Disruption Of Soil Aggregates Research Articles

Grazing amplifies the stimulating effects of N addition on N2O emissions in a temperate meadow steppe

Nitrogen (N) addition typically increases nitrous oxide (N2O) emissions, but the magnitude of this effect depends on land use type and intensity. Grazing has an important influence on grassland N dynamics, but it is not clear to what extent grazing may interact with N addition in affecting grassland N2O emissions. We investigated the mechanisms underlying the individual and interactive effects of N addition and cattle grazing (control (CK), N addition (N), grazing (G), and N addition plus grazing (N + G) treatments) on N2O emissions in a temperate meadow steppe by measuring N2O fluxes, weather factors, vegetation parameters and soil properties from October 2017 to September 2018. The studied temperate meadow steppe was a source of N2O emissions during the experimental period. Approximately 80 % of the annual N2O emissions were released during the plant growing season. The highest N2O emission was detected in the N + G plots, which indicated the grazing treatment amplified the stimulating effects of N addition on N2O emissions. The results from structural equation model (SEM) suggested that N addition enhanced N2O emission through increases in NH4+-N concentration and soil potential net nitrification rate (Nnitri); grazing promoted N2O emissions by increasing substrate availability directly via excreta or indirectly through trampling-associated soil aggregate disruption and substrate release. N2O emissions increased most strongly with N addition and grazing due to the synergetic effects between animal trampling, animal excreta and available N on the processes of nitrification and denitrification. Overall, our results suggest that the interactive effects of grassland management and global change factors on N2O emissions need to be considered for the accurate assessment of grassland greenhouse gas emissions.

The mechanisms underpinning microbial resilience to drying and rewetting – A model analysis

Soil moisture is one of the most important factors controlling the activity and diversity of soil microorganisms. Soils exposed to pronounced cycles of drying and rewetting (D/RW) exhibit disconnected patterns in microbial growth and respiration at RW. These patterns differ depending on the preceding soil moisture history, leading to contrasting amounts of carbon retained in the soil as biomass versus that respired as CO2. The mechanisms underlying these microbially-induced dynamics are still unclear. In this work, we used the process-based soil microbial model EcoSMMARTS to offer candidate explanations for: i) how soil moisture can shape the structure of microbial communities, ii) how soil moisture history affects the responses during D/RW, iii) what microbial mechanisms control the shape, intensity and duration of these responses, and iv) what carbon sources sustain the increased biogeochemical rates after RW. We first evaluated the response to D/RW in bacterial communities previously exposed to two different stress histories (‘moderate’ vs ‘severe’ soil moisture regimes). We found that both the history of soil moisture and the harshness of the dry period preceding the rewetting shaped the structure and physiology of microbial communities. The characteristics of these communities determined the harshness experienced and the nature of the responses to RW obtained. Modelled communities exposed to extended severe conditions showed a resilient response to D/RW, whereas those exposed to moderate environments exhibited a more sensitive response. We then interchanged the soil moisture regimes and found that the progressive adaptation of microbial physiology and structure to new environmental conditions resulted in a switch in the response patterns. These microbial changes also determined the contribution of biomass synthesis, osmoregulation, mineralization by cell residues, and disruption of soil aggregates to CO2 emissions.

A Novel Index (RI) to Evaluate the Relative Stability of Soils Using Ultrasonic Agitation

As soil stability is a complex phenomenon, various methods and indexes were introduced to assess the strength of soils. Because of the limitations of different stability methods and indexes (including wet sieving-based), we aimed to presents a relative stability index (RI) that was based on the estimated components of the soil overall disruptive characteristic curve (SODC): (1) soil disruption constant (Ki, that is based upon dispersion energy of soils); (2) resulting change in mean weight diameter (ΔMWD). To evaluate the effectiveness and limitations of RI as well as to compare it with classical soil stability indexes of mean weight diameter (MWD) and geometric mean diameter (GMD). Ultrasonic agitation (UA) along with a wet sieving method (followed by dry sieving) was applied against four different soils named on the basis of sample location, Qingling soil (QL), Guanzhong soil (GZ), Ansai soil (AS), and Jingbian soil (JB). To evaluate the relative strength of soils at different applied energies (increase in sonication duration usually resulted in increased input energy and temperature of soil–water suspension), soils were subjected to six sonication durations (0, 30, 60, 120, 210, and 300 s) with a fixed (and exact) initial amplitude and temperature. Output energy was calculated based on the amplitude and temperature of the suspension, vessel, and system. The most abrupt and maximum disruption of soil aggregates was observed at a dispersion energy level of 0–200 J g−1. The MWD value of surface and subsurface ranged between 0.58 to 0.15 mm and 0.37 to 0.17 mm, respectively, while GMD was ranged from 0.14 to 0.33 mm overall. The results for MWD and GMD showed a similar trend. MWD and GMD showed more strong associations with physicochemical characteristics of soil than RI. A non-significant correlation was found between RI and MWD/GMD. Contrary to MWD and GMD, RI was significantly positively correlated with sand content; this finding indicated the influential role of sand in assessing the soil’s relative strength. The results indicated that JB soil possessed the least MWD and GMD but proved to be relatively stable because of having the highest RI value.

Tillage induces rapid loss of organic carbon in large macroaggregates of calcareous soils

Soil organic carbon (SOC) sequestration and protection not only mitigate CO2 emission but also promote soil fertility. Naturally, calcareous soils in the karst area contain significantly higher SOC concentration than the red soils in non-karst area of the same region. After cultivation, calcareous soils are susceptible to SOC loss, with substantial SOC loss occurring within two years. However, the underlying mechanisms are not well understood. To expand our understanding of the process of rapid SOC loss and the role played by soil aggregate disruption, we conducted a 1-year in situ study in a karst area in Southwest China. We tilled the soil at different intervals—every 6 (T1), 4 (T2), 2 (T3), and 1 (T4) months—causing aggregate disruption of different intensities. No-tillage soil (T0) was treated as a control. After 1 year, the SOC concentration in the soil under the tillage treatments (T1, T2, T3, and T4) was significantly reduced; however, in the no-tillage soil, no significant change was observed. The SOC loss within the first 3 months accounted for approximately 70 %–84 % of the total SOC loss, indicating that SOC loss was most pronounced during the initial tillage period. SOC was mainly stored in the 2–8 mm soil aggregates. Tillage-induced disruption of the 5–8 mm aggregates resulted in a decline of the physical protection of SOC, leading to substantial SOC loss from these aggregates. Besides, the SOC loss was positively correlated to the amount of Ca2+ leaching, indicating that enhanced carbonate dissolution might contribute to the decline in aggregate stability and associated SOC loss. Our results emphasize the importance of zero-tillage management and maintenance of large soil macroaggregates for sustainable land use of calcareous soils in karst regions.

Long term effects of intensive biomass harvesting and compaction on the forest soil ecosystem

Forest soil ecosystems can be negatively affected by intensive biomass harvesting due to losses of organic inputs and soil compaction, ultimately leading to reduced forest productivity. In this research, we revisited a site from the North American Long-Term Soil Productivity study located on a sandy Spodosol within the Huron National Forest in Michigan, USA, to measure the effects of aboveground organic matter removal of different intensities (three levels: bole only; whole tree harvest; or whole tree harvest and forest floor removal) and soil compaction (2 levels: no or moderate compaction) nearly 20 years following the initial treatments. The effects of harvesting on the soil microbial community in surface and subsurface soils and on soil nutrient availability in surface soils were evaluated. Additionally, patterns of carbon and nitrogen distribution among soil organic matter pools in surface and subsurface soils were compared using a physical fractionation approach to isolate a free – light fraction of particulate organic matter external to aggregates, an occluded – light fraction, which represents particulate organic matter released from the disruption of soil aggregates, and a heavy or mineral-associated fraction. Whole-tree harvests had significantly different microbial community compositions than bole-only harvests (P = 0.02), a result driven by significantly lower abundance of arbuscular mycorrhizae and greater gram positive bacterial abundance in the whole-tree harvest relative to bole-only harvest conditions. Few differences in soil nutrient availability were apparent 20 years after organic matter manipulations, with the exception of reduced calcium availability where organic matter was removed. Soil compaction resulted in greater microbial biomass (0.19 versus 0.14 μmol g−1 soil), which may have also led to a reduced C:N ratio in the heaviest and oldest soil component and increased P availability as well. Nitrogen concentrations and stocks were greatest at the surface (0–10 cm depth) for the free and light soil fractions in bole-only removal treatments, in contrast to whole-tree harvest treatments where C and N concentrations and C stocks were greater in the subsurface soil (free - light fraction at 20–30 cm depth). The soil microbial community, soil fraction size, and soil C and N stocks differed between surface and subsurface soils, highlighting the soil forming processes at work in this Spodosol, and the importance of sampling multiple depths to address research questions. These results demonstrate the long-term effects of forest management on soil biological, physical, and chemical properties and are useful in evaluating sustainable biomass harvesting practices for comparable forests.

Replicability of aggregate disruption by sonication—an inter‐laboratory test using three different soils from Germany

AbstractSonication is widely used for disruption of suspended soil aggregates. Calorimetric calibration allows for determining sonication power and applied energy as a measure for aggregate disrupting forces. Yet other properties of sonication devices (e.g., oscillation frequency and amplitude, sonotrode diameter) as well as procedure details (soil‐to‐water ratio, size, shape, and volume of used containers) may influence the extent of aggregate disruption in addition to the applied energy. In this study, we tested potential bias in aggregate disruption when different devices or procedures are used in laboratory routines. In nine laboratories, three reference soil samples were sonicated at 30 J mL−1 and 400 J mL−1. Aggregate disruption was estimated based on particle size distribution before and after sonication. Size distribution was obtained by standardized submerged sieving for particle size classes 2000–200 and 200–63 µm, and by dynamic imaging for particles < 63 µm. Despite differences in sonication devices and protocols used by the participants, only 16 in 216 tests of samples of the size fractions 2000–200 and 200–63 µm were identified as outliers. For the size fraction < 63 µm, fewer outliers were detected (8 in 324 tests). Four out of nine laboratories produced more than two outliers. In these laboratories, sonication devices differed from the others regarding oscillation frequencies (24 or 30 kHz compared to 20 kHz), sonotrode diameters (10 and 14 mm compared to 13 mm), and sonication power (16 W compared to > 45 W). Thus, these sonication device properties need to be listed when reporting on sonication‐based soil aggregate disruption. The overall small differences in the degree of disruption of soil aggregates between different laboratories demonstrate that sonication with the energies tested (30 and 400 J mL−1) provides replicable results despite the variations regarding procedures and equipment.

Changes in snow cover alter nitrogen cycling and gaseous emissions in agricultural soils

Climate change-related increases in winter temperatures and precipitation, as predicted for eastern Canada, may alter snow cover, with consequences for soil temperature and moisture, nitrogen cycling, and greenhouse gas fluxes. To assess the effects of snow depth in a humid temperate agricultural ecosystem, we conducted a two-year field study with (1) snow removal, (2) passive snow accumulation (via snow fence), and (3) ambient snow treatments. We measured in situ N2O and CO2 fluxes and belowground soil gas concentration, and conducted denitrification and potential nitrification laboratory assays, from November through May. Snow manipulation significantly affected winter N2O dynamics. In the first winter, spring thaw N2O fluxes in snow removal plots were 31 and 48 times greater than from ambient snow and snow accumulation plots respectively. Mid-winter soil N2O concentration was also highest in snow removal plots. These effects may have been due to increased substrate availability due to greater soil frost, along with moderate gas diffusivities facilitating N2O production, in snow removal plots. In the second winter, spring thaw N2O fluxes and soil N2O concentration were greatest for ambient snow plots. Peak fluxes in ambient snow plots were 19 and 24 times greater than in snow accumulation and snow removal plots, respectively. Greater soil moisture in ambient snow plots overwinter could have facilitated denitrification both through decreased O2 availability and increased disruption of soil aggregates during freeze-thaw cycles. Overall, results suggest that effects of changing snow cover on N cycling and N2O fluxes were not solely a direct effect of snow depth; rather, effects were mediated by both soil water content and temperature. Furthermore, the fact that treatments with greatest mid-winter belowground N2O accumulation also had greatest spring thaw N2O fluxes in both years suggests the hypothesis that high spring thaw fluxes were due not only to spring soil conditions, but also to an effect of soil conditions in frozen soils that had facilitated N2O production throughout winter.

Soil organic carbon stocks and fractionation under different land uses in the Peruvian high-Andean Puna

Soil organic C (SOC), the largest terrestrial C pool, has been shown to be sensitive to agricultural disturbance. In the Andean Puna, agricultural expansion is thought to be jeopardizing the large SOC stock sequestered in its soils. In this paper, we assessed the impact changes in land use have on C sequestration, fractionation and δ13C natural abundance featuring two case studies in the Peruvian Central Puna. We found SOC stocks greater than average temperate grassland soils (between 123±4 and 136±4MgCha−1 in the 0–30cm soil profile); however, they did not differ between land uses. In the first case study, depletion of δ13C in mineral associated C in long fallow (>7yr) and maca (Lepidium meyenii Walpers) croplands was hypothesized to be caused by disruption of soil aggregates due to tillage and initial greater SOC concentration in maca plots. In addition, SOC loss was found in long fallow plots, and was correlated to steep and low plant cover plots; evidencing that soil degradation does not occur during cropping activity but due to erosion after land abandonment. In the second case study, a long-established perennial ryegrass (Lolium perenne L.) - white clover (Trifolium repens L.) association (>40yr) was studied. We did not find an increase in SOC stock under the cultivated pasture. However, we found a better soil aggregation and an increase of 17.3 and 12.2gCkg−1WSA in particulate organic C, and SOC within small macroaggregates in cultivated pasture soils. Soil δ13C natural abundance was about 1‰ depleted in all measured C fractions of cultivated pastures when compared to native grasslands, suggesting that pasture-fixed C forms labile and recalcitrant SOC fractions.

Molecular composition of soil organic matter with land-use change along a bi-continental mean annual temperature gradient

Soil organic matter (SOM) is critical for maintaining soil fertility and long-term agricultural sustainability. The molecular composition of SOM is likely altered due to global climate and land-use change; but rarely are these two aspects studied in tandem. Here we used molecular-level techniques to examine SOM composition along a bi-continental (from North to South America) mean annual temperature (MAT) gradient from seven native grassland/forest and cultivated/pasture sites. Biomarker methods included solvent extraction, base hydrolysis and cupric (II) oxide oxidation for the analysis of free lipids of plant and microbial origin, ester-bound lipids from cutin and suberin, and lignin-derived phenols, respectively. Solid-state 13C nuclear magnetic resonance (NMR) was used to examine the overall composition of SOM. Soil cultivation was found to increase the amount of microbial-derived compounds at warmer temperatures (up to 17% increase). The cultivated soils were characterized by much lower contributions of plant-derived SOM components compared to the native soils (up to 64% lower at the coldest site). In addition, cultivation caused an increase in lignin and cutin degradation (up to 68 and 15% increase, respectively), and an increase in the amount of suberin-derived inputs (up to 54% increase). Clear differences in the molecular composition of SOM due to soil cultivation were observed in soils of varying mineral composition and were attributed to disturbance, different vegetation inputs, soil aggregate destruction and MAT. A high organic allophanic tropical soil was characterized by its protection of carbohydrates and nitrogen-containing compounds. The conversion of native to cultivated land shows significant shifts in the degradation stage of SOM. In particular, cutin-derived compounds which are believed to be part of the stable SOM pool may undergo enhanced degradation with long-term cultivation and disruption of soil aggregates. On a per year basis, the total amount of cutin decreased only at the two forest sites that were converted to pasture, likely due to cutin degradation or to changes in vegetation and litter quality associated with land-use change. Overall, our study highlights that the implementation of different agricultural management practices enhances the degradation of recalcitrant SOM compounds that may become a source of atmospheric CO2 with increasing land-use and climate change.

Soil Quality Indicators as Affected by Shallow Tillage in a Vineyard Grown in a Semiarid Mediterranean Environment

AbstractWithin the Mediterranean basin, soil tillage enhances the mineralisation of soil organic matter. We assessed the short‐term impact of shallow tillage [field cultivator (FC), rotary tiller (RT) and spading machine (SM)] on some soil quality indicators [bulk density, water‐stable aggregates, total and labile organic C pools (microbial biomass and extractable organic C), soil respiration and related eco‐physiological indexes] in a Sicilian vineyard. Also no tillage was included. We hypothesized that (i) RT and FC worsened soil quality indicators more than SM, and (ii) within the same tillage system, labile C pools, soil respiration and eco‐physiological indexes will respond more efficiently than chemical and physical soil properties since the tillage starts. The experiment started at March 2009, and each tillage type was applied three times per year (March or April, May and June), with soil tilled up to 15‐cm depth. Soil was sampled (0–15 and 15–30‐cm depth) in March 2009, April 2010, May 2012 and June 2014. SM was very effective in preserving soil organic matter pool and in improving any monitored soil quality indicator, similarly to no tillage. By contrast, RT was the most deleterious machine as it worsened most investigated indicators. Such deleterious effects were due to drastic disruption of soil aggregates and consequent exposition of protected soil organic matter to further microbial mineralization. Labile organic C pools and microbial quotients were the most responsive soil parameters for assessing the impact of shallow tillage on soil quality, even in the short term (<5 years). Copyright © 2016 John Wiley & Sons, Ltd.

Biochar-induced N2O emission reductions after field incorporation in a loam soil

Biochar addition to soils is heralded to reduce N2O emissions, but still, the explanatory mechanisms have not been resolved. Moreover, it is uncertain whether N2O emission reductions would persist after prolonged biochar incorporation in the field. In this study, we incorporated four biochar types in a loam textured cropland field and intact soil cores were sampled to investigate the physical control of biochar on denitrification after 7months. During a first incubation experiment, we measured N2O emissions from undisturbed and disturbed (i.e. sieved (2mm) and grounded) soil cores. Both in the disturbed and undisturbed soil cores biochar at water filled pore space (WFPS) of 80% reduced the N2O emissions by 50–90%, refuting the hypothesis that biochar exerts an indirect physical control over soil denitrification several months after incorporation. Secondly, we hypothesized that biochar creates denitrification ‘hotspots’ in soil, where complete reduction of N2O to N2 is promoted compared to non-amended soil. In these hotspots biochar particles could act as microlocations with local anaerobic conditions and local higher pH, stimulating in this way complete denitrification. Via the acetylene inhibition method we did not observe a reduction in the N2O/(N2O+N2) ratio, which could suggest that biochar did not promote the reduction of N2O to N2. Manipulations likely to promote labile C bioavailability, here either by glucose addition or by soil particulate OM disclosure after disruption of soil aggregates, resulted in the most prominent biochar-induced N2O emission reductions.

VLIV NÍZKÝCH TEPLOT NA ROZPAD PŮDNÍCH ČÁSTIC TĚŽKÝCH PŮD

Heavy clay soils that are normally resistant to wind erosion, from study site Ostrožská Nová Ves si­tua­ted in the foothills of the Bílé Karpaty Mountains, Czech Republic, were a subject of laboratory analyses. The analyses should found out the influence of overwinter processes on disruption of soil aggregates and thus reason of vulnerability to soil loss by wind. Two overwinter processes were observed – freezing and thawing, and freeze-drying of the soil. Both processes have indicated the increasing of erodible fraction in dependence of water content of analysed soils. Exposed frozen clay soils that freeze-dries during the winter in the foothills of Bílé Karpaty, leaves soils highly erodible in late winter and early spring.

Recent research progress on soil microbial responses to drying–rewetting cycles

Climatic change, such as increases in extreme drought and rainfall events and changes in rainfall intensity and pattern, has been strongly influencing soil moisture. The climatic change impact is particularly common in arid, semi-arid and Mediterranean regions, which is causing dramatic changes in the intensity and frequency of soil drying–rewetting cycles. The soil drying–rewetting cycle is a natural phenomenon that the soil experiences drying, then wetting, and then drying and rewetting again and again. When a dry soil is being rewetted, the amount of soil microbial biomass and its activity can be sharply increasing in a short time period, and then a large amount of gaseous carbon (C) and nitrogen (N) erupts from the soil. The sudden release of gaseous C and N is caused by the stimulation of the soil microbes. Such a phenomenon is called “Birch effect”. The drying–rewetting cycles have direct and indirect effects on soil microbes, and soil microbial responses to the drying and rewetting events play an important role in the feedbacks of terrestrial ecosystems. From aspects of soil microbial biomass, microbial activities and microbial structure, we review recent advances on studies regarding microbial responses to soil drying–rewetting cycles. We interpret the microbial responses using five different types of mechanisms: (1) Microbial stress mechanism: when a soil becomes dry, microorganisms must accumulate compatible solutes such as carbohydrates and aminoacids so that the soil microbes can equilibrate with their environment in order to avoid dehydrating and being killed. When the soil is rewetted, soil microbes must dispose of those osmolytes rapidly by transforming them into carbon dioxide (CO2), dissolved organic carbon (DOC) and nutrients in order to prevent water from being flowing into the cells. (2) Substrate supply mechanism: low soil moisture may result in the physical disruption of soil aggregates which leads to the exposure of new soil surfaces and of previously protected organic matter. When the soil is rewetted, its physical structure is further disrupted by swelling. The increased new soil surfaces and previously protected organic matter will improve the microorganism’s nutrient availability. (3) Soil hydrophobicity mechanism: soil hydrophobicity can cause the reduction of soil moisture and nutrient availability and inhibition of microbial decomposition of soil organic matter. Therefore, soil hydrophobicity is an important factor of explaining the activity of microorganism in drying and rewetting events. (4) Diffusive limitations mechanism: transportation of the soil microbe is limited in a dry soil. When soil moisture is increasing, soil microbial activity is enhanced along with the increased availability of substrate nutrients. (5) Predation mechanism: a moist soil is usually conducive to the increase of bacteria and fungi populations. In response, protozoa and nematodes also increase, leading to the fluctuation of the soil microbial community structure. On the basis of the literature review, we propose five important aspects to be considered in the future: (1) assessing soil microbes’ concrete adapting ways to the drying–rewetting cycles, (2) evaluating the microbial responses to the drying–rewetting cycles based on suitable indicators, (3) interpreting microbial responses to the drying–rewetting cycles by combining field investigation and laboratory controlling experiment, (4) investigating the microbial responses to the drying–rewetting cycles at different temporal and spatial scales.

Soil Aggregate Destruction by Ultrasonication Increases Soil Organic Matter Mineralization and Mobility

Ultrasonication is widely used in soil organic matter (SOM) fractionation studies to break up soil aggregates to disperse the soil into its primary particles. An increasing number of studies also aim to incubate SOM fractions obtained by physical soil fractionation to study the bioavailability of different soil organic C (SOC) pools. To evaluate possible influences of ultrasonic soil disruption on short-term SOM bioavailability as well as the content and composition of water-extractable organic matter (WEOM) and salt-extractable (10 mM K2SO4) organic matter (SEOM), we conducted a laboratory incubation experiment with aggregated soil material from an Ap horizon of a cropland soil. Bulk soil and subsamples in which aggregates had been disrupted by ultrasonication were incubated in triplicate for 18 d at 20°C. During the incubation, CO2 production was measured continuously. At the beginning and at the end of the experiment, the chemical composition of SEOM was analyzed for the different experimental variants by ultraviolet (UV)-absorbance and solid state 13C cross polarization magic angle spinning nuclear magnetic resonance (13C-CPMAS NMR) spectroscopy. Additionally, we studied the effects of WEOM removal on heterotrophic soil respiration to assess artifacts by unavoidable WEOM removal during soil wet sieving, which is an important step in soil fractionation. After repeated leaching of unsonicated and sonicated soil samples, 0.5 to 1.0% of the total organic C (OC) pool was associated with the WEOM fraction. Ultrasonic disruption of soil aggregates resulted in increased amounts and changed composition of SEOM as well as in a 27% increase in CO2 production compared with intact soils. Incubation of ultrasonicated samples resulted in decreased O/N alkyl C and increased aromatic C contents in SEOM, indicating mineralization of potentially mobile organic matter (OM). Ultrasonic treatment during soil fractionation leads to enhanced dissolved OM (DOM) leaching and increased SOM bioavailability, which may bias results from incubation experiments on separated SOM fractions.

Evaluation of potentially labile soil organic carbon and nitrogen fractionation procedures

Abstract Particulate organic matter (POM) and light fraction (LF) organic matter are potentially labile (active) fractions of soil organic matter (SOM) that have been shown to be indicators of short-term changes in soil management practices (e.g. tillage, manure and fertilizer applications, and crop rotation). These two fractions consist mainly of partially decomposed plant residues, microbial residues, seeds, and spores forming organo-mineral complexes with soil mineral particles; however, they cannot be used as synonyms because of their different chemical composition and structure. Particulate-OM is recovered by size-based procedures while LF is generally recovered in two distinct fractions [free-LF (FLF) and occluded-LF (OLF)] using density-based solutions in conjunction with soil-aggregate disruption. Solutions used in these density-based separations have most commonly varied in density from 1.6 to 2.0 g cm −3 . Sodium iodide (NaI) and sodium polytungstate (SPT) are the chemicals most often used to prepare the density solutions in LF recovery but comparisons of the effectiveness of two solutions have not been conducted. The objectives of this research were: (1) compare the efficiency of similar density solutions of NaI and SPT in recovering FLF; and (2) compare POM, FLF, and OLF as possible sensitive indices of short-term soil changes due to tillage management. Soil samples were collected at 0–15 cm depth from a cropping system experiment conducted on a silt loam Ultisol. Plots selected for sampling had received either reduced till (RT) or no-till (NT), and cropping was continuous corn silage for a period of 3 years prior to sampling. Solutions of NaI and SPT at densities of 1.6 and 1.8 g cm −3 were used to recover FLF, and OLF was recovered with SPT solution at a density of 2.0 g cm −3 from the soil pellet remaining after FLF recovery with SPT 1.6 g cm −3 . The average total soil organic carbon (SOC) content of these samples was of 12.7 g kg −1 , and carbon-POM (C-POM), carbon-FLF (C-FLF), and carbon-OLF (C-OLF) represented 22.4, 5.5, and 5.2% of it, respectively. In general, C-FLF and nitrogen-FLF (N-FLF) contents recovered did not differ significantly between chemical solutions (NaI or SPT) adjusted to the same density (1.6 or 1.8 g cm −3 ). Increasing the density within a specific solution (NaI or SPT) resulted in significantly higher C-FLF and N-FLF recovery. For instance, C-FLF recovery averaged 637 and 954 mg kg −1 at 1.6 and 1.8 g cm −3 , respectively. For both chemicals increasing density from 1.6 to 1.8 g cm −3 reduced the variability in recovering C-FLF and N-FLF with coefficient of variation values decreasing from a range of 14.9–19.1% for densities of 1.6 g cm −3 to 6.7–10.4% when densities increased to 1.8 g cm −3 . In the present work, POM and OLF were more sensitive than FLF to changes in tillage management, with significantly greater amounts of the sensitive fractions in RT samples. A better sensitivity of FLF would be expected if treatments dealing with residue input (e.g. crop rotation and cover crop) were evaluated.

Nonpoint-source nitrogen and phosphorus behavior and modeling in cold climate: a review

Pollution from nonpoint-source (NPS) nitrogen (N) and phosphorus (P) are the main causes of eutrophication in lotic, lentic and coastal systems. The climate of cold regions might play an important role in disturbing environmental behavior of NPS N and P, influencing simulation of watershed scale hydrologic and nonpoint-source pollution models. The losses of NPS N and P increase in regions of cold climate. In cold seasons, accumulations of N and P are accelerated in soil with increasing fine root and aboveground biomass mortality, decreasing plant nutrient uptake, as well as freezing soil. N and P transformation is disturbed by soil frost and snow. Moreover, factors such as physical disruption of soil aggregates, pollutant accumulation in snowpack, and snow melting can all increase the NPS N and P losses to the waterbody. Therefore, NPS N and P in first flush are more serious in cold climate. All these effects, especially frozen soil and snowpack, make great challenges to watershed scale hydrologic and nonpoint-source pollution models simulation in cold climate. Model improvements of snowmelt runoff, nutrient losses in frozen soil, as well as N and P behavior have been initiated and will be continued to evaluate in terms of their performances and suitability with different scale, hydrologic and geologic conditions in the future.

Effects of compost input and tillage intensity on soil microbial biomass and activity under Mediterranean conditions

Organic amendment and tillage reduction are two common practices to contrast soil organic matter decline, thus promoting sustainable cropping and carbon sequestration. In a horticultural land use system under Mediterranean climate, we evaluated the 9-year effects of two compost inputs (15 and 30 t ha−1 y−1, low and high input, respectively) and two tillage intensities (intensive and reduced) on soil macronutrients concentration, microbial biomass and activity. Total organic C, total N and POlsen were smaller in plots amended at low input, whilst intensive tillage decreased them at both compost inputs. These decreases in intensively tilled plots was ascribed to the disruption of soil aggregates, with consequent microbial degradation of the physically protected organic matter by oxidative processes. On the contrary, reduced tillage increased the extractable C, likely due to a higher protection of the most labile soil C fraction from the mineralization. Similarly, microbial biomass C and N increased following both doubling compost input and reducing tillage intensity, with a greater effect by the first factor. The higher values of cumulative 10-day basal and 20-h glucose-induced respiration, and metabolic quotient in intensively tilled plots suggested that high tillage favoured soil aggregate disruption and C accessibility. This was also confirmed by higher values of dehydrogenase activity/total organic C in those plots. Intensive tillage caused a higher soil aeration and organic substrates accessibility to microflora, thus undoing the fertility benefits provided by the high compost input. However, also a low compost input coupled to reduced tillage seemed to accomplish soil sustainability needs.

Influence of different litter quality on the abundance of genes involved in nitrification and denitrification after freezing and thawing of an arable soil

Due to disruption of soil aggregates and cell lysis and the subsequent release of organic C and N, increased microbial N transformation processes can be observed after freeze–thaw cycles. In a microcosm study, we investigated the influence of plant residues with different C/N ratios (lucerne-clover-grass-mix and wheat straw) on N transformations and the abundance pattern of the corresponding functional genes in an arable soil after freezing and thawing. Unfrozen soil samples, continuously incubated at 10°C, served as control. Concentration of soil NH4+, NO3−, and water-extractable organic C (WEOC) as well as genes involved in nitrification and denitrification, quantified by real-time PCR, were determined before freezing and 1, 3, and 7 days after thawing. The amounts of inorganic N and WEOC as well as the investigated gene abundance pattern did hardly differ between control samples and samples subjected to freezing and thawing that have been amended with straw. In contrast, clear alterations of the measured parameters and abundances were observed after freezing and thawing in samples being amended with the lucerne-clover-grass-mix compared to the control samples.

Short term changes in dynamics of C and N in soil when crops are cultivated on permanent raised beds

Densely populated, intensively cropped highland areas in the subtropics are prone to erosion and declining soil fertility, making agriculture unsustainable. Permanent raised bed planting systems, as a form of conservation agriculture, have been developed to reduce production costs while conserving resources and sustaining the environment. In 2004, a new experiment with long term focus was started under rain fed conditions at El Batan (Mexico; 2,240 m a.s.l.; 19.31N, 98.50W; Cumulic Phaeozem), which aims at understanding the effects of (1) tillage (conventionally tilled or permanent raised beds), (2) residue management (retention or removal) and (3) N fertilizer application (0 or 120 kg N/ha) on N availability in a yearly maize/wheat rotation system. Incubation experiments were conducted to establish how the different treatments affect C and N dynamics in the soil. Tillage increases the availability of soil organic matter by soil aggregate disruption, enhancing C and N mineralization. Conventionally tilled raised beds with incorporation of crop residues increased the CO2 production rate. In both tillage systems, retention of maize or wheat residue without N fertilizer application led to N immobilization. In permanent raised beds, however, the immobilization due to residue retention could be compensated by application of N fertilizer, while conventionally tilled raised beds appeared to use the applied N fertilizer less efficiently.

Short-term temporal changes of soil carbon losses after tillage described by a first-order decay model

Tillage stimulates soil carbon (C) losses by increasing aeration, changing temperature and moisture conditions, and thus favoring microbial decomposition. In addition, soil aggregate disruption by tillage exposes once protected organic matter to decomposition. We propose a model to explain carbon dioxide (CO 2) emission after tillage as a function of the no-till emission plus a correction due to the tillage disturbance. The model assumes that C in the readily decomposable organic matter follows a first-order reaction kinetics equation as: d C sail( t)/d t = − kC soil( t) and that soil C-CO 2 emission is proportional to the C decay rate in soil, where C soil( t) is the available labile soil C (g m −2) at any time ( t). Emissions are modeled in terms soil C available to decomposition in the tilled and non-tilled plots, and a relationship is derived between no-till ( F NT) and tilled ( F T) fluxes, which is: F T = a 1 F NT e − a 2 t , where t is time after tillage. Predicted and observed fluxes showed good agreement based on determination coefficient ( R 2), index of agreement and model efficiency, with R 2 as high as 0.97. The two parameters included in the model are related to the difference between the decay constant ( k factor) of tilled and no-till plots ( a 2) and also to the amount of labile carbon added to the readily decomposable soil organic matter due to tillage ( a 1). These two parameters were estimated in the model ranging from 1.27 and 2.60 ( a 1) and −1.52 × 10 −2 and 2.2 × 10 −2 day −1 ( a 2). The advantage is that temporal variability of tillage-induced emissions can be described by only one analytical function that includes the no-till emission plus an exponential term modulated by tillage and environmentally dependent parameters.