LEGUME, GRASS, AND CONSERVATION RESERVE PROGRAM EFFECTS ON SOIL ORGANIC-MATTER RECOVERY

Active pools of soil organic matter (SOM) can recover to native levels on formerly cultivated fields that are abandoned for approximate to 5 0 yr, but the short-term (<10 yr) recovery dynamics of SOM and nutrien t supply have not been widely investigated. In several fields on a far m in southeastern Wyoming that had been involved in the Conservation R eserve Program (CRP, a federal program that pays landowners to convert cultivated land into revegetated grasslands), we compared C and N in several SOM pools (coarse particulate organic matter [POM, between 500 mu m and 2 mm], fine POM [53-500 mu m], and total SOM), and we compar ed potential C and N mineralization in active pools responsible for nu trient supply. The two CRP treatments, planted 6 yr prior to this stud y, were an approximate to 80% legume:20% grass mixture (HL CRP) and a 20% legume:80% grass mixture (LL CRP). To quantify SOM accumulations d irectly due to increased plant inputs within CRP fields, we also compa red SOM pools under legumes and grasses relative to plant interspaces, where we expected plant inputs to be minimal. The net impacts of incr eased plant inputs and the cessation of tillage generally increased po ols of mineralizable and coarse-POM C and N by factors of two to four relative to wheat-fallow fields (alternate years in winter wheat and i n fallow), but had negligible effects on total SOM. Recovery of micros ite (approximate to 10-cm scale) soil heterogeneity, an important stru ctural attribute of native arid and semiarid eco systems, was accelera ted under legumes, which produced more labile tissue than grasses. Soi ls under legumes contained larger pools of coarse-POM C and N and exhi bited higher net N mineralization rates than soil under grasses or in plant interspaces. Grasses grown in HL CRP soils, which had the highes t rates of potential net N mineralization, produced more labile tissue than the same grasses grown in the more nutrient-depleted LL CRP fiel ds, suggesting that plant/soil feedbacks were important. Therefore, re covery of labile soil and plant N was enhanced when the proportion of legumes was high, and this may lead to improved grain or animal N nutr ition if these CRP fields are subsequently cropped or grazed.