Effect Of Land Use Types On The Fixation Of Soil Organic Carbon

Soil is the core of the land ecosystem, so land use change controls the content and transformation of carbon in the land ecosystem by changing the soil organic matter input, improving the soil condition and changing the decomposition speed of soil organic carbon. So that understanding the fixation mechanism of soil organic carbon and its influencing factor in different land use is important for enhancing carbon fixation and slowing down climate change. However, there were little research on the stability and active component of soil organic carbon in deeper soil, especially on the fixation of organic carbon in paddy soils of chongqing hills under different land use. Therefore, selecting Chongqing Rongchang Wujia Daba's third and fifth society as study region and adopting the method of field sampling and indoor analyzing, the influence of land use change on the soil organic carbon fixation, soil organic carbon stability and active component were studied for providing theory on understanding the fixation mechanism of soil organic carbon in paddy soil and using land reasonably. The main result was as follows:1. The effect of land use on soil physical and chemical characteristics.The total N and alkali-hydrolyzable N in cropland, woodland, dryland and wasteland tended to decrease in deeper soils, however, in the woodland, total N and alkali-hydrolyzable N were more significantly affected by soil depth than in other land use types. At the same soil level, total N and alkali-hydrolyzable N in wasteland and dryland were significantly lower than in cropland and woodland, whose total N and alkali-hydrolyzable N in the top 10cm were higher in woodland than cropland, but significantly higher in cropland than woodland below 10cm. Although total P and available P in all land use tended to decrease with soil depth, but the decrease mainly happened below 20cm. However, Total P and available P were significantly different in the four land use types, displaying total P founded as the trend: dryland cropland＞wasteland＞woodland and available P founded as the trend: cropland＞dryland＞wasteland＞woodland. Total K was less affected by soil depth, but available K decreased with soil depth. At the same soil depth levels, total K displayed the trend: cropland＞dryland＞wasteland＞woodland and available K displayed the trend cropland＞dryland＞wasteland＞woodland. Bulk density increased with soil depth, and showed the trend: woodland＞dryland＞wasteland＞cropland, however, on the contrary, porosity decreased with soil depth and displayed the trend: cropland＞wasteland＞dryland＞woodland.2. Effect of land use on soil organic carbon in soilsBeside which in cropland showed the maximum in 10～20cm, soil organic carbon in all land use types decreased with soil depth. However, soil organic carbon in woodland and wasteland were significantly different at P-values of 0.05 between different soil depth levels, but the difference of which in cropland and dryland were mainly displayed between top 20cm and 20～40cm. Soil organic carbon in cropland and woodland were significantly higher than in wasteland and dryland, but which in woodland was only higher in the top 10cm and lower below 10cm than in cropland. Soil organic carbon storage displayed the trend: woodland＞cropland＞wasteland＞dryland in the top 10cm, but when the soil depth increased, which in woodland decreased gradually and increased in cropland to the maximum below 10cm. In a word, soil organic carbon storage above 5cm, 10cm and 20cm showed the trend: woodland＞cropland＞wasteland＞dryland, but which in cropland displayed the maximum in the entire profile. Total N, total P were positively correlated with soil organic carbon and therefore areas of higher soil organic carbon contained elevated total N and P levels. However, the increasing rate of total N with soil organic carbon displayed the trend: cropland＞wasteland＞dryland＞woodland, and the rate of total P increased displayed the trend: dryland＞wasteland＞cropland＞woodland. Bulk density was significantly negatively correlated with soil organic carbon, but porosity displayed the contrary relationship. C: N rate declined with soil depth especially below 20cm within all land use types, and displayed the trend: woodland＞cropland＞wasteland＞dryland. C: P rate declined with soil depth, but which in woodland and wasteland were more significantly affected by soil depth than the other land use types. However, in the entire profile, C: P rate in woodland was the highest, followed by cropland, which in dryland and wasteland were significant lower than in woodland and cropland.3. Effect of land use on soil organic carbon in soil aggregatesThe MWD in the four land use types were greater than 250um, but difference of which between land use types were significantly. MWD in woodland is greater than 2000urn, which in cropland and wasteland varied from 1000 to 1500urn and in dryland showed the minimum varying from 500 to 1000um, meanwhile, MWD declined with soil depth. The soil of four land use are mainly existed as macro-aggregate(＞0.25mm),but woodland soil was mainly existed as＞2mm aggregate whose proportion in soils was significantly higher than other land use types which mainly existed as 0.25-2mm aggregate. The proportions of＜0.25mm aggregate in cropland and woodland were lower than in dryland and wasteland, and which in dryland displayed the maximum. Soil organic carbon of macro-aggregate (＞0.25mm) in cropland, woodland and wasteland were higher than in micro-aggregate (0.053～0.25mm), however soil organic carbon in＞2mm aggregate was lower than in other three aggregate in dryland. Soil organic carbon of aggregate in different land use were significantly different. Beside soil organic carbon of＜0.053mm aggregate in woodland displayed the maximum, 0.053～0.25, 0.25～2 and＞2mm aggregate had the highest soil organic carbon in cropland and the lowest in dryland and wasteland whose aggregate had no significant difference in soil organic carbon. Macro-aggregate(＞0.25mm) maded up mostly soil organic carbon in cropland and woodland, and soil organic carbon in dryland mainly derived from 0.053～2mm aggregate, but the contribution of different size aggregate had no significant difference in wasteland. The contribution of＞2mm aggregate for soil organic carbon decreased with soil depth and which of＜0.25mm aggregate was less affected by soil depth. However, the contribution of 0.25～2mm in cropland and woodland increasedwith soil depth, but which in dryland and wasteland displayed the contrary result. Soil organic carbon was significantly positively correlated with MWD and＞2mm aggregate proportion, but negatively correlated with proportion of＜0.053, 0.053～0.25, 0.25-2mm aggregate.4. Effect of land use on active soil organic carbon in soilsSoil organic carbon oxidized with 333 mM KMnO4 and its fraction declined with soil depth, but fractionâ… (soil organic carbon oxidized with 33mM KMnO₄) and fractionâ…¡(soil organic carbon oxidized with 33～167mM KMnO₄) were significantly affected by soil depth. Calculating data indicated that top soil was in favor of accumulating fractionâ… and fractionâ…¡and bottom soils was in favor of accumulating fractionâ…¢(oxidized with 167～333mM KMnO₄), however soil organic carbon oxidized with 333mM KMnO4 was mainly existed with fractionâ… and fractionâ…¢in the entire profile. Soil organic carbon oxidized with 333mM KMnO₄ and its fraction of entire profile in woodland displayed the maximum, followed by cropland, which in wasteland and dryland were the lowest. However the dominance of Soil organic carbon oxidized with 333mM KMnO₄ and its fraction in woodland were mainly displayed in 0～20 and 0～5cm respectively, which in cropland displayed the maximum with increasing soil depth. Particulate organic carbon rapidly declined with soil depth and showed the significantly difference between different soil depth levels, however, particulate organic carbon in woodland was more significantly affected by soil depth than in dryland and cropland. Woodland showed the highest particulate organic carbon in the top 10cm, while cropland showed the highest in 10～40cm, but particulate organic carbon in dryland and wasteland were lower than in cropland and woodland. The ratio of soil organic carbon oxidized with 333mM KMnO₄ to soil organic carbon in woodland displayed the maximum to 27% which is 20%, 29% higher than in cropland and wasteland respectively. However the ratio of particulate organic carbon to soil organic carbon showed the order: woodland＞cropland＞wasteland＞dryland, but the difference mainly happened in the top 5cm. Particulate organic carbon, soil organic carbon oxidized with 333mM KMnO₄ and its fraction were all have positive correlativity with soil organic carbon, proportion of＞2mm aggregate and soil organic carbon in aggregate, but had negative eorrelativity with proportion of＜2ram aggregate. Meanwhile, there were significantly positive correlativity between particulate organic carbon, soil organic carbon oxidized with 333mM KMnO₄ and fractionâ… , fractionâ…¡. However increasing rate of particulate organic carbon with soil organic carbon was higher than soil organic carbon oxidized with 333mM KMnO₄ whose fractionâ… increased more rapid than other fractions.