Land management effects on wet aggregate stability and carbon content

Authors: Mikha, Maysoon; Jin, Virginia; Johnson, Jane; Lehman, R - Michael; Karlen, Douglas; Jabro, Jalal - Jay

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2021
Publication Date: 9/3/2021
Citation: Mikha, M.M., Jin, V.L., Johnson, J.M., Lehman, R.M., Karlen, D.L., Jabro, J.D. 2021. Land management effects on wet aggregate stability and carbon content. Soil Science Society of America Journal. 85(6):2149-2168. https://doi.org/10.1002/saj2.20333.
DOI: https://doi.org/10.1002/saj2.20333

Interpretive Summary: Land management practices can alter the stability of soil structure and nutrient dynamics by influencing soil organic C (SOC) [also referred to as soil organic matter (SOM) because SOC accounts for 58% of SOM]. Soil aggregate stability and size distribution are two measurements, expressed as mean weight diameter (MWD) and geometric mean diameter (GMD), used to characterize soil structure stability. Higher MWD and GWD values indicate the aggregates are less likely to be broken and washed away by rain or irrigation water. Tillage physically breaks aggregates into smaller sizes and hastens decomposition of plant and fungal materials that hold soil particles together as aggregates. Our objective was to quantify long-term soil management and slope on SOC, aggregate-size distribution, aggregate-associated C, and soil structure. Thirty-eight sites with moderate (5-9%) or high (14-18%) slope and 5 to 40 years of management history were evaluated. The management practice were business as usual (BAU) cropland, BAU pasture (BAU-Pasture), newly established conservation reserve program (CRP-New), and established CRP (CRP-Old) areas. Soil samples were taken from 0- to 5-cm (0- to 2-inches) and 5- to 15-cm (2- to 6-inches) depth increments. Several soil health indicator measurements were evaluated including separation of the soil samples into five aggregate size groups [>2000, 1000-2000, 500-1000, 250-500, and 53-250 microns]. Within the surface 5 cm Pasture and CRP-Old sites had 79% more macroaggregates (>250 micron),123% higher MWD, 38% higher GMD, and 47% higher SOC concentrations than Cropland or CRP-New sites. Soil from the 5- to 15-cm depth increment showed a lower but similar response for all indicators. Aggregate-associated C was quantified using a constant soil mass that reflected aggregate size distribution to prevent overestimating C content. Eliminating soil disturbance greatly improved all indicators. High microaggregate quantities associated with BAU-Crop and CRP-New indicated poor soil structure and a reduced ability to store soil carbon. Long-term erosion moved soil down slope, affecting soil properties and nutrient concentrations when compared to upper slope positions. Overall, our data confirms that land management decisions can influence soil structure stability and nutrient dynamics. It also shows that 10 to 40 years of CRP management may be required to return SOC content and soil structure stability to pre-cultivation prairie levels. To conserve land resources, enhance soil stability, and potentially provide cellulosic feedstocks for bioenergy or other bio-products, we recommend establishing grasses on sloping, highly erosive areas.

Technical Abstract: Land management affects soil structure and many other soil properties and processes. Our objectives were to evaluate soil organic carbon (SOC), aggregate-size distribution, aggregate-associated C, and soil structure as affected by long-term land management and slope. A chronosequence of 38 on-farm sites with low to high (5-18%) slopes was selected to evaluate 5 to 40 years of management. The sites were classified as business as usual (BAU) cropland (BAU-Crop), BAU pasture (BAU-Past), newly established conservation reserve program (CRP) areas (CRP-New), and established CRP (CRP-Old). Soil samples were collected from the 0 to 5- and 5 to 15-cm depth increments and processed for soil property measurements including fractionation by wet sieving into five aggregate-size classes (>2000, 1000-2000, 500-1000, 250-500, and 53-250 micron). Within the surface 5 cm, mean weight diameter (MWD) and geometric mean diameter (GMD) were used to characterize soil structural stability. BAU-Past and CRP-Old sites had 79% more macroaggregates (>2000, 1000-2000, 500-1000 micron, 123% higher MWD, 38% higher GMD, and 47% higher SOC than BAU-Crop or CRP-New sites. The 5- to 15-cm depth increment showed a similar but lower magnitude response. Aggregate-associated C was quantified using a constant soil mass that reflected aggregate size distribution to prevent overestimating C content. Lower slope locations had more SOC, more macroaggregates, more C associated with macroaggregates, and higher GMD and MWD compared to high slope locations across all management classifications and soil depths. The results support our hypothesis that the high slop soils may benefits from specific management decisions than the lower sloping soils as a function of landscape property. We recommend re-establishing grassland on sloping land that is susceptible to excessive soil erosion, although those practices will likely take a long time to restore soil structural stability and SOC content to pre-cultivation levels.