1a. Objectives (from AD-416):
The long-term research objective of this project is to develop and translate fundamental agroecological knowledge into recommendations and products to improve the economic position of organic farmers and to improve their ability to meet consumer demand for organic products. Objective 1 is to develop component technologies and management strategies that lead to improved productivity, enhanced soil and water conservation, and efficient nutrient cycling on organic farms. Objective 2 is to understand agroecological principles that drive the function of organic cropping systems and quantify ecosystem services.
1b. Approach (from AD-416):
Approaches to developing component strategies include A) incorporating legumes into organic crop rotations to maximize nitrogen fixation, B) composting that provides a productive and safe amendment for organic agriculture, and C) optimal agronomic practices for managing nutrients and production on organic farms. Approaches to determining agroecological principles include investigating the following variables within the Beltsville long-term Farming Systems Project that compares two conventional and three organic rotations, A) crop performance, B) soil nitrogen dynamics in relation to nitrogen inputs, C) soil carbon sequestration and greenhouse gas flux, D) soil biological community structure in relation to soil quality and production performance, and E) soil erosion and nutrient loss potential.
3. Progress Report:
Field experiments to evaluate animal manures and OMRI-approved fertilizers as sources of K and P in organic forage production are in their third year. We have cloned and sequenced five key flowering control genes from eight representative hairy vetch genotypes that differ in flowering time, information that could be used by crop breeders. The automated electronic control system for aeration supply for an in-vessel composting system was brought up to a fully operational status and two test batches were completed. Field studies comparing survival of generic and non-pathogenic strains of E. coli in soils managed organically and conventionally were initiated with poultry litter, dairy manure solids, dairy manure liquid and horse manure to compare baseline responses before composts made of these materials are introduced. Cacao pod husks, an organic residual typically left in the field after cacao beans are harvested, were evaluated as a potential source of compostable feedstock for use in producing an organic fertilizer for cacao farmers. Data are being collected for a third year in permanent weed-free and weedy check plots in organic and conventional systems in the FSP. We are monitoring population and community dynamics and weed-crop competition. A germinable assay is being conducted to determine long-term persistence of weed seedbanks. A University of Maryland student has completed the experimental phase of a project evaluating the influence of long-term soil management on weed-crop competition relationships. Two Master’s students at the University of Maryland are compiling 14 years of data to compare nutrient budgets (C,N,P,K) and evaluate carbon and nutrient management among cropping systems at the long-term FSP. A Brazilian PhD student is characterizing soil carbon fractions collected from diverse FSP systems using near infrared, mid infrared and pyrolysis spectra as part of our effort to evaluate mechanisms of soil carbon sequestration. A scientist from Brazil (EMBRAPA), in association with colleagues at the University of New Hampshire, is comparing output from the DNDC model with two years of soil N2O emissions data to evaluate model predictions. A University of Maryland Master’s student is evaluating the impact of cover crop mixes (rye, hairy vetch) and manure sub-surface banding on soil N2O emissions. The lead scientist is collaborating with scientists from Princeton University to compare a laser based method of measuring atmospheric N2O concentrations with the standard GC method. The lead scientist is collaborating with scientists from the University of Maryland to compare N2O emissions from organically-managed eggplant grown using four different cover crop practices. A visiting Brazilian PhD student is measuring soil N2O emissions following application of a conventional and a newly-designed slow-release nitrogen fertilizer. A high school intern is characterizing short term dynamics of N2O emissions from soil to better characterize factors controlling N2O emissions in the field.
1. Corn grain yield in a novel cover crop-based, organic rotational no-till corn production system is similar to that in traditional tillage-based systems. Although demand for organic grains is high, traditional tillage-based organic corn production practices have restricted the ability of existing producers from expanding their operations due to high labor requirements to manage weeds. Conventional no-till farmers considering transitioning to organic practices are concerned about the amount of tillage required for organic production due to risks of lowering soil quality. Novel rotational no-till organic systems may help alleviate these barriers to expansion of organic acreage. A hairy vetch cover crop coupled with high residue cultivation can provide both weed management and fertility requirements for corn. In a 3-yr field experiment on certified organic land, ARS researchers at Beltsville, Maryland, demonstrated that crop planting date had little effect on corn yield in either tilled or no-tilled treatments. Based on plots that received supplemented weed seedbanks, weed competition was the major driver determining corn yield in the no-till system, with 50-60% reductions in yield. With high-residue cultivation, yield reduction was still unacceptable at 25%. This work demonstrates to farmers that organic corn grain yield in reduced tillage systems can be similar to that in tillage-based systems when weed seedbanks are not large, and that there is a need to diversify weed management across the entire crop rotation to ensure optimal corn yields. This research should help organic farmers reduce labor costs while increasing the conservation benefits of organic corn production.
2. Emissions of N2O, a greenhouse gas, may be reduced by up to 50% in the central and eastern U.S. by reducing N fertilizer application rate to an economic optimum, reducing the proportion of high nitrogen demanding crops within a rotation, and possibly long-term no-tillage. While agricultural soils are the dominant source of nitrous oxide (N2O), a greenhouse gas and catalyst of stratospheric ozone decay, we have a very limited understanding of the impact of various management practices on N2O emissions or on emissions of CH4, another important greenhouse gas. ARS scientists from Beltsville, Maryland and Ames, Iowa summarized recent research findings from the central and eastern U.S., including previously unpublished data from Beltsville, to show that reducing N fertilizer application rate to an economic optimum, reducing the proportion of high nitrogen demanding crops within a rotation, and possibly long-term no-tillage could reduce N2O emissions by up to 50% in the central and eastern U.S. There is insufficient and/or inconsistent evidence indicating whether other proposed N2O mitigation strategies—nitrogen fertilizer and manure timing, placement or source selection, nitrification inhibitors, delayed release fertilizers, or cover crops—provide any mitigation, or what management practices might reduce CH4 emissions or increase CH4 uptake. Additional studies, including long-term studies incorporating sampling strategies that better capture temporal and spatial variability of GHG fluxes and factors controlling this variability, are needed to better quantify the impact of cropland management on GHG emissions. Results show that substantial additional research is needed to provide policymakers with recommendations about best management practices to reduce soil N2O and CH4 emissions from cropland soils.
3. Soil nitrogen availability in no-till soils increases following initiation of no-till such that test results may not be readily interpretable without considering whether test values have equilibrated or not over time. The influence of no-till (NT) management on soil nitrogen fertility is poorly defined. Many different soil testing procedures have been proposed and tested in search of a useful nitrogen fertility test but a robust solution to this important problem has remained elusive. Scientists from ARS, along with colleagues from Virginia Tech, sampled soils from farmer fields that had been in continuous NT for varying lengths of time and analyzed them using diverse soil nitrogen tests. Results indicate that a new equilibrium level for various nitrogen tests had not been reached after 11 years. These results indicate that absolute values for labile soil nitrogen tests may not be readily interpretable with respect to soil fertility recommendations without considering whether these values are changing over time or are at an equilibrium level. In addition, different patterns of change for different tests indicate that these nitrogen fractions are distinct and may provide different types of information with respect to quantifying changes in soil fertility with time under no-till. Results will help soil scientists develop useful soil nitrogen fertility tests, a goal that has proven elusive for many decades.
4. Cover crops may negatively influence vegetable germination: allelopathic effects of cover crop residues on vegetable crops. Cover crop residues often release allelochemicals (chemicals that reduce germination and seedling growth of plants) that help reduce weed pressure in cropping systems. These chemicals may also inhibit establishment of cash crops because they are not specifically targeted to weedy plants. Sunhemp is a tropical legume with significant potential for use as a cover crop in the southeast. Its residues have been shown to suppress weeds but their effects on cash crops are relatatively unknown. Scientists from the Sustainable Agriculture Systems Laboratory in collaboration with University of Georgia scientists evaluated the effects of sunhemp residues and cereal rye residues on weeds and vegetable crops in greenhouse and growth chamber experiments. In the greenhouse study field dried sunnhemp residues reduced germination of lettuce and smooth pigweed (a weed) to the same degree as rye residues. Sunhemp leaves had a greater effect compared to roots. In a second study, water extracts of sunhemp leaves significantly inhibited germination of bell pepper, tomato and onion and reduced turnip and okra germination by 50% while germination of sicklepod, a common weed in southern cropping systems, was not affected by the extracts. In addition, root elongation of lettuce, carrot, smooth pigweed and ryegrass was negatively affected by sunhemp leaf extracts. These results indicate that caution should be used when cropping some vegetables following sunhemp but the residues can also help reduce weed pressure.
5. Biochar source and production conditions influence change in soil water holding capacity. Biochar is produced during the pyrolysis of biomass for the manufacture of biofuels. When added to soil, biochar has potential for long-term sequestration of atmospheric CO2 (the dominant greenhouse gas contributing to global warming) because it is very resistant to microbial decomposition. Additional benefits of biochar include improving soil nutrient and water availability. However, there is sparse information identifying feedstocks and pyrolysis conditions that maximize these characteristics. ARS scientists from Florence, SC, Beltsville, MD, and New Orleans, LA along with university colleagues evaluated the effects of several biochars pyrolyzed from different feedstocks at two temperatures on soil-water storage capacity. In general, biochar amendments enhanced the moisture storage capacity of Ultisols and Aridisols, but the effect varied with feedstock selection and pyrolysis temperature. Switchgrass biochars resulted in the greatest increase in potential water holding capacity in the soils compared with the controls. Further work is needed to determine the optimum rates of biochar to provide economically useful improvements in water availability.
6. Automatic water level and rate measurement systems need frequent verification to avoid erroneous data. Use of digital electronic hardware and software to automatically monitor environmental variables can improve efficiency of field measurements. However there are few scientific evaluations of field performance, accuracy, reliability and limitations of such instruments. We used submerged pressure transducers connected to data loggers to automatically record water levels in streams, rivers, canals, etc., to estimate water flow rate, an important variable in water quantity and quality investigations. Electronically monitored flow rates from a spring and pond at the ARS facility in Watkinsville, Georgia were used to evaluate data integrity over a two year period. Data integrity was often but not always sufficient. An extreme case was a period during which average flow rate was underestimated by 17% or underestimated by 29% for the spring, and underestimated by 27% for pond outflow. Users should not assume field sensors, and pressure transducer-based automatic water level measurement systems in particular, continuously record data correctly. Routine verification of transducer output is crucial to avoid inaccurate (and possibly useless) water level and flow rate estimates.
7. Soil N2O emissions may be impacted by soil microbial community structure. Agricultural soils are the dominant source of atmospheric nitrous oxide (N2O), a greenhouse gas catalyst of stratospheric ozone decay. Nitrous oxide emissions are controlled by environmental conditions and soil denitrifying bacteria. Whether differences in denitrifying bacterial community structure impact soil N2O emissions is largely unknown but may be an important factor that is not accounted for in N2O emissions models. This research showed that season (spring, summer or fall) and crop plant species (wheat vs. soybean) are the main determinants of denitrification gene abundance. Cropping system, on the other hand, was the main determinant of denitrification gene phylogeny, meaning that regardless of season or crop species the denitrification genes were more closely related within any particular system than among systems. Results provide a foundation for further investigations into a potential important mechanistic link between soil bacterial community structure and function. This information may eventually be integrated into landscape level models used to predict N2O emissions.Cavigelli, M.A., Parkin, T.B. 2012. Agricultural management and greenhouse gas flux: cropland management in eastern and central US. In: Liebig, M.A., Franzluebbers, A.J., Follett, R.F., editors. Managing Agricultural Greenhouse Gases. London, England: Academic Press. p. 177-233.