R. R. Allmaras

Soil organic carbon and 13C abundance as related to tillage, crop residue, and nitrogen fertilization under continuous corn management in Minnesota

Long-term field experiments are among the best means to predict soil management impacts on soil carbon storage. Soil organic carbon (SOC) and natural abundance 13 C( d 13 C) were sensitive to tillage, stover harvest, and nitrogen (N) management during 13 years of continuous corn (Zea mays L.), grown on a Haplic Chernozem soil in Minnesota. Contents of SOC in the 0‐15 cm layer in the annually-tilled [moldboard (MB) and chisel (CH)] plots decreased slightly with years of corn after a low input mixture of alfalfa (Medicago sativum L.) and oat (Avena sativa L.) for pasture; stover harvest had no effect. Storage of SOC in no-till (NT) plots with stover harvested remained nearly unchanged at 55 Mg ha ˇ1 with time, while that with stover returned increased about 14%. The measured d 13 C increased steadily with years of corn cropping in all treatments; the NT with stover return had the highest increase. The N fertilization effects on SOC and d 13 C were most evident when stover was returned to NT plots. In the 15‐30 cm depth, SOC storage decreased and d 13 C values increased with years of corn cropping under NT, especially when stover was harvested. There was no consistent temporal trend in SOC storage and d 13 C values in the 15‐30 cm depth when plots received annual MB or CH tillage. The amount of available corn residue that was retained in SOC storage was influenced by all three management factors. Corn-derived SOC in the 0‐15 cm and the 15‐30 cm layers of the NT system combined was largest with 200 kg N ha ˇ1 and no stover harvest. The MB and CH tillage systems did not influence soil storage of corn-derived SOC in either the 0‐15 or 15‐30 cm layers. The corn-derived SOC as a fraction of SOC after 13 years fell into three ranges: 0.05 for the NT with stover harvested, 0.15 for the NT with no stover harvest, and 0.09‐0.10 for treatments with annual tillage; N rate had no effect on this fraction. Corn-derived SOC expressed as a fraction of C returned was positively biased when C returned in the roots was estimated from recovery of root biomass. The half-life for decomposition of the original or relic SOC was longer when stover was returned, shortened when stover was harvested and N applied, and sharply lengthened when stover was not harvested and N was partially mixed with the stover. Separating SOC storage into relic and current crop sources has significantly improved our understanding of the main and interacting effects of tillage, crop residue, and N fertilization for managing SOC accumulation in soil. Published by Elsevier Science B.V.

Conservation tillage systems and their adoption in the United States

Abstract Conservation benefits of conservation tillage had been developed long before the production disadvantages were removed. Even though, in some cases, there are still production disadvantages and lower yields, compared to conventional tillage, conservation tillage is attractive to farmers primarily because of the potential for reduced production costs; conservation benefits are of secondary interest in most cases even though they accrue from the use of conservation tillage. This farmer interest in cost reduction will most certainly guide research inputs. Surveys of farmers have shown that more emphasis must be placed on all of the technology needed for a production system. In order to avoid financially-disastrous consequences, associated risk assessments are even required during the adoption period, i.e., the period when conservation tillage is replacing the conventional tillage. When a conservation-tillage-planting system is defined rigorously, based on the requirement that at least 30% of the surface should be covered with crop residue, the adoption averages about 25% of the cropland in the United States. Nine tillage management regions (TMR) in the United States were delineated based on climate, adapted crops and cropping systems. Adoption of conservation-tillage-planting systems ranged widely from 22 to 45% of the cropland in a TMR. Full-width systems such as mulch till, in which the whole field is tilled, were used much more than partial-width systems such as no-till, ridge till and strip till in which only strips are tilled. Adoption of these forms of conservation tillage are sensitive to the dominant-cropping systems in a TMR. Variations in adoption were often well related to the problems and benefits discussed by research on tillage-planting systems in the TMR.

Modeling the incorporation of corn (Zea mays L.) carbon from roots and rhizodeposition into soil organic matter

Experimental data reported in the literature over the last decennium indicate that roots and rhizodeposition are important sources of carbon for the synthesis of soil organic carbon. Our objective was to verify the capability of the simulation model NCSWAP to reproduce the general conclusions from the experimental literature, and to gain some insight about the processes that control the incorporation of corn belowground production into the soil organic matter. The model was calibrated against the experimental data gathered from a long-term field experiment located near St. Paul, Minnesota. The simulation model updated daily the soil conditions to reproduce over a 13 year period the measured kinetics of seven variables: above-ground corn production, and the total soil organic matter, soil d value, and the soil organic matter derived from corn in the 0‐15 and 15‐30 cm depth. The simulation gave a root-plus-rhizodeposition 1.8 times larger than stalks plus leaves. The translocation efficiency of corn-C into soil organic C at the 0‐15 cm depth gradually decreased to 0.19 of the below-ground deposition. The sensitivity of below-ground photosynthate incorporation into the soil organic matter was analyzed relative to variations in the parameters that control the formation and decay of roots and rhizodeposition. Roots had a greater effect than rhizodeposition on the soil organic matter, though more photosynthates were translocated to rhizodeposition than to roots. q 2001 Elsevier Science Ltd. All rights reserved.

Persistence of subsoil compaction from heavy axle loads

Abstract As the persistence of subsoil compaction has a major impact on agricultural sustainability, responses of soil pore characteristics and plant roots to subsoil compaction were examined as related to heavy axle loads. A Ves clay loam was originally compacted with three axle loads of less than 4.5 (control), 9, and 18 Mg in the fall of 1981 (soil dry (D)) and in the fall of 1982 (soil wet (W)). The 9 Mg treatment was recompacted on both trials in the spring of 1988 with an 18 Mg axle load, and maize and soybean were grown in rotation. Field measurements in 1988 and 1989 included bulk density, biopore area, root number and location, ponded and negative water pressure infiltration, saturated hydraulic conductivity ( K sat ), and ped size distribution. Bulk densities measured under the row in 1989 for the W plots were increased in the 10–28 cm layer by the new 18 Mg load, whereas the original 18 Mg treatment had higher bulk densities in the 28–68 cm depth. The control had lower bulk densities than both compacted treatments for the 18–58 cm increment in 1989. The number and areas of biopores in the D control plots were at least ten times greater than in W control plots. Compaction at 35 cm reduced K sat and ponded infiltration comapred with the control. Mean ped diameter ranged from 1.3 to almost four times greater in the compacted treatments than in the control. In spite of increased soil density, a substantial reduction in root growth was not observed. Roots grew around dense clods in the compact treatments allowing continual vertical extension. There was no evidence of poor drainage hindering crop growth in the heavy axle load treatments.

Shirnkage and water retention characteristic in a fine-textured mollisol compacted under different axle loads

In areas where heavy vehicles are used, the subsoils often become very compacted. Freezing-thawing and wetting-drying have not been effective at reducing compaction. In this study, the type of soil shrinkage related to compaction was investigated to explain these amelioration failures. In conjunction with a shrinkage curve, the water retention characteristic was also measured because both can be related to compaction. Shrinkage and bulk density of undisturbed clods (about 200 cm3), as well as water retention of undisturbed cores, were measured to evaluate long-term compaction effects in two sets (better and poor tile drainage) of two axle-load treatments (9- and 18-Mg axle loads) relative to their control. Wet clods were sampled from the Ap (0 to 25 cm) and subsoil (30 to 45 cm) horizons of a Normania clay loam (fine-loamy, mixed, mesic Udic Haplustoll) in the spring of 1991 without fragmentation after the soil had a full winter to swell following moldboard plowing in the fall. Clods were further saturated, coated with a film, then allowed to air-dry. Mass and volume were determined periodically for eight months to measure shrinkage. Maximum volume reduction (m3m−3) of clods in the Ap layer (0.232) during shrinkage was significantly greater than in the subsoil layer (0.152), but compaction effects were not significant in either layer. Dry bulk density of subsoil clods (1.77 Mg m−3) was significantly greater than in the Ap layer (1.68 Mg m−3), but no statistical differences were observed among compaction treatments. Maximum shirnkage was always <1 and averaged 0.61 in the subsoil compared to averaged 0.80 in the Ap layer, which indicates nearly all structural and residual shirnkage after immediate air entry during gravity drainage. The water retention characteristic of the original compacted and control treatments were still significantly different in the better drained subsoil but not in the more poorly drained subsoil, which showed that the 9-Mg axle load overall since 1987 has compacted the subsoil nearly as much under wet soil conditions as the 18-Mg axle load initially. These soil structural measurements explain the failure of natural forces to reduce bulk density of the compacted subsoil.

Common root rot of pea (Pisum sativum L.): Oat pre-crop and traffic compaction effects in fine-textured mollisols

Common root rot of pea caused by Aphanomyces euteiches Drechs. is widespread and difficult to control. In many production areas, yearly losses have been estimated at 10% because of the disease. Cultural control is needed even when disease tolerant cultivars are planted. Soil compaction due to traffic is known to aggravate the disease. In a series of research studies in a heavily infested nursery and adjacent farm fields, it was shown that compaction aggravates the disease by decreasing drainage and thus providing more favourable soil water conditions for early infection of pea roots. Traffic compaction has also provided an adverse abiotic environment for plant stress due to poor aeration. A precrop of oat (Avena sativum L.), as a full-season or late-summer crop, suppressed the disease only if the oat residue was incorporated at a shallow depth late in the fall using a chisel. Incorporated oat residue reduced inoculum potential of A. euteiches above 10 cm when incorporated with a chisel and below 10 cm when incorporated with a moldboard plow. A rolled towel bioassay using a susceptible pea cultivar successfully estimated inoculum potential when the test soil was placed near the epicotyl of 7-day-old seedlings. Although A. euteiches is an aggressive disease, all of these findings focus on vulnerability during the infection process. These investigations were required to examine carefully the soil ecology pertaining to the host crop, the pathogen when in the saprophytic mode, and the host crop interaction with the pathogen.