Volcanic eruptions affect land and humans globally. When a volcano erupts, tons of volcanic ash materials are ejected to the atmosphere and deposited on land. The hazard posed by volcanic ash is not limited to the area in proximity to the volcano, but can also affect a vast area. Ashes ejected from volcano’s affect people’s daily life and disrupts agricultural activities and damages crops. However, the positive outcome of this natural event is that it secures fertile soil for the future. This paper examines volcanic ash (tephra) from a soil security view-point, mainly its capability. This paper reviews the positive aspects of volcanic ash, which has a high capability to supply nutrients to plant, and can also sequester a large amount of carbon out of the atmosphere. We report some studies around the world, which evaluated soil organic carbon (SOC) accumulation since volcanic eruptions. The mechanisms of SOC protection in volcanic ash soil include organo-metallic complexes, chemical protection, and physical protection. Two case studies of volcanic ash from Mt. Talang and Sinabung in Sumatra, Indonesia showed the rapid accumulation of SOC through lichens and vascular plants. Volcanic ash plays an important role in the global carbon cycle and ensures soil security in volcanic regions of the world in terms of boosting its capability. However, there is also a human dimension, which does not go well with volcanic ash. Volcanic ash can severely destroy agricultural areas and farmers’ livelihoods. Connectivity and codification needs to ensure farming in the area to take into account of risk and build appropriate adaptation and resilient strategy.

This study aims to assess the performance of a low‐cost, micro‐electromechanical system‐based, near infrared spectrometer for soil organic carbon (OC) and total carbon (TC) estimation. TC was measured on 151 soil profiles up to the depth of 1 m in NSW, Australia, and from which a subset of 24 soil profiles were measured for OC. Two commercial spectrometers including the AgriSpecTM (ASD) and NeoSpectraTM (Neospectra) with spectral wavelength ranges of 350–2,500 and 1,300–2,500 nm, respectively, were used to scan the soil samples, according to the standard contact probe protocol. Savitzky–Golay smoothing filter and standard normal variate (SNV) transformation were performed on the spectral data for noise reduction and baseline correction. Three calibration models, including Cubist tree model, partial least squares regression (PLSR) and support vector machine (SVM), were assessed for the prediction of soil OC and TC using spectral data. A 10‐fold cross‐validation analysis was performed for evaluation of the models and devices accuracies. Results showed that Cubist model predicts OC and TC more accurately than PLSR and SVM. For OC prediction, Cubist showed R2 = 0.89 (RMSE = 0.12%) and R2 = 0.78 (RMSE = 0.16%) using ASD and NeoSpectra, respectively. For TC prediction, Cubist produced R2 = 0.75 (RMSE = 0.45%) and R2 = 0.70 (RMSE = 0.50%) using ASD and NeoSpectra, respectively. ASD performed better than NeoSpectra. However, the low‐cost NeoSpectra predictions were comparable to the ASD. These finding can be helpful for more efficient future spectroscopic prediction of soil OC and TC with less costly devices.

The south-western slope of Anak Krakatau collapsed after the eruption on December 22nd, 2018 and reshaped the volcanic island landscape. This work focused on determining the geomorphological features of Mt. Anak Krakatau before and after the eruption. A total of 71 lapilli and 17 volcanic ash samples were collected from Anak Krakatau and Panjang islands on February 23, 2019, and March 14, 2019. Sentinel-2 and Planet Scope images were utilized to monitor thermal activities and the changes of the coastlines. Google Earth Pro was capitalized to determine the rills and gullies formation. After the December 2018 eruption, the height of Anak Krakatau was reduced from 258 to 126 m and, about 76 x 106 m3 of materials were eroded to the sea. The eruption caused Anak Krakatau to be covered by unconsolidated volcanic materials. About 214 of rills (dimension of 380 to 851 m and 30 to 100 cm) and 35 of the gully features (length from 150 to 841 m and width from 0.5 to 13 m) run from the highest peak to the coastline. This work can serve as a reference for predicting potentially disastrous events such as Anak Krakatau, which shows growth and destruction can be observed using remote sensing techniques.

Volcanic activity produces pyroclastic deposits when erupted and cover the surrounding area. The minerals contained in these deposits are the source of plant nutrients. The volcanic deposits weathered, release nutrients to the environment, and improve soil chemical properties. The eruption of Mt. Sinabung in 2018 covered an area of 30, 320 ha, while in 2019 was 1, 371 ha. The study aims to investigate the status of nutrient content and the volcanic ash weathering level in 2020. There were 16 samples taken from ash deposits at various depths, with a total area of 1, 585.31 ha. Samples were analyzed to determine the total elemental composition using X-ray fluorescence (XRF) spectrometer, nutrient reserves, and weathering indices. The results showed that the total elemental composition of SiO2 is 51.51-67.51% classified as mafic (basalt) to felsic (dacite) materials, Al2O3; 15.54-23.41%, Fe2O3; 2.84-10.02% and CaO; 3.94-6.46%. Mount Sinabung’s volcanic ash has a nutrient reserve capacity of MgO, CaO, P2O5, K2O, and SO3, respectively with the amount of 37, 384.17 kg/ha, 235, 794.99 kg/ha, 34, 293.12 kg/ha, 72, 357.39 kg/ha, and 70, 352.22 kg/ha. The weathering indices of volcanic ash of 2020 were determined with a value of 2.76-4.19 for Ruxton ratio and Product of Weathering Index (PWI) of 67.39-76.13, indicates the weathering rate of silicates from volcanic ash are still at initial stage and are still in the fresh condition.

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