Saturday, 15 July 2006

Short-Term Dynamics of Organic Matter and Microbial Biomass of Soils after Simulated Rainfall on Dry Cropland under Different Climate, Soil Texture, and Crop Residue Management.

Sugihara So1, Funakawa Shinya1, Kilasara Methods2, and Kosaki Takashi3. (1) Graduate School of Agriculture, Kyoto Univ, kitasirakawaoiwakecho sakyo-ku, Kyoto, Japan, (2) Dept of Soil Science, Sokoine Univ of Agriculture, P.O.Box3008, Morogoro, Tanzania, (3) Graduate School of Global Environmental Studies, Kyoto Univ, kitasirakawaoiwakecho sakyo-ku, Kyoto, Japan

To evaluate the short-term dynamics of soil organic matter (SOM) during rapid wetting/drying of soil, it is important to analyze the short-term dynamics of Microbial Biomass (MB) as well as CO2 emission rate because soil microbes would adapt to their respective environmental conditions and behave differently. We conducted the simulated rainfall experiments on croplands with different climate, soil texture, and crop residue management during dry season. The objectives of this study are (1) to evaluate the effects of rapid wetting/drying event on the dynamics of SOM and MB under field conditions, and (2) to comparatively analyze the behavior of soil microbes under different environmental conditions. Six experimental plots were installed as follows; (1) clayey soils under tropical savanna climate in Tanzania (MG) and Thailand (RP), (2) sandy soils under tropical savanna climate in Niger (SD) and Thailand (SJ), which had been received different amounts of crop residues in recent years, and (3) clayey soils under step climate in Kazakhstan (KZ) and Hungary (KC). Three treatments were imposed: (i) C plot, receiving no water, (ii) W plot, treated with 10 mm of rainfall water, and (iii) G plot, sprayed with glucose as a substrate together with the 10 mm of rainfall treatment. CO2 efflux rate and MB were measured at the field for 3 to 15 days. After the rainfall treatment, at all the W plots with clayey texture, a rapid CO2 flush occurred due to increased microbial activity, and it finished within 2 days. On the other hand, at one of the W plot with sandy texture (SJ), the CO2 flush lasted 4 days, and both the increases in MB and microbial activity contributed to the flush. At all the G plots, a large CO2 flush was observed due to the increase of MB. Such an increase of MB was observed more rapidly in the tropical soils than in the temperate soils. In addition, the amount of crop residues that had been added in recent years affected the microbial response, i.e. increase of MB. In conclusion, to simulate the SOM dynamics during wetting/drying cycles, we should consider the effect of the increase of MB on CO2 flush as well as the effect of microbial activity, especially when a large amount of carbon substrate is added to some tropical soils.

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