Abstract
AbstractLand management affects soil structure and many other soil properties and processes. Our objectives were to evaluate soil organic C (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 5–40 yr 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 (>2,000, 1,000–2,000, 500–1,000, 250–500, and 53–250 μm). Within the surface 5 cm, mean weight diameter (MWD) and geometric mean diameter (GMD) were used to characterize soil structural stability. The BAU‐Past and CRP‐Old sites had 79% more macroaggregates (>2,000, 1,000–2,000, and 500–1,000 μm), 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 with 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 reestablishing 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 precultivation levels.
Highlights
Land management had no significant effect on soil organic C (SOC) within the 5-to-15-cm depth increment (P = .1684), with business as usual (BAU)-Past and conservation reserve program (CRP)-Old having numerically higher SOC content (∼21% or 3.2 g C kg−1 soil) compared with BAU-Crop and Conservation Reserve Program–new (CRP-New) sites
Evaluating aggregate-associated C in specific size classes, we found that macroaggregates (250–500 μm) and microaggregates (53–250 μm) contained 16% greater aggregateassociated C (4.7 g C kg−1 aggregates) for Conservation Reserve Program–old (CRP-Old) and 13% greater aggregate-associated C (3.6 g C kg−1 aggregates) for BAU-Past than macroaggregates (>2,000 μm and 1,000–2,000 μm) at 0-to-5-cm depth
This study documented increased macroaggregate size associated with BAU-Past and CRPOld, which led to increased mean weight diameter (MWD) and geometric mean diameter (GMD) values and greater SOC storage within soil aggregates
Summary
Soil aggregation is an important indicator of soil structural stability (Kalhoro et al, 2017; Sekaran et al, 2021; Six et al, 2000; Tourn et al, 2019) that influences soil health factors, including (a) soil organic C (SOC) conservation and nutrient dynamics (Rodríguez et al, 2021; Somasundaram et al, 2017; Weidhuner et al, 2021; S. Xu et al, 2021), (b) porosity and water retention (Regelink et al, 2015; Sekaran et al, 2021), (c) water infiltration and surface runoff, and (d) soil erosion (Anderson et al, 2019). Microaggregates exhibit chemical bonding mechanisms that can withstand slaking and mechanical stress, enabling microaggregates to persist in soil for a long time (Totsche et al, 2018). They are the building blocks for macroaggregates (Totsche et al, 2018), being physically bound by SOM, plant root exudates, fungal hyphae (Angers, 1998; Jastrow et al, 1998; Miller & Jastrow, 1990), and microbial byproducts (Rillig et al, 2006). Microaggregates are continuously formed within macroaggregates, further contributing to aggregate size and SOC stabilization (Six et al, 2000, 2002)
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