Fallow Frequency Research Articles

Improving Sustainability of Cropping Systems in the Central Great Plains

ABSTRACT Rotations are changing in the semiarid Central Great Plains because of no-till systems and crop residue management. With improved precipitation storage in soil, producers now grow corn (Zea mays L.), proso millet (Panicum miliaceum L.), or sunflower (Helianthus annuus L.) in sequence with winter wheat (Triticum aestivum L.) and fallow. A long-term cropping systems study was started at Akron, Colorado in 1990 to evaluate rotations comprised of a diversity of crops, with the goal of developing rotations that minimize frequency of fallow. After 10 years, we examined ecological trends associated with soil structure, nutrient cycling, and pest management as affected by rotations. Soil structure and nutrient cycling improved with continuous cropping, whereas, arranging winter and summer annual crops in a cycle-of-four improved pest management. Producers are seeking rotations that not only are economical, but also improve soil quality; they view fallow and tillage as detriments to long-term sustainability. Therefore, we also suggest options for developing rotations with continuous cropping, based on insight gained from the Akron study. Soils in the Central Great Plains were severely damaged during the Dust Bowl era; producers seek to repair this damage with continuous cropping and no-till. But, a concern with continuous cropping is yield variability and financial risk, which producers previously managed with fallow. Crop diversity and sequencing in conjunction with residue management and no-till may provide advantages that minimize need for fallow in risk management.

Challenging Approaches to Nitrogen Fertilizer Recommendations in Continuous Cropping Systems in the Great Plains

Cropping systems in the Great Plains have evolved over the past two decades from reliance on summer fallowing to continuous cropping under reduced or no‐tillage. Most N recommendation models were developed in fallow systems under conventional tillage and were based on average yield goal, with adjustments for soil profile N content. The objective of this review is to examine the impact of continuous cropping on N requirements. With high‐residue continuous cropping systems, N requirements may increase because of increased annualized production, reduced contribution of N mineralization, and increased immobilization and volatilization potential of surface‐applied fertilizer N. Mitigating these effects on N availability and supplemental N requirements are the reduction in yield per crop, reduced nitrate (NO3) leaching potential, increased N use efficiency (NUE), and increased rates of N mineralization due to higher soil organic matter (OM) content. Unfortunately, increased year‐to‐year yield variability with continuous cropping increases the difficulty in accurately estimating yield goals. Also, reducing the frequency and duration of fallow may reduce the usefulness of the preplant soil N tests in estimating N availability. Recent research has evaluated the use of optical sensors during the growing season to assess N stress and to estimate crop N requirements. If proved feasible for many crops, this would provide a drastic change for determining N recommendations. In the absence of a reasonable yield goal and known residual soil N content, a fertilizer N rate near 70 kg N ha−1 or less was generally sufficient to optimize small‐grain or oilseed yields in several continuous cropping studies.

Soil organic matter status of Chernozem soil in North Kazakhstan: effects of summer fallow

In North Kazakhstan there is concern about the degradation of Chernozem soil and agricultural sustainability by the inclusion and frequency of summer fallows in crop rotations in terms of their influence on the changes of soil organic matter (SOM) quality and quantity. We examined five fallow-wheat ( Triticum aestivum L.) cropping systems with different frequencies of the fallow phase in Chernozem soil of North Kazakhstan; continuous wheat (CW), 6-y rotation (6R), 4-y rotation (4R), 2-y rotation (2R) and continuous fallow (CF). Soil samples were collected from the two phases of each rotation, pre- and post-fallow, and analyzed for potentially mineralizable C (PMC) and N (PMN), ‘light fraction’ organic matter (LF-OM), C (LF-C) and N (LF-N). Potentially mineralizable C was inversely proportional to the frequency of fallow and was highest in CW. Mineral N (min-N) and PMN were more responsive to rotation phase than other indices of SOM. Mineral N was higher in the post-fallow phase while PMN was higher in the pre-fallow phase. Light fraction organic matter was negatively correlated to the frequency of fallow and was higher in the pre-fallow than in the post-fallow phase in a rotation. The results suggested that the yearly input of plant residue in a less frequently fallowed system built up more PMC, whereas PMN was closely correlated to recent inputs of substrate added with plant residues. We conclude that a frequent fallow system may deplete SOM via accelerated mineralization. Also that LF-OM, PMC and PMN are more sensitive to detect subtle changes in SOM quality than total SOM. Our results may provide prediction of SOM response to fallow frequency in wheat-based rotation systems in Chernozem soils of semi-arid regions.

Cropping frequency effects on yield of grain, straw, plant N, N balance and annual production of spring wheat in the semiarid prairie

Producers in the semiarid Canadian prairies practice frequent summerfallow to conserve water and reduce the risk of crop failure, but this practice promotes soil degradation. In contrast, annual cropping enhances soil quality but results in greater economic risk. We need to know what is the most suitable cropping frequency for this region. In 1985, based on results of the first 18 yr of a long-term crop rotation experiment being conducted on a medium-textured, Orthic Brown Chernozem at Swift Current, Saskatchewan, we modified the experiment to allow comparison of four cropping frequencies over the period 1985–2002. These were fallow-spring wheat (Triticum aestivum L.) (F-W), F-W-W, F-W-W-W-W-W, and continuous wheat (Cont W). All systems received recommended rates of N and P fertilizer. Growing season precipitation during the 1985–2002 period was 10% above average so that grain yields were also above average for this region. We assessed yields of grain, straw and N in aboveground plant parts, N concentration in grain and straw, harvest index, nitrogen harvest index and water use efficiency, and the average annual production (on a rotation basis) of grain, straw and aboveground N yield, and related these characteristics to water deficit. Water deficit was the main factor responsible for annual variations in the characteristics assessed. Of these characteristics, cropping frequency influenced only the average annual productivity factors; it rarely influenced the characteristics when they were assessed on a rotation-phase basis. Assuming cropping frequency (x) for the 2-yr, 3-yr, 6-yr and Cont W rotations is 50%, 67%, 83%, and 100%, respectively, then average annual yields (y) in kg ha-1 yr-1 were related to frequency as follows: (1) for grain yield y = 768 + 10.7 x, r2 = 0.99; (2) for straw yield y = 1159 + 18.9 x, r2 = 0.99; and (3) for aboveground plant N yield y = 23.7 + 0.36 x, r2 = 0.99. The regression between annual grain production and cropping frequency at Swift Current for 1985 to 2002 had a much higher slope than the relationship for the same experiment in the much drier 1967 to 1984 period. Further, the equations for Swift Current in 1985–2002 were similar to the relationships obtained for systems in the subhumid Black Chernozems, indicating such relationships are greatly influenced by weather conditions. These relationships may be useful for conducting economic analyses and for modeling N balance. We conducted an apparent N balance analysis which indicated that all four rotations have lost N from the system in inverse proportion to the frequency of cropping. However, changes in total N in the 0- to 0.3-m depth of soil suggest that they have not lost N and that Cont W gained N over the course of the study. We are not able to explain this apparent anomaly and recommend further research on this question. Key words: Fallow frequency, grain yield, N concentration, N yield, water deficit, water use efficiency, N balance

Effect of tillage and crop rotations on the light fraction organic carbon and carbon mineralization in Chernozemic soils of Saskatchewan

Light fraction of soil organic C (LFOC) represents a major portion of labile soil organic C (SOC) and is a key attribute of soil quality. Soil respiration (Cmin) is an important index depicting the potential activity of the labile SOC. Six field experiments, varying in duration (8 to 25 yr), in location (Brown, Dark Brown and Black Chernozemic soil zones of Saskatchewan) and soil texture, were conducted to evaluate the impact of tillage and crop rotations on crop production and soil quality. We sampled the 0-7.5-cm depth of soil in these experiments to determine the treatment effects on LFOC, the proportion of LFOC in the SOC (LFOC/SOC) and Cmin. Increasing the frequency of summer fallow in cropping systems decreased the LFOC in all soil zones; it also decreased the proportion of LFOC in SOC and Cmin. Tillage had little impact on LFOC in the Brown and Dark Brown Chernozemic soil zones, although it significantly decreased LFOC in the Black Chernozemic soil zone. Thus, crop rotation had a greater impact on LFOC than tillage. Tillage did not influence Cmin in any soil zone. Because adoption of no-till management increased SOC in all soil zones, we concluded that LFOC was not a sensitive indicator of the impact of tillage on this soil quality attribute for these Chernozemic soils in Saskatchewan. We also found that LFOC/SOC is directly proportional to sand content. This relationship may assist us in partitioning SOC pools with differing turnover times when modeling SOC dynamics. Key words: Soil organic C, light fraction organic C, tillage, crop rotations, texture, mineralizable C

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

Evaluation of a simple model to describe carbon accumulation in a Brown Chernozem under varying fallow frequency

Soil organic C (SOC) is readily influenced by crop management practices, such as summerfallowing. On the Canadian prairies, the area summerfallowed has decreased significantly in recent years. Our objectives were to determine the influence of fallow frequency on the rate of change in SOC in an Orthic Brown Chernozem, and to test the effectiveness of an empirical equation developed in an earlier study for estimating SOC changes in these rotations over 33-yr period. The rotations, which were initiated in 1967, all received adequate N and P fertilizers. They were (i) fallow-spring wheat (Triticum aestivum L.) (F-W), F-W-W, F-W-W-W-W-W and W-lentil (Lens culinaris L.) (W-Lent). Soil organic C was measured in the 0- to 15-cm and 15- to 30-cm depths in 1976, 1981, 1984, 1990, 1993, 1996 and 1999. No measurements of SOC were made in 1967; we estimated SOC starting values to be 30.5 Mg ha–1 in the 0- to 15-cm depth. In the period 1967 to 1990, when growing season precipitation was near normal for this semiarid region, SOC in the four rotations approached a steady state. However, a decade of much more favourable growing season precipitation in the 1990s increased C inputs, which resulted in a marked increase in SOC in the treatments. The empirical equation suggests, and the F-W and W-Lent rotations appear to confirm, that these rotations are approaching a new steady state at a higher level of SOC, reflecting the decade of favourable precipitation. Measured SOC levels were quite variable, emphasizing the difficulty of relying on measurements made over short time frames (e.g., 5-6 yr) when quantifying SOC changes. The equation effectively simulated the trends in SOC changes in all rotations, but consistently underestimated SOC levels in the W-Lent rotation by about 2 Mg ha–1. Estimates of difference in SOC between treatments were generally similar whether expressed on a mass/fixed depth or a mass/equivalent depth basis. Based on the estimates derived by the empirical equation, we estimated rates of SOC sequestration during the 1967-1990 period to be 0.03 Mg ha–1 yr–1 for F-W, 0.10 Mg ha–1 yr–1 for F-W-W, and 0.15 Mg ha–1 yr–1 for W-Lent. If we include the decade of more favourable precipitation (1967-1999), the rates were between 0.05 Mg ha–1 yr–1 for F-W and 0.20 Mg ha–1 yr–1 for W-Lent. These values are much higher than those estimated by others using the CENTURY model. We concluded that (i) simple models, such as that used in this study, are very useful for estimating management effects on SOC changes, and (ii) we must be cautious in extrapolating C sequestration estimates based on data from short-term experiments because future weather conditions are not easily predicted and weather can have an important impact on C sequestration. Key words: C sequestration, cropping frequency, crop rotations, wheat, lentil, flax, oats

Effects of tillage method and fallow frequency on leaf spotting diseases of spring wheat in the semiarid Canadian prairies

Leaf spotting diseases of wheat ( Triticum spp.) are widespread in western Canada. Because these diseases are residue-borne, they are expected to be affected by changes in the quality and quantity of crop residues. A field study was conducted to determine the effects of summerfallow and tillage practices on leaf spotting diseases of spring wheat ( T. aestivum L.) in the semiarid area of the western Canadian prairies. Leaf spot severity was greater in wheat grown after fallow than in continuous wheat when these systems were managed using either cultivator- or zero-tillage methods. Disease severity in wheat after fallow was similar in all three tillage methods: cultivator-, reduced-, and zero-tillage. Pyrenophora tritici-repentis (Died.) Drechs. (tan spot) was the pathogen most commonly isolated from lesioned leaf tissue. Crop residues collected in the spring of 1995 and 1996 from cultivator- and zero-tillage treatments were examined for the presence and density of fungal infective structures. The density of mature and immature structures, especially of P. tritici-repentis, was greater in residues from two years previous than in those from the previous growing season. Most of the residues in the continuous wheat system were from the previous crop. The apparent lower amount of initial inoculum available in a continuous wheat system than in wheat grown after fallow would explain the higher leaf spotting severity in the latter system. In addition, lower levels of infective structures on residues were found in wheat after fallow in zero- rather than in cultivator-tillage. However, similar disease levels in cultivator- and zero-tillage treatments suggest that the more favourable microclimate for disease development in a zero-tillage system might have compensated for the lower amounts of residue-borne inoculum.

Soil Organic Matter Changes in Intensively Cropped Dryland Systems

AbstractContinuous cropping or decreasing the frequency of summer fallow (F) in cereal‐based dryland rotations may have benefits other than greater water utilization and erosion control. We hypothesized that rotations with no fallow or minimum fallow frequency can produce more biomass and cover than the traditional winter wheat (Triticum aestivum L.)‐summer fallow systems (W‐F), and ultimately, greater amounts of soil organic matter (SOM). To this end, we evaluated changes in various pools of SOM at the 0‐ to 5‐ and 0‐ to 15‐cm depths on a Weld loam (fine, smectitic, mesic aridic Paleustolls) that were caused by (i) decreasing fallow or increasing cropping intensities, (ii) specific rotations of the same length but with different crop sequencing, and (iii) accumulated residue and roots from reduced‐ or notillage from 1993 to 1997. Total soil organic carbon (SOC) and N for the 0‐ to 5‐cm depth increased by =20% with continuous cropping rotations compared with W‐F rotations. Particulate organic matter‐carbon (POM‐C) doubled, while POM‐N, and soluble organic C (OC) increased by one third for the same comparison. At the 0‐ to 15‐cm depth, SOC, POM‐C, and POM‐N did not differ among systems with fallow, nor among systems with cropping intensities greater than W‐F. Thus, significant differences always existed between W‐F and continuous cropping. Generally, fallow had a negative influence on SOC accumulation, and continuous cropping a positive influence on surface SOM. Changes in SOC did not correlate with yields in the five‐year comparison of this ongoing study.

Alternative Crop Rotations for the Central Great Plains

Winter wheat (Triticum aestivum L.) is the most common dryland crop grown in the central Great Plains. Producers in this region include fallow in the rotation to minimize yield variability due to erratic precipitation. However, fallow degrades soil quality by increasing erosion potential and loss of organic matter. Fortunately, minimum-till production systems and residue management improve water use efficiency by plants, thus producers can crop more frequently. We evaluated eight rotations comprised of various sequences of winter wheat (W), corn (Zea mays L.) (C), proso millet (Panicum miliaceum L.) (M), sunflower (Helianthus annuus L.) (S), and fallow (F) in comparison to W-F at Akron CO. Our goal was to identify rotations that can replace W-F to minimize the frequency of fallow. The soil was a Weld silt loam (Aridic Paleustoll). Continuously cropping with W-C-M and W-M almost doubled total grain yield compared with the conventional system of W-F. Other rotations such as W-C-F, W-C-S-F, and W-C-M-F yielded >60% more on an annualized basis than W-F. Winter wheat yield increased with longer time intervals between wheat crops. Sunflower yielded the most when grown only once every 4 yr; more frequent cropping favored diseases. Sunflower reduced yield of the following crop, especially during dry years. Yield variability was highest with corn and sunflower, whereas proso millet showed the least variability. Producers can manage yield variability by diversifying crops in the rotation, as annualized yield variability of W-M and W-C-M was similar to W-F. With residue maintenance and minimum tillage, producers can crop more frequently, thus increasing land productivity while minimizing the frequency of fallow in this semiarid region.

Reduced tillage and increasing cropping intensity in the Great Plains conserves soil C

Concern about soil organic matter losses as a result of cultivation has been voiced consistently since the early part of the 20th century. Scientists working in the US. Great Plains recognized that organic matter losses from an already small pool could have major negative consequences on soil physical properties and N supplying capacity. The advent of reduced- and no-till systems has greatly improved our ability to capture and retain precipitation in the soil during the non-crop periods of the cropping cycle, and has made it possible to reduce fallow frequency and intensify cropping systems. The purpose of this paper is to summarize the effects of reduced tillage and cropping system intensification on C storage in soils using data from experiments in North Dakota, Nebraska, Kansas, Colorado, and Texas. Decades of farming with the wheat ( Triticum aestivum L.)–fallow system, the dominant farming system in the Great Plains, have accentuated soil C losses. More intensive cropping systems, made possible by the greater water conservation associated with no-till practices, have produced more grain, produced more crop residue and allowed more of it to remain on the soil surface. Combined with less soil disturbance in reduced- and no-till systems, intensive cropping has increased C storage in the soil. We also conclude that the effects of cropping system intensification on soil C should not be investigated independent of residue C still on the surface. There are many unknowns regarding how rapidly changes in soil C will occur when tillage and cropping systems are changed, but the data summarized in this paper indicate that in the surface 2.5 cm of soil, changes can be detected within 10 years. It is imperative that we continue long-term experiments to evaluate rates of change over an extended period. It is also apparent that we should include residue C, both on the surface of the soil and within the surface 2.5 cm, in our system C budgets if we are to accurately depict residue–soil C system status. The accounting of soil C must be done on a mass basis rather than on a concentration basis.

Long-term effects of tillage and fallow-frequency on soil quality attributes in a clay soil in semiarid southwestern saskatchewan

Reduced tillage management is being adopted at an accelerated rate on the Canadian prairies. This may influence soil quality and productivity. A study conducted on a clay soil (Udic Haplustert) in southwestern Saskatchewan, Canada, to determine the effects of fallow frequency [fallow-wheat (F-W) vs. continuous wheat (Cont W)] and tillage [no-tillage (NT) vs. conventional (CT) or minimum tillage (MT)] on yields of spring wheat ( Triticum aestivum L.), was sampled after 3, 7 and 11 years to assess changes in selected soil quality attributes. Tillage had no effect on amount of crop residues returned to the land, but the tilled systems had significantly ( P<0.05) lower total organic C and N in the 0–7.5 cm soil depth, though not in the 7.5–15 cm depth. Further, these differences were observed after only 3 years and persisted for the entire 11 years of the study. For example, in the 0–7.5 cm depth, organic C in F-W (MT) after 3 years was 10 480 kg ha −1 and in F-W (NT) 13 380 kg ha −1, while in Cont W (CT) and Cont W (NT) corresponding values were 11 310 and 13 400 kg ha −1, respectively. After 11 years, values for F-W (MT) and F-W (NT) were 11 440 and 14 960 kg ha −1, respectively, and for Cont W (CT) and Cont W (NT), 12 970 and 16 140 kg ha −1, respectively. In contrast to total organic matter, two of the more labile soil quality attributes [i.e., C mineralization (C min) and N mineralization (N min)] did not respond to fallow frequency until after 7 years and only in the 0–7.5 cm depth. Microbial biomass (MB) and the ratio of C min to MB [specific respiratory activity (SRA)], two attributes also regarded as labile, were not influenced by the treatments even after 11 years. After 11 years, only C min and N min among the labile soil quality attributes responded to the treatments. Surprisingly, the labile attributes were no more sensitive to the treatments than was total organic C or N. More research is required to determine why responses in this soil differed from those reported elsewhere.

Soil quality—Effect of tillage and fallow frequency. Soil organic matter quality as influenced by tillage and fallow frequency in a silt loam in southwestern Saskatchewan

Characterization of organic matter is valuable for identifying the overall quality of soils. Management will influence soil quality. The rate at which quality attributes change may depend upon soil texture and environmental conditions. We assessed the effects of tillage (no-tillage (NT) vs conventional mechanical tillage (CT)) and fallow frequency (continuous wheat ( Triticum aestivum L.) (Cont W) vs fallow-wheat (F-W)) on selected soil quality attributes for a silt loam in semiarid southwestern Saskatchewan. Measurements were made in the 0–7.5 and 7.5–15 cm depths after 4, 8 and 12 y of study. After 4 and 8 y, only C mineralization (C min) and N mineralization (N min) showed significant treatment effects, but after 12 y specific respiratory activity (SRA) was also significant ( P<0.05). Total organic C and N and microbial biomass (MB) were not affected significantly by treatments ( P<0.05) even after 12 y. Cropping more frequently resulted in higher C min, N min and SRA, likely associated with higher crop residue production (e.g., correlation of C min and N min with mean annual straw production was 0.95, significant at P=0.05). Tillage did not influence straw production nor any of the soil quality attributes assessed. The labile attributes were more sensitive than total organic C and N to fallow frequency, but they were generally less sensitive to method of tillage management.

Tillage and fallow frequency effects on selected soil quality attributes in a coarse-textured Brown Chernozem

An 11-yr study was conducted on a coarse-textured Brown Chernozemic soil in the semiarid prairie of southwestern Saskatchewan. Soil was sampled after 3, 7 and 11 yr, and the results were used to assess the influence of fallow frequency and tillage on selected soil quality attributes [e.g., total soil organic C and N, microbial biomass C (MB-C) and microbial biomass N (MB-N), C mineralization (Cmin) and N mineralization (Nmin), and specific respiratory activity (SRA)] in the 0- to 7.5-cm and 7.5- to 15-cm depths. Although it took 11 yr before we observed significant treatment effects on total organic C or N, effects on Cmin and Nmin were observed in 7 yr in the 0- to 7.5-cm depth and by 11 yr, MB and SRA also showed significant treatment effects in this depth. Generally, soil quality attributes were greater in no-tillage (NT) systems than in conventional mechanical tillage (CT) or minimum tillage (MT), and greater in continuous wheat (Triticum aestivum L.) (Cont W) than in fallow-wheat (F-W) systems. With time, the labile constituents tended to increase under the Cont W cropping, but to decrease when F-W was coupled with MT. After 11 yr there was a strong, direct association between the labile attributes (viz., Cmin, Nmin and MB-C) in the 0- to 7.5-cm depth and the mean annual straw produced (kg ha−1yr−1) in the four cropping systems tested. Of the soil quality attributes tested, Cmin and Nmin were the most sensitive indices to tillage and fallow frequency effects. Key words: Mineralizable C, mineralizable N, microbial biomass, specific respiratory activity, crop residues

Tillage and crop rotation effects on soil organic C and N in a coarse-textured Typic Haploboroll in southwestern Saskatchewan

An 11 year study conducted in the semiarid prairie of southwestern Saskatchewan, Canada, to determine the influence of fallow frequency and tillage on yields and economics of durum wheat ( Triticum turgidum L.) production, was used to assess changes in soil organic C and N. The soil was a coarse-textured, Typic Haploboroll. Cropping frequency (fallow-wheat vs. continuous wheat) had no effect on organic C or N concentrations or amount in this study. Compared with conventional mechanical or minimum tillage, no-tillage increased organic C and N concentrations in the 0–7.5 cm soil depth during the final 4 years of the experiment when crop yields and residue production were above average. However, there was no effect of tillage on C or N concentration in the 7.5–15 cm depth, nor on the mass of organic C or N in either depth. The amount of organic C in the top 15 cm of soil increased by 1.6 t ha −1 over the 11 year period (average for all systems). Most of this increase occurred in the final 4 years and was generally related to the amount of crop residues returned to the soil. Because neither rotation nor tillage influenced crop production, these practices did not affect organic C or N. We concluded that there was limited opportunity to sequester additional C in coarse-textured soils located in semiarid climates if monoculture cereal systems are used under conventional (mechanical) or no-tillage.

Total and Light‐Fraction Carbon Dynamics during Four Decades after Cropping Changes

AbstractThe dynamics of soil organic C after a change in cropping practice are important in defining the potential impact of the new practice on soil behavior and net CO2 exchange. Our objective was to determine the duration and magnitude of a shift in soil organic C after a change in summer fallow frequency in spring wheat (Triticum aestivum L.) systems. Total organic and light‐fraction C in continuous wheat (W), fallow‐wheat‐wheat (FWW), and fallow‐wheat (FW) treatments were determined in soil samples collected periodically from 1954 to 1992 from a long‐term spring wheat study located in the northern Great Plains. Organic C concentrations in the 0‐ to 15‐cm depth were 11% lower in the FW and FWW treatments than in the W treatment from 1967 to 1992. Organic C concentrations in the 15‐ to 30‐cm depth were not affected by treatment, but varied among sampling years due to differences in effective sampling depth. Light‐fraction C concentrations were 38% lower in the FW and FWW treatments than in the W treatment from 1967 to 1992. Reduced light‐fraction C accounted for about one‐third of the decrease in total organic C due to inclusion of fallow in the crop rotation. The negative influence of fallow was primarily due to enhanced decomposition in the fallow year. We conclude that most losses of soil organic C due to introduction of a fallow system occur within two decades, and that further loss of organic C is negligible provided soil erosion is minimal.

Effect of Cropping and Fertilization on Plant and Soil Phosphorus

AbstractKnowledge regarding the effects of long‐term cropping practices and fertilization on the fate of P applied to the soil is required to aid in the prediction of how such practices influence the quality and sustainability of the environment. A 24‐yr crop rotation experiment, conducted on a Swinton silt loam (Aridic Haploboroll) in southern Saskatchewan, was used to determine uptake of P by crops, its distribution in grain and straw, and the rate of change and nature of the residual P in the soil. Phosphorus concentration in the grain relative to that in the straw was 7:1 to 12:1 for spring wheat (Triticum aestivum L.), 6:1 for flax (Linum usitatissimum L.), and 5:1 for lentil (Lens culinaris Medikus) and fall rye (Secale cereale L.). Grain P concentration was lowest in cereal crops (3.6 g kg−1) and highest in flax and lentil (4.4 g kg−1). Phosphorus removed annually with the grain increased with P fertilization and decreased with an increase in fallow frequency; it was 4.8 to 5.3 kg P ha−1 for continuous wheat, 3.5 to 3.8 kg P ha−1 for fallow‐wheat‐wheat, and 3.1 kg P ha−1 for fallow‐wheat. Trend analysis showed that fertilizer P increased the Olsen P levels in soil at rates of 1.0 to 1.7 kg ha−1 yr−1, but in the systems receiving no P, Olsen P remained constant and did not decline as suggested by a P balance sheet. Residual fertilizer P accumulated in labile forms, dominated by sorbed P and microbial P, rather than in forms dominated by Ca phosphate precipitates or other organic forms.

Labile soil organic matter as influenced by cropping practices in an arid environment

The dynamics of organic matter (OM) in prairie soils play an important role in long-term soil productivity and in the global carbon cycle. Temporal fluctuations in OM occur primarily in the readily-decomposable, labile fractions. We analyzed soils from a long-term study on a Brown Chernozem (Aridic Haploboroll) in southwestern Saskatchewan to determine the effect of cropping practices on OM content and composition under arid conditions. Soils (0−7.5 and 7.5–15 cm layers) from various rotations of spring wheat ( Triticum aestivum L.), winter wheat, flax ( Linum usitatissimum L.), lentil ( Lens culinaris Medikus) and fallow were analyzed for organic C, total N and selected indicators of labile OM [light fraction(LF)-C, LF-N, mineralizable-C, mineralizable-N, microbial biomass(MB)-C and MB-N]. The indicators of labile OM were all more sensitive than total OM to agronomic variables. Treatment effects on all characteristics were usually similar in the 0−7.5 and 7.5–15 cm layers, but effects in the surface layer were often more highly significant. Frequency of fallow in the rotation was the dominant factor influencing labile OM. For example, LF-C in the 0−7.5 cm layer of well-fertilized continuously-cropped spring wheat, bare fallow-wheat-wheat and bare fallow-wheat was 3.15, 1.55 and 1.17 mg C kg −1 soil, respectively. The corresponding values for CO 2-C mineralized at 21°C in 30 days were 371, 184 and 158 mg C kg −1 soil and those for cumulative net N mineralized in 16 weeks at 35°C were 126, 96 and 80 mg N kg −1 soil. MB-C and MB-N exhibited similar trends (e.g. MB-C was 368, 256 and 257 mg C kg −1 soil for the three treatments, respectively). Application of N fertilizer and substitution of winter wheat (with chemical fallow) for spring wheat (with tilled bare fallow) tended to increase labile OM, while substitution of flax or lentil for spring wheat had little effect or reduced labile OM. Differences in labile OM appeared to be related, not only to residue inputs, but also to moisture and temperature conditions. Our findings suggest that it may be possible to manipulate the timing of residue inputs and moisture through cropping practices and thereby maintain adequate labile OM concentrations and improve the synchrony of mineralization with crop requirements. Our results also imply that LF and mineralizable fractions may be useful as early indicators of OM change.

Sensitivity of total, light fraction and mineralizable organic matter to management practices in a Lethbridge soil

Crop management influences the quantity and quality of organic matter in agricultural soils. A crop rotation study established at Lethbridge, Alberta in 1951 was sampled in September 1992 to determine the effect of crop management on total, light fraction and mineralizable (10-wk) organic matter contents. Spring wheat was the dominant cropping system; treatments examined include fallow frequency, forage hay production in rotation, manure amendment, N fertilizer application, and native grass. The two latter treatments were introduced in 1985. Total and light fraction organic matter did not vary among phases of the rotation whereas mineralized C tended to be lowest during and shortly after a fallow phase. When averaged across rotation phases, total, light fraction, and mineralized organic matter were enhanced by reduced fallow frequency, manure additons, hay production and native grass. Highest concentrations of total and labile organic concentrations in the 0- to 7.5-cm soil depth were generally found in the continuously-cropped wheat and native grass treatments. Hay production significantly increased soil organic matter in the 15- to 30-cm soil depth. Nitrogen fertilization did not increase soil organic matter in this study, likely because of minimal yield response over the treatment period. Sensitivity of the various indicators to treatment [(highest–lowest)/lowest] was 0.2 for total, 2.5 for light fraction, and 1.5 for mineralized soil organic C. Light fraction organic matter was the most robust indicator of management-induced effects on soil organic matter. Key words: Labile organic matter, fallow frequency, forage hay production, manure, native grass

Long‐Term Management and Clay Dispersibility in Two Haploborolls in Saskatchewan

AbstractThe water dispersibility of clay influences soil physical properties such as water infiltration, surface crusting, and water erosion. The development of the Water Erosion Prediction Project (WEPP) model, which uses water‐dispersible clay in the algorithm for estimating interrill erosion, has created a need to understand the factors controlling clay dispersibility. This study evaluated the effects of long‐term (≥24 yr) crop rotations and management on water‐dispersible clay in two contrasting soil types in southern Saskatchewan. Cultivated soils were 0.4 to 21 times more dispersive than nearby grassland soils. Among the rotation treatments, a 6‐yr rotation, which included 1 yr of summer fallow, 2 yr of spring wheat (Triticum aestivum L.), and 3 yr of bromegrass (Bromus inermis Leyss.)‐alfalfa (Medicago sativa L.) hay, had the greatest impact, giving dispersible clay values (151–244 g kg−1) that were substantially lower than those for continuous wheat (361–441 g kg−1) and wheat‐fallow rotations (354–483 g kg−1). Fallow frequency and fertilizer treatment had little effect. Water‐dispersible clay was negatively correlated (r = −0.79*** and −0.86***) with wet aggregate stability. The results suggested that both of these characteristics are influenced by organic binding agents produced during crop growth. Although soil management had a major effect on clay dispersibility, total clay content may be the dominant factor when a wide range of cultivated soil types are considered. However, management history and aggregate stability data should be taken into account when attempting to predict dispersible clay from total clay content.