The Effect Of Land Use Change On The Soil Organic Carbon And The Soil Aggregate Light Fraction Carbon

?The mountains are hight and steep in the red soil region of sub-tropic in China. The rainfall is great and concentrates. But with the growing population and excessive pursuit of economic interests, the human activity in the region had greatly changed the mode of land use. In this study, we collected 0～10cm and 10～20cm layers of the soil which was different utilization patterns including: the Schima superba and Cunninghamia lanceolata piantation forest, Enforcement of the forest, Cirtrus aurantium orchards, Tea Garden and the steep farmland in Jian'ou City of Fujian Province. With the method of dry and wet sieving, we got >2mm, 2～0．5mm, 0．5～0．25mm, <0．25mm four different sizes of the soil aggregates, the quality aggregates percentage, the contents of the organic carbon, total nitrogen, and the soil aggregate light fraction carbon were analyzed; the soil samples of total organic carbon, total nitrogen, total phosphorus, and total potassium were also analyzed. The stability of the soil aggregates, the distribution and the contents of the organic carbon, total nitrogen and the light fraction carbon in soil aggregates were discussed.The major results obtained were summarized as follows:1．Land use change affected the soil organic carbon accumulation. The contents of the soil organic carbon were higher in the woodland than the Cirtrus aurantium orchards and the steep farmland ,the Tea Gardenwas minimum.The ratio of C/N can indicated the turnover of the soil organic carbon, the ratio of C/N under different utilization patterns: the Schima superba (9．07) > Enforcement of the forest (8．53) >Cunninghamia lanceolata piantation forest (8．35) >Cirtrus aurantium orchards (7．93) >the steep farmland (6．28) >Tea Garden (6．22) .There was a significant linear correlation between the contents of the soil organic carbon and the soil nitrogen (R~2=0．9666．p<0．01) .2．Land use change affected the composition and quantity of the soil aggregates with the method of dry sieving. The contents of >0．25mm soil aggregates: Cunninghamia lanceolata piantation forest > the Schima superba > Enforcement of the forest > Tea Garden > Cirtrus aurantium orchards>the steep farmland. The amount of >2mm soil aggregates was up to 50．56%～68．63%, more than one half under the woodland; the amount of >2mm aggregates under Cirtrus aurantium orchards, Tea Garden, the steep farmland was 14．02%～25．96%. The woodland was conducive to the formation of the large-size particle aggregates; and Cirtrus aurantium orchards, Tea Garden citrus orchards, the steep farmland with the smaller particle sizes. 3．Land use change affected the quanlity of the soil aggregates. The contents of >2mm water-stable soil aggregates and the weighted mean dameter: the Schima superba>Enforcement of the forest >Cunninghamia lanceolata piantation forest>Cirtrus aurantium orchards > Tea Garden > the steep farmland. There was a significant linear correlation between the weighted mean dameter and the amount of >0．25mm water stable soil aggregates (R~2=0．9245, p<0．01) .4．There was a significant correlation between the contents of the soil aggregate organic carbon and total organic carbon. With comparison, we could see through the woodlands help protect against loss of the soil organic carbon, the soil carbon was fixed. With the soil aggregate size decreasing, the content of the aggregates nitrogen showed an upward trend; There was a significant linear correlation between the contents of the soil aggregates organic carbon and the total nitrogen (R~2=0．9105,p<0．01) .5．The enrichment coefficient can indicated the accumulation or consumption of the soil organic carbon. The radio of the different size soil aggregate organic carbon/the soil of total organic carbon could calculate it. the Esoc>1 was clearly in > 0．25mm soil aggregates under the woodland,it showed that the soil organic carbon in the soil aggregats tended to accumulate. Almost all Esoc<1 was clear in > 0．25mm soil aggregates under the steep farmland and Tea Garden, it showed that the soil organic carbon in soil aggregates tended to consume. The radio of C/N in <0．25mm soil aggregates was the smallest, > 2mm soil aggregates was maximum. The radio of C/N was higher under the woodland; it was beneficial to the soil carbon storage.6．The soil light fraction had declined markedly with the soil depth increasing under the differentt utilization patterns. There was a negatively linear correlation between the contents of the soil light fraction and the soil bulk density (R~2 = 0．2333, p <0．05) , and the contents of the soil organic significant positive correlation (R~2 = 0．8735, p <0．05) , the contents of the soil nitrogen with a significant positive correlation (R~2 = 0．8113, p <0．05) .7．Land use change was a major factor affecting the composition and distribution of the light fraction in the soil aggregates. The contents of the soil aggregate light fraction carbon were the largest under the Schima superba piantation, the steep farmland were the smallest. The soil aggregates under the woodland intimated the soil light fraction to formate. The radio of C/N in the soil aggregate light fraction was higher than the soil aggregates, about 2～5 times. The radio of C/N was maximum in <0．25mm soil aggregate light fraction. In addition to Tea Garden, The radio of C/N was maximum in the 2～0．5mm soil aggregates in the soil light fraction.