| Soil aggregation is related to the cementation of various cementing agents which are affected by soil management regimes. This study investigated effects of different management regimes on aggregate cementing agents and the relationship between aggregate’s cementing agents and soil properties and mean weight diameter by using structural equation model to thereby understand the mechanism of the formation and distribution of aggregates on Lou soil based on a long-term experiment in Guanzhong Plain, Shaanxi province, China. The treatments included bare fallow(Fallow), cropland abandonment(Abandonment) and winter wheat rotated with summer maize(Cropping), and Cropping included nine nutrient management regimes, i.e. control(CK, no nutrient input), nitrogen only(N), nitrogen and potassium(NK), phosphorus and potassium(PK), NP, NPK and straw plus NPK(SNPK) and two levels of manure plus NPK(M1NPK and M2NPK), totally eleven treatments. The undisturbed soil samples was taken at 0-10,10-20 and 20-30 cm depths to measure organic cementing agents(e.g. fungal hyphal density, pentose, hexose, humic acid, fulvic acid and organic carbon), inorganic cementing agents(e.g. calcium carbonate, clay, free iron oxide and free aluminum oxide) and soil properties(p H, electric conductivity, available phosphorus and potassium). The main results were as follows:1) Soil management regimes had significantly effects on contents of organic cementing agents. Compared with Cropping, both Fallow and Abandonment generally decreased contents of fulvic acid and pentose, increased hyphal density(except Fallow) and hexose. There were no significant differences in the content of humic acid between treatments. Different management regimes had no significantly effects on inorganic cementing agents. Compared with Cropping, both Fallow and Set aside had no significantly effects on contents of free iron oxide and free aluminum oxide(except 10-20 cm depth).The contents of calcium carbonate and clay were affected differently in the different soil layers.2)Different fertilization treatments had significantly effects on organic cementing agents. Compared with CK, SNPK, M1 NPK and M2 NPK treatments significantly decreased hyphal density, but significantly increased contents of soil pentose, hexose, humic acid and fulvic acid on the whole. Long-term application of NP and NPK basically increased contents of soil pentose, humic acid and fulvic acid, decreased hyphal density, and had no effect content of hexose. Long-term application of N, NK and PK also increased content of pentose(except 20-30 cm), but had no significant effects on fungal hyphal density, hexose(except 10-20 cm), humic acid and fulvic acid(except 0-10 cm). Different fertilization treatments also had effects on inorganic cementing agents. Compared with CK, treatments of SNPK, M1 NPK and M2 NPK decreased content of free aluminum oxide(0-10 cm), basically had no significant effects on contents of clay and free iron oxide. The content of calcium carbonate was affected differently in the different soil layers. Long-term application of NP and NPK had no significant effects on free iron oxide and free aluminum oxide(except 10-20 cm). The contents of calcium carbonate and clay were also affected differently in the different soil layers. Long-term application of N, NK and PK increased contents of calcium carbonate(except 0-10 cm) and free iron oxide, had no significant effects on content of free aluminum oxide. The content of clay was affected differently in the different soil layers.3) The structural equation model established by organic cementing agents showed that aggregate mean weight diameter was significantly affected by organic carbon, fulvic acid and hyphal density at 0-10 cm soil layer. The fulvic acid had negative effect. While at 10-20 cm soil layer, aggregate mean weight diameter was significantly affected by the organic carbon, polysaccharide and hyphal density. The polysaccharide had negative effect. The structural equation model established by inorganic cementing agents showed that the aggregate mean weight diameter was only affected by electric conductivity significantly at 0-10 cm soil layer. The structural equation model established by combining organic cementing agent, inorganic cementing agent and soil properties showed that aggregate mean weight diameter was significantly affected by organic carbon, humic acid, fulvic acid, available phosphorus, available potassium, p H, electric conductivity and calcium carbonate at 0-10 cm soil layer. Fulvic acid, available phosphorus, p H and electric conductivity had the negative effects. At 10-20 cm soil layer, organic carbon, humic acid, polysaccharide, hyphal density, p H, calcium carbonate and clay had significant effects on the aggregate mean weight diameter. Polysaccharide and humic acid had the negative effects. Finally, the comprehensive model for 0-20 cm soil layer showed that the aggregate mean weight diameter was significantly affected by organic carbon content, hyphal density, polysaccharide content and calcium carbonate content.All in all, organic carbon, hyphal density, polysaccharide and calcium carbonate were the main factors of significantly impacting aggregate stability on the tested soil. While some other factors may also play an important role in aggregate stability, further study should be needed in future. |
