Effect of Tillage Practices on Microbial Biomass Dynamics

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, CO₂ respiration was proportional to microbial biomass. Nitrogen mineralization, however, was not proportional to microbial biomass. Immobilization of added ¹⁵N was highest for the native sod treatment and amounts of ¹⁵N 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.