Joseph G. Benjamin

Advances and challenges in predicting agricultural management effects on soil hydraulic properties

Agricultural management practices can significantly affect soil hydraulic properties and processes in space and time. These responses are coupled with the processes of infiltration, runoff, erosion, chemical movement, and crop growth. It is essential to quantify and predict management effects on soil properties in order to model their consequent effects on production and the environment. We present work done thus far on this topic area along with the challenges that lie ahead. The effects of tillage and reconsolidation, wheel-track soil compaction, crop residue management, macropore development and management interactions with natural sources of variability, such as topography, are addressed. Whether explicitly or implicitly, the available field studies include interactions between treatments, such as tillage, crop rotation and residue management. Controlled equipment traffic has been shown to have significant effects on soil compaction and related hydraulic properties in some soils and climates, but in others, landscape and temporal variability overwhelm any effects of wheel tracks. New research results on wheel-track effects in Colorado are highlighted along with initial attempts to predict their effects on hydraulic properties. The greatest challenge for the future is improved process-based prediction using a systems approach to include tightly coupled process interactions in space and time.

Cropping System Influence on Planting Water Content and Yield of Winter Wheat

wheat yields were reduced by 79 kg ha 1 for every centimeter that soil water at wheat planting was reduced by Many dryland producers in the central Great Plains of the USA sunflower (Helianthus annuus L.) ahead of wheat in express concern regarding the effect that elimination of fallow has on soil water content at winter wheat (Triticum aestivum L.) planting rotation. In southwestern Kansas, Norwood (2000) simiand subsequent yields. Our objectives were to quantify cropping syslarly showed lower winter wheat yields when the previtem effects (fallow weed control method and crop sequence), including ous crop was sunflower or soybean compared with corn corn (Zea mays L.) (C) and proso millet (Panicum miliacium L.) (M), or grain sorghum [Sorghum bicolor (L.) Moench]. These on soil water at winter wheat planting and subsequent grain yield, and reductions in wheat yield were related to lower soil to determine the frequency of environmental conditions which would water at planting. Lyon et al. (1995) showed that soil cause wheat yield to drop below 2500 kg ha 1 for various cropping water at planting was strongly correlated with yield of systems. Crop rotations evaluated from 1993 through 2001 at Akron, short season summer crops [pinto bean (Phaseolus vulCO, were W-F, W-C-F, W-M-F, and W-C-M (all no-till), and W-F garis L.), proso millet] but only weakly related to yield (conventional till). Yields were correlated with soil water at planting: of long season summer crops (sunflower, grain sorghum, kg ha 1 373.3 141.2 cm (average and wet years); kg ha 1 897.9 39.7 cm (dry years). Increasing cropping intensity to two corn). They attributed this result in part to shorter seacrops in 3 yr had little effect on water content at wheat planting and son crops having more soil water available at the critical subsequent grain yield, while continuous cropping and elimination of reproductive growth stage than longer season crops, fallow reduced soil water at planting by 11.8 cm and yields by 450 which used much of the initial soil water for stover to 1650 kg ha 1, depending on growing season precipitation. No-till production and did not have it available for grain develsystems, which included a 12to 15-mo fallow period before wheat opment. planting nearly always produced at least 2500 kg ha 1 of yield under In addition to differences in previous crop water use, normal to wet conditions, but no cropping system produced 2500 kg soil water content at wheat planting can also be affected ha 1 under extremely dry conditions. by differences in tillage and crop residue effects on precipitation storage efficiency. Precipitation storage efficiency increases as tillage is reduced during the sumT traditional wheat–fallow production system used mer fallow period before wheat planting (Smika and in the central Great Plains of the USA was develWicks, 1968; Tanaka and Aase, 1987; Norwood, 1999). oped in the 1930s as a strategy to minimize incidence of Crop residues reduce soil water evaporation by shading crop failures resulting from erratic precipitation (Hinze the soil surface and reducing convective exchange of and Smika, 1983). The use of herbicides to control weeds water vapor at the soil–atmosphere interface (Greb et in this system reduced or eliminated tillage, and led to al., 1967; Aiken et al., 1997; Van Doren and Allmaras, greater precipitation storage efficiencies, such that more 1978). Additionally, reducing tillage and maintaining frequent cropping could be successfully employed (Halsurface residues reduce precipitation runoff and invorson and Reule, 1994; Peterson et al., 1993; Anderson crease infiltration, thereby increasing precipitation storet al., 1999; Norwood et al., 1990; Smika, 1990; Farahani age efficiency (Unger and Stewart, 1983). et al., 1998). Both producers and agricultural lenders would like While more intensive cropping is gradually replacing to have a means of assessing the risk level that might W-F in the central Great Plains, many producers still be incurred in moving from conventional wheat–fallow express concern regarding the effect that more frequent production systems to more intensively cropped no-till cropping has on soil water content at planting and subsesystems. Part of that risk assessment involves quantifyquent winter wheat yields. Previous research has shown ing the effects of cropping system on wheat yields. Thererelationships between available soil water and yield of fore, the objectives of this study were to (i) quantify some crops. Nielsen et al. (1999) reported that winter effects of cropping system (crop sequence and fallowseason weed-control method [i.e., tillage vs. no-till]) on D.C. Nielsen, M.F. Vigil, R.A. Bowman, and J.G. Benjamin, USDAsoil water content at winter wheat planting and subseARS, Central Great Plains Res. Stn., 40335 County Road GG, Akron, quent effects on grain yield, and (ii) determine freCO 80720; R.L. Anderson, USDA-ARS, Northern Grain Insects Res. quency of environmental conditions that cause wheat Lab., 2923 Medary Ave., Brookings SD 57006; and A.D. Halvorson, USDA-ARS, Soil–Plant–Nutrient Research Unit, P.O. Box E, 301 S. Howes, Ft. Collins, CO 80522. Received 21 Jan. 2002. *Corresponding Abbreviations: CT, conventional tillage; W-C-F, wheat–corn–fallow; author (dnielsen@lamar.colostate.edu). W-C-M, wheat–corn–millet; W-F, wheat–fallow; W-M-F, wheat–millet– fallow; NT, no-till. Published in Agron. J. 94:962–967 (2002).

Quantifying effects of soil conditions on plant growth and crop production

Soil management decisions often are aimed at improving or maintaining the soil in a productive condition. Several indicators have been used to denote changes in the soil by various management practices, but changes in bulk density is the most commonly reported factor. Bulk density, in and of itself, gives little insight on the underlying soil environment that affects plant growth. We investigated using the Least Limiting Water Range (LLWR) to evaluate changes in the soil caused by soil management. The LLWR combines limitations to root growth caused by water holding capacity, soil strength and soil aeration into a single number that can be used to determine soil physical improvement or degradation. The LLWR appeared to be a good indicator of plant productivity when the full potential of water holding capacity on available water can be realized, such as with wheat (Triticum aestivum, L.) grown in a no-till system when the wheat followed a fallow period. A regression of wheat yield to LLWR gave an r 2 of 0.76. The LLWR was a poorer indicator of plant productivity when conditions such as low total water availability limited the expression of the potential soil status on crop production. Dryland corn (Zea mays, L.) yields were more poorly correlated with LLWR (r 2 =0.18), indicating that, under dryland conditions, in-season factors relating to water infiltration may be more important to corn production than water holding capacity. An improved method to evaluate in-season soil environmental dynamics was made by using Water Stress Day (WSD). The WSD was calculated by summing the differences of actual water contents in the field from the limits identified by the LLWR during the growing season. A regression of irrigated corn yield with LLWR as the soil indicator of the soil environment resulted in an r 2 of 0.002. A regression of the same yield data with WSD as the indicator of the soil environment resulted in an r 2 of 0.60. We concluded that the LLWR can be a useful measure of management effects on soil potential productivity. Soil management practices that maximize the LLWR can maximize the potential of a soil for crop production. Knowledge of the LLWR for a soil can help the farm manager optimize growing conditions by helping schedule irrigation and for making tillage decisions. The WSD, calculated from the LLWR and in-season water dynamics, allows us to evaluate changes in the

A method to separate plant roots from soil and analyze root surface area

Analysis of the effects of soil management practices on crop production requires knowledge of these effects on plant roots. Much time is required to wash plant roots from soil and separate the living plant roots from organic debris and previous years’ roots. We developed a root washer that can accommodate relatively large soil samples for washing. The root washer has a rotary design and will accommodate up to 24 samples (100 mm diam. by 240 mm long) at one time. We used a flat-bed scanner to digitize an image of the roots from each sample and used a grid system with commercially-available image analysis software to analyze each sample for root surface area. Sensitivity analysis and subsequent comparisons of ‘dirty’ samples containing the roots and all the organic debris contained in the sample and ‘clean’ samples where the organic debris was manually removed from each sample showed that up to 15% of the projected image could be coveredwith debris without affecting accuracy and precision of root surface area measurements. Samples containing a large amount of debris may need to be partitioned into more than one scanning tray to allow accurate measurements of the root surface area. Sample processing time was reduced from 20 h, when hand separation of roots from debris was used, to about 0.5 h, when analyzing the image from an uncleaned sample. The method minimizes the need for preprocessing steps such as dying the roots to get better image contrast for image analysis. Some information, such as root length, root diameter classes and root weights, is not obtained when using this technique. Root length measurements, if needed, could be made by hand on the digital images. Root weight measurement would require sample cleaning and the advantage of less processing time per sample with this method would be lost. The significance of the tradeoff between information not obtained using this technique and the ability to process a greater number of samples with the time and personnel resources available must be determined by the individual researcher and research objectives.

Nitrogen uptake and partitioning under alternate- and every-furrow irrigation

Alternate-furrow irrigation, combined with fertilizer placement in the non-irrigated furrow, has the potential to reduce fertilizer leaching in irrigated corn (Zea mays L.). The potential also exists, however, for reduced N uptake under alternate-furrow irrigation. This study examined the effects of fertilizer placement and irrigation treatment on N uptake, roota→shoot→root circulation, and partitioning between reproductive and vegetative tissues. Rainfall was above average in both years of the study, especially during May and June, so that root growth beneath the non-irrigated furrow was equal to root production beneath the irrigated furrow. Under those conditions, soil NO3 concentration in the fertilized furrow during late-vegetative and reproductive growth was greater in the alternate-furrow compared with the every-furrow treatment, resulting in increased fertilizer N uptake during reproductive growth and increased N partitioning to reproductive tissues under alternate-furrow irrigation. About 80% of the fertilizer N found in roots had first been translocated to the shoot and then returned via the phloem to the root system. Nitrogen cycling from root to shoot to root was not affected by irrigation treatment. Alternate-furrow irrigation successfully increased N uptake and reduced the potential for NO3leaching when environmental conditions allowed adequate root development in the non-irrigated furrow, and when the growing season was long enough to allow the crop to reach physiological maturity.

Root distribution following spatial separation of water and nitrogen supply in furrow irrigated corn

Proper management of water and fertilizer placement in irrigated corn (Zea mays L.) has the potential to reduce nitrate leaching into the groundwater. Potential management practices tested in a two year field experiment included row or furrow fertilizer placement combined with every or alternate furrow irrigation. To understand how fertilizer availability to plants could be affected by these management practices, root growth and distribution in a Ulm clay loam soil were examined. Spring rains were greater than normal in both years providing adequate moisture for early root growth in both irrigated and non-irrigated furrows. As the non-irrigated furrow began to dry, root biomass increased as much as 126% compared with the irrigated furrow. The greatest increase was at lower depths, however, where moisture was still plentiful. When early season moisture was available, roots proliferated throughout the soil profile and quickly became available to take up fertilizer N in both irrigated and non-irrigated furrows. Root growth responded positively to fertilizer placement in the furrow in 1996 but not in 1995. Excessive N leaching in 1995 may have limited the response to fertilizer N.

Modelling corn rooting patterns and their effects on water uptake and nitrate leaching

Water and nitrogen absorption by corn (Zea mays L.) are partly determined by the region in the soil containing roots and, as a result, rooting patterns could change water availability and leaching of nitrates. A two-dimensional model of corn root growth was developed and linked to a two-dimensional model for water, heat and solute transport in soil. The model was calibrated with root distribution and soil environment data obtained in a Mollisol at Lamberton, MN. Changing the root growth parameters allowed the model to be used to compare water uptake and NO3 leaching between a shallow, dense root system and a deep, sparse root system. For the rainfall conditions used in model validation, the model predicted a small amount of water absorption from lower in the soil profile with the deep, sparse root system compared with the shallow root system, but that most of the water for transpiration would come from shallow depths directly below the plant. Nitrate leaching was almost identical for both root systems. However, the model predicted reduced downward movement of N when plant uptake of water occurred than with no plant water uptake. The bulk volume of soil explored by the root system may be more important for determining water availability and possible plant water stress during dry periods than for decreasing fertilizer or pesticide leaching. The model should be useful for other examinations of water and chemical movement in the soil by including the effects of the plant in the system. The model also allows at least a preliminary examination of soil management effects on water and nutrient availability.

A comparison of corn (Zea mays L.) residue and its biochar on soil C and plant growth.

In order to properly determine the value of charring crop residues, the C use efficiency and effects on crop performance of biochar needs to be compared to the un-charred crop residues. In this study we compared the addition of corn stalks to soil, with equivalent additions of charred (300 °C and 500 °C) corn residues. Two experiments were conducted: a long term laboratory mineralization, and a growth chamber trial with proso millet plants. In the laboratory, we measured soil mineral N dynamics, C use efficiency, and soil organic matter (SOM) chemical changes via infrared spectroscopy. The 300 °C biochar decreased plant biomass relative to a nothing added control. The 500°C biochar had little to no effect on plant biomass. With incubation we measured lower soil NO3 content in the corn stalk treatment than in the biochar-amended soils, suggesting that the millet growth reduction in the stalk treatment was mainly driven by N limitation, whereas other factors contributed to the biomass yield reductions in the biochar treatments. Corn stalks had a C sequestration use efficiency of up to 0.26, but charring enhanced C sequestration to values that ranged from 0.64 to 1.0. Infrared spectroscopy of the soils as they mineralized showed that absorbance at 3400, 2925-2850, 1737 cm-1, and 1656 cm-1 decreased during the incubation and can be regarded as labile SOM, corn residue, or biochar bands. Absorbances near 1600, 1500-1420, and 1345 cm-1 represented the more refractory SOM moieties. Our results show that adding crop residue biochar to soil is a sound C sequestration technology compared to letting the crop residues decompose in the field. This is because the resistance to decomposition of the chars after soil amendment offsets any C losses during charring of the crop residues.

Switchgrass Biochar Effects on Plant Biomass and Microbial Dynamics in Two Soils from Different Regions

Abstract Biochar amendments to soils may alter soil function and fertility in various ways, including through induced changes in the microbial community. We assessed microbial activity and community composition of two distinct clayey soil types, an Aridisol from Colorado (CO) in the U.S. Central Great Plains, and an Alfisol from Virginia (VA) in the southeastern USA following the application of switchgrass ( Panicum virgatum ) biochar. The switchgrass biochar was applied at four levels, 0%, 2.5%, 5%, and 10%, approximately equivalent to biochar additions of 0, 25, 50, and 100 t ha −1 , respectively, to the soils grown with wheat ( Triticum aestivum ) in an eight-week growth chamber experiment. We measured wheat shoot biomass and nitrogen (N) content and soil nutrient availability and N mineralization rates, and characterized the microbial fatty acid methyl ester (FAME) profiles of the soils. Net N mineralization rates decreased in both soils in proportion to an increase in biochar levels, but the effect was more marked in the VA soil, where net N mineralization decreased from −2.1 to −38.4 mg kg −1 . The 10% biochar addition increased soil pH, electrical conductivity, Mehlich- and bicarbonate-extractable phosphorus (P), and extractable potassium (K) in both soil types. The wheat shoot biomass decreased from 17.7 to 9.1 g with incremental additions of biochar in the CO soil, but no difference was noted in plants grown in the VA soil. The FAME recovery assay indicated that the switchgrass biochar addition could introduce artifacts in analysis, so the results needed to be interpreted with caution. Non-corrected total FAME concentrations indicated a decline by 45% and 34% with 10% biochar addition in the CO and VA soils, respectively, though these differences became nonsignificant when the extraction efficiency correction factor was applied. A significant decline in the fungi:bacteria ratio was still evident upon correction in the CO soil with biochar. Switchgrass biochar had the potential to cause short-term negative impacts on plant biomass and alter soil microbial community structure unless measures were taken to add supplemental N and labile carbon (C).

Incorporation of Biochar Carbon into Stable Soil Aggregates: The Role of Clay Mineralogy and Other Soil Characteristics

Aggregation and structure play key roles in water-holding capacity and stability of soils. In this study, the incorporation of carbon (C) from switchgrass biochar into stable aggregate size fractions was assessed in an Aridisol (from Colorado, USA) dominated by 2:1 clays and an Alfisol (from Virginia, USA) containing weathered mixed 1:1 and 2:1 mineralogy, to evaluate the effect of biochar addition on soil characteristics. The biochar was applied at 4 levels, 0, 25, 50, and 100 g kg−1, to the soils grown with wheat in a growth chamber experiment. The changes in soil strength and water-holding capacity using water release curves were measured. In the Colorado soil, the proportion of soil occurring in large aggregates decreased, with concomitant increases in small size fractions. No changes in aggregate size fractions occurred in the Virginia soil. In the Colorado soil, C content increased from 3.3 to 16.8 g kg−1, whereas in the 2 000 µm fraction. The greatest increase (from 6.2 to 22.0 g kg−1) occurred in the 53–250 µm fraction. The results indicated that C was incorporated into larger aggregates in the Virginia soil, but remained largely unassociated to soil particles in the Colorado soil. Biochar addition had no significant effect on water-holding capacity or strength measurements. Adding biochar to more weathered soils with high native soil organic content may result in greater stabilization of incorporated C and result in less loss because of erosion and transport, compared with the soils dominated by 2:1 clays and low native soil organic content.