The Michigan NRCS has a website with program sheets for each of the different types of conservation practices that are included in the CRP program. These sheets define each practice, what to plant, how to manage, etc.  The site is: http://www.mi.nrcs.usda.gov/programs/crp.html

The linear habitats with the most acreage (U.S.) in the CRP program appear to be the riparian buffers and filter strips.  However, filter strips can also include buffers around wetlands which would not really be linear.  Also note that for most of the CRP programs farmers cannot receive any payments for remnant habitat - most enrolled land has to have been converted from cropland.

Riparian forest buffer (CP22): " A riparian forest
buffer is an area of trees and shrubs located adjacent to streams, lakes,
ponds, sinkholes and karst areas, or wetlands.  Riparian
forest buffers intercept sediment, nutrients, pesticides, and other materials
in surface runoff and reduce nutrients and other pollutants in shallow
subsurface water flow.  Woody vegetation in buffers provides food and cover for
wildlife, helps lower water temperatures by shading the water body, stabilizes
stream banks, and slows out-of-bank flood
flows. In addition, the vegetation closest to the water body provides litter
fall and large woody debris important to aquatic organisms. Some trees
established or managed in a riparian forest buffer can also provide timber,
wood fiber, and/or horticultural products after the CRP contract expires."

Filter
strips are typically located on cropland immediately adjacent and parallel to
streams, lakes, ponds, ditches, sinkholes, wetlands, or groundwater recharge
areas."