Abstract The reliability of sequential phosphorus fractionation in calcareous soils remains insufficiently evaluated, particularly in resolving mineral P fractions beyond the liable pool. This study investigated the influence of increasing CaCO 3 content on inorganic phosphorus (IP) distribution and fraction‐specific response behavior in a clay soil from the central clay plain of Sudan. Surface soil (0–25 cm) from Attra (Gezira State) was amended with CaCO 3 at 1%, 1.8%, 3%, 5%, and 6% (w/w). Inorganic P fractions, including loosely bound P (LO‐P; NH 4 Cl), Al‐bound P (NH 4 F), Fe‐bound P (NaOH), reductant‐soluble P (citrate–dithionite–bicarbonate), and Ca‐bound P (H 2 SO 4 ), were sequentially extracted and quantified using the ascorbic acid method. Data were analyzed using analysis of variance and segmented regression analysis was applied to identify threshold‐dependent responses along the CaCO 3 gradient; the statistical significance was declared at p ≤ 0.05. Increasing CaCO 3 significantly altered IP distribution, but responses were highly fraction‐dependent and nonlinear. Segmented regression revealed distinct breakpoint behavior between 1% and 3% CaCO 3 across fractions, indicating a shift in phosphorus dynamics beyond critical carbonate levels. Post‐threshold responses differed markedly among fractions, with LO‐P exhibiting accelerated depletion, iron‐bound phosphorus (Fe‐P) showing an enhanced depletion rate, aluminum‐bound phosphorus (Al‐P) and reductant‐soluble phosphorus attenuated depletion, and Ca‐demonstrating stabilized positive accumulation. Total inorganic phosphorus (T‐IP) showed an integrated accelerated depletion pattern, confirming system‐level sensitivity to CaCO 3 enrichment. Overall, the results demonstrate that the non‐calcareous inorganic P fractionation procedure showed fraction‐specific responses to increasing CaCO 3 concentrations, with a critical threshold of approximately 3% observed for Fe‐P, Al‐P, Ca‐P, and total inorganic P. Beyond this threshold, CaCO 3 was associated with nonlinear changes in P distribution among the inorganic P fractions.
Abstract Crops grown on over 40% of arable land are limited by low levels of bioavailable phosphorus (P). Additionally, about 80% of applied P becomes fixed in soil through sorption, precipitation, and complexation with humic–metal complex surfaces. This accumulated P, excluding geogenic P, is known as “legacy P” and can contribute to crop's needs if it can be made bioavailable. However, the understanding needed to manage the bioavailability of legacy P for crop is hindered by lack of information on P chemical forms in soil. To address this gap, this study evaluated three soil types—acidic (inceptisols), organic (histosols), and calcareous (entisol) from three distinct areas of the United States which vary in biogeography, economic conditions, and land uses for comparison. The objective was to characterize the legacy P species across three locations with different land uses using solution‐state 31 P nuclear magnetic resonance spectroscopy, coupled with indicators of P dynamics and potential bioavailability, including phosphatase activity and Mehlich‐3 P. Inorganic orthophosphate (20%–96%) and monoesters (2%–80%) were dominant forms in acidic soil (mean 7% organic matter), whereas in calcareous soil, orthophosphate was the dominant form, ranging from 79% to 100%. In organic soil, organic monoesters and diesters collectively predominated over orthophosphate, ranging from 18% to 70%, with a high average phosphomonoesterase activity of 2.81 µmol pNP/g soil/h. Overall, legacy P was mainly inorganic and more labile in acidic and calcareous soils, whereas in organic soils it was less mobile because of relatively high recalcitrant P forms and relatively high phosphomonoesterase activity.
Abstract Soils change over time and most change is accelerated by agricultural activities. Short‐term changes have been well‐documented, but there is a dearth of long‐term data. Here we report on continuously cultivated fields in southern Wisconsin in which soils were sampled 12 years apart, and we compare the change to soils under adjacent forest that were never cultivated. Total carbon (TC) had decreased significantly at 0–20 cm from 2.2% to 1.6% over the 12‐year period, but no statistical difference in TC stocks was noted as the bulk density of the soils increased. Changes in soil texture and Ap‐horizon thickness were small. Spatial patterns of soil properties remained controlled by topography, drainage, and erosion–deposition processes. Mid‐infrared spectra confirmed that the most significant temporal changes occurred in the organic fraction rather than the mineral matrix. Paired sampling indicated substantially higher pH and carbon contents in cultivated soils relative to adjacent soils under forest. Annual rates of change demonstrated that carbon‐related properties changed more rapidly than morphological or textural properties. These findings demonstrate that, even after more than 150 years of continuous cultivation, soilscape patterns remain strongly influenced by topography, whereas agricultural management has primarily affected soil carbon.
Abstract Nucleotide‐based organic phosphorus (P o ) cycling in the environment contributes to significant amounts of plant‐available inorganic phosphorus (P i ). Enzymatic degradation of P o to plant‐available soluble P i can be hindered when P o is adsorbed on soil minerals. Thus, understanding adsorption mechanisms of nucleotides, one of the major sources of P o in soil, could help in predicting biogeochemical cycling of phosphorus in the environment. Most published studies reported that the phosphate moiety of the nucleotide is the principal reactive component in the adsorption mechanism, ignoring the participation of purine or pyrimidine moieties of the nucleotides in the adsorption mechanisms. Thus, to evaluate the importance of these moieties in the adsorption mechanisms, we studied the surface interaction mechanisms of a selected ribonucleotide, adenosine‐5′‐monophosphate (AMP), on gibbsite, a common aluminum hydroxide mineral present in the soil. Both macroscopic adsorption and attenuated total reflectance Fourier transform infrared spectroscopic studies were employed to determine the role of the purine moiety (i.e., the adenine portion of AMP) in the adsorption mechanism. Our results indicated a significant IR band shift at location 1460 cm −1 in aqueous AMP to 1479 cm −1 in gibbsite‐adsorbed AMP, suggesting the involvement of purine moieties in the adsorption mechanism. Spectroscopic results of adenine adsorption on gibbsite corroborated our findings for AMP. Our results will help to improve future research aimed at in‐depth understanding of adsorption mechanisms of nucleic acids and nucleotides on soil minerals.
Abstract Soil organic matter (SOM) is crucial for soil fertility and is strongly linked to soil productivity. The heterogeneity of soil at different depths leads to variations in spectral characteristics. This study addresses the challenge of using near‐infrared (NIR) spectroscopy to estimate SOM across heterogeneous soil profiles. Analyzing spectral data from soil cores (≈150 cm), samples were categorized into surface (0–30 cm), subsurface (>30 cm), and complete profile datasets. Optimal models for each layer were achieved using partial least squares regression with Savitzky–Golay smoothing and variable selection via the uninformative variable elimination‐successive projections algorithm. These layer‐specific models accurately predicted SOM within their own layer but performed poorly when directly applied to the contrasting layer. The spiking method significantly improved this cross‐layer prediction. To balance cost and accuracy, we recommend incorporating 20 (surface) and 30 (subsurface) spiked samples into the respective calibration sets. This increased the prediction coefficient of determination () by 11.0% and 1.6% and reduced the root mean square error of prediction by 29.7% and 6.7%, compared to unspiked models. The effectiveness of spiking was inversely related to calibration set size, and an extra‐weighting technique offered no additional benefit for small sets. Furthermore, calibration models constructed from the combined profile dataset generally provided more robust predictions across various depths than models based on individual layers. This work provides a practical framework for accurate SOM estimation throughout the soil profile and validates spiking as an effective strategy to enhance the transferability of NIR spectroscopy models between distinct soil layers.
