Effect of Tillage Practices on Microbial Biomass Dynamics

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Abstract
Changes in microbial biomass dynamics and N cycling were studied in soils from a site derived from native grassland in western Nebraska managed under three tillage systems: no‐till, stubble mulch, and moldboard plow (bare fallow). A treatment of native sod was maintained in each replication. The site was in native sod until 1970 when it was plowed and an alternate wheat ( Triticum aestivum L.)‐fallow rotation established. After 16 yr of cultivation, total N concentration in the top 10 cm of soil had decreased to 73, 68, and 50% of native sod in the no‐till, stubble mulch, and plow treatments, respectively. Soil microbial biomass levels were decreased to 57, 52, and 36% for the corresponding tillage treatments as compared to the native sod. In laboratory incubation studies, CO 2 respiration was proportional to microbial biomass. Nitrogen mineralization, however, was not proportional to microbial biomass. Immobilization of added 15 N was highest for the native sod treatment and amounts of 15 N extracted by KCl were inversely related to microbial biomass. Respiration rates per unit of mineralized N (r/N min ratio) increased in the order of plow, stubble mulch, no‐till, and sod treatments, suggesting that C availability for microbial growth declined with increased tillage intensity. In these tillage experiments, increased tillage intensity decreased the ability of soil to immobilize and conserve mineral N.