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In Argentina, wheat (Triticum aestivum L.) is cropped over a wide range of climatic conditions. Considerable variability in the ratio of dry weight produced per unit of transpired water, usually referred to as water‐use efficiency (WUE), is expected as variation in climatic factors affects photosynthesis and transpiration in different ways. Also, previous studies have shown that water supply limitations may affect WUE in wheat. The objective of this study was to quantify the effects of climatic environment and water availability on WUE in wheat crops. Six experiments were conducted at different locations of the Argentine wheat belt and crop dry weight and water use were measured in periods when water use was dominated by transpiration. Three of the experiments included both irrigated and rainfed treatments. Mean daily values of (i) pan evaporation, (ii) relative humidity, (iii) potential water use, and (iv) vapor pressure deficit, were used to find a general relationship that explained effects of the climatic environment on WUE. For experiments with high water availability, daytime vapor pressure deficit was better related to WUE than the other climatic factors. WUE was greater for experiments with water limitation, probably because stomatal closure to restrict transpiration rate occurred around midday when vapor pressure deficit was highest. As a consequence, relative dry weight under water limitation was not linearly related to relative water use as proposed in previous studies. A quadratic relationship that better represented this response was derived.

resulting in an estimated 1066 kg ha Ϫ1 (17 bu ac Ϫ1 ) yield loss over 354 commercial fields. Lauer and Rankin Corn (Zea mays L.) grain yields are known to vary from plant to (2004) and Liu et al. (2004) had differing results, noting plant, but the extent of this variability across a range of environments has not been evaluated. This study was initiated to evaluate by-plant that PSV did not significantly alter grain yields in Wiscorn grain yield variability over a range of production environments consin and Ontario, Canada, respectively. Nafziger et and to establish the relationships among mean grain yield, standard al. (1991) noted that uneven emergence of corn can deviation, coefficient of variation, and yield range. A total of fortyoccur when soils are dry at the time of planting and six 8-to 30-m corn transects were harvested by plant in Argentina, could lead to decreased grain yields. It is generally ac-Mexico, Iowa, Nebraska, Ohio, Virginia, and Oklahoma from 2002 cepted that when adjacent plants differ by more than to 2004. By-plant corn grain yields were determined, and the average two leaf stages, the younger plant may not develop to individual plant yields were calculated. Over all sites in all countries its fullest potential. A two leaf stage difference can result and states, plant-to-plant variation in corn grain yield averaged 2765 from delayed emergence ranging from 5 to 10 d, which kg ha Ϫ1 (44.1 bu ac Ϫ1 ). At the sites with the highest average corn can cause a 1% yield loss for each 1-d delay (Robert grain yield (11 478 and 14 383 kg ha Ϫ1 , Parana Argentina, and Phillips, NE), average plant-to-plant variation in yield was 4211 kg ha Ϫ1 (67 bu L. Nielsen, Purdue University, personal communication, ac Ϫ1 ) and 2926 kg ha Ϫ1 (47 bu ac Ϫ1 ), respectively. As average grain 2004). Tollenaar and Wu (1999) found increased stress yields increased, so did the standard deviation of the yields obtained tolerance in corn when plant-to-plant variability was within each row. Furthermore, the yield range (maximum corn grain lower. In general, these statistics identify a twofold probyield minus the minimum corn grain yield per row) was found to lem: first, the need to homogenize plant spacing and increase with increasing yield level. Regardless of yield level, plantemergence and second, the need to recognize differto-plant variability in corn grain yield can be expected and averaged ences in yield potential that clearly exist by plant. more than 2765 kg ha Ϫ1 over sites and years. Averaging yield over Some technologies in precision agriculture have been distances Ͼ0.5 m removed the extreme by-plant variability, and thus, driven commercially. The most notable has been comthe scale for treating other factors affecting yield should be less than bine yield monitors. Depending on combine speed, 0.5 m. Methods that homogenize corn plant stands and emergence may decrease plant-to-plant variation and could lead to increased Published in Agron.

Soil carbon sequestration is a viable shortterm option to mitigate increased atmospheric CO 2 . In agriculture, strategies to increase the soil carbon (C) sink include no-tillage, cover crops, and improved crop rotation. The objective of this study was to determine the influence of tillage systems on SOC and total N, soil aggregation and aggregate associated C and N in three soil types: Oxisol (Brazil), Vertisol (Argentina), and Mollisol (USA). Long-term tillage experiments included tilled (T) and no-till (NT) systems. A native grassland was included for comparison in each site. Soil samples were taken at 0-5, 0-15, and 15-30 cm depths.Water-stable aggregates (WSA) were separated using a wet-sieving method. Total C and total N were determined by dry combustion. A shift from native grassland to an agroecosystem decreased microbial biomass, but this decrease was less pronounced under NT. Cultivation reduced the mass of macroaggregates and the concentration associated C and N; however among agroecosystems, NT, regardless soil type, tended to be more similar to the native grassland sites. Agroecosystems reduced TOC and total N stocks, regardless of soil type, compared to the native grassland. This effect followed: Mollisol [ Oxisol [ Vertisol, and was more pronounced at the 0-5 cm soil depth than at deeper depths. This loss of C and N was associated with the decrease in the mass of macroaggregates and lower C and N concentrations of the aggregates. Macroaggregation was related to TOC and microbial biomass in the Mollisol, suggesting that the biological process of aggregate formation is the principal mechanism of C protection in these soils. The relationship between TOC and large macroaggregates showed lower values for the Oxisol and Vertisol, indicating that in these soils TOC has a complementary role in macroaggregation.

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