Long-term cropping system impact on quality and productivity of a Dark Brown Chernozem in southern Alberta

Transcription

1 Long-term cropping system impact on quality and productivity of a Dark Brown Chernozem in southern Alberta Elwin G. Smith, H. Henry Janzen, and Francis J. Larney Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 Agriculture and Agri-Food Canada, Lethbridge Research Centre, PO Box, Lethbridge, Alberta, Canada TJ B ( elwin.smith@agr.gc.ca); Lethbridge Research Centre Contribution Number 878. Received 8 October, accepted January 5. Published on the web March 5. Smith, E. G., Janzen, H. H. and Larney, F. J. 5. Long-term cropping system impact on quality and productivity of a Dark Brown Chernozem in southern Alberta. Can. J. Soil Sci. 95: Long-term cropping system studies offer insights into soil management effects on agricultural sustainability. In 995, a 6-yr bioassay study was superimposed on a long-term crop rotation study established in 95 at Lethbridge, Alberta, to determine the impact of past cropping systems on soil quality, crop productivity, grain quality, and the relationship of yield productivity to soil quality. All plots from longterm crop rotations were seeded to wheat (Triticum aestivum L.) in a strip plot design [control, nitrogen (N) fertilizer]. Prior to seeding, soils were sampled to determine soil chemical properties. Total wheat production for the last yr of the study was used as the measure of productivity. The 995 soil analysis indicated crop rotations with less frequent fallow and with N input had higher soil quality, as indicated by soil organic carbon (SOC) and light fraction carbon (LF-C) and N (LF-N). SOC had a positive relationship to total wheat yield, but was largely masked by the application of N in this bioassay study. Frequent fallow in the previous crop rotation lowered productivity. The concentration of LF-C had a negative relationship, whereas LF-N had a positive relationship to total wheat yield, with and without N fertilization in this bioassay study. Grain N concentration was higher with applied N and when the long-term rotation included the addition of N by fertilizer, livestock manure, annual legume green manure or legume hay. This study determined that long-term imposition of management practices have lasting effects on soil quality and crop productivity. Key words: Soil quality, organic carbon, bioassay, wheat yield, soil productivity Smith, E. G., Janzen, H. H. et Larney, F. J. 5. Impact des systèmes agricoles a` long terme sur la qualite et la productivite d un tchernoziom brun fonce dans le sud de l Alberta. Can. J. Soil Sci. 95: L e tude des systèmes agricoles a` long terme aide a` mieux cerner les répercussions de la gestion des terres sur la pe rennite de l agriculture. En 995, les auteurs ont combine une étude biologique de six ans a` une e tude sur des assolements a` long terme amorce e en 95, a` Lethbridge, en Alberta, en vue de pre ciser l impact des anciens systèmes agricoles sur la qualite du sol, le rendement des cultures, la qualite du grain et les liens entre le rendement et la qualite du sol. Les parcelles de assolements a` long terme ont toutes e té emblave es avec du ble (Triticum aestivum L.) dans le cadre d une expe rience sur bande e chantillon [te moin, amendement azote (N)]. Avant l ensemencement, on a e chantillonne le sol afin d en de terminer les proprie te s chimiques. La production totale de blé des quatre dernie` res anne es de l e tude a servi a` jauger la productivite. L analyse du sol re alise e en 995 indique que les assolements avec jache` re moins fréquente et apport d engrais azote be ne ficient d un sol de meilleure qualite, comme le re vèlent la concentration de carbone organique (COS) ainsi que la fraction le ge` re du carbone (FL-C) et du N (FL-N). Le COS pre sente une corre lation positive avec le rendement total de ble, mais celle-ci se dissimule dans une large mesure derrie` re l application d engrais N dans l e tude biologique. Une jache` re fréquente dans l assolement ante rieur diminue la productivite. La concentration de la FL-C est ne gativement corre lée au rendement total de ble alors que celle de FL-N y est corre lée de fac on positive, avec ou sans amendement azote dans l e tude biologique. La teneur en N du grain est plus e leve e quand on applique un engrais N et quand l assolement a` long terme inclut un apport de N sous forme d engrais, de fumier, d engrais vert de légumineuses annuelles ou de foin de le gumineuses. Cette e tude a permis d e tablir que l imposition de pratiques de gestion à long terme a un effet durable sur la qualite du sol et la productivite des cultures. Mots clés: Qualite du sol, carbone organique, essai biologique, rendement du blé, productivite du sol Soil quality is a major determinant of crop productivity and sustainability. Management practices imposed on the soil, including crop rotation, fertility management and tillage, will impact current and future productivity (Johnston et al. 995; Halvorson et al. ; Campbell et al. 5; Van Eerd et al. ). Since soil quality parameters change very slowly over time in response to management practices, long-term cropping system studies are required to document cumulative changes critical for maintaining productivity (Janzen 987a). The productivity of soils has been shown to benefit from improved soil quality parameters, such as higher Abbreviations: LF-C, light fraction carbon; LF-N, light fraction nitrogen; LTR, long-term rotation; SOC, soil organic carbon; TN, total nitrogen Can. J. Soil Sci. (5) 95: 7786 doi:./cjss-- 77

2 Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 78 CANADIAN JOURNAL OF SOIL SCIENCE soil organic carbon (SOC), less inorganic carbon, and a higher percent of light fraction organic C (LF-C) and N (LF-N) (Janzen 987b; Bremer et al. 99; Larney et al., 9; Zvomuya et al. 8). Factors such as macronutrients, bulk density, soil ph and microbial biomass also influence soil productivity (Karlen et al. 6). Soil quality and crop productivity were reduced on soils degraded by erosion (Larney et al. ). Inorganic fertilizer amendments have been proposed as a means of restoring soils, but fertilizer alone was found not to restore the quality of eroded soils, nor was its application to these soils economical (Smith et al. b). The value of maintaining SOC, determined by integrating responses from multiple studies, estimated long-term benefits to exceed a net present value of $ ha for each additional unit of SOC concentration (g kg soil) (Smith et al. a). This value was higher with higher grain and nitrogen (N) fertilizer prices. There have been studies that evaluated the impact of crop management systems on soil quality parameters (Campbell et al. 99; Bremer et al. 99) and other studies have evaluated soil quality on productivity (Zvomuya et al. 8; Larney et al. 9). There is a need to directly link the impact of management practices on crop productivity, through soil quality. The production of spring wheat on a set of longterm crop rotations with differing chemical and physical properties was used as a bioassay to determine the contribution of various soil quality parameters on crop productivity. The specific objectives of this study were: (i) to determine the impact of long-term rotations (LTR) on soil chemical properties, (ii) to determine the impact of LTR on productivity, as measured by the yield of spring wheat, (iii) to determine the LTR and N fertilizer application in a bioassay study commencing in 995 on wheat yield, grain N concentration and total N yield, (iv) to evaluate whether N fertilizer can overcome productivity differences resulting from soil quality differences due to the LTR, and (v) to estimate the relationship between spring wheat yield and parameters of soil quality. Table. Long-term rotations from 985 through 99, and the year established Number Long-term rotation Designation Established Continuous wheat W 95 Fallow-Wheat FW 95 Fallow-Wheat-Wheat FWW 95 Continuous wheat, plus N W(N) Fallow-Wheat, plus N FW(N) Fallow-Wheat-Wheat, plus N FWW(N) Lentil (green manure)-wheat G M W Lentil (green manure)-wheat- G M WW 985 Wheat 9 Fallow (manure)-wheat- F M WW 95 Wheat Fallow-Wheat-Wheat-Hay- FWWHHH 95 Hay-Hay Wheat (if adequate spring soil Wc 985 moisture) Wheat (if adequate spring soil Wc(N) 955 moisture), plus N Native grass G 985 MATERIALS AND METHODS Field Experiment A 6-yr study was established in 995 at the Agriculture and Agri-Food Canada Research Centre at Lethbridge, AB (lat N, long..7758w) on a LTR site (plot size was. m6.6 m). The soil was an Orthic Dark Brown Chernozem clay loam with level and uniform topography, and calcareous subsoil. The long-term site, established in 95, included seven rotations that were reflective of common rotations at that time (Smith et al. ). There were four replications of each rotation and all phases were present each year. In 985, two of the original 95 rotations plus five adjoining plots for each replicate were randomly reassigned to seven new rotations (Table ). One of the discontinued rotations experienced susceptibility to winter-kill and winter annual grass weeds, and the second with crested wheatgrass (Agropyron cristatum L.) was less productive than a mixture of crested wheatgrass and alfalfa (Medicago sativa L.). The changes eliminated crop sequence duplication, added alternate crops being grown commercially, and added N management options to maintain crop productivity.as a result, there were rotations in the study from 985 through 99 and all phases of the rotations were present each year (Table ). From 995 to, all 6 plots were no-till seeded to spring wheat cultivar Katepwa. The seed rate was kg ha in 995 and was 8 kg ha in all subsequent years. Seeding dates from 996 through ranged from May to May 6, but in 995 seeding was later (May ) to facilitate soil sampling. A pre-seed burn-off of glyphosate was applied to control weeds. In-crop weeds were controlled with appropriate herbicides at label rates, and the herbicide varied by year depending on the weeds present. Phosphorus (P) fertilizer was applied with the seed to all plots as 5, at the rate of 6.7 kg P ha in 995, 6. kg P ha in 996, 7.9 kg P ha in 997, and 9. kg P ha in 998, 999 and. The rotation plots for each replicate were divided into two randomly assigned strips, one with zero N fertilizer (control) and one strip with N fertilizer (as NH NO ) applied at the rate of 5 kg N ha in 995, 56 kg N ha in 996, 998, 999 and, and 59 kg N ha in 997. Harvest was by a small plot combine harvesting the center rows of the plot. A sample of wheat from each plot was oven dried at 68C for7dto obtain oven dry weight. In the spring of 995, the perennial crops of alfalfa (rotation ) and native grass (rotation ) were terminated by herbicides. Since the native grass was not actively growing when terminated, much of it survived

3 SMITH ET AL. * CROPPING IMPACT ON SOIL PRODUCTIVITY 79 Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 and reduced wheat yield in 995 (Fig. ). In the spring of 996 the native grass plots were disced twice to improve the seedbed and ensure the perennial forage growth was terminated prior to seeding. Also in the first year (995), wheat was planted on plots that either had livestock manure applied, were fallow, or were sown to crops, annual green manure, or perennial forage in the previous year. These land use practices the previous year likely had a short-term impact on wheat yield. Soil moisture will be higher on fallow land in a semi-arid environment, and available soil N will depend on previous nutrient application and land use. Precipitation Yield (Mg ha ) 5 during the 6 yr varied with the total May, June and July precipitation at 87% of normal for 995, 56% for 996, 6% for 997, 55% for 998, % for 999, and 7% for. The first yr of yield were excluded from the bioassay analysis of total wheat yield because of confounding short-term effects of land use the year prior to the study (as outlined above), and the problem of seedbed preparation of the native grass plots the first yr. An analysis of total yield over the 6 yr, excluding the native grass treatment (not reported), indicated yield differences for the remaining rotations were consistent whether using all Rotation (without N fertilizer) Rotation (with N fertilizer) Fig.. Average wheat yield with standard deviation by year, crop rotation and without or with N fertilizer applied in this bioassay study (rotations are listed in Table ).

4 Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 8 CANADIAN JOURNAL OF SOIL SCIENCE 6 yr or the last yr. As a result, the last yr of yield data (997) were used in the analysis so the native grass plots could be included. Soil Sampling and Analysis In the spring of 995 soil samples were taken at depths of 7.5 cm, 7.55 cm and 5 cm. Each plot sample was a composite of four soil cores. Soil samples were dried, weighed to determine bulk density, and coarsely ground (B mm) in a rotating sieve. The coarsely ground samples were subsampled and ground further to pass through a 5-mm screen. The samples were analyzed for total C and N by dry combustion with a CNS analyzer (Carol Erba, Milan, Italy). Inorganic C concentration was determined by adding 5 ml of.5 M HCl to.5 g of soil in a sealed 66-mL jar to react and release CO. A gas chromatograph syringe was used to remove.5 ml of gas volume that was injected into the gas chromatograph (Amundson et al. 988). Concentrations of inorganic N (NO and NH ) were determined by. M KCl extraction. The concentrations of SOC and organic N were calculated by subtracting inorganic C and N from the total. Light fraction C (LF-C) and N (LF-N) were determined for the - to 7.5-cm and 7.5- to 5-cm depths using the method of Strickland and Sollins (987). Subsamples ( g) of coarsely ground soil (B mm) were added to a solution of NaI ( ml) that had a specific gravity of.7. Suspensions were allowed 8 h to equilibrate before removing the light fraction material by vacuum. This process was repeated on the unsuspended material to recover any remaining light fraction material. The light fraction material was washed, dried, ground and analyzed for total C and N using dry combustion with a CNS analyzer. The measurement unit for soil quality used in this study was concentration because mass applies more to measuring changes to stocks. Grain Analysis The N concentration of wheat grain was determined for all plots each year. The grain samples were oven dry. A sample of grain was fine ground and then analyzed for total C and N by dry combustion with the CNS analyzer. Total N harvested was estimated as grain yieldgrain N concentration. Statistical Analysis To determine whether the crop rotation prior to the bioassay study impacted chemical properties of the soil, SOC at three depths, and LF-C and LF-N at two depths were analyzed by analysis of variance. The previous rotation was the fixed effect and replication was random. To determine productivity impacts from the current bioassay study, total wheat yield, N concentration and total grain N over the last yr (997 to ) were analyzed by analysis of variance for a strip plot design using PROC MIXED of SAS software (SAS Institute Inc. ) with rotation and N fertilizer as fixed effects and replication as random. Contrasts were used to evaluate pre-determined comparisons of total wheat yield, grain N concentration, and total grain N. The SOC data for rep of the W(N) rotation were deleted from the analysis because it appeared from the measurements the - to 7.5-cm and the 5- to -cm depth samples were switched. The relationship between total yield and soil properties was estimated by regression analysis. Equations were estimated separately without added N fertilizer and with added N fertilizer in the bioassay study. Total wheat yield was regressed against the soil quality parameters: concentration of SOC, LF-C, LF-N, and the frequency of fallow in the LTR. Highly non-significant variables were excluded. The soil data for the regression analysis were all for the - to 7.5-cm depth. The soil depth of 7.55 cm and the - to 5-cm depth combined were tested but had lower explanatory power. The frequency of fallow was the proportion of fallow in the rotation (. for fallow wheatwheat, for example). The frequency of fallow was included to account for fallow impacts that would not be measured by soil chemical properties alone. The soil property variables used in the equation were somewhat related, but the correlation among them was low enough that it did not cause estimation issues. RESULTS AND DISCUSSION Soil The crop rotation history significantly impacted the concentrations of SOC at all three depths, the LF-C at the 7.5- to 5-cm depth, and LF-N at the two depths (Table ). The concentration of SOC, LF-C and LF-N declined with depth of soil. The SOC concentration was higher for historic LTR that added nitrogen to the system and had a lower fallow frequency. The measured soil parameters by LTR were consistent with those reported by Bremer et al. (99), as would be expected since the sampling for their study was only mo prior to the sampling for this study. The mean values of the soil parameters and the least significant difference (LSD) are provided in Table. Augmenting N fertility has been shown to increase SOC (Janzen et al. 998; Halvorson et al. 999), and continued use of crop fallow systems reduces SOC (Halvorson et al. ). SOC was higher for systems that added more carbon into the system, either by more available plant nutrients (N sources) from more plant biomass or fallow was less frequent. The LF-C concentration for the - to 7.5-cm depth was not impacted by the previous rotation, but the concentration at the 7.5- to 5-cm depth had a marginal impact (P.95). The trend was grass and fertilized continuous wheat rotations had higher, and frequently fallowed rotations had lower LF-C concentrations. The concentration of LF-N was higher when the LTR added N to the system (perennial legume forage, applied N, or applied livestock manure).

5 SMITH ET AL. * CROPPING IMPACT ON SOIL PRODUCTIVITY 8 Table. Long-term rotational influence on soil organic carbon concentrations for three soil depths, and light fraction carbon and light fraction nitrogen concentrations for two soil depths, 995 Light fraction Soil organic carbon (SOC) 7.5 cm 7.55 cm 7.5 cm 7.55 cm 5 cm LF-C LF-N LF-C LF-N Rotation z (g kg ) (g kg ) (g kg ) (g kg ) (g kg ) (g kg ) (g kg ) Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 W FW FWW W(N) FW(N) FWW(N) G M W G M WW F M WW FWWHHH Wc Wc(N) G Rotation effect *** *** ** NS ** * ** LSD Coefficient of Variation (%) z The historic rotations were those in place prior to this study (see Table ). *, **, *** P5., P5.5, and P5., respectively; NS, not significant (P.). Wheat Yield Wheat yield varied over time due to growing season precipitation (Fig. ). The bioassay N fertilized treatment had higher -yr total wheat yield than without N (985 kg ha vs. 6 kg ha, PB.). Total wheat yield over yr was significantly impacted by the historic LTR and whether N fertilizer was applied in this bioassay study (Table ). Without the application of N fertilizer in this bioassay study, total yield was highest for the FWWHHH rotation (Table ; Fig., Rotation ). The next highest total wheat yield was on plots that had native grasses and continuous wheat with previously added N [G and W(N)] rotations (Fig., Rotations, and ). The absence of N additions to the system and high fallow frequency (FW, FWW) in the historical LTR resulted in lower total yield, regardless of N application in this bioassay study (Table contrasts). Smith et al. (5) observed wheat yield responses to N fertility and frequency of fallow consistent with this study. The fallow-based LTR with green manure (G M W and G M WW) and applied livestock manure (F M WW) had higher yield than unfertilized fallow-based rotations, Table. Probabilities for sources of variation explaining total wheat yield by rotation and application of N fertilizer in this bioassay study Source of variation Probability Long-term rotation (LTR) B. Nitrogen fertilizer (N).9 LTRN B. but less than fertilized wheat [W(N) and Wc(N)] and FWWHHH. The average -yr total yield difference between rotations FWWHHH and FW and FWW without N application in this bioassay study was 56 kg ha (887 kg ha yr ), a substantial difference in productivity and profitability of wheat production. Wheat yield was higher on historic LTRs that replenished N, whether N fertilizer, livestock manure, or nitrogen fixing legumes, and those with less frequent fallow. The application of N fertilizer during the bioassay years negated many of the historical LTR impacts on total -yr wheat yield that were evident without the added N. Except for the fallow-based rotations without previous N inputs (FW and FWW), FW(N) and G M WW, total yield was equal across all rotations when N was added. In this study, N fertilizer was unable to compensate for the lower productivity of fallow-based rotations that did not replenish N removed by the crop, or for rotations that were in fallow one-half of the time and had added N [FW(N)]. With applied N in this bioassay study, the total -yr yield difference between the highest- and the lowest-yielding rotations that previously included N replenishment [Wc(N)FW(N)] was kg ha (86 kg ha yr ). Without previous N replenishment [Wc(N)FW] the difference was 6 kg ha ( kg ha yr ). Frequent fallow reduced crop productivity. Contrasts to evaluate predetermined LTR comparisons, with and without N fertilizer applied during the study years, confirmed the above findings (Table ). The first set of contrasts compared wheat and fallowwheat

6 Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 8 CANADIAN JOURNAL OF SOIL SCIENCE Table. Total -yr wheat production (997) by N fertility treatment in this bioassay study and long-term rotation Long-term rotation z Without N (kg ha ) With N (kg ha ) W 69def 959abcd FW 65h 855e FWW 5gh 876de W(N) 699bcd 995ab FW(N) 55fg 9cde FWW(N) 66ef 959abc G M W 675cde 9abc G M WW 6de 96bcd F M WW 66de 99abc FWWHHH 857a 985abc Wc 5979defg 97abcd Wc(N) 75bc 85a G 7675b 99abcd Phase effect NS NS Contrasts y Replenished nitrogen W(N)&Wc(N)W&Wc 5*** 586* FW(N)&FWW(N)FW&FWW 865*** 669*** G M W&G M WWFW&FWW 9*** 696*** F M WWFW&FWW *** 77*** Fallow W&WcFW&FWW *** 8*** W(N)&Wc(N)FW(N) 8*** 76*** &FWW(N) W(N)&Wc(N)G M W&G M WW 855*** 7** &F M WW Forage rotation (FWWHHH) W(N)&Wc(N) 6*** 58** FW(N)&FWW(N) 68*** NS G M W&G M WW 55*** NS FW&FWW 56*** 8*** Native grass (G) W(N)&Wc(N) NS NS FW(N)&FWW(N) 88*** NS z The historic rotations were those in place prior to the study (see Table ). y Predetermined contrasts were weighted by the number of years in the crop rotation. Four groups of contrasts and statistical significance were reported without and with N fertilizer application in this bioassay study. The longterm rotations listed under Forage rotation are contrasted to the forage rotation, similar for native grass. ah Yields followed by the same letter within a column are not significantly different. *, **, *** P5., P5.5, and P5., respectively; NS, not significant (P.). rotations, with or without added N in this study. The historic LTR with N replenishment (fertilizer, green manure or livestock manure) had higher total -yr yield without (865 to 5 kg ha ) and with (586 to 77 kg ha ) N fertilizer applied in this bioassay study. For the continuous wheat LTR, total -yr yield with replenishing N was higher without (5 kg ha ) and with (586 kg ha ) the application of N in this bioassay study. The yield benefit from past management was greater when N was not applied in this study; however, the addition of N in this bioassay study did not fully compensate for previous management that lowered soil quality. The application of N to a cropping system cannot completely compensate for lower soil quality caused by previous production practices. The inability of N to suppress the negative impact of previous soil management was also noted for eroded soils (Tanaka and Aase 989; Larney et al. 995). The second set of contrasts compared the frequency of fallow in the rotation. The inclusion of fallow in the unfertilized LTR reduced total -yr yield without ( kg ha ) and with (8 kg ha ) the application of N fertilizer in this bioassay study. The -yr yield benefit persisted for rotations that had applied N fertilizer [W(N) and Wc(N) vs. FW(N) and FWW(N)] (8 kg ha without N and 76 kg ha with N in this study). Fertilized continuous wheat had a -yr yield benefit over green manure and livestock manure systems of 855 kg ha without and 7 kg ha with N in this bioassay study. The third set of contrasts compared the LTR containing alfalfa hay (FWWHHH) with other rotations. The LTR with alfalfa had higher total -yr yield (6 to 56 kg ha ) than any of the contrasted rotations when N fertilizer was not applied in this study. When N fertilizer was applied in this study, total -yr yield was higher for the alfalfa rotation when compared with the unfertilized FW and FWW rotations (8 kg ha ), but the alfalfa rotation had lower total -yr yield than W(N) and Wc(N) (58 kg ha ). The fourth set of contrasts was for native grass. Total -yr yield was higher on plots previously in native grass than for fertilized fallow-based rotations only when N fertilizer was not applied during this study (88 kg ha ). When N fertilizer was applied in this study, there was no difference in yield between the native grass plots and rotations with previous N replenishment. The yield differences from this study demonstrated that previous N fertility management and crop rotation impacted current crop yield. Management practices that replenished N to the system had higher current yield, while rotations with fallow reduced current yield. The yield reduction with fallow was greater when combined with no N replenishment to the system. The current application of N fertilizer could not overcome the yield disadvantage from previous systems with frequent fallow and without replenish of N. Alfalfa hay in crop rotations can increase productivity when inorganic N fertilizer is not applied to crops, as would be the case for organic producers. Producers need to recognize the importance of crop rotation and N fertility management on both long-term and current crop productivity. Nitrogen Concentration and Total Harvested N The N concentration of wheat was significantly impacted the LTR, but not by the application of N in this bioassay study (Table 5). Current N application had no impact on N concentration, indicting the added N uptake was diluted by higher grain yield (Campbell et al. 997). The N concentration was weighted by yield to be consistent

7 SMITH ET AL. * CROPPING IMPACT ON SOIL PRODUCTIVITY 8 Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 Table 5. Probabilities for sources of variation explaining N concentration and total harvested N for wheat by rotation and application of N fertilizer in this bioassay study Source of variation N concentration Total harvested N Long-term rotation (LTR) B. B. Nitrogen fertilizer (N).965 B. LTRN.65 B. with the -yr total grain yield and harvested N. The concentration of N varied by year, consistent with that reported by Campbell et al. (997), but in this analysis the yearly variation was removed by using the -yr weighted concentration. The LTR of FWWHHH and Wc(N) had the highest N concentration (Table 6). The historic LTR that did not replenish N had the lowest grain N concentration. Grain N concentration below. g kg would indicate protein below.5% (based on % moisture in commercial grain). The historic rotations W, Wc, FW and FWW had low protein and the wheat price would be discounted. The -yr total grain N harvested followed the pattern of yield and N concentration. A higher grain N concentration was associated with higher total wheat yield. The LTR, previous N addition, and N fertilizer application significantly impacted total grain N harvested (Table 5). The N fertilized plots in this study yielded more grain N than unfertilized plots (.6 kg N ha vs kg N ha, over the rotations). The highest harvested N was from FWWHHH, W(N) and Wc(N) when N was applied in this bioassay study (Table 6). The lowest N yield was the LTR that had not replenished N and contained at least one-third fallow (FW, FWW). For example, the total harvested N for rotation FWWHHH was 8% and 5% higher than FW, respectively, without and with N in the bioassay study. Table 6. Wheat nitrogen concentration and total harvested nitrogen (997) by previous crop rotation, oven dry weight of wheat Total harvested N (kg N ha ) Rotation z N concen. (g kg ) Without N With N W.9defg def 6c FW.g g 9e FWW.5fg 5fg de W(N) 6.9bc 8bc 78ab FW(N).de ef cd FWW(N).7de de 5c G M W 5.de 6cd 6c G M WW.9def de c F M WW.de 6de 8c FWWHHH 8.9a a 79a Wc.5efg 6def cd Wc(N) 7.6ab b 8ab G 5.6cd 9bc 9bc Contrasts y Replenished nitrogen W(N)&Wc(N)W&Wc.5*** 57.*** 6.6*** FW(N)&FWW(N)FW&FWW.9*** 9.7*** 7.7*** G M W&G M WWFW&FWW.9***.***.*** F M WWFW&FWW.7*** 5.8*** 8.6*** Fallow W&WcFW&FWW.***.5***.*** W(N)&Wc(N)FW/W(N).69*** 57.9***.9*** W(9N)&Wc(N)G M W&G M WW&F M WW.95*** 8.6***.*** Forage rotation (FWWHHH) W(N)&Wc(N).66*** 5.*** NS FW(N)&FWW(N).5***.***.*** G M W&G M WW.5*** 9.8*** 9.8*** FW&FWW 6.***.8*** 8.*** Native grass (G) W(N)&Wc(N).68** NS.** FW(N)&FWW(N) NS 5.*** NS z The long-term rotations were those in place prior to this study (see Table ). y Predetermined contrasts were based on four groups and statistical significance was reported without and with N fertilizer application in this bioassay study. The long-term rotations listed under Forage rotation are contrasted to the forage rotation, similar for native grass. ag Nitrogen concentration and total harvested N followed by the same letter within a column are not significantly different (P.5). N concentration was based on dry weight. *, **, *** P5., P5.5, and P5., respectively; NS, not significant (P.).

8 8 CANADIAN JOURNAL OF SOIL SCIENCE Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 Contrasts showed the LTR that replenished N had higher grain N concentration and total N harvested, regardless of N application in this bioassay study. The results of the contrasts were consistent with the yield contrasts, so will not be described in detail. Total N harvest was higher when N was replenished in the LTR, there was less fallow, and alfalfa forage was in the system. Yield Response to Soil Quality Factors The impact of soil quality variables on crop productivity was estimated by regression analysis. The parameter estimates for the yield equation without and with added N Soil Organic Carbon (g kg ) fertilizer in this bioassay study are reported in Table 7. There was considerable variability in total yield that was not explained by the regression model, as demonstrated by the R, especially when N fertilizer was applied in this bioassay study. The significant soil properties were 9 8 retained in the final equation. 7 Total -yr wheat yield responded to increased SOC. Without N 6 When fertilizer N was not applied in this bioassay study With N yield increased at a decreasing rate as the concentration 5 of SOC increased (Fig. ). The -yr total yield response due to an increase in the concentration of SOC declined from 5 kg ha for a one-unit increase in SOC (g kg ) when SOC was 5. g kg (FW) to. kg ha for a oneunit Light Fraction Carbon (g kg ) increase in SOC (g kg ) when SOC was 8. g kg [FMWW, Wc(N)]. The SOC concentration at which there was no longer a yield benefit from higher SOC (8. g kg 9 ) was about % lower than that reported by Zvomuya et al. (8) for total biomass. When N fertilizer 8 was applied in this bioassay study, there was a 7. kg ha yield increase for a one-unit increase in SOC 7 6 Without N (g kg ) concentration. The benefit from increased SOC With N 5 concentration was higher (.7%) when SOC was low combined with no N application. The -yr yield benefit from the lowest (FW) to the highest (G) SOC concentrations when fertilizer was applied in this study was Light Fraction Nitrogen (g kg ) kg ha. A higher concentration of LF-C reduced yield Fig.. Yield response to soil organic carbon, light fraction (Fig. ). When N was not applied in this study, an carbon and light fraction nitrogen without and with N additional. units of LF-C concentration reduced yield fertilizer in this bioassay study. 9 kg ha when LF-C was. g kg to 6 kg ha when LF-C as. g kg (Table 7). When N was Table 7. Parameter estimates to explain total wheat yield (kg ha )by N fertilization with and without N fertilizer applied in this bioassay study Variable Without N fertilizer With N fertilizer Intercept.* 8 79.*** SOC (g kg ) 6.** 7.* SOC 6.9*** LF-C 99.*** 9.6** (LF-C).9*** LF-N 5 77*** 5 55.** (LF-N) 5 766** Fallow frequency 8.*** 76.9*** R.6. All soil quality variables were for the - to 7.5-cm depth. *, **, *** P5., P5.5, and P5., respectively. Wheat Yield (kg ha ) Wheat Yield (kg ha ) Wheat Yield (kg ha ) Without N With N applied in this study, an additional. units of LF-C concentration reduced total yield 9 kg ha. Light fractions have been reported to be a sensitive indicator of SOC (Janzen et al. 99). A higher concentration of LF-N increased yield (Fig. ). The higher concentration of LF-N was an indication of the ability of the soil to supply N to the crop. When N was not applied in this study, an additional. units of LF-N concentration increased yield 8 kg ha when LF-N was. g kg, to kg ha when LF-N was. g kg. When N was applied in this study, an additional. units of LF-N concentration increased yield 55 kg ha. The historic rotations with the lowest LF-C and highest LF-N concentration, the combination that had higher

9 SMITH ET AL. * CROPPING IMPACT ON SOIL PRODUCTIVITY 85 Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 yields, were FWWHHH and G. The rotation FW had low concentrations of both LF-C and LF-N, with the lower yield associated with low LF-N dominating the benefit of lower LF-C. The light fraction component of SOC decomposes at a faster rate and is a better indicator of supplied nutrients. More frequent fallow in the rotation reduced yield. When N was not applied in this bioassay study, the fallow yield penalty ranged from 57 kg ha for the FWWHHH rotation to 79 kg ha for the FW, FW (N) and G M W rotations. When N was applied in this study, the fallow impact was less, kg ha for FWWHHH and 68 kg ha for FW, FW(N) and G M W rotations. CONCLUSIONS Previous cropping systems, including rotation and the addition of N, altered soil quality measures, including SOC, LF-C, and LF-N. Cropping systems that improved soil chemical properties, such as including legume forages and replenishing N with fertilizer, manure or annual legume crops, resulted in higher total wheat yield. The benefit of past practices that increased soil quality parameters was greater if additional N was not applied to the current cropping system. The presence of frequent fallow in crop rotations had a negative effect on total wheat yield, long-term productivity and grain N concentration, even with past and current application of N fertilizer. The historic cropping system was demonstrated to impact current productivity through soil quality changes (LF-C, LF-N, and SOC), and also through quality factors that were not measured in this study but were reflected through the frequency of fallow. The determination of past and current practices on crop productivity was only possible because of the long-term nature of the crop rotation study. As with crop yield, higher levels of soil quality increased grain N concentration. The higher concentration was more evident without the application of N fertilizer. Producers need to be aware that current land use practices will impact future crop productivity, and the production system needs to replace N removed by grain harvest. ACKNOWLEDGMENTS We thank L. Kremenik, Y. Bruinsma, and L. Selinger for their collection of soil samples and laboratory analyses, and yield collection required for this study. Amundson, R. G., Trask, J. and Pendall, E A rapid method of soil carbonate analysis using gas chromatography. Soil Sci. Soc. Am. J. 5: Bremer, E., Janzen, H. H. and Johnston, A. M. 99. Sensitivity of total, light fraction and mineralizable organic matter to management practices in a Lethbridge soil. Can. J. Soil Sci. 7: 8. Campbell, C. A., Biederbeck, V. O., Zentner, R. P. and Lafond, G. P. 99. Effect of crop rotations and cultural practices on soil organic matter, microbial biomass and respiration in a thin Black Chernozem. Can. J. Soil Sci. 7: 676. Campbell, C. A., Selles, F., Zentner, R. P., McConkey, B. G., McKenzie, R. C. and Brandt, S. A Factors influencing grain N concentration of hard red spring wheat in the semiarid prairie. Can. J. Plant Sci. 77: 56. Campbell, C. A., Zentner, R. P., Selles, F., Jefferson, P. G., McConkey, B. G., Lemke, R. and Blomert, B. J. 5. Longterm effect of cropping system and nitrogen and phosphorus fertilizer on production and nitrogen economy of grain crops in a Brown Chernozem. Can. J. Plant Sci. 85: 89. Halvorson, A. D., Reule, C. A. and Follett, R. F Nitrogen fertilization effects on soil carbon and nitrogen in a dryland cropping system. Soil Sci. Soc. Am. J. 6: 997. Halvorson, A. D., Wienhold, B. J. and Black, A. L.. Tillage, nitrogen, and cropping system effects on soil carbon sequestration. Soil Sci. Soc. Am. J. 66: 969. Janzen, H. H. 987a. Effect of fertilizer on soil productivity in long-term spring wheat rotations. Can. J. Soil Sci. 67: 657. Janzen, H. H. 987b. Soil organic matter characteristics after long-term cropping to various spring wheat rotations. Can. J. Soil Sci. 67: Janzen, H. H., Campbell, C. A., Brandt, S. A., Lafond, G. P. and Townley-Smith, L. 99. Light-fraction organic matter in soils from long-term crop rotations. Soil Sci. Soc. Am. J. 56: Janzen, H. H., Campbell, C. A., Izaurralde, R. C., Ellert, B. H., Juma, N., McGill, W. B. and Zentner, R. P Management effects on soil C storage on the Canadian prairies. Soil Tillage Res. 7: 895. Johnston, A. M., Janzen, H. H. and Smith, E. G Longterm spring wheat response to summerfallow frequency and organic amendment in southern Alberta. Can. J. Plant Sci. 75: 75. Karlen, D. L., Hurley, E. G., Andrews, S. S., Cambardella, C. A., Meek, D. W., Duffy, M. D. and Mallarino, A. P. 6. Crop rotation effects on soil quality at three northern corn/soybean belt locations. Agron. J. 98: 895. Larney, F. J., Janzen, H. H. and Olson, B. M Efficacy of inorganic fertilizers in restoring wheat yields on artificially eroded soils. Can. J. Soil Sci. 75: Larney, F. J., Janzen, H. H., Olson, B. M. and Lindwall, C. W.. Soil quality and productivity responses to simulated erosion and restorative amendments. Can. J. Soil Sci. 8: 555. Larney, F. J., Janzen, H. H., Olson, B. M. and Olson, A. F. 9. Erosion-productivity-soil amendment relationships for wheat over 6 years. Soil Tillage Res. : 78. SAS Institute Inc.. SAS 9.. Product Documentation. [Online] Available: [ Jan 7]. Smith, E. G., Janzen, H. H., Ellert, B. H. and Nakonechny, D. J.. Rotation Lethbridge Alberta. Prairie Soils Crops 5: 556. Smith, E. G., Lerohl, M., Messele, T. and Janzen, H. H. a. Soil quality attribute time paths: Optimal levels and values. J. Agric. Res. Econ. 5: 7. Smith, E. G., Peng, Y., Lerohl, M. and Larney, F. J. b. Economics of N and P fertilization to restore wheat yields on three artificially eroded sites in southern Alberta. Can. J. Soil Sci. 8: Smith, E. G., Janzen, H. H. and Kro bel, R. 5. Yield and profitability of fallow and fertilizer inputs in long-term wheat

10 Can. J. Soil. Sci. Downloaded from pubs.aic.ca by Agriculture and Agri-food Canada on 5//5 86 CANADIAN JOURNAL OF SOIL SCIENCE rotation plots at Lethbridge, Alberta. Can. J. Plant Sci. 95: (in press). Strickland, T. C. and Sollins, P Improved method for separating light- and heavy-fraction organic material from soil. Soil Sci. Soc. Am. J. 5: 99. Tanaka, D. L. and Aase, J. K Influence of topsoil removal and fertilizer application on spring wheat yields. Soil Sci. Soc. Am. J. 5: 8. Van Eerd, L. L., Congreves, K. A., Hayes, A., Verhallen, A. and Hooker, D. C.. Long-term tillage and crop rotation effects on soil quality, organic carbon, and total nitrogen. Can. J. Soil Sci. 9: 5. Zvomuya, F., Janzen, H. H., Larney, F. J. and Olson, B. M. 8. A long-term field bioassay of soil quality indicators in a semiarid environment. Soil Sci. Soc. Am. J. 7: DOI:.6/sssaj7.8.