Long-Term Effects of Tillage on Soil Aggregates and the Distribution of Soil Organic Carbon, Total Nitrogen, and Other Nutrients in Aggregates on the Semi-Arid Loess Plateau, China

Cultivating pasture changes the turnover of aggregates and aggregate-binding agents (soil organic carbon [SOC], total nitrogen [TN]), and affects soil nutrient levels. We tested the hypothesis that soil aggregate fractions and soil nutrient concentrations would exhibit temporal variation using a 55-year chronosequence of cultivated sites with silty clay loam soils on China's Loess Plateau. As cultivation duration increased, large aggregates (>1 mm) broke into smaller aggregates; the mean weight diameter (MWD) and geometric mean diameter (GMD) declined monoexponentially, reaching equilibrium after 32.9 and 35.5 years, respectively, in the surface soil and after 35.5 and 38.4 years in the subsurface soil. SOC and TN decreased over time following monoexponential curves; equilibrium concentrations for both occurred after 28.8 years in the surface soil versus 25.6 and 10.3 years, respectively, in the subsurface soil. The surface soil lost 170 and 7.6 kg ha-1 of SOC and TN, respectively, versus 45.6 and 6.7 kg ha-1 from the subsurface soil after 55 years of cultivation. In contrast, available phosphorus (AP) and potassium (AK) contents increased by 0.22 and 2.47 kg ha-1, respectively, in the surface soil, versus 0.24 and 1.43 kg ha-1 in the subsurface soil. Aggregates >2 mm and 0.25-0.106 mm contained the most SOC, TN, and AP, whereas AK was stored in the 0.25-0.106 mm aggregates. As in the bulk soils, cultivation decreased levels of aggregate-protective compounds (SOC and TN), but increased P and K in the aggregates. Our results indicate that SOC and TN concentrations must be managed to protect the aggregate structure.