All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. N itrogen is an important and costly input for nonleguminous grain crops, and producers are applying N fertilizer in large amounts to ensure high yields over a range of environmental conditions (Kyveryga et al., 2007). However, excessive N fertilization may lead to runoff , leaching, and nitrate pollution. A delayed N application and the use of remote sensing tools might allow a producer to apply a more economically benefi cial N rate to their fi elds. Scharf and Lory (2002) gave several reasons to delay N applications, including avoiding extra work during the busy planting season and lowering the in-season N loss during wet years. Th ey also suggested that diagnostic tools for plant N might increase fertilizer use effi ciency, and these tools include the SPAD meter, refl ectance measurements, and color analysis. Th e SPAD meter is used to make an optimum fertilizer N-rate decision by measuring N stress relative to an optimum N-rate strip within a fi eld (Hawkins et al., 2007). Th e SPAD meter is well documented as an accurate measure of the N status of corn at diff erent developmental stages (Piekielek and Fox, 1992; Blackmer et al., 1994; Schepers, 1994). Piekielek et al. (1995) showed that SPAD values expressed relative to SPAD values from a high-N strip (relative or normalized SPAD) could be compared over a wide range of sampling times when using a common critical value. Normalized SPAD values lessen the eff ect of diff erences in hybrid, soil type, growth stage, or environmental conditions (Piekielek et al., 1995). Scharf et al. (2006) found that the relationship between SPAD values and economically optimum N rate was much stronger when using normalized values as opposed to absolute values. Th e SPAD meter is a useful tool, but it has some potential limitations. Th e SPAD meter costs about $1500 USD, has a small sampling area (6 mm 2), is subject to operator bias, and Zhang et al. (2008) showed that SPAD meters have diffi culty in estimating chlorophyll levels when they are near or above optimum. Th eir observations indicate that increases in chlorophyll are not necessarily associated with increases in yield. Spectral refl ectance of crop leaves can be a valuable tool to estimate plant N status (Li et al., 2005). Spectral refl ectance is the refl ectance of certain plant components that are controlled by their visual properties and radiant energy exchange in a canopy (Huete, 1988). Th e refl ectance of certain wavelengths is related to diff erent amounts of chlorophyll a and b, which can be used to estimate the N status of certain crops (Huete, 1988). Th is method shows great potential because it off ers a method to deliver variable-rate N applications from a vehicle-mounted sensor (Kitchen et al., 2010). Tools for measuring refl ectance, howe...

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Burning of wheat (Triticum aestivum L.) residue followed by plowing is a common management practice in wheat–soybean [Glycine Max (L.) Merrill] production systems in the mid‐southern USA. However, this residue management practice is not environmentally friendly and may not be sustainable. The objectives of this study were to (i) evaluate the effects of N fertilization of wheat, residue burning, and tillage on soil surface carbon dioxide (CO2) flux in a wheat–soybean double‐crop production system, and (ii) evaluate the role of soil temperature and soil moisture in controlling CO2 flux in a relatively warm, subhumid environment. Soil surface CO2 flux was measured nine times between June 2002 and October 2003 during the soybean growing season under all combinations of conventional‐ (CT) and no‐tillage (NT) at high and low N fertilization levels with and without residue burning at two locations in the Mississippi River delta region of eastern Arkansas. Soil surface CO2 flux was 37.6% higher (P < 0.01) from CT than from NT and 6.1% higher (P < 0.05) from the low than the high N rate treatment at one location, but not at the other. Burning did not affect soil surface CO2 flux except for a significant burning × N rate (P = 0.016) and burning × time interaction (P = 0.032) at one location, but not at the other. Both soil temperature and moisture parameters were significantly negatively correlated with temperature‐normalized soil surface CO2 flux, but soil temperature, particularly at the 10‐cm depth, explained more of the variation than did soil moisture parameters. The results of this study indicate that tillage and N fertilization of prior wheat, but not residue burning, affect the loss of C as CO2 from the soil and that there are additional soil and/or environmental factors, other than near‐surface soil moisture and temperature fluctuations, perhaps microbial biomass, that contribute to controlling soil surface CO2 flux in wheat‐soybean production systems in the subhumid region of southern USA.

Nutrient‐rich biochar produced from animal wastes, such as poultry litter, may increase plant growth and nutrient uptake although the role of direct and indirect mechanisms, such as stimulation of the activity of mycorrhizal fungi and plant infection, remains unclear. The effects of poultry litter biochar in combination with fertilizer on mycorrhizal infection, soil nutrient availability and corn (Zea mays L.) growth were investigated by growing corn in a loam soil in a greenhouse with biochar (0, 5 and 10 Mg/ha) and nitrogen (N) and phosphorus (P) fertilizer (0, half and full rates). Biochar did not affect microbial biomass C or N, mycorrhizal infection, or alkaline phosphomonoesterase activities, but acid phosphomonoesterase activities, water‐soluble P, Mehlich‐3 Mg, plant height, aboveground and root biomass, and root diameter were greater with 10 Mg/ha than with no biochar. Root length, volume, root tips and surface area were greatest in the fully fertilized soil receiving 10 Mg/ha biochar compared to all other treatments. The 10 Mg/ha biochar application may have improved plant access to soil nutrients by promoting plant growth and root structural features, rather than by enhancing mycorrhizal infection rates.