Impact of long-term fertilization practices on the soil aggregation and humic substances under double-cropped rice fields

Soil organic matter (SOM) content and soil aggregation are essential components of soil structure, which plays an important role in soil quality and fertility. Also, the SOM content, aggregation, and humus substances in paddy field were affected by application of fertilization practices. However, there is still limited information about the effects of long-term different fertilization practices on soil aggregation and carbon content in the humic acid (C-HAF), fulvic acid (C-FAF), and humin (C-HUM) fractions under double-cropping rice (Oryza sativa L.) system in Southern China paddy fields. Therefore, the effects of long-term fertilizer application on soil aggregation and C-HUM, C-HAF, and C-FAF contents in 0–5-, 5–10-, and 10–20-cm soil depth under double-cropped rice fields in Southern China were investigated. The experiment located at NingXiang County in Hunan Province, China begins in 1986 and the experiment includes five treatments: without fertilizer input (CK), mineral fertilizer alone (MF), rice straw residues and mineral fertilizer (RF), 30% organic matter and 70% mineral fertilizer (LOM), and 60% organic matter and 40% mineral fertilizer (HOM). The results showed that the soil total organic carbon content in paddy soils with RF, LOM, and HOM treatments was significant higher (P < 0.05) than that of the CK treatment at early and late rice maturity stages. The different sizes of soil aggregates with different fertilization treatments were decreased as HOM > LOM > RF > MF > CK. The HOM treatment had the highest percentage of soil aggregates in each size class and the CK treatment had the lowest percentage of soil aggregates in each size class in 0–5-, 5–10-, and 10–20-cm soil depth at early and late rice maturity stages. The soil C-HAF, C-FAF, and C-HUM contents were increased by long-term combined application of manure with mineral fertilizer practices. Meanwhile, the results indicated that the soil C-HAF, C-FAF, and C-HUM contents with RF, LOM, and HOM treatments were significantly higher (P < 0.05) than that of the CK treatment at early and late rice maturity stages. As a result, the soil total organic carbon content, each size class of soil aggregates, and soil C-HAF, C-FAF, and C-HUM contents were increased by long-term combined application of manure with mineral fertilizer in double-cropped rice fields.