Microbial biomass and N cycling under native prairie, conservation reserve and no-tillage in Palouse soils

Abstract

Tillage management practices that improve soil quality are needed to maintain agricultural productivity. Soil quality can be improved through increases in microbial biomass carbon (MBC) and the nitrogen (N) contents in different pools of soil organic matter (SOM). The objective of our experiment was to study the dynamics of MBC and N cycling in soils collected from the Palouse area of eastern Washington State. Soil managements were: no tillage for 4 (NT4) and 28 (NT28) years, conservation reserve program (CRP), native prairie (NP), and conventional tillage (CT). Microbial biomass carbon was 50% less in the CT soil than the NP soil and there was 74% less total soil N (TSN) in the surface CT soil compared to the NP soil. Conversion of CT to CRP, NT4, and NT28 increased MBC in the 0–5 cm depth by 40, 6 and 78%, respectively. Compared to CT, the TSN content was 20, 57, and 94% higher in CRP, NT4 and NT28, respectively. The net N mineralized (Nmin) over 180 days in the surface 5 cm was highest (69.5 µg N/cm 3) in NT28 soil and lowest (14.2 µg/cm 3) in CRP. Interestingly, the CT soil mineralized as much N as the NT systems but had less TSN than NT. Compared to NP, CT had a 70% reduction in particulate organic matter nitrogen (POM-N) to a depth of 20 cm. The conversion of CT soil to CRP, NT28, and NT4 enhanced the POM-N content by 65, 216, and 101% at 0–5 cm. The proportion of POM-N to TSN was lowest in CT soil whereas the NT and CRP soils were considerably higher. The lower Nmin/MBC ratios (qN) of the conservation systems imply a higher immobilization capacity and tighter soil N cycling. Mineralized N per unit of respiration (Nmin/CO 2-C) was highest in CT and NT28 management systems suggesting different factors were responsible in these systems for exhibiting almost similar values. The Nmin ratios along with MBC and POM-C relationships could be useful soil quality indicators in judging tillage-induced changes in soil management systems.