Catherine,
Thank you for your questions. The overall and range in crop N uptake at the Cook Farm appears to be similar to what is seen in other comparable agricultural systems. A global estimate reported crop N uptake of ~33% (of the N applied, which can be used as an estimate of nitrogen use efficiency). It is common to see ranges in crop N uptake due to the specific crop and environmental conditions (from less than 33% to over 75% at the Cook Farm). Reducing the amount of N added to the system is the main way in which greenhouse gas emissions are being reduced in agricultural systems. However, this must be done without a reduction in yield. Research at the Cook Farm is looking at a management strategy to reduce N fertilizer additions by looking at site-specific nitrogen requirements of the crop. This will require understanding the ecosystem drivers impacting nitrogen need/use at the different “microsites” across the field. For example, preliminary work I am involved in has identified areas in the field where yield is more limited by water rather than nitrogen. Nitrogen rate trials are in place to determine if we can reduce N additions in these areas and still maintain or even increase yield.

Thank you, Tabitha.
Catherine

The best nitrogen management strategy for any farm is to meet the crops demands at the right time. For crops planted in the fall, like winter wheat, fertilizer applications can be split between a fall application and spring application. This method allows the farmer to get a starter does of fertilizer in the fall to meet the early demands of the crop as well as a spring application at the beginning of the growing season. By splitting applications there is less nitrogen fertilizer applied to the soil prior to the onset of the winter precipitation.

Probably the best nitrogen management practice is the adoption of precision farming. This way instead of a one size fits all fertilizer application, farmers can meeting the demands of each zone of their farm to maximize production and minimize over application of fertilizer.

I forgot to mention, we have also compared water and nitrate fluxes for five years before and after planting a vegetated buffer strip above the tile drain. We found there was no statistically significant difference in water or nitrate fluxes attributed to the addition of the buffer strip. We believe this was because the majority (>90%) of the yearly water and nitrate fluxes occur from January through May when the buffer strip is not actively growing.

We could also consider using time-released fertilizers in the spring split application to improve availability of N during high crop N demand. This should work as long as we maintain enough surface soil moisture for the chemical conversion. In our rain-fed agricultural systems this can be a challenge (and unpredictable with year to year variability in precipitation amounts and timing).

The use of cover crops is gaining popularity in the region though there is little new research to understand how these will fit into our current agricultural systems. If cover crops can scavenge nitrogen before winter it could reduce N loss to the tile drains but this will not remove N that is deeper in the soil profile (and likely entering the tile drain). Great question.

My contribution to this project has been linking the hydrologic and nitrogen cycles, focusing on nitrate transport and leaching. In this area we have definitely seen some interesting temporal trends. From my earlier work we found a seasonal shift in the source of nitrate leached from the system. During the high-discharge season (January through May) we found the source was nitrification of reduced nitrogen fertilizer (ammonium/ ammonia). During the low-discharge season (June through December) the source was mineralized soil organic matter. From my current research the preliminary data is suggesting this switch between the two sources is controlled by soil hydrologic conditions and precipitation rates. During the high-discharge season the saturated soils allow for nitrate transport from the fertilizer zone (where nitrification of fertilzier is the dominant source of nitrate). During the low-discharge season, vertical transport from the fertilizer application zone decreases as soils dry out and the majority of water comes from the deeper soil/ shallow groundwater system where mineralization of organic matter is the dominant source of nitrate.

Thank you for your interest Dr. Lefticariu. Other temporal trends that affect N cycling in this system include crop selection and climate. The crop rotation impacts the amount or frequency of N fertilizer additions. No N fertilizer is added in years when the crop grown can make its own nitrogen (called nitrogen fixation and plants referred to as nitrogen fixers or legumes). Since N fertilizer is the main N input to the system it should impact the N cycle. As a rain-fed agricultural system, the amount of precipitation and timing can impact the ability of crops to use added nitrogen. The amount and timing of precipitation can vary from year to year and contribute to N loss pathways. Our next steps will include analyzing and quantifying how these relate to N cycle processes (transformations, losses, and pool sizes).

Year to year variability in leaching is very large, largely due to the variability in precipitation. For example, over the last decade yearly precipitation has ranged from ~380 mm to ~590 mm. The variability in precipitation has a huge impact on leaching rates, we have found for each water year the soil will store the first ~150 mm of precipitation before tile-drain discharge begins.

We are still learning about how storm frequency and intensity affect water and nitrate fluxes. This is our second water year with a flume and pressure transducer monitoring discharge at 15 minute intervals and an ISCO sampler collecting samples at weekly intervals and at 0.2 inch changes in flume water depth. From this we have found once the tile-drain discharge begins for the year, each precipitation event stimulates increased discharge within 12 – 48 hours, depending on previous soil water content conditions. We have also found that nitrate concentration also increased with increasing discharge during these events. Based on preliminary numbers we have found a few big storm-events can account for the majority of losses for the whole year, and are previous estimates (based on hand sampling weekly to bi-weekly) are probably a lower estimate.

Thank you Philomena. I am glad you enjoyed our “N” formation. It was harder to make than it looks!

Tony, I have worked with farmers on problems with nitrate leaching and water quality issues in the Southern WIllamette Valley of Oregon during a 3-month policy fellowship required by my IGERT program. I was actually quiet surprised, most of the farmers I worked with were well aware of the environmental consequences of over fertilization. The difference was, I worried about water quality first and they (the farmers) worried about wasting fertilizer first, either way we had the same goal. We both want to keep fertilizer in the soil where it can be used by crops.

Tony,
We have not yet presented this research to farmers but we will be later this summer at a field tour of the Cook Farm. My past experience with farmers has been that they talk about how the climate has been changing in the region (e.g., rain coming at a different time than historically, colder and more unpredictable spring temperatures) but on the other hand they indicate they do not believe in climate change. I tend to focus on rising levels of greenhouse gases when discussing this with farmers rather than climate change. It is an interesting and sensitive conversation to navigate. Some growers in our area are involved in C trading through building soil organic matter but I am not sure how they stand on climate change. Thanks for your post and feedback Tony.

Ken, Thanks for watching our video. Our IGERT faculty actually asked us to participate in this competition because we are in the process of writing a Carbon, Nitrogen, and Water budget paper for the Cook Farm. We still have a lot of number crunching to do, but we are making progress.

Ken,
Some of the collaborators are stream ecologists and they have been looking at stream nitrate levels as well as stream processing of nitrate (for example, denitrification). Now that we have the bigger picture we are hoping to dive into things in greater detail across the soil-atmospheric-aquatic continuum to more fully capture how N fertilizer management can be improved. We are hoping we can scale this up to the larger watershed that the stream ecologists have been studying. I look forward to sharing the future results with you.

Excellent. I’m looking forward to that paper. It would certainly be useful to be able to use your research to create a model that could be applied to different farms or watersheds throughout the region. And you’re right, Christopher, the way to a farmer’s heart is through his or her wallet. If we can show them that they are potentially losing 30% of their fertilizer to the environment perhaps they would be more willing to modify their application practices. Was the fertilizer on this particular farm primarily anhydrous ammonia applied in the fall?

Ken, The kind of fertilizer has changes over the years and timing of fertilizer depends on the crop. For the timing of fertilizer application, winter seeded crops like winter wheat is fertilizer in the fall. For the spring seeded crops like chickpeas, spring wheat, and spring barley fertilizer is applied in the spring. The Cook Farm is split into several areas with crops rotated though each region, so each year there is a section of winter wheat, spring what, chickpea, and sometimes barley.