Journal Paper Digests 2026 #17
- Assessing Precipitation Effectiveness in the Context of Drought
- From Light Scan to Flow: Estimating Soil Hydraulic Properties Through VNIR-SWIR Spectroscopy and Calibration-Free PTFs
- Trends in Soil Health in the European Union
- Clay content, not microbial community composition, regulates carbon stabilisation along a soil carbon and texture gradient impacted by reduced precipitation
Clay content, not microbial community composition, regulates carbon stabilisation along a soil carbon and texture gradient impacted by reduced precipitation
Soils are the largest terrestrial carbon (C) reservoir and play a crucial role in climate regulation; yet, sustaining soil organic carbon (SOC) stocks in agricultural systems remains a challenge. Although advances have been made in understanding SOC dynamics, the mechanisms controlling C stabilisation and decomposition remain uncertain, especially regarding the interplay of SOC content, soil texture, microbial community composition and environmental stressors such as drought. In this study, we addressed the key question whether high-SOC, high-clay soils support greater C decomposition of added litter due to additional breakdown of native SOC, or whether these soils have smaller C decomposition due to more C stabilisation. Additionally, the legacy effect of 120-day reduced precipitation was studied. We conducted a six-month incubation study using soils collected in a barley field along a natural gradient of SOC, clay and pH, with precipitation reduced by rainout shelters. Soils were amended with 13C-enriched plant litter to trace fresh C inputs into different C pools. We found that litter-derived CO2 production was highest in coarse-textured, low-SOC soils, supporting our hypothesis that clay content controls C stabilisation and decomposition. Accordingly, high-SOC, high-clay soils supported increased formation of mineral-associated organic carbon (MAOC), indicating rapid stabilisation and protection of fresh C by minerals. MAOC formation efficiency correlated positively with clay content and native SOC, suggesting that increased C input could further enhance SOC storage. Reduced precipitation did not affect litter decomposition or C stabilisation, indicating that short-term precipitation reduction has little effect on soil C turnover in cool-humid upland soils. Metagenomic and amplicon analyses revealed that microbial community structure and functional potential were largely stable across the gradient and unaffected by reduced precipitation. Our findings suggest that clay content, rather than native SOC or microbial community composition, is the primary factor shaping litter-C turnover and stabilisation on a field scale.
Trends in Soil Health in the European Union
Healthy soils are essential for our environment and society, as they deliver crucial ecosystem services. However, soils across the European Union (EU) are increasingly affected by multiple soil degradation processes. As a result of these increasing degradation incidences, recent EU policy initiatives have been taken to protect and restore soils, such as the good agricultural and environmental conditions under the Common Agricultural Policy, as well as the Soil Monitoring and Resilience Directive, the Mission Soil and the EU Soil Strategy for 2030, aiming to reach healthy soils by 2050. Soil monitoring through time as well as studying changes in soil status is needed to verify whether EU soils are on track to reach a healthy state by 2050. In this study, trends of seven soil degradation processes were analysed, using EU-scale spatial layers and data points produced from harmonised repeated monitoring surveys. Results suggest that the range of unhealthy soils decreased in the past decades regarding three indicators, namely soil erosion by water, harvest erosion and phosphorus deficiency. Conversely, results suggest that the area with unhealthy soils increased regarding loss of soil organic carbon, phosphorus excess and soil sealing. No clear trends were observed for soil pH. The temporal coverage of the available datasets only marginally covers the period since the start of the Mission Soil and the EU Soil Strategy. Thus, evaluating the effectiveness of these policy initiatives in protecting soils was not possible in this study. Projections suggest that four studied soil degradation processes will further deteriorate in the upcoming decades due to climate change, continued unsustainable management practices, land use changes and urbanisation. This shows the importance of the Soil Monitoring and Resilience Directive, the EU Soil Strategy and the Mission Soil to continue the investment in soil monitoring and related research, to accelerate the implementation of sustainable soil management practices, and to bring all indicators on track to reach healthy soils by 2050.
From Light Scan to Flow: Estimating Soil Hydraulic Properties Through VNIR-SWIR Spectroscopy and Calibration-Free PTFs
This study presents a three-step approach for estimating soil-water retention (WRF) and hydraulic conductivity functions (HCF) by integrating VNIR-SWIR reflectance spectroscopy with semi-physical, calibration-free pedotransfer functions (PTFs). A total of 135 soil samples from the Alento Observatory (southern Italy) were analyzed. In the first step, diffuse reflectance spectroscopy was used to derive particle-size distribution (PSD) by combining five spectral pretreatment algorithms and three machine learning techniques. The best-performing model demonstrated excellent predictive accuracy, achieving a coefficient of determination (R2) of 0.945 and a Root Mean Square Error (RMSE) of 0.051 for PSD mass fractions. In the second step, two semi-physical PTFs—the Arya-Heitman (PTFWRF-AH) and Mohammadi-Vanclooster (PTFWRF-MV) models—were applied to estimate the WRF using spectrally-derived PSD together with measured soil bulk density and saturated water content. The PTFWRF-MV model outperformed the AH variant, yielding a lower RMSE (0.049 cm3 cm−3) and higher R2 (0.676). The third step involved estimating the HCF by applying the Arya and Heitman PTF alongside measured saturated hydraulic conductivity values. This process relied on the flow-similarity hypothesis, which assumes that water flow partitioning within pore domains is equivalent across idealized and natural-structure soils. However, the HCF predictions for both models exhibited uncertainties greater than one order of magnitude and R2 values under 0.50. These findings underscore the efficacy of spectroscopy for soil texture characterization, while highlighting persistent limitations in predicting hydraulic properties of structured soils, likely due to violations of flow-similarity assumptions.
Assessing Precipitation Effectiveness in the Context of Drought
The effect of intense rainfall on drought is complex and less well understood than impacts on flooding. Limits to water storage under heavy rainfall can result in less infiltration and more runoff with increasing rainfall intensity. This response to increasing precipitation intensity can reduce the usefulness of widely used precipitation-focused metrics for drought monitoring and early warning. Precipitation effectiveness could provide a useful framework to account for the impact of precipitation intensity on drought conditions and soil moisture recharge. In this study, we assess how precipitation effectiveness varies across space and time and define the impact of rainfall intensity using station observations of precipitation and soil moisture at 3 sites in Illinois. We develop a random forest (RF) model to estimate precipitation effectiveness and compare the model to both observations and precipitation effectiveness estimates using the SCSC Curve Number (CN) method. Observed precipitation effectiveness substantially varies between precipitation events and growing seasons at all three study sites, and this variability is not well represented in total precipitation. We find the RF method can provide a useful measure of the effectiveness of precipitation events for maintaining, improving, or deteriorating drought conditions, especially in lieu of dense, widespread root zone soil moisture observations. Further evaluation and optimization of the RF method for different climates, soil types, and land uses, as well as over larger spatial scales like watersheds is necessary to realize the operational monitoring capabilities of precipitation effectiveness for drought monitoring in a changing climate.