Assessing the Impacts of Fertilization Regimes on Soil Aggregate Dynamics in Northeast China

Determining the effects of fertilization regimes on soil aggregates, carbon (C) and nitrogen (N) distribution, and pH is essential for improving soil structure and soil organic carbon (SOC) accumulation to help in proper soil fertility management. Based on a 41-year field fertilization experiment conducted on dark brown soil in northeast China, we examined the soil aggregate size distribution and associated C, N, and pH to provide a scientific basis for elucidation of the mechanisms underlying the effects of fertilization treatments on soil structure and fertility. Six different fertilization treatments included no fertilizer (CK), low-dose chemical fertilizer (NP), moderate-dose chemical fertilizer (2NP), high-dose chemical fertilizer (4NP), normal-dose organic fertilizer (M), and normal-dose organic fertilizer plus moderate-dose chemical fertilizer (M+2NP). Our findings showed that compared to CK, M and M+2NP significantly increased the proportion of macroaggregates by 40% and 28%, respectively, whereas 4NP significantly decreased it by 19%. The mean weight diameter (MWD) and geometric mean diameter (GMD) under M and M+2NP were significantly higher than that under CK, at 12–21% and 24–36%, respectively. The fractal dimension (D) value of M+2NP was significantly lower than those of 2NP and 4NP by 4% and 5%, respectively. Soil pH under the M treatment was highest, followed by M+2NP. Soil pH under 2NP and 4NP more significantly decreased, by 0.1 and 0.2 units, than under M treatment. Soil pH values were correlated with the proportion of soil macroaggregates, MWD, and GWD, respectively (p < 0.05). Relative to CK, M and M+2NP increased the contents and stocks of SOC (by 40–49% and 89–93%, respectively) and total N (59–68% and 119–123%, respectively). Furthermore, the contents and stocks of aggregate-associated SOC and total N decreased following the order: NP > 2NP > 4NP. Overall, the long-term application of organic fertilization regimes (M and M+2NP) effectively improved soil aggregation as well as SOC accumulation and decreased soil acidification in dark brown soil in northeast China.