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The effects of fertilization and residue retention practices on enhancing crop yield and soil productivity, have been extensively investigated in various cropping system studies. However, various potential interactions exist among system components (soil, plant, microbial). The multitude and complexity of these interactions results in variability in agroecological responses, such as the soil extracellular enzyme activities (EEAs). Using a meta-analysis of 85 peer-reviewed articles published in the past three decades, we synthesized the EEAs of four major soil enzymes (urease, phosphatase, invertase, and peroxidase), which are affected by various fertilization and residue management practices used in cropping systems of China. The combined application of straw residues and chemical fertilizers, and straw residues only significantly increased soil organic carbon (by 38 and 47%, respectively) and total nitrogen (by 26 and 57%, respectively) as compared to the non-fertilized control. Manure plus chemical fertilizer treatments showed a consistent increase in urease, phosphatase and invertase activity by 86, 34 and 37%, respectively, as compared to the non-fertilized control. The use of inorganic fertilizers increased the activity of soil EEAs involved in carbon- and phosphorus-cycling, but did not decrease EEAs involved in nitrogen-cycling as expected. The experimental duration had a significant impact on urease activity, particularly with unbalanced application of chemical fertilizers. Therefore, along with managing fertilizer type, considerable attention should be given to adopting alternative production systems and increasing cropping intensity (preferably double cropping), to enhance soil EEAs.

A comparative study on the effect of elevated CO2 environment on soil nitrogen availability in different rice planting system is needed to develop nutrient management strategies in future climate scenarios. A field experiment was conducted inside open top chambers (OTC) to study the effect of elevated CO2 environment with varying nitrogen management on soil NH4+–N and NO3−–N status in two planting system of rice, direct-seeded rice (DSR) and puddled transplanted rice (PTR). The nitrogen management included chemical fertilizer (CF) at 100% (CF100) and 150% (CF150) of the recommended dose, integrated nitrogen management including organic fertilizer (OF) and CF as CF75+ OF75, and site-specific N management through CF using SPAD meter. The soil NH4+–N content was higher in PTR, but NO3−–N was higher in DSR. The soil NH4+–N and NO3−–N content decreased significantly under elevated CO2 environment as compared to ambient in both planting system, except the NO3−–N content at flowering in DSR. The decrease was around 8% for NH4+–N and 5% for NO3−–N content. Soil nitrogen content in DSR can be maintained by following integrated nutrient management (CF75 + OF75) and SPAD-based nitrogen management for sustainable yield. Grain yield, in general, increased with CO2 elevation in both planting system. Under ambient environment, CF150 increased the grain yield by 23% as compared to CF100 in DSR, but no change was noted in PTR. However, under elevated CO2 environment, CF150 increased the grain yield by 13% in PTR. Under elevated CO2 environment, the yield increase of the hybrid rice to additional N fertilizer application was noted in PTR but not in DSR. This study suggests that for sustainable rice production under increasing CO2 environment in future climate scenarios, higher dose of N fertilizer is recommended in PTR, but normal dose in DSR production system.

The combination of extended dry periods and high intensity rainfall, common in the southeastern US, leads to greater variability in soil moisture and consequently increases uncertainty to microbial processes pertinent to soil carbon (C) mineralization. However, field-based findings on soil moisture sensitivity to soil C cycling are very limited. Therefore, a field experiment was conducted in 2018 and 2019 on a soybean (Glycine max L.) cropland in the southeastern US with three soil moisture treatments: drought (simulated using rainout-shelter from June to October in each year), rainfed (natural precipitation), and irrigated (irrigation and precipitation). Soil respiration was measured weekly from May to November in both years. Soil samples were collected multiple times each year from 0–5, 5–15, and 15–30 cm depths to determine microbial biomass C (MBC), extractable organic C (EOC), hydrolytic enzyme activities, and fungal abundance. The cumulative respiration under drought compared to other treatments was lower by 32% to 33% in 2018 and 38% to 45% in 2019. Increased MBC, EOC, and fungal abundance were observed under drought than other treatments. Specific enzyme activity indicated fewer metabolically active microbes under drought treatment compared to rainfed and irrigated treatments. Also, maintenance of enzyme pool was observed under drought condition. These results provide critical insights on microbial metabolism in response to soil moisture variation and how that influences different pools of soil C under field conditions.

Combined application of biochar and nitrogen (N) fertilizer has the potential to reduce N losses from soil. However, the effectiveness of biochar amendment on N management can vary with biochar types with different physical and chemical properties. This study aimed to assess the effect of two types of hardwood biochar with different ash contents and cation exchange capacity (CEC) on soil N mineralization and nitrous oxide (N2O) production when applied alone and in combination with N fertilizer. Soil samples collected from a temperate pasture system were amended with two types of biochar (B1 and B2), urea, and urea plus biochar, and incubated for 60 days along with soil control (without biochar or urea addition). Soil nitrate N, ammonium N, ammonia-oxidizing bacteria amoA gene transcripts, and N2O production were measured during the experiment. Compared to control, addition of B1 (higher CEC and lower ash content) alone decreased nitrate N concentration by 21% to 45% during the incubation period while the addition of B2 (lower CEC and higher ash content) alone increased the nitrate N concentration during the first 10 days. Biochar B1 also reduced the abundance of amoA transcripts by 71% after 60 days. Compared to B1 + urea, B2 + urea resulted in a significantly greater initial increase in soil ammonium and nitrate N concentrations. However, B2 + urea had a significantly lower 60-day cumulative N2O emission compared to B1 + urea. Overall, when applied with urea, the biochar with higher CEC reduced ammonification and nitrification rates, while biochar with higher ash content reduced N N2O production. Our study demonstrated that biochar has the potential to enhance N retention in soil and reduce N2O emission when it is applied with urea, but the specific effects of the added biochar depend on its physical and chemical properties.

Quantifying the upper limit of stable soil carbon storage and relative saturation is essential for guiding policies designed to increase soil carbon storage, such as ‘4 per 1000’ sequestration initiative. Carbon stabilization processes are diverse, but one particular pool of carbon that is considered stable across climate zones and soil types is the mineral-associated fraction, measured using density or size fractionation. Some soil carbon decomposition models assume sorption to minerals is the main form of stabilization in this fraction. We estimate the global capacity of mineral soils in six soil orders to sorb additional dissolved organic carbon (DOC). We gathered data from 400 DOC sorption experiments representing 133 soil profiles across six soil orders. We used the relationship between DOC added and DOC sorbed to calibrate a modified Langmuir sorption equation, from which we quantified the DOC sorption potential in each soil. We found that the sorption potential is empirically related to climate variables (including mean annual temperature and mean annual precipitation) and soil geochemical variables (chiefly, percent clay, pH, and soil order). From this relationship, we then estimated the DOC sorption potential for 14631 profiles distributed globally. This amount was 1.4 (global median; 95% CI: 0.50, 2.8) kg C m-3, totaling 102 Pg C globally across six soil orders, representing up to a 7% increase in the existing total C stock. We show that there is greater capacity for additional DOC sorption in subsoils (30cm-1m) compared to top-soils (0-30cm). The gap between the modest potential of mineral sorption processes found in this study and the large total capacity of long-term organic matter stabilization (2541 Pg C for the six soil orders of this study) indicates that other mechanisms such as aggregation, the sorption of microbial necromass, layering, and co-precipitation also play a critical role in stable organic matter formation and persistence.

Weed communities may potentially reduce row‐crop yields resulting in a loss of revenue for producers. Studies have reported a decrease in the efficacy of chemical methods of weed control due to increased weed resistance. With the goal of reducing reliance on chemical weed control and promoting sustainable crop production in mind, we evaluated different winter cover crop (CC) types (single‐, double‐, and multi‐species) and CC planting methods (broadcasting vs. drilling) on CC and weed groundcover and row‐crop yield in a no‐till corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation system in Tennessee from 2013 to 2017. Our results showed that winter CC reduced spring weed ground coverage by 64–76% compared with winter fallow with no herbicide application. A five‐species CC mixture increased soybean yields by 6 to 8.5% across seeding method treatments compared with other CC treatments and winter fallow. This yield increase was more visible in a drought year. Corn, grown in years with normal weather patterns, did not show a yield response to CC types. From this study, cover cropping was found to be an effective strategy for weed control compared with leaving the field fallow without herbicide application following row‐crop harvest; however, no consistent species‐specific trends were observed on weed suppression and row‐crop yield. Future research should focus on understanding CC species composition and biomass production in double‐ and multi‐species mixtures from this field experiment to draw robust conclusions on agronomic responses of individual CC species and planting methods.

Genetic engineering has been used to decrease the lignin content and to change the lignin composition of switchgrass (Panicum virgatum L.) to decrease cell wall recalcitrance to enable more efficient cellulosic biofuel production. Previous greenhouse and field studies showed that downregulation of the gene encoding switchgrass caffeic acid O-methyltransferase (COMT) and overexpression of the switchgrass PvMYB4 (MYB4) gene effectively improved ethanol yield. To understand potential environmental impacts of cultivating these transgenic bioenergy crops in the field, we quantified the effects of field cultivation of transgenic switchgrass on soil organic carbon (SOC) dynamics. Total and active SOC as well as soil respiration were measured in soils grown with two COMT-downregulated transgenic lines (COMT2 and COMT3), three MYB4-overexpressed transgenic lines (L1, L6, and L8), and their corresponding non-transgenic controls. No differences in total SOC, dissolved organic carbon (DOC), and permanganate oxidizable carbon (POXC) were detected between transgenic and non-transgenic treatments for both COMT (10.4–11.1 g kg−1 for SOC, 60.0–64.8 mg kg−1 for DOC, and 299–384 mg kg−1 for POXC) and MYB4 lines (6.89–8.21 g kg−1 for SOC, 56.0–61.1 mg kg−1 for DOC, and 177–199 mg kg−1 for POXC). Soil CO2-carbon (CO2-C) production from the COMT2 transgenic line was not significantly different from its non-transgenic control. In contrast, the COMT3 transgenic line had greater soil CO2-C production than its non-transgenic control (210 vs. 165 µg g−1) after 72 days of laboratory incubation. Combining the improvement in ethanol yield and biomass production reported in previous studies with negligible change in SOC and soil respiration, COMT2 could be a better biofuel feedstock than COMT3 for environmental conservation and cost-effective biofuel production. On the other hand, MYB4 transgenic line L8 produced more biomass and total ethanol per hectare while it released more CO2-C than the control (253 vs. 207 µg g−1). Long-term in situ monitoring of transgenic switchgrass systems using a suite of soil and environmental variables is needed to determine the sustainability of growing genetically modified bioenergy crops.

Crop residue retention and minimum tillage (including no-tillage, NT, and reduced tillage, RT) are common conservation tillage practices that have been extensively practised for improving soil health and reducing the negative environmental impact caused by intensive farming. However, the complex effect of conservation tillage practices on soil organic carbon (SOC) storage has not been systematically analyzed, and particularly, the synergistic effect of crop residue retention and minimum tillage on SOC storage remains nonexistent. We conducted a global meta-analysis using a dataset consisting of 823 pairs of data points from 164 studies. We analyzed the effect of crop residue retention and minimum tillage on SOC storage and how the above effects were influenced by various soil/environmental (soil sampling depth, soil texture, and climate) and management conditions (cropping intensity and treatment duration). We found that either residue retention or minimum tillage alone increased SOC stock, while the former increased SOC more. The NT and RT increased SOC stock by 10 and 6%, respectively, in comparison to conventional tillage (CT). The NT plus residue retention (NTS) and RT plus residue retention (RTS) resulted in 20 and 26% more increase in SOC than NT and RT, respectively. Compared with CT, NTS and RTS further increased SOC stock by 29 and 27%, respectively. The above effects were greater in the topsoil than in the subsoil. Availability of initial soil nutrient played a greater role in affecting SOC stock than climatic conditions and management practices. Both residue retention and NT increased SOC rapidly in the first 6 years regardless of soil texture or climate condition, followed by a period of slower sequestration phase before reaching a slow steady rate. Double cropping generally increased SOC stock across all conservation tillage practices as compared to single or multiple cropping. Therefore, we conclude that minimum tillage coupled with residue retention in a double cropping system is the most beneficial management system for increasing cropland SOC storage, which can inform sustainable soil management practices aimed at increasing global C sequestration.

The North Central Education and Research Activity Committee (NCERA-59) was formed in 1952 to address how soil organic matter formation and management practices affect soil structure and productivity. It is in this capacity that we comment on the science supporting soil quality and associated soil health assessment for agricultural lands with the goal of hastening progress in this important field. Even though the suite of soil quality indicators being applied by U.S. soil health efforts closely mirrors the “minimum data set” we developed and recommended in the mid-1990s, we question whether the methods or means for their selection and development are sufficient to meet current and emerging soil health challenges. The rush to enshrine a standard suite of dated measures may be incompatible with longer-term goals. Legitimate study of soil health considers soil change accrued over years to decades that influence on- and off-site function. Tailoring of methods to local conditions is needed to effectively apply and interpret indicators for different soil resource regions and land uses. Adherence to a set suite of methods selected by subjective criteria should be avoided, particularly when we do not yet have adequate data or agreed upon interpretive frameworks for many so-called “Tier 1” biological indicators used in soil health assessment. While pooling data collected by producer-groups is one of the most exciting new trends in soil health, standardizing methods to meet broad inventory goals could compromise indicator use for site or application-specific problem solving. Changes in our nation's research landscape are shifting responsibility for soil stewardship from national and state government backed entities to public-private partnerships. As a result, it is critical to ensure that the data needed to assess soil health are generated by reproducible methods selected through a transparent process, and that data are readily available for public and private sector use. Appropriate methods for engagement need to be applied by public-private research partnerships as they establish and expand coordinated research enterprises that can deliver fact-based interpretation of soil quality indicators within the type of normative soil health framework conceived by USDA over 20 years ago. We look to existing examples as we consider how to put soil health information into the hands of practitioners in a manner that protects soils' services.

Resilient agroecosystems are foundational for stable and profitable food, feed, and fiber production in the face of increasing climatic perturbations and environmental stresses. Enhanced soil and environmental benefits of cover crops applied to conservation tillage systems has been well documented. However, little is known about the timespan for interactions of no tillage and cover crops to achieve enhanced yield and yield stability while lowering N fertilization. Using a long-term continuous cotton experiment in southeastern USA, we analyzed yield data collected from 1986 to 2018 from 32 management systems to identify how management duration controls the synergistic effect of applied mineral N rates (0, 34, 67, and 101 kg ha−1), cover crops (no cover [NC], hairy vetch [HV], crimson clover [CC], and winter wheat [WW]), and tillage practices (no tillage [NT] and conventional tillage [CT]) on cotton yield and yield stability. Yield stability was analyzed using Finlay—Wilkinson regression model, Wricke’s Ecovalance, and coefficient of variation, and a mixed model approach was used to compare the yield and yield stability within three time phases (1−10 years, 11–20 years, and 21−33 years) at the 95% confidence level. During the initial 10 yr period (phase 1) CT resulted in greater cotton yield (7%) and yield stability than NT. However, in phase 2 (11–20 years) and phase 3 (21−33 years) NT led to greater yield (7%) and yield stability for almost all cover crop × N interactions, except for zero N following NC and WW. Increased N during the initial phase reduced both yield and yield stability under legume cover crops. During phases 2 and 3, however, the higher N rates (67 and 101 kg N ha−1) increased cotton yield, although management systems with 0 and 101 kg N ha−1 showed the largest temporal yield variability. Legume cover crops increased yield and yield stability under low N rates. The maximum combined yield and yield stability benefit was obtained from HV cover on NT with additional application of 34 kg N ha−1. Our results suggest that after the initial phase NT delivers the most consistent yield benefits while enhancing yield stability against unfavorable environmental conditions. Long-term integration of legume cover crops (particularly HV) to NT systems was effective in maintaining high yield and increasing yield stability while lowering N rates.

Monitoring long-term grazing management practices influence on soil quality (SQ) is essential to ensuring pasture sustainability, which is the largest land use in world agroecosystems. The aim of this study was to quantify SQ based on long-term (15-years) conservation pasture management and landscape position using the Soil Management Assessment Framework (SMAF). Treatments were setup in 15 watersheds (0.14 ha each, 8% slope) in a completely randomized design with five pasture management practices: continuously grazed (CG), hayed (H), rotationally grazed (R), rotationally grazed with an unfertilized buffer strip (RB), and rotationally grazed with an un-grazed, unfertilized, and fenced riparian strip (RBR). Each watershed was divided in three zones (A, B, and C), with the riparian buffer strip (RBS) corresponding to the RBR D zone. Selected soil chemical, physical, and biological properties were determined on soil samples collected in 2017 (0−15 cm depth) per zone. Total P, organic C, and total suspended solids (TSS) were measured in 2017 runoff samples. The SMAF SQ scores were evaluated individually and as an overall SQ index (SQI). Exponential models were used to investigate the relationship between SQI and total P, organic C runoff, and TSS loads. Continuously grazed watersheds had improved soil fertility, with greater nutrient concentration at the shoulder landscape position (zone A). Degradation of soil physical properties were not observed for this practice. After 15 years of continuous management, CG and R watersheds had the greatest SQI (7.07 and7.05, respectively), not differing from RBR (6.93), likely owing to cattle manure deposition for these treatments. Increased SQI in RBS (7.33) improved SQI for RBR watersheds. Differences in SQI were mostly driven by changes in pH, electrical conductivity, soil P and K concentrations. The exponential models indicated that 34 and 28 % of the variation in P and TOC runoff loads, respectively, can be explained by the SMAF SQI (p

Crop residue retention and minimum tillage (including no-tillage, NT, and reduced tillage, RT) are common conservation tillage practices that have been extensively applied for improving soil health and reducing the negative environmental impact caused by intensive farming. However, the effects of minimum tillage, coupled with crop residue retention (including no-tillage plus residue retention, NTR, and reduced tillage plus residue retention, RTR), on soil organic carbon (SOC) stock have not been systematically analyzed. Using a dataset consisting of 1928 pairs of data points from 243 studies, we conducted a global meta-analysis to evaluate the effects of crop residue retention and minimum tillage on SOC stock in the 0-30 cm soil and how these effects varied with soil (soil sampling depth and texture), environmental (climate) and crop management conditions (cropping intensity), as well as treatment duration. We found that regardless of the climatic condition, crop management, or residue retention, minimum tillage alone increased the overall mean SOC stock. Specifically, NT and RT increased SOC stock by 11 and 6%, respectively, in comparison to conventional tillage (CT). Compared with CT, NTR and RTR increased SOC stock by 13 and 12%, respectively. The above effects were greater in the topsoil (62% of data points from the 0-15 cm depth) than in the subsoil (38% of data points from the 15-30 cm depth). Moreover, residue retention enhanced the resistance of SOC turnover to agricultural and environmental factors; mean annual temperature (coefficient = 0.15), soil pH (0.14), and experimental duration (0.08) were critical for increasing SOC stock with minimum tillage alone, while the response ratio of SOC stock under coupled residue retention and minimum tillage was insensitive to changes in those factors. Additionally, double cropping generally increased SOC stock cross all conservation tillage practices compared to multiple cropping. Therefore, we conclude that minimum tillage coupled with residue retention in a double-cropping system is the most promising management system for increasing SOC stocks in the 0-30 cm soil in croplands Our finding can inform sustainable soil management practices aimed at increasing resistance of SOC in croplands to climate change and soils degradation induced by intensive agriculture.

Agricultural management practices control soil organic carbon (SOC) content in croplands. Long-term cropping system experiments offer a great opportunity to understand the magnitude and direction of SOC change in response to management practices. Such information is very limited from the southeastern US, a region with warm and humid climatic conditions that typically favor SOC decomposition over accumulation. Therefore, this study was conducted to assess the effect of 39 years of chisel plow (CP), disc plow (DP), moldboard plow (MP), no-tillage (NT), NT with winter wheat (Triticum aestivum L.) cover crop (NTW), and NT with wheat-soybean (Glycine max L.) double crop (NTWD) on total SOC and SOC fractions including permanganate oxidizable C (POXC), water extractable C (WEC), resistant C (RC), and aggregate-associated SOC in a continuous soybean system. Additionally, aggregate size distribution, mean weight diameter (MWD), and wet aggregate stability (WAS) were determined. Results showed that NTW and NTWD significantly increased SOC and POXC compared to MP with mean SOC (g kg⁻¹ soil) of 12.2 (NTW) ≥10.9 (NTWD) >7.2 (MP) and mean POXC (mg kg⁻¹ soil) of 465 (NTWD) ≥418 (NTW) >252 (MP). The WEC and RC fractions did not differ among treatments. Across the treatments, the greatest aggregate-associated SOC concentration was found in microaggregates (0.053–0.25 mm) and the lowest in clay- and silt-size particles ( 0.6, p

Grazing land ecosystem services including food provision and climate regulation are greatly influenced by soil health. This paper provides a condensed review of studies on the response of three important soil properties related to soil health to grazing land management: water infiltration, carbon (C) sequestration, and nitrogen use efficiency (NUE). Impacts of management strategies that are often used in grazing lands are discussed in this review including vegetation composition, grazing methods, and other factors such as fertilizer use and climatic conditions. In general, proper grazing management such as continuous moderate grazing and rotational/deferred-rotational grazing with low or moderate stocking rates tends to benefit all three soil properties. Water infiltration can usually be increased with full vegetation cover, increased soil C, and aggregate stability, or be decreased with greater soil bulk density. Adoption of highly productive plant species with faster turnover rates can promote soil C sequestration by increasing C input. However, excessive C removal from ecosystems due to overgrazing or improper soil fertilization management results in higher C loss, which can have detrimental effects on soil C sequestration. Proper stocking rate and a balanced manure/fertilizer management was found to be critical for enhancing NUE. Grazing land management sometimes simultaneously influence the three soil properties. Techniques that can increase soil C such as introduction of high productive plant species can often promote water infiltration and soil nitrogen (N). Some other practices such as adoption of N fertilizer may enhance C sequestration while being detrimental to NUE. An integrated management plan for a specific location or farm should be considered carefully to improve soil health as well as ecosystem production. This review provides farmers and policy makers the current state of general knowledge on how health-related soil processes are affected by grazing land management.

Excessive crop residue returned to the soil hinders farm operations, but residue removal can affect soil quality. In contrast, cover cropping can return additional residue to the soil and improve soils and environmental quality compared with no cover cropping. Residue and cover crop impacts on soil surface greenhouses gas (GHG) emissions are undetermined and site specific. Thus, the present study was conducted to investigate the impacts of corn ( L.) residue management and cover cropping on GHG fluxes. The fluxes were measured from 2013 to 2015 using static chamber under corn and soybean [ (L.) Merr.] rotation initiated in 2000 at Brookings, SD. Treatments included two residue management levels (residue returned [RR] and residue not returned [RNR]) and two cover cropping (cover crops [CC] and no cover crops [NCC]). Results showed that RR under corn and soybean phases significantly reduced cumulative CO fluxes (2681.3 kg ha in corn and 2419.8 kg ha in soybeans) compared with RNR (3331.0 kg ha in corn and 2755.0 kg ha in soybeans) in 2013. The RR emitted significantly less cumulative NO fluxes than RNR from both the phases in 2013 and 2014, but not in 2015. The CC treatment had significantly lower cumulative NO fluxes than the NCC for corn and soybean phases in 2013 and 2014. We conclude that crop residue retention and cover cropping can mitigate the GHG emissions compared with residue removal and no cover cropping.

The rate and extent of decomposition of soil organic carbon (SOC) is dependent, among other factors, on substrate chemistry and microbial dynamics. Our objectives were to understand the influence of substrate chemistry on microbial decomposition of carbon (C), and to use model fitting to quantify differences in pool sizes and mineralization rates. We conducted an incubation experiment for 270 days using four uniformly labeled 14C substrates (glucose, starch, cinnamic acid and stearic acid) on four different soils (a temperate Mollisol, a tropical Ultisol, a sub-arctic Andisol, and an arctic Gelisol). The 14C labeling enabled us to separate CO2 respired from added substrates and from native SOC. Microbial gene copy numbers were quantified at days 4, 30 and 270 using quantitative polymerase chain reaction (qPCR). Substrate C respiration was always higher for glucose than other substrates. Soils with cinnamic and stearic acid lost more native SOC than glucose- and starch-amended soils. Cinnamic and stearic acid amendments also exhibited higher fungal gene copy numbers at the end of incubation compared to unamended soils. We found that 270 days were sufficient to model the decomposition of simple substrates (glucose and starch) with three pools, but were insufficient for more complex substrates (cinnamic and stearic acid) and native SOC. This study reveals that substrate quality exerts considerable control on the microbial decomposition of newly added and native SOC, and demonstrates the need for multi-year incubation experiments to constrain decomposition parameters for the most recalcitrant fractions of SOC and complex substrates.

Dissolved organic carbon (DOC) transported from the soil surface is stabilized in deeper soil profiles by physico-chemical sorption processes. However, it is unclear how different forms of organic carbon (OC) compounds common in soil organic matter interact with soil minerals in the surface (A) and subsurface (B) horizons. We added four compounds (glucose, starch, cinnamic acid and stearic acid) to the silt- and clay-sized fraction (fine fraction) of A and B horizons of eight soils from varying climates (3 temperate, 3 tropical, 1 arctic and 1 sub-arctic). Equilibrium batch experiments were conducted using 0 to 100 mg C L− 1 of 14C-labeled compounds for 8 h. Sorption parameters (maximum sorption capacity, Qmax and binding coefficient, k) calculated by fitting sorption data to the Langmuir equation showed that Qmax of A and B horizons was very similar for all compounds. Both Qmax and k values were related to sorbate properties, with Qmax being lowest for glucose (20–500 mg kg− 1), highest for stearic acid (20,000–200,000 mg kg− 1), and intermediate for both cinnamic acid (200–4000 mg kg− 1) and starch (400–6000 mg kg− 1). Simple linear regression analysis revealed that physico-chemical properties of the sorbents influenced the Qmax of cinnamic acid and stearic acid, but not glucose and starch. The sorbent properties did not show predictive ability for binding coefficient k. By using the fine fraction as sorbent, we found that the mineral fractions of A horizons are equally reactive as the B horizons irrespective of soil organic carbon content.

Conservation tillage practices, here defined as no-tillage (NT) or reduced tillage (RT) with/without residue retention, have been widely used to alleviate the negative effects caused by intensive tillage practices. Implementing effective and sustainable agriculture requires a deeper understanding of the impacts of conservation tillage practices on soil physical properties. This study examined the effects of conservation tillage practices on soil physical properties, including soil bulk density, aggregate size and stability, hydraulic properties, and soil pH; based on data collected from 264 studies published worldwide since 1980. The results indicated that no-tillage (NT), NT with residue retention (NTS), and reduced tillage (RT) increased bulk density by 1.4, 2.6, and 2.1%, respectively, compared with conventional tillage (CT). Soil bulk density decreased by 2.9% in NTS compared with NT, and 3.9% in RT with residue retention (RTS) compared with RT. The effect size of bulk density significantly decreased with the increasing experimental duration under NT and NTS practices. Compared to CT, conservation tillage practices increased aggregate mean weight diameter (MWD), geometric mean weight diameter and water stable aggregate (WSA) regardless of the residue retention or minimum tillage systems. The largest effect size of MWD (51.9%) and WSA (54.9%) appeared under NTS as compared to the CT. The effect size of MWD and WSA increased under NT with the increasing experimental duration. NT increased saturated hydraulic conductivity by 24.6% compared to CT. All conservation tillage practices increased soil available water capacity (AWC) compared with CT and NTS with a 10.2% increase in AWC compared with NT. The effect size of AWC increased under RT and NT practices with the increasing experimental duration. Soil pH decreased by 1.7 and 1.0% under RTS compared with RT and CT, respectively; and NT led to a 2.8% reduction in soil pH compared with CT. The effect size of soil pH decreased under RT and NT treatments with the increasing experiment duration. Overall, conservation tillage practices positively affected many soil physical properties; and the extent of the effects varied with the duration of the experiment.

Soil organic carbon (SOC) content is strongly influenced by agricultural management practices. Leveraging three long-term field experiments in Tennessee, this study analyzed the effect of crop rotation, tillage, and cover crops on SOC and permanganate-oxidizable C (POXC) at various soil depth increments (0–2.5, 2.5–5, 5–7.5, 7.5–10, 10–15, 15–22.5, 22.5–30, 30–45, and 45–60 cm), as well as at the 0- to 15-cm profile. Corn ( L.) and cotton ( L.)–based systems showed increased SOC from 0 to 22.5 cm compared with soybean [ (L.), Merr.]–based systems. Additionally, no-till systems accumulated more SOC than tilled systems from 0 to 5 cm, but cover cropping showed no effects. Results also showed a significant positive relationship between SOC and POXC ( < 0.0001, = 0.93). This study revealed that SOC and POXC are strongly influenced by management practices in the surface shallower depth increments of southeastern US croplands.

There have been increasing efforts to understand the dynamics of organic carbon (OC) associated with measurable fractions of bulk soil. We compared the decomposition of native OC (native C) with that of an added substrate (glucose) on physically separated fractions of a diverse suite of soils. Five soil orders were selected from four contrasting climate zones (Mollisol from temperate, Ultisol and Oxisol from tropics, Andisol from sub-arctic, and Gelisol from arctic region). Soils from the A horizon were fractionated into particulate OC (POC) and mineral-associated OC (MOC) by a size-based method. Fractions were incubated at 20 °C and 50 % water-holding capacity in the dark after the addition of unlabeled d-glucose (0.4 mg C g−1 fraction) and U–14C glucose (296 Bq g−1 fraction). Respiration of glucose 14C indicated 64 to 84 % of added glucose 14C which was respired from POC and 62 to 70 % from MOC within 150 days of incubation, with more than half of the cumulative respiration occurring within 4 days. Native C respiration varied widely across fractions: 12 to 46 % of native C was respired from POC and 3 to 10 % was respired from MOC fractions. This suggested that native C was more stabilized on the MOC than on the POC, but respiration from the added glucose was generally similar for MOC and POC fractions. Our study suggests a fundamental difference between the behavior of freshly added C and native C from MOC and POC fractions of soils.

Organic carbon (OC) stabilization in soils plays a significant role in the global C cycle, therefore understanding the structure and function of the OC–soil mineral interface is of high importance. To study the interface, films of simple OC compounds and natural organic matter (NOM) were deposited onto a soil mineral analogue (Al2O3) using spin coating and were exposed to humidity. The thickness, density and structure of the films were studied using a depth-sensitive, nano-scale technique of neutron reflectometry. A single homogenous layer was observed when NOM and glucose (GL) were adsorbed onto Al2O3. However, when stearic acid (SA) was added to either NOM or GL, separate layers attributed to SA and either NOM or GL were detected. The formation of distinct, immiscible layers is due to insolubility of SA with NOM and GL. In contrast, GL and NOM are both water-soluble, and therefore soluble with each other, forming a homogenous layer on the mineral surface. Our results suggest that the extent of complex layering formed on the OC–mineral interface may depend on the relative solubility of the compounds.

© 2016 The Authors. Reoxidation and mobilization of previously reduced and immobilized uranium by dissolved-phase oxidants poses a significant challenge for remediating uranium-contaminated groundwater. Preferential oxidation of reduced sulfur-bearing species, as opposed to reduced uranium-bearing species, has been demonstrated to limit the mobility of uranium at the laboratory scale yet field-scale investigations are lacking. In this study, the mobility of uranium in the presence of nitrate oxidant was investigated in a shallow groundwater system after establishing conditions conducive to uranium reduction and the formation of reduced sulfur-bearing species. A series of three injections of groundwater (200 L) containing U(VI) (5 μM) and amended with ethanol (40 mM) and sulfate (20 mM) were conducted in ten test wells in order to stimulate microbial-mediated reduction of uranium and the formation of reduced sulfur-bearing species. Simultaneous push-pull tests were then conducted in triplicate well clusters to investigate the mobility of U(VI) under three conditions: 1) high nitrate (120 mM), 2) high nitrate (120 mM) with ethanol (30 mM), and 3) low nitrate (2 mM) with ethanol (30 mM). Dilution-adjusted breakthrough curves of ethanol, nitrate, nitrite, sulfate, and U(VI) suggested that nitrate reduction was predominantly coupled to the oxidation of reduced-sulfur bearing species, as opposed to the reoxidation of U(IV), under all three conditions for the duration of the 36-day tests. The amount of sulfate, but not U(VI), recovered during the push-pull tests was substantially more than injected, relative to bromide tracer, under all three conditions and further suggested that reduced sulfur-bearing species were preferentially oxidized under nitrate-reducing conditions. However, some reoxidation of U(IV) was observed under nitrate-reducing conditions and in the absence of detectable nitrate and/or nitrite. This suggested that reduced sulfur-bearing species may not be fully effective at limiting the mobility of uranium in the presence of dissolved and/or solid-phase oxidants. The results of this field study confirmed those of previous laboratory studies which suggested that reoxidation of uranium under nitrate-reducing conditions can be substantially limited by preferential oxidation of reduced sulfur-bearing species.

A better understanding of the effect of long-term tillage management on soil properties and yield is essential for sustainable food production. This research aimed to evaluate the 37-year impact of different tillage systems and cover cropping on soil hydro-physical properties at 0&ndash, 15 and 15&ndash, 30 cm, as well as on soybean [Glycine max (L.) Merr] yield. The long-term experiment was located in Jackson, TN, and the different treatments involved in this study were no-tillage (NT), disk (DP), chisel (CP), moldboard plow (MP), and no-tillage with winter wheat [Triticum aestivum (L.)] cover crop (NTW). Forty-five days after the tillage operation, MP showed a comparable bulk density (BD) with NT, NTW, and CP at 0&ndash, 15 cm depth. At surface depth, No-tillage systems increased cone penetration resistance (PR) by 12% compared with the reduced tillage systems, and 47% relative to MP. Wet aggregate stability (WAS) at surface depth was 27% and 36% greater for NT systems than for reduced and conventional tillage systems, respectively. Similarly, the geometric mean diameter (GMD) of aggregates was significantly higher under NT and NTW. However, water infiltration and field-saturated hydraulic conductivity (Kfs) did not differ significantly among tillage systems. The greatest soybean yield was obtained from CP and DP, producing 10% higher yield than NTW. Overall, 37 years of no-tillage, with or without simplified cover cropping did not result in a consistent improvement in soybean yield and soil physical properties with the exception of having improved soil aggregation.

Soil organic carbon (SOC) is distributed heterogeneously among different-sized primary particles and aggregates. Further, the SOC associated with different physical fractions respond differently to managements. Therefore, this study was conducted with the objective to quantify the SOC associated with all the three structural levels of SOC (particulate organic matter, soil separates and aggregate-size fractions) as influenced by long-term change in management. The study also aims at reevaluating the concept that the SOC sink capacity of individual size-fractions is limited. Long-term tillage and crop rotation effects on distribution of SOC among fractions were compared with soil from adjacent undisturbed area under native vegetation for the mixed, mesic, Typic Fragiudalf of Wooster, OH. Forty five years of no-till (NT) management resulted in more SOC accumulation in soil surface (0–7.5 cm) than in chisel tillage and plow tillage (PT) treatments. However, PT at this site resulted in a redistribution of SOC from surface to deeper soil layers. The soils under continuous corn accumulated significantly more SOC than those under corn–soybean rotation at 7.5–45 cm depth. Although soil texture was dominated by the silt-sized particles, most of the SOC pool was associated with the clay fraction. Compared to PT, the NT treatment resulted in (i) significantly higher proportion of large macroaggregates (>2,000 μm) and (ii) 1.5–2.8 times higher SOC concentrations in all aggregate-size classes. A comparative evaluation using radar graphs indicated that among the physical fractions, the SOC associated with sand and silt fractions quickly changed with a land use conversion from native vegetation to agricultural crops. A key finding of this study is the assessment of SOC sink capacity of individual fractions, which revealed that the clay fraction of agricultural soils continues to accumulate more SOC, albeit at a slower rate, with progressive increase in total SOC concentration. However, the clay fraction of soil under native woodlot showed an indication for SOC saturation. The data presented in this study from all the three structural levels of SOC would be helpful for refining the conceptual pool definitions of the current soil organic matter prediction models.

Continued loss of topsoil due to erosion is a threat to sustaining row-crop production in soils of the US Cornbelt. The on-site impacts of soil erosion under natural field conditions, despite the confounding effects of many interacting factors, can be simulated by creating a range of topsoil depths (TSD) through soil removal from or addition to the existing soil surface. Thus, this study was conducted on two Alfisols in central Ohio, located at Waterman Farm of the Ohio State University, Columbus (site 1) and Western Agricultural Experiment Station of the Ohio Agricultural Research and Development Center at South Charleston (site 2) with the objectives to assess (1) impacts of differences in TSD after 10 years of creating simulated erosion on crop yields and soil properties and (2) effectiveness of using organic manure and synthetic fertilizer in restoring the quality of eroded soil. The treatments included (1) three levels of TSD including removal of 20 cm (8 in) of topsoil, undisturbed soil, and addition of 20 cm (8 in) of topsoil; and (2) two amendment types including organic manure and synthetic fertilizer. The results indicated that at site 1, the grain yield response followed the order: topsoil removal (3.1 Mg ha-1 [1.38 tn ac-1])

Evaluating the effects of management practices on soil physical and chemical properties would be valuable to explain field-level variability in crop production. A 23-year-old experiment on a Muscatune soil (fine-silty, mixed, superactive, mesic, Aquic Argiudolls) in Illinois with five N rates [0 (N 0 ), 70 (N 1 ), 140 (N 2 ), 210 (N 3 ) and 280 (N 4 ) kg N ha −1 ] and two cropping systems [continuous corn ( Zea mays L.) (CC), and corn–soybean ( Glycine max (L.) Merr.) rotation (CS)] was evaluated. Specific objectives were to: (i) evaluate the effects of long-term N fertilization and cropping systems on field level changes in soil physical and chemical properties and crop yield, (ii) identify the most responsive soil physical and chemical properties to N fertilizer and crop management, and (iii) investigate the relationship between the selected soil properties and crop yield. Soil was collected in May 2004 to 30 cm depth and 20 soil physical and chemical properties were measured. The univariate analysis indicated that 14 soil properties were significantly influenced by at least one treatment effect (crops, N or crops × N). Due to multicollinearity among soil properties, principal component analysis (PCA) was used to group correlated properties, resulting in five soil properties such as soil organic carbon stock (OC stock), mean weight diameter (MWD), soil C:N ratio, exchangeable potassium (K + ) and gravimetric moisture content ( ω ). Finally, the multiple regression analysis performed between PCA derived soil properties and corn and soybean yields retained all the representative soil properties from PCA except ω as yield predictors for corn ( P R 2 = 0.39) from CC system, whereas none of the soil properties were significantly related to corn and soybean yields from CS system. The soil properties most influenced by long-term N fertilization of continuous corn were successfully identified with PCA and multiple regression. The insignificant relationship between corn and soybean yields from CS system and PCA derived soil properties might be due to the lack of response of soybean to N fertilization. This study shows the integrated use of multivariate and regression analyses in identifying yield determining soil properties by eliminating the multicollinearity among soil properties.

Agricultural soils can be a major sink for atmospheric carbon (C) with adoption of recommended management practices (RMPs). Our objectives were to evaluate the effects of nitrogen (N) fertilization and cropping systems on soil organic carbon (SOC) and total N (TN) concentrations and pools. Replicated soil samples were collected in May 2004 to 90 cm depth from a 23-year-old experiment at the Northwestern Illinois Agricultural Research and Demonstration Center, Monmouth, IL. The SOC and TN concentrations and pools, soil bulk density ( ρ b ) and soil C:N ratio were measured for five N rates [0 (N 0 ), 70 (N 1 ), 140 (N 2 ), 210 (N 3 ) and 280 (N 4 ) kg N ha −1 ] and two cropping systems [continuous corn ( Zea mays L.) (CC), and corn–soybean ( Glycine max (L . ) Merr.) rotation (CS)]. Long-term N fertilization and cropping systems significantly influenced SOC concentrations and pools to 30 cm depth. The SOC pool in 0–30 cm depth ranged from 68.4 Mg ha −1 for N 0 to 75.8 Mg ha −1 for N 4 . Across all N treatments, the SOC pool in 0–30 cm depth for CC was 4.7 Mg ha −1 greater than for CS. Similarly, TN concentrations and pools were also significantly affected by N rates. The TN pool for 0–30 cm depth ranged from 5.36 Mg ha −1 for N 0 to 6.14 Mg ha −1 for N 4 . In relation to cropping systems, the TN pool for 0–20 cm depth for CC was 0.4 Mg ha −1 greater than for CS. The increase in SOC and TN pools with higher N rates is attributed to the increased amount of biomass production in CC and CS systems. Increasing N rates significantly decreased ρ b for 0–30 cm and decreased the soil C:N ratio for 0–10 cm soil depth. However, none of the measured soil properties were significantly correlated with N rates and cropping systems below 30 cm soil depth. We conclude that in the context of developing productive and environmentally sustainable agricultural systems on a site and soil specific basis, the results from this study is helpful to strengthening the database of management effects on SOC storage in the Mollisols of Midwestern U.S.

Multispecies cover cropping has become popular in recent years because of the multiple ecosystem benefits compared with single- or double- species cover cropping. However, scientific studies on the effects of multispecies cover cropping—especially in the southern United States—are limited. A field study was initiated in 2013 at the University of Tennessee’s Research and Education Center in Milan, TN, to assess the agronomic and soil responses from single-, double-, and multispecies cover cropping in corn ( L.)–soybean [ (L.) Merr.] systems. After 3 yr, we found that a multispecies mixture of legumes, grasses, and spp. significantly increased soybean yield, gravimetric soil water content, and soil inorganic nitrogen as compared to the less-diverse treatments and a no-cover control. However, after 3 yr, cover cropping did not increase soil organic carbon. Although multispecies cover cropping exhibited a positive effect on yield and some soil properties after 3 yr, we plan to continue collecting multiple years of data from this field trial.

Climate feedbacks from soils can result from environmental change followed by response of plant and microbial communities, and/or associated changes in nutrient cycling. Explicit consideration of microbial life-history traits and functions may be necessary to predict climate feedbacks owing to changes in the physiology and community composition of microbes and their associated effect on carbon cycling. Here we developed the microbial enzyme-mediated decomposition (MEND) model by incorporating microbial dormancy and the ability to track multiple isotopes of carbon. We tested two versions of MEND, that is, MEND with dormancy (MEND) and MEND without dormancy (MEND_wod), against long-term (270 days) carbon decomposition data from laboratory incubations of four soils with isotopically labeled substrates. MEND_wod adequately fitted multiple observations (total C-CO2 and (14)C-CO2 respiration, and dissolved organic carbon), but at the cost of significantly underestimating the total microbial biomass. MEND improved estimates of microbial biomass by 20-71% over MEND_wod. We also quantified uncertainties in parameters and model simulations using the Critical Objective Function Index method, which is based on a global stochastic optimization algorithm, as well as model complexity and observational data availability. Together our model extrapolations of the incubation study show that long-term soil incubations with experimental data for multiple carbon pools are conducive to estimate both decomposition and microbial parameters. These efforts should provide essential support to future field- and global-scale simulations, and enable more confident predictions of feedbacks between environmental change and carbon cycling.

The complexity of the mineral-organic carbon interface may influence the extent of stabilization of organic carbon compounds in soils, which is important for global climate futures. The nanoscale structure of a model interface was examined here by depositing films of organic carbon compounds of contrasting chemical character, hydrophilic glucose and amphiphilic stearic acid, onto a soil mineral analogue (Al2O3). Neutron reflectometry, a technique which provides depth-sensitive insight into the organization of the thin films, indicates that glucose molecules reside in a layer between Al2O3 and stearic acid, a result that was verified by water contact angle measurements. Molecular dynamics simulations reveal the thermodynamic driving force behind glucose partitioning on the mineral interface: The entropic penalty of confining the less mobile glucose on the mineral surface is lower than for stearic acid. The fundamental information obtained here helps rationalize how complex arrangements of organic carbon on soil mineral surfaces may arise.

Enzyme dynamics are being incorporated into soil carbon cycling models and accurate representation of enzyme kinetics is an important step in predicting belowground nutrient dynamics. A scarce number of studies have measured activation energy (Ea) in soils and fewer studies have measured Ea in arctic and tropical soils, or in subsurface soils. We determined the Ea for four typical lignocellulose degrading enzymes in the A and B horizons of seven soils covering six different soil orders. We also elucidated which soil properties predicted any measurable differences in Ea. β-glucosidase, cellobiohydrolase, phenol oxidase and peroxidase activities were measured at five temperatures, 4, 21, 30, 40, and 60°C. Ea was calculated using the Arrhenius equation. β-glucosidase and cellobiohydrolase Ea values for both A and B horizons in this study were similar to previously reported values, however we could not make a direct comparison for B horizon soils because of the lack of data. There was no consistent relationship between hydrolase enzyme Ea and the environmental variables we measured. Phenol oxidase was the only enzyme that had a consistent positive relationship between Ea and pH in both horizons. The Ea in the arctic and subarctic zones for peroxidase was lower than the hydrolases and phenol oxidase values, indicating peroxidase may be a rate limited enzyme in environments under warming conditions. By including these six soil types we have increased the number of soil oxidative enzyme Ea values reported in the literature by 50%. This study is a step towards better quantifying enzyme kinetics in different climate zones.

Accurate quantification of different soil organic carbon (SOC) fractions is needed to understand their relative importance in the global C cycle. Among the chemical methods of SOC fractionation, oxidative degradation is considered more promising because of its ability to mimic the natural microbial oxidative processes in soil. This study focuses on detailed understanding of changes in structural chemistry and isotopic signatures of SOC upon different oxidative treatments for assessing the ability of these chemicals to selectively isolate a refractory fraction of SOC. Replicated sampling (to ~1 m depth) of pedons classified as Typic Fragiudalf was conducted under four land uses (woodlot, grassland, no-till and conventional-till continuous corn [Zea mays L.]) at Wooster, OH. Soil samples (

Physico-chemical sorption of dissolved organic carbon (DOC) on soil minerals is one of the major processes of organic carbon (OC) stabilization in soils, especially in deeper layers. The attachment of C on soil solids is related to the reactivity of the soil minerals and the chemistry of the sorbate functional groups, but the sorption studies conducted without controlling microbial activity may overestimate the sorption potential of soil. This study was conducted to examine the sorptive characteristics of a diverse functional groups of simple OC compounds (D-glucose, L-alanine, oxalic acid, salicylic acid, and sinapyl alcohol) on temperate climate soil orders (Mollisols, Ultisols and Alfisols) with and without biological degradative processes. Equilibrium batch experiments were conducted using 0-100 mg C L-1 at a solid-solution ratio of 1:60 for 48 hrs and the sorption parameters were calculated by Langmuir model fitting. The amount of added compounds that remained in the solution phase was detected by high performance liquid chromatography (HPLC) and total organic C (TOC) analysis. Soil sterilization was performed by -irradiation technique and experiments were repeated to determine the contribution of microbial degradation to apparent sorption. Overall, Ultisols did not show a marked preference for apparent sorption of any of the modelmore » compounds, as indicated by a narrower range of maximum sorption capacity (Smax) of 173-527 mg kg soil-1 across compounds. Mollisols exhibited a strong preference for apparent sorption of oxalic acid (Smax of 5290 mg kg soil-1) and sinapyl alcohol (Smax of 2031 mg kg soil-1) over the other compounds. The propensity for sorption of oxalic acid is mainly attributed to the precipitation of insoluble Ca-oxalate due to the calcareous nature of most Mollisol subsoils and its preference for sinapyl alcohol could be linked to the polymerization of this lignin monomer on 2:2 mineral dominated soils. The reactivity of Alfisols to DOC was in between that of Ultisols and Mollisols. HPLC results revealed significantly higher sorption of D-glucose and L-alanine than did TOC results, and duplicate experiments with sterilized soils confirmed that glucose and alanine were mineralized leading to higher apparent sorption values via HPLC. This study demonstrated that three common temperate soil orders experienced differential sorption of simple OC compounds, indicating that sorbate chemistry plays a significant role in the sorptive stabilization of DOC.« less