Alain Brauman

Comparative distribution of organic matter in particle and aggregate size fractions in the mounds of termites with different feeding habits in Senegal: Cubitermes niokoloensis and Macrotermes bellicosus

Abstract The comparative distribution of organic components in different soil fractions as a result of the activities of two of the most representative species of termites (Cubitermes niokoloensis and Macrotermes bellicosus) in the semi-arid savanna of Senegal was assessed by physical fractionation. The impact of these two species on soil organic matter (SOM) differed. For the soil feeding C. niokoloensis, the internal and external wall of the mound contained five times higher carbon and 10 times higher nitrogen concentrations than the reference soil. In contrast, the mounds of the fungus-growing M. bellicosus had lower C content than the reference soil. Although both species select fine soil particles for mound construction, their strategy differs. Particle-size fractions finer than 50xa0μm represented 75% of the total mass of soil sampled from the internal and external walls of the mound of soil feeding termites, while clay size particles were the most abundant fraction of these compartments of the fungus growing mound. For the soil feeding species, the particle-size fractions contained 3–10 times higher C concentrations than the fractions isolated from the reference soil, the enrichment being the highest for clay fractions. About 60% of these clay particles remained aggregated within the coarser structures of the silt (2–50xa0μm) and the sand size particles (50–200xa0μm). The latter represented 60% of the total soil mass and contained 50% of the total carbon. No impact of M. bellicosus on soil aggregation was recorded. These results clearly reveal the contrasting effects (positive for soil feeding, negative for fungus feeder) of these two species on SOM dynamics. The use of the aggregate fraction of the coarse silt size (20–50xa0μm) as an indicator of the activity of soil feeding termites is suggested.

Differential responses of nitrate reducer community size, structure, and activity to tillage systems.

ABSTRACT The main objective of this study was to determine how the size, structure, and activity of the nitrate reducer community were affected by adoption of a conservative tillage system as an alternative to conventional tillage. The experimental field, established in Madagascar in 1991, consists of plots subjected to conventional tillage or direct-seeding mulch-based cropping systems (DM), both amended with three different fertilization regimes. Comparisons of size, structure, and activity of the nitrate reducer community in samples collected from the top layer in 2005 and 2006 revealed that all characteristics of this functional community were affected by the tillage system, with increased nitrate reduction activity and numbers of nitrate reducers under DM. Nitrate reduction activity was also stimulated by combined organic and mineral fertilization but not by organic fertilization alone. In contrast, both negative and positive effects of combined organic and mineral fertilization on the size of the nitrate reducer community were observed. The size of the nitrate reducer community was a significant predictor of the nitrate reduction rates except in one treatment, which highlighted the inherent complexities in understanding the relationships the between size, diversity, and structure of functional microbial communities along environmental gradients.

Impact of irrigation water quality on soil nitrifying and total bacterial communities

Disturbance induced by two contrasting irrigation regimes (groundwater versus urban wastewater) was evaluated on a sandy agricultural soil through chemical and microbial analyses. Contrary to wastewater, groundwater displayed very high nitrate contents but small amounts of ammonium and organic matter. Despite these strong compositional shifts, soil organic carbon and nitrogen, nitrate and ammonium contents were not significantly different in both types of irrigated plot. Moreover, neither microbial biomass nor its activity, determined as fluorescein diacetate hydrolysis activity, was influenced by irrigation regimes. Bacterial community structure, assessed by denaturing gradient gel electrophoresis (DGGE) of 16S ribosomal DNA fragments, was also weakly impacted as molecular fingerprints shared an overall similarity of 85%. Ammonia-oxidizing bacterial community (AOB) was monitored by DGGE of the functional molecular marker amoA gene (alpha subunit of the ammonia monooxygenase). Surprisingly, no amoA signals were obtained from plots irrigated with groundwater, whereas signal intensities were high in all plots under wastewater. Among the last, compositional shifts of the AOB community were weak. Overall, impact of irrigation water quality on soil chemistry could not be evidenced, whereas effects were low on the total bacterial compartment but marked on the AOB community.

Nitrous Oxide (N2O) Emissions by Termites: Does the Feeding Guild Matter?

In the tropics, termites are major players in the mineralization of organic matter leading to the production of greenhouse gases including nitrous oxide (N2O). Termites have a wide trophic diversity and their N-metabolism depends on the feeding guild. This study assessed the extent to which N2O emission levels were determined by termite feeding guild and tested the hypothesis that termite species feeding on a diet rich in N emit higher levels of N2O than those feeding on a diet low in N. An in-vitro incubation approach was used to determine the levels of N2O production in 14 termite species belonging to different feeding guilds, collected from a wide range of biomes. Fungus-growing and soil-feeding termites emit N2O. The N2O production levels varied considerably, ranging from 13.14 to 117.62 ng N2O-N d-1 (g dry wt.)-1 for soil-feeding species, with Cubitermes spp. having the highest production levels, and from 39.61 to 65.61 ng N2O-N d-1 (g dry wt.)-1 for fungus-growing species. Wood-feeding termites were net N2O consumers rather than N2O producers with a consumption ranging from 16.09 to 45.22 ng N2O-N d-1 (g dry wt.)-1. Incubating live termites together with their mound increased the levels of N2O production by between 6 and 13 fold for soil-feeders, with the highest increase in Capritermes capricornis, and between 14 and 34 fold for fungus-growers, with the highest increase in Macrotermes muelleri. Ammonia-oxidizing (amoA-AOB and amoA-AOA) and denitrifying (nirK, nirS, nosZ) gene markers were detected in the guts of all termite species studied. No correlation was found between the abundance of these marker genes and the levels of N2O production from different feeding guilds. Overall, these results support the hypothesis that N2O production rates were higher in termites feeding on substrates with higher N content, such as soil and fungi, compared to those feeding on N-poor wood.