Nematodes enhance plant growth and nutrient uptake under C and N-rich conditions

Abstract

The role of soil fauna in crucial ecosystem services such as nutrient cycling remains poorly quantified, mainly because of the overly reductionistic approach adopted in most experimental studies. Given that increasing nitrogen inputs in various ecosystems influence the structure and functioning of soil microbes and the activity of fauna, we aimed to quantify the role of the entire soil nematode community in nutrient mineralization in an experimental set-up emulating nutrient-rich field conditions and accounting for crucial interactions amongst the soil microbial communities and plants. To this end, we reconstructed a complex soil foodweb in mesocosms that comprised largely undisturbed native microflora and the entire nematode community added into defaunated soil, planted with Lolium perenne as a model plant, and amended with fresh grass-clover residues. We determined N and P availability and plant uptake, plant biomass and abundance and structure of the microbial and nematode communities during a three-month incubation. The presence of nematodes significantly increased plant biomass production (+9%), net N (+25%) and net P (+23%) availability compared to their absence, demonstrating that nematodes link below- and above-ground processes, primarily through increasing nutrient availability. The experimental set-up presented allows to realistically quantify the crucial ecosystem services provided by the soil biota.

Figure 1. Mean nematode abundance of each trophic group in reinoculated samples (+Nem) on the 7th, 47th and 105th days after incubation.

The −Nem treatment is not presented as this treatment was without nematodes. The error bars indicate the standard error of the mean (n = 3).

Figure 2. The evolution of total PLFA (a) and PLFA biomarker concentrations of the major microbial groups G+ve, G−ve bacteria and actinomycetes (b), Saprophytic fungi and AMF (c) and protozoa nad FB ratio (d) over time in +Nem and −Nem treatments.

The asterisk symbol (*) indicates statistically significant differences between treatments on the corresponding sampling dates.

Figure 3. The evolution of dry biomass which is the sum of both the shoot and root biomass (a), and total nutrient mineralization (sum of available nutrients in the soil and plant uptake at each sampling occasion) for Nitrogen (μg N g⁻¹ soil), (b) and Phosphorus (μg P g⁻¹ soil), (c) in +Nem and −Nem treatments.

The error bars indicate the standard error of the mean (n = 4).

Figure 4. The main mechanisms by which nematode functional groups link below- and above-ground processes under resource-rich conditions.

Root herbivory (1) increases root exudation stimulating bacterial and fungal growth (2). Microbial communities then immobilize (3) N and P from the bio-available pool and N and P acquired from that of the externally added fresh OM (4) and the native SOM (5) through a mechanism known as root priming effect. Bacterivores and fungivores feed on the microbial communities (6) and excrete N and P in excess of their metabolic needs through a mechanism known as microbial loop (7) which otherwise would have been locked up in the microbial biomass. By increasing available nutrients, nematodes enhance plant nutrient availability and uptake. Depending on the chemical composition, the fresh OM would stimulate the microbial biomass and activity (8) in a way similar to that of exudates.