Feng Gu

The semi-arid region of North China has undergone extensive afforestation to prevent land degradation. Although afforestation was considered an effective way to improve soil water retention, the mechanism by which it affects soil hydraulic properties remained uncertain. In this study, soil water retention curve (SWRC), soil water-stable aggregates, and other soil physicochemical properties were determined in short-term abandoned cropland (AC), shrubland (SL), and woodland (WL) that had been converted from cropland for 1, 8, and 24 years, respectively. Pearson correlation analysis and partial least-squares structural equation modeling methods were used to identify the main factors affecting soil hydraulic properties. Results showed that the SWRCs of all three land uses were well-fitted by a double-exponential model. The WL and SL land uses exhibited higher soil field capacity (0.33–0.37 cm³ cm⁻³), wilting point (0.20–0.23 cm³ cm⁻³), and available water content (0.13–0.15 cm³ cm⁻³). Surface soil exhibits a more pronounced trend in water retention capacity changes compared to subsoil under vegetation restoration. The WL and SL land uses showed more soil macroaggregates and intra-aggregate pores at surface layers, which mainly explained the variations in hydraulic properties. The main factors influencing soil hydraulic properties were soil aggregates, matrix and structural porosity, soil organic carbon (SOC), and soil bulk density (BD). Overall, afforestation can improve soil hydraulic properties and could be an effective practice for soil and water conservation in the semi-arid region of North China.

Biochar, which contains abundant nutrients like phosphorus (P), is considered a potential source of nutrients for plant use after its application in soil. However, the localized distribution of P in the vicinity of biochar (that is, the charosphere) is poorly understood. In this study, the biochars derived from pinewood (PWB), maize straw (MSB), and chicken manure (CMB) were granulated and investigated for their short-term charosphere effect, using an integrated imaging technique in situ. The results showed that biochars significantly elevated the soil pH and P concentration around their granules, leading to the development of a charosphere over several days. Both pH and P were distributed in a gradient in the charosphere: the values were highest where the biochars were located and declined with increasing distance from the biochar. A highly positive correlation (p < 0.01) was found between the pH, electric conductivity (EC), ash content of the biochar, and the scope of the pH gradient in the charosphere. Moreover, the scope of the P gradient was positively related to the pH and EC values of the biochar as well (p < 0.05). The charosphere effect was prominent around the CMB and MSB granules because of their high ash content, an especially soluble component. The P diffusion in the charosphere was inferred to be influenced by the soil pH as well as the endogenous P species in the biochars. These findings provide new insight into the localized effect of biochars in soil, which is conducive to understanding the nutrient availability to plants after biochar application.

Core Ideas: Calcareous concretions (CCs) could maintain a considerable amount of water and the water content of CCs decreased with an increase in CC sizes. The addition of CCs to soils changed the WRC and the pore distribution of soils significantly. The available water content (AWC) of soil–CC mixtures decreased when the CC content increased. A linear equation, which included CC content and CC water content, was proposed to estimate the AWC of soils containing CCs accurately. The soil water retention curve (WRC) is one of the most important properties in soil hydraulic studies. Current studies normally focus on fine soils, ignoring the water‐holding capacity of coarse fragments (>2 mm) and their effects on soil available water content (AWC). Calcareous concretions (CC) are coarse inclusions distributed widely in soils around the world. This study was conducted to quantify the effects of the proportion and size of CCs on the soil WRC and AWC of a calcic vertisol. The WRCs of CCs were measured using the sand box and ceramic plate method, and the WRCs of fine soils and soil–CC mixtures of various sizes (2–5, 5–8, and 8–30) and proportions (5, 10, 15, and 30% on a total mass basis) of CC were determined with the centrifuge method. The WRCs were fitted with the van Genuchten equation. The results showed that CCs retained a considerable amount of water. The saturated water contents of CCs of 2‐ to 5‐, 5‐ to 8‐ and 8‐ to 30‐mm were 0.251, 0.220, and 0.196 cm ³ cm ^–3 , respectively. The addition of CCs changed the WRC and pore distribution of soils significantly. The AWC of soil–CC mixtures decreased with increasing CC content. A linear equation, which included the fraction of CCs and water content in CCs, was proposed to estimate the AWC of soils containing CCs accurately. Accounting for CCs in the calcic vertisol improved the accuracy of soil hydraulic property estimations.