The Soil Organic Carbon network (SOCnet): Farmers building soil assets to help mitigate and adapt to climate change in the North Central US

SOCnet will consist of three research and engagement Tiers (Figure 2). This collaborative 3-Tiered framework integrates multi-location on-farm experiments with ongoing SOC monitoring and cropping systems research in Wisconsin (WICST), Iowa (Marsden Farm), and Minnesota (LTARN). WICST¹ (Wisconsin Integrated Cropping Systems Trial) was established in 1990 and consists of cropping systems broadly representative of the North Central U.S. that include cash-grain, dairy-forage, grazing, and native grasslands. The Marsden Farm² long-term experiment was established in 2001 and compares cropping systems of varying diversity including corn, soybean, small grains, and cover crops in Central Iowa. LTARN³ (the Long-Term Agricultural Research Network) is a network of three long-term experiments that were established across Minnesota in 2013 to evaluate cropping systems that represent a range of diversification strategies. Work at each long-term experiment has focused on understanding the productivity, profitability, and environmental benefits of alternative production strategies within cropping systems that are broadly reflective of regional practices. Each site employs a randomized complete block design, providing a robust statistical framework with which to evaluate the impact of agricultural management practices on SOC stocks. WICST, Marsden Farm, and LTARN (3 sites) will make up the SOCnet Tier 1 “hubs” (Figure 2, red).

Each Tier 1 hub will be linked to three Tier 2 on-farm locations (Figure 2, orange), creating the core layer of SOCnet. To expand the scope of study and increase our ability to scale our results across the region, we will prioritize Tier 2 sites with soils that differ significantly from the soils at the respective Tier 1 hubs. Each Tier 2 site will compare a “business-as-usual” treatment (BaU) representing typical on-farm practices with a farmer-defined “change in practice” (CiP) geared specifically for SOC accrual. Tier 2 locations will be selected so that on-farm cropping systems broadly align with those at the nearby long-term hub. An example of the broad cropping system categories at each Tier 1 hub are show in Table 1.

Table 1. Cropping system categories evaluated at each of the Tier 1 long-term hubs.

The Tier 1 hubs and Tier 2 on-farm sites will provide the foundation for meeting SOCnet objectives 1 and 2.

Over the initial 3-year grant cycle, we will expand the on-farm network to a third Tier (Figure 2, yellow), which will dovetail with ongoing regional soil health projects and on-farm networks tracking SOC like the extensive FFAR funded collaborative effort between our research team, the Soil Health Institute (SHI), and the Dairy Research Institute (DRI). Other collaborative networks associated with the three-state research team include Grassland 2.0 (SAS CAP, 2019-68012-29852), the cover crop research and outreach project (CCROP, WI), producer-led watershed groups (WI), Great River Greening (MN), Practical Farmers of Iowa (IA), and several recent or ongoing USDA HATCH (WIS03057), USDA CIG (NR183A750008G011) and SARE (LNC20-440) projects evaluating SOC stocks in paired on-farm settings associated with grazing of cool-season pastures and cover crops. We will solicit participation in Tier 3 with the help from the Tier 2 CFT and above-mentioned networks as well as through stakeholder meetings and field days. Tier 3 sites will be used to increase the diversity of soil types, climate, and management practices within SOCnet, thereby solidifying the anticipated outcomes for objectives 2 and 3 and maximizing the overall impact of SOCnet.

All soil sampling and processing will follow standard protocols. At the Tier 1 sites, sampling will occur in established research plots, with a minimum of two soil cores taken in random locations per plot per collection time. Soil sampling at the Tier 2 and Tier 3 fields will be based on the Terraton Experiment™ protocol as a guide to ensure a link with developing carbon market standards. In line with guidance from Terraton, Climate Action Reserve, and Verified Carbon Standard, Tier 2 and Tier 3 fields will be stratified into areas that are expected to contain relatively homogenous soils.⁴ Stratification will be based on the soil series given in the USDA NRCS Soil Survey Geographic Database (SSURGO), which integrates critical properties including slope, drainage classification, and soil texture. Two soil cores will be taken for each 5-acre area, with a minimum of one soil sample for strata smaller than 2.5 acres (see Figure 3). In fields larger than 20 acres, soil sampling will be restricted to a 20-acre area that is deemed to be representative of the entire field based on soil properties in the SSURGO database. This hybrid approach will allow for robust determination of SOC change while keeping to a manageable number of soil samples. The location of all soil samples will be georeferenced with a high precision GPS, and all future soil samples will be collected within 1 m of the original samples.

Most efforts to track changes in SOC focus on surface soils and ignore deeper SOC stocks, often for logistical reasons. However, changes in shallow SOC stocks (e.g., 0 to 30 cm) are not always related to changes in deeper SOC stocks, and losses at depth can offset gains in surface soils⁵. All soil samples collected for SOCnet will therefore include as much of the soil profile as possible while recognizing that shallow soils may occasionally limit sampling depth. Deep soil cores (0 to 100 cm) will be collected at Tier 2 and Tier 3 sites beginning in fall 2022 and fall 2023, respectively (Table 2). Deep soils collection at the Tier 1 sites will commence in 2024, as those sites have all recently collected deep cores during independent sampling campaigns. Re-sampling at all sites will occur at three-year intervals. Once collected, all cores will be stored at -4 °C until further processing can occur. Deep soil cores will be divided into four depth increments (0 to 15 cm, 15 to 30 cm, 30 to 60 cm, 60 to 100 cm). Field moist soil mass will be recorded, and a representative aliquot will be dried at 105 °C to determine total dry soil mass. The remaining sample will be sieved to 2 mm to remove gravel and plant debris, and the gravel-free dry weight will be used to calculate soil bulk density. A subsample of each soil sample will be dried, pulverized, and analyzed for total carbon (TC) and total N using flash combustion elemental analysis. Total soil organic carbon (SOC) will be directly measured on a subset of 10% of each site’s soil cores by acid fumigating an additional soil aliquot prior to elemental analysis⁶. If soil inorganic carbon (SIC) is detected by this method, additional soil cores from that site will be acid fumigated. If not, we will assume SOC is equal to TC. SOC stocks will be calculated by combining the SOC values with the gravel-free bulk density and SOC stocks will be presented in equivalent soil mass form to account for any changes in soil bulk density that could otherwise bias estimates of temporal SOC change⁷. Where present, SIC stocks and temporal changes will also be documented.

Table 2. Soil sampling details including sampling strategy, depth, and response variables for each of three Tiers.

A suite of additional soil analyses will be performed to evaluate links between soil properties and SOC, elucidate the mechanisms underlying SOC change, and assess the temporal changes in SIC (Table 2). Soil texture analysis using the hydrometer method, and soil pH will be quantified in a 1:1 soil-to-water slurry (all Tiers). Agronomic nutrients (pH, OM%, P, K, Ca, Mg) will be quantified for the 0 to 15 cm depth using standard methods at a regional soil laboratory (Tiers 1 and 2). Particulate organic matter (POM) will be isolated from bulk 0 to 15 cm soils with a size-based fractionation scheme⁸, and organic carbon concentrations will be determined (Tiers 1 and 2). Mineral associate organic matter (MOAM) will be estimated as the difference between SOC and POM. Bulk dried samples will be archived for potential future analyses. All analyses less Agronomic nutrients will be conducted at the Jackson Lab (UW-Madison).

Crop yields from all plots and fields will be recorded, and residue return to soil will be estimated based on crop-specific harvest indices. All data will be curated by project personnel at the University of Wisconsin-Madison and stored redundantly in a shared cloud space. Data analysis will be overseen by Dr. von Haden working closely with Dr. Sanford and other members of the scientific team. Specific analyses will assess the response of SOC to climate, soil, and management factors as well as investigate the biological and physical mechanisms that may mediate the SOC response. Moreover, feedback between SOC change and yield response will be elucidated, as this relationship has significant economic implications.

Farmers will be invited to participate in the analytical processes, provided annually with updated data, and given complete access to the anonymized network database. The research team will help participating farmers quantitively track and interpret SOC changes in their fields. Researchers will also help farmers determine which land management practices are most effective at promoting SOC storage and whether differences in SOC storage rates varies across their fields. Farmers will be provided training in, and access to, decision support tools (DSTs) such as Smartscape™⁹ and Grazescape™. These two powerful decision support tools will allow farmers to evaluate how on-farm land management, including their selected CiP, will translate into a broad suite of environmental and economic outcomes. These economic outcomes not only include traditional crop revenue but also the value of ecosystem services such as SOC storage, reduced greenhouse gas emissions, and water quality improvements. Measurements of SOC change from all SOCnet sites will be used to enhance the underlying Smartscape™ and Grazescape™ biogeochemical model framework, which will in turn improve the accuracy and value of these tools.

References

1. Wisconsin Integrated Cropping Systems Trial. University of Wisconsin-Madison. https://wicst.wisc.edu/
2. Marsden Long-Term Rotation Study. Iowa State University. https://www.cals.iastate.edu/inrc/marsden-long-term-rotation-study
3. Minnesota Long-Term Agricultural Research Network. University of Minnesota. https://ltarn.cfans.umn.edu/
4. Lawrence, P.G., et al. 2020. Guiding soil sampling strategies using classical and spatial statistics: A review. https://doi.org/10.1002/agj2.20048
5. Sanford, G.R., et al. 2012. Soil carbon lost from Mollisols of the North Central U.S.A. with 20 years of agricultural best management practices. https://doi.org/10.1016/j.agee.2012.08.011
6. Harris, D., et al. 2001. Acid fumigation of soils to remove carbonates prior to total organic carbon or carbon-13 isotopic analysis. https://doi.org/10.2136/sssaj2001.1853
7. von Haden, A.C., et al. 2020. Soils’ dirty little secret: Depth-based comparisons can be inadequate for quantifying changes in soil organic carbon and other mineral soil properties. https://doi.org/10.1111/gcb.15124
8. Poeplau, C., et al. 2018. Isolating organic carbon fractions with varying turnover rates in temperate agricultural soils – A comprehensive method comparison. https://doi.org/10.1016/j.soilbio.2018.06.025
9. Tayyebi, A., et al. 2016. SmartScape™: A web-based decision support system for assessing the tradeoffs among multiple ecosystem services under crop-change scenarios. https://doi.org/10.1016/j.compag.2015.12.003