| Soil is a huge pool of organic carbon, which occupies the global land total carbon pool 2/3-3/4 and is the atmospheric carbon library three times, the land of the biomass about 2.5 times. The soil carbon storehouse’s small change may cause the atmospheric CO2 density the remarkable change. Organic carbon is mainly affected by natural factors and human factors, a period of time in the future human activities will be the main factors affecting the soil carbon cycle. Human on soil tillage, fertilization, irrigation and other management measures will affect the changes in soil organic carbon pool, while farming is often considered as the main reason for decline of organic matter content in farmland soil.The impacts of different tillage systems on SOC and active organic carbon (included Readily Oxidized Carbon-ROC, Dissolved Organic Carbon-DOC, Microbial Biomass Carbon-MBC, Particulate Organic Carbon-POC and Light Fraction Organic Carbon-LFOC) were studied in this paper. The experiment was initiated in 1990 and included four tillage treatments:1) conventional tillage with rotation of rice and winter fallow system (DP), where regular tillage practices were used for rice with three times of plowing and harrowing annually, and after the rice harvest, the field was submerged with water; 2) conventional tillage with rotation of rice and rape system (SH), where tillage was the same as in the DP treatment, but the field was alternately submerged and drained for rice and rape cultivation; 3) no-till and ridge culture with rotation of rice and rape system (LM), where ridges (five in each plot) with the top of 25 cm width were intervened with the ditches of 30 cm width and 35 cm depth, with no tillage practices performed, rape cultivated on the top of the ridges with the water level being maintained just to the bottom of the ditch, and the field submerged in water to cultivate rice after rape being harvested; 4) tillage and ridge culture with rotation of rice and rape system (LF), where ridges were made as in LM treatment, but ridges were made every year for cultivating rice after rape harvest. At the same time the seasonal dynamics of SOC, ROC, MBC, DOC, LFOC and POC in a purple paddy soil were studied under the SH system. Results were showed as following: 1. The impacts of different tillage systems on SOC and active organic carbon.(1) The contents of SOC decreased with the increase of soil depth under LF systems, and LM treatment enriched SOC content near the soil surface. In the whole soil layer (0-60 cm), the differences of SOC content in the same soil layer among the four tillage systems were the largest in 0-10 cm soil layer and the lowest in 50-60 cm soil layer. In the whole soil layer (0-60 cm), the order of SOC contents was LM (17.57 g-kg-1)> DP (13.91 g-kg-1)> LF (12.50 g-kg-1)> SH (11.29 g-kg-1), SOC storage was LM (158.52 Mg C·hm-2)> DP (106.74 Mg C·hm-2)> LF (93.11 Mg C·hm-2)> SH (88.59 Mg C-hm-2), LM treatment significantly increased the content and storage of SOC.(2) The ROC content increased along with the depth of soil displayed decreasing progressively. Under the different tillage the active organic carbon’s average content is: DP (3.67 g-kg-1)> LF(3.49 g-kg-1)> LM (3.28 g-kg-1)> SH (2.69 g-kg-1), the percentage of ROC in SOC is:LF (25%)=DP (25%)> SH (22%)> LM (14%). Compares with other processing, the LM system reduced the proportion of ROC to SOC (p< 0.05), but did not significantly affect on the ROC content (p= 0.830).(3) The differences of MBC content in the same soil layer among the four tillage systems were the largest in 0-10 cm soil layer and the lowest in 50-60 cm soil layer. MBC contents under DP, SH and LF treatments decreased equably with the increase of soil depth, however no-tillage practices increased the SOC concentration in soil surface. In the whole soil layer (0-60 cm), the order of MBC contents was LM (259 mg-kg-1)> SH (213 mg-kg-1)> LF (160 mg-kg-1)> DP (144 mg-kg-1). Compared with the other three treatments, LM treatment significantly increased the content MBC.(4) The contents of DOC under four tillage systems decreased equably with the increase of soil depth. From 0 to 60 cm, DOC content ranked in the following order: LM (32.92 mg-kg-1)> DP (32.63 mg-kg-1)> SH (26.79 mg-kg-1)> LF (22.10 mg-kg-1). The single factor variance analysis showed that, four kinds of tillage soil had no significant influence on DOC content.(5) Physical fractionation of SOC is usually divided into the free light fraction (F-LF), occluded light fraction, (O-LF) and heavy fraction (HF).Results indicated that most carbon (69.6%-92.7%) was held in the HF, carbon dynamics in the whole soil were controlled by the behavior of this fraction. The proportion of F-LF carbon to the total SOC was 5.03%-26.43%. O-LF contained the least amount of organic carbon (1.37%-4.93%) of total SOC. In the four tillage soils the greatest contents of F-LFC, O-LFC and HFC were found in the 0-10cm depth. In the whole soil layer (0-60 cm), the order of F-LFC contents was LM (4.36g-kg-1)> DP (2.11g-kg-1)> LF (1.74 g-kg-1)> SH (1.46 g-kg-1), and the corresponding proportions of F-LFC to total SOC were 17.12%, 14.00%,12.17% and 11.29%, respectively. The order of O-LFC contents was LM (0.82 g-kg-1)> DP (0.51 g-kg-1)> LF (0.36 g-kg-1)> SH (0.34 g-kg-1), and the corresponding proportions were 3.36%,3.45%,3.00% and 2.71%, respectively. Compared with the other tillage systems, LM treatment significantly increased the contents and proportions of the two light fraction carbons(6) The mass fraction of particulate soil range from 30.38% to 45.65%, under the 0-60 cm soil layer, the basic change trends of all the values are increased with the soil depth decreases. In the whole soil layer (0-60 cm), the content of POC is:LM (48.56 g-kg-1)> LF (35.84 g-kg-1)> DP (32.61 g-kg-1)> SH (25.77 g-kg-1), Compared with the other three treatments, LM treatment significantly increased the content POC.(7) Stratification ratios of soil organic carbon (CSR) were 2.14 and 2.65 under LF and LM, meanwhile the values under DP and SH were only 1.54,1.92. Setting CSR>2 be an indication that soil quality might be improving. Conventional tillage DP and SH treatment both resulted in degraded conditions, however LM and LF improved soil quality.2. The seasonal dynamics of SOC and active organic carbon.(1) The contents were much higher in the early and later stages than in the middle stage of the rape growth. SOC, ROC, MBC and POC all achieved the highest values of 16.20 g-kg-1,3.58 g-kg-1,309.70 mg-kg-1 and 6.11 g-kg-1 at the end of growing period, respectively. DOC content presented as a single peak seasonal change and reached to the highest value of 37.64 mg-kg-1 at the middle stage of the rape growth. The content of LFOC was highest in the middle stage of the rape growth, achieved the highest values of 3.72 g-kg-1.The temporal dynamics of the allocation ratios of ROC, MBC, DOC, LFOC and POC were similar as their contents.(2) The influencing factors of SOC and ROC contents were the underground 5cm soil temperature, soil total nitrogen content and pH. MBC content was jointly impacted by the underground 5cm soil temperature, root biomass and its contents of C and N. DOC content was mainly affected by soil moisture. POC content was mainly affected by soil total nitrogen content and pH. The root biomass and its contents of C and N were the main factors affecting soil LFOC content. |
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