No abstract

The positive effects of soil organic matter (OM) on soil properties that influence crop performance are well documented. But definitive and quantitative information of differential effects of soil OM contents is lacking for the northern Great Plains. The objective of this study was to quantify the contribution of a unit quantity of soil OM to productivity. Experiments were conducted on Williams loam (fine‐loamy, mixed, Typic Argiboroll) for 4 yr in the same field. The variables were soil OM content of the upper 30.5 cm together with all combinations of three postplanting soil available N levels (55, 90, and 125 kg N ha−1 as NO3‐N to 1.2 m) and three water levels. Water levels were uniformly maintained with a trickle system that independently metered water to each plot for each soil available N level. Pretillering spring wheat (Triticum aestivum L.) plant population decreased as soil OM content decreased in 3 of 4 yr. On an annual basis, highest total aerial dry matter and grain yields were associated with highest OM contents. The contribution of 1 Mg OM ha−1 to soil productivity, across the range of 64 to 142 Mg OM ha−1, was calculated as equivalent to 35.2 kg ha−1 for spring wheat total aerial dry matter and 15.6 kg ha−1 for grain yield. Loss of productivity associated with a depletion of soil OM in the northern Great Plains is primarily a consequence of a concomitant loss of fertility.

Seasonal variability in climate within and between the major spring wheat (Triticum aestivum L.) growing regions of the world causes large differences in plant development patterns. A major need of models attempting to describe crop growth as a dynamic process is an evaluation of phenological development. Researchers modeling growth and yield would benefit from a meteorologically-based phenological index that accurately and reliably predicts the development stage. The objective of this study was to develop capability to estimate leaf growth rate and crop growth stage from planting to an thesis under variabie management and climatic conditions. Aboveground morphological entities of 16 hard red spring (HRS) and 3 durum (T. durum Desf.) wheats were rated in Haun scale designations in seven field trials grown on Williams loam (fine-loamy, mixed Typic Argiboroll). These designations were regressed with growing degreedays (GDD), ph()ltothermal units (PTU), and days (DAYS) after emergence. Time to emergence did not differ among cultivars within a trial. Among the seven trials, emergence occurred 7 to 15 days after planting; accumulated GDD ranged from 94 to 138, and averaged 106. Eighteen of the cultivars produced eight leaves, 'James' produced seven. Main-stem phyllochron interval was 73 GDD for the HRS cvs. Butte and Waldron combined for analysis, and about fl1 GDD for either "other" eight-leaved HRS cultivars or for the durums combined for analysis, and for James. The R 2 value for any combined analysis was no less than 0.970**, significant at the 0.01 level. Soil water level and fertilizer N rate had no effect on main stem development rate, but did affect the maximum number of tillers produced per plant. The GDD accumulated per plant growth unit from emergence to anthesis (leaves plus four stages after expansion of the flag leaf) was the same as from emergence through full expansion of the flag leaf. Precision of estimatiag growth rates and stages with GDD was the same as with PTU, and both were superior to DAYS.

With increased cropping intensity, one would expect that more crop residue and C would be added to the Soil C sequestration can improve soil quality and reduce agriculsoil than with a crop-fallow system (Campbell et al., ture's contribution to CO 2 emissions. The long-term (12 yr) effects of tillage system and N fertilization on crop residue production and 1995, 2000b; Janzen et al., 1998a; Peterson et al., 1998). soil organic C (SOC) sequestration in two dryland cropping systems As the amount of crop residue returned to the soil is in North Dakota on a loam soil were evaluated. An annual cropping increased, SOC sequestration is expected to increase if (AC) rotation [spring wheat (SW) (Triticum aestivum L.)-winter the residue C is not lost as CO 2 to the atmosphere wheat (WW)-sunflower (SF) (Helianthus annuus L.)] and a spring because of tillage induced decomposition (Larney et al., wheat-fallow (SW-F) rotation were studied. Tillage systems included 1997; Reicosky, 1997a,b). Research in the Great Plains conventional-till (CT), minimum-till (MT), and no-till (NT). Nitrogen has shown that SOC sequestration is enhanced by N rates were 34, 67, and 101 kg N ha Ϫ1 for the AC system and 0, 22, fertilization (Campbell and Zentner, 1993; Campbell et and 45 kg N ha Ϫ1 for the SW-F system. Total crop residue returned

Stubble mulching is advocated as a desirable soil management practice, but no data are available for the northern Great Plains showing the effect of its long‐term use on soil properties. Neither have any comparisons been made of organic carbon (C) levels in cropland and virgin grassland in this region since 1947.Four farm field sites, each of moderately coarse‐, medium‐, and fine‐textured soils under conventional tillage, stubble mulch tillage, and virgin grassland (never cultivated) were sampled at 0 to 7.6, 7.6 to 15.2, 15.2 to 30.5, and 30.5 to 45.7 cm to measure organic C, total nitrogen (N), and bulk density.Average organic C and total N to a depth of 45.7 cm, % by weight, were significantly higher under stubble mulch than conventional tillage management. Organic C concentration in moderately coarse‐ and fine‐textured soils was 44 and 13% higher, respectively, under stubble mulch than conventional management, but it did not differ between the two management systems in medium‐textured soils.The percentage loss of C and N from the 0‐ to 7.6‐ and 7.6‐ to 15.2‐cm soil zones due to cropping as compared to virgin grassland was no greater than the percentage loss found in 1947 at field stations after nearly 40 years of cropping. In another comparison considering only medium‐textured soils on farm fields, percentage loss of organic C after about 70 years of cropping, using 1979 virgin grasslands as a reference, was no greater than loss after about 40 years of cropping, using 1950 virgin grasslands. Continued cultivation of dryland soils, especially with stuhble mulching, has maintained organic C and total N at higher levels than was projected from earlier research.Cropland bulk densities to 30.5 cm ranged from 7 to 20% higher than on grassland but did not differ between conventional and stubble mulch tillage management.

A major cause of soil productivity loss through erosion is attributed to changes in soil water‐holding properties or a reduction in thickness of the plant rooting zone. The objective of this study was to measure the effect of management‐induced changes in soil organic carbon (OC) concentration on soil characteristics affecting the available water capacity. Four sites each of moderately coarse (sandy), medium, and moderately fine or fine (fine) soils were sampled in four increments to 0.457 m within each of two cropland management systems (conventionally cultivated and stubble mulched) and each of two virgin grassland management systems (grazed virgin and relict virgin). Measurements of the water concentration by weight (Pw) at field capacity (FC) and permanent wilting point (PWP) were made on disturbed soil samples. Bulk density decreased with increasing OC concentration. The magnitude of change was greatest in sandy and least in the medium‐textured soils. For all soils combined, a change in sand fraction accounted for about 75% of the change in Pw at both the FC and the PWP. The change in Pw was greater at FC than the PWP. A unit change in OC concentration in the sandy soils caused a greater change in Pw at FC than at the PWP, but in the medium and fine soils the change in Pw at FC essentially paralleled the change at the PWP. An increase in OC concentration did not change the available water capacity in the sandy group and decreased it in the medium and fine textural groups. Loss of soil productivity induced by erosion in the northern Great Plains is probably more closely associated with a decline in nutrients and biological activity than from a change in available water capacity.

to optimize crop yields in a dryland annual cropping system. Long-term N fertility studies have shown that Increasing the frequency of cropping in dryland systems in the residual soil NO 3 -N levels increase when N fertilization northern Great Plains requires the application of N fertilizer to maintain optimum crop yields. A 12-yr annual cropping rotation [spring rates exceeded that needed for maximum yield (Halvorwheat (Triticum aestivum L.)-winter wheat-sunflower (Helianthus son and Reule, 1994; Porter et al., 1996; Raun and Johnannuus L.)] under dryland conditions was monitored to determine son, 1995; Westerman et al., 1994). Black et al. (1981) the influence of tillage system [conventional till (CT), minimum till reported more efficient water use with more intensive (MT), and no till (NT)] and N fertilizer rate (34, 67, and 101 kg N cropping systems and increased yields with N fertilizaha Ϫ1 ) on N removed in grain and annual changes in postharvest soiltion. In addition to more efficient water use, more inten-NO 3 -N. Nitrogen removal in the grain increased with increasing N sive MT and NT cropping systems have the potential rate in most years. Total grain N removal was lowest with NT at the to be more profitable and reduce soil erosion potential lowest N rate and highest with NT at the highest N rate compared (Dhuyvetter et al., 1996;Merrill et al., 1999). with CT. Total grain N removal after 12 cropping seasons was 144, 84, and 61% of the total N applied for the 34, 67, and 101 kg N ha Ϫ1 Long-term applications of N fertilizer to dryland cropfertilizer rates, respectively. Residual soil NO 3 -N levels were not ping systems can influence the level of residual soil affected by N rate or tillage system in the first 3 yr, but they increased NO 3 -N in the profile. Increasing levels of residual soil significantly following consecutive drought years. Residual NO 3 -N in NO 3 -N in the lower part of the root zone increases the the 150-cm soil profile tended to be higher with CT and MT than potential of leaching NO 3 -N below the root zone and with NT. Soil NO 3 -N movement below the crop root zone may have into shallow water tables, creating environmental conoccurred in 1 or 2 yr when precipitation was above average. Results cerns (Keeney and Follett, 1991; Peterson and Power, indicate that NT, with annual cropping, may reduce the quantity of 1991). Information is limited on the effects of tillage residual soil NO 3 -N available for leaching compared with MT and system and long-term N fertilizer application on soil CT systems.

We hypothesized that drought accelerates wind erosion by increasing plant and soil factors of erodibility together, compounding the erosion hazard. Erodibility factors measured in biennial spring wheat–fallow on Pachic and Typic Haploborolls soil were (i) soil‐inherent wind erodibility (SIWE) by rotary sieving, (ii) surface roughness by pin meter and chain methods, (iii) standing residue profile, and (iv) residue coverage photographically. Four tillage treatments ranged from low residue (LR) to no‐till (NT). The erodible fraction of surface soil (a SIWE measure) changed from 53% during a dry period (1989–1990) to a less erodible 26% during a wet period (1992–1994). Median erosion protection values calculated from flat and standing residue measurements made after seeding were, respectively, 16 and 43% in 1989 to 1990, and 80 and 76% in 1992 to 1994. Soil losses estimated by RWEQ model equations were 11 to 6100 times greater during 1989 to 1990, compared with 1992 to 1994. No‐till was protective, and estimated soil losses on LR were up to 3000 times greater than those on NT. However, low residue yields in 1988 (930 vs. 3640 kg/ha avg.) resulted in inadequate protection after seeding in 1990, even in NT; and soil losses in LR and NT were 13 and 8 Mg/ha, respectively. Results indicate biennial small grain–fallow is nonsustainable in the long term from a soil‐erosion perspective.