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Soil water repellency (SWR) was measured for a 28 yr field study under irrigation on a clay loam Dark Brown soil in southern Alberta. The objectives were to study the effect of legume–cereal crop rotations, feedlot manure, and phosphorus (P) fertilizer application on soil hydrophobicity (SH) and soil water repellency index (RI) under irrigation. Mean SH and RI were similar (P > 0.05) for a legume–cereal and cereal rotation and were unaffected by P fertilization. However, P fertilization shifted the RI classification from slight to subcritical. In contrast, SH was significantly greater for manured than nonmanured treatments, while RI was unaffected. Soil organic carbon (SOC) concentration was significantly (P ≤ 0.05) correlated with SH (r = 0.74), but not with RI (r = −0.17). This suggested a closer association between the quantity of SOC and quantity of hydrophobic compounds (SH method) compared with the hydrophobic coatings inhibiting infiltration of water (RI method). No significant correlation between SH and RI (r = −0.09) suggests that SH is not a good predictor of SWR using the RI method. Overall, manure application increased SH and P fertilization shifted the RI classification from slight to subcritical. In contrast, legume–cereal rotations had no influence on SH and SWR using RI method compared with continuous cereal.

Crop residues and N fertilizer under no-till may increase soil water repellency (SWR) and soil hydrophobicity, but few studies have examined these two treatment factors and their interaction. A laboratory study was conducted using a long-term (since 1999) field experiment on a clay loam soil to determine the effect of three crop residues and two N fertilizer levels on SWR and soil hydrophobicity under no-till within the Dark Brown soil zone of the semi-arid Canadian prairies. The three residue treatments were residues removed from soil (Rx0), residues returned to soil (Rx1), and residues supplemented to soil (Rx2). The two fertilizer N treatments were 0 (N0) and 45 kg N ha−1 (N1). Surface (0–10 cm) soil samples were taken in the spring of 2017 after 17 yr. Laboratory measurements were conducted on air-dried and sieved ( 1.95) SWR. Similar (P > 0.05) RI values were found for the three residue and two N fertilizer treatments, but the trend was for greater RI with increased residue addition (by 12%–26%) and N fertilizer (by 8%). Soil hydrophobicity was significantly greater by 47%–82% for straw returned or supplemented than straw removed treatments, and by 33% for fertilized than unfertilized treatments. Overall, greater residues and N fertilizer had no effect on SWR, but significantly increased soil hydrophobicity.

Crop residues are an important resource for maintaining soil productivity. The decay of crop residues is linked to many ecosystem functions, affecting atmospheric CO2, nutrient release, microbial diversity, and soil organic matter quality. The rate of decay, in turn, is regulated by soil type, management, and environmental variables, some of which will be changing in the future. Our objective in this study was to evaluate effects of soil type, climate, residue placement on the decomposition and retention of residue-derived C. 13C-labelled barley straw was either placed at the surface or mixed to 10 cm in soils at four sites in Canada and one site in New Zealand representing different soil types and climates. Soils were collected periodically over 10 yr to determine 13C remaining. The loss of C from crop residues occurred quickly, most (70-75%) within the first 2 yrs but with only 5-10% remaining after 10 yrs. There were large losses of C from the mixed treatments within the first year, with 20-50% lost after 6 months over winter and 50-70 % lost after one year; after that decomposition slowed. Temperature was the single most important factor regulating the rate of residue decay. Thermal time, expressed as cumulative degree days, explained more of the variability in residue C recovered than time (in calendar years). Slower decay of surface-placed residues may be attributed to lower mean annual precipitation at those sites. Thermal time is a robust, consistent way of predicting crop residue decay rates (or C storage) for comparing C kinetics across sites with different soils and climates.

The effects of grazing on grassland ecosystems must be understood to develop effective soil conservation measures and sustain livestock production. The objective of this research was to characterize the impact of dairy cow grazing intensity on C and N in soil and vegetation on a meadow steppe. The meadow steppe site, located near Hailaer, Inner Mongolia, China, has been grazed by dairy cattle for more than 30 years, with the last 23 years managed by the same herders. The study site never received any form of mineral fertilizer. Grazing intensity was classified as light (LG), moderate (MG) and heavy (HG) according to the vegetation utilization rate. Grazing affected biomass, total N (TN) and 15N levels, but not 13C, of above-ground plants. The above-ground biomass and δ15N decreased but TN content increased with the grazing intensity for all nine non-legume species while five legume species were not affected. Coarse root biomass also decreased with grazing intensity while TN or δ15N responses to grazing showed no consistent patterns. Soil OC and total N content decreased significantly with grazing intensity for all three sampling depths (0–10, 10–20 and 20–30 cm) while nitrate, ammonium and 13C levels were not affected by grazing intensity in all sampling depths. The decreases in soil OC suggest carbon storage in the meadow steppe declined as grazing intensity increased. Combined with the decreases in soil TN content, this also means heavy grazing could lead to decreases in soil quality and fertility. Uneven grazing intensity is an inherent feature of a season-long grazing system and there are risks to soil quality and environmental health associated with HG. We need to find the proper balance between light to heavy grazing intensity through proper grazing management and in some cases using judicious herding to distribute livestock. Some combination of HG, MG and LG could be desirable in sustainable rangeland grazing production.

Agricultural land currently is used not only for producing food, fiber, and other bioproducts, but also for disposing of and recycling industrial, municipal, and agricultural wastes. Because such uses may potentially have negative effects on the environment, the fate of applied elements or of those originally present in the soil are under intensive scrutiny. Temporal change in the mass of a given soil element or constituent per unit area typically is calculated as the difference between constituent masses in a fixed soil depth at two sampling times. This method, however, is adequate for only rare cases when soil volume remains unchanged (i.e., when there are no changes in soil bulk density or thickness). Other methods based on "fixed soil mass," "equivalent soil depth," or "cumulative mass coordinate" have been developed to account for bulk density changes but they still do not account for changes in soil mass, such as those associated with waste inputs or soil redistribution through erosion and deposition, or imports and exports. We propose an alternative method based on elevation-based soil sampling to account for the effects of changes in both soil bulk density and soil mass. Unlike methods that assume soil mass remains unchanged, the proposed method would also be applicable to sites with appreciable additions or removals of soil mass or volume. We discuss the merits of elevation-based soil sampling to assess temporal changes in soil constituents, and present an example of its application to a site receiving heavy applications of livestock manure for 30 yr.

The Haber–Bosch process for synthesizing fertilizer nitrogen (N) is among the most important modern discoveries because it has enabled us to grow enough food for several billion more of us. At the same time, however, profusion of added “reactive” N has become a prominent ecological threat, globally, because a large fraction of applied N is lost from agricultural ecosystems. Nitrogen added to agricultural soils, in organic forms or as synthetic N fertilizers, has one of four fates; it can be assimilated by plants, lost to surface- or groundwater, retained in the soil, or lost to the atmosphere. The last of these is particularly worrisome because of links to climate change and other threats to the biosphere. Our aim is to summarize briefly the processes of atmospheric N emissions to the air from agricultural ecosystems, and to consider how management practices might reduce those emissions. Nitrogen gases emitted from soil emanate from naturally occurring biological processes regulated largely by three interactive factors: substrate availability, aeration, and temperature. Although these factors are partly dictated by weather and intrinsic soil properties, they are also influenced by management so that emissions can be heavily influenced by practices imposed on the land. Variables to consider in devising systems with reduced emissions include: forms, rate, and timing of fertilizer; tillage and residue management; crop rotation, including the use of legumes; and manuring practices. All of these need to be considered together to devise systems, tuned to local conditions, which not only reduce emissions but also meet growing demands for agricultural yields. Developing such systems, based on holistic understanding from many disciplines, is now critical to sustain the long-term productivity and vitality of our ecosystems.

Crested wheatgrass (Agropyron cristatum [L.] Gaertn.) and Russian wildrye (Elymus junceus Fisch.) are commonly used for reseeding in the more xeric Mixed Prairie of the Canadian prairies because they are perceived to be more productive than native species. However, they have been implicated in soil deterioration. The objectives of our study were to compare the aboveground net primary production and soil organic carbon (C) among monoculture communities of selected native grass species, crested wheatgrass, and Russian wildrye and to compare the native grass monocultures with their mixtures. In 1995, a 5-year study was initiated on Dark Brown Chernozemic (Typic Haploboroll) soil near Lethbridge, Alberta. Ten treatments consisting of monocultures of introduced and selected native species and mixtures of native species were established in a randomized complete block design with 4 replications. Aboveground net primary production and soil organic C were measured. Monocultures of 2 native species, green needlegrass (Stipa viridula Trin.) and blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Steud.), were more productive than crested wheatgrass or Russian wildrye under both normal moisture and drought conditions. Monocultures of these native species also tended to be more productive than their mixtures. The western wheatgrass (A. smithii Rydb.) monoculture and the western wheatgrass–blue grama mixture experienced the greatest yield reduction as a result of drought. Treatment effects on soil organic C were not detected (P . 0.05) 5 years after seeding. Soils of the June grass (Koeleria macrantha [Ledeb.] J.A. Schultes f.) community had less (P , 0.05) macro-organic C than most other treatments. Resumen

The regional scale response of soil carbon mass to long-term grazing exclusion was investigated in the Canadian Great Plains. Vegetation, litter, macro-organic matter and soil were sampled in paired grazed and ungrazed treatments from 9 independent locations along an environmental gradient in southern Alberta. Vegetation and litter carbon mass were greater on ungrazed treatments, but no consistent grazing effect was observed for macro-organic matter (roots, subsurface litter) or soil (fine particles < 2mm) carbon mass per equivalent soil mass. Soil carbon in mixed grass prairie was positively correlated with clay content, but no grazing effect could be detected when this subset (n = 7) was analyzed by ANCOVA. Comparison of multiple sites with a consistent sampling and reporting method revealed no general trend in the response of soil carbon to grazing. Current range management practices to maintain range types in good to poor condition appear to be consistent with maintaining the soil organic mat...

Differences in 13C inputs between two adjacent Canadian mixed grass prairie communities were used to estimate turnover of two soil organic matter fractions with increasing depth below the surface. Partial cultivation and abandonment of a C4-dominated grassland ca. 1930, and natural recovery to a native C3-dominated grassland changed the 13C/12C ratio (reported in units of δ13C‰) of plant residue inputs to soil organic matter. After 70 years, macro-organic matter carbon (MOMC) differed up to 5.6‰, and fine soil organic carbon (FSOC) by 2.9‰ near the surface; differences declined to a depth of 1 m. Mean residence time estimates differed among fractions from a short 35 years for MOMC 0–5 cm, to a longer 413 years for FSOC 5–10 cm. The background carbon pool on this site was substantial, and the minimum change in soil carbon of 10.29 Mg C ha−1 over 70 years represented less than 7% of the carbon pool to a 1-m depth. Considerable vertical and horizontal variation in soil organic carbon mass and 13C content were observed. Variation due to small sample size was a more substantial limitation than cumulative analytical variation for calculating turnover estimates >500 years or at depths >20 cm. Corrections for bias were also necessary since imperfect physical separation of soil organic matter fractions, or CO32− contamination, were issues.

Nitrous oxide (N2O) emission from farmland is a concern for both environmental quality and agricultural productivity. Field experiments were conducted in 1996–1997 to assess soil N2O emissions as affected by timing of N fertilizer application and straw/tillage practices for crop production under irrigation in southern Alberta. The crops were soft wheat (Triticum aestivumL.) in 1996 and canola (Brassica napusL.) in 1997. Nitrous oxide flux from soil was measured using a vented chamber technique and calculated from the increase in concentration with time. Nitrous oxide fluxes for all treatments varied greatly during the year, with the greatest fluxes occurring in association with freeze-thaw events during March and April. Emissions were greater when N fertilizer (100 kg N ha−1) was applied in the fall compared to spring application. Straw removal at harvest in the fall increased N2O emissions when N fertilizer was applied in the fall, but decreased emissions when no fertilizer was applied. Fall plowing also increased N2O emissions compared to spring plowing or direct seeding. The study showed that N2O emissions may be minimized by applying N fertilizer in spring, retaining straw, and incorporating it in spring. The estimates of regional N2O emissions based on a fixed proportion of applied N may be tenuous since N2O emission varied widely depending on straw and fertilizer management practices.

The soil environment is a primary component of the global biogeochemical sulfur (S) cycle, acting as a source and sink of various S species and mediating oxidation state changes. However, ecological significance of the various S forms and the impacts of human intervention and climate on the amount and structural composition of these compounds are still poorly understood. We investigated the long-term influences of anthropogenically mediated transitions from natural to managed ecosystems on molecular-level speciation, biogeochemical dynamics, and the apparent temperature sensitivity of S moieties in temperate, subtropical, and tropical environments with mean annual temperature (MAT) ranging from 5 degrees C to 21 degrees C, using elemental analysis and X-ray absorption near-edge structure (XANES) spectroscopy. Land-use and land-cover changes led to the depletion of total soil S in all three ecoregions over a period of up to 103 years. The largest decline occurred from tropical forest agroecosystems (67% Kakamega and 76% Nandi, Kenya), compared to losses from temperate (36% at Lethbridge, Canada, and 40% at Pendleton, USA) and subtropical (48% at South Africa) grassland agroecosystems. The total S losses correlated significantly with MAT. Anthropogenic interventions profoundly altered the molecular-level composition and resulted in an apparent shift in oxidation states of organic S from native ecosystems composed primarily of S moieties in intermediate and highly reduced oxidation states toward managed agroecosystems dominated by organic S rich in strongly oxidized functionalities. The most prominent change occurred in thiols and sulfides, the proportion of which decreased by 46% (Lethbridge) and 57% (Pendleton) in temperate agroecosystems, by 46% in subtropical agroecosystems, and by 79% (Nandi) and 81% (Kakamega) in tropical agroecosystems. The proportion of organic S directly linked to O increased by 81%, 168%, 40%, 92%, and 85%, respectively. Among the various organic S functionalities, thiols and sulfides seem to have higher apparent temperature sensitivity, and thus these organic S moieties may become prone to losses due to land-use changes, even from the cooler regions of the world if MAT of these regions rise in the future.