Mechanism Of Poorly Crystalline Minerals In Stabilizing Organic Carbon In Yellow-brown Soils

Four yellow-brown soils were graded into particle-size fractions, then an stable organic C and N of the different particles were gained by oxidation of 6% NaOCl. Soil organic C and N, CEC, SSA as well as dithionite-citrate-,oxalate- and pyrophosphate- extractable elements were tested. The relationships between the contents of Fe,Al oxides, CEC, SSA and the soil organic C and N were investigated. The major results obtained were summarized as follows:(1) Amounts of total organic C and N in yellow-brown soils were 1.5~27.75 g.kg-1 and 0.18~3.78 g.kg-1, Amounts of stable organic C and N in yellow-brown soils were 0.75~22.1 g.kg-1 and 0.05~2.51 g.kg-1. The contents of organic C and N in yellow-brown soils decreased with the thickening of the particles and the increasing depth., and most of the organic C and N in yellow-brown soils was distributed in microaggregates <2μm, next was <20μm. The stabilization index of the organic carbon and nitrogen (SIC and SIN) were 38%~90% and 19%~79%, and they decreased with the thickening of the particles.(2) 20~250μm and 2~20μm microaggregates were respectively advantage microaggregates of upland soil land and paddy field soil, they also were the fixed place of new organic carbon in yellow-brown soils. The organic carbon in <0.2μm microaggregates was comparatively stable, but it had little value to the accumulation of total organic carbon. Silt particles (2~20μm) and coarse clay particles (0.2~2μm) had a great contribution to the accumulation of total organic carbon, they also had good stablitily.(3) The CEC and SSA of yellow-brown soils decreased with the thickening of the particles. CEC was significantly related to SSA by a log-log linear. The CEC and SSA of yellow-brown soils were significantly and positively correlated with the contents of organic carbon and nitrogen, and the SSA of minerals are the physical basis of keeping SOC. Mechanism of minerals in stabilizing organic carbon maybe caused by surface adsorption of minerals, or caused by organic-mineral complex.(4) Amounts of Fed, Alo and Alp were generally higher in paddy yellow-brown soils than in arid yellow-brown soils, and the amounts of Feo And Fep were generally lower in paddy yellow-brown soils than in arid yellow-brown soils. Flooding conditions may be in favour of the activation and complexation of Fe.(5) The distribution rule of Fe,Al oxides was was basically consistent to the stable carbon and nitrogen in yellow-brown soils. The contents of various Fe,Al oxides were significantly and positively correlated with the stable carbon and nitrogen. Oxalate-extractable elements can much better preserve the stable carbon and nitrogen. The contents of Fe,Al oxides and the stable carbon and nitrogen in caly particles were hightest, this indicates that Fe,Al oxides in caly particles play an important role in preserving organic carbon and nitrogen.