This ped is my ped: Visual separation and near infrared spectra allow determination of the origins of soil macroaggregates
Introduction
Aggregation is an important attribute of soils that likely determines such important properties as infiltration and retention of water, and C storage in soils (Elliott and Coleman, 1988; Denef et al., 2001; Blanco-Canqui and Lal, 2004; Lavelle et al., 2006). These properties in turn contribute to the provision by soils of such ecosystem services as climate regulation, flood and erosion control or nutrient cycling. Research so far has been mainly focused on the global assessment of soil aggregation by physical methods and the elucidation of physical as well as root- and microbially driven processes that stabilize aggregates. The hierarchical organization of aggregates has been stressed and mechanisms for microaggregate stabilization have been proposed (Tisdall and Oades, 1982; Chenu, 1993; Six et al., 2002). Much attention has been paid to mechanisms whereby organic debris incorporated into soils by tillage form nuclei around which microbial activities aggregate the soil (Gale et al., 2000; Golchin et al., 1994; Plante and McGill, 2002a, Plante and McGill, 2002b). These aggregates seem to have relatively short life spans, of less than a few weeks.
Less attention, however, has been paid so far to the macroaggregates created by ecosystem engineers through their bioturbation and other mechanical activities. Blanchart et al. (1999) demonstrated that the ca. 1000 mg casts egested by earthworms in an African savannah soil (Lavelle, 1978) soon become relatively stable macroaggregates that comprise ca. 40% of the soil volume. Several authors have emphasized the great contribution of earthworms and other soil ecosystem engineers to the formation of stable aggregates (Bossuyt et al., 2005; Pulleman et al., 2005). Dramatic, although reversible, changes in soil aggregation have been observed after colonization of an Amazonian pasture by the invasive earthworm Pontoscolex corethrurus (Chauvel et al., 1999; Barros et al., 2001). In a recent review, Six et al. (2002) proposed a double origin for soil macroaggregates, depending on the process that produced them: either large structures created by ecosystem engineers and further stabilized by a secondary microaggregation, or an agglomeration of microbial microaggregates into larger macroaggregate structures.
There is growing awareness that the dynamics of aggregate production and destruction over time is important to their function as microsites for C sequestration (Six et al., 2000). The effect of different land use practices on their dynamics at different scales and the combined effects of physical, chemical and biological processes involved are also of great importance (Pulleman et al., 2005). A major obstacle to understanding aggregate dynamics is our inability to identify the origins of the different types of aggregates found in soils, their turnover times and positions within the soil matrix.
Here, we propose a general methodology aimed at identifying and quantifying soil macroaggregates according to their origin. Research conducted at different sites provided examples to illustrate the methodology and assess its ability to detect changes in soil macroaggregation as well as identify the origins of these aggregates using specific morphological and spectral characteristics.
A visual method of aggregate separation derived from the highly detailed Topoliantz et al. (2000) assessment technique is proposed and validated across a wide range of sites in Nicaragua. At those sites, we tested the hypothesis that soil aggregate morphology assessed with our simple field method would be significantly different in sites dedicated to different types of land use. We also hypothesized a significant relationship between invertebrate communities and soil morphology.
The same hypotheses were tested in a field experiment conducted at the site of Benfica in the Brazilian state of Pára. At this site, soil previously covered with a pasture of the African grass Brachiaria bryzantha was planted to four different plants (two herbs, B. bryzantha and the Leguminosae Arachis pintoi and two shrubs, Solanum nigris and the Leguminosae Leucaena leucocephala) alone or in all possible combinations.
We then tested the hypothesis that biogenic structures produced by a wide diversity of soil invertebrates in forest and pasture ecosystems at the Benfica site had specific Near Infrared spectrometry (NIRS) spectral signatures, irrespective of the effects of soil and vegetation types. We finally compared spectral signatures of the different macroaggregate fractions separated from the soil at the Brazilian site with signatures of the large casts deposited by the earthworm Andiodrilus pachoensis at the soil surface to check whether they contributed to the biogenic aggregate fraction found in these soils.
Section snippets
Sites description
Research was conducted at two different sites in Tropical America, Wibuse in Nicaragua and Benfica in the state of Para (Brazil).
Relationships between macroinvertebrate communities and soil aggregate morphology at Wibuse (Nicaragua)
Macroinvertebrate communities: Average macroinvertebrate density was highest in coffee plantations (976 m−2) and fallows (939 m−2), and lowest in house yards (342 m−2) and eroded soil (74 m−2). Pastures (577 m−2), mixed crops (562 m−2), corn crops (383 m−2) and forests (366) had intermediate densities (Table 1). Ants (56.0%) and earthworms (19.2%) were the most abundant invertebrates overall, followed by Coleoptera (10.6%), Isoptera (3.5%), Diptera larvae (1.9%), Diplopoda (1.8%) and Arachnida (1.6%).
Discussion
The proposed methodology, NIRS signatures, aimed to assist in the identification of the origin of macroaggregates separated visually from small soil volumes. Methods used so far, based on the physical properties of aggregates, generally only evaluate the proportion of soil comprised by stable aggregates and the average size of these elements. Our methodology has a different aim as it first seeks to describe the origin of the aggregates. Data thus generated would allow for the assessment of the
Conclusion
The pattern of soil aggregation determined by visual separation using a simplified version of Ponge's small volume technique was significantly different in soils under different types of land use. Soil aggregation assessed this way was also significantly related to soil macroinvertebrate communities. The aggregates had significantly different NIRS spectral signatures indicating probable differences in their origins. Since fresh biogenic structures also have highly specific spectral signatures,
Acknowledgements
This work is dedicated to David Coleman, a great friend and colleague who has had considerable influence on soil ecology developments over the last few decades. We are grateful to CIAT (TSBF-CIAT Institute at Cali), Museu Paraense Emilio Goeldi and IRD (LEST at Bondy) for providing access to the field, logistics and laboratory facilities.
We thank Thierry Desjardins for his assistance in sending the samples from Brazil to France, and Alister Spain for language editing.
Funding was provided by the
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