Effects of long-term fertilization regimes on changes of aggregate cementing agent of Lou Soil

【Objectives】 Soil aggregate is a basic unit of soil structure, its formation and stability is mainly related to soil cementation of various cementing agent which is affected by soil management regimes. Fertilization practice, as one of soil management regimes has an important effect on aggregate cementing agent due to different input levels and subsequent effects on crop production. This study investigated effects of different fertilization regimes on aggregate cementing agent and was to thereby understand mechanism of formation and distribution of aggregates in Lou soil based on a long-term experiment under winter wheat and summer maize cropping system in Guanzhong Plain, Shaanxi province, China.【Methods】 Different fertilization regimes included 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 nine treatments. The determination of aggregate cementing agents included fungal hyphal density, pentose, hexose, calcium carbonate, clay, free iron oxide and free aluminum oxide. 【Results】 The results show that different fertilization treatments have significantly effects on the aggregate cementing agents in the surface and subsurface soil layers (0-10 and 10-20 cm). In the 0-10 cm soil layer, compared with CK, the SNPK, M1NPK and M2NPK treatments significantly decrease the hyphal density and the content of calcium carbonate, and significantly increase the contents of the soil pentose and hexose. There are no significant differences in the contents of clay (except M2NPK), free iron oxide and free aluminum oxide. The long-term application of chemical nitrogen together with phosphorus (NP and NPK) also significantly increases the soil pentose content, but significantly decreases the content of calcium carbonate. It also increases the content of clay,but decreases the soil hyphal density. There are no significant differences in the content of hexose, free iron oxide and free aluminum oxide. The long-term application fertilization without nitrogen or phosphorus (N, NK, PK) significantly increases the content of pentose, increases the contents of clay and free iron oxide, and have no significant effects on the fungal hyphal density (except PK), the contents of hexose, calcium carbonate and free aluminum oxide relative to CK treatment. In the 10-20 cm soil layer, compared with CK, the SNPK, M1NPK and M2NPK treatments increase the content of pentose, significantly increase the content of hexose, but significantly decrease the hyphal density and the content of free aluminum oxide, and have no effects on the contents of calcium carbonate, clay and free iron oxide. The hyphal density and free aluminum oxide are lower, but the contents of pentose under NP and NPK treatments are significantly higher than those under the CK treatment. However, there are no significant differences in the contents of hexose, calcium carbonate, clay and free iron oxide. Fertilization without nitrogen or phosphorus (N, NK and PK) increases the contents of soil pentose, hexose, calcium carbonate and free iron oxide, but significantly decreases the content of free aluminum oxide, and have no effects on the fungal hyphal density and clay content. The linear correlation analysis between different aggregate cementing agents and mean weight diameter of aggregates(MDW) shows that MWD of dry sieving is significantly and positively correlated with the calcium carbonate content (P0.01), significantly and positively correlated with the free iron oxide content (P0.05), but negatively correlated with the pentose and hexose contents (P0.05). The MWD of wet sieving is significantly and positively correlated with the hyphal density and calcium carbonate (P0.01), significantly and positively correlated with the free iron oxide contents (P0.05), but negatively correlated with the pentose content (P0.05). 【Conclusions】 Further stepwise multiple regression shows that the calcium carbonate is the unique factor 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.