Protozoan Pulses Unveil Their Pivotal Position Within the Soil Food Web

Abstract

Protozoa are one of the most abundant groups of bacterivores within the soil and are responsible for mineralisation of bacterial biomass, having a large impact on C and N cycling. Little is known of their contribution to soil nutrient transfers or the identity of their consumers. Here, for the first time indigenous flagellates and ciliates, enriched to 83 atom% for (13)C and 10 atom% for (15)N, were introduced to soil cores from two different land managements, grassland and woodland with the same soil type, to trace the flow of protozoan C and N through the soil food web. Nematodes, Collembola, earthworms and insect larvae obtained the greatest amounts of C and N of protozoan origin, either through direct consumption or uptake of biomass post-cell death. Our results show that changes in management, affect the functioning of the soil food web and the utilisation of protozoa as a food source.

Divergence of feeding channels within the soil food web determined by ecosystem type

“…The δ 13 C and δ 15 N values were significantly different though, with those of the grassland being lower compared with the woodland ( δ 13 C F 1,10 = 86.10; P < 0.001 and δ 15 N F 1,10 = 43.09; P < 0.001; Table S1, other soil characters (Crotty et al (2012)).…”

“…Collembola are generally considered to be fungivorous; however, studies have found them to consume large amounts of bacteria (Murray et al 2009; Crotty et al 2011) and protozoa (Crotty et al 2012). Subtle differences in the microbial community between habitats have the potential to affect the isotopic composition of a taxon, making it conceivable that they appear to be acting at different trophic levels dependent on habitat type.…”

“…It is probable that the same organisms are utilizing different food sources in the different habitats – due to different basal resources or potentially the taxa act as more generalist feeders in the woodland compared to the grassland. There are no specific predator–prey relationships within the soil (Crotty et al 2012), this is reflected in stable isotope analysis at natural abundance where there appears to be a continuum of decomposition and predation. There is a lack of steps between trophic levels, with a truly omnivorous diet leading to isotopic signatures having a preferred feeding type as opposed to a definitive one.…”

“…The willow woodland was planted approximately 25 years ago. Details of the soil characteristics and weather conditions at the sites can be found in Crotty et al (2012). …”

Using Stable Isotopes to Differentiate Trophic Feeding Channels within Soil Food Webs

“…These experiments showed the movement of bacterial C and N through various invertebrate trophic levels, through the addition of an enriched culture of Pseudomonas lurida to form part of the bacterial feeding channel with results consistent across habitats (Crotty et al. ) and nutrient regimes (Murray et al. ).…”