SOIL ORGANIC-MATTER RECOVERY ON CONSERVATION RESERVE PROGRAM FIELDS IN SOUTHEASTERN WYOMING

Soil C and N changes following cessation of cultivation in semiarid so ils is not well understood. We hypothesized that returning cultivated fields in southeastern Wyoming to perennial grasses through the Conser vation Reserve Program (CRP) would (i) increase labile pools of soil o rganic matter (SOM), and (ii) increase small-scale heterogeneity of SO M. Carbon and N in labile and passive pools of SOM were measured in CR P fields seeded with perennial grasses intermediate wheatgrass (Elytri gia intermedia [Host] Nevski ssp. intermedia), pubescent wheatgrass (E lytrigia intermedia [Schur.] A. Love ssp. barbulata) and smooth brome (Bromus inermis Leysser), and in winter wheat (Triticum aestivum L.)-f allow fields. Mineralizable C increased from 0.37 g m(-2) d(-1) in whe at-fallow fields to 0.99 g m(-2) d(-1) in CRP fields; mineralizable N and coarse particulate C were consistently but not significantly highe r in CRP fields. Fine particulate and total soil C and N were not sign ificantly different between CRP and wheat-fallow. Within CRP fields, m ineralizable C was significantly higher under gasses than in interspac es (1.96 vs. 0.73 g m(-2) d(-1), respectively), and mineralizable N an d coarse particulate C and N were consistently but not significantly h igher under grasses than in interspaces. Soil C and N have increased o nly slightly after 6 yr of CRP management, and future changes in land use management on these CRP fields, including grazing and cropping, ma y accrue some small benefits associated with improved soil fertility s tatus.