.4 Mg C ha −1 , which replicated the application rate used in previous field experiments (Silver et al., 2018) and was the rate recommended by rangeland managers (Mayer & Silver, 2022).…”

“…One promising nature‐based climate solution that has been intensively investigated in California is compost amendments to rangelands (Mayer & Silver, 2022; Ryals & Silver, 2013; Ryals et al., 2014). Previous work has shown that compost amendments act as a slow‐release fertilizer (Claassen & Carey, 2007), releasing nutrients that are often limited in California's annual grasslands, such as nitrogen and phosphorus, while increasing water‐holding capacity (Blair et al., 2006; Gerzabek et al., 1997; Ryals & Silver, 2013).…”

Soil carbon sequestration in global working lands as a gateway for negative emission technologies

“…Tables S1 and S2) and their capacity to continue net C sequestration is estimated to last for decades or longer (Mayer & Silver, 2022;Poeplau et al, 2011;Ryals et al, 2015). Some of the most common practices in the United States today, like cover cropping and no-till, may provide short-lived C benefits by reducing the rate that soil C is converted to CO 2 and increasing the annual production of organic C in above-and belowground biomass (Mutegi et al, 2013).…”

“…Approaches that can rapidly store large C stocks over years to decades, such as many SCS strategies (Figure 1), can be deployed immediately while additional NETs are developed and scaled up. Many working land NET practices generate detectable increases in SCS within 3–5 years (Figure 1; Tables S1 and S2) and their capacity to continue net C sequestration is estimated to last for decades or longer (Mayer & Silver, 2022; Poeplau et al, 2011; Ryals et al, 2015). Some of the most common practices in the United States today, like cover cropping and no‐till, may provide short‐lived C benefits by reducing the rate that soil C is converted to CO 2 and increasing the annual production of organic C in above‐ and belowground biomass (Mutegi et al, 2013).…”

Climate seasonality and extremes influence net primary productivity across California’s grasslands, shrublands, and woodlands

“…Much of what we know about the potential for ecosystems to store carbon in California is focused on forests due to their high NPP and large standing biomass (Pan et al 2011, Gonzalez et al 2015. However, California contains over 17 million hectares of grasslands, woodlands, and shrublands that may show greater resilience to a drying climate compared to forests and serve as important carbon sinks (Dass et al 2018, Mayer andSilver 2022). Grasslands, woodlands, and shrublands in California span broad gradients of topography, soil, and climate conditions and show high spatial and temporal variation in productivity (Murphy 1970, Larsen et al 2015, Becchetti et al 2016.…”