What was done
The authors measured vertical profiles of nitrate nitrogen in a number of U.S. desert soils to depths of several meters.

What was learned
Walvoord et al. discovered, in their words, that "a large reservoir of bioavailable nitrogen (up to ~10⁴ kilograms of nitrogen per hectare, as nitrate) has been previously overlooked in studies of global nitrogen distribution."  This amount of new nitrogen, they say, "raises estimates of vadose-zone nitrogen inventories by 14 to 71% for warm deserts and arid shrublands worldwide."  Viewed another way, the 2,000 to 10,000 kilograms of nitrogen per hectare they find at depth vastly overshadows the 25 to 250 kilograms of nitrogen typically applied each year to the surfaces of agricultural fields by farmers.

What it means
In our Editorial of 21 May 2003, The Inexorable Greening of Earth's Arid Lands, we briefly review the history of the "greening of the earth" concept that was developed several years ago by Idso (1982, 1986), wherein he postulated a "reverse desertification" phenomenon that promotes vegetation encroachment upon the planet's deserts as a consequence of the dramatic increase in plant water use efficiency that is being brought to pass by the ongoing rise in the air's CO2 content (see Water Use Efficiency in our Subject Index).  Although the gradual relaxation of the water deficit aspect of this concept is easy enough to understand, there has been continued concern over the nutrient deficit aspect; for the world's deserts have long been thought to be notoriously lacking in nitrogen, and in their upper reaches indeed have been (West and Skuijins, 1975).  From whence, therefore, the concern went, will the needed nitrogen come?
Well, wonder no more.  The deep reservoir of bioavailable nitrogen, as Walvoord et al. refer to it, is a "potential bonanza," in the words of Stokstad (2003), just waiting to be tapped.  Indeed, Stokstad says Duke University ecologist Robert Jackson "wonders if the pool of nitrate could help explain why deep-rooted woody plants have invaded the Southwest over the past century or so."  Whether it can or can't [see Trees (Range Expansion) in our Subject Index], the worldwide encroachment of deep-rooted woody plants upon the world's arid lands that has accompanied the development and progression of the Industrial Revolution will provide the physical pump required to transport the long-hidden nitrogen from several meters' depth to the soil's surface layer, where all plants may share in the blessings it provides, as the greening of the earth continues.

References
Idso, S.B. 1982.  Carbon Dioxide: Friend or Foe?  IBR Press, Tempe, Arizona, USA.

Idso, S.B. 1986.  Industrial age leading to the greening of the Earth?  Nature 320: 22.

Stokstad, E. 2003.  Unsuspected underground nitrates pose a puzzle for desert ecology.  Science 302: 969.

West, N.E. and Skuijins, J.J.  1975.  Nitrogen in Desert Ecosystems.  Dowden, Stroudsburg, PA, USA.