Managing Crop Residues for Soil Health

The research portion of this proposal addresses three objectives:

1. To assess the impacts of different crop’s post-harvest residue and root biomass on soil activity during the following growing season;
2. To measure soil health traits that contribute to soil’s capacity to capture and retain water and promote nutrient cycling across crop rotations;
3. To identify target crop rotations and their residue metrics that contribute to soil health

Site Description

This research was conducted on two farms in Montana that incorporate a variety of cropping systems, management practices, and harvest techniques, allowing for a comparison of soil health traits in response to crop rotations and post-harvest organic matter inputs. Both farms produce grains, legumes, and oil seed crops. However, the two sites differ in their management practices, from crops included in rotations to inclusion of occasional grazing, and often in harvest practices, impacting the crop residue left on the fields after harvest. Further, these sites were selected because of the producer’s commitment to incorporating soil health principles in their land management practices.

Cavin Steiger’s farm is near Forsyth, Montana, along the Yellowstone River. This area of east central Montana receives an average of 12.6” of precipitation annually, with an average temperature of 46.8 °F, and a cold semi-arid climate type. The soil is a frigid, Aridic Ustifluvent loam. Steiger incorporates 17 crops into rotation, including winter wheat, spring wheat, malt barley, sunflowers, canola, lentils, alfalfa hay, yellow peas, many cover crop mixes, and occasionally rotates with fallow for moisture accumulation. He utilizes a stripper header to harvest grains and has been no-till since 2007, maximizing the crop residue left on the fields after harvest for erosion protection and moisture retention. This diverse crop rotation and harvest technique allows for a comparison of various legumes, oil seed crops, and cereals in rotation with their residual impacts on surface residues and soil health characteristics. 

Casey Baileys’s farm is located in Fort Benton, in the "Golden Triangle" region of north central Montana, near the upland slopes of the Missouri River. This area receives an average of 13.2” of precipitation each year, with an average annual temperature of 45.6 °F, and a cold semi-arid climate. The soil type is a frigid, Typic Argiustoll clay loam. Bailey includes wheat, barley, Kernza®, alfalfa hay, lentils, and oil seed crops. Additionally, he incorporates no-till strategies, buffer strips between fields, and some grazing. Bailey's array of crop rotations and management techniques provide a different gradient of residue cover and crop rotations to assess a relationship to soil health traits.

Objective 1: To assess the impacts of different crop’s post-harvest residue on soil activity during the following growing season.

The specific fields and rotation of crops selected were informed by planting decisions made by the participating farmers in spring of 2022, which were impacted by spring precipitation patterns, particularly because of the record drought during the last growing season and the relatively dry winter.

At the Steiger farm, six plots were delineated within five fields at a different stage of their crop rotation, representing four crop types: cereal (wheat), legume (lentils or peas), oil seed crop (canola or flax), and sugar beets. At the Bailey farm, fields strip planted in perennial wheat, annual wheat, and perennial alfalfa were selected for sampling to assess impacts of annual vs. perennial crop types. In each of the plot within the five fields, quantity of residue inputs was measured by the following methods: 

• Crop residue biomass: In spring 2022 and spring 2023, aboveground residue biomass was sampled by hand in the 6 plots within each of the crop rotation categories to assess residue that had overwintered. The residue biomass associated with each root core was collected, including the loose residue, plant litter, and standing stalks cut at the soil surface. This residue was dried for 72 hours at 60 °C and weighed immediately out of the oven.
• Root biomass: During spring 2022 sampling, we extracted root cores (root auger, 3” diameter, 20” depth) at a randomized location within each of the six plots. Roots were cleaned and analyzed for root biomass to assess the belowground biomass inputs to the soil for each crop rotation.

Objective 2: To measure soil health traits that contribute to soil’s capacity to capture and retain water and promote nutrient cycling across crop rotations.

In spring 2023, composited soil samples (1” diameter, 6” depth) were collected from the six plots in each of the crop rotation treatment groups. Soil cores were taken from the same subplots where biomass residue was collected (Objective 1). The soil biological indicators that were measured in this study were selected based on their responsiveness to management practices and crop rotation sequences, correlation with ecosystem processes, and integration of a soil’s chemical, physical, and biological properties. Residue management, tillage, and other farming practices can change soil characteristics and microbial community structure because of their impacts to food supply, changes in the populations of decomposers, altered physical properties, and changes to SOM distribution (Busari et al., 2015). As such, the following biological parameters included on the NRCS list of indicators were sampled for soil health assessment: potentially mineralizable nitrogen (PMN), five soil enzymes, soil organic matter (SOM) proxies such as permanganate oxidizable carbon (POXC), and total carbon and nitrogen (NRCS, 2015). 

Mineralizable nitrogen (N) is a measure of the microbial activity that makes N available from SOM, and has been shown to be correlated with crop growth (Wang et al. 2018). Potentially mineralizable nitrogen (PMN) provides an estimate of the N available for plant-uptake throughout the growing season. PMN was analyzed using a 14-day anaerobic lab incubation at 30 °C on 5-gram field-moist soil samples, following methods from Keeney and Nelson (1982). Soils were analyzed for ammonium, NH4+, at day zero and at day 14, using a 1M potassium chloride (KCl) extraction and Lachat autoanalyzer (Lachat Instruments, Loveland, CO) in MSU’s Environmental Analytical Laboratory. Plant available nitrogen is calculated from the difference in pre- and post-incubation NH4+ concentrations.

Soil enzymes are a catalyst for nutrient cycling and are an indication of microbial activity, as they are released into soils primarily by soil bacteria and fungi. We have used them as an indicator of impacts of cover crops on soils in dryland production (Housman et al. 2021). Enzymes respond quickly to disturbances, providing an early indication of the trajectory of soil quality with changes in land management practices and crop rotation (reference). Soil extracellular enzyme activity was analyzed in 1-gram of field-moist soil and incubated with the p-nitrophenol enzyme-specific substrate at 37 °C for 1-hour, and assayed for a colorimetric response spectrophotometrically. Controls and duplicates for each sample were analyzed following the procedure outlined by Dick et al. (1996) and Parham and Deng (2000). Five enzymes were analyzed, including β-1,4,-N-acetyl glucosaminidase (nitrogen cycle), β-1,4-glucosidase (carbon cycle), arylsulfatase (sulfur cycle), and alkaline and acid phosphatases (phosphorus cycle), in the Zabinski Laboratory. The geometric mean will be used to compare enzyme activity between the different crop rotations and residual organic matter inputs at each site. 

Soil organic matter (SOM) is the portion of soil composed of plant residue and decomposing organisms, humus, and the active decomposing organic matter. Post-harvest crop residue provides a source of SOM and ample nutrient supply, water holding capacity and increased infiltration rate, soil aggregation, and erosion prevention. Because of the difficulty in directly measuring SOM, permanganate oxidizable carbon (POXC) was measured as a proxy for SOM. Soil POXC was analyzed in duplicates with 2.5-gram of air-dried and pulverized soil, ground for 12-hours to a fine consistency of <0.1-mm. The soils were combined with 18-mL of deionized water and 2-mL of 0.2 M potassium permanganate (KMnO4) in 1 M calcium chloride (CaCl2). A diluted aliquot of the sample was assessed for a colorimetric response that was measured spectrophotometrically, following methods developed by Weil et al. (2003).  This method is more sensitive to land management impacts than TOC, and has been closely linked to soil respiration, microbial biomass, and soil aggregation (Bongiorno et al. 2019).

Timeframe of Sampling for Objectives 1 & 2

**This sampling event and its related analyses are not included in this proposal and are already a part of the applicant’s masters thesis work. However, these results will be utilized in the statistical analysis of this project for comparison to spring 2023 soil analyses.

Objective 3: To identify target crop rotations and their residue metrics that contribute to different components of soil health.

This research will inform the understanding of the effects of specific crop rotations on organic matter inputs and soil health. Specifically, the results of this project are expected to assist in guiding management decisions for crop rotations in respect to farmer’s specific rotation and soil health goals. The residue biomass and soil analyses will be compared within the treatments at each site to identify trends that will inform target rotation and residue metrics for reaching soil health goals. 

At each of the sites in Forsyth and Fort Benton, MT, the four treatment groups of differing crop rotation (i.e. cereals, legumes, oil seeds, and fallow) will be compared for differences in crop residue and root biomass. Analysis of variance (ANOVA) will be utilized to test for differences in residue quantity between the rotation groups at each site. Further, an assessment for correlations between residue quantity and soil health parameters within each crop rotation will be performed. This will guide the process for identifying target crop rotations and their respective residue metrics for specific soil health goals. It is possible that biomass quantity has more of an effect on soil quality than the crop type itself. Research in semi-arid systems with cover crops has suggested that in biomass-limited systems, the quantity of the biomass produced as residue may be more important than the identity of the species for soil health traits (Housman et al., 2021). 

We expect that legumes will have the biggest impact on soil biological activity, specifically on PMN and enzymatic activity, because of their capacity to add nitrogen to the system when used as a legume green manure (Miller et al., 2006). Soil organic matter will likely decrease following crop types with lower residue contributions because there are limited residue contributions to the SOM pool (Acosta-Martinez et al., 2007). Further, it is anticipated that within cereals and oil seeds fields, areas associated with higher quantities of crop residue biomass will result in an increase in labile carbons due to organic matter inputs (Vázquez et al., 2016).