The retention of carbon (C) and nitrogen (N) in soil maintains soil fertility while withholding C- and N-based pollutants (e.g., CO₂, NO₃) from air and water.  This realization encourages conversion of land into uses wherein soil pools of C and N increase.  Whether such amendments are retained over the long term depends on whether they are stored in stable or labile forms.  In arid central Arizona, USA, total soil C and N pools are elevated (relative to the native desert) on human-modified lands (see JP Kaye presentation, this conference).  Here, we compare labile soil C and N among different paths of land transition; our 2 x 2 design examines the effects of land use history (desert vs. agrarian in 1912) and conversion status in 2004 (still in original 1912 use vs. converted to residential).

Our results suggest three conclusions.  (1) In arid biomes, anthropogenic land conversion produces potentially greater fluxes of C and N from soil.  Thus, (2) greater total soil C and N sequestration in arid-land agriculture is of uncertain benefit for drawing down atmospheric CO₂.  Finally, (3) classifying land by contemporary use for biogeochemical purposes is overly simplistic, as residential lands are extremely variable owing to differences in pre-urban land use.  Specific results follow.

Agrarian history and contemporary residential land use each increased the mass of labile C and N—respired CO₂ and leached C and N—in 1 y soil incubations.  Respired CO₂-C, leached dissolved organic C (DOC), and leached dissolved inorganic N (DIN) were greater by a multiple of 1.3-1.4 from soil with any agrarian history compared to soil with no agrarian history; leached DIC was unaffected by agrarian history.  Respired CO₂-C was greater by a multiple of 1.3 from residential soil compared to non-residential soil (i.e., soil still in original 1912 agrarian or desert use).  Leached DOC and DIN were unaffected by residential development.  Residential development and agrarian history interacted to influence leached DIC, which was greater by a multiple of 2 from residential soils than from non-residential soil if the 1912 land use was desert; present-day residential development had no effect on leached DIC if the 1912 land use was agrarian.

When considering the fate of soil C and N additions caused by land transition, it is perhaps more relevant to express labile C and N as proportions of total C and N.  While agrarian history resulted in elevated masses of respired CO₂-C and leached DOC and DIN, it had no effect when labile C and N are expressed proportions of total C and N (again, DIC is the exception, as 0.9% of C was lost as DIC from soil with agrarian history, while only 0.5% of C was lost as DIC from soil with no agrarian history).  Conversion of land use residential development reduced the labile proportion of soil C and N.  From residential soil, 6.8% of total soil C was lost as CO₂-C and 0.7% was lost as DOC, while 4.7% of total soil N was lost as DIN; respective values for non-residential soil are higher, at 9.4, 1.1, and 7.9%.