A. J. M. Smucker

Soil carbon pools and fluxes in long-term corn belt agroecosystems

The dynamics of soil organic carbon (SOC) play an important role in long-term ecosystem productivity and the global C cycle. We used extended laboratory incubation and acid hydrolysis to analytically determine SOC pool sizes and fluxes in US Corn Belt soils derived from both forest and prairie vegetation. Measurement of the natural abundance of 13 C made it possible to follow the influence of continuous corn on SOC accumulation. The active pools (Ca) comprised 3 to 8% of the SOC with an average field mean residence time (MRT) of 100 d. The slow pools (Cs) comprised 50% of SOC in the surface and up to 65% in subsoils. They had field MRTs from 12‐28 y for C4-C and 40‐80 y for C3-derived C depending on soil type and location. Notill management increased the MRT of the C3-C by 10‐15 y above conventional tillage. The resistant pool (Cr) decreased from an average of 50% at the surface to 30% at depth. The isotopic composition of SOC mineralized during the early stages of incubation reflected accumulations of labile C from the incorporation of corn residues. The CO2 released later reflected 13 C characteristic of the Cs pool. The 13 C of the CO2 did not equal that of the whole soil until after 1000 d of incubation. The SOC dynamics determined by acid hydrolysis, incubation and 13 C content were dependent on soil heritage (prairie vs. forest), texture, cultivation and parent material, depositional characteristics. Two independent methods of determining C3 pool sizes derived from C3 vegetation gave highly correlated values. # 2000 Elsevier Science Ltd. All rights reserved.

Root growth and water uptake during water deficit and recovering in wheat

Root growth and soil water content were measured in a field experiment with wheat subjected to two periods of water deficit. The first period was induced early in the season between the early vegetative stage (22 DAS) and late terminal spikelet (50 DAS), the second period at mid-season between terminal spikelet (42 DAS) and anthesis (74 DAS). Total root growth was reduced under water deficit by a reduction in the top 30 cm, while the root system continued to grow in the deeper soil profile between 30 and 60 cm. Shortly after rewatering, the growth pattern reverted to fastest root growth rates in the shallow soil layers. In relative terms, the total root system increased in relation to the above ground dry matter under water shortage. The early-, the mid-season water deficit treatments, and the control treatment had total root length of 27.4, 19.4 and 30.6 km m-2, respectively, about 2 wk before maturity. Evapotranspiration declined under water deficit, but water uptake in deeper layers increased. Water uptake per unit root length was reduced with water deficit and was still low shortly after rewatering. Remarkable was the increase in water uptake at 2–3 weeks after rewatering, both deficit treatments exceeded the control by almost 100%. This increase in water uptake followed the burst of new root growth in the upper regions of the soil. However, water uptake rates subsequently declined towards maturity, being between 0.15 L km-1 d-1 and 0.17 L km-1 d-1 for the early and mid-season water deficit treatments, slightly higher than the control, 0.12 L km-1 d-1. The results showed that the crop subjected to early water deficit could compensate for some of the reductions in root growth during subsequent rewatering, but the impact of the mid-season water deficit treatment was more severe and permanent.

Root recolonization of previous root channels in corn and alfalfa rotations

Distribution of root systems through soils and recolonization of root channels by successive crops are fundamental, though difficult to study, processes of soil ecology. This article reports a minirhizotron (MR) study of corn and alfalfa root systems throughout the soil profile of Kalamazoo loam (fine-loamy, mixed, mesic Typic Hapludalf) monolith lysimeters for a three-year succession of corn, alfalfa and corn. Multiple-date comparisons within and between years were conducted to estimate total root densities in each soil horizon. Root recolonization was assessed by comparing every video frame of paired minirhizotrons, from recordings conducted one growing season apart. Distributions of corn root systems were modified by tillage practices. In 1994, root populations of corn in the Bt1 horizon peaked 75–90 days after planting (DAP). Numbers of corn roots per m2 in the Bt1 horizon were consistently higher for no-tillage (NT) than for conventional tillage (CT) lysimeters, in 1994 and 1996. Distribution of alfalfa roots within the soil profile was not significantly modified by tillage. However, alfalfa root decomposition rates responded to conventional and no-tillage practices and were specific for each soil horizon. Corn root systems growing in soils previously cropped with alfalfa presented similar patterns of root distribution by horizons as that of the previous alfalfa crop. Successive corn root systems did not display similar distribution patterns throughout the soil profile from one growing season to the next. Proportions of roots of the current crop recolonizing root induced macropores (RIMs) of the previous crop averaged 18% for corn after corn, 22% for alfalfa after corn and 41% for corn after alfalfa, across Bt horizons and tillage treatments. In conclusion, distribution of corn root systems appeared to be modified by tillage practices and root recolonization of RIMs was controlled by the preceding crop.

Temporal trends in nitrogen isotope values of nitrate leaching from an agricultural soil

The concentration and d 15 No f NO y in leachate from two undisturbed and unfertilized soil lysimeters, one convention- 3

DYNAMIC ROOT RESPONSES TO WATER DEFICITS

Water absorption is influenced by the geometric distribution of viable roots and the availability of water at root surfaces. Utilization of additional plant energy for the extensive and localized development of root systems is a high risk investment of carbon by the plant, especially during short-term water deficits. Nonuniform root distribution, resulting from root growth within networks of continuous soil macropores, reduces the efficiency of water absorption by minimizing contact between roots and increasing the geometric mean pathway of water from the bulk soil to the root. Root clustering in these soil niches causes greater intraspecific competition for the limited water, especially as soil water deficits develop more deeply within the soil profile. Current methods for quantifying and predicting plant root and soil water relationships are reviewed, and some research and modeling imperatives are presented.

A comparison between minirhizotron and monolith sampling methods for measuring root growth of maize (Zea mays L.)

Transparent plastic minirhizotron tubes have been used to evaluate spatial and temporal growth activities of plant root systems. Root number was estimated from video recordings of roots intersecting minirhizotron tubes and of washed roots extracted from monoliths of the same soil profiles at the physiological maturity stage of a maize (Zea mays L.) crop. Root length was measured by the line intercept (LI) and computer image processing (CIP) methods from the monolith samples.There was a slight significant correlation (r=0.28, p<0.005) between the number of roots measured by minirhizotron and root lengths measured by the LI method, however, no correlation was found with the CIP method. Using a single regression line, root number was underestimated by the minirhizotron method at depths between 0–7.6 cm. A correlation was found between root length estimated by LI and CIP. The slope of estimated RLD was significant with depth for these two methods. Root length density (RLD) measured by CIP showed a more erratic decline with distance from the plant row and soil surface than the LI method.

Anaerobic stimulation of root exudates and disease of peas

SummaryThe relationships between root exudation, root disease and anaerobic root stresses were investigated. Sand culture and mist chamber studies demonstrated that low O2 and high CO2 reduced plant growth and increased the exudation of ethanol, amino acids, and sugars by pea roots. The relative loss of ethanol by roots was much greater in treatments with atmospheres of N2 containing 30% CO2 than in treatments of air containing 30% CO2 or N2. Ethanol was not detected in the nutrient solution of aerated plant roots. Atmospheres of N2 plus 30% CO2 caused 500% greater mycelial growth ofFusarium solani f. sp.pisi and 400% more disease of inoculated pea roots. Relative losses of four amino acids and four sugars were much greater in atmospheres of N2 plus 30% CO2 than in N2 or air.

Automated image analyses for separating plant roots from soil debris elutrated from soil cores

Historically, destructive root sampling has been labor intensive and requires manual separation of extraneous organic debris recovered along with the hydropneumatic elutriation method of separating plant roots from soils. Quantification of root system demographics by public domain National Institute of Health (NIH-Image) and Root Image Processing Laboratory (RIPL) image processing algorithms has eliminated much of the labor-intensive manual separation. This was accomplished by determining the best length to diameter ratio for each object during image analyses. Objects with a length to diameter ratio less than a given threshold are considered non-root materials and are rejected automatically by computer algorithms. Iterative analyses of length to diameter ratios showed that a 15:1 ratio was best for separating images of maize (Zea mays L.) roots from associated organic debris. Using this threshold ratio for a set of 24 soil cores, a highly significant correlation (r2 = 0.89) was obtained between computer image processed total root length per core and actual root length. A linear relationship (r2 = 0.80) was observed between root lengths determined by NIH-Image analyses and lengths determined independently by the RIPL imaging system, using the same maize root + debris samples. This correlation demonstrates that computer image processing provides opportunities for comparing root length parameters between different laboratories for samples containing debris.

Tillage and Compactive Modifications of Gaseous Flow and Soil Aeration

Oxygen influx and carbon dioxide efflux from the rhizosphere are adversely influenced when tillage and traffic reduce soil porosity. Greater soil anaerobiosis inhibits plant growth/development and modifies aerobic microbial populations and activities. Although greater volumetric soil water contents of compacted soils may improve seed germination and perhaps emergence, root systems of young seedlings generally encounter adverse soil gaseous concentrations below the seed zone. Temporarily saturated zones at the interface of deeper compacted areas of the rhizosphere (e.g., tillage pans, wheel tracks, etc.) often result in lower soil oxygen contents, causing reductions in metabolic energy levels of roots growing in those regions. Concurrently, carbon dioxide, ethylene, and other soil biogases accumulate sufficiently to cause additional physiological and morphological changes in root development; disease susceptibility is also increased.

Fibrous carrot root responses to irrigation and compaction of sandy and organic soils

Field experiments were performed in Southern Finland on fine sand and organic soil in 1990 and 1991 to study carrot roots. Fall ploughed land was loosened by rotary harrowing to a depth of 20 cm or compacted under moist conditions to a depth of 25–30 cm by three passes of adjacent wheel tracks with a tractor weighing 3 Mg, in April were contiguously applied across the plot before seed bed preparation. Sprinkler irrigation (30 mm) was applied to fine sand when moisture in the 0–15 cm range of soil depth was 50% of plant-available water capacity. For root sampling, polyvinyl chloride (PVC) cylinders (30 × 60 cm) were installed in the rows of experimental plots after sowing, and removed at harvest. Six carrot plants were grown in each of in these soil colums in situ in the field.Fine root length and width were quantified by image analysis. Root length density (RLD) per plant was 0.2–1.0 cm cm-3 in the 0–30 cm range. The fibrous root system of one carrot had total root lengths of 130–150 m in loose fine sand and 180–200 m in compacted fine sand. More roots were observed in irrigated than non-irrigated soils. In the 0–50 cm range of organic soil, 230–250 m of root length were removed from loosened organic soils and 240–300 m from compacted soils. Specific root surface area (surface area divided by dry root weight) of a carrot fibrous root system averaged 1500–2000 cm2 g-1. Root length to weight ratios of 250–350 m g-1 effectively compare with the ratios of other species.Fibrous root growth was stimulated by soil compaction or irrigation to a depth of 30 cm, in both the fine sand and organic soils, suggesting better soil water supply in compacted than in loosened soils. Soil compaction increased root diameters more in fine sand than it did in organic soil. Most of the root length in loosened soils (fine sand 90%, organic soil 80%) and compacted soils (fine sand 80%, organic soil 75%) was composed of roots with diameters of approximately 0.15 mm. With respect to dry weight, length, surface area and volume of the fibrous root system, all the measurements gave significant resposes to irrigation and soil compaction. Total root volumes in the 0–50 cm of soil were 4.3 cm3 and 9.8 cm3 in loosened fine sand and organic soils, respectively, and 6.7 cm3 and 13.4 cm3 in compacted sand and organic soils, respectively. In fine sand, irrigation increased the volume from 4.8 to 6.3 cm3.