Status Of Soil Organic Matter And Nitrogen, Phosphorus, Potassium And Their Influence Factors In Tibet

According to the analysis data in the soil survey of Tibet including 16 soil classes, 168 soil profiles and 676 soil layers, this paper studied the status of soil organic matter (SOM) and nitrogen, phosphorus, potassium and their influence factors in Tibet. The results showed that:1. The change of content of SOM in alpine and subalpine soils in Tibet, which occupied 82.6% of the whole area, indicated the horizontal zone distribution of SOM. From southeast to northwest, the content of SOM rapidly decreased along with the climate changed from wet and semi-wet to semi-dry and dry, the vegetation changed from meadow to steppe and desert. The SOM content in surface horizon was: Dark felty soils and felty soils (96-130g kg-1)old calcic soils and frigid calcic soils(40-49g g-1)>Cold desert soils (6g g-1 ). At the same time, the SOM in southeast had clear vertical zone distribution. From base belt yellow earths to upward yellow-brown earths, brown earths, dark-brown earths and podzolic soils, the SOM content had increase trend. But crossing the forest line to Dark felty soils and felty soils, the SOM content decreased again down to frigid frozen soils. The SOM content of cinnamon soils and cold brown calcic soils, which were located in shrub meadow vegetation in dry valley, was lower than wet forest soil. But the SOM content of grey-cinnamon soils, Cold calcic soils and frigid calcic soils was increased again. In a word, the vertical zone distribution of SOM in Tibet was very complicated.2. Different soil had different soil active humus, which was extracted by 1mol L-1 NaOH. The extraction ratio, which meant the percentages that extracted C occupied the whole quantities of soil organic carbon, was: acid soils (soil of Dark felty soils, felty soils and wet forest soils)> alkaline soils which was mainly consisted of calcium soils. HA/FA showed the composition of soil humus which was mostly below 1.0, this meant humus was mainly composed of FA. HA/FA of soil humus in felty soils and frigid calcic soils was clearly lower than the black soils and chestnut soils in northeast China, which had similar vegetation with felty soils and frigid calcic soils. This showed that the high and cold condition in Tibet was unfit to the formation of HA. Simultaneously, the optical properties of HA:E4/E6 showed that the molecular weight and aromaticity of HA were lower than the black soils and chestnut soils.3. The content of soil total nitrogen and organic matter had line positive correlation(r=0.940**, n=685). so the spatial distribution trend of soil total nitrogen and organic matter was the same. Besides, soil available nitrogen and total nitrogen had highly positive correlation (coefficient of determination r20.9), this indicated that soil total nitrogen was a determinant of available nitrogen. Most available nitrogen hadvery significant positive correlation with active humus, but had disrelated with humin; this showed that available nitrogen mostly came from active humus in SOM. C/N of SOM was an important index to evaluate the humification of humus. This was an important characteristic of SOM accumulation in high, cold and wet district.4. The soil total phosphorus content in Tibet was higher in common. This had a relationship with the content of phosphorus in parent material. In addition, the content of soil total phosphorus in surface horizon was higher than in parent material horizon, this showed that soil total phosphorus existed a phenomenon that accumulating at surface. Soil total phosphorus had a significant positive correlation with SOM (r=0.431**, n=465), this indicated that the surface accumulation of soil total phosphorus was biology accumulation.Then, The soil total phosphorus with the accumulation of SOM and total phosphorus in parent materials had very significant positive correlation, and the determination coefficient was 0.701.The content of available phosphorus in surface horizon was higher than in parent material horizon. Most soil available phosphorus had high correlation with total phosphorus and SOM for...