Rapid immobilization of applied nitrogen in saline–alkaline soils

The dynamics of inorganic N are important in soil, and this applies particularly to the saline–alkaline soils of the former lake Texcoco in Mexico with high pH and salinity where a forestation program was started in the 1970s. In soils of lake Texcoco, in Mexico, more than 50% of applied N could not be accounted for one day after application of 200 mg kg−1 soil along with glucose amendment. It was not clear whether this was due to abiotic or biotic processes, the form of inorganic N applied or the result of applying an easily decomposable substrate. We investigated this by adding glucose and 200 mg kg−1 soil as (NH4)2SO4-N or KNO3-N to sterilized and unsterilized soil. The changes in inorganic and ninhydrin N, microbial biomass C and production of CO2 were then monitored. Between the time of applying N and extraction with 0.5 M K2SO4, i.e., after ca 2 h, approximately 110 mg NH4+-N kg−1 dry soil could not be accounted for in the unsterilized and sterilized soil and that remained so for the entire incubation in the sterilized soil. After 1 day this increased to 140 mg NH4+-N kg−1 dry soil in the unsterilized control and 170 mg NH4+-N kg−1 dry soil in C amended soil. Volatilization of NH3 accounted for 56 mg NH4+-N kg−1 so the rest appeared to be adsorbed on the soil matrix. The NH3 volatilization and NH4+ fixed in the soil matrix remained constant over time and no oxidation to NO2− or NO3 − had occurred, so unaccounted N in unsterilized soil was probably incorporated into the microbial biomass in excess of what was required for metabolic activity. The unaccounted N was ca 70 mg NO3−–N in nitrate amended soil after 3 days and 138 NO3−–N when glucose was additionally added. Losses through abiotic processes were absent as inferred from changes in sterilized soil and the aerobic incubation inhibited possible losses through denitrification. It was inferred that NO3− that could not be accounted for was taken up by micro-organisms in excess of what was required for metabolic activity