Publication

Active pools of soil organic matter (SOM) can recover to native levels on formerly cultivated fields that are abandoned for approximate to 50 yr, but the short-term (<10 yr) recovery dynamics of SOM and nutrient supply have not been widely investigated. In several fields on a farm in southeastern Wyoming that had been involved in the Conservation Reserve 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 compared potential C and N mineralization in active pools responsible for nutrient supply. The two CRP treatments, planted 6 yr prior to this study, were an approximate to 80% legume:20% grass mixture (HL CRP) and a 20% legume:80% grass mixture (LL CRP). To quantify SOM accumulations directly due to increased plant inputs within CRP fields, we also compared SOM pools under legumes and grasses relative to plant interspaces, where we expected plant inputs to be minimal. The net impacts of increased plant inputs and the cessation of tillage generally increased pools 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 in fallow), but had negligible effects on total SOM. Recovery of microsite (approximate to 10-cm scale) soil heterogeneity, an important structural attribute of native arid and semiarid eco systems, was accelerated under legumes, which produced more labile tissue than grasses. Soils under legumes contained larger pools of coarse-POM C and N and exhibited higher net N mineralization rates than soil under grasses or in plant interspaces. Grasses grown in HL CRP soils, which had the highest rates of potential net N mineralization, produced more labile tissue than the same grasses grown in the more nutrient-depleted LL CRP fields, suggesting that plant/soil feedbacks were important. Therefore, recovery 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 nutrition if these CRP fields are subsequently cropped or grazed.