Drying-wetting cycles influence both soil organic matter (SOM) decomposition and rhizosphere processes. Rhizosphere processes also affect SOM decomposition through rhizosphere priming. However, little is known about how drying-wetting cycles regulate SOM decomposition with rhizosphere priming, because most previous studies incubated root-free soils and omitted the rhizosphere effect. To investigate the effect of drying-wetting cycles on SOM decomposition in the presence of plants, we grew sunflower (Helianthus annuus) and soybean (Glycine max) in a sandy loam soil under the treatments of either constant moisture or 12 drying-wetting cycles, and used a continuous 13C-labeling method to partition soil respiration into rhizosphere respiration and SOM decomposition. We found that compared to the constantly-moist treatment, the severe drying-wetting cycles in soils planted with sunflower significantly reduced shoot biomass (32%), root biomass (52%), rhizosphere respiration (29%), and SOM decomposition (22%), while the moderate drying-wetting cycles in soils planted with soybean did not significantly affect these variables. Moreover, SOM decomposition rates in the planted treatment subjected to constantly-moist or drying-wetting conditions were significantly higher compared with the constantly-moist unplanted treatment, indicating a positive rhizosphere priming effect under both soil moisture regimes. However, drying-wetting reduced the rhizosphere priming of sunflower (69% versus 33%) likely due to lower plant biomass and rhizodeposition, but produced similar rhizosphere priming of soybean (82% versus 85%). Overall, drying-wetting cycles significantly modulated rhizosphere respiration and SOM decomposition, with the magnitude depending on soil drying intensity and plant growth performance. © 2013.

Recent studies have indicated that the hematopoietic stem/progenitor cell (HSPC) niche, consisting of two major crucial components, namely osteoblasts (OBs) and mesenchymal stromal cells (MSCs), is responsible for the fate of HSPCs. Thus, closely mimicking the HSPC niche ex vivo may be an efficient strategy with which to develop new culture strategies to specifically regulate the balance between HSPC self-renewal and proliferation. The aim of this study was to establish a novel HSPC three-dimensional culture system by co-culturing bone marrow-derived MSCs and OBs differentiated from MSCs without any cytokines as feeder cells and applying bio-derived bone from human femoral metaphyseal portion as the scaffold. Scanning electron microscopy revealed the excellent biocompatibility of bio-derived bone with bone marrow-derived MSCs and OBs differentiated from MSCs. Western blot analysis revealed that many cytokines, which play key roles in HSPC regulation, were comprehensively secreted, while ELISA revealed that extracellular matrix molecules were also highly expressed. Hoechst 33342/propidium iodide fluorescence staining proved that our system could be used to supply a long-term culture of HSPCs. Flow cytometric analysis and qPCR of p21 expression demonstrated that our system significantly promoted the self-renewal and ex vivo expansion of HSPCs. Colony-forming unit (CFU) and long-term culture-initiating cell (LTC-IC) assays confirmed that our system has the ability for both the expansion of CD34+ hematopoietic stem cells (HPCs) and the maintenance of a primitive cell subpopulation of HSCs. The severe-combined immunodeficient mouse repopulating cell assay revealed the promoting effects of our system on the expansion of long-term primitive transplantable HSCs. In conclusion, our system may be a more comprehensive and balanced system which not only promotes the self-renewal and ex vivo expansion of HSPCs, but also maintains primitive HPCs with superior phenotypic and functional attributes.

© 2015 Springer International Publishing Switzerland The decomposition of soil organic carbon (SOC) is intrinsically sensitive to temperature. However, the degree to which the temperature sensitivity of SOC decomposition (as often measured in Q10 value) varies with soil depth and labile substrate availability remain unclear. This study explores (1) how the Q10 of SOC decomposition changes with increasing soil depth, and (2) how increasing labile substrate availability affects the Q10 at different soil depths. We measured soil CO2 production at four temperatures (6, 14, 22 and 30 °C) using an infrared CO2 analyzer. Treatments included four soil depths (0–20, 20–40, 40–60 and 60–80 cm), four sites (farm, redwood forest, ungrazed and grazed grassland), and two levels of labile substrate availability (ambient and saturated by adding glucose solution). We found that Q10 values at ambient substrate availability decreased with increasing soil depth, from 2.0–2.4 in 0–20 cm to 1.5–1.8 in 60–80 cm. Moreover, saturated labile substrate availability led to higher Q10 in most soil layers, and the increase in Q10 due to labile substrate addition was larger in subsurface soils (20–80 cm) than in surface soils (0–20 cm). Further analysis showed that microbial biomass carbon (MBC) and SOC best explained the variation in Q10 at ambient substrate availability across ecosystems and depths (R2 = 0.37, P < 0.001), and MBC best explained the variation in the change of Q10 between control and glucose addition treatment (R2 = 0.14, P = 0.003). Overall, these results indicate that labile substrate limitation of the temperature sensitivity of SOC decomposition, as previously shown in surface soils, is even stronger for subsoils. Understanding processes controlling the labile substrate availability (e.g., with rising atmospheric CO2 concentration and land use change) should advance our prediction of the fate of subsoil SOC in a warmer world.

The breakdown and decomposition of plant inputs are critical for nutrient cycling, soil development, and climate-ecosystem feedbacks, but uncertainties persist in how the rates and products of litter decomposition are affected by soil temperature, rhizosphere, and depth of input. We investigated the effects of soil warming (+ 4 °C), rhizosphere, and depth of litter placement on the decomposition of Avena fatua (wild oat grass) root litter in a Mediterranean grassland ecosystem. Field lysimeters were subjected to three environmental treatments (heating, control, and plant removal) and three ¹³C-labeled root litter addition treatments (to A horizon, to B horizon, and no-addition disturbance control) for each of two harvest time points. We buried root litter in February 2014 and measured loss of ¹³C in CO₂ from the soil surface and in leachate as dissolved organic carbon (DOC) over two growing seasons. At the end of each growing season we recovered the ¹³C remaining in the soil. Loss of root litter C occurred almost entirely via heterotrophic respiration, with an estimated < 2% lost as DOC during the initial decay period. The added roots were broken down and incorporated into bulk soil material very quickly; only ~ 30% of added root was visible after 6 months. In the first growing season, decomposition occurred faster in the B than in the A horizon, the latter having greater moisture limitation. Subsequently, there was almost no further decomposition in the B horizon. After two growing seasons, less than 20% of the added root litter C remained in the A or B horizons of all environmental treatments. Heating did not stimulate decomposition, likely because it exacerbated the moisture limitation. However, while plots without plants dried down more slowly than plots with plants, their decomposition rate was not significantly greater, possibly due to the lack of rhizosphere processes such as priming. We conclude that in this Mediterranean grassland ecosystem, soil moisture, which is affected by season, depth, heating, and rhizosphere, plays a dominant role in mediating the effect of those factors on root litter decomposition, which after two seasons did not differ by depth or by treatment.

Living roots and their rhizodeposits affect microbial activity and soil carbon (C) and nitrogen (N) mineralization. This so-called rhizosphere priming effect (RPE) has been increasingly recognized recently. However, the magnitude of the RPE and its driving mechanisms remain elusive. Here we investigated the RPE of two plant species (soybean and sunflower) grown in two soil types (a farm or a prairie soil) and sampled at two phenological stages (vegetative and mature stages) over an 88-day period in a greenhouse experiment. We measured soil C mineralization using a continuous 13C-labeling method, and quantified gross N mineralization with a 15N-pool dilution technique. We found that living roots significantly enhanced soil C mineralization, by 27-245%. This positive RPE on soil C mineralization did not vary between the two soils or the two phenological stages, but was significantly greater in sunflower compared to soybean. The magnitude of the RPE was positively correlated with rhizosphere respiration rate across all treatments, suggesting the variation of RPE among treatments was likely caused by variations in root activity and rhizodeposit quantity. Moreover, living roots stimulated gross N mineralization rate by 36-62% in five treatments, while they had no significant impact in the other three treatments. We also quantified soil microbial biomass and extracellular enzyme activity when plants were at the vegetative stage. Generally, living roots increased microbial biomass carbon by 0-28%, β-glucosidase activity by 19-56%, and oxidative enzyme activity by 0-46%. These results are consistent with the positive rhizosphere effect on soil C (45-79%) and N (10-52%) mineralization measured at the same period. We also found significant positive relationships between β-glucosidase activity and soil C mineralization rates and between oxidative enzyme activity and gross N mineralization rates across treatments. These relationships provide clear evidence for the microbial activation hypothesis of RPE. Our results demonstrate that root-soil-microbial interactions can stimulate soil C and N mineralization through rhizosphere effects. The relationships between the RPE and rhizosphere respiration rate and soil enzyme activity can be used for explicit representations of RPE in soil organic matter models. © 2014.

The paucity of investigations of carbon (C) dynamics through the soil profile with warming makes it challenging to evaluate the terrestrial C feedback to climate change. Soil microbes are important engines driving terrestrial biogeochemical cycles; their carbon use efficiency (CUE), defined as the proportion of metabolized organic C allocated to microbial biomass, is a key regulator controlling the fate of soil C. It has been theorized that microbial CUE should decline with warming; however, empirical evidence for this response is scarce, and data from deeper soils are particularly scarce. Here, based on soil samples from a whole-soil-profile warming experiment (0 to 1 m, +4 °C) and 18O tracing approach, we examined the vertical variation of microbial CUE and its response to ~3.3-y experimental warming in an alpine grassland on the Qinghai-Tibetan Plateau. Microbial CUE decreased with soil depth, a trend that was primarily controlled by soil C availability. However, warming had limited effects on microbial CUE regardless of soil depth. Similarly, warming had no significant effect on soil C availability, as characterized by extractable organic C, enzyme-based lignocellulose index, and lignin phenol-based ratios of vanillyls, syringyls, and cinnamyls. Collectively, our work suggests that short-term warming does not alter microbial CUE in either surface or deep soils, and emphasizes the regulatory role of soil C availability on microbial CUE.

Over 70% of soil organic carbon (SOC) is stored at a depth greater than 20 cm belowground. A portion of this deep SOC actively cycles on annual to decadal timescales and is sensitive to global change. However, deep SOC responses to global change likely differ from surface SOC responses because biotic controls on SOC cycling become weaker as mineral controls predominate with depth. Here, we synthesize the current information on deep SOC responses to the global change drivers of warming, shifting precipitation, elevated CO2, and land use and land cover change. Most deep SOC responses can only be hypothesized because few global change studies measure deep soils, and even fewer global change experiments manipulate deep soils. We call on scientists to incorporate deep soils into their manipulations, measurements, and models so that the response of deep SOC can be accounted for in projections of nature-based climate solutions and terrestrial feedbacks to climate change.

Even though over half of the world's soil organic carbon (SOC) is stored in subsoils (>20 cm deep), and the old ages of subsoil OC indicate its cycling differs from surface SOC, there are few studies examining in situ decomposition processes in deep soils. Here, we added ¹³C-labeled fine roots to 15, 55, and 95 cm depths of a well-characterized coniferous forest Alfisol and monitored the amount of root-derived C remaining over 6, 12, and 30 months. We recovered the root-derived C in microbial phospholipid fatty acids (PLFAs) after 6 months and in coarse (>2 mm) particulate, fine (<2 mm) particulate, and dense, mineral-associated pools after 6, 12, and 30 months. Overall, root decomposition in the first 6 months was similar among all depths but significantly diverged at 30 months with faster decomposition at 15 cm than at 95 cm. There were more fungal and Gram negative-associated PLFAs at 15 cm than at 95 cm, and ¹³C analysis revealed those microbial groups preferred the added root carbon to native SOC. Mineral-associations were not the cause of slower decomposition at depth because similar amounts of applied root C was recovered in the dense fraction at all depths. The largest difference among depths was in the amount of root C recovered in the coarse particulate fraction, which was greater at 95 cm (50%) than at 15 cm (15%). Slower decomposition of the particulate pool at depth likely contributed to the increase in C:N ratios and depletion of δ¹³C values below 60 cm depth in our soil profiles. Simulations of these soils using the CORPSE model, which incorporates microbial priming effects and mineral stabilization of SOC, reproduced patterns of particulate and mineral-associated SOC over both time and depth and suggested that a lack of priming by root exudates at depth could account for the slower decomposition rate of particulate root material. Decomposition of deep particulate SOC may increase if root exudation or dissolved OC transport to depth increases.

The sensitivity of soil organic carbon (SOC) decomposition in seasonally frozen soils, such as alpine ecosystems, to climate warming is a major uncertainty in global carbon cycling. Here we measure soil CO2 emission during four years (2018-2021) from the whole-soil warming experiment (4 °C for the top 1 m) in an alpine grassland ecosystem. We find that whole-soil warming stimulates total and SOC-derived CO2 efflux by 26% and 37%, respectively, but has a minor effect on root-derived CO2 efflux. Moreover, experimental warming only promotes total soil CO2 efflux by 7-8% on average in the meta-analysis across all grasslands or alpine grasslands globally (none of these experiments were whole-soil warming). We show that whole-soil warming has a much stronger effect on soil carbon emission in the alpine grassland ecosystem than what was reported in previous warming experiments, most of which only heat surface soils.