Spatial Heterogeneity Among Soil Processes at Sub-Centimeter Scale.

Spatial heterogeneity occurs among processes in soil organic matter turnover. Herein relationships were observed among processes and parameters in 7 X 7 mm micro-cores recovered with modified microtiter hardware. Cores (~ 2 g) from three soils were transferred to deep-well microtiter blocks, pulverized, and mixed with tap water to collect roots for gravimetric analysis. Soil suspensions were filtered and assayed for urease, protease, and cellulase activities. Similarly processed samples from replicate plates were weighed (0.25 g) into each of four microtiter plates. Wells were amended with ¹⁴C-glucose, ¹⁴C-2, 4-D, ¹⁴C-metolachlor, or a control solution without substrates and incubated aerobically. ¹⁴C mineralization and total respiration (BaCl₂ precipitation and turbidimetry) were measured periodically in headspace traps using a microplate scintillation counter. Fe(II) and inorganic nitrogen were measured in solution (micro-colorimetry). After 14 days, headspace was flushed with nitrogen, sealed for anaerobic incubation and analyses continued. Urease activity was relatively uniform, whereas protease activity was variable, correlated positively with cellulase and negatively with inorganic nitrogen. Limited variability was observed in aerobic respiration or glucose mineralization, except for elevated activity in adjacent wells where the greatest root biomass was found. Herbicide degradation rates varied spatially. 2,4-D was mineralized in all wells, whereas metolachlor mineralization was only observed in 30-53% of wells among soils. Variable accumulation of Fe(II) was observed in 51-100% among soils after nitrate was depleted. 2,4-D mineralization rates decreased under anaerobiosis while metolachlor mineralization increased in many wells that had been active (and many inactive) under aerobic conditions. MPN counts of 2,4-D and metolachlor degraders correlated with degradation rates. Mineralization lag periods were observed for MPN counts below 100 CFU/g. Results suggest spatial variability is greater in processes mediated by uncommon microorganisms. Anaerobic and aerobic degradation processes were uncorrelated in space. Protease activity was related to carbon and nitrogen utilization.