Linking Soil Microbial Activity to Water‐ and Air‐Phase Contents and Diffusivities

Quantification of in situ soil microbial activity is indispensable to improve manipulation of nutrient turnover in soil and optimize crop nutrient supply. We sampled 100‐cm 3 cores of undisturbed arable soil at three locations along a naturally occurring clay gradient (L1: 11% clay; L3: 22% clay; L5: 34% clay). The cores were drained to seven different matric potentials in the range −15 to −1500 hPa and gas diffusion determined prior to a 4‐wk incubation at 20°C in the dark. For all soils the net nitrification increased with water content to a maximum (L1, 12.1 μg NO 3 –N g −1 soil; L3, 10.3 μg NO 3 –N g −1 soil; and L5, 8.2 μg NO 3 –N g −1 soil) and then decreased with further increase in water content. The water content at maximum nitrification was 0.26, 0.37, and 0.42 m 3 m −3 , respectively. Calculations of water‐filled pore space (WFPS) did not normalize soil type differences in optima for microbial activity. The matric potential at peak net nitrification was −140, −170, and −430 hPa, respectively. No single correlation between CO 2 evolution and soil water content existed across soil types. The relative solute diffusivity estimated by recently developed models offered a better description of CO 2 evolution. The relative gas diffusivity was a better predictor of the increase in net nitrification than was the soil air content. A conceptual model balancing the effects of solute and gas diffusivity indicated that the relative trend in the observed optima of water contents across soil types was as expected. We advocate the use of the conceptual model including soil type dependent expressions for solute and gas diffusivity in future studies of aerobic microbial activity.