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

Abstract 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. Keywords: available phosphorusavailable potassiumLoess Plateausoil aggregatessoil organic carbontillagetotal nitrogen Acknowledgement We thank all the farmers who allowed us to sample their fields, Dr. Qiong Li and Cheng Jin in Lanzhou University for measuring the soil properties, and Dr. Zhenkuan Gu in Lanzhou University for preparing the samples for aggregate separation and soil nutrient analysis. We would also like to thank Geoff Hart in Canadian Forest Service for his assistance with English language and grammatical editing of the manuscript. We also thank anonymous reviewers and Prof. Tóth for their valuable comments on the manuscript. Notes a pH was determined in a soil/water suspension of 1:10. b CaCO3 was analyzed by measuring the volume of CO2 released from the reaction between the soil and HCl. c CEC means the cation-exchange capacity determined by the NH4OAc method; Values of soil properties are means, with the standard deviation in parentheses (n = 3 or 5). a Values are means, with the standard deviation in parenthesis (n = 3); Values within a row followed by different uppercase letters are significantly different (p < 0.05); values within a column followed by different lowercase letters are significantly different (p < 0.05). a k means the rate constant. b X e means the equilibrium value. c t e means the equilibrium time. d R 2 is the regression goodness of fit, and all regressions were statistically significant (p < 0.05). e MWD and GMD are the abbreviations for mean weight diameter (mm) and geometric mean diameter (mm). f SOC and TN are the abbreviations for soil organic carbon (g kg−1) and total nitrogen (g kg−1). Values of results are means, with standard errors given in parentheses. a Values in the same row followed by different lowercase letters are significantly different for a given aggregate size class (p < 0.05). Values in the same column followed by different uppercase letters are significantly different for a given cultivation duration (p < 0.05). b SOC, soil organic carbon; TN, total nitrogen; AP, available phosphorus; AK, available potassium. a SOC, soil organic carbon; TN, total nitrogen; AP, available phosphorus; and AK, available potassium represent the concentrations in the bulk soil; C/N is the ratio of SOC to TN. MWD, mean weight diameter, mm; GMD, geometric mean diameter, mm; CEC, cation-exchange capacity. **correlation is significant at p < 0.01 (one-tailed); *correlation is significant at p < 0.05 (one-tailed).