Mineralization of 14 C-labelled maize in alkaline saline soils

Abstract

The turnover of organic material determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. Uniformly labelled [14C] maize and its neutral detergent fibre (NDF) fraction, mainly containing cellulose and hemi-cellulose, were added to these soils to investigate dynamics of C and N and the importance of the NDF fraction. Soil with electrolytic conductivity (EC) of 1.2, 3.2, 24.6 and 32.7 dS m−1 was incubated aerobically, while CO2 and 14CO2 production, and inorganic N dynamics (NH4 +, NO2 −, NO3 −) were monitored. The amount of 14C-labelled maize mineralized after 97 days was >500 mg C kg−1 dry soil (D.S.) of the 1000 mg C kg−1 D.S. added in soils with EC≤ 24.6 dS m−1, but only 257 mg C kg−1 D.S. in soil with EC 32.7 dS m−1. The decomposition of the NDF fraction showed a lag, greatest in the soil with the largest EC and the amount of 14C-labelled NDF fraction mineralized after 97 days was > 300 mg C kg−1 D.S. in soils with EC ≤ 3.2 dS m−1, but in the soil with EC 32.7 dS m−1 it was only 118 mg C kg−1D.S. Application of 14C-labelled maize and the NDF fraction induced a priming effect, most accentuated at the onset of the incubation. The ratio between the amount of CO2 produced due to the priming effect and the 14CO2 produced was 16-times larger when 250 mg maize-C kg−1 D.S. was added and only 3-times when 2000 mg maize-C kg−1 D.S. was added. Oxidation of NO2 − occurred in soil with EC 32.7 dS m−1 as witnessed by decreases in concentration of NO2 − and increases in concentration of NO3 −. It was found that EC affected the decomposition of maize, the NDF fraction and the priming effect. Decomposition of cellulose and oxidation of NO2 − occurred in soil with EC 32.7 dS m−1 although cellulolytic micro-organisms and autotrophic NO2 − oxidizers could previously not be isolated from this soil.