How deep is the soil studied – an analysis of four soil science journals

Huang J., Desai A.R., Zhu J., Hartemink A.E., Stoy P., Loheide II S.P., Zhang Y., Zhang Z., Arriaga F.J.

Weiss M., Jacob F., Duveiller G.

Agriculture provides humanity with food, fibers, fuel, and raw materials that are paramount for human livelihood. Today, this role must be satisfied within a context of environmental sustainability and climate change, combined with an unprecedented and still-expanding human population size, while maintaining the viability of agricultural activities to ensure both subsistence and livelihoods. Remote sensing has the capacity to assist the adaptive evolution of agricultural practices in order to face this major challenge, by providing repetitive information on crop status throughout the season at different scales and for different actors. We start this review by making an overview of the current remote sensing techniques relevant for the agricultural context. We present the agronomical variables and plant traits that can be estimated by remote sensing, and we describe the empirical and deterministic approaches to retrieve them. A second part of this review illustrates recent research developments that permit to strengthen applicative capabilities in remote sensing according to specific requirements for different types of stakeholders. Such agricultural applications include crop breeding, agricultural land use monitoring, crop yield forecasting, as well as ecosystem services in relation to soil and water resources or biodiversity loss. Finally, we provide a synthesis of the emerging opportunities that should strengthen the role of remote sensing in providing operational, efficient and long-term services for agricultural applications.

McMahon D.E., Vergütz L., Valadares S.V., Silva I.R., Jackson R.B.

Intensive management in tropical plantation forestry has increased global wood production per unit of time and land. Eucalyptus trees in southeastern Brazil grow exceptionally fast, even on the highly weathered and nutrient-poor soils of the Atlantic Forest and Cerrado biomes. By remeasuring plantation soils after twelve years and 1–2 rotations, we investigated how established plantations alter soil stocks of carbon, nitrogen, calcium, potassium, and phosphorus, and whether any changes might limit future plantation productivity. We hypothesized that each harvest cycle would deplete soil stocks of nitrogen, because less nitrogen is often added in fertilizer than is removed in wood, and that the balance between harvest and fertilizer would also dictate changes in stocks of other nutrients. In 2004 and 2016, we sampled soils to a depth of 100 cm in plantations and adjacent pastures and native vegetation reserves, and compared total nutrient stocks across time and vegetation type. We found that nutrients were not significantly depleted over time, and that soil stocks of carbon and nutrients in the plantations all tended to increase, with significant increases in the top 20 cm of 20% for potassium in the Atlantic Forest biome, and 23% for carbon and more than 500% for calcium in the Cerrado. Changes in soil nutrient stocks can be attributed in part to both fertilizer inputs and redistribution from changing stocks of biomass. We also observed changes over time and substantial spatial heterogeneity in nutrient stocks under non-plantation vegetation, highlighting the difficulties of using other vegetation types as static “controls” to assess the effects of plantations on soils. Overall, soil nutrient depletion does not appear to threaten sustainability in these intensive plantation forests over the time period studied.

Goebes P., Schmidt K., Seitz S., Both S., Bruelheide H., Erfmeier A., Scholten T., Kühn P.

Soil properties and terrain attributes are of great interest to explain and model plant productivity and community assembly (hereafter P&CA). Many studies only sample surface soils, and may therefore miss important variation of deeper soil levels. We aimed to identify a critical soil depth in which the relationships between soil properties and P&CA were strongest due to an ideal interplay among soil properties and terrain attributes. On 27 plots in a subtropical Chinese forest varying in tree and herb layer species richness and tree productivity, 29 soil properties in six depth columns and four terrain attributes were analyzed. Soil properties varied with soil depth as did their interrelationships. Non-linearity of soil properties led to critical soil depths in which different P&CA characteristics were explained best (using coefficients of determination). The strongest relationship of soil properties and terrain attributes to most of P&CA characteristics (adj. R2 ~ 0.7) was encountered using a soil column of 0–16 cm. Thus, depending on the biological signal one is interested in, soil depth sampling has to be adapted. Considering P&CA in subtropical broad-leaved secondary forests, we recommend sampling one bulk sample of a column from 0 cm down to a critical soil depth of 16 cm.

Chen S., Mulder V.L., Martin M.P., Walter C., Lacoste M., Richer-de-Forges A.C., Saby N.P., Loiseau T., Hu B., Arrouays D.

Soil thickness (ST) is a crucial factor in earth surface modelling and soil storage capacity calculations (e.g., available water capacity and carbon stocks). However, the observed depths recorded in soil information systems for some profiles are often less than the actual ST (i.e., right censored data). The use of such data will negatively affect model and map accuracy, yet few studies have been done to resolve this issue or propose methods to correct for right censored data. Therefore, this work demonstrates how right censored data can be accounted for in the ST modelling of mainland France. We propose the use of Random Survival Forest (RSF) for ST probability mapping within a Digital Soil Mapping framework. Among 2109 sites of the French Soil Monitoring Network, 1089 observed STs were defined as being right censored. Using RSF, the probability of exceeding a given depth was modelled using freely available spatial data representing the main soil-forming factors. Subsequently, the models were extrapolated to the full spatial extent of mainland France. As examples, we produced maps showing the probability of exceeding the thickness of each GlobalSoilMap standard depth: 5, 15, 30, 60, 100, and 200 cm. In addition, a bootstrapping approach was used to assess the 90% confidence intervals. Our results showed that RSF was able to correct for right censored data entries occurring within a given dataset. RSF was more reliable for thin (0.3 m) and thick soils (1 to 2 m), as they performed better (overall accuracy from 0.793 to 0.989) than soils with a thickness between 0.3 and 1 m. This study provides a new approach for modelling right censored soil information. Moreover, RSF can produce probability maps at any depth less than the maximum depth of the calibration data, which is of great value for designing additional sampling campaigns and decision making in geotechnical engineering.

Brewer T.E., Aronson E.L., Arogyaswamy K., Billings S.A., Botthoff J.K., Campbell A.N., Dove N.C., Fairbanks D., Gallery R.E., Hart S.C., Kaye J., King G., Logan G., Lohse K.A., Maltz M.R., et. al.

AbstractWhile most bacterial and archaeal taxa living in surface soils remain undescribed, this problem is exacerbated in deeper soils owing to the unique oligotrophic conditions found in the subsurface. Additionally, previous studies of soil microbiomes have focused almost exclusively on surface soils, even though the microbes living in deeper soils also play critical roles in a wide range of biogeochemical processes. We examined soils collected from 20 distinct profiles across the U.S. to characterize the bacterial and archaeal communities that live in subsurface soils and to determine whether there are consistent changes in soil microbial communities with depth across a wide range of soil and environmental conditions. We found that bacterial and archaeal diversity generally decreased with depth, as did the degree of similarity of microbial communities to those found in surface horizons. We observed five phyla that consistently increased in relative abundance with depth across our soil profiles: Chloroflexi, Nitrospirae, Euryarchaeota, and candidate phyla GAL15 and Dormibacteraeota (formerly AD3). Leveraging the unusually high abundance of Dormibacteraeota at depth, we assembled genomes representative of this candidate phylum and identified traits that are likely to be beneficial in low nutrient environments, including the synthesis and storage of carbohydrates, the potential to use carbon monoxide (CO) as a supplemental energy source, and the ability to form spores. Together these attributes likely allow members of the candidate phylum Dormibacteraeota to flourish in deeper soils and provide insight into the survival and growth strategies employed by the microbes that thrive in oligotrophic soil environments.ImportanceSoil profiles are rarely homogeneous. Resource availability and microbial abundances typically decrease with soil depth, but microbes found in deeper horizons are still important components of terrestrial ecosystems. By studying 20 soil profiles across the U.S., we documented consistent changes in soil bacterial and archaeal communities with depth. Deeper soils harbored distinct communities compared to the more commonly studied surface horizons. Most notably, we found that the candidate phylum Dormibacteraeota (formerly AD3) was often dominant in subsurface soils, and we used genomes from uncultivated members of this group to identify why these taxa are able to thrive in such resource-limited environments. Simply digging deeper into soil can reveal a surprising amount of novel microbes with unique adaptations to oligotrophic subsurface conditions.

Yost J.L., Huang J., Hartemink A.E.

Knowledge of soil water storage and deep drainage is important for improving irrigation efficiency, maximizing crop water use, and understanding groundwater table fluctuation. This is particularly important in sandy soils that depend on irrigation to produce high crop yields. In sandy soils under potato in the Wisconsin Central Sand Plains, soil water storage using tension probes was measured in three irrigation zones over the 2014 and 2015 growing seasons. Soil water storage was estimated across a 78 ha field using apparent electrical conductivity maps. Deep drainage was estimated using the Richards’ equation and Hydrus-1D software. It was found that the average soil water storage ranged from 74 to 110 mm in the top 0.45 m in three irrigation zones in 2014, and from 70 to 95 mm in 2015. Rainfall and irrigation was 387 and 269 mm in 2014, and 328 and 281 mm in 2015. Estimated deep drainage was uniform in three irrigation zones, and ranged from 222 to 244 mm in 2014, and from 167 to 180 mm in 2015. A negative correlation was found between soil water storage and potato yields possibly due to over-irrigation. The methods used in this study can be applied to improve irrigation and water use efficiency.

Yost J.L., Hartemink A.E.

Quantifying soil water storage is important for plant growth and agricultural production. This is particularly important in sandy soils because they are widely cultivated and prone to drought. We investigated the effect of soil organic carbon (SOC), soil texture and gravel content on soil moisture characteristics in sandy soils. Soil moisture characteristics were analysed using standard soil physical characterization as well as visible–near infrared (vis–NIR) spectroscopy and pedotransfer functions (PTFs). Volumetric water content (−10, −33, −1500 kPa) and available water capacity (AWC) increased by 0.01 to 0.02 m³m⁻³ with a 2% increase in silt plus clay content. Available water capacity increased by about 0.05 m³m⁻³ with a 1% increase in SOC. A 5% increase in gravel in the subsoil decreased the volumetric water content of the soil by 0.01 m³m⁻³. Predictions of volumetric water contents and AWC by vis–NIR were the best when using a random forest model. Pedotransfer functions for AWC predictions performed best when sand, silt, clay, bulk density and SOC were included. Soil moisture storage ranged from 44 to 65 mm in the top 50 cm. At mean evapotranspiration (4.2 mm day⁻¹), irrigation requirements were reduced by 6 days in the sandy soils that have larger SOC and silt contents in the topsoil. These soils are sandy throughout but have subtle differences in particle‐size distribution, SOC concentrations and gravel content. These differences affect soil moisture storage considerably. Soil management practices that increase the SOC contents in these sandy soils favour moisture storage and tend to reduce irrigation requirements. HIGHLIGHTS: SOC, soil texture and gravel affect soil moisture characteristics in sandy soils. With vis–NIR and PTFs, soil water storage, water characteristics and AWC in soils with >80% sand can be quantified and predicted. Available water capacity increased by about 0.05 m³m⁻³ with a 1% increase in SOC. Subtle differences in particle‐size distribution affect soil moisture storage and reduce irrigation requirements.

Li Y., Liu H., Pan H., Zhu X., Liu C., Zhang Q., Luo Y., Di H., Xu J.

There is increasing evidence to suggest that viruses may influence the succession of individual populations of microorganisms, biogeochemical cycles and, ultimately, microbial community structure. However, it is still not well understood if T4-type viruses can affect the bacterial communities of terrestrial ecosystems. Here, we report an investigation of the impact of T4-type phage and bottom-up (environmental factors) controls on bacterial community structures along a 2000-year paddy soil chronosequence. T4-type myoviral and bacterial communities were evaluated by clone sequencing and high-throughput sequencing of the gene encoding the major capsid protein (g23) and 16S ribosomal DNA, respectively. Long-term (centurial/millennial) anthropogenic managements of paddy soils resulted in an accumulation of nutrients and soil acidification. Significant shifts in soil bacterial and phage communities were detected during the development of paddy soils at millennial time scales. The Mantel test and variation partitioning analysis (VPA) suggested that the profile of bacterial community composition was strongly affected by both T4-type phage and environmental variables. Network analysis between phage and bacterial taxa indicated that six bacterial families were implicated as potential hosts of T4-type phages. These results suggest that phage lysis is important in shaping bacterial communities in the soil environment.

Singh G., Schoonover J.E., Williard K.W., Kaur G., Crim J.

Labile and bulk pools of carbon and nitrogen (C and N) play different functional roles in soil organic matter dynamics and nutrient cycling. The objectives of this study were to evaluate the (i) effects of land use [corn (Zea mays L.)–soybean [Glycine max (L.) Merr.], alfalfa (Medicago sativa L.), and black walnut (Juglans nigra L.) plantation] and (ii) vertical distribution of labile [potassium permanganate oxidizable carbon (POXC), water extractable organic carbon (WEOC), water extractable nitrogen (WEN)] and bulk [total carbon (TC), total nitrogen (TN)] pools of C and N in soil to a depth of 105 cm at different topographic positions within a watershed. Alfalfa had 10.02 to 14.86 Mg ha⁻¹ greater TC than corn–soybean and black walnut plantation on the shoulder position in the surface horizon (0–15 cm), whereas the subsurface horizon (15–105 cm) showed no significant differences for TC measured at all topographic positions. Soil POXC was significantly higher in alfalfa than corn–soybean in the surface layer of 0 to 15 cm by 1748.6, 1904.03, and 2878.67 kg ha⁻¹ at the shoulder, backslope, and footslope positions, respectively. However, no differences were observed for POXC at the shoulder position when all subsurface layers were combined at 15- to 105-cm depth. In general, the labile pools of C and N showed differences with land use and topographic positions and followed alfalfa > black walnut plantation > corn–soybean. The study results suggest that for accurate assessment of land use on C and N gains and/or losses both bulk and labile pools should be measured including the entire root zone depth. Topographic differences should be accounted for assessing C and N pools at the watershed scale.

Nishigaki T., Tsujimoto Y., Rinasoa S., Rakotoson T., Andriamananjara A., Razafimbelo T.

Phosphorus (P) deficiency is a major constraint for rice production in the tropics. Field-specific P management is key for resource-limited farmers to increase yields with minimal inputs. We used soil P fractionation analysis to identify the relevant factors controlling P uptake and the responses to P fertilization of rice in flooded and highly weathered soils. Phytometric pot-based experiments and a modified Hedley fractionation analysis were repeated for soils from extensive regions and from geographically adjacent fields in Madagascar. Large field-to-field variations in indigenous P supply from soils (total P uptake of rice when P is omitted) and fertilizer-P recovery efficiencies (increased P uptake when P is applied) were observed not only for soils with various geological backgrounds but also for soils from adjacent fields. Regression models indicated that the indigenous P supply in soils was largely controlled by readily available inorganic and organic P pools (r2 = 0.72), whereas fertilizer-P recovery efficiencies were controlled by the abundance of oxalate-extractable aluminum and iron in soils (r2 = 0.81). Spatial heterogeneity even within adjacent fields leads to benefits from field-specific fertilizer management based on indigenous P supply from soils and fertilizer-P recovery efficiencies evaluated by different soil properties.

Bonfatti B.R., Hartemink A.E., Vanwalleghem T., Minasny B., Giasson E.

Soil thickness is an important soil characteristic changing over space and time. In this study, we used a mechanistic soil landscape models to predict soil thickness and show it under development over time. The study was conducted in an 8,118 ha area in Vale dos Vinhedos, Rio Grande do Sul State, Brazil. Different soil production functions (SPF) combined with a landscape evolution model (LEM) were explored. The SPF calculated the soil production rates and LEM calculated erosion and deposition patterns. We evaluated two types of model. Model 1 was used to predict the current soil thickness. The model equals the erosion estimations (by a LEM) to the soil production rate (by a SPF). Three types of SPF were tested, based on a spatial variation of soil moisture. A steady-state condition was assumed, considering soil production rates similar to erosion rates. The model simulated erosion events to 1 year, using a Digital Elevation Model (DEM). A soil survey with observed soil thickness was used to validate the different models. Model 2 used the soil thickness estimation from Model 1 to simulate the soil thickness changes up to 100 kyr, considering the balance between soil production rate and soil eroded or deposited. The soil thickness changes were evaluated in different landscape positions. In Model 1, the linear correlation between observed and predicted soil thickness varied between 0.25 and 0.49, with higher linear correlation in models using soil moisture data. The RMSE under different models varied between 34 cm and 37 cm. Overall, soil depth was more accurately predicted in the upland areas than in the valley bottom areas. Model 2 suggested that the soil thickness variation largely depended on the landscape position. The average soil thickness changed from initial 67 cm (0 Kyr) to 103 cm (100 kyr).

Spohn M., Klaus K., Wanek W., Richter A.

Processing of organic carbon (C) by soil microorganisms is a key process of terrestrial C cycling. For this reason we studied (i) microbial carbon use efficiency (CUE) defined as C allocated to growth over organic C taken up by the microbial community, and (ii) the turnover time of microbial biomass in a pasture and in two forest soils. We hypothesized that microbial CUE decreases in mineral soils with depth from the topsoil to the subsoil, while the turnover time of the microbial biomass increases due to energetic constrains. We determined microbial CUE and turnover of microbial biomass C using a novel substrate-independent method based on incorporation of 18O from labeled water into microbial DNA with concurrent measurements of basal respiration. Microorganisms showed decreasing C uptake rates with decreasing C contents in the deeper soil layers. In the forest soils, no adaptation of microbial CUE with soil depth took place, i.e., microbes in the forest topsoil used C at the same efficiency as microbes in the subsoil. However, in the pasture soil, microbial CUE decreased in the lower soil layers compared to the topsoil, indicating that microorganisms in the deeper soil layers allocated relatively more C to respiration. In the organic soil layer, microorganisms respired more per unit microbial biomass C than in the subsoil, but had a similar CUE despite the high C-to-nitrogen and C-to-phosphorus ratios of the litter layers. The turnover time of microbial biomass increased with soil depth in the two forest soils. Thus, in the forest soils, a lower microbial C uptake rate in the deeper soil layers was partially compensated by a longer turnover time of microbial biomass C. In conclusion, our findings emphasize that in addition to microbial CUE, the turnover time of the microbial biomass strongly affects soil C cycling.

Stone M.M., DeForest J.L., Plante A.F.

Extracellular enzymes in soils mediate the decomposition of organic matter and catalyze key transformations in carbon, nitrogen and phosphorus cycling. However, most studies of extracellular enzyme activity have focused exclusively on relatively carbon and nutrient-rich surface soils. In tropical forests, several centimeters of nutrient-rich surface soil can overlay meters of resource-poor subsoil, of which the microbial ecology is poorly characterized. The goal of this study was to determine how extracellular enzyme activity changes as a function of depth across two soil orders (Oxisols and Inceptisols) and two forest types that occur at different elevations (Tabonuco, lower elevation; Colorado, higher elevation) at the Luquillo Critical Zone Observatory in northeast Puerto Rico. We excavated three soil pits to 140 cm at four different sites representing the four soil × forest combinations, and measured potential activities of four carbon-acquiring enzymes (α-glucosidase, β-glucosidase, β-xylosidase, cellobiohydrolase), one nitrogen-acquiring enzyme (N-acetyl glucosaminidase) and one organic phosphorus-acquiring enzyme (acid phosphatase) at six discrete depth intervals. We used phospholipid fatty acid (PLFA) analysis to assess viable microbial biomass and community structure. Overall, microbial biomass, specific enzyme activities and community structure were similar across the two soil and forest types, in spite of higher carbon concentrations and C:N ratios in the Colorado forest soil. Soil nutrients, microbial biomass and potential enzyme activities all declined exponentially with depth. However, when normalized to microbial biomass, specific enzyme activities either did not change with depth (β-glucosidase, β-xylosidase, cellobiohydrolase and N-acetyl glucosaminidase) or increased significantly with depth (α-glucosidase and acid phosphatase, P

Serrano J., Shahidian S., Silva J.

Feng W., Ai J., Sánchez-Rodríguez A.R., Li S., Zhang W., Yang H., Apostolakis A., Muenter C., Li F., Dippold M.A., Zhou J., Dittert K., Wang H.

Philipp L., Sünnemann M., Schädler M., Blagodatskaya E., Tarkka M., Eisenhauer N., Reitz T.

Yuan F., Xu X., Liu Z., Sa R., Sun C., Liu J., Li N., Zhang Y., Zhang T., Xing T., Ren J., Tang S., Jin K.

Abe Y., Nakayama M., Atarashi-Andoh M., Tange T., Sawada H., Liang N., Koarashi J.

Xiang M., Luo R., Wu J., Niu B., Pan Y., Zhang X., Duo L., Ma T., Han C.

Babu A.R., Mohebbanaaz, Lalitha T., Anjali B., Sree U.C.

Blakemore R.J.

Basic inventory is required for proper understanding and utilization of Earth’s natural resources, especially with increasing soil degradation and species loss. Soil carbon is newly refined at >30,000 Gt C (gigatonnes C), ten times above prior totals. Soil organic carbon (SOC) is up to 24,000 Gt C, plus plant stocks at ~2400 Gt C, both above- and below-ground, hold >99% of Earth’s biomass. On a topographic surface area of 25 Gha with mean 21 m depth, Soil has more organic carbon than all trees, seas, fossil fuels, or the Atmosphere combined. Soils are both the greatest biotic carbon store and the most active CO2 source. Values are raised considerably. Disparity is due to lack of full soil depth survey, neglect of terrain, and other omissions. Herein, totals for mineral soils, Permafrost, and Peat (of all forms and ages), are determined to full depth (easily doubling shallow values), then raised for terrain that is ignored in all terrestrial models (doubling most values again), plus SOC in recalcitrant glomalin (+25%) and friable saprock (+26%). Additional factors include soil inorganic carbon (SIC some of biotic origin), aquatic sediments (SeOC), and dissolved fractions (DIC/DOC). Soil biota (e.g., forests, fungi, bacteria, and earthworms) are similarly upgraded. Primary productivity is confirmed at >220 Gt C/yr on land supported by Barrow’s “bounce” flux, C/O isotopes, glomalin, and Rubisco. Priority issues of species extinction, humic topsoil loss, and atmospheric CO2 are remedied by SOC restoration and biomass recycling via (vermi-)compost for 100% organic husbandry under Permaculture principals, based upon the Scientific observation of Nature.

Pan J., Zhang X., Liu S., Liu N., Liu M., Chen C., Zhang X., Niu S., Wang J.

Asensio H.A., McSweeney K., Brown T., Barker D., Charuc J., Lombardini L., Margenot A.J.

Moreland K., Dove N.C., Yan Q., Hou T., Barnes M.E., Hart S.C., Filley T., Kumar P., Asefaw Berhe A.

Soil is the thin, vital layer of Earth’s surface that forms the foundation of the Critical Zone and sustains life. This chapter explores the intricate dynamics of soil organic matter within the critical zone, focusing on three key thematic areas: deep soil organic matter, wildfire organic matter interactions, and organic matter erosion. Soil organic matter, although a small fraction of soil mass, plays a crucial role in soil function and ecosystem stability. The complexity of soil organic matter arises from its diverse chemical composition and interactions with minerals, which influence its persistence in the environment. This chapter begins by examining deep soil organic matter, which constitutes a significant portion of global carbon storage. We discuss how deep soil organic matter, typically isolated from surface processes, may become vulnerable to decomposition and carbon release due to disturbances. Next, we explore the impacts of fire on soil organic matter, particularly the formation and stability of pyrolyzed organic matter. The decomposition of pyrolyzer organic matter is influenced by its chemical composition and the surrounding environmental conditions, with implications for carbon cycling and soil fertility in post-fire ecosystems. Finally, the chapter addresses the role of erosion in soil organic matter dynamics. Erosion, accelerated by human activities, redistributes soil organic matter across landscapes, affecting its turnover and the broader biogeochemical cycles. We consider how micro-topographic features and erosion processes interact to influence soil OM stability and carbon sequestration. By synthesizing recent advances in these areas, this chapter provides a comprehensive overview of the complex and dynamic nature of soil organic matter in the critical zone, highlighting its importance in understanding the critical zone in the face of environmental change.

Wahab L.M., Chacon S.S., Kim S.L., Berhe A.A.

AbstractThere are major gaps in our understanding of how Mediterranean ecosystems will respond to anticipated changes in precipitation. In particular, limited data exists on the response of deep soil carbon dynamics to changes in climate. In this study we wanted to examine carbon and nitrogen dynamics between topsoils and subsoils along a precipitation gradient of California grasslands. We focused on organic matter composition across three California grassland sites, from a dry and hot regime (~ 300 mm precipitation; MAT: 14.6 $$\boldsymbol{^\circ{\text{C}} }$$ ∘ C ) to a wet, cool regime (~ 2160 mm precipitation/year; MAT: 11.7 $$\boldsymbol{^\circ{\text{C}} }$$ ∘ C ). We determined changes in total elemental concentrations of soil carbon and nitrogen, stable isotope composition (δ13C, δ15N), and composition of soil organic matter (SOM) as measured through Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS) to 1 m soil depth. We measured carbon persistence in soil organic matter (SOM) based on beta ($${\varvec{\beta}}$$ β ), a parameter based on the slope of carbon isotope composition across depth and proxy for turnover. Further, we examined the relationship between δ15N and C:N values to infer SOM’s degree of microbial processing. As expected, we measured the greatest carbon stock at the surface of our wettest site, but carbon stocks in subsoils converged at Angelo and Sedgwick, the wettest and driest sites, respectively. Soils at depth (> 30 cm) at the wettest site, Angelo, had the lowest C:N and highest δ15N values with the greatest proportion of simple plant-derived organic matter according to DRIFTS. These results suggest differing stabilization mechanisms of organic matter at depth across our study sites. We infer that the greatest stability was conferred by associations with reactive minerals at depth in our wettest site. In contrast, organic matter at our driest site, Sedgwick, was subject to the most microbial processing. Results from this study demonstrate that precipitation patterns have important implications for deep soil carbon storage, composition, and stability.

McMurtry A.R., Kasmerchak C.S., Vaughan E.A., Dolui M., Phillips L.M., Mueller C.W., Pett-Ridge J., Berhe A.A., Mason J.A., Marín-Spiotta E., de Graaff M.

Large quantities of soil carbon (C) can persist within paleosols for millennia due to burial and subsequent isolation from plant-derived inputs, atmospheric conditions, and microbial activity at the modern surface. Erosion exposes buried soils to modern root-derived C influx via root exudation and root turnover, thus stimulating microbial activity leading to SOC decomposition and accumulation through organo-mineral stabilization of modern C. With this study we aim to quantify how modern root-derived C inputs impact paleosol C decomposition and stabilization across varying degrees of isolation from modern surface conditions in southwestern Nebraska, USA, where hillslope erosion is bringing a buried Late-Pleistocene-early Holocene paleosol (the "Brady Soil") closer to the modern surface. We collected Brady Soil samples from 0.2 m, 0.4 m, and 1.2 m below the modern surface and conducted two lab-based incubations. Soils were amended with either (1) a lab-synthesized mixture of low molecular weight compounds (12 atom% 13C), or (2) 13C enriched root residues (92 atom% 13C), in 30-day and 240-day incubation experiments, respectively. We determined microbial responses to synthetic root exudates and residues by partitioning the 13C label from Brady Soil C, including measurements of total, root, and primed C respiration, microbial biomass C (MBC), microbial C use efficiency (CUE). To assess the capacity of isolated paleosols to accrue modern plant C, we used Nano-scale Secondary Ion Mass Spectrometry imaging. We found that: (1) adding root-derived C inputs primed Brady Soil C across all depths, and was mediated by depth and composition of root additions; (2) root-derived C inputs stimulated microbial biomass C (MBC) growth similarly across depths, but the magnitude of CUE and MBC varied by chemistry of root-derived additions; (3) new particulate organic matter was incorporated into mineral-associated pools over time; (4) material from the added root residues was found in association with bacterial cells and fungal hyphae as well as with soil aggregate and mineral surfaces. Our study shows that paleosols defy expectations of C content and reactivity with depth, and changes in land cover and climate will expose buried paleosols to modern surface conditions, increasing respired C. This work highlights the importance of evaluating the role resurfacing buried soils through landscape change plays in C cycle feedbacks to the climate system.

Garrett L.G., Byers A.K., Wigley K., Heckman K.A., Hatten J.A., Wakelin S.A.

Forests are the reservoir for a vast amount of terrestrial soil organic carbon (SOC) globally. With increasing soil depth, the age of SOC reportedly increases, implying resistance to change. However, we know little about the processes that underpin deep SOC persistence and what deep SOC is vulnerable to climate change. This review summarizes the current knowledge of deep forest SOC, the processes regulating its cycling, and the impacts of climate change on the fate of deep forest SOC. Our understanding of the processes that influence deep SOC cycling and the extent of SOC stores is limited by available data. Accordingly, there is a large degree of uncertainty surrounding how much deep SOC there is, our understanding of the influencing factors of deep SOC cycling, and how these may be distinct from upper soil layers. To improve our ability to predict deep SOC change, we need to more accurately quantify the deep SOC pool and deepen our knowledge of how factors related to the tree root–soil–microbiome control deep SOC storage and cycling. Thereby, addressing the uncertainty of deep SOC contribution in the global C exchange with climate change and concomitant impacts on forest ecosystem function and resilience.

Anuo C.O., Sleem M., Fossum B., Li L., Cooper J.A., Malakar A., Maharjan B., Kaiser M.

• In the A horizon, OC in all fractions was lower in arable than in prairie soils. • OC loss in arable topsoils was mainly driven by losses in mineral-associated OC. • Mineral-associated OC of prairie topsoils correlated most strongly with mineral characteristics. • In the C horizon, water-extractable OC was higher in prairie than in arable soils. Improved understanding of land use derived changes in soil organic matter (OM) compartments stabilized to different degrees against microbial decomposition is required for outlining efficient land use strategies aimed at improving soil ecosystem functions that are strongly coupled to gains and losses of soil organic carbon (OC). However, such data is scarce, particularly in subsoil environments. Consequently, in this study, we analyzed OC storage in topsoils and subsoils, as well as OM fractions with different OC turnover dynamics, including particulate (free and occluded), water-extractable, and mineral-associated OM. We sampled soils under native prairie (10 sites) and long-term arable use (> 40 years, 10 sites) to a depth of 3 m in the central U.S. Our results showed that the arable bulk soils had significantly lower OC content in the A horizon and across all analyzed OM fractions compared to native prairie soils. This reduction was primarily derived from OC losses in the mineral-associated OM (arable: 7.2 ± 0.5 g kg −1 ; native prairie: 12 ± 0.7 g kg −1 ), which retained the most significant portion (50–56 %) of bulk soil OC among all fractions. No significant impact of land use on OC storage in the bulk soil and fractions was observed in the subsoil B and C horizons, except for water-extractable OM, which had lower amounts in arable soils in the C horizon than native prairie soils. This underscores the relevance of this fraction for the translocation of OC across the soil profile in undisturbed systems. Our results highlight the crucial role of mineral-associated OM for soil OC storage, but also its sensitivity to land use change, especially in the topsoil, suggesting this fraction is highly relevant for strategies aiming at restoring pre-disturbance soil OC levels.

Anuo C.O., Li L., Moreland K.C., McFarlane K.J., Malakar A., Cooper J.A., Maharjan B., Kaiser M.

Land use change from native grasslands to arable lands globally impacts soil ecosystem functions, including the storage of soil organic carbon (SOC). Understanding the factors affecting SOC changes in topsoil and subsoil due to land use is crucial for effective mitigation strategies. We determined SOC storage and persistence as affected by land use change from native prairies to arable lands. We examined SOC stocks, soil δ13C and ∆14C signatures, microbial communities (bacteria and fungi), and soil mineral characteristics under native prairies and long-term arable lands (i.e., > 40 years) down to 3 m in the U.S. Midwest. Native prairie soils had higher SOC stocks in the A horizon and 0–50 cm depth increment than arable soils. For both land use types, the δ13C and ∆14C values significantly decreased with depth, with the latter pointing towards highly stabilized SOC, especially in the B- and C-horizons. Analysis of the microbial communities indicated that the diversity of bacteria and fungi decreased with increasing soil depth. The content of oxalate soluble Al appeared to be the single most important predictor of SOC across horizons and land use types. Our data suggest that most SOC gains and losses and transformation and translocation processes seem to be restricted to the uppermost 50 cm. Increasing SOC retention in the A and B horizons within the 0–50 cm depth would enhance organic material serving as substrate and nutrients for microbes and plants (A horizon) and facilitate long-term SOC storage in the subsoil (B horizon).