As a proximal soil sensing technique, laboratory visible and near-infrared (Vis-NIR) spectroscopy is a promising tool for the quantitative estimation of soil properties. However, there remain challenges for predicting soil phosphorus (P) content and availability, which requires a reliable model applicable for different land-use systems to upscale. Recently, a one-dimensional convolutional neural network (1D-CNN) corresponding to the spectral information of soil was developed to considerably improve the accuracy of soil property predictions. The present study investigated the predictive ability of a 1D-CNN model to estimate soil available P (oxalate-extractable P; Pox) content in soils by comparing it with partial least squares (PLS) and random forest (RF) regressions using soil samples (n = 318) collected from natural (forest and non-forest) and cultivated (upland and flooded rice fields) systems in Madagascar. Overall, the 1D-CNN model showed the best predictive accuracy (R2 = 0.878) with a highly accurate prediction ability (ratio of performance to the interquartile range = 2.492). Compared to the PLS model, the RF and 1D-CNN models indicated 4.37% and 23.77% relative improvement in root mean squared error values, respectively. Based on a sensitivity analysis, the important wavebands for predicting soil Pox were associated with iron (Fe) oxide, organic matter (OM), and water absorption, which were previously known wavelength regions for estimating P in soil. These results suggest that 1D-CNN corresponding spectral signatures can be expected to significantly improve the predictive ability for estimating soil available P (Pox) from Vis-NIR spectral data. Rapid and accurate estimation of available P content in soils using our results can be expected to contribute to effective fertilizer management in agriculture and the sustainable management of ecosystems. However, the 1D-CNN model will require a large dataset to extend its applicability to other regions of Madagascar. Thus, further updates should be tested in future studies using larger datasets from a wide range of ecosystems in the tropics.

Given the finite nature of P fertilizer resources, it is imperative to investigate effective P management practices in order to achieve sustainable rice production. This study was conducted (1) to assess the effect of dipping rice seedlings in P-enriched slurry before transplanting (P-dipping, hereafter) on initial plant growth and (2) to determine the optimum P concentration and dipping duration. In the P-dipping treatments, four P2O5 concentrations in the slurry (4.3%, 5.0%, 6.0%, and 7.5%) and four dipping durations (0.5 h, 2 h, 4 h, and 8 h) were investigated. After the treatments, the seedlings were transplanted into 1/5000 Wagner pots and grown under flooded conditions for 42 days and they were compared with plants under conventional P incorporation at the rate of 300 mg P2O5 pot−1 and with plants under no P application. The amount of P2O5 attached to P-dipped seedlings, or locally applied in the rhizosphere at transplanting, increased with higher P concentrations in the slurry, ranging from 87.5 to 112.2 mg pot−1. Shoot biomass at 42 days after transplanting (DAT) was greatly increased in plants under the P-dipping treatments, compared to that in plants with no P application and was comparable to or greater than that in plants under conventional P incorporation, even when P levels were 2.5 to 3 times lower. Among the P-dipping treatments, we observed some significant effects of P concentrations and dipping durations on seedling P uptake and shoot biomass, without any interaction between these variables. Seedling P uptake and biomass tended to be higher with higher P concentrations in slurry and longer dipping durations. Conversely, the shoot biomass at 42 DAT was significantly lower in plants under the highest P concentration treatment (7.5% P2O5) compared to that in other plants and tended to be lower with longer dipping durations (4 h and 8 h). These negative effects can be attributed to the slow recovery from transplanting shock because of the chemical damage of seedlings exposed to higher salt concentrations for longer durations. The present study highlights that (1) P-dipping could be an effective approach to increase transplanted rice production with minimal P inputs, and (2) this effect could be higher with a low P-concentration in the slurry (4.3% P2O5) and a short dipping duration (0.5 h). Based on the obtained results, further on-farm trials are expected to assess farmers’ appreciation and the potential constraints of adopting this technique.

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Purpose The response of rice plants to P deficiency together with rhizosphere microorganisms is yet to be explored in flooded soil cultures. This study aimed to identify how alterations in bacterial diversity are associated with underground metabolic and chemical reactions in the rhizosphere of rice. Methods Two rice varieties were grown in a split-root box under flooded conditions and filled with either P-applied or non-P-applied soils with or without γ-irradiation. At 41–42 days after transplanting, plant biomass, P uptake, exudation rates of total carbon and low-molecular-weight organic acids (LMWOAs) from the roots, and bacterial diversity, P fractions, and phosphatase activity in the rhizosphere were determined. Results γ-irradiation significantly decreased plant P uptake and biomass of both varieties and reduced the relative abundance of Alphaproteobacteria and microbiome beta diversity in the rhizosphere. Concurrently, the use of insoluble P in soils was evidently reduced by irradiation. These microbiological and chemical changes in the γ-irradiated rhizosphere occurred in both P-applied and non-P-applied soils. On the other hand, the proportion of LMWOAs and exudation of citric acid, high in P-solubilization capacity were increased in the γ-irradiated and P-applied rhizosphere soils at the expense of other carbons. No differences were detected in phosphatase activity in any treatments. Conclusion Dysbiosis in the rhizosphere microbiome negatively affected rice growth and the use of insoluble P pools in flooded soil cultures. With a reduction in microbiome diversity, rice plants may have a complementary strategy to increase the exudation proportion of citric acid toward the P-rich soil area.