关键词: 长期施肥, 红壤性水稻土, 团聚体, 全氮储量, 亚热带

Abstract:

Fertilization greatly influences soil structure and nutrients accumulation in soil aggregates. In this study, the effects of long-term fertilization on aggregates composition and total nitrogen stock in a reddish paddy soil in the subtropical area of China were investigated. The fertilization treatments included CK (without fertilization), NK (nitrogen and potassium fertilizer application), NPK (balanced application of nitrogen, phosphorous, and potassium fertilizers), LOM (combined application of NPK and 30% organic manure fertilizers) and HOM (combined application of NPK and 60% organic manure fertilizers). Undisturbed soil samples in two layers were collected and then separated into five aggregate-size classes (>5 mm, 2~5 mm, 0.5~2 mm, 0.25~0.5 mm, <0.25 mm) by the dry sieving method. The amount and storage of nitrogen in each aggregate fraction were measured. The results indicate that the >5 mm blocky fraction was the dominant aggregates component, accounting for 65.7%~83.4% of dry soil mass in the two layers. It sequestrated 63.1%~82.7% of the total nitrogen in the soils and is considered the major carrier of soil nitrogen. Deeper in the soil, the content of >5 mm blocky fraction increased, while the percentages of the other aggregate-size groups decreased. For each soil layer, the contents of soil total nitrogen in different sizes of soil aggregates were significantly different and increased in the order of >5 mm, 2~5 mm, 0.5~2 mm, 0.25~0.5 mm, and decreased from 0.25~0.5 mm to <0.25 mm. However, the total nitrogen storage value first decreased and then increased and the minimum value occurred in the size group of 0.25~0.5 mm. Compared to the CK treatment, long-term fertilization was in favor of increasing the proportion of aggregates in the size group of 0.25~5 mm, while reducing the aggregate fractal dimensions. Moreover, the total soil nitrogen contents and storages in 0.25~5 mm aggregates and in the whole soils were increased significantly (P<0.05) after fertilizer application and the effects show an increasing sequence of NK<NPK<LOM<HOM. There was a significant positive correlation between the contents of 2~5 mm aggregates and total nitrogen storages in the soils, suggesting that the newly enriched organic nitrogen mostly appeared in 2~5 mm aggregates. Overall, in comparison with the plow pan, the amounts and total nitrogen contents of different aggregates in the plough layer were more sensitive to fertilization. The NPK fertilizers mixed with organic manure application greatly (P<0.05) promoted the total nitrogen storages in the plow pan and the whole soil layers when compared to single chemical fertilizer application and can be considered the best fertilization mode for improving soil structure and nitrogen fertility and that can be widely applied in the reddish paddy soil distrribution areas in subtropical China.

Key words: long-term fertilization, reddish paddy soil, aggregates, total soil nitrogen storage, subtropics