- Author: Sara Tiffany
- Author: Dr. Martin Burger
Soil nitrate sampling protocol
For maximum accuracy that can reliably predict nitrate availability in the soil, growers should sample according to the following protocol:
- For fields with 60-inch beds: soil cores should be taken at 3 lateral distances from drip tape, in at least 4 locations within a field.
- For fields with 80-inch beds: soil cores should be taken 2 lateral distances from drip tape, in at least 3 locations within a field.
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Research on agricultural greenhouse gas emissions in tomatoes
- The adoption of subsurface drip irrigation substantially reduces greenhouse gas emissions in tomato production (compared to furrow irrigation).
- Use of nitrification inhibitors lowers nitrous oxide emissions in tomato fields with subsurface drip irrigation.
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UC Davis researcher Dr. Martin Burger presented the results of a survey conducted by post-doctoral scholar Cristina Lazcano on pre-plant nitrate, phosphorus (Olson-P), and exchangeable potassium levels in 16 processing tomato fields in Yolo, San Joaquin and Fresno counties. The purpose of the study was to develop an economical sampling protocol that reliably predicts nitrate availability and allows growers to adjust fertilizer rates taking the residual soil nitrate into account.
While the conversion to subsurface drip irrigation has enabled growers to precisely deliver water and nutrients close to plant roots, there is still pressure for growers to increase nitrogen use efficiency, for example to reduce the risk of nitrate leaching. Previously, the spatial distribution of macronutrients in fields under drip irrigation was not well known. One concern has been that nitrate may accumulate at the periphery of the wetted soil volume, whereas the less mobile nutrients phosphorus and potassium may be depleted near the drip tape where roots can be expected to proliferate.
According to the survey encompassing more than 1000 soil analyses, pre-plant nitrate levels in the 16 fields varied widely, ranging from 45 – 438 lbs NO3- - N per acre in the top 20 inches of soil, with higher levels of nitrate found in fields under consecutive tomato cultivation. No depletion of Olsen-P or potassium in the root feeding areas close to the drip tape was detected. The majority of the fields showed phosphorus concentrations lower than 15 ppm, which based on earlier research is the threshold below which a yield response can be expected from a P addition. In contrast, potassium levels were higher than previously reported values, ranging from 293 ppm on average in Yolo County to 468 ppm in Fresno County.
The nitrate sampling protocol was based on a Minimax analysis by selecting the minimum number of samples within the field and locations within the beds (i.e. lateral distance from the drip tape). The combination of samples with the lowest relative error across all fields (< 5% from the field average) and the lowest number of samples taken was selected as the best sampling procedure to estimate average soil NO3-N. The analysis showed that soil cores should be taken at three (60-inch beds) or two (80-inch beds) lateral distances in at least four (60-inch beds) or three (80-inch beds) locations within a field.
Table 1. Pre-plant nitrate sampling protocol for 60-inch beds in Yolo (Y), San Joaquin (SJ), and Fresno (F) County SDI tomato fields.
Table 2. Pre-plant nitrate sampling protocol for 80-inch beds in Yolo (Y), San Joaquin (SJ), and Fresno (F) County SDI tomato fields.
***The full article about this study will appear in the Oct-Nov-Dec 2015 issue of California Agriculture.
An update on agricultural greenhouse gas emissions research included results of field studies testing a nitrification inhibitor for mitigation of nitrous oxide in subsurface drip irrigated tomato.
Nitrous oxide (N2O) is arguably the most important greenhouse gas produced in the agriculture sector, with its global warming potential 300 times that of Carbon Dioxide. N2O is produced by soil microbes during N transformations. N2O is a by-product of nitrification and denitrification.
Recent studies have shown that N2O produced during nitrification can be as important as that resulting from denitrification (Zhu et al., 2013). The highest rates of N2O emissions typically occur shortly after N fertilizer applications when soils are re-wet. The main regulatory factor is the availability of oxygen since microbes use nitrate (denitrification) and nitrite (nitrification) as electron acceptors of respiration when oxygen is in short supply. Soil processes that consume oxygen, such as the presence of a carbon source, and conditions that limit replenishment of oxygen levels in the soil, such as high soil water content, promote N2O production in soil. Compacted soils lead to rapid depletion of oxygen because of the reduced air spaces and greater tortuosity of pathways of oxygen diffusion.
Although the use of the nitrification inhibitor significantly lowered nitrous oxide emissions in SDI tomato in one of the two years of the study, the reduction in absolute values is rather small (64 lbs carbon dioxide per acre) to make a significant contribution to California's greenhouse gas inventory. With the adoption of subsurface drip irrigation, tomato growers have already lowered the impact of greenhouse gas emissions from tomato production substantially as furrow irrigation generated leads to greater nitrous oxide emissions than SDI.
Zhu, X., Burger, M., Doane, T.A., Horwath, W.R., 2013. Ammonia oxidation pathways and nitrifier denitrification are significant sources of N2O and NO under low oxygen availability. Proceedings of the National Academy of Sciences of the United States of America 110, 6328-6333.