Factors influencing nitrogen concentrations in soil water in a North American tallgrass prairie

Abstract

The influence of roots, soil fauna and microbes on nitrogen concentrations in soil water in annually-burned tallgrass prairie were evaluated using porous cup lysimeters and a series of manipulations involving an insecticide, clipping of the vegetation, and C and N additions to the soil surface. An organophosphate insecticide (isofenphos), which significantly reduced densities of certain nematode and arthropod groups, resulted in small but statistically significant declines in soil water NO 3 − concentrations during 2 of 3 yr of study. Organic N, the dominant form of soil water N, was unaffected by treatment. Clipping the foliage weekly during the spring resulted in significant reductions in annual foliage production, but failed to change concentrations of soil water NO 3 − or organic N. Nitrogen additions (10gNm −2 as NH 4NO 3) greatly increased soil water NO 3 − concentrations; However, when 250 g C sucrose m −2 were concurrently added to plots, there was a significant reduction in soil water NO − 3 concentrations. Soil water NH 4 + concentrations were unaffected by treatment. These studies emphasize the importance of microbes as the dominant factor affecting soil waler N concentrations in carbon-rich soils. Plants, soil fauna and nitrogen fertilizers do affect N dynamics, but. under annually-burned conditions in the tallgrass prairie, these effects are obscured by microbial processes.