North Central Soil Conservation Research Laboratory

Crop residue management received little attention until about 1970. Records of crop residue production are limited, but crop yield databases have been available since 1865. Carbon sequestration and other conservation benefits require a detailed knowledge of crop residue production and management. Our objectives are to: (i) review grain and biomass yield, harvest index (HI), and root C/shoot C ratios ( k ) of major grain crops in the USA; (ii) discuss historical agricultural‐practice impacts on soil organic C (SOC); and (iii) compare estimates of total (above‐ and belowground) source C production (ESC) relative to minimum source C inputs required to maintain SOC (MSC). Aboveground MSC input averaged 2.5 ± 1.0 Mg C ha −1 yr −1 ( n = 13) based on moldboard plow sites and 1.8 ± 0.44 Mg C ha −1 yr −1 ( n = 5) based on no‐till and chisel plow sites. These MSC values included only aboveground source C, thus underestimate the total MSC. When ESC is estimated from k , including rhizodeposition ( k rec ), the true magnitude of the C cycle is at least twice that when ESC is estimated using k excluding rhizodeposition ( k his ). Neglecting rhizodeposition C underestimates the net production of C in cropland. Current yields and measured MSC predict continued SOC loss associated with soybean [ Glycine max (L.) Merr.] and some wheat ( Triticum aestivum L.) production management unless conservation tillage is used and ESC is increased. The adequacies of ESC to maintain SOC has direct implications for estimating the amount of crop residue that can be harvested and yet maintain SOC.

Sustainable aboveground crop biomass harvest estimates for cellulosic ethanol production, to date, have been limited by the need for residue to control erosion. Recently, estimates of the amount of corn ( Zea mays L.) stover needed to maintain soil carbon, which is responsible for favorable soil properties, were reported (5.25–12.50 Mg ha −1 ). These estimates indicate stover needed to maintain soil organic carbon, and thus productivity, are a greater constraint to environmentally sustainable cellulosic feedstock harvest than that needed to control water and wind erosion. An extensive effort is needed to develop advanced cropping systems that greatly expand biomass production to sustainably supply cellulosic feedstock without undermining crop and soil productivity.

High levels of charcoal C resulting from repeated historical burning of grasslands, open woodlands, and agricultural crop residues have been reported in soils from Australia and Germany. In this study, five U.S. soils were selected from long‐term research plots in widely different agricultural areas. The charcoal C content was estimated on each soil using a combination of physical separation, high energy photo‐oxidation and solid‐state 13 C nuclear magnetic resonance (NMR) spectroscopy. These analyses showed that all five soils contained measurable amounts of charcoal C, <53 μm in size and ranging from 1.8 to 13.6 g C kg −1 soil and constituted up to 35% of the soil total organic C (TOC). Scanning electron microscopy showed that the charcoal material had a plant‐like morphology but were blocky and had fractured edges. These particles were similar in morphology to those separated from Australian and German soils. The implications of this material, which must be highly resistant to microbiological decomposition, to the soil C cycle are discussed.

Understanding the interaction between plant components and their subsequent decomposition provides insights on how plant quality differences may influence C sequestration within a given management system. Our hypothesis was that decomposition is a function of biochemical composition when all other variables are constant (e.g., particle size, temperature and moisture). Recognizing the challenges of reconciling laboratory and field studies, this study examined the decomposition dynamics of five selected crops with varying composition under controlled temperature and moisture regimes. Residue materials were partitioned into leaf, stem, and root organs to give a clearer indication of compositional control on decomposition. Plant quality varied among species (alfalfa [ Medicago sativa L.], corn [ Zea mays L.], cuphea [ Cuphea viscosissima Jacq. · Cuphea lanceolata W.T. Aiton], soybean [ Glycine max (L.) Merr.] and switchgrass [Panicum virgatum L.]). A two‐component litter decomposition model was used to describe decomposition observed during 498 d. Stepwise multivariate regression indicated initial N concentration, starch, total lignin, and acid‐insoluble ash (AI ash) were the four best predictors ( r 2 = 0.83) of the rate of active component decomposition ( k a ); however, initial composition poorly predicted the rate of passive decomposition ( k p ). The best four‐component model ( r 2 = 0.43) identified by stepwise multiple regression for k p included AI ash, hemicellulose, N concentration, and C/N ratio. Rate constants are a function of the incubation period, thus making direct comparison among separate experiments difficult. Chemical recalcitrance appears to slow root decomposition; such chemical recalcitrance to decay may partially explain why roots have been found to contribute more C to the SOC pool than surface residues.

The species composition and density of weed seed in the soil vary greatly and are closely linked to the cropping history of the land. Altering tillage practices changes weed seed depth in the soil, which plays a role in weed species shifts and affects efficacy of control practices. Crop rotation and weed control practices also affect the weed seedbank. Information on the influence of cropping practices on the weed seedbank should be a useful tool for integrated weed management. Decision aid models use information on the weed seedbank to estimate weed populations, crop yield loss, and recommend weed control tactics. Understanding the light requirements of weed seed may provide new approaches to weed management. Improving and applying our understanding of weed seedbank dynamics is essential to developing improved weed management systems. The principles of plant ecology must be integrated with the science of weed management to develop strategies that take advantage of basic plant responses in weed management systems for agronomic crops.

Abstract The increasing concern for rising CO 2 concentrations from agricultural activities has prompted the need to better understand the flux of greenhouse gases to the atmosphere. This work determines the effect of four fall tillage methods on short‐term CO 2 flux from a Hamerly clay loam (fine‐loamy, frigid Aeric Calciaquoll) in the northern Corn Belt. Moldboard plow only, moldboard plow plus disk harrow twice, disk harrow once, and chisel plow once using standard tillage equipment following a wheat ( Triticum aestivum L.) crop were compared with no‐tillage. The CO 2 flux was measured with a large portable system commonly used to measure canopy gas exchange of field crops. Measurements of CO 2 flux were initiated within 5 min after tillage completion for each tillage treatment and continued intermittently for 19 d. Moldboard plow had the roughest soil surface and the highest initial CO 2 flux (29 g m −2 h −1 ) and maintained the highest flux throughout the study. Moldboard plow plus disking twice and chisel plow had similar initial rates (7 and 6 g m −2 h −1 , respectively) that were greater than disk harrow and no‐tillage. The high initial CO 2 fluxes were more related to depth of soil disturbance that resulted in a rougher surface and larger voids than to residue incorporation. The differences in CO 2 flux between tillage treatments were small but consistent 19 d after initial tillage and 64 mm rain. Lower CO 2 flux rates caused by tillage were associated with low soil disturbance and/or small voids. Tillage methods affected the initial CO 2 flux differently and suggest improved soil management can minimize agriculture's impact on global CO 2 increase.

Abstract In many crop models, light intercepted by a canopy (IPAR) is calculated from a Beer's Law equation: IPAR = PAR × [1 − exp(− k × LAI)], where k is the extinction coefficient, PAR the photosynthetically active radiation, and LAI the leaf area index. The first objective of this study was to investigate the effect of row spacing on k for corn ( Zea mays L.), sorghum [ Sorghum bicolor (L.) Moench], soybean [ Glycine max (L.) Merr.], and sunflower ( Helianthus annuus L.) to provide information for modeling. Data from literature and from an experiment conducted at Temple, TX, were evaluated. The second objective was to investigate effects of time of day and stage of crop development on k for different row spacings. Seeds of all four species were sown in rows 0.35, 0.66, or 1.00 m apart. Measurements of canopy light interception were taken near solar noon on two dates before anthesis. At anthesis, extinction coefficients were determined at 0845, 1015, and 1145 h (solar time). The extinction coefficient showed a linear decrease as row spacing increased. For each crop, the effect of row spacing on k was described by one linear regression for most data. Stage of crop development and stage of development × row spacing interaction did not significantly affect k during the period of measurements. The effect of time of day was significant for all four crops, and the time of day × row spacing interaction was significant for soybean and sunflower. Thus, modeling light interception for different row spacings should account for these effects.

Society is facing three related issues: overreliance on imported fuel, increasing levels of greenhouse gases in the atmosphere, and producing sufficient food for a growing world population. The U.S. Department of Energy and private enterprise are developing technology necessary to use high‐cellulose feedstock, such as crop residues, for ethanol production. Corn ( Zea mays L.) residue can provide about 1.7 times more C than barley ( Hordeum vulgare L.), oat ( Avena sativ a L.), sorghum [ Sorghum bicolor (L.) Moench], soybean [ Glycine max (L.) Merr.], sunflower ( Helianthus annuus L.), and wheat ( Triticum aestivum L.) residues based on production levels. Removal of crop residue from the field must be balanced against impacting the environment (soil erosion), maintaining soil organic matter levels, and preserving or enhancing productivity. Our objective is to summarize published works for potential impacts of wide‐scale, corn stover collection on corn production capacity in Corn Belt soils. We address the issue of crop yield (sustainability) and related soil processes directly. However, scarcity of data requires us to deal with the issue of greenhouse gases indirectly and by inference. All ramifications of new management practices and crop uses must be explored and evaluated fully before an industry is established. Our conclusion is that within limits, corn stover can be harvested for ethanol production to provide a renewable, domestic source of energy that reduces greenhouse gases. Recommendation for removal rates will vary based on regional yield, climatic conditions, and cultural practices. Agronomists are challenged to develop a procedure (tool) for recommending maximum permissible removal rates that ensure sustained soil productivity.

Anecdotal accounts regarding reduced US cropping system diversity have raised concerns about negative impacts of increasingly homogeneous cropping systems. However, formal analyses to document such changes are lacking. Using US Agriculture Census data, which are collected every five years, we quantified crop species diversity from 1978 to 2012, for the contiguous US on a county level basis. We used Shannon diversity indices expressed as effective number of crop species (ENCS) to quantify crop diversity. We then evaluated changes in county-level crop diversity both nationally and for each of the eight Farm Resource Regions developed by the National Agriculture Statistics Service. During the 34 years we considered in our analyses, both national and regional ENCS changed. Nationally, crop diversity was lower in 2012 than in 1978. However, our analyses also revealed interesting trends between and within different Resource Regions. Overall, the Heartland Resource Region had the lowest crop diversity whereas the Fruitful Rim and Northern Crescent had the highest. In contrast to the other Resource Regions, the Mississippi Portal had significantly higher crop diversity in 2012 than in 1978. Also, within regions there were differences between counties in crop diversity. Spatial autocorrelation revealed clustering of low and high ENCS and this trend became stronger over time. These results show that, nationally counties have been clustering into areas of either low diversity or high diversity. Moreover, a significant trend of more counties shifting to lower rather than to higher crop diversity was detected. The clustering and shifting demonstrates a trend toward crop diversity loss and attendant homogenization of agricultural production systems, which could have far-reaching consequences for provision of ecosystem system services associated with agricultural systems as well as food system sustainability.

Losses in grain yield are particularly severe when low water potentials (low ψ W ) occur at anthesis in maize ( Zea mays L.). The losses can result from delays in floral development and from a failure in grain development when flowering appears normal. The causes of the latter problem are unknown, but could involve either the male or the female flower. Therefore, we made reciprocal crosses at various stigma (silk) and pollen ψ W to determine which flower part failed at low ψ W . The plants grew in soil in a controlled environment where silk ψ W was between −0.3 and −0.5 MPa and pollen ψ W between −1.5 and −12.5 MPa during the day. Water was withheld for a few days at anthesis after which water was resupplied and grain production was evaluated. At high pollen and silk ψ W plants produced about 550 grains ear −1 . At low pollen ψ W (to −12.5 MPa), plants also produced grain at a similar high rate. However, at low silk ψ W (−1.2 MPa), grain did not develop. The failure of the grain to grow could not be attributed to insufficient water on the silk surfaces because pollen germinated and the pollen tube grew within the silks. The egg sac invariably was fertilized but the embryo, endosperm, and seedcoat did not develop beyond 2 or 3 days. Therefore, the failure to produce grain at low ψ W was attributed to factors in the female flower that allowed fertilization to occur but prevented embryo development. Because both embryonic and maternal tissues were involved, the effect suggests a general starvation for substrate, which could have been caused either by a lack of photosynthate or by a blockage of translocation. Earlier work from our laboratory showed that photosynthesis was inhibited at these ψ W , and that carbohydrate reserves were low at this time. However, reserves accumulated at later stages of grain fill and could support grain growth when low ψ W occurred. Therefore, a lack of photosynthate rather than a blockage of translocation may have caused the failure in grain development when low ψ W were present at anthesis.

A better understanding of C turnover, with estimates of root‐derived C, is needed to manage soil C sequestration. The objective was to evaluate the long‐term treatment and environmental effects on unharvestable soil C components. Two N fertilizer treatments and a control were imposed during 29 yr of continuous corn ( Zea mays L.) with stover removal as silage vs. stover return during grain harvest with moldboard plow (MB) tillage. Soil organic carbon (SOC) declined and natural 13 C abundance (δ 13 C) increased during the 29‐yr period. Field averages of SOC and δ 13 C (0–30 cm) were 96.4 Mg ha −1 and −17.3‰ in 1965; respective values in 1995 were 78.9 Mg ha −1 and −16.6‰. Loss of SOC was greater with stover removed or no fertilization, but δ 13 C increased for all treatments. Stover yield (SY), SOC, and δ 13 C data were applied to a model to estimate unharvestable C and predict total source C (SC) input from corn. The SC for 29 yr totaled 172 to 189 Mg ha −1 when stover was harvested and 268 to 284 Mg ha −1 when stover was returned. The SC input from unharvestable sources was 1.8 times more than SC from aboveground stover when N was added and 1.7 when N was not added. The root‐to‐shoot ratio was 1.1 when N was added and 1.2 with no N. Only 5.3% of the SC was retained as SOC. Unharvestable C contributions to rhizodeposition are much larger than suggested from controlled studies including C‐enriched CO 2 followed by soil respiration or CO 2 efflux measurements.

An estimate of soil mineralizable N is needed to determine crop needs for N fertilizer. The objective of this research was to estimate soil net N mineralization in soils maintained in continuous corn ( Zea mays L.) (CC), corn–soybean [ Glycine max (L.) Merr.] (CS), and corn–soybean–wheat ( Triticum aestivum L.)/alfalfa ( Medicago sativa L.)–alfalfa (CSWA) rotations that have been managed since 1990 with zero N (0N), low N (LN), and high N (HN) fertilization. Soil samples were taken from 0‐ to 20‐cm depth in plots planted to corn in 1998. In order to produce more realistic time‐series data of net N mineralization, soils were incubated in filtration units in a variable‐temperature incubator (VTI) that mimicked field soil temperatures under a growing corn canopy. Rotation and N fertilization significantly affected net N mineralization in soil samples. Cumulative net N mineralized in a 189‐d field temperature incubation averaged 133 ± 6 kg ha −1 in CC, 142 ± 5 kg ha −1 in CS, and 189 ± 5 kg ha −1 in CSWA. Across rotations, average net N mineralized was 166 ± 9 kg ha −1 in 0N plots, 147 ± 10 kg ha −1 in LN plots, and 152 ± 10 kg ha −1 in HN plots. Inclusion of a legume, particularly alfalfa, in the rotation increased net N mineralized. Generally, more net N was mineralized from plots receiving no fertilizer N than from soil with a history of N fertilization. Variable‐temperature incubation produced realistic time‐series data with low sample variability.

Smokeless biomass pyrolysis for biochar and biofuel production is a possible arsenal for global carbon capture and sequestration at gigatons of carbon (GtC) scales. The United States can annually harvest over 1.3 Gt (gigaton) of dry biomass. Use of the smokeless (clean and efficient) biomass-pyrolysis technology would enable the United States to converts its 1.3 Gt of annually harvestable biomass to biochar products equivalent to 325 million tons of stable carbon plus significant amount of biofuels including syngas and bio-oils. Currently, the world could annually harvest more than 6.5 GtC y−1 of biomass. The 6.5 GtC y−1 of biomass could be converted to biochar (3.25 GtC y−1) and biofuels (with heating value equivalent to that of 6500 million barrels of crude oil). Because biochar is mostly not digestible to microorganisms, a biochar-based soil amendment could serve as a permanent carbon-sequestration agent in soils/subsoil earth layers for thousands of years. By storing 3.25 GtC y−1 of biochar (equivalent to 11.9 Gt of CO2 per year) into soil and/or underground reservoirs alone, it would offset the world's 8.67 GtC y−1 of fossil fuel CO2 emissions by about 38%. The worldwide maximum capacity for storing biochar carbon into agricultural soils (1411 million hectares) is estimated to be about 428 GtC. It may be also possible to provide a global carbon “thermostat” mechanism by creating biochar carbon energy storage reserves. This biomass-pyrolysis “carbon-negative” energy approach merits serious research and development worldwide to help provide clean energy and control global warming for a sustainable future of human civilization on Earth.

ABSTRACT Crown buds of field‐grown leafy spurge ( Euphorbia esula L.) were examined to determine relationships between carbohydrate metabolism and gene expression throughout para‐, endo‐, and eco‐dormancy during the transition from summer, autumn, and winter, respectively. The data indicates that endo‐dormancy plays a role in preventing new shoot growth during the transition from autumn to winter. Cold temperature was involved in breaking endo‐dormancy, inducing flowering competence, and inhibiting shoot growth. An inverse relationship developed between starch and soluble sugar (mainly sucrose) content in buds during the shift from para‐ to endo‐dormancy, which continued through eco‐dormancy. Unlike starch content, soluble sugars were lowest in crown buds during para‐dormancy but increased over two‐ to three‐fold during the transition to endo‐dormancy. Several genes ( AGPase , HK , SPS , SuSy , and UGPase ) coding for proteins involved in sugar metabolism were differentially regulated in conjunction with well‐defined phases of dormancy in crown buds. Marker genes for S‐phase progression, cell wall biochemistry, or responsive to auxin were also differentially regulated during transition from para‐, endo‐, and eco‐dormancy. The results were used to develop a model showing potential signalling pathways involved in regulating seasonal dormancy status in leafy spurge crown buds.

Summary: Résumé: Zusammenfassung Bioeconomic models for weed management ultimately require knowledge of weed densities. Weed seedling populations may be predicted by multiplying emergence rates by seedbank densities. However, emergence rates vary according to species, year and management. Furthermore, seedbank estimates may vary with sampling technique, size, number and date. These variables must be quantified before bioeconomic models can be used profitably. Consequently, two experiments were initiated, both in conventionally managed maize. The first experiment documented proportional seedling emergence across years and sites for three taxa: foxtail ( Setaria glauca [L.] Beauv. and S. viridis [L.] Beauv. combined), pigweed ( Amaranthus retroflexus L.), and lambsquarters ( Chenopodium album L.). The second experiment was devoted to methodology for estimation of seedbanks: sampling date (autumn or spring), technique (seed extraction or glasshouse germination), and soil sample size for the seed extraction technique. For emergence rates, the following order was observed: foxtail > lambs‐quarters > pigweed. Emergence rates for each species were related in a parabolic manner to growing degree‐days in April. The glasshouse technique appeared to be more reliable than seed extraction for correlation with field seedling densities. For the seed extraction technique, a minimum soil sample size of 100 g was necessary. A spring sampling date appears to be more reliable than an autumn date, probably because many seemingly viable seeds die during winter. Prevision de la densité deplantules d'adventices a partir des reserves de graines enfouies Les modèles bioéconomiques en matière de desherbage impliquent la connaissance des densités d'adventices. Les populations de plantules adventices peuvent être prévues à partir des taux de levées multipliés par les densités du stock grainier. Cependant, les taux de levée varient avec les espèces, l'année, et les méthodes culturales; et les estimations du stock grainier peuvent varier avec la technique d'échantillonnage, son nombre, sa taille et sa date. Ces variables doivent être quantifiées avant que les modèles bioéconomiques puissent être utilisés avec profit. En conséquence, deux expérimentations ont été mises en place, les deux en culture de maïs conventionnelle. La première expérimentation a indiqué le taux de levée des plantules pour les différentes années et sites de 3 taxons: setaires ( Setaria glauca [L.] Beauv. et S. viridis [L.] Beauv., en association), amaranthe réfléchie ( Amaranthus retroflexus L.) et chenopode blanc ( Chenopodium album L.). La seconde expérimentation était consacrée à la méthodologie d'estimation du stock grainier: date d'échantillonnage (automne ou printemps); technique (extraction des graines ou germination en serre) et taille de l'échantillon de sol pour la technique df'extraction des graines. Pour les taux de levée: sétaires > chénopode blanc > amaranthe. Les taux de levée pour chaque espèce sont liés de façon parabolique avec les degrés/jour de croissance en Avril — la technique en serre apparaît plus fiable que l'extraction des graines pour la corrélation avec les densités d'adventices au champ. Pour la technique d'extraction des graines, la taille minimum de l'échantillon nécessaire était de 100 g: un échantillonnage de printemps apparait meilleur qu'un d'automne, probablement parce que de nombreuses semences paraissant viables meurent durant l'hiver. Prognose der Keimpflanzendichte von Unkräutern nach dem Samenvorrat im Boden Bioökomische Modelle in der Unkrautbekämpfung setzen die Kenntnis der Unkrautdichte voraus. Die Keimpflanzendichte kann mit dem Produkt aus der Samendichte im Boden mit der Keimrate errechnet werden. Die Keimrate variiert jedoch je nach Pflanzenart, Jahr und Kulturbedingungen, und die Bestimmung der Samenbank hängt von der Probennahmetechnik, ‐größe, ‐zahl und ‐zeit ab. Diese Variablen müssen quantifiziert werden, um brauchbare bioökonomische Modelle zu erhalten. Deshalb wurden in normal angebautem Mais 2 Versuche angelegt, um zum einen die Keimpflanzendichte von 3 Arten(gruppen), Fuchsrote Borstenhirse ( Setaria glauca (L.) Beauv.)+Grüne B. ( S. viridis (L.) Beauv.), Zurückgekrümmter Amarant ( Amaranthus retroflexus L.) und Weißer Gänsefuß ( Chenopodium album L.), über die Jahre und an verschiedenen Punkten festzustellen und sich zum anderen mit der Methodik der Bestimmung der Samenbank zu befassen: Probennahmezeit (Herbst und Frühjahr), ‐technik (Samenauswaschung oder Keimung im Gewächshaus) und Bodenprobenumfang. Die Keimraten (im April) waren: Borstenhirse > Gänsefuß > Amarant. Die Keimung im Gewächshaus ergab für die Korrelation mit der Keimpflanzendichte im Feld bessere Werte als die Samenauswaschung. Für die Auswaschung waren mindestens 100 g Boden erforderlich. Probennahmen im Frühjahr erwiesen sich als günstiger als im Herbst, vermutlich wegen des Absterbens vieler keimfähiger Samen über Winter.

Low water potentials (ψ w ) during anthesis and early grain fill can decrease yields in grain crops. If photosynthesis is inhibited under these conditions, carbohydrate reserves may become limiting for grain growth. We measured the mobilization of stored nonstructural carbohydrates in the maize plant ( Zea mays L.) during low ψ w , imposed at silk emergence (T1), early grain fill (T2), and mid‐grain fill (T3) and maintained to maturity. low leaf ψ w , completely inhibited photosynthesis, and reproductive development depended entirely on reserves. The control yield was equivalent to 9910 kg ha −1 . Mean yields on a per‐plant basis were 139, 89, 25, and 0 g for the control, T3, T2, and T1 treatments, respectively. Yield losses resulted from decreased seed size in T3, decreased seed size and number in T2, and a cessation of silk and ear development in T1. After anthesis, dry matter accumulated in the stems and leaves and was mobilized to the ear at low ψ w in T3 and T2 but not in T1. All the mobilized dry matter from the stems and a sizable fraction from the leaves was carbohydrate. By maturity, the mobilized carbohydrates had been depleted to the level at anthesis. In T1, there was no mobilization and the carbohydrates remained at the low anthesis levels until maturity. The low carbohydrate at T1 indicates that reserves were not sufficient to support reproductive development when photosynthesis was inhibited at low ψ w . The lack of reserves may explain the high sensitivity of anthesis and early grain fill to low ψ w , as well as previous reports of the inability of silks to maintain turgor by osmotic adjustment. The decreasing sensitivity of grain development to low ψ w as reproduction progresses may result from an increasing availability of reserves.

Drought during rain filling decreases final kernel mass in maize ( Zea mays L.). Lack of assimilates or an unfavorable water status within the embryo or endosperm could limit kernel development. To test these possibilities, remobilization of reduced C and N as well as kernel and embryo water status were measured in plants exposed to a water deficit during rain fill. Irrigation was withheld from field‐grown plants after final kernel number was established. This treatment resulted in a soil moisture deficit of 224 mm and decreased endosperm and embryo mass by 16%, compared with controls. The water deficit shortened the effective filling period, but did not alter the rate of dry matter accumulation in either the endosperm or embryo. Carbohydrate reserves in leaf and stalk tissues as well as N stored in the leaves were remobilized to support kernel growth. However, grain filling ceased before these reserves were depleted completely. Grain filling continued in both well.watered and water‐deficient plants until the moisture content of the endosperm and embryo decreased to 280 and 430 g kg −1 fresh wt., respectively. Water‐deficient plants reached these values 10 d earlier because water loss from the endosperm began sooner after anthesis and maximum water content of the embryo was lower, compared with the controls. Kernel and embryo osmotic potentials (ψ s ) decreased rapidly late in grain filling and were − 2.2.to − 2.6 MPa when growth ceased. The results indicate that kernel water status is affected directly by drought and may be an important determinant of kernel development. They suggest that a water deficit after anthesis shortens the duration of grain filling by causing premature desiccation of the endosperm and by limiting embryo volume.

Various thermal properties of soils accessible through fairly simple measurements are described. These properties include the insulating effect of soil, the existence of the soil temperature wave, and the propagation of this wave through the soil.

Plant water deficits during flowering cause maize ( Zea mays L.) kernels to abort. Lack of current and reserve photosynthate account for much of the kernel loss, but partitioning to ovaries at low ovary water potential (ψ w ) may also be limited by lack of assimilate demand. To test this possibility, we measured the water status, carbohydrate content, and growth of ovaries on plants grown in the field in pots containing 22 kg of soil under one of three light environments [ Control , 75‐cm rows, 43 055 plants ha −l ; Shade, same as Control except under shade cloth (55% fight interception) from the sixth leaf stage until physiological maturity; or Isolated , 122‐cm rows, 6727 plants ha −1 ] and exposed to a water deficit during pollination. Water was withheld at silk emergence and plants were hand‐pollinated 4 d later when silk ψ was ≈ − 1.1 MPa, leaf ψ w was ≈ − 1.8 MPa, and photosynthesis was completely inhibited. The water deficit decreased kernel set, which was correlated with the inhibition of ovary dry matter accumulation. The concentration of sucrose and glucose increased in ovaries of water‐deficient plants, and ovary turgor remained at or above control levels. Thus, inhibition of ovary growth at low ψ w was not related to a loss of turgor, nor was it caused by a depletion of ovary sugars. Sugar accumulation at low ψ w , suggested that metabolism may have been impaired. Coupled with a low level of reserves, failure to utilize available sugars at low ψ w would severely inhibit assimilate flux to the ear and render kernel set highly vulnerable to water deficits during pollination.

Camelina ( Camelina sativa L.), a member of the Brassicaceae family, can potentially serve as a low‐input alternative oil source for advanced biofuels as well as food and other industrial uses. Winter annual camelina genotypes may be economically and environmentally advantageous for the northern Corn Belt, but little is known about their agronomic potential for this region. A 2‐yr field study was conducted in western Minnesota to determine optimum fall sowing time for yield and oil content of two winter camelina cultivars in a no‐tillage (NT) and chisel‐plowed (CP) system. Seeding dates ranged from early September to mid‐October. Plants reached 50% flowering as much as 7 d earlier in the NT than the CP system. Plant stands were generally greatest in the NT system, but yields were only greater than those in the CP system during the second year of the study, possibly due to differences in water logging of soil between tillage systems. Seed yield and oil content increased with sowing date up to early October. When sown in October, seed yield and oil content ranged from 419 to 1317 kg ha −1 and 282 to 420 g kg −1 , respectively. Results indicate that camelina is a viable winter crop for the northern Corn Belt and that seed yields and oil content tended to be greatest when sown in early to mid‐October. Moreover, fall‐seeded camelina offered good weed suppression without the use of herbicide, supporting the contention that it can be successfully produced with low agricultural inputs.