05 April 2009

Farm Nitrogen Balance for CA

I am working on an article for the upcoming Nitrate issue of Southwest Hydrology. One of the big picture numbers that I have been following is the statewide farm nitrogen balance in California. Here is the back-of-the-envelope computation that I am coming up with - somebody correct me if I am way wrong:

The average farm acreage with fertilizer use (1980-1984) was 6.066 million acres with an average of 595,000 tons of N fertilizer sold (Norman, Hargett, Berry, Fertilizer Summary Data, National Fertilizer Development Center, TVA, Alabama), which amounts to 197 lbs/acre. A similar number was reported by Jeffrey Swanson and Dale Dahl (on p.24): 217 lbs/acre. The crop acreage with fertilizer use in the 2007 Ag Census was somewhat higher(6.728 million acres), while fertilizer use has leveled off since the 1980s. For 2007, nitrogen farm use was 740,000 tons (CDFA). Total irrigated crop acreage in the 2007 Ag Census was 8 million acres, of which 740,000 acres are pasture and other (non-crop) land.

The fertilizer use efficiency in California is reportedly on the order of 44% (my 2002 notes refer to "DANR Bulletin 1887, Table 9"). Assuming that it has improved to roughly 50%, this leaves over 100 lbs N/acre for leaching. If 25% of that goes to volatilization and denitrification, average nitrogen fertilizer leaching on California farmland amounts to something on the order of 75 lbs N/acre/yr. At an average recharge rate (under irrigation) of 1 acre-foot/acre/yr, this is nearly 30 mg/L of nitrate-nitrogen in recharge. This estimate is somewhat higher than what has been measured - on average - in some agricultural groundwater monitoring networks.

Dairy cows. They are the biggest animal industry in the state. Some 1.8 million milking cows as of last year, producing 41.2 billion lbs of milk (yes, I drink a lot of it - and do it my share of cheese), which contains 3.15% protein or 100,000 tons of nitrogen. The amount of nitrogen excreted by a milking cow is approximately three times more than that going to milk. In other words, a milking cow has a 25% nitrogen efficiency - 75% of the N consumed goes out the back end.

Hence, adult milking cows (including non-lactating or dry cows) produce about 300,000 tons of nitrogen. Then there is about as many support cattle (calves, heifers) as there are milking cows, which produce another 100,000 tons of nitrogen - a total of 400,000 tons of manure nitrogen.

We obtain the same number based on average estimated excreted nitrogen: about 1 lbs/day for a milking cow (300 days per year), 0.45 lbs/day for non-lactating adult cows (65 days per year), and the average for support stock is about 0.33 lbs/day. At 1.8 million adult cows and 1.8 million support cattle, this also adds to 400,000 tons of N excreted.

In a recent UC report, we estimated that 20%-40% of the N volatilizes in the production area (flush lanes, dry lots, lagoons). Hence, 240,000 tons manure N are applied to field crops. The same report estimated that another 25% either volatilizes or denitrifies after application to field forage crops. That leaves 180,000 tons of manure N for plant uptake or leaching to groundwater (most dairies are located on relatively flat land and must minimize runoff losses).

The 2007 Ag Census reports that 645,000 acres of farmland are treated with manure. At 180,000 tons manure N applied, that is 750 lbs N/acre/year. A double-crop of summer corn and winter grain will take up about 400 - 450 lbs N/acre/year. Which - on manured dairy forage crops - leaves at least 300 lbs N/acre/year for leaching to groundwater (not including any additional fertilizer). In an acre-foot of recharge that is more than 100 mg/L nitrate-nitrogen. This is consistent with our groundwater monitoring data in Merced and Stanislaus County, where we measured an average of 64 mg/L of nitrate-nitrogen under dairy forage crops in highly vulnerable groundwater conditions and with average recharge rates closer to 2 acre-feet/acre. We are seeing much lower nitrate-nitrogen concentrations (around 20 mg/L nitrate-N) in the upper-most groundwater under dairies with much less vulnerable conditions in Tulare and Kings County - groundwater levels are nearly 100 feet below ground surface and manure is spread over larger acreages (relative to the number of cows).

(Note that 27 lbs N in 1 acre-foot of water equals 10 mg/L nitrate-N).

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