Nitrifiers and mycorrhizae in pristine and grazed pinyon‐juniper ecosystems

Abstract

We compared soils from grazed and pristine pinyon‐juniper ecosystems for the numbers of chemoautotrophic nitrifying bacteria and determined the distribution of vesicular‐arbuscular (VA) mycorrhizae and ectomycorrhizae (EM). Additionally, we report on the presence of allelopathic substances in organic horizons. Ammonium (NH4⁺)‐oxidizing bacteria were in greater numbers in the interspaces between the trees, ranging from 9.60 × 10⁴ to 2.13 × 10⁵ bacteria^‐1 soil, than under canopies, 3.5 × 10⁴ to 4.8 × 10⁴ bacteria^‐1 soil. Grazed interspace soils had over twice the number of NH4⁺oxidizing bacteria than any other location. There were no differences in the numbers of nitrite‐oxidizing bacteria between either vegetative covers or sites. The rate of nitrate production did not correlate with the number of nitrifiers. However, there was a significant correlation (r² = 0.85) between mineralization coefficients of total nitrogen and the total number of nitrifiers. Fourteen known and ten unknown monoterpenes were found in soils from under pinyon pine (Pinus edulis Engelm.) and juniper [Juniperus osteosperma (Torr.) Little] canopies. VA mycorrhizal spore numbers were significantly greater under canopies than in interspace soils at both sites, with the lowest number recorded in grazed interspace soils. No differences were found in the degree of colonization of either EM or VA mycorrhizae on pinyon pine or juniper, respectively, between each site. The differences in the spatial distribution of nitrifiers and mycorrhizae emphasize the complexities of the belowground ecosystem in these woodlands despite their apparent aboveground structural simplicity.