Claudia Hidalgo

NIR Spectroscopy: An Alternative for Soil Analysis

Advances in laboratory instrumentation and chemometrics provide alternatives to traditional methods of conducting soil chemical analysis. One of these is infrared diffuse reflectance spectroscopy in the near-infrared spectral range (NIRS). Herein we report the results of a multinational study to develop useful calibrations associating NIRS spectra with laboratory-measured results for total soil carbon (C), total soil nitrogen (N), δ13C, and δ15N from a single soil site in Mexico subjected to zero- and conventional-tillage regimens with and without crop residues and crop rotations of maize and wheat across 16 years. Modified partial least squares regression (MPLS) was used to obtain useful NIR predictions for total soil C and N, with ratio performance deviation (RPD) values of 6.8 and 2.6, respectively. Corresponding multiple correlation coefficients (RSQs) for C and N were 0.98 and 0.85, with standard errors of prediction (SEPs) of ±0.45 g C kg–1 and ±0.09g Nkg–1, respectively. The generation of δ15N and δ13C models produced different NIR recordings in soils with and without crop residues. Application of discriminant partial least squares (DPLS) statistics to the NIR spectral data allowed us to discriminate soils with and without residues. The prediction confidence for stable isotopes was 90% (internal validation) and 94% (external validation). Modified partial least squares regression was used to estimate δ15N and δ13C. Ratio performance deviation, RSQ, and SEP values obtained for δ13C and δ15N were 2.44 and 3.57, 0.83 and 0.81, ±0.5‰ (parts per thousand) and ±0.45‰ in soils with residues and 2.5 and 3.8, 0.93 and 0.92, and ±0.2‰ and ±0.23‰ in soils without residues, respectively. Overall, results obtained with NIRS were comparable to those obtained using conventional analytical methods, a finding that has wide relevance to agricultural soils and environmental studies in tropical locations. However, further testing is necessary to confirm that the calibration models are neither site nor instrument specific.

Soil carbon dynamics in high-elevation temperate forests of Oaxaca (Mexico): thinning and rainfall effects

Resumen en: To explore the forest harvest effects on biologically active fractions of soil organic matter dynamics, we evaluated soil total carbon (C-total) and soil...

The effects of precipitation regime on soil carbon pools on the Yucatan Peninsula

The effects of precipitation regime on the size of soil carbon (C) pools were compared in mature tropical dry forests of the Yucatan Peninsula. Our study included three forest stands in each, a dry site (potential evapotranspiration ratio = 3.2 mm mm ―1 ; mean annual precipitation = 537 mm), a wetter site (2.0 mm mm ―1 : 993 mm) and a site in which water was comparatively less limiting (1.3 mm mm ―1 : 1086 mm). At each site. soil C pools in dead fallen phytomass (includes leaves, flowers, fruits, small twigs and deadwood debris) deposited on the litter layer and in roots and organic matter of the mineral soil (from the upper 10 cm) were measured in samples collected during the dry season. A high proportion of the total C pool (93―95%) was in the top 10 cm of soil in all forest sites. The smallest C pool was in roots (1.8―2.4% of the total C), meanwhile the C in the litter layer represented 3―5% of the total pool. These patterns were observed irrespective of study site. However, distribution of C (i.e. wood debris vs. fine litter) varied across sites; the proportion of the forest-floor C pool in wood debris decreased from 80% in the driest site, to 51% and 42% in 993-mm and 1086-mm sites, respectively. Overall, we observed that three pools (wood debris, roots and soil organic C) provide evidence for the significant decrease in soil C storage with increase in mean annual precipitation in Yucatan Peninsula. A potential explanation for this unexpected pattern includes an increasing C turnover time with decreasing mean annual precipitation, resulting in higher C accumulation per unit of C input in the driest site.

The natural abundance of 13C with different agricultural management by NIRS with fibre optic probe technology.

In the present study the natural abundance of (13)C is quantified in agricultural soils in Mexico which have been submitted to different agronomic practices, zero and conventional tillage, retention of crop residues (with and without) and rotation of crops (wheat and maize) for 17 years, which have influenced the physical, chemical and biological characteristics of the soil. The natural abundance of C13 is quantified by near infrared spectra (NIRS) with a remote reflectance fibre optic probe, applying the probe directly to the soil samples. Discriminate partial least squares analysis of the near infrared spectra allowed to classify soils with and without residues, regardless of the type of tillage or rotation systems used with a prediction rate of 90% in the internal validation and 94% in the external validation. The NIRS calibration model using a modified partial least squares regression allowed to determine the delta(13)C in soils with or without residues, with multiple correlation coefficients 0.81 and standard error prediction 0.5 per thousand in soils with residues and 0.92 and 0.2 per thousand in soils without residues. The ratio performance deviation for the quantification of delta(13)C in soil was 2.5 in soil with residues and 3.8 without residues. This indicated that the model was adequate to determine the delta(13)C of unknown soils in the -16.2 per thousand to -20.4 per thousand range. The development of the NIR calibration permits analytic determinations of the values of delta(13)C in unknown agricultural soils in less time, employing a non-destructive method, by the application of the fibre optic probe of remote reflectance to the soil sample.

Land use impacts on physical-based soil organic matter fractions on three hillside Ferrasols in Mexico

The effects of cultivation in hillside (> 30% slope) on soil C and N stocks after forest clearance and subsequent cultivation in new re-growth forest sites not always induce rapid soil organic matter (SOM) loss in subtropical areas. In the present study we evaluated the sensitivity to SOM changes of material floatable in water (Fw), coarse sand (> 250 im), fine sand (50-250 im), silt (2-50 im) and clay (0-2 im) fractions by the impact of cultivation of Mexican Ferrasols. This research also determined the relative degree of C and N saturation in the silt and clay size particles. The soil physical fractions were obtained after ultrasonic vibration and sedimentation. Soil organic matter stock (0-20 cm depth) in cultivated sites ranged from 54 to 146 Mg C ha-1 and from 3.9 to 7.1 Mg N ha-1 and in the forest soil 46-94 Mg C ha-1 and 3.7-5.1 Mg N ha-1. The relative distribution of C and N was lower in the coarse sand and highest in the silt fraction; the latter paralleled the changes of whole soil C and N. Coarse sand was the most sensitive fraction responding to cultivation, thus representing a diagnostic pool to assess the SOM shifts under cultivation. Half of our studied soil showed a low degree of C and N saturation in the silt and clay fraction indicating potential for accumulation of C and N, while the other half were well above the saturation limit.

Dynamics of Soil K Release

Abstract It is well known that soil management and potassium (K) fertilizer applications affect soil‐K pools. However, it is not quite clear how K is distributed among K pools in different soil types shortly after its application and after many years of frequent K fertilizer applications. The objective of this work was to determine the size of three soil‐K pools after recent and long‐term K fertilizer applications to identify K distribution patterns. The information generated could be useful for modeling. Soluble and exchangeable K (Ks and Ke) release curves of two Mollisols and an Entisol from Hungary, collected in long‐term experiments (Nagyhörcsök, Kompolt, and Orbottyán), and of Vertisol, Mollisol, and Entisol from Mexico, collected in farmer fields, were determined. The treatments sampled in the Hungarian experiments included checks that had not received K fertilizer or had received very low rates (60–80 kg ha−1 K2O) every second or third year during the last 10–30 yr and plots heavily fertilized with K (1440 up to 8800 kg ha−1 K2O) during the same period. The Ks pool (the sum of several sequential extractions made with CaCl2 [0.01 M]) and Ke pool (the sum of several sequential extractions made with NH4OAc [1N pH7]) were determined in samples without and with added K fertilizer (100 ppm K) before and after a 50‐day incubation (35°C, field capacity) period. Non‐exchangeable K was determined by the HNO3 (1N) procedure. In general, slightly cultivated Mexican soils contained more K than the soil of the long‐term Hungarian experiment either fertilized with K or not. Soil incubation without added K slightly increased the Ks and Ke; however, a relatively large increment was observed in non‐exchangeable K (Kne). When K was added to the soil samples of check plots of Mollisols from Nagyhörcsök and Kompolt before incubation, Ks and Ke increased, but a decrease was observed in the Entisol from Orbottyán. The Ks and Ke released as a function of the sequential extractions followed a negative power function in all cases. When the Ks and Ke values were expressed as a fraction of total Ks or Ke (the sum of all sequential fractions), only one model was necessary to account for all soils and treatments. Hungarian and Mexican soils behave in the same way. These results suggest that the release mechanism is independent of the soil type, clay mineral type, and previous management. The actual Ks and Ke being released in each extraction depends only on the size of their respective pools. This information is useful for modeling purposes.

NIR spectroscopy to identify and quantify imazapyr in soil

The analysis of herbicide residues applied to the soil is commonly conducted by using chromatographic techniques which use organic solvents, are costly and time consuming. Having a simple and inexpensive discriminant method to determine samples with and without herbicide residues would be highly beneficial to lower costs of analysis and save time. The general objective of the present work was to develop such methodology. In particular, the presence of imazapyr was quantified using a near-infrared spectroscopy (NIRS) and a chemometrics approach. The experimental soil for a qualitative experiment of discrimination was Phaeozem with different agricultural managements and the following treatments: dry soil doped with imazapyr, rehydrated soil also doped with the herbicide, and control soil without the chemical. A discriminant algorithm was developed to allow the identification of soils containing imazapyr and those which did not contain the herbicide. The qualitative approach was based on a discriminant partial least squares (DPLS). An accurate identification of 100 percent of the soils containing imazapyr and 98 percent of those which did not contain the pesticide was achieved. Quantification of imazapyr was performed in five soil types (Andosol, Vertisol, Acrisol, Cambisol and Phaeozem) with different concentrations of the herbicide with an equation generated by modified partial least squares (MPLS) regression. As a reference for the above analytical method, a high performance liquid chromatography (HPLC) analysis was performed. The resulting equation had a RSQ of 0.81 and a prediction capacity (RPD) of 2.4. The results showed that with NIR spectroscopy it was possible to discriminate soils that contained imazapyr from those that did not contain the herbicide. This finding allows for a reduction in the number of samples to be evaluated to determine their concentration. Moreover, with this technique it is also possible to estimate the amount of herbicide in the soil with similar results to those accomplished using the reference method (HPLC), but faster and at lower cost.