Root Influence on Nitrogen Mineralization and Nitrification in Avena barbata Rhizosphere Soil

Abstract

Micro–¹⁵N pool dilution was used to quantify rates of gross N mineralization, consumption, and nitrification in bulk soil and in soil within 2 mm of root sections of Avena barbata (slender wild oats), an annual grass common to California oak woodland‐savannas. Rates of gross N mineralization in rhizosphere soil (9.2 mg N kg⁻¹d⁻¹) were about ten times higher than in bulk soil (1.0 mg N kg⁻¹d⁻¹). Total bacterial numbers in soil adjacent to roots were slightly higher than in bulk soil; protozoa biomass was not measurably different. Changes in bacterial numbers or standing stocks of bacterial N could not account for rates of N mineralization. Nitrification potential values were similar in bulk and rhizosphere soil, yet gross rates of nitrification were highly dependent on location along the root. Gross nitrification rates in soil near the root tip were the same as those in bulk soil, while rapid uptake of NH₄ by older sections of root (8–16 cm from the tip), appeared to limit nitrification rates. Only small differences in microbial community structure between bulk and rhizosphere soil were detected by terminal restriction fragment length polymorphism (TRFLP) analysis. While the small increases in bacterial numbers and changes in community composition may in‐part explain the increased rates of N mineralization, other microbial‐root interactions are likely involved in accelerating the flux of N from organic sources to the plant‐available NH₄ pool. The high rates of N mineralization observed in soil immediately adjacent to roots should facilitate plant access to N. Most of the stocks and fluxes determined in these studies exhibited distinct spatial patterns along the plant root that may have significantly impacted N‐availability to the plant.