Effects Of Land Use Regimes On Soil Carbon In The Loess Plateau, Gansu, China

Land use mode is one of the biggest factors which affect carbon cycle in terrestrial ecosystems.Soil is the largest and the most active organic carbon pool in terrestrial ecosystems,the most important material basis in soil fertility as well.In this study,two types of rain-fed rotated farmland(cultivated and fallowed) which at least have 50 years traditional farming history and two types grassland system that have been returned from farmland to grassland for 20 years(fenced and grazing) were chosed as the research objects in the Loess Plateau in Gansu.Based on 2 years field investigation,laboratory test and data analysis,the characteristics of soil organic carbon(SOC),nitrogen(TN),soil microbial biomass carbon(SBMC),K₂SO₄--extracted carbon(K₂SO₄-C),soil aggregates organic carbon(SAOC),seasonal dynamics and influencing factors of water-soluble organic carbon(WSOC) and soil microbial biomass carbon were discussed in this paper.The main conclusions are as follows:1.Different land uses resulted in significant differences in SOC,TN,total phosphorous,available phosphorous,soil bulk density and soil pH.In surface soil(0-10 cm),SOC contents were significantly higher in grasslands than that in croplands,while TN show no significance.In the soil layers from 20 to 100cm,SOC and total nitrogen contents were significantly higher in croplands than that in grasslands(P<0.05).The soil total phosphorous was higher in croplands than that in grasslands but the difference was not significant.In the soil profile down to 100cm,the C/N ratios in croplands were much higher than that in grasslands,except in 0-10cm layer.The soil pH in grasslands was significant higher than that in croplands in each layer(P<0.05).The soil bulk densities in the grasslands were higher than that in croplands,except in 10-20 cm layer.The SOC and TN decreased with the increase of soil depth in grasslands,while this phenomenon was only found in the soil layers below 30cm in croplands;in the upper 30cm soil layers,the SOC increased with depth.Total phosphorous content did not show significant differences in different soil layers in grasslands.In croplands,the changes of total phosphorous showed the same trends with that of SOC and total nitrogen.The available phosphorous contents decreased with soil depth in both grasslands and croplands while the soil pH showed completely converse trend.Results showed that in both of grasslands and croplands,SOC,TN,total phosphorous and available phosphorous showed significant negative correlations with soil BD and soil pH.There were significant positive correlation between soil BD and soil pH.The significant positive correlations were also found among SOC,TN,total phosphorous and available phosphorous.2.Two years study showed that SMBC was mainly influenced by soil water content and plant biomass,not significantly correlated with temperature.In May and November, SMBC content in the 0-10cm layer was significantly higher in the fenced grassland than in other plots,while the lowest value was found in the fallow.The big difference in plant biomass was the main reason(P＜0.05).In July and September,when the soil temperature and soil water content were relatively higher,the sequence was millet＞fenced grassland＞grazing grassland＝fallow,and there was same trend among all plots: September＞July＞May≥(≤) November,there was significant difference among all months(P＜0.05).In 0-10 cm layer,the ranges of SMBC content of the four plots(fenced grassland,grazing grassland,millet cropland and fallow) in two years were 183.49-576.72mg/kg,121.13-366.76mg/kg,127.99-725.62mg/kg,44.05-354.58mg/kg, respectively.In each plot,SMBC content decreased with the depth in all seasons.3.In all plots,soil WSOC was mainly influenced by soil water content,and it did not correlate with soil temperature,while in millet cropland,it also tightly correlated with plant biomass.In September,when the soil water content was highest,WSOC was significantly higher than in other months in all plots(P＜0.01),and there was no significant difference among May,July and November.In 0-10cm soil layer,the ranges of WSOC content in fenced grassland,grazing grassland,millet cropland and fallow were 31.88-13.58mg/kg,9.28-22.22mg/kg,8.34-23.72mg/kg,13.00-26.0mg/kg, respectively.Along the soil profile in each plot,WSOC content was highest in 0-10cm layer,and decreased sharply in May and slightly in rainy season with soil depth. Compared with surface soil,WSOC content in 30-60 cm layer was only 0.8%lower in September(at this time of year,precipitation was much higher) in millet cropland. WSOC content in each layer of millet cropland was higher than that in other plots. WSOC leak resulting from precipitation may be one reason why SOC content in 10-100cm soil layer of cropland system was higher than in grassland.4.The change of land uses affected the content of soil aggregates significantly. Tillage activities could decrease small aggregates and increase large aggregates,resulting in the higher content of soil aggregates of＞0.5mm particle size in upper soil layer (0-20cm) in grasslands than that in croplands,while soil aggregates＜0.05mm in 0-20cm layer in croplands were higher than that in grasslands.The content of soil aggregates of different particle size except that bigger than 1mm were higher in croplands than in grasslands,while that of bigger than lmm were higher in grasslands(P＜0.05). Distribution of different particle size aggregates mainly concentrated in＜0.25mm micro-aggregates,ranging between 87.89%and 96.22%,while the large aggregates (＞0.25mm) content was less than only 13%.5.The distribution of SAOC along soil profile showed same pattern with that of SOC.In 0-10 cm layer,the content of soil organic carbon in soil aggregates of different particle sizes was higher in fenced grassland than in cropland;while in 10-40 cm layer, the content of soil aggregates of different particle size were higher in croplands than in grasslands except that bigger than lmm which was at the same level in the two plots, especially for that of＜0.25mm which account for 93.61%.The content of SAOC of＞0.25mm were significantly higher than that of＜0.25mm micro-aggregates in all plots (P＜0.05).The distribution of SAOC of different particle size along soil profile showed different pattern.The content of SAOC of＞0.5mm decreased with soil depth while that of＜0.5mm showed the same pattern with SOC.The content of SAOC of different particle sizes showed significant positive correlation with SOC in grasslands,but the same correlation was only found for the soil aggregates with particle size＜0.25mm in croplands.