Equivalent Mass Basis Research Articles

Long-term adoption of bed planted conservation agriculture based maize/cotton-wheat system enhances soil organic carbon stabilization within aggregates in the indo-gangetic plains

Sustainability of contemporary crop establishment and management practices is questioned due to soil degradation, higher carbon emission and declining soil productivity. Hence, this study was conducted to address the impacts of conservation agriculture (CA) practices like permanent broad beds (PBB), permanent narrow beds (PNB) and zero tilled flat beds (ZT) with residue retention on soil organic carbon (SOC) protection within aggregates in the Indo-Gangetic Plains (IGP). Compared to conventionally tilled (CT) plots, the total SOC content was ∼27%–33% higher in the CA plots on equivalent mass basis. The soil physical properties, such as soil aggregation and mean weight diameter were considerably improved under the CA practices. The macroaggregates were ∼41, 37% and 27% higher in the PBB with residue (PBB + R), PNB with residue (PNB + R) and ZT with residue (ZT + R) plots (CA plots), respectively, than the CT plots in the surface soil (0–15 cm). The plots under PBB + R had ∼31% higher microaggregates within macroaggregates than the CT plots (24.4 g 100 g−1) soil. An increase in SOC content by ∼72, 55% and 69% was observed in the PBB + R, PNB + R and ZT + R plots over the CT plots in microaggregates within macroaggregates (3.02 Mg ha−1). However, plots under PBB + R, PNB + R and ZT + R had only ∼11, 3% and 23% more SOC within silt + clay fraction, respectively, than CT plots (5.85 Mg ha−1). Thus, SOC stabilization within microaggregates inside macroaggregates was the major mechanism, and not the chemical stabilization within silt + clay, of C sequestration under CA. As aggregate-associated carbon is an ecosystem property that strongly affects organic carbon stabilization, water holding capacity and resistance to erosion, growing maize/cotton–wheat system under PBB + R practice is a viable option for carbon sequestration in the IGP and similar agro-ecologies.

Wastewater monitoring of SARS-CoV-2 RNA at K-12 schools: comparison to pooled clinical testing data.

Wastewater measurements of SARS-CoV-2 RNA have been extensively used to supplement clinical data on COVID-19. Most examples in the literature that describe wastewater monitoring for SARS-CoV-2 RNA use samples from wastewater treatment plants and individual buildings that serve as the primary residence of community members. However, wastewater surveillance can be an attractive supplement to clinical testing in K-12 schools where individuals only spend a portion of their time but interact with others in close proximity, increasing risk of potential transmission of disease. Wastewater samples were collected from two K-12 schools in California and divided into solid and liquid fractions to be processed for detection of SARS-CoV-2. The resulting detection rate in each wastewater fraction was compared to each other and the detection rate in pooled clinical specimens. Most wastewater samples were positive for SARS-CoV-2 RNA when clinical testing was positive (75% for solid samples and 100% for liquid samples). Wastewater samples continued to test positive for SARS-CoV-2 RNA when clinical testing was negative or in absence of clinical testing (83% for both solid and liquid samples), indicating presence of infected individuals in the schools. Wastewater solids had a higher concentration of SARS-CoV-2 than wastewater liquids on an equivalent mass basis by three orders of magnitude.

Land Use Effects on Soil Organic Carbon Fractions in the Pampas of Argentina Determined via Inverse Analysis of Carbon Mineralization Data

ABSTRACT Understanding the long-term effects of land use on soil organic carbon (SOC) is of interest due to the possibility of using soils as carbon (C) sinks. We determined how the C levels in the labile (C0) and stable (Cr) fractions of the organic matter of soils of the Pampas were affected by land use, applying the inverse analysis methodology. Soil samples (0–50 cm) taken from 198 sites under different land uses (forest, grassland, and cultivated soil) were incubated for six months, adjusting a one pool exponential model to C mineralization data. The model allowed a good estimation of C0 (R2 = 0.96–0.98) and Cr was calculated by difference with SOC. The stocks of C0 and Cr were calculated on an equivalent mass basis (5000 t soil ha−1) for comparisons. About 80% of the SOC is found in the Cr pool in the Pampas soils. Both C0 and Cr were higher in forest and grassland than in cultivated soils. Soil cultivation produced an average reduction of 30% of the C0 pool and 18% of the Cr pool in relation to grassland soils. Consequently, most of the SOC lost due to agriculture came from the Cr pool. In mixed production systems, no differences of the C pools were detected between the crop or pasture phase of the rotation. The parameters C0 and k (mineralization rate constant) of the exponential model were negatively correlated, so the comparison of C0 between different land uses must be careful. The k parameter was similar among land uses when the comparison was made for similar values of C0, suggesting that land use affected the size of the C pools not their stability. Forest and grasslands were effective systems for sequestering C in stable forms. Expanding the cultivated area in the Pampas should be avoided if it is desired not to emit C into the atmosphere from the soil.

Equivalent-mass versus fixed-depth as criteria for quantifying soil carbon sequestration: How relevant is the difference?

Soil organic carbon (SOC) sequestration is often quantified through the comparison of SOC stocks down to a fixed depth, usually 30 cm. This approach gives biased estimates because SOC sequestration changes soil bulk density. To account for these biases, the comparison of SOC stocks on an equivalent mass basis is recommended. How big are the biases due to the use of a fixed-depth approach (either under- or overestimations of SOC sequestration), and whether these biases imply that the current estimates of SOC sequestration worldwide need an overhaul, are the two questions addressed in this work.We collected from literature 140 pairs (initial-final) of plots affected by land use changes: crop to forest, crop to grassland, etc. Peatlands were excluded. The datasets were detailed enough to calculate SOC sequestration by both approaches: fixed-depth (down to 30 cm) and equivalent-mass (amount of fine mineral earth at the top 30 cm of the initial stage). Differences between both approaches were often small, but relevant in many cases: namely, when changes in soil compaction were relevant too. The fixed-depth approach overestimates SOC sequestration when soil compaction occurs, and underestimates it when soil expands. However, the average error is low, because over- and underestimations of sequestered SOC partly compensate: the fixed-depth approach underestimates SOC sequestration by about 0.326 ± 1.016 kg SOC m−2 (overall amount), or 14 g SOC m−2 yr−1 (rate). This suggest that the corrections needed by current estimates of SOC sequestration worldwide—at least, those related to the problem studied in this paper—would be low.From our data, the equivalent-mass approach is compulsory in studies about SOC sequestration. Nevertheless, it seems less adequate than fixed-depth as an approach to compare SOC stocks worldwide. Both approaches should not be considered as opposite views, but as complementary tools for the study of carbon cycle in soils.

Soil carbon stock changes due to afforestation in Japan by the paired sampling method on an equivalent mass basis

To identify the soil carbon stock changes from croplands/grasslands to forestlands in Japan, we compared the soil carbon stock of a cropland/grassland with that of an adjacent forestland at 22 different sites. With respect to a 0–30-cm depth basis, the soil carbon stock in the cropland/grassland was the same as that in the forestland; however, it was less than that in the forestland when an equivalent mass approach was used. The ratio of the soil carbon stock in the forestland to that in the cropland/grassland was 1.11 on average, which is comparable with previous mass-corrected paired sampling studies. The ratio was affected by two factors: tree age and current vegetation. Mean annual temperature, mean annual precipitation, phosphate absorption coefficient, litter quantity, and former land use did not affect the ratio. According to a comparison with the results of a paired study for deforestation, the application of the reciprocal of the soil carbon stock ratio of deforestation to the ratio of afforestation was slightly overestimated. Further studies are necessary to establish a general conclusion. Some previous studies, including those involving non-mass-corrected data, are possibly biased, and more studies using the paired sampling method with equivalent mass basis must be conducted to provide the general factor for soil carbon stock change in the future.

No-till and carbon stocks: Is deep soil sampling necessary? Insights from long-term experiments

Confusion persists regarding the potential for no-till (NT) farming to accumulate soil organic C (SOC) and mitigate climate change. This uncertainty could be partly due to shallow sampling (<40 cm soil depth) employed in most previous SOC publications. The specific objective of this study was to gain additional knowledge of SOC accumulated by a wide range of tillage systems in surface and deeper soil profiles in the long term (> 34 yr). We quantified SOC stock (Mg ha−1) on an equivalent mass basis for the 0–60 or 100 cm soil depths for NT, disk, double disk, chisel plow, and moldboard plow under continuous corn and corn-soybean on silt clay loams after 34 yr and 39 yr in the western US Corn Belt. Results showed that NT did not increase SOC stocks near the surface but it did with cumulative depth relative to moldboard plow. No-till accumulated SOC at a rate of 0.68 Mg ha−1 yr-1 relative to inversion tillage (moldboard plow). However, NT did not store more SOC than common conventional (chisel plow) or reduced (disk) till practices for the whole soil profile. Results indicate that depth of soil sampling can affect conclusions about tillage system impacts on SOC stocks. For example, if we had sampled to 20 cm soil depth only, we would have incompletely concluded that NT does not significantly accumulate more SOC than moldboard plow. Results after 34 and 39 yr of tillage management also indicate that non-inversion tillage, particularly disk, can be as effective as NT for accumulating SOC. Overall, deep soil sampling can tell a different story about NT impacts on SOC stocks from shallow sampling.

Are grassland soils of the pampas sequestering carbon?

A belief that grassland soils act as carbon sinks and mitigate greenhouse gas emissions generated by livestock production is common among some farming organizations. A recent theoretical analysis proposed that grazing lands in South America sequester carbon as soil organic matter but this result is controversial. Here, we search for empirical evidence of changes in the carbon stock in the grassland soils of the Pampean region of Argentina. For this purpose, 22 natural grassland sites were selected, distributed across the region. The sites were first sampled in 2007 as part of a regional survey of soil carbon stocks; they were re-sampled in 2019. The sites represent a wide range of soil and climate conditions. Samples were taken to a depth of 50 cm. Bulk density, and inorganic and organic carbon, were determined, and the carbon stocks were estimated on an equivalent mass basis. The results were analyzed using ANOVA. No statistically significant differences were found in either inorganic or organic carbon stocks between the sampling times. Across all the sites, the average organic carbon stock was 59.0 t ha−1 in 2007 and 56.2 t ha−1 in 2019. These results suggest that the Pampas grasslands are not sequestering carbon.

Carbon stocks in riparian buffer systems at sites differing in soil texture, vegetation type and age compared to adjacent agricultural fields in southern Ontario, Canada

In any given watershed, riparian buffer systems (RBSs) can influence terrestrial carbon (C) sequestration and potentially enhance terrestrial C sinks. This study looked at the effects of soil texture, vegetation type and age on C stock sequestration in eight RBSs located within the Grand River Watershed (GRW), southern Ontario, Canada and compared the C stock with adjacent agricultural fields. Soil organic carbon (SOC) stocks on an equivalent mass basis were significantly higher (p<0.05) in mature buffers (192.7 Mg C ha-1) compared to adjacent agricultural fields (corn [Zea mays] –soybean [Glycine max] –wheat [Triticum aestivum] rotation), at 0-30 cm soil depth (88.2 Mg C ha-1), irrespective of soil texture and vegetation type of RBS. Sixty-year old deciduous tree buffers on clayey soils had the highest SOC concentration (5.94 %, p<0.05) at 0-30 cm soil depth while the young six-year old coniferous tree buffers on loamy soil had the lowest SOC concentration (2.87%). In mature deciduous [major species; sugar maple (Acer saccharum) and ash (Fraxinus excelsior)] buffers, biomass C stocks were 247 Mg C ha-1, which is up to two and half times greater than mature coniferous [major species; eastern white cedar (Thuja occidentalis) and white pine (Pinus strobus)] buffers (100 Mg C ha-1). Overall results support that increasing RBS within Ontario watershed areas can enhance C stocks and may contribute to Canada’s climate change mitigation efforts. In the context of riparian areas, deciduous species should be promoted over coniferous species to obtain higher terrestrial C sequestration within Ontario watersheds.

Integrated crop‐livestock effects on soil carbon and nitrogen in a semiarid region

AbstractIntegrated crop−livestock (ICL) system effects on soil organic carbon (SOC) and total nitrogen (TN) are highly variable due to differences in ecoregion‐specific management practices, historical land use, and inherent climatic/edaphic attributes. Few studies have evaluated ICL system effects on SOC and TN in semiarid regions, where soil attributes change slowly. This study quantified ICL system effects on SOC and TN in a long‐term experiment near Mandan, ND. Effects of three residue management treatments (Grazed, Removed, Control) on SOC and TN were determined at five depths (0−0.08, 0.08−0.15, 0.15−0.31, 0.31−0.61, and 0.61−0.91 m) in 1999 and 2014. No‐till practices were employed throughout the study following conversion of perennial grass pastures. A significance criterion of P ≤ .10 was used to assess treatment effects as the experimental design was constrained to two replications. Residue management affected SOC and TN in near‐surface depths only, with greater concentration of SOC and TN at 0−0.08 m under Grazed compared to Removed in 2014 (P = .0844 and .0576). When expressed on an equivalent mass basis, SOC stocks were greater under Grazed and Control compared to Removed. Soil organic C and TN stocks increased over time in the Control by 11.2 Mg C ha−1 and 1.2 Mg N ha−1, respectively (P = .0728 and .0868). Soil organic C stocks also increased over time under the Grazed treatment by 10 Mg C ha−1 (P = .0630). Study results highlighted the importance of residue retention and livestock grazing for increasing SOC and TN in ICL systems under semiarid conditions.

Long-term no-till and stover retention each decrease the global warming potential of irrigated continuous corn.

Over the last 50years, the most increase in cultivated land area globally has been due to a doubling of irrigated land. Long-term agronomic management impacts on soil organic carbon (SOC) stocks, soil greenhouse gas (GHG) emissions, and global warming potential (GWP) in irrigated systems, however, remain relatively unknown. Here, residue and tillage management effects were quantified by measuring soil nitrous oxide (N2 O) and methane (CH4 ) fluxes and SOC changes (ΔSOC) at a long-term, irrigated continuous corn (Zea mays L.) system in eastern Nebraska, United States. Management treatments began in 2002, and measured treatments included no or high stover removal (0 or 6.8MgDMha-1 yr-1 , respectively) under no-till (NT) or conventional disk tillage (CT) with full irrigation (n=4). Soil N2 O and CH4 fluxes were measured for five crop-years (2011-2015), and ΔSOC was determined on an equivalent mass basis to ~30cm soil depth. Both area- and yield-scaled soil N2 O emissions were greater with stover retention compared to removal and for CT compared to NT, with no interaction between stover and tillage practices. Methane comprised <1% of total emissions, with NT being CH4 neutral and CT a CH4 source. Surface SOC decreased with stover removal and with CT after 14years of management. When ΔSOC, soil GHG emissions, and agronomic energy usage were used to calculate system GWP, all management systems were net GHG sources. Conservation practices (NT, stover retention) each decreased system GWP compared to conventional practices (CT, stover removal), but pairing conservation practices conferred no additional mitigation benefit. Although cropping system, management equipment/timing/history, soil type, location, weather, and the depth to which ΔSOC is measured affect the GWP outcomes of irrigated systems at large, this long-term irrigated study provides valuable empirical evidence of how management decisions can impact soil GHG emissions and surface SOC stocks.

Agglomeration of Hematite Concentrate by Starches

ABSTRACTStarches of varying solubilities were investigated as a partial and full bentonite replacements. They were found to be suitable for the pelletization of iron ore concentrate. When utilizing starches of varying solubility as a direct bentonite replacement, dry compression strengths increased by nearly 300 N, as compared to bentonite alone, with the highest solubility starch. This implies that on an equivalent mass basis, starch is a far superior binder than bentonite prior to 500 °C. Above this temperature bentonite became the dominating binding factor as starch had combusted and the strengths due to starch were los(t. As a partial bentonite replacement, starch again shows increasing compression strength. The highest strengths were observed where the highest solubility starches were used. Further study has indicated that starch is comprised of insoluble starch granules and soluble starch chains, the latter being the only viable binding agent.

Long-term toxicity of naturally occurring asbestos in male Fischer 344 rats

ABSTRACTNaturally occurring asbestos (NOA) fibers are found in geologic deposits that may be disturbed by mining, earthworks, or natural processes, resulting in adverse health risks to exposed individuals. The toxicities of Libby amphibole and NOA samples including Sumas Mountain chrysotile (SM), El Dorado tremolite (ED), and Ontario ferroactinolite cleavage fragments (ON) were compared in male Fischer 344 (F344) rats 15 mo after exposure. Rat-respirable fractions of LA and SM displayed greater mean lengths and aspect ratios than ED and ON. After a single intratracheal (IT) instillation (0.5 or 1.5 mg/rat), persistent changes in ventilatory parameters and a significant increase in lung resistance at baseline and after methacholine aerosol dosing were found only in rats exposed to 1.5 mg SM. High-dose ED significantly elevated bronchoalveolar lavage lactate dehydrogenase (LDH) activity and protein levels, while high-dose SM increased γ-glutamyl transferase and LDH activities. A moderate degree of lung interstitial fibrosis after exposure to 1.5 mg SM persisted 15 mo after exposure, unchanged from previous findings at 3 mo. LA induced mild fibrosis, while ED and ON produced minimal and no apparent fibrosis, respectively. Bronchioloalveolar carcinoma was observed 15 mo after exposure to LA or ED. Data demonstrated that SM, given by bolus IT dosing on an equivalent mass basis, induced greater pulmonary function deficits, airway hyperresponsiveness, and interstitial fibrosis than other NOA, although unlike LA and ED, no apparent evidence for carcinogenicity was found. All NOA samples except ON cleavage fragments produced some degree of long-term toxicity.

Gowanus Canal Superfund Site. I: NAPL Mobility Testing of MGP-Impacted Sediments

The Gowanus Canal superfund site is located in Brooklyn, New York. The site is impacted by nonaqueous phase liquid (NAPL) from historic manufactured gas plant (MGP) sites and other sources. One of the primary objectives for the selected remedy included eliminating the migration of NAPL into the canal. Accordingly, the amount of NAPL present in sediment samples was quantified and physical testing was conducted to determine a threshold amount of NAPL that could be potentially mobilized into the canal as a result of upwelling groundwater. Undisturbed core samples of native sediment were collected and analyzed for NAPL, water, and air pore fluid saturation (PFS) using the Dean-Stark extraction method, and the total petroleum hydrocarbon (TPH) content. Sediment samples were found to contain NAPL saturations ranging from nondetectable (<0.12%) to 52%. A relatively good correlation (R2=0.77) was observed between the Dean-Stark and TPH results when compared on an equivalent mass basis. The NAPL mobility was evaluated for induced groundwater fluxes ranging from average existing site conditions to a factor of 10 higher (13–130 cm/day). Consistent NAPL mobility was observed when the NAPL saturation exceeded 21%. However, when the NAPL saturation results were converted to TPH on a mass basis (milligrams of NAPL per kilogram of sediment), the results were a poor indicator of NAPL mobility.

Comparison of depth- and mass-based approaches for estimating changes in forest soil carbon stocks: A case study in young plantations and secondary forests in West Java, Indonesia

Soil carbon (C) stocks in forest ecosystems have been widely estimated to a fixed soil depth (i.e., 0–30 cm) to clarify temporal changes in the C pool. However, surface elevations change as a result of compaction or expansion of the soil under forest management and land use. On the other hand, the calculation of soil C stocks based on “equivalent soil mass” is not affected by compaction or expansion of forest soil. To contribute to the development of a forest C accounting methodology, we compared changes in soil C stocks over 4 years between depth- and mass-based approaches using original soil data collected at 0–30 cm depths in young plantations and secondary forests in West Java, Indonesia. Our methodology expanded on the mass-based approach; rather than using one representative value for the mass-based calculation of soil C stocks, we adjusted individual values, maintaining the coefficient of variance in soil mass. We also considered the effect of an increase or decrease in soil organic matter on equivalent soil mass. Both increasing and decreasing trends in soil C stocks became clearer when the mass-based approach was used rather than the depth-based approach. The trends in soil C stocks based on equivalent soil mass were particularly evident in the surface soil layers (0–5 cm) and in plantation sites, compared with those for soil profiles including subsurface soil layers (0–30 cm) and in secondary forests. These trends in soil C stocks corresponded with temporal trends in litter stocks. We suggest that equivalent mass-basis soil C stock for the upper 30 cm of soil be calculated based on multiple soil layers to reduce estimation errors. Changes in soil organic matter mass had little effect on the estimation of soil C stock on an equivalent mass basis. For the development of a forest C accounting system, the mass-based approach should be used to characterize temporal trends in soil C stocks and to improve C cycle models, rather than simpler methods of calculating soil C stocks. These improvements will help to increase the tier level of country-specific forest C accounting systems.

Evaluating the Effect of Tillage on Carbon Sequestration Using the Minimum Detectable Difference Concept

Three long-term field trials in humid regions of Canada and the USA were used to evaluate the influence of soil depth and sample numbers on soil organic carbon (SOC) sequestration in no-tillage (NT) and moldboard plow (MP) corn (Zea mays L.) and soybean (Glycine max L.) production systems. The first trial was conducted on a Maryhill silt loam (Typic Hapludalf) at Elora, Ontario, Canada, the second on a Brookston clay loam (Typic Argiaquoll) at Woodslee, Ontario, Canada, and the third on a Thorp silt loam (Argiaquic Argialboll) at Urbana, Illinois, USA. No-tillage led to significantly higher SOC concentrations in the top 5 cm compared to MP at all 3 sites. However, NT resulted in significantly lower SOC in sub-surface soils as compared to MP at Woodslee (10–20 cm, P = 0.01) and Urbana (20–30 cm, P < 0.10). No-tillage had significantly more SOC storage than MP at the Elora site (3.3 Mg C ha −1) and at the Woodslee site (6.2 Mg C ha −1) on an equivalent mass basis (1350 Mg ha −1 soil equivalent mass). Similarly, NT had greater SOC storage than MP at the Urbana site (2.7 Mg C ha −1) on an equivalent mass basis of 675 Mg ha −1 soil. However, these differences disappeared when the entire plow layer was evaluated for both the Woodslee and Urbana sites as a result of the higher SOC concentrations in MP than in NT at depth. Using the minimum detectable difference technique, we observed that up to 1 500 soil sample per tillage treatment comparison will have to be collected and analyzed for the Elora and Woodslee sites and over 40 soil samples per tillage treatment comparison for the Urbana to statistically separate significant differences in the SOC contents of sub-plow depth soils. Therefore, it is impracticable, and at the least prohibitively expensive, to detect tillage-induced differences in soil C beyond the plow layer in various soils.

Towards accurate measurements of soil organic carbon stock change in agroecosystems

In response to Kyoto Protocol commitments, countries can elect agricultural carbon sinks to offset emissions from other sectors, but they need to verify soil organic carbon (SOC) stock change. We summarize issues we see as barriers to obtaining accurate measures of SOC change, including: soil depth, bulk density and equivalent soil mass, representation of landscape components, experimental design, and the equilibrium status of the SOC. If the entire plow depth is not considered, rates of SOC storage under conservation compared with conventional tillage can be overstated. Bulk density must be measured to report SOC stock on an area basis. More critical still is the need to report SOC stock on an equivalent mass basis to normalize the effects of management on bulk density. Most experiments comparing SOC under differing management have been conducted in small, flat research plots. Although results obtained from these long-term experiments have been useful to develop and validate SOC prediction models, they do not adequately consider landscape effects. Traditional agronomic experimental designs can be inefficient for assessing small changes in SOC stock within large spatial variability. Sampling designs are suggested to improve statistical power and sensitivity in detecting changes in SOC stocks over short time periods. Key words: Soil organic carbon change, agroecosystems, experimental design, sampling depth

Changes in Soil Organic Carbon Contents and Nitrous Oxide Emissions after Introduction of No‐Till in Pampean Agroecosystems

We reviewed published results to estimate no-till effects on SOC and denitrification in the Argentine Pampas and the potential of no-till to mitigate the global warming effect. On an equivalent mass basis, 42 paired data sets were used for SOC comparisons of no-till vs. plow till (moldboard plow or disk plow), 18 paired data for comparison of no-till vs. reduced till (chisel plow or harrow disk), and 20 paired data for comparison of plow till vs. reduced till. Twenty-six denitrification data sets were used for evaluation of tillage system and fertilization effects on N2O emission. Changes in SOC under no-till were not correlated to time since initiation of experiments. Averaged over years a 2.76 Mg ha(-1) SOC increase (P = 0.01) was observed in no-till systems compared with tilled systems, but no differences were detected between plow and reduced till. The SOC under tillage explained most of the SOC variation under no-till (R2= 0.94, P = 0.01). The model had a positive intercept and predicted a relatively higher increase of SOC in areas of low organic matter level. The conversion of the whole pampean cropping area to no-till would increase SOC by 74 Tg C, about twice the annual C emissions from fossil fuel consumption of Argentina. Emissions of N2O were greater under no-till with a mean increase of 1 kg N ha(-1) yr(-1) in denitrification rate for humid pampean scenarios. The increased emissions of N2O might overcome the mitigation potential of no-till due to C sequestration in about 35 yr, and therefore no-till might produce global warming.

Persistence of Soil Organic Carbon after Plowing a Long‐Term No‐Till Field in Southern Ontario, Canada

There is abundant evidence that minimizing soil disturbance reduces mineralization of organic matter and can result in larger storage of soil organic carbon (SOC) relative to conventional tillage. However, little is known about the persistence of SOC when no‐till lands are plowed periodically. This study set out to determine the change in SOC when a long‐term (22 yr) no‐till field in southern Ontario, Canada, was plowed once. Four plots were located within three textural classes [sandy loam (SL), sandy clay loam (SCL), and silty clay loam (SiCL)] within two hydrologic conditions (well‐ and poorly drained) in the field. The plots were sampled before and three times after (3 d, 7 mo, and 18 mo) the no‐till field was moldboard plowed. The single tillage event homogenized the SOC through the profile and reduced the stratification. When calculated on an equivalent mass basis beyond the plow depth, there was no significant change in SOC 18 mo after plowing the SCL, SiCL, and SL high SOC plots. However, in the SL plot with low SOC, there was a loss of about 3 Mg SOC ha−1 after 18 mo, and the loss occurred primarily between the 15‐ and 30‐cm depths in the profile. The loss may have accounted for as much as two‐thirds of the SOC gained from no‐tillage. This study also emphasized the need for additional care to account for changes in bulk density when comparing the quantity of soil constituents, such as SOC, after plowing.

Carbon sequestration in a long-term conventional versus conservation tillage experiment

The impact of conservation tillage practices on carbon sequestration has been of great interest in recent years. Changes in the soil organic carbon (SOC) as influenced by tillage, is more noticeable under long-term rather than short-term tillage practices. This experiment analyzed the organic carbon status of soils sampled at depth increments from 0 to 60 cm after 25 years of five tillage treatments in a silt loam soil. Zero tillage (ZT) treatment was compared to conventional tillage practices of mouldboard and chisel plow operations conducted either during the fall or spring season in a randomized complete block design with four replications. The SOC was calculated on depth and equivalent soil mass bases. Contrast analysis showed a significantly (5%) higher soil bulk density for zero versus fall and zero versus chisel tillage operations at 5–10 cm soil depth. The SOC concentration was dependent on the depth of tillage operation and followed the trend of higher SOC for zero, chisel, and mouldboard tillage at 0–5, 5–10, and 20–40 cm depth, respectively. There were more significant differences in the SOC storage when expressed on depth compared to an equivalent soil mass basis. SOC storage was significantly higher for ZT at the 0–5 cm soil depth compared to conventional tillage practices. Contrast analysis on an equivalent mass basis showed that SOC storage was significantly higher for spring tillage compared to fall tillage at 0–60 cm depth. In conclusion, ZT practices increased SOC concentration and storage compared to conventional tillage operations only for the surface layer but not for the entire soil profile.

Short-term impact of fallow frequency and perennial grass on soil organic carbon in a Brown Chernozem in southern Alberta

Maximum sequestration of atmospheric C in agricultural soils of the Canadian prairies can be achieved by the replacement of crop rotations that include summer fallow with continuous cropping or perennial forages. However, few studies have documented potential gains of soil organic C (SOC) in the region with the greatest use of fallow, the Brown soil zone. A dryland crop rotation study was initiated in 1992 on a Chin clay loam (Orthic Brown Chernozem) in southeastern Alberta on a site that had previously been under conventional irrigated cereal production. The study included fertilized and unfertilized treatments of the following crop rotations: fallow-wheat (FW), fallow-wheat-wheat (FWW), continuous wheat (W) and grass (G). The grass rotation consisted of pubescent wheatgrass (Agropyron trichophorum) harvested for hay. Total organic-, light fraction- and 10-wk mineralized-C and -N were determined in fall 1997, 6 yr after study initiation. Growing season precipitation (April through August) was 43% above long-term normals in the first 2 yr of this study, supporting excellent grass establishment and high crop yields, but was below normal thereafter. Fertilization with N and P increased average rotation yields by 21 to 56%, with the greatest yield increases in the G and W rotations. Total SOC to 15 cm (equivalent mass basis) was 1.5 Mg ha-1 higher (P = 0.03) in the W rotation and 3.0 Mg ha-1 higher (P = 0.0001) in the G rotation than in the FW rotation, but was not affected by fertilizer treatment or the interaction of fertilizer and crop rotation treatments. Low variability in measured SOC allowed us to detect differences as low as 1.3 Mg ha-1 among crop rotations, demonstrating that small effects of crop management on SOC can be detected using appropriate methods at uniform sites. Much of the gain in SOC in the G treatment was in mineralizable and light fraction material, indicating that this gain in SOC would be readily lost upon reversion to annual cropping. A simple calculation of residue C inputs and conversion to SOC showed that most of the gain in SOC in the W treatment was due to increased crop residue inputs, but that both increased inputs and reduced decomposition rates likely affected SOC gains in the G treatment. The average gain in SOC during the 6-yr period of this study was 0.25 Mg ha-1 yr-1 due to the elimination of fallow and 0.5 Mg ha-1 yr-1 due to establishment of a perennial forage. Key words: Soil organic matter, carbon sequestration, summer fallow, light fraction