Effect of long-term fertilization on soil aggregate formation in greenhouse and farmland conditions in the North China Plain

The status of soil aggregates is an important soil physical property and the amount of soil aggregates is a critical index for measuring and evaluating soil fertility. Organic fertilizer application is an essential measure for improving soil organic carbon (SOC) content, promoting soil aggregate formation and improving soil structure. A long-term experiment was conducted in greenhouse soil and in farmland soil under three fertilizer treatments in Quzhou County in the North China Plain in order to investigate the effects of different modes of fertilization on the content, distribution and stability of soil water-stable aggregates using the wet-sieving method under different land use types. Results indicated that continuous organic matter application (OM) significantly decreased soil bulk density and significantly increased soil organic matter content, compared with chemical fertilizer application (NP) and mixed organic manure with chemical fertilizer treatment (NPM) in both greenhouse and farmland soils (P < 0.05). This effect was obviously for the 0-10 cm soil layer. The bulk density of the 0-10 cm soil layer under greenhouse conditions with organic fertilize application treatment was 1.17 g·cm^-3, which decreased respectively by 12.0% and 8.6% compared with those under chemical fertilizer and mixed organic manure with chemical fertilizer treatments. The content of organic matter in the 0-10 cm soil layer of greenhouse with organic fertilize application treatment was 54.81 g·kg^-1, which increased respectively by 104.8% and 35.7% compared with those under chemical fertilizer and mixed organic manure with chemical fertilizer treatments. Also the bulk density of soil in the 0-10 cm layer of farmland with organic fertilize application was 1.19 g·cm^-3, which decreased respectively by 8.5% and 7.0% compared with those under chemical fertilizer and mixed organic manure with chemical fertilizer treatments. The contents of farmland organic matter in the 0-10 cm soil layer with organic fertilize application was 22.67 g·kg^-1, which increased respectively by 23.1% and 15.0% compared with those under chemical fertilizer and mixed organic manure with chemical fertilizer treatments. The mean weight diameter (MWD) and geometric mean diameter (GMD) of water-stable aggregates of both greenhouse and farmland soils changed in the following order: OM > NPM > NP. Fractal dimension (D) of water-stable aggregates under OM treatment was lowest while that under NP treatment was highest. Treatments with OM significantly decreased D value in the 0-20 cm soil layer, which effect was most obvious for the 0-10 cm soil layer where soil structure improvement was also very obvious. Compared with farmland soil, changes of soil stability indexes and the effects of aggregate structure were more obvious for greenhouse soil. The most significant correlation was between soil organic matter content and the content of > 0.25 mm soil aggregates, which indicated that the more soil organic matter, the greater stability the soil structure. In conclusion, the application of organic matter not only increased the content of soil organic matter and available nutrients, but also promoted the formation of macro-aggregates and improved aggregate stability. It was an effective measure to improve the stability of farmland soil, which also was good for sustaining soil development, especially for soils under greenhouse conditions in the North China Plain.