Organic Matter Associated with Soil Aggregate Fractions of a Black Soil in Northeast China: Impacts of Land-Use Change and Long-Term Fertilization

Soil organic matter plays a pronounced role in soil aggregation, showing a wide variation depending upon soil-management practices. This study was conducted to characterize organic-matter changes in aggregate fractions in response to land-use change and long-term fertilization. Two experimental sites were established for this study: Site 1 included grassland (GL) and bare land (BL); site 2 comprised three treatments under cropland: no fertilizer application (NF), nitrogen and phosphorus fertilizer application (NP), and NP amended with organic manure (NPM). There was significantly increased carbon (C) sequestration (P < 0.001) in particulate organic matter (POM) observed under grassland and NPM relative to other plots, especially in large macroaggregates, attributable to lack of disturbance and organic input. The protected coarse and fine POM-C together accounted for 15% on average of soil organic carbon, ranging from 10.1 to 18.6% for all plots. The enhanced correlation occurred between protected POM-C fractions and soil C stocks and soil aggregation (calculated as mean weight diameter, MWD) (P = 0.000) relative to other C fractions, indicating that the increases in POM fractions resulting from long-term vegetation restoration and organic amendment enhance soil aggregation and C sequestration in this black soil. In contrast, the significantly negative relationship between MWD and fine-to-coarse POM-C ratio implied that this ratio might account for the decreased soil aggregation. Principal component analysis (PCA) showed that three PCs accounted for 42.6, 25.7, and 11.8%, respectively, and together more than 80% of the total variance. The protected POMs with significantly greater positive PC1 loadings (>0.8), particularly for large macroaggregates, were highly sensitive to changes induced by land use and fertilizer-management practices, leading to the wide variations in soil properties. The interrelation of organic matter with soil aggregation helps us to better understand the mechanisms of C protection and restoration in this black soil in the context of soil degradation and climate change.