Soil organic matter and water-stable aggregates under different tillage and residue conditions in a tropical dryland agroecosystem

Changes in the proportions of water-stable soil aggregates, organic C, total N and soil microbial biomass C and N, due to tillage reduction (conventional, minimum and zero tillage) and crop residue manipulation (retained or removed) conditions were studied in a tropical rice–barley dryland agroecosystem. The values of soil organic C and total N were the highest (11.1 and 1.33 g kg⁻¹ soil, respectively) in the minimum tillage and residue retained (MT+R) treatment and the lowest (7.8 and 0.87 g kg⁻¹, respectively) in conventional tillage and residue removed (CT−R) treatment. Tillage reduction from conventional to minimum and zero conditions along with residue retention (MT+R,ZT+R) increased the proportion of macroaggregates in soil (21–42% over control). The greatest increase was recorded in MT+R treatment and the smallest increase in conventional tillage and residue retained (CT+R) treatment. The lowest values of organic C and total N (7.0–8.9 and 0.82–0.88 g kg⁻¹ soil, respectively) in macro- and microaggregates were recorded in CT−R treatment. However, the highest values of organic C and total N (8.6–12.6 and 1.22–1.36 g kg⁻¹, respectively) were recorded in MT+R treatment. The per cent increase in the amount of organic C in macroaggregates was greater than in microaggregates. In all treatments, macroaggregates showed wider C/N ratio than in microaggregates. Soil microbial biomass C and N ranged from 235 to 427 and 23.9 to 49.7 mg kg⁻¹ in CT−R and MT+R treatments, respectively. Soil organic C, total N, and microbial biomass C and N were strongly correlated with soil macroaggregates. Residue retention in combination with tillage reduction (MT+R) resulted in the greatest increase in microbial biomass C and N (82–104% over control). These variables showed better correlations with macroaggregates than other soil parameters. Thus, it is suggested that the organic matter addition due to residue retention along with tillage reduction accelerates the formation of macroaggregates through an increase in the microbial biomass content in soil.     

Animal manure and anhydrous ammonia amendment alter microbial carbon use efficiency,microbial biomass,and activities of dehydrogenase and amidohydrolases in semiarid agroecosystems

The potential negative impact of agricultural practices on soil and water quality is of environmental concern. The associated nutrient transformations and movements that lead to environmental concerns are inseparable from microbial and biochemical activities. Therefore, biochemical and microbiological parameters directing nitrogen (N) transformations in soils amended with different animal manures or inorganic N fertilizers were investigated. Soils under continuous corn cultivation were treated with N annually for 5 years at 56, 168, and 504 kg N ha⁻¹ in the form of swine effluent, beef manure, or anhydrous ammonia. Animal manure treatments increased dehydrogenase activity, microbial biomass carbon (C_mic) and N (N_mic) contents, and activities of amidohydrolases, including l-asparaginase, urease, l-glutaminase, amidase, and β-glucosaminidase. Soils receiving anhydrous ammonia demonstrated increased nitrate contents, but reduced microbiological and biochemical activities. All treatments decreased C_mic:organic C (C_org) ratios compared with the control, indicating reduced microbial C use efficiency and disturbance of C equilibrium in these soil environments. Activities of all enzymes tested were significantly correlated with soil C_org contents (P < 0.001, n = 108), but little correlation (r = 0.03, n = 36) was detected between C_mic and C_org. Activities of amidase and β-glucosaminidase were dominated by accumulated enzymes that were free of microbial cells, while activities of asparaginase and glutaminase were originated predominately from intracellular enzymes. Results indicated that soil microbial and biochemical activities are sensitive indicators of processes involved in N flow and C use efficiency in semiarid agroecosystems.     

Effect of litter quality and soil fungi on macroaggregate dynamics and associated partitioning of litter carbon and nitrogen

We investigated the effect of plant residue decomposability and fungal biomass on the dynamics of macroaggregate (250–2000 μm) formation in a three months' incubation experiment and determined the distribution of residue-derived C and N in the microbial biomass and in aggregate size fractions (250–2000 μm, 53–250 μm and <53 μm) using ¹³C and ¹⁵N data. A silty loam soil (sieved <250 μm) was incubated with and without addition of ¹⁵N labelled maize leaves (C/N = 27.4) and roots (C/N = 86.4). Each treatment was carried out with and without fungicide application. The addition of maize residues enhanced soil respiration and microbial biomass C and N and resulted in increased macroaggregate formation with a higher and more rapid maximum macroaggregation in the soil amended with maize leaves than in that with addition of roots. Fungicide application led to a significant decline of microbial biomass C and mineralization of the added residues compared to untreated soils, which demonstrates a successful suppression of part of the active microbial biomass by the fungicide. However, this was not confirmed by a generally lower ergosterol concentration. Consequently, ergosterol was no reliable fungal biomarker in periods of rapid decline of the fungal biomass. A single addition of fungicide was insufficient for continued inhibition of the fungal biomass. Yet, a significant delay (28–42 days) in macroaggregation in fungicide treated compared to untreated samples highlighted the importance of the fungal biomass in macroaggregate formation. Macroaggregates were enriched in maize-derived ¹³C and ¹⁵N compared to microaggregates or the fraction < 53 μm. They turned over rapidly with decreasing substrate availability, which entailed a transfer of maize-derived C and N stored within macroaggregates during the first weeks of incubation to microaggregates with proceeding incubation time. Our results indicate that this transfer happened within macroaggregates, because no considerable amount of free particulate organic matter (POM) was released upon macroaggregate breakdown. We conclude that substrate decomposability and fungal activity are key factors determining extent and dynamics of macroaggregation during decomposition processes. Macroaggregate formation implied rapid incorporation and thereby short-term protection of maize-derived C and N. Moreover, macroaggregates allowed a transfer of maize-derived organic matter into microaggregates within macroaggregates, which prevented the release of significant amounts of free POM upon macroaggregate breakdown. Consequently, macroaggregates constitute to the transfer of recently added C into more stable soil organic matter fractions.     

Effect of conventional and minimum tillage on physical and biochemical stabilization of soil organic matter

The objectives were to investigate (1) to which extent water-stable macro- and microaggregates sequester organic matter (OM) in a minimum tillage (MT) system compared to a conventional tillage (CT) system and (2) if the content of biochemically stabilized OM differs between both tillage systems, and (3) to study the temporal dynamics of the distribution of aggregate size classes and of storage of OM within aggregates in the field. Surface soils (0–5 cm) and subsoils (10–20 cm) were sampled after fallow (March 2007) and directly after tillage (November 2007) from a long-term experimental field near Göttingen, Germany. Macroaggregates (>0.25 mm) were in general less abundant after fallow than directly after tillage. In March, only 21% (CT) and 45% (MT) of C_org was stored within macroaggregates in the surface soil, whereas in November, the percentages increased to 58% and 73%, respectively. CT and MT soils of both depths were incubated as bulk soil (CT_bulk, MT_bulk) and with macroaggregates disrupted (<0.25 mm) (CT_md, MT_md) for 28 days at 22°C and water content of 50% of the maximum water holding capacity. For the MT_bulk and MT_md surface soils, C mineralization was significantly higher compared to the CT soils. Incubation of md soils did not generally result in a significantly higher C mineralization compared to the respective bulk soils, except for the MT_md subsoil. Acid hydrolysis showed that the proportion of biochemically stabilized, nonhydrolysable, C_org to total C_org was lower in the MT than in the CT soils. Overall, the data indicate that the effect of physical stabilization of OM stored in the macroaggregates may not be a mechanism protecting very labile C with a turnover time of weeks, but that longer preservation likely occurs after macroaggregate transformation into microaggregates, and the surplus of OM found in the surface soil of MT does not only depend on the biochemically stabilized OM. Finally, our data suggest that the temporal variability of distribution of aggregate size classes in the field is large, but spatial and operator variability also contributed to the observed differences.     

Influence of reduced tillage on earthworm and microbial communities under organic arable farming

Although reduced tillage is an agricultural practice reported to decrease soil erosion and external inputs while enhancing soil fertility, it has still rarely been adopted by European organic farmers. The objective of this study was to assess the long-term interactive effects of tillage (conventional (CT) vs. reduced (RT)) and fertilization (slurry (S) vs. composted manure/slurry (MCS)) on earthworms and microbial communities in a clay soil under spelt in an organic 6-year crop rotation. Earthworm populations (species, density and biomass, cocoons) were investigated by handsorting the soil nine years after initial implementation of the treatments. Soil microbial carbon (C_mic) and nitrogen (N_mic) were measured by chloroform-fumigation extraction and a simplified phospholipid fatty acid (PLFA) analysis was used to separate for populations of bacteria, fungi and protozoa. Significantly increased total earthworm density in RT plots was mainly attributed to increased numbers of juveniles. Moreover, we found five times more cocoons with RT. Species richness was not affected by the treatments, but tillage treatments had differentially affected populations at the species-level. In addition, cluster analysis at the community level revealed two distinct groups of plots in relation to tillage treatments. In RT plots C_mic increased in the 0–10 cm and 10–20 cm soil layers, while PLFA concentrations indicative of Gram-negative bacteria, fungi and protozoa only increased in the topsoil. Lower bacteria-to-fungi ratios in the upper soil layer of RT plots indicated a shift to fungal-based decomposition of organic matter whereas a higher C_mic-to-C_org ratio pointed towards enhanced substrate availability. Slurry application decreased microbial biomass and enhanced density of juvenile anecic earthworms but overall fertilization effect was weak and no interactions with tillage were found. In conclusion, tillage is a major driver in altering communities of earthworms and microorganisms in arable soils. The use of reduced tillage provides an approach for eco-intensification by enhancing inherent soil biota functions under organic arable farming.     

Decomposition kinetics of soil carbon of different age from a forest exposed to 8 years of elevated atmospheric CO2 concentration

Ecosystem exposure to elevated atmospheric CO₂ concentration can often leads to increased ecosystem carbon (C) fluxes, as well as greater net primary production. Changes in the soil C pool with elevated [CO₂] are more difficult to measure and therefore remain poorly understood. In this study, we carried out a series of laboratory soil incubations, in order to determine whether 8 years of ecosystem exposure to elevated [CO₂] altered decomposition dynamics of two age classes of soil C in a temperate coniferous forest. Our objectives were to determine whether there were differences in the decomposition kinetics of soil C up to 8 years old (C_post-tr) and soil C older than 8 years (C_pre-tr), in the absence of concurrent plant activity. We collected soil from the Duke Forest Free Air CO₂ Enrichment site in North Carolina and incubated whole and crushed (all macroaggregates dispersed) soil from two depth increments (0–5 cm and 5–15 cm) for 102–127 days. We found that mineral soil from the treatment plots had higher respiration rates in the absence of concurrent plant activity than mineral soil from plots under ambient CO₂ conditions. These differences in respiration rate were only significant in 0–5 cm soil and could be largely explained by higher initial respiration rates of soil collected from the CO₂-treated plots. Disruption of soil macroaggregates did not result in a difference in efflux rate in soil from this forest under ambient or elevated CO₂ conditions at either depth. The specific respiration rate of C_post-tr was higher than that of C_pre-tr in the top 5 cm of soil, while the opposite was true for 5–15 cm of soil. Even though C_post-tr was assimilated by the ecosystem more recently than C_pre-tr, their decay constants were similar at both depths. These results suggest that, in the absence of plant activity, the mineralization of soil C of different ages in this forest may be under similar biological and/or biochemical control. Therefore, if the higher initial rates of decomposition of C_post-tr seen in these experiments are sustained in the field, greater labile pool size of recently added C, and potentially faster cycling of this pool, may in part explain higher soil respiration rates and limited soil C accumulation under elevated [CO₂] in this forest.     

Soil organic matter assessment in natural regrowth,Pueraria phaseoloides and Mucuna pruriens fallow

Biological and chemical components of soil fertility were quantified under three different fallow types and related to soil quality of an Ultisol in southern Cameroon at the end of a 9-month fallow. Soil organic matter (SOM), soil exchangeable Ca²⁺, Mg²⁺ and K⁺ and available P concentrations, effective cation exchange capacity (ECEC) and, soil acidity in the 0–10 and 10–20 cm layers were evaluated under: natural regrowth mainly composed of Chromolaena odorata and the legume cover crops velvet bean (Mucuna pruriens var. utilis) and kudzu (Pueraria phaseoloides). SOM quality was assessed by C mineralisation during a 4-week incubation at 28°C in the laboratory. In addition, particulate organic matter (POM), the most active part of SOM, was fractionated by wet sieving into coarse (4000–2000 μm), medium (2000–250 μm) and fine (250–53 μm) particle size classes and analysed for C and N contents. Under Mucuna, Ca²⁺, K⁺ and P concentrations, ECEC and soil pH were higher and C mineralisation was lower than under natural regrowth and Pueraria in 0–10 cm depth. Soil under natural regrowth had a higher C mineralisation in 0–10 cm indicating more labile SOM than in Pueraria and Mucuna fallow. There was no difference in weight of total POM, for any of the fractions between the three fallow types. However, both leguminous fallow species increased POM quality through a higher N content. Compared to natural regrowth, Pueraria increased N content in coarse POM by 36% in the 0–10 cm layer and by 19% (coarse POM) and 35% (medium POM) in the 10–20 cm layer. Mucuna increased N content in the 0–10 cm layer by 12% (coarse POM), and by 19% (fine POM), compared to natural regrowth. According to the differences in nutrient concentrations, soil acidity and the biological stability of SOM, the three fallow types ranked: Mucuna≥Pueraria>natural regrowth. However, in terms of POM quality the ranking was: Pueraria>Mucuna>natural regrowth.     

Microbial respiration activities of soils from different climatic regions of European Russia

The aim of this study was to survey and evaluate the microbial respiration of main soil types (gleyic Cryosols, umbric Albeluvisols, albic Luvisols, luvic Chernozems, Kastanozems) across European Russia, from semiarid to polar climatic zones. Soil was sampled from 0–5 and 5–10 cm layers at natural (forest, grassland, fallow) and corresponding sites under agricultural land use. Soil microbial biomass carbon (C_mic) determined by the substrate-induced respiration method and basal respiration (BR) were measured under standardized laboratory conditions (22 °C, 60% WHC). The ratios of BR/C_mic and C_mic/C_org were also calculated. C_mic and BR were highest in polar (gleyic Cryosols) and temperate (albic Luvisols, luvic Chernozems) climatic zones, the lowest were in boreal (umbric Luvisols) and semiarid (Kastanozems). C_mic, BR and C_mic/C_org ratios were higher in 0–5 cm layers compared to the corresponding 5–10 cm and in natural sites versus in arable. Principal component analysis yielded a clear separation of the vegetation zones with respect to the several principal components (PC). PC 1 was composed of C_mic, BR, soil chemical (C_org, N_tot) and texture parameters. PC 2 was composed of climatic (MAT, MAP) and soil pH variables. Three-way ANOVA indicated that “soil type”, “ecosystem” and “layer” factors, and their interactions accounted for almost 98 and 99% of the total variance in C_mic and BR, respectively.     

Bacterial community structure in soil microaggregates and on particulate organic matter fractions located outside or inside soil macroaggregates

Soil aggregates and particulate organic matter (POM) are thought to represent distinct soil microhabitats for microbial communities. This study investigated whether organo-mineral (0–20, 20–50 and 50–200 μm) and POM (two sizes: >200 and <200 μm) soil fractions represent distinct microbial habitats. Microbial habitats were characterised by the amount and quality of organic matter, the genetic structure of the bacterial community, and their location outside or inside macroaggregates (>200 μm). The denaturing gradient gel electrophoresis (DGGE) profiles revealed that bacterial communities structure of organo-mineral soil fractions were significantly different in comparison to the unfractionated soil. Conversely, there were little differences in C concentrations, C:N ratios and no differences in DGGE profiles between organo-mineral fractions. Bacterial communities between soil fractions located inside or outside macroaggregates were not significantly different. However, the bacterial communities on POM fractions were significantly different in comparison to organo-mineral soil fractions and unfractionated soil, and also between the 2 sizes of POM. Thus in the studied soil, only POM fractions represented distinct microhabitats for bacterial community, which likely vary with the state of decomposition of the POM.     

Land-use/cover change effects and carbon controls on volcanic soil profiles in highland temperate forests

The stabilization of SOM by Al–humus complexes and non-crystalline minerals is a key issue to explain the soil-C variability and the biogeochemical processes that determine the fate of soil C following land-use/cover change (LUCC) in volcanic landscapes. In an altitudinal gradient of volcanic soils (2550–3500 masl), we quantified the total soil C (C_T) concentrations and stocks in soil pits sampled by genetic horizons. We performed analyses at landscape and local scales in order to identify and integrate the underlying environmental controls on C_T and the effects of LUCC. We selected four sites, two on the upper piedmont, one on the lower mountain slope and one on the middle mountain slope at Cofre de Perote volcano (eastern central Mexico) where temperate forests are the natural vegetation. At each site we selected three to five units of use/cover as a chronosequence of the LUCC pathways. In each soil horizon chemical characteristics (i.e. N, C/N ratio, pH, exchangeable bases) were determined and mineralogical properties were estimated from selective Al, Fe and Si oxalate and pyrophosphate extractions (i.e. the Alp/Alo ratio, the active Al related to non-crystalline minerals as Al_o ? Al_p, the allophane concentration, and the non-crystalline Al and Fe minerals as Al_o + 1/2Fe_o). At landscape scale, the Al–humus complexes were strongly related to the C_T concentration in topsoil (A horizons) but this relationship decreased with depth. In turn, the non-crystalline minerals and the C/N ratio explained the variability of the C_T concentrations in C horizons. At local scale, C_T concentrations and stocks were depleted after conversion of forest to agriculture in Vitric Andosols at the upper piedmont but this was not observed in Silandic Andosols. However, in Vitric Andosols the reduction of the C_T stocks is partially recovered throughout the regeneration/reforestation processes. The results suggest that the lower vulnerability of Silandic Andosols than Vitric Andosols to changes in the C_T after LUCC is due to the higher levels of SOM stabilized by Al–humus complexes and non-crystalline minerals in the Silandic soils. Furthermore, the importance of the allophane to explain the C_T stocks in the Silandic Andosols of the middle slopes suggests that the C_T stabilized by this mineral fraction in the subsoil adds an important fraction of the C_T to the estimates of the stocks.     

Earthworm populations in relation to soil organic matter dynamics and management in California tomato cropping systems

Earthworms are key regulators of soil structure and soil organic matter (SOM) dynamics in many agroecosystems. They are greatly impacted by agricultural management, yet little is known about how these factors interact to control SOM dynamics. This study sought to explore linkages between agricultural management, earthworms and aggregate associated SOM dynamics through a survey of tomato (Solanum lycopersicum L.) cropping systems in northern California. Earthworms and soil samples were collected between February and April of 2005 from 16 fields under one of three types of residue management: (1) tomato mulch – no postharvest tillage and tomato residues left on the soil surface, (2) cover crop – tomato residues tilled in and leguminous cover crop planted, and (3) bare fallow – tomato residues tilled in and soil surface left exposed throughout the winter. Earthworms were collected via hand-sorting and identified to species, while soils were wet sieved to yield four aggregate size classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm) and the silt and clay fraction (<53 μm). The combined large and small macroaggregate fraction was then fractionated into coarse particulate organic matter (cPOM; 250 μm), microaggregates within macroaggregates (mM; 53–250 μm) and macroaggregate occluded silt and clay (Msc; <53 μm). The earthworms identified in this survey were composed entirely of exotic species and were dominated by Aporrectodea caliginosa. Earthworm abundance was related to residue management, with the tomato mulch systems averaging 4.5 times greater fresh earthworm biomass than bare fallow (P = 0.024). Aggregate stability and total soil C and N also appeared to be influenced by residue management, such that the tomato mulch system displayed significantly greater mean weight diameters than the bare fallow system (P = 0.049), as well as more than 50% greater total soil C and N (P = 0.049 and P = 0.036; respectively). Earthworm biomass was also found to be positively correlated with total soil C (P = 0.009, R² = 0.39) and N (P = 0.010, R² = 0.039) as well as the proportion of macroaggregate C in the cPOM fraction (P = 0.028, R² = 0.30). Our findings suggest that residue handling and the associated management practices (e.g., tillage, organic vs. conventional agriculture) are important for both earthworm populations and SOM storage. Although earthworms are known to influence SOM in many ways, other factors appear to play a more prominent role in governing aggregate associated SOM dynamics.     

Surface soil responses to silage cropping intensity on a Typic Kanhapludult in the piedmont of North Carolina

Although reduced tillage itself is beneficial to soil quality and farm economics, the amount of crop residues returned to the soil will likely alter the success of a particular conservation tillage system within a farm operation. We investigated the impact of three cropping systems (a gradient in silage cropping intensity) on selected soil physical, chemical, and biological properties in the Piedmont of North Carolina, USA. Cropping systems were: (1) maize (Zea mays L.) silage/barley (Hordeum vulgare L.) silage (high silage intensity), (2) maize silage/winter cover crop (medium silage intensity), and (3) maize silage/barley grain—summer cover crop/winter cover crop (low silage intensity). There was an inverse relationship between silage intensity and the quantity of surface residue C and N contents. With time, soil bulk density at a depth of 0–3 cm became lower and total and particulate C and N fractions, and stability of macroaggregates became higher with lower silage intensity as a result of greater crop residue returned to soil. Soil bulk density at 0–3 cm depth was initially 0.88 Mg m⁻³ and increased to 1.08 Mg m⁻³ at the end of 7 years under high silage intensity. Total organic C at 0–20 cm depth was initially 11.7 g kg⁻¹ and increased to 14.3 g kg⁻¹ at the end of 7 years under low silage intensity. Stability of macroaggregates at 0–3 cm depth at the end of 7 years was 99% under low silage intensity, 96% under medium silage intensity, and 89% under high silage intensity. Soil microbial biomass C at 0–3 cm depth at the end of 7 years was greater with low silage intensity (1910 mg kg⁻¹) than with high silage intensity (1172 mg kg⁻¹). Less intensive silage cropping (i.e., greater quantities of crop residue returned to soil) had a multitude of positive effects on soil properties, even in continuous no-tillage crop production systems. An optimum balance between short-term economic returns and longer-term investments in improved soil quality for more sustainable production can be achieved in no-tillage silage cropping systems.     

Influence of dry–wet cycles on the interrelationship between aggregate,particulate organic matter,and microbial community dynamics

Aggregate dynamics and their relationship to the microbial community have been suggested as key factors controlling SOM dynamics. Dry–wet (DW) cycles are thought to enhance aggregate turnover and decomposition of soil organic matter (SOM), particularly in tilled soils. The objective of this study was to evaluate the effects of DW cycles on aggregate stability, SOM dynamics, and fungal and bacterial populations in a Weld silt loam soil (Aridic Paleustoll). Samples, taken from 250 μm sieved air-dried soil (i.e. free of macroaggregates > 250 μm), were incubated with ¹³C-labeled wheat residue. In one set of soil samples, fungal growth was suppressed using a fungicide (Captan) in order to discern the effect of dry–wet cycles on fungal and bacterial populations. Aggregate formation was followed during the first 14 d of incubation. After this period, one set of soil samples was subjected to four DW cycles, whereas another set, as a control, was kept at field capacity (FC). Over 74 d, total and wheat-derived respiration, size distribution of water stable aggregates and fungal and bacterial biomass were measured. We determined native and labeled C dynamics of three particulate organic matter (POM) fractions related to soil structure: the free light fraction (LF), and the coarse (250–2000 μm) and fine (53–250 μm) intra-aggregate POM fraction (iPOM). In the fungicide treated soil samples, fungal growth was significantly reduced and no large macroaggregates (> 2 mm) were formed, whereas without addition of fungicide, fungi represented the largest part of the microbial biomass (66%) and 30% of the soil dry weight was composed of large macroaggregates. During macroaggregate formation, labeled free LF-C significantly decreased whereas labeled coarse iPOM-C increased, indicating that macroggregates are formed around fresh wheat residue (free LF), which is consequently incorporated and becomes coarse iPOM. The first drying and wetting event reduced the amount of large macroaggregates from 30 to 21% of the total soil weight. However, macroaggregates became slake-resistant after two dry-wet cycles. Fine iPOM-C was significantly lower in soil after two dry–wet cycles compared to soil kept at FC. We conclude that more coarse iPOM is decomposed into fine iPOM in macroaggregates not exposed to DW cycles due to a slower macroaggregate turnover. In addition, when macroaggregates, subjected to dry–wet cycles, became slake-resistant (d 44) and consequently macroaggregate turnover decreased, fine iPOM accumulated. In conclusion, differences in fine iPOM accumulation in DW vs. control macroaggregates are attributed to differences in macroaggregate turnover.     

Effects of residue location on soil organic matter turnover: results from an incubation experiment with 15N‐maize

Differences in the mechanisms of storage and decomposition of organic matter (OM) between minimum tillage (MT) and conventional tillage (CT) are generally attributed to differences in the physical impact through tillage, but less is known about the effects of residue location. We conducted an incubation experiment at a water content of 60% of the maximum water‐holding capacity and 15°C with soils from CT (0–25 cm tillage depth) and MT fields (0–5 cm tillage depth) with ¹⁵N‐labeled maize straw incorporated to different depths (CT simulations: 0–15 cm; MT simulations: 0–5 cm) for 28 d in order to determine the effects of the tillage simulation on (1) mineralization of recently added residues, (2) the dynamics of macroaggregate formation and physical protection of OM, and (3) the partitioning of maize‐derived C and N within soil OM fractions. The MT simulations showed lower relative C losses, and the amount of maize‐C mineralized after 28 d of incubation was slightly but significantly lower in the MT simulations with maize added (MT_maize) than in the respective CT (CT_maize) simulations. The formation of new water‐stable macroaggregates occurred during the phase of the highest microbial activity, with a maximum peak 8 d after the start of incubation. The newly formed macroaggregates were an important location for the short‐term stabilization of C and N with a higher importance for MT_maize than for CT_maize simulations. In conclusion, our results suggest that a higher amount of OM in MT surface soils compared with CT surface soils may not only result from decreased macroaggregate destruction under reduced tillage but also from a higher efficiency of C retention due to a more concentrated residue input.     

In-field management of corn cob and residue mix: Effect on soil greenhouse gas emissions

In-field management practices of corn cob and residue mix (CRM) as a feedstock source for ethanol production can have potential effects on soil greenhouse gas (GHG) emissions. The objective of this study was to investigate the effects of CRM piles, storage in-field, and subsequent removal on soil CO₂ and N₂O emissions. The study was conducted in 2010–2012 at the Iowa State University, Agronomy Research Farm located near Ames, Iowa (42.0°′N; 93.8°′W). The soil type at the site is Canisteo silty clay loam (fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquolls). The treatments for CRM consisted of control (no CRM applied and no residue removed after harvest), early spring complete removal (CR) of CRM after application of 7.5 cm depth of CRM in the fall, 2.5 cm, and 7.5 cm depth of CRM over two tillage systems of no-till (NT) and conventional tillage (CT) and three N rates (0, 180, and 270 kg N ha⁻¹) of 32% liquid UAN (NH₄NO₃) in a randomized complete block design with split–split arrangements. The findings of the study suggest that soil CO₂ and N₂O emissions were affected by tillage, CRM treatments, and N rates. Most N₂O and CO₂ emissions peaks occurred as soil moisture or temperature increased with increase precipitation or air temperature. However, soil CO₂ emissions were increased as the CRM amount increased. On the other hand, soil N₂O emissions increased with high level of CRM as N rate increased. Also, it was observed that NT with 7.5 cm CRM produced higher CO₂ emissions in drought condition as compared to CT. Additionally, no differences in N₂O emissions were observed due to tillage system. In general, dry soil conditions caused a reduction in both CO₂ and N₂O emissions across all tillage, CRM treatments, and N rates.     

Nitrogen fertilization and cropping systems effects on soil organic carbon and total nitrogen pools under chisel-plow tillage in Illinois

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 (N₀), 70 (N₁), 140 (N₂), 210 (N₃) and 280 (N₄) 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 N₀ to 75.8 Mg ha⁻¹ for N₄. 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 N₀ to 6.14 Mg ha⁻¹ for N₄. 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.     

Influence of mouldboard plough and rotary harrow tillage on microbial biomass and nutrient stocks in two long-term experiments on loess derived Luvisols

The nutrient-specific effects of tillage on microbial activity (basal respiration), microbial biomass (C, N, P, S) indices and the fungal cell-membrane component ergosterol were examined in two long-term experiments on loess derived Luvisols. A mouldboard plough (30 cm tillage depth) treatment was compared with a rotary harrow (8 cm tillage depth) treatment over a period of approximately 40 years. The rotary harrow treatment led to a significant 8% increase in the mean stocks of soil organic C, 6% of total N and 4% of total P at 0–30 cm depth compared with the plough treatment, but had no main effect on the stocks of total S. The tillage effects were identical at both sites, but the differences between the sites of the two experiments were usually stronger than those between the two tillage treatments. The rotary harrow treatment led to a significant increase in the mean stocks of microbial biomass C (+18%), N (+25%), and P (+32%) and to a significant decrease in the stocks of ergosterol (−26%) at 0–30 cm depth, but had no main effect on the stocks of microbial biomass S or on the mean basal respiration rate. The mean microbial biomass C/N (6.4) and C/P (25) ratios were not affected by the tillage treatments. In contrast, the microbial biomass C/S ratio was significantly increased from 34 to 43 and the ergosterol-to-microbial biomass C ratio significantly decreased from 0.20% to 0.13% in the rotary harrow in comparison with the plough treatment. The microbial biomass C-to-soil organic C ratio varied around 2.1% in the plough treatment and declined from 2.6% at 0–10 cm depth to 2.0 at 20–30 cm depth in the rotary harrow treatment. The metabolic quotient qCO₂ revealed exactly the inverse relationships with depth and treatment to the microbial biomass C-to-soil organic C ratio. Rotary harrow management caused a reduction in the microbial turnover in combination with an improved microbial substrate use efficiency and a lower contribution of saprotrophic fungi to the soil microbial community. This contrasts the view reported elsewhere and points to the need for more information on tillage-induced shifts within the fungal community in arable soils.     

Denitrification in a vertisol under long-term tillage and no-tillage management in dryland agricultural systems: Key genes and potential rates

The impact of tillage practices on microbial N transformations in Vertisols is poorly understood and data from long-term field experiments are scarce, particularly in semiarid regions. We evaluated the effects of traditional tillage (TT) vs no-tillage (NT) on denitrification in a long-term field experiment under a rainfed crop rotation system (cereal-sunflower-legumes) on a Vertisol (SW Spain). In general, the abundance of denitrifiers and the respective potential denitrification rates was higher under NT compared to TT during the vegetation period, but not after harvesting. However differences in denitrifier numbers were within the same order of magnitude (0.5–3 × 10⁷ copies g soil dw). The abundance of nitrite reducers and N₂O reducers was relatively similar. In addition, N₂O/N₂ ratios between 1 and 2 were found for both treatments. These results emphasize that NT has a limited impact on denitrification in Vertisols under fertilizer regime and legume-crop rotation and thus losses of N₂O are expected to be comparable to those of traditional tillage systems.     

The influence of soluble carbon and fertilizer nitrogen on nitric oxide and nitrous oxide emissions from two contrasting agricultural soils

Contradictory effects of simultaneous available organic C and N sources on nitrous oxide (N₂O), carbon dioxide (CO₂) and nitric oxide (NO) fluxes are reported in the literature. In order to clarify this controversy, laboratory experiments were conduced on two different soils, a semiarid arable soil from Spain (soil I, pH=7.5, 0.8%C) and a grassland soil from Scotland (soil II, pH=5.5, 4.1%C). Soils were incubated at two different moisture contents, at a water filled pore space (WFPS) of 90% and 40%. Ammonium sulphate, added at rates equivalent to 200 and 50 kg N ha^?1, stimulated N₂O and NO emissions in both soils. Under wet conditions (90% WFPS), at high and low rates of N additions, cumulative N₂O emissions increased by 250.7 and 8.1 ng N₂O–N g^?1 in comparison to the control, respectively, in soil I and by 472.2 and 2.1 ng N₂O–N g^?1, respectively, in soil II. NO emissions only significantly increased in soil I at the high N application rate with and without glucose addition and at both 40% and 90% WFPS. In both soils additions of glucose together with the high N application rate (200 kg N ha^?1) reduced cumulative N₂O and NO emissions by 94% and 55% in soil I, and by 46% and 66% in soil II, respectively. These differences can be explained by differences in soil properties, including pH, soil mineral N and total and dissolved organic carbon content. It is speculated that nitrifier denitrification was the main source of NO and N₂O in the C-poor Spanish soil, and coupled nitrification–denitrification in the C-rich Scottish soil.     

Soil organic carbon and nitrogen in a Mollisol in central Ohio as affected by tillage and land use

Minimum tillage practices are known for increasing soil organic carbon (SOC). However, not all environmental situations may manifest this potential change. The SOC and N stocks were assessed on a Mollisol in central Ohio in an 8-year-old tillage experiment as well as under two relatively undisturbed land uses; a secondary forest and a pasture on the same soil type. Cropped systems had 51±4 (equiv. mass) Mg ha⁻¹ lower SOC and lower 3.5±0.3 (equiv. mass) Mg ha⁻¹ N in the top 30 cm soil layer than under forest. Being a secondary forest, the loss in SOC and N stocks by cultivation may have been even more than these reported herein. No differences among systems were detected below this depth. The SOC stock in the pasture treatment was 29±3 Mg ha⁻¹ greater in the top 10 cm layer than in cultivated soils, but was similar to those under forest and no-till (NT). Among tillage practices (plow, chisel and NT) only the 0–5 cm soil layer under NT exhibited higher SOC and N concentrations. An analysis of the literature of NT effect on SOC stocks, using meta-analysis, suggested that NT would have an overall positive effect on SOC sequestration rate but with a greater variability of what was previously reported. The average sequestration rate of NT was 330 kg SOC ha⁻¹ year⁻¹ with a 95% confidence interval ranging from 47 to 620 kg SOC ha⁻¹ year⁻¹. There was no effect of soil texture or crop rotation on the SOC sequestration rate that could explain this variability. The conversion factor for SOC stock changes from plow to NT was equal to 1.04. This suggests that the complex mechanisms and pathways of SOC accrual warrant a cautious approach when generalizing the beneficial changes of NT on SOC stocks.