Agrophysical Properties of Ordinary Slightly Eroded Chernozem in a Long-Term Experiment in Rostov Oblast

The research data on the fields (situated on the slope of the Bol’shoi Log gully, Aksai district, Rostov oblast, Russia) of the experiment registered in the Russian Geographic Network of long-term experiments with fertilizers (certificate no. 169) are described. The data on organic carbon content (Corg), exchangeable bases, and characteristics of structure and aggregate composition of old arable lands located on a slightly eroded slope of the southeastern aspect with a slope gradient of 3.5°–4.0° over a 35-year period are analyzed. The examined soil is an ordinary carbonate heavy loamy chernozem on loess-like loam, or Haplic Chernozem (Loamic). A long-term use of various farming technologies on the slightly eroded slope has changed the properties of the arable layer. Degradation processes in the form of soil loss have been observed in 74.2% of the analyzed years depending on the prevailing meteorological conditions. An extensive farming system in the crop rotations with different erosion tolerance is accompanied by a decrease in the Corg content from 3.80–3.83 to 3.58–3.69%. The use of landscape specific farming systems has reduced soil loss. Application of soil-protective crop rotations on an erosion-prone slope reduces soil erosion there by 35.0–52.5%. Organomineral fertilizers applied at medium doses (N46P24K30) stabilize the Corg content at a level of 3.78–3.88%. A 1.5-fold increase in the dose of fertilizers and the introduction of 40% of perennial grasses into the crop rotation raises the Corg content to 4.09–4.12%. A strong correlation (r = 0.90) between total exchangeable bases and Corg content is observed. These changes in the agrochemical properties influence soil structure. A decrease in the number of lumpy structural aggregates (>10 mm) caused by an increase in the share of aggregates of agronomically valuable size (7–2 mm) and silt fraction (<0.25 mm) is recorded. Statistical analysis shows two principal components (PCs) describing 79.1% of the total variance of the aggregates (PC1, 40.6% and PC2, 38.5%). The increase in mesoaggregates in PC1 with the decrease in the share of larger aggregates (>10 mm) suggests soil degradation processes. The fractions with the sizes of 5–7, 2–3, and 0.25–0.5 mm are the most important contributors to the variance described by the two first PCs.