- Author: Mike Hsu
Light irrigation before flooding stimulates microbes to remove nitrates from soil
With California enduring record-breaking rain and snow and Gov. Gavin Newsom recently easing restrictions on groundwater recharge, interest in “managed aquifer recharge” has never been higher. This process – by which floodwater is routed to sites such as farm fields so that it percolates into the aquifer – holds great promise as a tool to replenish depleted groundwater stores across the state.
But one concern, in the agricultural context, is how recharge might push nitrates from fertilizer into the groundwater supply. Consumption of well water contaminated with nitrates has been linked to increased risk of cancers, birth defects and other health impacts.
“Many growers want to provide farmland to help recharge groundwater, but they don't want to contribute to nitrate contamination of the groundwater, and they need to know how on-farm recharge practices might affect their crops,” said Matthew Fidelibus, a University of California Cooperative Extension specialist in the UC Davis Department of Viticulture and Enology.
A recently published study by UC scientists sheds new light on how nitrates move through an agricultural recharge site and how growers might reduce potential leaching. Researchers analyzed data from two grapevine vineyards at Kearney Agricultural Research and Extension Center in Fresno County – one flooded for two weeks, and other for four.
Understanding initial nitrate levels crucial
A key factor in mitigating contamination is understanding how much nitrate is in the soil at the outset, said study author Helen Dahlke, a UC Davis hydrologist and leader of UC Agriculture and Natural Resources' strategic initiative on water. In areas with little precipitation and cropping systems that require greater amounts of synthetic fertilizer, the accumulation of residual nitrate – resulting from nitrogen in the fertilizer not taken up by the plants – can be quite high.
“The percentage of nitrates in some soils can really increase over the years, particularly if you have many dry years in a row where you don't have access to irrigation water or natural precipitation flushing some of those nitrates out of the soil,” Dahlke said.
While intense rains in recent weeks have helped dilute nitrate concentrations naturally, farmers looking to participate in recharge during the dry years ahead should consider flooding their fields with greater volumes of water.
“If you're doing this for the first time – on-farm recharge in the winter – check your residual soil nitrate levels because if they're very high, you should apply a lot of water in order to make sure that the residual nitrate is diluted down,” said Dahlke, who also added that growers should check their soil properties for suitability of recharge projects.
She recommended using, as a “good first approximation,” the online Soil Agricultural Groundwater Banking Index map, a project led by Toby O'Geen, a UC Cooperative Extension soil resource specialist.
Researchers looking at other ways to reduce nitrates
Even before flooding the fields for recharge, there are several practices that can lower initial nitrate levels and risk of leaching. Cover crops such as alfalfa and triticale, for example, can help take up residual nitrates that accumulate from fertilizing a main crop over time.
Dahlke and Fidelibus – a co-author of the San Joaquin Valley vineyard study – both pointed to pre-flooding irrigation that encourages denitrification, a process in which soil microbes transform nitrates into gaseous forms of nitrogen.
“Those denitrifying microbes need to be stimulated to do the work,” said Dahlke. “What we have found is that if you do a little bit of irrigation before you start the flooding, increasing the soil moisture can get those microbes started and they can take out more nitrate from the soil.”
The timing and quantity of fertilizer applications are also major factors in reducing leaching. Although more growers are following high-frequency, low-concentration practices to maximize uptake by crops, Dahlke said there needs to be more emphasis on incorporating nitrogen transformation processes – such as denitrification – in the nutrient management guidelines that farmers follow.
“Implementing thoughtful nutrient management plans will play a particularly important role in participating farms,” Fidelibus added.
A more holistic view of groundwater recharge
In short, choices made during the growing season can affect those in the winter recharge season – and vice versa. For example, applying compost or other organic amendments to soil can give microbes the “fuel” they need for sustained denitrification.
“What we have found is that our denitrifying bacteria often run out of steam because they don't have enough carbon to do the work,” Dahlke said. “Like us, microbes need energy to do the work, and for microbes this energy comes from soil carbon.”
Then, adding moisture via recharge to that field with high organic content can stimulate mineralization and nitrification, processes in which microbes transform the organic nitrogen into ammonium – and subsequently nitrates – that the plants can then take up. Those naturally occurring nitrates would thus reduce the need for the grower to apply synthetic fertilizer.
“The winter on-farm recharge experiments have shown that altering the moisture regime in the winter has consequences for the nitrogen budget in the summer growing season,” Dahlke explained. “Theoretically, what we need to be doing is better integrating both seasons by keeping an eye on the soil-nitrogen balance across the whole year so that we can ensure, at the end of the growing season, the residual nitrate in the soil is minimized.”
The study, published in the journal Science of The Total Environment, was part of the post-doctoral work of former UC Davis researcher Elad Levintal. In addition to Fidelibus and Dahlke, other authors are Laibin Huang, Cristina Prieto García, Adolfo Coyotl, William Horwath and Jorge Rodrigues, all in the Department of Land, Air and Water Resources at UC Davis.
/h3>/h3>/h3>/h3>- Author: Mark Bolda
While most of the strawberry growing community is nestled around the warm fire roasting chestnuts, drinking hot toddies and reflecting on the past year, I've been managing a fairly large amount of communication concerning plant dieback which is almost certainly attributed to high nitrate accumulation from pre-plant fertilizers. Those queries which are accompanied by soil analyses say as much with EC's above four, and nitrates well above the 40 ppm what I would see to be of concern, with one sample even setting my heart racing with a stratospheric print of 220 ppm.
This has been discussed pretty thoroughly in this space before, so I simply provide the links here:
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=6538
and a closer analysis of the issue, including soil samples, here:
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=6608
We are not looking at getting much benefit from rain for a while, so it's time to run the overhead sprinklers to leach all of this stuff out if high nitrates are the issue in a plant dieback scenario.
- Author: Janet L. White
In most California homes there are few commodities more precious, or more taken for granted, than clean tap water. We use it without hesitation for drinking, cooking and washing produce.
![120px-Flash photo shower faucet Watr200 120px-Flash photo shower faucet Watr200](http://ucanr.org/blogs/food/blogfiles/4004.jpg)
However, recent news reminds us that not everyone can take clean water for granted. In rural California, where some households rely on well water, up to 2 million people have been exposed to unhealthy levels of nitrates in their water at some time during the last 15 years.
Synthetic fertilizers used in agriculture are major contributors of nitrates to our water, but a growing number of farmers are taking steps to reduce this problem by adopting micro and drip irrigation technologies, and by cultivating noncrop vegetation (buffer strips and cover crops) and “constructed wetlands.” All these measures reduce not only nitrates but many other pollutants from runoff.
"Nitrate problems start when applied fertilizer moves outside the root zones of plants," says Toby O'Geen, UC Davis soil resource specialist. "If nitrate reaches the root zone of actively growing crops, much of it is taken up by roots. But because of its high mobility, nitrate occurring outside the root zones can be transported by irrigation or rain as surface or subsurface runoff. "
Technologies like drip irrigation deliver water and fertilizer precisely to root zones. From 1991 to 2001, Califiornia growers decreased surface irrigation (furrow and flood) about 28 percent and increased sprinkler and drip/microirrigation about 28 percent (Orang et al 2008).
In addition, growers are using buffer strips, cover crops, vegetated waterways and constructed wetlands to purify water, reduce erosion, and remove hazardous contaminants such as pesticides, metals, pathogens and fertilizers. The Farm Bill funds some of these programs through the Environmental Quality Improvement Program and the Wetland Reserve Program.
"Farmers receive technical advice and funding to take marginal land out of production and put it into noncrop vegetation and creation of habitat, including constructed wetlands," O'Geen said.
The effectiveness of such measures depends on the site and the design, but they are probably the best way to reduce water pollution from runoff. (Taking regulatory action against any particular grower is not practical because the sources of nitrate pollution are widespread and occur regionally. Even isotope "fingerprints" often give mixed results because sources mix in the environment.)
Another way that agriculture helps remove pollutants of all kinds, including not only fertilizers and pesticides but pathogens, is through natural filtering by rangelands, including oak woodlands. (See details of how oaks help nitrogen cycling.) More than two-thirds of California's drinking-water supply passes through or is stored in oak woodlands. In fact, rangeland in California (land suitable for grazing livestock) comprises more than half of the state's 101 million acres and typically is not fertilized. It forms our major drainage basins – filtering the drinking water for millions of people. (see California Agriculture).![Border strips, shown on a Central Coast farm, <br> help improve water quality by filtering. Border strips, shown on a Central Coast farm, <br> help improve water quality by filtering.](/blogs/blogcore/blogfiles/4001.jpg)
help improve water quality by filtering.