D. W. Reeves

The role of soil organic matter in maintaining soil quality in continuous cropping systems

Abstract Maintenance and improvement of soil quality in continuous cropping systems is critical to sustaining agricultural productivity and environmental quality for future generations. This review focuses on lessons learned from long-term continuous cropping experiments. Soil organic carbon (SOC) is the most often reported attribute from long-term studies and is chosen as the most important indicator of soil quality and agronomic sustainability because of its impact on other physical, chemical and biological indicators of soil quality. Long-term studies have consistently shown the benefit of manures, adequate fertilization, and crop rotation on maintaining agronomic productivity by increasing C inputs into the soil. However, even with crop rotation and manure additions, continuous cropping results in a decline in SOC, although the rate and magnitude of the decline is affected by cropping and tillage system, climate and soil. In the oldest of these studies, the influence of tillage on SOC and dependent soil quality indicators can only be inferred from rotation treatments which included ley rotations (with their reduced frequency of tillage). The impact of tillage per se on SOC and soil quality has only been tested in the ‘long-term’ for about 30 yrs, since the advent of conservation tillage techniques, and only in developed countries in temperate regions. Long-term conservation tillage studies have shown that, within climatic limits: Conservation tillage can sustain or actually increase SOC when coupled with intensive cropping systems; and the need for sound rotation practices in order to maintain agronomic productivity and economic sustainability is more critical in conservation tillage systems than conventional tillage systems. Long-term tillage studies are in their infancy. Preserving and improving these valuable resources is critical to our development of soil management practices for sustaining soil quality in continuous cropping systems.

USING WINTER COVER CROPS TO IMPROVE SOIL AND WATER QUALITY

This article reviews literature about the impacts of cover crops in cropping systems that affect soil and water quality and presents limited new information to help fill knowledge gaps. Cover crops grow during periods when the soil might otherwise be fallow. While actively growing, cover crops increase solar energy harvest and carbon flux into the soil, providing food for soil macro and microrganisms, while simultaneously increasing evapotranspiration from the soil. Cover crops reduce sediment production from cropland by intercepting the kinetic energy of rainfall and by reducing the amount and velocity of runoff. Cover crops increase soil quality by improving biological, chemical and physical properties including: organic carbon content, cation exchange capacity, aggregate stability, and water infiltrability. Legume cover crops contribute a nitrogen (N) to subsequent crops. Other cover crops, especially grasses and brassicas, are better at scavenging residual N before it can leach. Because growth of these scavenging cover crops is usually N limited, growing grass/legume mixtures often increases total carbon inputs without sacrificing N scavenging efficiency. Cover crops are best adapted to warm areas with abundant precipitation. Water use by cover crops can adversely impact yields of subsequent dryland crops in semiarid areas. Similarly, cooler soil temperatures under cover crop residues can retard early growth of subsequent crops grown near the cold end of their range of adaptation. Development of systems that reduce the costs of cover crop establishment and overcome subsequent crop establishment problems will increase cover crop utilization and improve soil and water quality.

Effects of residue management and controlled traffic on carbon dioxide and water loss

Management of crop residues and soil organic matter is of primary importance in maintaining soil fertility and productivity and in minimizing agricultural impact on the environment. Our objective was to determine the effects of traffic and tillage on short-term carbon dioxide (CO2) and water (H2O) fluxes from a representative soil in the southeastern Coastal Plain (USA). The study was conducted on a Norfolk loamy sand (FAO classification, Luxic Ferralsols; USDA classification, fine-loamy siliceous, thermic Typic Kandiudults) cropped to a corn (Zea mays L.) — soybean (Glycine max (L.) Merr) rotation with a crimson clover (Trifolium incarnatum L.) winter cover crop for eight years. Experimental variables were with and without traffic under conventional tillage (CT) (disk harrow twice, chisel plow, field cultivator) and no tillage (NT) arranged in a splitplot design with four replicates. A wide-frame tractive vehicle enabled tillage without wheel traffic. Short-term CO2 and H2O fluxes were measured with a large portable chamber. Gas exchange measurements were made on both CT and NT at various times associated with tillage and irrigation events. Tillage-induced CO2 and H2O fluxes were larger than corresponding fluxes from untilled soil. Irrigation caused the CO2 fluxes to increase rapidly from both tillage systems, suggesting that soil gas fluxes were initially limited by lack of water. Tillage-induced CO2 and H2O fluxes were consistently higher than under NT. Cumulative CO2 flux from CT at the end of 80 h was nearly three times larger than from NT while the corresponding H2O loss was 1.6 times larger. Traffic had no significant effects on the magnitude of CO2 fluxes, possibly reflecting this soil’s natural tendency to reconsolidate. The immediate impact of intensive surface tillage of sandy soils on gaseous carbon loss was larger than traffic effects and suggests a need to develop new management practices for enhanced soil carbon and water management for these sensitive soils. # 1999 Elsevier Science B.V. All rights reserved.

SOIL CARBON RELATIONSHIPS WITH TERRAIN ATTRIBUTES, ELECTRICAL CONDUCTIVITY, AND A SOIL SURVEY IN A COASTAL PLAIN LANDSCAPE

Soil organic carbon (SOC) estimation at the landscape level is critical for assessing impacts of management practices on C sequestration and soil quality. We determined relationships between SOC, terrain attributes, field scale soil electrical conductivity (EC), soil texture and soil survey map units in a 9 ha coastal plain field (Aquic and Typic Paleudults) historically managed by conventional means. The site was composite sampled for SOC (0-30 cm) within 18.3 × 8.5-m grids (n = 496), and two data sets were created from the original data. Ordinary kriging, co-kriging, regression kriging and multiple regression were used to develop SOC surfaces that were validated with an independent data set (n = 24) using the mean square error (MSE). The SOC was relatively low (26.13 Mg ha−1) and only moderately variable (CV = 21%), and showed high spatial dependence. Interpolation techniques produced similar SOC maps but the best predictor was ordinary kriging (MSE = 9.11 Mg2 ha−2) while regression was the worst (MSE = 20.65 Mg2 ha−2). Factor analysis indicated that the first three factors explained 57% of field variability; compound topographic index (CTI), slope, EC and soil textural fractions dominated these components. Elevation, slope, CTI, silt content and EC explained up to 50% of the SOC variability (P ≤ 0.01) suggesting that topography and historical erosion played a significant role in SOC distribution. Field subdivision into soil map units or k-mean clusters similarly decreased SOC variance (about 30%). The study suggests that terrain attributes and EC surveys can be used to differentiate zones of variable SOC content, which may be used as bench marks to evaluate field-level impact of management practices on C sequestration.

Wheel-traffic effects on corn as influenced by tillage system

Surface and subsoil compaction limit crop productivity on many soils of the southeastern Coastal Plain of the United States. Deep tillage, and to a lesser extent, controlled traffic have been utilized to manage soil compaction on these soils, but there is a need to develop tillage systems that integrate conservation tillage practices with deep tillage and controlled traffic. In 1988, a study was initiated with a wide-frame (6.3 m) vehicle to determine the interactive effects of traffic, deep tillage, and surface residues on corn (Zea mays L.) grown on a Norfolk loamy sand (fine-loamy, siliceous, thermic, Typic Kandiudults). Corn was planted into a winter cover crop of ‘Cahaba White’ vetch (Vicia sativa L.) Treatments included: traffic (conventional equipment or no traffic); deep tillage (no deep tillage, in-row subsoiling, or complete disruption); surface tillage (no surface tillage or disk and field cultivate). Complete disruption was accomplished by subsoiling at a depth of 43 cm on 25-cm centers. Although tillage × traffic interactions significantly affected soil strength and soil water, the only grain yield response both years was due to a surface tillage × deep tillage interaction. In a drought year (1988), with surface tillage, yields averaged 3.54, 2.75, and 1.41 t ha−1 with complete disruption, in-row subsoiling, and no deep tillage, respectively. Without surface tillage, respective yields averaged 3.77, 3.14, and 1.12 t ha−1. In 1989 when rainfall amount and distribution were excellent, yields with complete disruption, in-row subsoiling, and no deep tillage averaged 7.79 t ha−1, 7.08 t ha−1, and 6.44 t ha−1, respectively, with surface tillage; and 7.40 t ha−1, 6.91 t ha−1, and 4.70 t ha−1 respectively, without surface tillage. Soil strength and soil water measurements confirmed the detrimental effect of traffic after disking and field cultivation; however, soil water measurements and the lack of any yield response to applied traffic suggest that corn compensated for reduced rooting capacity in wheeled interrows by increased rooting in non-wheeled interrows.

Elevated atmospheric carbon dioxide effects on sorghum and soybean nutrient status 1

Abstract Increasing atmospheric carbon dioxide (CO2) concentration could have significant implications on technologies for managing plant nutrition to sustain crop productivity in the future. Soybean (Glycine max [L.] Merr.) (C3 species) and grain sorghum (Sorghum bicolor [L.] Moench) (C4 species) were grown in a replicated split‐plot design using open‐top field chambers under ambient (357 μmol/mol) and elevated (705 μmol/mol) atmospheric CO2. At anthesis, leaf disks were taken from upper mature leaves of soybean and from the third leaf below the head of sorghum for analysis of plant nutrients. Leaf greenness was measured with a Minolta SPAD‐502 chlorophyll meter. Concentrations of chlorophylls a and b and specific leaf weight were also measured. Above‐ground dry matter and seed yield were determined at maturiry. Seed yield of sorghum increased 17.5% and soybean seed yield increased 34.7% with elevated CO2. There were no differences in extractable chlorophyll concentration or chlorophyll meter readings du...

Tillage intensity effects on chemical indicators of soil quality in two coastal plain soils

Few experiments in the coastal plain region of the southeastern United States have reported the effect of long-term tillage and tillage intensity on chemical soil quality indicators. The purpose of this study was to determine the 17-year influence of four tillage systems on chemical soil quality indicators in a Benndale fine sandy loam (coarse-loamy, siliceous, semiactive, thermic, Typic Paleudults) and a Lucedale very fine sandy loam (fine-loamy, siliceous, subactive, thermic, Rhodic Paleudults) in the coastal plain region of Alabama. Tillage systems were no-tillage, disk, moldboard plow, and chisel plow under varied double-cropping in a randomized complete block design with four replications. Soil pH, sum of extractable bases, soil organic carbon (SOC), and soil nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K), zinc (Zn), manganese (Mn), copper (Cu), and iron (Fe) were determined on soil samples collected at depths of 0–2.5, 2.5–7.5, 7.5–15.0, 15.0–22.5, and 22.5–30 cm. Soil carbon (C) accumulation occurred primarily in the top 2.5 cm, varied by soil type, and was inversely proportional to tillage intensity. The coarse textured Benndale soil averaged 27.6, 13.1, 12.7, and 10.4 g C kg− 1 soil with no-tillage, disk, chisel, and moldboard plow management, respectively, in the top 2.5 cm. The finer textured Lucedale soil averaged 16.7, 10.0, 9.8, and 6.9 g C kg− 1 soil, for the same treatments and depth, respectively. Surface applications of lime maintained soil pH at an acceptable level within the plow layer on both soil types and all tillage systems. Extractable P was higher with no-tillage than moldboard plowing to the 22.5 cm depth on the Lucedale soil. On the Benndale soil, P tended to accumulate at the 15 to 22.5 cm depth with tillage systems other than moldboard plowing, and no-tillage had the most extractable P at these depths. Soil C and pH combined proved effective as continuous pedotransfer functions, predicting 73% and 86% of the variation in sum of extractable bases for the Benndale and Lucedale soils, respectively. As determined from chemical indicators of soil quality, adoption of conservation tillage with doublecropping is a sustainable practice for these soils.

Cotton response to in-row subsoiling and potassium fertilizer placement in Alabama

In the USA a suggested method for correcting late season K deficiencies in cotton (Gossypium hirsutum L.) is by in-row deep placement of K fertilizer. Experiments were conducted on three Alabama soils (southeastern USA) for 3 years to evaluate cotton response to K fertilizer when surface broadcast with and without in-row subsoiling (to 38 cm depth) or deep placed in the in-row subsoil channel. Potassium was applied at rates ranging from 0 to 84 kg K ha−1. Deep placement was achieved with a fertilizer applicator developed to distribute dry fertilizer at three depths down the back of the subsoil shank. All three soils also had deep placement treatments of 1680 kg ha−1 agricultural limestone with and without 84 kg K ha−1. Soils were an Emory silt loam (fine-silty, siliceous Fluventic Umbric Dystrochrepts), a Norfolk sandy loam (fine-loamy, siliceous Typic Kandiudults), and a Lucedale sandy clay loam (fine-loamy, siliceous Rhodic Paleuduts). All three soils had medium soil test K concentrations in the plow layer and medium or low concentrations of K at greater depths. The Norfolk soil had a well-developed traffic pan and in-row subsoiling increased seed cotton yields by an average of 22% during the 3 years of the study. Cotton responded to K fertilization in 2 out of 3 years at each location (6 out of 9 site-years) regardless of the method of K application. Annual applications of 84 kg K ha−1 increased 3 year average seed cotton yields by 17%, 10% and 19% on the Emory, Norfolk and Lucedale soils, respectively. Deep placement of agricultural limestone with or without K fertilizer for cotton did not increase cotton yields.

USING SITE-SPECIFIC SUBSOILING TO MINIMIZE DRAFT AND OPTIMIZE CORN YIELDS

Subsoiling is often required to alleviate soil compaction; however, deep tillage can be expensive and time-consuming. If this tillage operation is conducted deeper than the compacted soil layer, energy is wasted. However, if this tillage operation is conducted shallower than the compacted soil layer, energy is again wasted, and plant roots may be prevented from penetrating the compacted layer. Technologies are now available that allow subsoiling to be conducted at the specific depth of the compacted layer, which would conserve natural resources without sacrificing crop yields. An experiment was conducted over four years in a field located in southern Alabama to evaluate whether the concept of site-specific subsoiling (tilling just deep enough to eliminate the hardpan layer) would reduce tillage draft and energy requirements and/or reduce crop yields. Average corn (Zea mays L.) yields over this four-year period showed that site-specific subsoiling produced yields equivalent to those produced by the uniform deep subsoiling treatment while reducing draft forces, drawbar power, and fuel use.

Mineralogy of eroded sediments derived from highly weathered Ultisols of central Alabama

Coarse-textured surface horizons are common in highly weathered southeastern US coastal plain soils. These soils have historically been managed under conventional tillage practices, but conservation tillage management practices are increasing. Although clay (<2 �m) contents are low in these surface horizons (typically < 1 0 0gk g −1 ), the reactive nature of this fraction tends to play a dominant role in colloidal facilitated pollutant transport. Studies have suggested that enrichment of certain minerals occurs in transported sediments, thus, we evaluated sediment size and the mineralogical partitioning of clay minerals of soil versus runoff sediment under simulated rainfall. In addition, because water dispersible clay (WDC) has been shown to be correlated with soil erodibility, we evaluated WDC differences as a function of tillage practices. Plots were established at a site in the upper coastal plain of central Alabama, where soils classified as coarse-loamy, siliceous, subactive, thermic Plinthic Paleudults and Typic Hapludults (FAO-Acrisols). Surface tillage treatments were established in 1988, and included conventional tillage (CT) versus no surface tillage (NT) treatments, with crop residue remaining or removed, and with or without paratilling (non-inversion subsoiling). Within these plots, simulated rainfall (target intensity = 50 mm h −1 for 2 h) was applied to replicated 1 m × 1 m areas, and runoff and sediment were collected. Mineralogical analyses of soils and sediment were conducted using thermogravimetric analysis (TGA) and X-ray diffraction (XRD) techniques. WDC quantities were highly correlated with percent soil organic carbon (SOC) (r 2 = 0.76), which was a function of tillage treatment. Although no differences in the mineralogy of the <2 �m sediment were observed between tillage treatments, runoff sediments (<2 �m) were enriched in quartz (qtz) and relatively depleted with kaolinite compared to in situ soils. These findings will facilitate development of mechanistic models that predict sediment attached losses of nutrients and pesticides. Published by Elsevier Science B.V.