Short-Term response of soil microbial biomass and gaseous emissions to different chabazite zeolitite amendments

Zeolitites (ZTs) are rocks containing more than 50 % of zeolites and are worldwide recognized as a suitable and valuable soil amendment. Once homogenized in the soil or in the cultivation substrate, ZTs enhance soil physicochemical properties and nitrogen (N) use efficiency. However, little is known about their effects on soil microbial biomass and on how they influence soil gaseous emissions, which may represent an important contribution of greenhouse effect. This study aims at i) evaluating short-term effects of different chabazite-rich ZT (CHAZT) amendments on soil microbial biomass and activity and ii) evaluating the effects of different CHAZT amendments on soil gaseous emissions (CO2, N2O, NOx and NH3) soon after the application. To reach these goals a silty-clay agricultural soil was amended with different percentages of natural CHAZT (NZ, at 5 and 15 wt%) and NH4-enriched CHAZT (CZ, at 10 wt%) in two separate incubation experiments. In the first incubation experiment, soil dissolved organic carbon (C), total dissolved N, NH4, NO3, NO2, microbial biomass C and N, and ergosterol were measured periodically over a 16 day period. To verify the microbial immobilization of the N derived from CZ, a naturally high 15N source (pig slurry) was used for enriching the mineral and microbial biomass 15N signature was monitored over the incubation. In the second incubation experiment, an investigation of soil CO2, N2O, NOx and NH3 fluxes was carried out for a total of 24 h both immediately after the application of urea and without a further N input. Concerning the effect on soil microbial biomass (first experiment), ergosterol content increased in the soil amended with 5 % NZ while no clear trends were observed in the soil amended with 15 % NZ, suggesting that fungal biomass was favored at lower application rate. CZ amended soil showed evidence of nitrification, since microbial biomass N was directly related to NO3 production and inversely related to NH4. Isotopic measurements confirmed immediate assimilation of N derived from CZ. In the second experiment, immediate CO2, N2O, NOx and especially NH3 emissions after fertilizer application were generally reduced (up to 60 %) in soils amended with NZ, indicating it as a valuable material for reducing soil C-N gaseous losses. CZ application lowered CO2 and N2O emissions, but very high NOx fluxes occurred even without applying any further N input. NH3 emissions were higher in NH4-enriched zeolites amended soil, but if the amendment is performed without further N inputs, the emissions can be significantly lowered with respect to a conventional urea fertilization. These results suggested that the CZ used in this study supplied an immediately available N pool to the microbial biomass and that NZ can be a suitable material for mitigating gaseous N and C losses from soil or substrates.