Distribution of organic carbon and humic acids in aggregates of Mollisol as affected by amendments with different rates of organic manure plus mineral fertilizer

Objectives Aggregates provide important physical protection for soil organic carbon. Organic amendments strongly influence aggregate formation and its organic carbon stability, quantifying the relationships between organic manure addition rate and stability of organic carbon in aggregates could provide valuable information for better understanding in soil organic carbon sequestration mechanism.

Methods This experiment was carried out based on a 10-year field experiment in National Observation Station of Hailun Agroecology System. Soils were selected from four treatments amended with different rate of organic manure 0, 7.5, 15, 22.5 t/ (hm²·a) plus mineral fertilizer. Aggregate fractionation and humic acids extraction was used to analyze the distribution of soil organic carbon and content and optical characteristics of humic acid in different size of aggregate fractions.

Results 1) Compared with application of mineral fertilizer alone, addition of mineral fertilizers plus organic manure increased the proportion of macroaggregates ( > 0.25 mm) and mean weight diameter which was gradually increased with the addition rate of organic manure. Significant linear correlation between the value of mean weight diameter and organic manure addition rate was observed (P=0.03). 2) Soil organic carbon was mainly distributed in 2-0.25 mm aggregates, accounting for about 64.8%-68.8%. Organic carbon content and stock in macroaggregates increased with the increasing organic manure rate. However, no significant differences were showed both in organic carbon content and stock in small microaggregates among treatments. 3) Humic carbon content in microaggregate (0.25-0.053 mm) was higher than other aggregates, but no significant difference in the percentage of humic carbon was found among different aggregate fractions. The content of humic carbon in each aggregate fraction increased significantly after application of organic manure plus mineral fertilizer, thus its capacity in organic C retention increased. The humic carbon content gradually increased with the increasing addition rate of organic manure. 4) Application of organic manure plus mineral fertilizer increased the E4/E6 ratio of humic acid in each aggregate fraction, particularly in the aggregate of > 2 mm and 0.25-0.053 mm, which indicated that the structure of humic acid tended to become simpler.

Conclusions In black soil, continuous application of organic manure plus mineral fertilizers is effective to improve surface soil structure through promoting the formation of aggregates, increase carbon sequestration in aggregates, and simplify the structure of humic acid in aggregates, these beneficial effects are greater in soil amended with high rate of organic manure. Therefore, the addition rate of organic manure could be properly increased in order to improve soil fertility and carbon sequestration.