| As the largest carbon (C) pools in terrestrial ecosystems, soil organic carbon (SOC) pools play important roles in the balance of global carbon pool, the development of soil fertility, the diversity of soil microorganisms and the sustainment of ecological system. Atmosphere CO2 is photosynthetically fixed by plant and then transformed into the soil in the form of rhizodeposition and plant residues. The distribution and immobilization of photosynthetically fixed C in plant- soil- microbial system control the cycling, turnover and dynamics of SOC pools. Plastic film mulching (mulching) is used to increase the crop yield because the mulching can increase the soil temperature and conserve soil misture. However, the land is intensively cultivated and is less fertilized for a long time (more than 50 years) which leads to the sharp decrease of SOC and soil feryility. The application of organic manure and the amendment of straw to soil increase the content of SOC and soil fertility. It is not clear that the dynamics of distribution and microbial immobilization of photosynthetically fixed C in plant-soil system after application of straw or organic manure combined with mulching. The in situ 13CO2 pulse labeling was conducted in the long-term Brown Earth Experiment Station in Shenyang Agricultural University during the maize growing stages (seedling, shooting, tasseling stage). The objects of this study were to trace the fate of 13C in plant- soil- microbial system, quantify the distribution of 13C in this system, and estimate the effect of mulching and fertilizer on belowground allocation and microbial immobilisation of photosynthetically fixed C. An in situ incubation was set up to trace the distribution of straw C to SOC pools, quantify its relative contribution to SOC pools, and discuss the microbial immobilization and distribution of straw C in the different fertility soils added with 13C labeled maize straw. All the results showed that:(1) The in situ labeling technique enhanced the efficiency of photosynthetically fixed C. The 13C recovery was 50% ～ 80% on the 1st day after labeling during the seedling stage,36% ～70% on the 1st day after labeling during the shooting stage, and 39%～56% during the maturing stage. The photosynthetically fixed C was very rapidly transferred belowground. The average 6.0% and 11% of photosynthetically fixed C was incorporated into roots, average 5.2% and 8.9% remained in rhizosphere soil, and average 0.65% and 2.6% allocated to bulk soil, respectively, on the 1st day during the maize seedling and shooting stage.(2) The distribution proportion of net fixed 13C belowground was increased from 12% on the 1st day to 15% on the 15th day after the labeling during the seedling stage. The proportion of net fixed C allocated to roots, rhizosphere soil and bulk soil was 7.9%,3.9%, and 2.9%, respectively. More than 60% of 13C retained in soil was incorporated into microbial biomass C (MBC) on the 1st day, but only less than 27% on the 15th day. Organic manure application with mulching showed the greatest proportion of photosynthetically fixed 13C incorporated belowground (including root, rhizosphere soil and bulk soil) and the largest amount of MBC (28 μg kg-1 soil and 22 μg kg-1 soil, respectively, in rhizosphere soil and bulk soil) derived from rhizodeposits.(3) After the labeling during the shooting stage,23% of net fixed 13C was incorporated belowground on the 1st day and 27% on the 6th day. On average,14%,10% and 3.3% of net fixed 13C was distributed to roots, rhizosphere soil and bulk soil, respectively, on the 6th day. Organic manure with or without mulching incorporated 76% of 13 C remained in SOC into MBC on the 1st day and 43% on the 6th day. However, the proportion of 13 C of MBC in 13C of SOC was increased from 24% on the 1st day to 35% (especially 50% in the bulk soil) on the 6th day in the no fertilizer treatment. Mulching and organic manure increased the amount of microbial immobilization of rhizodeposits, wherease, no fertilizer with or without mulching increased the incorporation of photosynthetically fixed 13C belowground (about more than one third 13C belowground).(4) During the maturing stage, average 4% of net fixed 13C was transferred to seeds and 20% was input belowground. On average,10% and 30% of 13C in SOC was input to MBC, respectively, in rhizosphere soil and bulk soil. The largest proporation (34%) of net fixed 13C was incorporated belowground, including 7% to rhizosphere soil and 18% to bulk soil. The amount of 13C in MBC was lower 19% and 62% in organic manure treatment than in no fertilizer treatment under the same mulching. The amount of 13C in MBC was 126 μg kg-1 soil and 225 μg kg-1 soil, respectively, in rhizosphere soil and bulk soil in the organic manure and mulching treatment.(5) The proportion of straw C distributed to soil was average 82%, 68%, and 86%, respectively, in high, medium, and low fertility level soils at day 30 and average 28% in the three fertility soils at day 365. Only 2.4%～5.0% of straw C retained in MBC and less than 0.5% was present in dissolved organic C (DOC).(6) The amount of MBC was decreased with the decreasing of soil fertility level after the soils added with 13C labeld maize straw. At the high and medium fertility levels, MBC contained, on average,58% of straw C and 42% of native SOC, respectively, whereas more than 75% of MBC was derived from straw C at the low fertility level.These results indicate that the distribution and immobilization of phosynthetically fixed C were controlled by the crop growing stages, soil nutrients, soil fertility levels, and cultivation measures such as mulching, organic manure application, amendment of straw. Mulching and orgniac manure application increased the microbial activity and the proportion of active organic C pools, whereas, mulching and no fertilizer application increased the net fixed 13C incorporated belowground. In the low fertility soils, the addition of crop straw significantly promoted the activity and growth of soil microorganisms and provided a potential positive feedback to soil fertility. It needs to further study the dynamics of belowground incorporation and microbial immobilization of photosynthetically fixed C using the stable isotope probing technique. |
