Dominique Gillon

Using near-infrared reflectance spectroscopy to predict carbon, nitrogen and phosphorus content in heterogeneous plant material

Abstract The aim of this study was to produce calibration equations between near-infrared reflectance (NIR) spectra and the concentrations of carbon, nitrogen, and phosphorus in heterogeneous material: from living needles to litter in Pinus halepensis stands subjected to prescribed burnings. The aim was to determine whether calibrations should be conducted within each stage in the transformation of needles (local calibrations), giving relationships that were accurate but valid only for each particular stage, or whether it was possible to integrate the various forms of variation in needles (global calibrations) while retaining an acceptable accuracy. A principal component analysis calculated from the sample spectral data was used to distinguish three different sets, each sharing spectral characteristics and corresponding to three categories of needle: needles collected on the pines (N), falling needles (F), and litter (L), and each containing samples collected from the burnt sites and a control site. Samples representative of all the forms of variation in spectral properties were selected from within each category and their carbon, nitrogen, and phosphorus concentrations were measured using standard wet chemistry methods; these constituted the calibration sets n, f, and l. Calibrations were produced between the nutrient concentrations and the NIR spectra of the calibration sets n, f, and l and the grouped sets (n+f, f+l, n+f+l). The results of local calibrations made from each individual category showed that the carbon, nitrogen, and phosphorus concentrations were accurately predictable by NIR spectra. The global calibrations made by lumping together several categories were valid for a wider range of concentrations and for spectrally heterogeneous materials and in most cases were just as accurate as the local calibrations produced from each individual category.

CAN LITTER DECOMPOSABILITY BE PREDICTED BY NEAR INFRARED REFLECTANCE SPECTROSCOPY

Several decomposition experiments were used to explore the potential of near infrared reflectance spectroscopy (NIRS) for predicting litter decomposability. A first experiment was conducted on a calibration set, and predictive equations were established between the NIR spectral data of the initial litters and the decay descriptors. These equations were then applied to two validation sets. The 34 litters of the calibration set were incubated for eight weeks in microcosms in the laboratory. The values of litter mass remaining (LMR) in relation to incubation time were fitted to the single-exponential decay model (LMR = e−k t ). The litter mass loss (LML) values at different sampling dates and the rate constant k were strongly correlated with the initial litter characteristics. The calibrations carried out between measurements of the NIR spectra on the initial litters and the different descriptors of the decomposition patterns of the 34 litters showed that the LML values at the different sampling dates and the rate constant k were correlated with the spectra of the initial litters more closely than with their initial characteristics. The descriptors of litter decomposition patterns could therefore be predicted by NIRS. Among these descriptors, which were all correlated with one another, the easiest to measure (LML after one week incubation in the microcosm) was tested as a litter decomposability index (LDI). LDI was predicted by NIRS on the initial litters in two validation sets, the first consisting of 12 litters incubated under the same conditions in microcosms over a longer period (14 mo), the second consisting of four litters incubated in the field for one year at three stations situated along a climatic gradient. The results showed that the values of LDI predicted by NIRS were significantly correlated with the decay rate constant k in the two validation sets. The values of LDI predicted by NIRS therefore expressed a decomposability scale that was valid for litters decomposing over a longer period under the same incubation conditions and for litters subjected to different incubation conditions. The possibility of predicting litter decomposability by NIRS provides many opportunities, firstly for studying spatial and temporal variability in the rate of recycling of organic matter, and secondly for characterizing the gradual changes in litter quality during decomposition.

Changes in litter during the initial leaching phase: An experiment on the leaf litter of Mediterranean species

Summary-A laboratory experiment was carried out with the litter of 7 Mediterranean species of plants to compare their mass losses during the initial leaching phase of decomposition. To this end, the leaf and needle litter of deciduous, marcescent (deciduous species in which the withered leaves remain on the tree. for several months) and evergreen broad-leaved species and pine was immersed for 10 days in demineralized water, in the dark at 22°C. Samples were taken at 1,6, 24,72, 168 and 240 h. Depending on the species, the litter lost between ‘T-15% of initial dry mass, and water content values were between 130-360% of dry mass. The water absorption rate was positively correlated with the initial cellulose content of the litter and negatively correlated wil:h the initial leaf density, but the water content at the end of the experiment, the leaching rate and the total mass loss were uncorrelated with any of the initial litter characteristics studied. For 100 g of litter at the start (as dry matter), the losses after 240 h of immersion in water were, when they were significant, 5-8 g of C, 70-l 30 mg of N (significant for 3 species), 4-40 mg of soluble sugars and 3 g of inorganic elements (significant for only 2 species). In contrast, the quantities of lignin and cellulose in the litter remained unchanged. Most of the leaching losses (SCrSO%) therefore consisted of C. During leaching, the litter showed a tendency to become more concentrated in cellulose and lignin and poorer in sugars. Pine needle litter was distinguished by its low water absorption and low mass loss during leaching. In general, the litters of deciduous species were more absorbent than the litters of evergreen species. In contrast, the losses of water-soluble substances were not significantly different between these 2 groups of litters. In the broad-leaved species, the losses of water-soluble substances were slower and 2-3 times lower, compared to results already published for species from mesic biomes. ResumU par contre la teneur en eau atteinte g la fin de l’exp&imentation, la vitesse de perte en masse ou la perte de masse totale ne sont corrtlCes g aucune des caractiristiques initiales ttudites. Pour 100 g de litiltre au dCpart (en mat&e s&he), les pertes au bout de 240 h d’immersion dans l’eau ont CtB, lorsque les diffkrences btaient significatives, de 5-8 g de C, de 70-130 mg d’azote (significatives chez 3 es&es), de 4-40 mg de sucres solubles, et de 3 g d’61Cments mindraux (significatives chez seulement 2 eseces). Par contre, les quantitts de lignine et de cellulose n’ont pas changC dans les lit&es. L’essentiel des pertes (S&SO%) a done Ctb constitut par le C. Au tours du lessivage, les lit&es ont montrt une tendance B se concentrer en cellulose et en lignine, et g s’appauvrir en sucres. La 1itBre d’aiguilles de pin s’est distingute par une faible absorption d’eau et une faible perte en masse au tours du lessivage. En g&&al, les lititres des espices d&cidues ont ttC plus absorbantes que les lit&es des esptrces sempervirentes. Par contre, les pertes en substances hydrosolubles n’ont pas ttt significativement diffkrentes entre ces deux groupes de lit&es. Chez les espixes feuillues, les pertes en substances hydrosolubles ont btt plus lentes et 2-3 fois plus faibles, compar&es aux r&ultats dijjn connus sur des esp&es de biomes mbiques.

Annual feeding rate of the millipede Glomeris marginata on holm oak (Quercus ilex) leaf litter under Mediterranean conditions

Summary The annual consumption and assimilation rates of Quercus ilex leaf litter by the millipede Glomeris marginata were estimated under Mediterranean conditions simulated in the laboratory. Measurements were made on six occasions throughout the year to take into account seasonal variations in (i) the quality of decomposing leaves, (ii) the physiological state of the animals, and (iii) temperature plus photoperiod. The mean weight-specific consumption rate was 14 g (dry weight) g−1 (live weight) y−1, which is much higher than published figures for Mediterranean oak leaf litter. The consumption rate was highest in spring (51 mg g−1 d−1) and lowest in summer (28 mg g−1 d−1), the summer consumption rate being even lower when determined with air-dried leaves (17 mg g−1 d−1). Assimilation efficiency averaged 6% for the year, with a maximum in spring (9%) and a minimum in winter (1%). A much higher assimilation efficiency (29%) was measured with freshly fallen leaves, but a food preference experiment showed that these leaves were generally avoided and that G. marginata preferred more decomposed leaves despite their lower digestibility. It is concluded that these saprophagous macroarthropods – with field biomasses of 7–11 g (live weight) m−2 – produce considerable amounts of faecal pellets in Mediterranean oak forests.

The use of near infrared reflectance spectroscopy to study chemical changes in the leaf litter consumed by saprophagous invertebrates

Chemical changes in holm oak leaf litter that was consumed by the millipede Glomeris marginata were studied using near infrared reflectance spectroscopy. Compared to leaf litter, faecal pellets were significantly richer in lignin and poorer in nitrogen and readily assimilable constituents. The C:N ratio was significantly higher in the faeces. A NIRS-predicted index of litter decomposability was found to be lower in the faeces than in uneaten litter.

Effects of pesticides on organic matter recycling and microarthropods in a maize field: use and discussion of the litterbag methodology

The effects of two herbicides (atrazine and alachlore) and two insecticides (fipronil and carbofuran) on organic matter decomposition and soil mesofauna were evaluated in a maize field under normal agriculture conditions. Soil mesofauna were studied using the litterbag method. Near infrared reflectance spectrophotometry was used to study the effects on decomposition parameters (litter mass and nitrogen dynamic). The effects on soil microarthropods varied depending on both taxon and pesticide, with fipronil having a particularly strong effect. Differences were also observed between treatments for organic matter decomposition parameters, especially with the use of alachlore. Therefore, the analysis of microarthropods and the decomposition parameters provides complementary information on the effects of these chemical treatments.

Actual litter decomposition rates in salt marshes measured using near-infrared reflectance spectroscopy

Abstract Near-infrared reflectance spectroscopy (NIRS) has been widely applied as a holistic tool to investigate decomposition processes in terrestrial ecosystems. The objectives of this research were to determine the potential of NIRS to predict (1) the halophytic litter chemistry (i.e., carbon and nitrogen content) during decomposition, and (2) the stage of decomposition of halophytic litter. Decomposition experiments were conducted in the laboratory with microcosms placed under a wide range of physical characteristics and in the field with litterbags located along the elevation gradient (i.e., low to upper marsh). Microcosm experiments were used to calibrate the predictive equations. These calibration equations were then applied to the field data to test their capacity to predict %C, %N, and litter mass loss (LML). NIRS can be successfully applied to predict chemical composition of halophyte litter during decomposition processes. We hypothesized that the use of litterbags in the field might lead to a 20–40% overestimation of the decay rate as fine organic debris are lost through the meshes of the litterbags. NIRS can be used as a fast and nondestructive method to more accurately predict decay rates, and thus microbial consumption in aquatic environments.

Short-term effect of defoliation on terpene content in Thuja plicata

Abstract Insect herbivory or mechanical wounding in conifers can induce monoterpene biosynthesis. Low risk of herbivory, coupled with low availability of resources, is hypothesized to favour induced responses and to decrease constitutive defences. We studied the response to defoliation in western redcedars (Thuja plicata) from two regions: the Haida Gwaii archipelago, where mammalian herbivores were lacking until black-tailed deer were introduced at the end of the 19th century and previous work indicated that trees were less well defended, and the north coast mainland (British Columbia, Canada). We predicted that higher induced defences in the island population would compensate for reduced constitutive defences. We used one- and two-year-old nursery-grown seedlings to test i) whether defoliation would cause a short-term chemical response in island western redcedar and ii) whether mainland western redcedars that have always been exposed to large mammalian herbivores respond differently. The concentration in monoterpene and diterpenes did not vary significantly in response to defoliation over the 5-day period analyzed regardless of the defoliation intensity or the plant’s origin.