Nitrogen mineralization: root and microbial interactions in pitch pine microcosms

In forest ecosystems, mineralization of native organic N provides the primary plant N nutrient source. The rate of this N mineralization is affected directly and indirectly by interactions with plant root biomass and is also probably impacted by interactions with the soil microbial community. Thus, the objectives of this study were to evaluate the relationship of N-mineralization rates in an acid forest soil (Lakehurst sand, a mesic, coated Haplaquodic Quartzipsamment) with microbial metabolic and enzymological activities and to determine the impact of proximity to the growing roots of pitch pine (Pinus rigida Miller) on these interactions. Potential proteolytic activity, amino acid metabolism, N mineralization, microbial growth rate, glucose metabolism and microbial biomass N were measured in microcosms planted with varying densities of pitch pine seedlings. Nitrogen mineralization, amino acid metabolism and proteolytic activity in the organic-matter-poor A horizon soil were 0.01 to 0.5 of the levels in the organic-matter-rich O horizon of the soil. After 21 to 22 wk of growth of pine seedlings in the soil amino acid metabolic rates in the O horizon soil were not affected by root mass. Amino acid metabolism rates declined between 10 and 25% in soils from the A horizon with increasing root density. Proteolytic activity was reduced between 10 and 50% in both sods due to root growth. In the O horizon soil amino acid metabolic rates correlated with microbial growth rate, glucose metabolism, and microbial biomass C and N, but not with protease activity in both rhizosphere and near-rhizosphere soils. Proteolytic activity varied independently of microbial growth rate, glucose metabolism, and biomass C and N. Except for a highly significant relationship to microbial growth rate, net N-mineralization rates were not correlated with the measured microbial and enzymological activities. Glucose metabolism and microbial biomass N correlated with protease activity in rhizosphere soils, but not in near-rhizosphere soils; otherwise, proximity to the pine tree roots had little effect on the relationship of N-metabolism parameters in the acid soils used in this study.