Microbial CO₂ production from surface and subsurface soil as affected by temperature, moisture, and nitrogen fertilisation

The Sanjiang Plain of north-east China is presently the second largest freshwater marsh in China. The drainage and use of marshes for agricultural fields occurred in the past 50 years, resulting in the increase in cultivated land from about 2.9 x 10⁸ m² in 1893 to 4.57 x 10¹⁰ m² in 1994. Under human disturbance in the past half century, the environment in Sanjiang Plain has had significant change. We hypothesised that environmental factors such as soil moisture, soil temperature, and soil N levels affect the rates of soil organic C mineralisation and the nature of the controls on microbial CO₂ production to change with depth through the soil profile in the freshwater marsh in the Sanjiang Plain. In a series of experiments, we measured the influence of soil temperature, soil water content, and nitrogen additions on soil microbial CO₂ production rates. The results showed that Q₁₀ values (the factor by which the CO₂ production rate increases when the temperature is increased by 10°C) significantly increased with soil depth through the soil profile (P < 0.05). The average Q₁₀ values for the surface soils were 2.7 (0-0.2 m), significantly lower than that (average Q₁₀ values 3.3) for the subsurface samples (0.2-0.6 m) (P < 0.05), indicating that C mineralisation rates were more sensitive to temperature in subsurface soil horizons than in surface horizons. The maximum respiration rate was measured at 60% water hold capacity for each sample. The quadratic equation function adequately describes the relationship between soil respiration and soil water content, and the R² values were > 0.80. The sensitivity of microbial CO₂ production rate response to soil water content for surface soils (0-0.2 m) was slightly lower than for subsurface soils (0.2-0.6 m). The responses of actual soil respiration rates to nitrogen fertilisation were different for surface and subsurface soils. In the surface soils (0-0.2 m), the addition of N caused a slight decreased in respiration rates compared with the control, whereas, in the subsurface soils (0.2-0.6 m), the addition of N tended to increase microbial CO₂ production rates, and the addition of 10 μg N/g soil treatment caused twice the increase in C mineralisation rates of the control. Our results suggested that the responses of microbial CO₂ production to changes in soil moisture, soil temperature, and soil N levels varied with soil depth through the profile, and subsurface soil organic C was more sensitive to temperature increase and nitrogen inputs in the freshwater marsh of the Sanjiang Plain.