Search

Net CO₂ exchange data of legume crops at 17 flux tower sites in North America and three sites in Europe representing 29 site-years of measurements were partitioned into gross photosynthesis and ecosystem respiration by using the nonrectangular hyperbolic light-response function method. The analyses produced net CO₂ exchange data and new ecosystem-scale ecophysiological parameter estimates for legume crops determined at diurnal and weekly time steps. Dynamics and annual totals of gross photosynthesis, ecosystem respiration, and net ecosystem production were calculated by gap filling with multivariate nonlinear regression. Comparison with the data from grain crops obtained with the same method demonstrated that CO₂ exchange rates and ecophysiological parameters of legumes were lower than those of maize (Zea mays L.) but higher than for wheat (Triticum aestivum L.) crops. Year-round annual legume crops demonstrated a broad range of net ecosystem production, from sinks of 760 g CO₂ m–² yr–¹ to sources of –2100 g CO₂ m–² yr–¹, with an average of –330 g CO₂ m–² yr–¹, indicating overall moderate CO₂–source activity related to a shorter period of photosynthetic uptake and metabolic costs of N₂ fixation. Perennial legumes (alfalfa, Medicago sativa L.) were strong sinks for atmospheric CO₂, with an average net ecosystem production of 980 (range 550–1200) g CO₂ m–² yr–¹.

Establishing cover crops in a corn (L.)–soybean (L.) rotation in northern climates can be difficult due to the short time between harvest and freezing temperatures. Aerial seeding into standing crops is one way to increase the time for germination and growth. Field studies were conducted to characterize the physical and chemical properties that affect winter rye (L.) establishment in corn and soybean, while a germination experiment was designed to determine optimal temperature and surface soil moisture content needed for successful germination. In the field study, 31 field-scale sites (22 corn and nine soybean) were aerially seeded in southeastern Minnesota during late August to early September 2009, 2010, and 2011. Aboveground biomass was collected before the ground froze, and multiple regression analysis was used to relate biomass to multiple soil and weather conditions. Total N uptake also was determined. Overall, precipitation the week after seeding was the most important factor in determining rye establishment, although our model accounted for only 43% of the variation in biomass. The germination study characterized winter rye germination on the surface of three different soils equilibrated to –50, –200, and –500 kPa water potential placed in three low-temperature incubators at 10, 18, and 25°C. Total germination was decreased by decreasing water potential in the sandy loam but not the clay or silt loam, suggesting that moisture content may be more important than water potential at the soil surface. Generally, germination was drastically reduced below a moisture content of 0.083 g g.

Interest in renewable energy sources derived from plant biomass is increasing, raising concerns about fuel vs. food competition. One strategy to produce additional cellulosic biomass without reducing food-harvest potential is to grow winter cover crops after harvest of the primary summer crop. This study estimated biomass accumulation of a fall-planted winter rye (L.) double crop across the United States on corn (L.)–soybean [ (L.) Merr.] croplands. We identified corn and soybean areas by county using National Agricultural Statistics Service data, excluding irrigated land and areas already supporting a winter small-grain crop. Within this area, we calculated biomass production after corn harvest and before the subsequent corn or soybean crop for 30 locations using RyeGro, a cover-crop simulation model. Average RyeGro biomass yields for a 14-yr period were used to develop a regression model based on temperature and precipitation, which was used to determine rye biomass potential in each county. The spatial analysis indicated that 7.44 Mha in continuous corn and 31.7 Mha in a corn–soybean rotation are suitable for producing winter rye. The average RyeGro biomass yield for the 30 locations for six planting–harvest date scenarios is 4.2 Mg ha. The regression modeling results projected that 112 to 151 Tg (120–170 million U.S. tons) of rye biomass can be harvested from this land base 14 to 7 d, respectively, before spring crop planting. The study demonstrates the sizable potential for this strategy to produce cellulosic biofuel feedstock without redirecting the primary food crop to fuel.

Recent proliferation of large dairies has prompted concern regarding environmental impacts of associated corn silage production and high-rate manure application. Our objectives were to compare environmental impacts and forage production of monocrop corn (L.) silage and rye (L.)–corn silage double-crop systems with multiple corn planting dates and high-rate manure application near Morris, MN. From 2007 to 2009, corn for silage was seeded into a silt loam as a monocrop in early and mid-May and as a double-crop after rye in mid-May and early June. Manure was fall applied annually at average total N and P rates of 393 and 109 kg ha, respectively. Double-cropping reduced total forage dry matter (DM) yield 2 of 3 yr and reduced corn DM yield 15 to 25%. Soil NO–N to 90 cm accumulated at an average rate of 71 kg N ha yr with monocropping, but accumulation was not observed with double-cropping. Soil organic C concentration from 0 to 5 cm increased in the monocrop (18%) and double-crop (26%) systems over 3 yr. Average soil solution NO–N concentration was high with monocropping (52 mg L) and double-cropping (37 mg L), but estimated leaching load averaged only 8 kg ha yr. Fall and spring ground cover was often less than 10% with monocropping but was usually greater than 30% with double-cropping. The primary environmental concerns identified for monocrop corn silage were soil NO–N buildup and inadequate ground cover. Double-cropping addressed each concern but often decreased forage production.

A winter rye (Secale cereale L.) cover crop can be seeded after corn (Zea mays L.) silage to mitigate some of the environmental concerns associated with corn silage production. Rye can be managed as a cover crop by chemical termination or harvested for forage. A field study was conducted in Morris, MN in 2008 and 2009 to determine the impact of killed vs. harvested rye cover crops on soil moisture and N0 3 —N, and to monitor the impact of the rye on subsequent corn yield. Corn for silage was seeded either after winter fallow (control), after a rye cover crop terminated 3 to 4 wk before corn planting (killed rye), or after a rye forage crop harvested no more than 2 d before corn planting (harvested rye). Soil moisture after killed rye was similar to the control, but after harvested rye was 16% lower. Available soil NO 3 ―N was decreased after both killed rye (35%) and harvested rye (59%) compared to the control. Corn biomass yield after killed rye was similar to the control, but yield following harvested rye was reduced by 4.5 Mg ha ―1 . Total forage biomass yield (silage + rye) was similar for all treatments. This work demonstrates that the environmental benefits of a winter rye cover crop can be achieved without impacting corn yield, but the later termination required for rye forage production resulted in soil resource depletion and negatively impacted corn silage yield.

Harvesting feedstock for biofuel production must not degrade soil, water, or air resources. Our objective is to provide an overview of field research being conducted to quantify effects of harvesting corn (Zea mays L.) stover as a bioenergy feedstock. Coordinated field studies are being conducted near Ames, IA; St. Paul and Morris, MN; Mead, NE; University Park, PA; Florence, SC; and Brookings, SD., as part of the USDA-ARS Renewable Energy Assessment Project (REAP). A baseline soil quality assessment was made using the Soil Management Assessment Framework (SMAF). Corn grain and residue yield for two different stover harvest rates (~50% and ~90%) are being measured. Available soil data remains quite limited but sufficient for an initial SMAF analysis that confirms total organic carbon (TOC) is a soil quality indicator that needs to be closely monitored closely to quantify crop residue removal effects. Overall, grain yields averaged 9.7 and 11.7 Mg ha -1 (155 and 186 bu acre -1 ) in 2008 and 2009, values that are consistent with national averages for both years. The average amount of stover collected for the 50% treatment was 2.6 and 4.2 Mg ha -1 for 2008 and 2009, while the 90% treatment resulted in an average removal of 5.4 and 7.4 Mg ha -1 , respectively. Based on a recent literature review, both stover harvest scenarios could result in a gradual decline in TOC. However, the literature value has a large standard error, so continuation of this long-term multi-location study for several years is warranted.

Kura clover (Trifolium ambiguum M. Bieb.) living mulch has potential to improve the environmental impact of corn (Zea mays L.) production, especially in the context of corn silage or stover harvest. Our objective was to determine the effects ofkura clover living mulch on the water balance and nitrate leaching under corn near Arlington, WI. Treatments in the 2.5-yr experiment (April 2006-November 2008) were N-fertilized no-till corn following killed kura clover as the control and no-till corn in living mulch with fertilizer rates of 0 and 90 kg N ha -1 yr -1 . Soil water storage was 37 to 50 mm lower under the living mulch than the control in the spring, while the control experienced 29 to 36 mm greater soil water depletion than the living mulch in the summer. Evapotranspiration was similar across treatments, except in May when it was greater under the living mulch by 11 to 41 mm. The living mulch did not appreciably reduce drainage. Nitrate-N storage in the soil profile and nitrate-N concentrations in the soil solution at 1-m depth were significantly reduced (p = 0.10) under both living mulch treatments relative to the control. Flow-weighted nitrate-N concentrations were 23 mg L -1 for the control, 17 mg L -1 for the living mulch with 90 kg N ha -1 yr -1 , and 6 mg L -1 for the living mulch with 0 kg N ha - 1 yr -1 . Total nitrate-N leached was reduced 31 and 74% relative to the control under the living mulch with 90 and 0 kg N ha -1 yr -1 . respectively.

Frontiers in any science are generally defined by measurement limitations, and that is especially true in environmental biophysics. Among the more persistent issues are surface–atmosphere exchange, soil water and solute fluxes, plant water status, and plant/soil nutrient status. Measurement of surface–atmosphere exchange is particularly critical to global climate change research. Despite advances in instrumentation, accuracy of flux measurements, particularly eddy covariance, remains unacceptable, partly because the underlying assumptions of stationarity and surface homogeneity are so restrictive. Even when these assumptions are valid, the method appears to systematically underestimate for reasons that are not yet well understood. Similarly, soil water and solute fluxes cannot yet be measured accurately and routinely, hampering water quality research. Recent advances in tension lysimetry offer hope for improvement, but most field experiments still rely on modeling of water and solute flow, supported by indirect measurements of ancillary variables, e.g., soil water content, soil water potential, and solute concentration, at discrete points in time and space. A third area of ongoing concern is that of plant water status. The major uncertainty here concerns which property should be measured. Nearly all of the effort over the past 30 yr has been directed at measuring water potential, but water potential measurements are equilibrium measurements, and plants operate in dynamic environments. Furthermore, many physiological processes appear to be more related to relative water content than to water potential. Finally, more accurate and more timely (e.g., in situ) measurements of plant/soil nutrient status are sorely needed to take advantage of the promise of precision agriculture.

Testing of simulation models against field-measured data is an important step that must be taken before a model can be used as a management tooL Field soil water contents, intensively measured by time-domain reflectometry (TDR) on a Zimmerman fine sand soil (Argic Udipsamments) in the Minnesota Management System Evaluation Area (MSEA), were used to evaluate the Root Zone Water Quality Model (RZWQM) over two rotated crop growing seasons in 1992 (corn, Zea mays L.) and 1993 [soybean, Glycine max (L.) Merr.]. The model was evaluated based on laboratory-measured soil hydraulic properties. Reasonable agreement exists between the daily TDR-measured and RZWQM-predicted water contents for the upper soil depths. However, the model overestimated the lower soil profile, leading to a consistent overestimation of the total water depth (TWD) in the entire root zone (0 to 150 cm) during both the 1992 and 1993 growing seasons. The maximum errors in TWD for the entire root zone were 2.47 and 2.77 cm, respectively, in 1992 and 1993. Predictions of solute transport by RZWQM were examined for three herbicides: atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2.4-di-amine], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], and metribuzia [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)]. Comparison between measured and predicted soil herbicide concentrations averaged over the top 15-cm layer showed that the model matched the temporal changes of the field measurements reasonably well, although the measurements showed that all pesticides were somewhat more persistent over time than the model predicted.

Data from agroecological experiments are typically stored in a collection of minimally documented computer files, with additional information entered into field or lab books. As a result of this fragmentation of data and lack of proper documentation, information may be lost and data manipulation is generally cumbersome, error-prone, and hard to automate. Modern database technology has the potential to resolve these issues. Storing experiment data in a database solves the problem of fragmentation because all data are in the database; the problem of documentation is solved by making the relations between different items of information explicit during the design of the data-base : and the problem of manipulation is solved by the powerful query languages available with modern database management systems. As a first step in the construction of a generally applicable database for use in agroecological research, we used a formal method to design a data model that explicitly describes the types of information (entities') one may want to remember about experiments and the relationships between these entities. The data model described here consists of 40 entities and 54 relationships. The entities are classified in five categories: (i) experiments, including statistical design; (ii) objects on which measurements are made; (iii) measurement protocol and equipment: (iv) measurements; and (v) field operations. We describe in detail how the information from several common types of measurements is stored using the proposed data model and conclude that the data model adequately describes the information that scientists in agroecological disciplines need to remember about their experiments.

With the goal of creating a generally applicable database for agroecological research data, we developed and described a data model. The objective of the present work was to determine the practical usefulness of a database implemented from that data model. We used a commercially available relational database management system and mapped the entities of the logical data structure to tables, entity attributes to table columns, and entity relationships to foreign keys. The attributes of the entity representing measurement values were distributed over two tables, both to reduce the size of the database and to reduce the response time of queries involving this entity. We found that loading data into the database was the most significant hurdle to its use, and so developed a set of stored procedures that function as a data input language. The input language and other methods were used to load the data from an intensively monitored, multisite, multiyear experiment into the database. The database was used to manage, explore, and analyze the data from these experiments, as well as to share the data between collaborators and with others in an effective way. We conclude that a database implemented from the previously designed data model is a practical and extremely useful tool for research.

Atmospheric CO 2 is important to plant growth and also plays a key role in the global energy balance. Thus, there are needs for reliable methods for measuring CO 2 concentrations, [CO 2 ]. One approacb has been the conductimetric method, where sampled gas is bubbled through deionized water. Some of the CO 2 in the air dissolves and ionizes, causing an increase in solution electrical conductivity. The method is inexpensive relative to other techniques, but usage has been limited, possibly due to its apparently empirical basis and suggestions that it must be frequently checked to correct for shifts due to temperature and other effects. We have derived the fundamental basis for the method and equations that allow [CO 2 ] to be directly determined from measurements of solution electrical conductivity and temperature, with no empirical calibration. The equilibrium constants and ionic conductivities that are used are temperature dependent, but those dependences are well known and easily computed. The approach was tested with a system in which the conductivity was measured with time-domain reflectometry (TDR), using a coaxial cell through which the aerated water was circulated. To maximize sensitivity, a long cell (1 m) of low impedance (19.3 Ω) was used. The system was compared against an infrared gas analyzer (IRGA) over a range of [CO 2 ] from 0 to 1000 μmol mol -1 , at three different temperatures (5, 19, and 34°C). Regression of [CO 2 ] calculated directly from the conductivity measurements against the IRGA measurements produced a slope of 1.00, r 2 of 0.997 and a standard error of estimate of 16.1 μmol mol -1 . Resolution was approximately 1 to 2 μmol mol -1 , too large for micrometeorological flux measurements but sufficient for many monitoring applications. The approach should work with any accurate device for measuring solution conductivity.

The use of net radiation as a critical variable in models of surface-atmosphere exchange is routine; however, its measurement is not. The discrete measurement of incoming and outgoing solar radiation with pyranometers and long-wave radiation with pyrgeometers is widely thought to be the most accurate method for estimating net radiation, but deployment at multiple sites is cost prohibitive. A recently developed single-sensor net radiometer differs from other commonly used net radiometers in that it has no domes covering the thermopile sensor. Long-term field data from this net radiometer were compared to reference net radiation calculated as the sum of the four independently measured components. The domeless net radiometer agreed well with the reference measurements, out-performing a similarly priced, widely used, domed net radiometer under most conditions. However, our analyses suggest that the domeless net radiometer has differing sensitivities to short and long-wave radiation, which is not accounted for in the calibration. Additionally, precipitation affected the domeless net radiometer much more severely than the domed model, which could be a significant limitation in areas with frequent rainfall. Despite these shortfalls, the domeless net radiometer evaluated here could be useful in a variety of field settings.

(...) When the potential gradient between center and outer compartments reached 1.0 MPa, the gamma readings showed overnight movement of water from wet to dry soil that could only have taken place through the root systems of the outer row plants. The overnight transfer of water, while significant, was not sufficient to fully supply the day-time transpiration demand of the unirrigated center row plants (...)