Search

Roots naturally exert axial and radial pressures during growth, which alter the structural arrangement of soil at the root–soil interface. However, empirical models suggest soil densification, which can have negative impacts on water and nutrient uptake, occurs at the immediate root surface with decreasing distance from the root. Here, we spatially map structural gradients in the soil surrounding roots using non‐invasive imaging, to ascertain the role of root growth in early stage formation of soil structure. X‐ray computed tomography provided a means not only to visualize a root system in situ and in 3‐D but also to assess the precise root‐induced alterations to soil structure close to, and at selected distances away from the root–soil interface. We spatially quantified the changes in soil structure generated by three common but contrasting plant species (pea, tomato, and wheat) under different soil texture and compaction treatments. Across the three plant types, significant increases in porosity at the immediate root surface were found in both clay loam and loamy sand soils and not soil densification, the currently assumed norm. Densification of the soil was recorded, at some distance away from the root, dependent on soil texture and plant type. There was a significant soil texture × bulk density × plant species interaction for the root convex hull, a measure of the extent to which root systems explore the soil, which suggested pea and wheat grew better in the clay soil when at a high bulk density, compared with tomato, which preferred lower bulk density soils. These results, only revealed by high resolution non‐destructive imagery, show that although the root penetration mechanisms can lead to soil densification (which could have a negative impact on growth), the immediate root–soil interface is actually a zone of high porosity, which is very important for several key rhizosphere processes occurring at this scale including water and nutrient uptake and gaseous diffusion. By high resolution imagery, we observed across the three plant types significant increases in porosity at the immediate root surface, irrespective of soil texture and bulk density, and not soil densification, the currently assumed norm. This has profound implication for several key rhizosphere processes occurring at this scale including water and nutrient uptake and gaseous diffusion that should be considered in plant breeding efforts. [ABSTRACT FROM AUTHOR]

Soil structure influences the soil hydraulic properties, aeration and resistance to root penetration. Many indicators can be used to investigate the changes of soil structure, but using a single indicator or method has limitations. This study evaluates the effects of 15-years tillage management on soil structure with soil bulk density (BD) , penetrometer resistance (PR), soil water retention curve (SWRC), least limiting water range (LLWR) and X-ray computed tomography (CT). The treatments include no-tillage (NT), rotary tillage (RT), and mouldboard plough (MP). Soil bulk density of the 0–30 cm soil profile was measured after the establishment of tillage treatments for 7, 9, 12 and 14-years. The values of PR , SWRC, LLWR, and 3-D pore geometry of the 5–10 and 15–20 cm layers were determined in the 15th year of the experiment. The greater BD in the 0–5 cm layer of NT was not observed after 9 years, but an increased BD in the 5–20 cm layer was found in most years. Tillage had no effect on PR in wet soil. When soil had dried, the NT treatment had a greater PR and it was the lower limiting factor of LLWR, ultimately leading to a narrower LLWR of NT. Both NT and RT showed lower proportions of structural but higher textural pores. The macroporosity (> 50 µm) from X-ray CT was greater than that estimated from SWRC of the 15–20 cm layer. The characteristics of pore morphology were similar under three tillage treatments. Therefore, NT resulted in a narrower LLWR due to the denser soil layer, yet similar macropores may alleviate the negative impacts on crop growth caused by lower LLWR. It was suggested that using LLWR and characteristics of macropores from X-ray CT at the same time is a suitable way to assess soil structure under various tillage practices. • After 15-years no-tillage, the physical quality is poor in 5–20 cm layer reflected by greater BD, PR, and lower LLWR. • Similar properties of macropore weak the usefulness of LLWR for assessing soil structure under no tillage treatment. • Combining LLWR with macropores from X-ray CT is a suitable way to assess soil structure under various tillage practices. [ABSTRACT FROM AUTHOR]

• Aggregates taken from the longest experiments in the world were scanned using X-ray CT. • Transport ability of each aggregate was calculated from pore-scale simulations. • Soil organic carbon and bulk diffusion coefficient of the aggregates are positively correlated. • Bulk diffusion coefficient of the aggregates tends to plateau as soil organic carbon increases. A key finding in soil carbon studies over the past decade is that soil organic carbon (SOC) stabilization is not controlled by its molecular complexity or adsorption to clay, but by its physicochemical protection including occlusion in aggregates and sorption-precipitation with organo-mineral associations. The organo-mineral complexes and the adsorbed SOC can be dissolved microbially under anoxic conditions, which is an important pathway in carbon cycle but has been overlooked by most carbon models. As organo-mineral associations are reported to form in aerobic conditions and can be lost under anaerobic conditions, there should be a positive correlation between SOC and ability of the aggregates to transport dissolved oxygen. We develop a simulation model to test this using soil structural data from two long-term experiments which naturally created a SOC gradient: One is a winter wheat experiment established in 1843 to compare the effects of different fertilizations on the yield of winter wheat and the other one is a ley-arable experiment established in 1948 to investigate the consequence of cropping system changes for ecological yield. Aggregates from different treatments on the two experiments were scanned using X-ray Computed Tomography to simulate oxygen transport using a pore-scale model. We compared porosity and diffusion coefficient of all aggregates and linked them to SOC measured from the two experiments. The agronomic practice changes which occurred 67 or 172 years ago substantially reshaped the intra-aggregate structure (<2 mm), and the accrual of SOC is positively correlated with diffusion coefficient of the aggregates to transport oxygen. However, the diffusion coefficient increases with SOC asymptotically, plateauing when SOC exceeds a threshold value. We also found the diffusion coefficient of the aggregates in chemically fertilized soils trended with their porosity approximately in the same way, deviating from those for other non-cropped treatments or fertilized with farmyard manure. [ABSTRACT FROM AUTHOR]

• Erosion induced greater total and labile SOC storage on the depositional position. • Old C contributed to a large proportion of labile SOM on the eroding position. • New C was preferentially preserved in macroaggregates. • Aggregates played important roles in the physical sequestration of SOC. • Both silt and clay fractions and aggregates protected labile SOM. The characteristics of soil organic matter (SOM) such as stocks, sources and stability, are important for understanding soil functioning and C cycling in an eroding field. In this study we investigated the characteristics of SOM in bulk soil and aggregates collected from three positions (shoulder, back, and foot) on a sloping field in Northeast China using stable and radioactive isotope (13C and 14C) and thermogravimetry (TG) techniques. The field had originally been populated with C 3 grassland plants before conversion to C 4 corn dominated arable cropping approximately 80 years ago. Soil samples were collected from 0–120 cm soil profiles in 20 cm depth intervals. Erosion rates were estimated by 137Cs analysis to be 0.21 cm yr–1 and 0.08 cm yr–1 at the shoulder and back positions, respectively. A greater proportion of small macroaggregates (2–0.25 mm) and microaggregates (0.25–0.053 mm) were found in the topsoil and subsoil layers at the foot position compared with the shoulder position. The concentration and storage of soil organic C (SOC) in the bulk soil was significantly lower at the shoulder and back positions compared with the foot position. Greater thermal-labile SOM concentration and storage were observed at the foot position compared with the shoulder and back positions. Positive relationships between C 4 -derived SOC and the proportion of thermal-labile SOM suggested that most thermal-labile SOM was C 4 -derived from the recent corn (young SOC), while most stable SOM fractions were C 3 -derived from previous grassland vegetation (old SOC). However, labile SOM from old SOC was also an important part of total thermal-labile SOM, particularly in deep soil horizons (below 40 cm) at all sampling positions. Small macroaggregates had a greater SOC content, δ 13C, and thermal-labile SOM proportion, and younger SOC age, compared with microaggregates and silt + clay (<0.053 mm) fractions, except at the 40–60 cm depth at the foot position. As the mass of aggregates accounted for 32–50% of the bulk soil, about half of thermal-labile and total SOC were stored as associations with silt + clay fractions at erosional and depositional points. Our results indicated that: (1) more thermal-labile and total SOM were stored at depositional position, (2) compared with new carbon sources, old SOM contributed to a large proportion of thermal-labile SOM on an eroding slope, particularly in deep soil, and, (3) soil aggregates and silt + clay fractions had equivalent roles in SOC sequestration in both erosional and depositional locations. [ABSTRACT FROM AUTHOR]