Nitrogen Addition Regulates Soil Nematode Community Composition through Ammonium Suppression

Abstract

Nitrogen (N) enrichment resulting from anthropogenic activities has greatly changed the composition and functioning of soil communities. Nematodes are one of the most abundant and diverse groups of soil organisms, and they occupy key trophic positions in the soil detritus food web. Nematodes have therefore been proposed as useful indicators for shifts in soil ecosystem functioning under N enrichment. Here, we monitored temporal dynamics of the soil nematode community using a multi-level N addition experiment in an Inner Mongolia grassland. Measurements were made three years after the start of the experiment. We used structural equation modeling (SEM) to explore the mechanisms regulating nematode responses to N enrichment. Across the N enrichment gradient, significant reductions in total nematode abundance, diversity (H' and taxonomic richness), maturity index (MI), and the abundance of root herbivores, fungivores and omnivores-predators were found in August. Root herbivores recovered in September, contributing to the temporal variation of total nematode abundance across the N gradient. Bacterivores showed a hump-shaped relationship with N addition rate, both in August and September. Ammonium concentration was negatively correlated with the abundance of total and herbivorous nematodes in August, but not in September. Ammonium suppression explained 61% of the variation in nematode richness and 43% of the variation in nematode trophic group composition. Ammonium toxicity may occur when herbivorous nematodes feed on root fluid, providing a possible explanation for the negative relationship between herbivorous nematodes and ammonium concentration in August. We found a significantly positive relationship between fungivores and fungal phospholipid fatty acids (PLFA), suggesting bottom-up control of fungivores. No such relationship was found between bacterivorous nematodes and bacterial PLFA. Our findings contribute to the understanding of effects of N enrichment in semiarid grassland on soil nematode trophic groups, and the cascading effects in the detrital soil food web.

Responses of soil enzyme activity and microbial community compositions to nitrogen addition in bulk and microaggregate soil in the temperate steppe of Inner Mongolia

“…The soil was a chestnut soil in Chinese classification or Calcic-orthic Aridisol in US soil taxonomy classification system [67]. The perennial rhizomatous grass Leymus chinensis and the perennial bunchgrass Stipagrandis are the dominant plant species, accounting for 60-75% of the total aboveground biomass [68].…”

“…Anthropogenic N enrichment can reveal dramatic impact on the alteration of soil enzyme activities and microbial compositions in N-limited systems [68]. The change of N availability in soil may increase carbon (C) input through enhancing plant biomass, microbial access to nutrients and soil organic dynamics [25,33,43].…”

Dynamics of soil nematode communities in wheat fields under different nitrogen management in Northern China Plain

“…However, the magnitude and direction of nematodes in response to N addition are still controversial, varying with the levels of N addition and among different ecosystems. In semiarid grasslands, for example, 25 and 50 kg N ha À1 yr À1 of N additions show no effects [14], but 120 kg N ha À1 yr À1 of N addition significantly suppresses soil nematode abundance [15]. By contrast, N enrichment of 300 kg N ha À1 yr À1 stimulates the population size of soil nematodes in an acid soil of subtropical China [11].…”

“…Elevated N can facilitate plant growth and indirectly benefit soil biota [12,19]. However, the direct effects of N addition such as soil acidification and ammonium toxicity may be harmful to soil nematodes [15,20,21]. The determinants of soil nematode abundance under N addition may depend on the level of N fertilization [22].…”

“…bacterivores, fungivores, plant parasites, and omnivoresepredators). For example, significant reductions in nematode community diversity (H 0 and taxonomic richness), maturity index (MI), and the abundance of root herbivores, fungivores and omnivoresepredators were found across an N enrichment gradient in a semiarid grassland ecosystem [15]. The relative abundances of fungivores (Fu%) and plant parasite (PP%) increase, whereas the relative abundances of bacterivores (Ba%) and omnivoresepredators (Om%) decrease following N fertilization in a vegetable greenhouse experiment and in a hybrid napiergrass study [22,23].…”