Location: Soil, Water & Air Resources Research
2022 Annual Report
Objectives
Objective 1: Assess humic products and cover crops as management options for promoting soil carbon pools, nutrient cycling for crop growth, and increased yield.
Objective 2: Assess the effectiveness of grass buffers, blind inlets, and carbon-based reactive barriers in the form of filter socks for phosphorus management.
Objective 3: Quantitatively characterize soil health and crop productivity responses to carbon input from crop sequences and carbon losses from tillage, crop residue management and harvest.
Approach
To enhance management of soil carbon, laboratory analyses for specific carbohydrates, amino compounds, phenols, and fatty acids will be used to distinguish between labile and recalcitrant fractions of soil organic matter (SOM). Effects of applying humic products and cover crops on short- (e.g., nutrient cycling and soil structure) and long-term (e.g., carbon sequestration) soil changes will be determined. Humic product application will be evaluated as a mitigation strategy for SOM loss due to excessive crop residue removal for bioenergy production. Filter socks filled with wood chips and bark will be evaluated as a means to efficiently catch dissolved and sediment-bound phosphorus moving with runoff water into surface inlets, subsurface drainage systems, and ultimately natural water bodies. Process-level knowledge from laboratory and plot-scale research will enhance development of site-specific subfield management strategies for increasing producer profitability and providing sustainable feedstock supplies at field and landscape scales. Several different crop rotations utilizing corn, soybean, alfalfa, wheat, rye, field pea, and tillage radish will be evaluated with various levels of stover harvest. No-tillage, strip-tillage, and chisel-plow management as well as effects of biochar will be determined. Effects on nutrient cycling, soil carbon stocks, and soil health (using the Soil Management Assessment Framework) will be quantified. Project results will provide critical information needed to elucidate the effects of crop residue management and carbon-based amendments on soil physical and biochemical properties, economic returns, and long-term sustainability of corn-based Midwestern cropping systems.
Progress Report
During the 5-year duration of this project, progress was made on all three objectives, their hypotheses, and goals.
Objective 1. Soil samples were extracted to recover two size classes of particulate organic matter and the mobile humic acid fraction from three field trials near Ames, Iowa to determine the effect of humic product application on soil carbon. Properties of these fractions of young soil organic matter (SOM) are more responsive to land management practices than are those of total SOM. The fractions are separated from the soil through both physical and chemical extraction steps, a promising integrated approach.
Humic product application in the three fields for 4 to 5 years had modest benefits on the amounts of young SOM fractions. The benefits were clearer in lower-carbon soils than in higher-carbon soils. Biochemical characterization of the SOM fractions indicated humic product application increased their contents of plant material.
In one of the fields the benefits of humic product application to soil physical properties were studied. Dry aggregate stability, penetration resistance, water-holding capacity, and bulk density showed modest benefits in subsoil depths with humic product application. Benefits were negligible in the surface depth, where native SOM was most abundant. Carbon inputs through humic product application were small, hence these subsoil benefits likely arise from greater root growth with humic product application.
In some of these same field trials, corn leaf and root samples were collected at multiple growth stages to measure plant hormone concentrations. Results suggest a novel causal mechanism for plant growth responses to humic products, involving specific hormones. Results are being written into a manuscript for publication. Agronomic, biochemical and root data sets from earlier humic product field trials were published.
The same three fractions of SOM were extracted from soil samples of a 20-year rye cover crop field trial near Ames, Iowa, at three crop growth stages in one growing season. A fourth, more recalcitrant fraction was also extracted at the third sampling time, near the end of the growing season. Its extraction is relatively laborious, limiting its study. The fractions and whole soils were analyzed for their compositions of carbohydrates and phenols. The activities of enzymes whose substrates are components of the young fractions were also measured at the same sampling times. Analyses of the phenol and enzyme concentrations are still ongoing and will not be reported here. All measurements were performed in two pairs of field treatments-with and without rye cover crop for either (1) standard harvest of only grain, or (2) harvest of grain plus also much of the stover for producing corn silage. Soil measurements were taken at both the 0-5 cm and 5-15 cm depths to account for vertical stratification of crop residues in this no-tilled field.
The two physically extracted fractions-the two size classes of particulate organic matter-responded to the treatments entirely through their mass, not their composition. The youngest fraction, the 500-µm particulate organic matter, increased in mass with cover cropping most clearly at the earlier sampling times. The slightly older 53-um fraction also increased in mass with cover cropping. These responses to the rye cover crop were clearer in the treatment pair having corn silage harvesting, thus accentuating the relative importance of the cover crop inputs to the soil compared to the treatment pair having standard grain harvest. Whole soil and the humic fractions responded to cover cropping in their carbohydrate contents, both in plant-associated and also microbial-associated carbohydrates. All trends were clearer in the 0-5 cm depth than in the 5-15 cm depth. In summary, rye cover cropping helped maintain soil carbohydrate concentrations with corn silage production, while cover cropping had muted benefits with harvesting of only corn grain.
To determine whether humic product use can enable decreased input rates of nitrogen fertilizer while maintaining corn yield targets, a nitrogen fertilizer response trial was established near Ames, Iowa, having five fertilizer rates. Each fertilizer rate main plot had subplots with or without humic product application. Corn grain yield increased with the humic product at the higher nitrogen rates, while grain yield decreased with the humic product at one suboptimal fertilizer rate, perhaps because the humic product overly stimulated vegetative growth beyond the short nitrogen supply. This complex trend was repeated in two years of mechanized combine harvesting and one year of hand-collected yield component samples. This field experiment is being continued into the new project plan: 5030-12210-004-000D, “Optimizing Carbon Management for Enhancing Soil and Crop Performance.”
Objective 2. The quality of water entering surface inlets was monitored as affected by three alternative inlet designs-blind inlet, filter socks, and grass buffer. Of the three practices, the filter socks, which were amended with alum (an aluminum-based material), were the most effective at reducing dissolved phosphorus (by 41-70%). Blind inlets were more effective than filter socks at reducing total water phosphorus (by 56 vs. 33%, respectively) and sediment (56% vs. 29%, respectively). The grass buffers reduced sediment by 60% and total phosphorus by 46%, but they were the least effective practice for reducing dissolved phosphorus (by 42%). The interim results of this research were presented at a farmer-oriented field day, and a brochure highlighting the findings was produced in collaboration with the citizen-led group, the Southfork Watershed Alliance. Additionally, the results were invited for presentations to the Iowa Secretary of Agriculture in December 2016 and to the Iowa State Soil Conservation Committee in March 2017. A field study was initiated to compare the capacities of filter socks amended with either woodchips or biochar to reduce losses of herbicides and nutrients transmitted to subsurface drains via surface inlets. Responsibility for this experiment was transferred to the USDA-ARS National Sedimentation Laboratory upon retirement of the incumbent in June 2018.
Objective 3. The economic feasibility of cellulosic biomass harvest was studied in two field studies (2008 to 2014, and 2011 to 2016, respectively) in central Iowa with moderate (~50%) and high (~90%) corn stover harvest. The first study, with and without an unharvested winter rye cover crop, resulted in average grain yields ranging from 7.55 to 10.80 Mg ha-1 with no reduction in yield due to the rye cover crop. This represented current practices, which may or may not be adopted to protect the soil resource because stover income alone must offset cover crop seed, herbicide, and labor input costs. The second study, which included soybean in the rotation and for which the rye cover crop was harvested as additional feedstock, resulted in an average corn grain yield of 9.72 Mg ha-1, moderate and high stover yields of 3.99 and 4.98 Mg ha-1, rye biomass yield of 4.00 Mg ha-1, and soybean grain yield of 2.90 Mg ha-1. Using prices from the Agricultural Marketing Service cornstalk bioenergy market, moderate corn residue, high corn residue, and high corn residue + cover crop harvest strategies would net US $11.47, $14.47, and $16.47 Mg-1, respectively, thus helping conservation practices pay for themselves and enhancing environmental and economic feasibility of cellulosic biomass harvest in central Iowa.
Two other cropping system studies, with 10- and 5-year historical records summarizing tillage, crop rotation, biochar, and crop residue harvest treatment effects, addressed the sustainability of stover removal for feedstock production in the Midwest, thus contributing to the Rural Energy for America Program database. Systems evaluated in Study 1 included: (1) chisel-plow and no-till production of continuous corn with no, moderate or high stover removal; (2) an oat/alfalfa-alfalfa–alfalfa–corn–corn rotation with corn stover harvest; (3) continuous corn with legacy biochar treatments and no, moderate, or high stover harvest rates; and (4) a corn–winter rye–soybean-winter wheat–cover crop (radish, pea, and oat) rotation where corn stover, vegetative rye, wheat straw and cover crop biomass were harvested as potential animal feed or cellulosic feedstock sources. Study 2 focused on a corn and soybean rotation using strip-tillage with no, moderate, or high-stover removal treatments. Surface (0 to 5- and 5 to 15-cm) and deep core (0 to 120-cm) soil samples were collected in fall 2017 to continue long-term monitoring of a suite of soil health (SH) indicators. Results for this central Iowa site indicated that diversified rotations can reduce N fertilizer requirements and improve several SH indicators. A 10-year assessment of tillage and stover harvest practices within Study 1 showed equivalent returns on investment for conventional (chisel plow) and no-tillage practices. Using no-tillage and an average annual stover harvest of 1.5 tons acre-1 (3.4 Mg ha-1) resulted in an average corn grain yield of 9.5 Mg ha-1. Collectively, these field studies confirm that a sustainable rate of corn stover can be harvested as a bioenergy or bio-product feedstock while still promoting soil health.
After 13 years of tillage and crop residue management practices, the plots in Study 1 were planted to an oat-clover rotation in 2020 to quantify residual impacts. Oat yield was measured, and samples were collected from a subset of the plots to quantify various SH indicators including subsurface compaction. Oat grain yield, soil organic C concentrations, and soil penetration resistance below the plow layer were not significantly different between the two tillage practices, suggesting either practice can be used to manage stover biomass without affecting plant productivity or soil health.
Accomplishments
