Recipient Organization
UNIVERSITY OF OKLAHOMA
(N/A)
NORMAN,OK 73019
Performing Department
(N/A)
Non Technical Summary
Soils are the largest terrestrial sink of carbon, and respiration by roots and heterotrophic microorganisms is the largest flux of C from soils. For this reason, soil respiration is a key component of the global carbon (C) cycle and represents soil's ecosystem services related to regulation of the global climate. Indeed, soil respiration is an accepted and commonly-used indicator of biological soil health. However, land use and climate change can shift soil respiration rates such that they outpace the rate of C inputs, depleting soil C stocks. Further, reactions within the soil system involving inorganic carbon in the solid and aqueous phases can modulate gaseous soil CO2 efflux, obscuring ecosystem response to global change and confounding efforts to characterize soil health. Therefore, with this work we will unravel the roles of climate and land-management in controlling the deviation of soil CO2 efflux from soil respiration rates by tracking both organic and inorganic C in solid, gaseous, and aqueous phases in a grassland agroecosystem of the Southern Great Plains, United States.We will use both field experiments and laboratory incubations to investigate the roles of drought, nitrogen fertilization, and hay harvest in controlling organic and inorganic C cycling in soils, and therefore the deviation of CO2 efflux from soil respiration rates. To expand knowledge of key concepts surrounding soil helalth and soil respiration, we will additionally develop laboratory activities and coursework about soil biogeochemistry and soil health directed towards introductory level undergraduate courses in the environmental sciences. Because C flux partitioning is directly related to our understanding of indicators of soil health, and soil's role in regulating the global C cycle, this work directly aligns with the listed priorities of Soil Health and will represent a significant advance in our understanding of pasture and rangeland response to climate and land use changes.
Animal Health Component
(N/A)
Research Effort Categories
Basic
100%
Applied
(N/A)
Developmental
(N/A)
Goals / Objectives
The overarchinig goal of this work is to unravel the roles of climate and land management in controlling the deviation of soil CO2 efflux from soil respiration rates by tracking both organic and inorganic C in solid, gaseous, and aqueous phases in a grassland agroecosystem of the Southern Great Plains (SGP). Ultimately, the team will identify and communicate management strategies that maximize soil C storage, forage quality, and soil health under climate and land use scenarios expected to become more likely in the SGP. Below are the 4 major goals of the project followed by the objectives that will be completed in order to accomplish such goalsGOAL 1: Measure the true change in soil respiration in grasslands in response to drought and pasture management intensification simulated by N fertilization and biomass removal in JulyInstallation of rainfall exclusion shelters and instrumentation.In the first quarter of the first year of the project, we will install rainfall exclusion shelters and establish the precipitation, N addition, and biomass management treatments in a fully factorial design. Within each replicate we will install one lysimeter in the C horizon, three soil gas collection tubes, one soil CO2 efflux collar, and one clear acrylic 100-cm minirhizotron tube.We will install one soil moisture sensor in the rooting zone, one soil moisture sensor beneath the rooting zone, one precipitation gauge, one photosynthetically active radiation (PAR) sensor, and one soil temperature and relative humidity sensor.Biweekly Sampling. Every other week during the growing season we will collect soil porewaters from lysimeters, sample soil pCO2and pO2from the gas collection tubes, measure soil CO2efflux, and measure root biomass production using the CI-600in situroot biomass imager.Seasonal Sampling.At the beginning and end of each growing season soils from each treatment will be sampled at 0-5, 5-15, 15-30; 30-50, 50-70, and 70-100 cm depth increments. Bulk density will be measured for each depth increment, and each sample will be assessed for pH in water, and total C, total N, and soil organic C (SOC). Soil samples will also be assessed for POM and MAOM.Biomass clipped in July will be dried, and total biomass yield will be measured.Data AnalysisRepeated measures analysis of variance will be used to assess changes in soil and plant parameters over the course of the experimentGOAL 2: Investigate ecosystem C flux partitioning in grasslands that are recovering from both drought and intensification of pasture management.Removal of existing rainfall exclusion shelters and installation of instrumentation.We will remove the DroughtNet rainfall exclusion shelters that have been in place for the past eight years.Following the design of Objective 1, within each replicate we will install one lysimeter in the C horizon, three soil gas collection tubes, one minirhizotron tube, and one soil CO2 efflux collar. We will install soil gas collection tubes at 20 cm, 40 cm, and within the C horizon (around 100 cm). Soil moisture sensors are already installed in the DroughtNet plots to measure differences in soil moisture across all treatments.Biweekly sampling.Following the removal of the rainfall exclusion shelters and installation of monitoring equipment, every other week during the growing season we will collect soil porewaters from lysimeters, sample soil pCO2 and pO2 from the gas collection tubes, measure root biomass production using the minirhizotrons, and measure soil CO2 efflux at the installed gas collars. Sampling and data analysis will follow same protocol as the biweekly sampling objective of goal 1Seasonal sampling.At the beginning and end of each growing season soils from each treatment will be sampled at 0-5, 5-15, 15-30; 30-50, 50-70, and 70-100 cm depth increments. Sampling and data analysis will follow the same protocol and methods as outlined in the seasonal sampling objective of goal 1.Data analysisRepeated measures analysis of variance will be used to assess changes in soil and plant parameters over the course of the experimentGOAL 3: Establish laboratory experiments to constrain the relative importance of the reactions in soils that generate deviations of soil CO2 efflux from soil respiration.Drought by nitrogen addition incubations.University of Science and Arts of Oklahoma undergraduate students will collect 25 cm by 5 cm soil cores and subject them toto the following treatments in a fully factorial design: 10%, 30%, 50%, and 70% of water-filled pore space; and 0, 30, 60, and 90 kg ha-1 N addition. Soil cores will be incubated for two months. During the incubation, soil porewaters will be sampled every other day using the mini-lysimeters. After porewater sampling, the top cap will be fitted onto the soil core to allow respired CO2 to accumulate. On opposite days of porewater sampling, headspace pCO2 and pO2 will be sampled and cores will be uncapped to allow for O2 diffusion. Every week, the mass of water lost due to porewater sampling and evaporation will be replaced with additional simulated rainfall.Drought recovery incubations. soil cores will be collected from each active rainfall exclusion treatment. As with goal 3.1, cores will be sealed on the bottom and mini-lysimeters will be installed in the side. Caps will be made with septa installed for sampling of headspace gases. Additional soil cores will be collected for destructive sampling. These cores will be divided into 0-5, 5-15, and 15-25 cm depth increments, and SIC concentration, soil organic C concentration, and soil pH will be measured.To simulate recovery from drought, soil cores will all be incubated for two months under the same 50% water filled pore space conditions. Porewater will be sampled every other day, and headspace pCO2 and pO2 will be measured on the days porewater is not sampled. Water mass lost from porewater sampling and evaporation will be replaced with additional simulated rainfall weekly. Major porewater ion concentrations will be measured by ion chromatography, and DIC will be modeled. The headspace pCO2 and pO2 will be used to calculate ARQ. Following incubations, each core will be air-dried and divided into 0-5, 5-15, and 15-25 cm increments. Each increment will be homogenized and SIC, SOC, POM, MAOM, and pH will be measured.Data analysis.Repeated measures analysis of variance will be used to assess changes in measured parameters over the course of the two experimentsGOAL 4: Translate Results for Students and Land ManagersBuild soil microbial activity assessment contraptions (SMAACs) and develop soil science and biogeochemistry laboratory activities utilizing SMAACs to understand soil respiration, carbon cycling, and soil health.Share laboratory activities and lesson plans on open access educational resource repository, such as the open education resource commons.Present results in years 2 and 3 at the South Central Climate Adaptation Science Center's annual meeting.Generate soil inorganic carbon factsheet for land managers in the Southern Great Plains, and disseminate via South Central Climate Adaptation Science Center.
Project Methods
For the two field experiments, we will employ the following methods.Every other week during the growing season we will collect soil porewaters from lysimeters, sample soil pCO2 and pO2 from the gas collection tubes, measure soil CO2 efflux at the installed gas collars using a LICOR Smart Chamber attached to a LICOR LI-7815, and measure root biomass production using the CI-600 in situ root biomass imager. Porewaters will be analyzed for NO3-- and NH4+-N concentrations colorimetrically in the laboratory of PI Hodges. These data will be used to assess nitrification of applied fertilizer. Porewaters will additionally be analyzed for DOC and major ion constituents in the Aqueous Geochemistry Laboratory at the University of Oklahoma so that DIC can be calculated (Hodges et al., 2021). Major ion concentrations (Ca+2, Mg+2, K+, Na+, Cl-, SO4-2, NO3-) will be measured by Ion Chromatography, and these data will be used to calculate alkalinity. Because porewater samples will be collected under vacuum and there is a large differential in pCO2 between soil and atmosphere, some DIC may degas from samples prior to sampling. Therefore, DIC and pH will be modeled using Geochemist's Workbench from the alkalinity of the samples and pCO2 of the soil atmosphere at the depth of the lysimeter (measured from the gas tubes). Soil pO2 will be measured using a portable analyzer. Soil pCO2 samples will be collected in the field using two-way lock syringes and measured for pCO2 using the LICOR LI-7815 with a small sample introduction module that is in the laboratory of PD Hodges. Soil pCO2 and pO2 data will be interpreted for signatures of SIC and SOC cycling using the principles of apparent respiratory quotient (ARQ; Angert et al., 2015; Hodges et al., 2019). Apparent respiratory quotient is defined as the change in soil pCO2 from ambient concentrations divided by the change in soil pO2 from ambient concentrations, multiplied by the negative ratio of the diffusion constants of CO2 and O2.By correcting for the differences in diffusion rates, ARQ equals about 1 when aerobic respiration by roots and microorganisms predominates within the soil system because aerobic respiration generates one mole CO2 for every mole O2 consumed. However, a range of biotic and abiotic reactions can shift ARQ to be above or below 1. For example, because CO2 is much more soluble in porewaters than O2, dissolution of respired CO2 in porewaters results in an ARQ < 1. On the other hand, degassing of DIC following carbonate mineral dissolution would result in an ARQ > 1 (Hodges et al. 2019). These soil gas data will provide evidence of the key processes of SIC cycling at play across the treatments.The modeled DIC concentrations will be used with the ARQ calculations to determine the predominance of dissolution of respired CO2 across treatments. The modelled pH and DIC along with the concentration of NO3- in porewaters will be used with the ARQ calculations to determine the likelihood of degassing of carbonate-derived CO2 into the soil atmosphere. Finally, the modelled DIC concentrations, and whether they are close to saturation will be used to predict the likelihood of precipitation of pedogenic carbonates. Soil CO2 efflux measurements, along with ARQ will be used to estimate the 'missing' CO2 that dissolved in porewaters, and/or the additional abiotic flux of CO2 in the case of degassing carbonate minerals. One can calculate actual soil respiration by multiplying soil CO2 efflux by soil ARQ. Therefore, by difference between the soil CO2 efflux and soil respiration, we will calculate the magnitude of the effect of the reactions of interest on measurements of soil respiration rate by the efflux method.At the beginning and end of each growing season soils from each treatment will be sampled at 0-5, 5-15, 15-30; 30-50, 50-70, and 70-100 cm depth increments. Bulk density will be measured for each depth increment, and each sample will be assessed for pH in water, and total C, total N, and soil organic C (SOC) using the Elementar Vario EL Cube in the lab of PD Hodges. For total organic C determination, inorganic C will be removed by repeated HCl addition and heating treatments.SIC will be calculated by subtraction of total C from SOC concentrations. Soil inorganic C and SOC stocks will be calculated using the measured bulk densities and C concentrations. SOC stocks of each treatment will be further fractionated into POM and MAOM (Cambardella & Elliot, 1992). Increases in the concentration of SIC will be used as evidence of precipitation of pedogenic carbonates and decreases in the concentration of SIC will be used as evidence of dissolution of pedogenic carbonates. Soil pH will be used to assess the probability of reactions 1-4, and changes in pH over time will be measured as a response variable to drought, N addition, and biomass clipping. Biomass clipped in July will be dried, and total biomass yield will be measured. A subset of the biomass will be ground and sent to an analytical lab for determination of CNS concentrations, and indicators of forage quality, including crude protein, neutral detergent fiber, acid detergent fiber, and total digestible nutrients.For the laboratory experiments, we will employ the following methodsSoil cores will be incubated for two months. During the incubation, soil porewaters will be sampled every other day using the mini-lysimeters. After porewater sampling, the top cap will be fitted onto the soil core to allow respired CO2 to accumulate. On opposite days of porewater sampling, headspace pCO2 and pO2 will be sampled and cores will be uncapped to allow for O2 diffusion. Every week, the mass of water lost due to porewater sampling and evaporation will be replaced with additional simulated rainfall. The students conducting the experiments at USAO will measure porewater chemistry using the spectrophotometer to be acquired for the research and teaching laboratory of Co-PD Sanders, and the funded postdoctoral scholar will work with the USAO undergraduate researchers to model DIC concentrations in the porewaters. Students will visit OU to measure pCO2 and pO2 of the headspace gases, which will be used to calculate the ARQ of the incubation at each timestep. Following incubations, each core will be air-dried and divided into 0-5, 5-15, and 15-25 cm increments. Each increment will be homogenized and SIC, SOC, POM, MAOM, and pH will be measured on each increment of the cores.Analysis of DataRepeated measuresanalysis of variance (ANOVA) will be used to examine the treatment effects on all measured response variables for both the laboratory and field experiments