The path of nitrogen from biological fixation to plants and above ground through the soil food web: a synthesis of nitrogen stable isotope studies in soil ecology

Abstract In most terrestrial ecosystems, nitrogen (N) is an essential and often limiting element for living organisms. There is competition for the available N between roots, rhizosphere organisms and the overall soil food web. N derived from biological fixation in the soil reaches the above ground terrestrial food web through two pathways: from root uptake, and by successive predation on soil organisms. Considering the importance of N in terrestrial ecosystems, investigations of food web interactions for N through the food web in time and space are a prerequisite for better understanding of primary production and nutrient cycling. Here we reviewed N partitioning in the soil based on stable isotope (15N and 13C) ratio mass spectrometry studies of the food webs. Bacteria stratification in peds and down the soil profile, as represented by Pourbaix diagrams, delineate metabolic functions and N fixation parameters. We proposed a model for the path of N through the soil to the above ground sub-system. There are two main paths: one from root nutrient uptake to the aerial parts of the plant, and the other through the soil food web from successive consumption by predation of soil organisms, to the above-ground terrestrial food web. There are insufficient data available on the bacteria-bacterivore feeding pathway, but it would appear that besides specialized bacterivorous protists, other protists and nematodes tend to be more generalized microbivores. Understanding the pathway of N transfer better, and its regulating parameters for functional guilds within trophic levels, has implications for the sustainable management of our environment, agriculture sustainability, forest soil management, and remediation of disturbed environments.