Soil C and N changes following cessation of cultivation in semiarid soils is not well understood. We hypothesized that returning cultivated fields in southeastern Wyoming to perennial grasses through the Conservation Reserve Program (CRP) would (i) increase labile pools of soil organic matter (SOM), and (ii) increase small-scale heterogeneity of SOM. Carbon and N in labile and passive pools of SOM were measured in CRP fields seeded with perennial grasses intermediate wheatgrass (Elytrigia intermedia [Host] Nevski ssp. intermedia), pu- bescent wheatgrass (Elytrigia intermedia [Schur.] A. Love ssp. barbu- lata) and smooth brome (Bromus inermis Leysser), and in winter wheat (Triticum aestivum L.)-fallow fields. Mineralizable C increased from 0.37 g m~2 d-1 in wheat-fallow fields to 0.99 g m~2d-1 in CRP fields; mineralizable N and coarse particulate C were consistently but not significantly higher in CRP fields. Fine particulate and total soil C and IN were not significantly different between CRP and wheat-fallow. Within CRP fields, mineralizable C was significantly higher under grasses than in interspaces (1.96 vs. 0.73 g m-2 d-1, respectively), and mineralizable N and coarse particulate C and N were consistently but not significantly higher under grasses than in interspaces. Soil C and N have increased only slightly after 6 yr of CRP management, and future changes in land use management on these CRP fields, including grazing and cropping, may accrue some small benefits associated with improved soil fertility status.
