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The variation in the soil microbial community composition over time was assessed at monthly time steps for 1 year in three neighboring Mollisols spanning a drainage gradient. This was done to distinguish between natural oscillations in the community composition versus lasting adaptations to environmental factors such as soil water availability. To isolate soil water availability as a controlling factor, we selected three soils sharing the same soil order (fine‐silty, superactive Argixerolls/Argialbolls); slope (0%–1%); temperature regime (mesic); moisture regime (xeric); and land use history (continuous grassland for the past 10 years) but differing in drainage class (well‐drained vs. moderately well‐drained vs. poorly drained). Changes in microbial diversity were quantified by monitoring the bacterial community at monthly intervals for 1 year. Within individual soils, α‐diversity varied little with season and drainage classes. Despite the three soils experiencing the same climate regime and vegetation/land use, they exhibited distinct community composition and turnover, which we attribute to differences in moisture availability across drainage and seasons. We posit that a seasonal recurring drop in soil redox potential induced by seasonal water saturation in the poorly drained soil is the most probable cause setting the microbial community of that soil apart from those in the better drained soils. Our investigation suggests that not all indicators of microbial diversity share the same sensitivity to seasonal and drainage‐related soil moisture variations. Core Ideas: Seasonal moisture fluctuations have a small effect on bacterial phylogenetic diversity.Phylogenetic diversity may increase with improved drainage.Impacts of seasonal saturation on the microbial community persist year‐round.Responsiveness to seasonal moisture fluctuations can influence the viability of microbial abundance as a bioindicator. [ABSTRACT FROM AUTHOR]

We used mid‐infrared (MIR) spectra (4000–600 cm−1) to identify and classify soil orders and soil horizons from 102 pedons across five soil orders (Alfisols, Entisols, Mollisols, Spodosols, and Histosols). The soils were analyzed for texture, total carbon, pH, and elemental properties. Random forest models were used to group the spectra of master horizons (O, A, E, B, and C), B horizons (Bs, Bt, and Bw), and the five soil orders. The prediction accuracies for the master horizons and B horizons were 0.81 and 0.89, respectively. The Kappa coefficient was 0.71 for the prediction of master horizons and 0.73 for the prediction of B horizons. The soil orders had an overall accuracy of 0.73 and a Kappa coefficient of 0.64. Histosols exhibited unique absorption characteristics at 2930 and 2860 cm−1 that differed distinctly from mineral soils. The MIR spectra accurately distinguished the O horizons. The spectral curve of topsoil of Spodosols was comparable to the O horizons. Spodosols under forest had A horizons with high organic matter and were classified accurately. Entisols (Psamments) displayed absorption peaks associated with sand, facilitating their differentiation from the other soil orders. The model struggled to discern subtle differences among some soil orders, and identification is hampered if soils undergo irreversible changes upon drying. However, our results showed that MIR spectra can be used for effectively identifying and classifying soil orders as well as soil horizons. Core Ideas: Mid‐infrared (MIR) spectra measured from 102 pedons across nine regions in Wisconsin.Unique adsorption characteristics in MIR spectra reflected pedogenetic features and horizons.MIR spectra and random forest machine learning correctly identified soil horizons and orders.Master horizons were classified with an accuracy of 0.81 and B horizons with an accuracy of 0.89.Soil orders were classified with an accuracy of 0.73. [ABSTRACT FROM AUTHOR]

The extent to which cover crops (CCs) accumulate soil organic carbon (SOC) in the entire soil profile is still unclear. We measured SOC, permanganate oxidizable C (POX‐C), and particulate organic matter (POM) concentrations down to 60‐cm soil depth in early [2–3 week before corn (Zea mays L.) planting]‐ and late‐terminated (at corn planting) winter rye (Secale cereale L.) CCs in rainfed and irrigated no‐till continuous corn systems in the U.S. Corn Belt after 10 years. CCs increased SOC stock and SOC, POX‐C, and POM concentrations but only in the irrigated system in the upper 5‐cm depth. Late‐terminated CC increased SOC concentration by 4.710 ± 3.501 g kg−1 and accumulated SOC at 0.207 ± 0.145 Mg C ha−1 year−1. It increased POX‐C and POM concentrations, on average, by 1.194 times. CCs likely increased SOC in the irrigated system by producing more biomass (2.247 ± 0.370 Mg ha−1) than in the rainfed system (0.949 ± 0.338 Mg ha−1). At least 2 Mg ha−1 of CC biomass may be needed to increase SOC. Because winter CCs often produce <1 Mg ha−1 of biomass when typically planted late and terminated early, extending the CC growing window by terminating CCs at or after crop planting (planting green) may boost CC biomass and SOC accumulation, although high‐C soils or Mollisols, such as our study soils (>22 g C kg−1), may limit SOC gains. We submit CCs would sequester more SOC in low‐C, eroded, and low‐fertility soils. Overall, winter rye CCs minimally alter soil C in the soil profile in no‐till continuous corn systems after 10 years. Core Ideas: Late‐terminated cover crop (CC) accumulates 0.207 ± 0.145 Mg C ha−1 year−1 at one of two sites but only in the top 5 cm of the soil profile.CCs may not accumulate soil C if they produce <2 Mg biomass ha−1.Early‐terminated CC (2–3 weeks before corn planting) does not accumulate soil C due to low biomass production.Extending CC growing window by terminating CC at or after corn planting can favor gains in total SOC and labile C.CCs minimally alter soil C in the soil profile in no‐till continuous corn systems after 10 years. [ABSTRACT FROM AUTHOR]

Soil fertility diagnosis often omits subsoil measurements, impacting precision. Our objective was to compare the vertical distribution of nitrate and sulfate in agricultural Mollisols. Both anions were measured in 34 Mollisols of the Pampean region (Argentina) sampled to 160‐cm depth at 20 cm intervals. Nitrate exhibited a continuous downward trajectory, with maximum values at 0–20 cm (12.7 mg N kg−1) and minimum values at 140–160 cm (3.3 mg N kg−1). Sulfate displayed a sinuous pattern, with a minimum at 60–80 cm (3 mg S kg−1). The 60–160/0–160 cm concentration ratio was 42% for nitrate and 60% for sulfate, indicating greater topsoil stratification for nitrate. Predicting deep‐layer nitrate concentrations from topsoil data was more accurate than for sulfate. This poses a challenge for assessing soil S bioavailability, as subsoil sulfates go undetected in conventional sampling. Core Ideas: Nitrate content consistently decreased from the shallowest to the deepest soil layer.In contrast, sulfate exhibited a sinuous trajectory, with minimum values at 60–80 cm.Compared to sulfates, the stratification toward the topsoil was 35% higher for nitrates.Sulfates in the upper layers failed to predict sulfate concentrations in deep layers.Nitrate at 20–60 cm was an acceptable predictor of subsoil nitrate (60–160 cm). [ABSTRACT FROM AUTHOR]

Most synthetic nitrogen (N) fertilizers used for corn (Zea mays L.) production supply ammonium (NH4+), but N availability can be reduced by volatilization or microbial immobilization. The former process is eliminated and the latter reduced by fertilizing with nitrate (NO3−), which is best done in‐season to promote crop uptake while minimizing N loss by leaching or denitrification. To compare 15N uptake for in‐season surface applications of labeled potassium nitrate (KNO3), urea‐ammonium nitrate (UAN), and urea, field studies were conducted in two growing seasons on Mollisols and Alfisols located in production fields under second‐ or third‐year corn or a corn–soybean (Glycine max L. Merr.) rotation. In contrast to dry matter production and total N uptake, the three 15N sources often differed significantly in N derived from the labeled fertilizer (NDF15F) and fertilizer 15N uptake efficiency (F15NUE), both of which decreased in the order: KNO3 > UAN > urea. As evidenced by a laboratory incubation study using the same three 15N sources, this trend was attributed to differences in NH3 volatilization and microbial immobilization that was greater for ammoniacal than NO3− fertilizers. When surface applying a sidedressing to corn in the Midwestern United States, N uptake efficiency can be increased by the use of NO3− fertilizers. Core Ideas: A 2‐year 15N field study was conducted to compare ammoniacal and NO3− fertilizer sources applied in‐season.Fertilizer 15N uptake efficiency ranged from 17% to 50% for grain and from 22% to 55% for total aboveground biomass.Fertilizer 15N uptake decreased in the order: KNO3 > urea‐NH4NO3 > urea. [ABSTRACT FROM AUTHOR]

Moreover, soil acidification, with soil pH ranging from 4.0 to 4.5, simplified network structure and compelled denitrifiers to cooperate with each other in contrast with other soil pH ranges. Soil tillage practices and the resulting increase in water erosion is one of the most important anthropogenic processes to drive soil nutrients and SOC turnover in agricultural land. Abbreviations AOC aggregate-associated carbon iPOC intra-aggregate particulate organic carbon OC organic carbon SOC soil organic carbon SOM soil organic matter Mollisols are inherently productive and fertile soils that are extensively distributed on the steppes of North America, South America, Ukraine, and Northeast China. [Extracted from the article]

A mechanistic understanding of nutrient movement associated with the erosion process is required to formulate precision soil conservation measures. We explored the response of surface soil nutrients and soil organic carbon (SOC) fractions to tillage erosion and water erosion. Tillage and water erosion rates were estimated by the directional tillage erosion model and revised universal soil loss equation, respectively. One hundred and twelve surface soil samples (0–20 cm) were collected from a sloping farmland (3.6 ha) in the Mollisols region of China. Soils were analyzed for total nitrogen (TN), total phosphorus (TP), total SOC, particulate organic carbon (POC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC). Results showed that no significant relation between TN and tillage or water erosion rates exists at any slope position. The TP distribution is more affected by water erosion than tillage erosion. Water erosion also played a greater role in controlling distribution of DOC than tillage erosion, whereas POC distribution was more sensitive to tillage erosion. In addition, we observed a contrasting relationship between MBC and water erosion for the mild erosion (r = −.43, P <.05) vs. intense erosion scenario (r =.38, P <.05). This shift indicates a possible dual role of microbes in SOC cycling associated with water erosion: mild erosion (averaged 17.4 t ha−1 yr−1) depletes microbial biomass and contributes to SOC mineralization, whereas intense erosion (averaged 54.6 t ha−1 yr−1) may lead to a shift in microbial structure thus promoting the dynamic replacement of SOC at eroding sites. Core Ideas: Distribution of total P is more related to water erosion than tillage erosion.Water erosion controlled distribution of DOC, whereas POC was sensitive to tillage erosion.The role of soil microbes in SOC cycling might depend on erosion intensity. [ABSTRACT FROM AUTHOR]

Tillage practices and water erosion are the most important anthropogenic and natural processes, respectively, driving soil organic C turnover in agricultural land. The aim of this study was to explore the responses of soil organic C (SOC) turnover to tillage and runoff by comparing the variation of soil aggregate‐associated organic C (AOC) and intra‐aggregate particulate organic C (iPOC) under simulated tillage and runoff conditions. Soil samples were collected from a native vegetation land with no cultivation history in the Mollisol region of Northeast China. After a series of simulated tillage (ST) and simulated runoff (SR) treatments, the samples were incubated for 30 d and then separated through 2‐, 1‐, 0.25‐, and 0.053‐mm sieves by wet‐sieving to obtain different aggregate size fractions. Each aggregate fraction was subsequently shaken for 18 h in 0.5% hexametaphosphate to get different intra‐aggregate particle size fractions. The proportion of the fractions and their AOC and iPOC were determined. The ST treatment promoted the reaggregation of macroaggregates (>2 mm) by accelerating the turnover of their coarse iPOC (0.25–2 mm), leading to a lower concentration of AOC. Runoff transformed larger aggregates (>0.25 mm) to smaller particles (<0.25 mm) without catalyzing C turnover. Fine iPOC (0.053–0.25 mm) could serve as an indicator for AOC (>1 mm) dynamics, especially associated with tillage operations. Our findings highlight the different influences of tillage and runoff, and the negative effect of tillage on SOC dynamics. Core Ideas: The responses of SOC turnover to tillage and runoff are different.Tillage accelerates the turnover of >2 mm soil aggregate and its coarse iPOC (0.25–2 mm).Runoff transforms larger aggregates (>0.25 mm) into smaller particles (<0.25 mm) without catalyzing C turnover.Fine iPOC (0.053–0.25 mm) could serve as an indicator for AOC (>1 mm) dynamics. [ABSTRACT FROM AUTHOR]

Land use change influences soil organic carbon (SOC) distribution in soil matrix. Conversion of cropland to grassland or forestland has the potential to sequester more carbon and influence the carbon allocation. Soil carbon sequestration happens within different physical and chemical fractions. This study aimed to investigate the vertical distribution of SOC fractions under 29‐yr of cropland, grassland, and forestland of a typical Mollisol and to derive specific factors for SOC fraction changes associated with land use conversion. Soil samples within 100 cm soil depth were fractionated by density and humic methods. Biomass of litter residues and plant roots were also measured. Together with plant root biomass, the contents of total SOC and organic carbon (OC) fractions decreased along soil profile under three land uses. At each soil depth, total SOC and OC fractions followed the same order of grassland > forestland > cropland except for higher OC contents in heavy fraction, fulvic acid, and humin in cropland than those in forestland at the 0–10 cm layer. Compared with forestland and cropland, 29‐yr grassland exerted stronger effects on the profile distribution of labile OC (water‐soluble and light OC fractions) than recalcitrant OC (humic and heavy fractions), significances were observed at 0–80 cm layers for the labile OC and at 0–40 cm layers for the recalcitrant OC (P <.05). Larger root biomass and better soil structure in grassland might facilitate such changes. Grassland restoration, with greater C sequestration potential and larger liable OC contents, may help improve soil quality better than forestland and cropland in this Mollisol region. Core Ideas: Vertical distribution of SOC fractions under 29‐yr land use regimes was studied. 29‐yr grassland restoration accumulated more liable SOC fractions in 0–80 cm depth. Recalcitrant OC fractions at 0–40 cm depth were affected by land use changes. [ABSTRACT FROM AUTHOR]

Decreasing physical quality of Mollisols in the Argentinean Pampas region is observed due to simplified crop rotations. The main objective of this work was to evaluate the effects of cover cropping management on soil water capture, transport, and storage as compared with different crop rotations with bare fallow in two different and representative Mollisols of the Argentinean Pampas region (one Typic Argiudoll [TA] and one Typic Hapludoll [TH]). Water capture, transport, and storage processes were assessed through soil sorptivity, infiltration tests at different pressures heads, and soil water retention curve determination. In addition, aggregate stability and soil organic carbon (SOC) were determined and the relationship between studied variables and processes was evaluated. It was observed that soybean [Glycine max (L.) Merr.] monocultures jeopardize Mollisols conservation, decreasing water capture and transport and SOC content. The inclusion of cover cropping management increased the soil water transport in the TA and water capture in the TH, as compared with bare fallow rotations. In this sense, our results show that cover cropping could be a suitable management in order to recover degraded soils due to simplified crop rotations in Mollisols from the Argentina Pampas region. Core Ideas: Effects of cover crops on soil hydraulic properties depended on the soil type.Soybean monocultures decreased water capture and transport.Cover crops increased water transport in the Typic Argiudoll.Cover crops increased water capture in the Typic Hapludoll. [ABSTRACT FROM AUTHOR]

Recently, the USDA-NRCS National Cooperative Soil Survey Soil Characterization Database (NSCD) was reported to provide evidence that total nitrogen (TN) stocks of agricultural soils have increased across the Mississippi basin since 1985. Unfortunately, historical changes in methods used to measure TN were not fully accounted for in that report. We used NSCD archives to calibrate between wet (pre-1995) and dry (post-1995) digestion methods used in measuring TN and soil organic carbon (SOC), then evaluated temporal trends in SOC and TN stocks with data from 423 Alfisol and 900 Mollisol profiles representing the US Corn Belt. Data were grouped by moisture regime, farming history (presence of Ap horizon), and depth (0-20, 20-60, and 60-100 cm). Regressions showed geographic and textural influences on SOC, and that SOC increased with time among farmed soils, particularly at 20 to 60 cm. Soil TN was dependent on SOC, especially at the surface (R² > 0.71), and decreasing TN trends with time were found among farmed soils above 60-cm depth. Increases in C to N ratios further suggested TN has been slowly stabilizing within soils of the US Corn Belt. However, C to N ratios <9.0 were prevalent at 60-100 cm depth (>60% frequency), indicating large TN stores remain in these soils. An increasing trend in SOC, TN, and C to N ratios among non-farmed, aquic Mollisols suggested SOC and TN accumulations in wet soils typically located below croplands. Results suggest slow improvement in agricultural soils. However, resampling has not been broadly undertaken for NSCD soil profiles, hence use of this database to detect soil change should be approached cautiously. [ABSTRACT FROM AUTHOR]

The role of different soil properties on defining the P sorption capacity was investigated in a group of Mollisols of the Pampean Region (Argentina). Twosingle point and three-multiple point P sorption indices were evaluated. Both simple and multiple regression models indicated that ammonium oxalate extractable Al (Alox) and ammonium oxalate extractable Fe (Feox) were the main properties defining P sorption. Ammonium oxalate extractable Al prevailed as the best predictor of short-term P sorption indices and Feox as the best predictor for those indices that involves longer interaction periods. After Alox and Feox, the best correlations were obtained for the sum of Ca and Mg. The higher P retention of soils located in the Southern part of the Region was associated with the parental material rather than with anthropogenic effects. Obtained information can strengthen P fertilization programs by providing a methodology to estimate fertilizer P requirements to optimize crop yields while reducing environmental risks. [ABSTRACT FROM AUTHOR]