Land management affects soil structural stability: Multi-index principal component analyses of treatment interactions

Soil structure mediates soil functioning and can be influenced by agricultural management practices (cropping intensity, tillage, and nitrogen source). This study evaluated the effectiveness of multiple soil aggregate indices to quantify soil structural development using long-term data from three locations within the central Great Plains: Alternative Crop Rotation (ACR) and Long-Term Tillage (LTT) near Akron, Colorado, and Knorr-Holden (KH) near Mitchell, Nebraska. Tillage treatments include no-tillage (NT), reduced tillage (RT), conventional tillage (CT), and moldboard plow (MP). Commercial mineral fertilizer (F) was used as a nitrogen source in ACR and LTT sites while manure (M) plus F treatments were used in KH. Soil aggregate size classes were measured in the laboratory, and aggregate stability index (ASI), mean weight diameter (MWD), geometric mean diameter (GMD), and fractal dimension (FD) were calculated to evaluate soil aggregation using linear regression and principal component analysis (PCA). At 0-15 cm, intensive tillage treatments (CT and MP) in ACR and LTT, reduced (P < 0.05) ASI by 46.7%, MWD by 21.0% and GMD by 8.4% and increased FD by 0.77% compared with NT and RT treatments. The addition of manure increased (P < 0.05) ASI by 72.2%, MWD by 65.6%, GMD by 32.8%, and reduced FD by 5.5% compared with tillage treatments in ACR and LTT. The PCA indicated clear effects of M on aggregate structure and stability, indicating that manure addition could compensate for the tillage operations in sustaining soil structural stability. Explanatory variables FD and ASI were orthogonal across the three sites, while FD was negatively correlated with MWD and GWD; thus, FD provides information not captured by ASI and complements MWD and GWD. The evaluated indices, including FD, are effective in measuring soil aggregation and structural stability and should be considered further in management decisions to sustain soil resources and enhance economic returns. The methods developed here can be applied to any site where soils are sampled and analyzed for aggregates.