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.

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.

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.

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.

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.