Dynamics of 14C-labelled Glucose and NH+4 in a Regularly Flooded Extremely Alkaline Saline Soil

Abstract

Flooding an extremely alkaline (pH 10.6) saline soil of the former Lake Texcoco to reduce salinity will affect the soil carbon (C) and nitrogen (N) dynamics. A laboratory incubation experiment was done to investigate how decreasing soil salt content affected dynamics of C and N in an extremely alkaline saline soil. Sieved soil with electrical conductivity (EC) of 59.2 dS m−1 was packed in columns, and then flooded with tap water, drained freely and conditioned aerobically at 50% water holding capacity for a month. This process of flooding-drainage-conditioning was repeated eight times. The original soil and the soil that had undergone one, two, four and eight flooding-drainage-conditioning cycles were amended with 1000 mg glucose-14C kg−1 soil and 200 mg NH+4-N kg−1 soil, and then incubated for 28 d. The CO2 emissions, soil microbial biomass, and soil ammonium (NH+4), nitrite (NO−2) and nitrate (NO−3) were monitored in the aerobic incubation of 28 d. The soil EC decreased from 59.2 to 1.0 dS m−1 after eight floodings, and soil pH decreased from 10.6 to 9.6. Of the added 14C-labelled glucose, only 8% was mineralized in the original soil, while 24% in the soil flooded eight times during the 28-d incubation. The priming effect was on average 278 mg C kg−1 soil after the 28-d incubation. Soil microbial biomass C (mean 66 mg C kg−1 soil) did not change with flooding times in the unamended soil, and increased 1.4 times in the glucose-NH+4-amended soil. Ammonium immobilization and NO−2 concentration in the aerobically incubated soil decreased with increasing flooding times, while NO−3 concentration increased. It was found that flooding the Texcoco soil decreased the EC sharply, increased mineralization of glucose, stimulated nitrification, and reduced immobilization of inorganic N, but did not affect soil microbial biomass C.