"Voluntary" does not mean free. The word voluntary can be deceptive. In this setting, "discretionary" would probably be a more appropriate term. Look here for one way to distinguish between mandatory, discretionary and voluntary incentives. Nevertheless, in this context, voluntary incentives imply an inducement is being offered so that land owner will chooses to comply rather than being required to comply. In the case of programs to reduce non-point pollution from agriculture, voluntary incentives are touted in a publication from the USDA for its potential success in an area where options are problematic.

The costs to the landowner are being compensated for in some way; usually the intent is that public funds (and taxpayers) will bear the costs. Conservation programs such as the Wildlife Habitat Incentives Program (WHIP) of the Natural Resources Conservation Service (NRCS), which creates 5 to 15 year agreements with landowners involving wildlife habitat improvements, encourages enrollment in the program by providing technical assistance and a 75% cost share on all land improvements. The program cost taxpayers just over $33 million dollars in 2005. Another voluntary conservation program administered by NRCS called the Wetlands Reserve Program (WRP) encourages ranchers and farmers to convert frequently flooded, marginal land to its former wetland condition. The WRP cost taxpayers nearly $240 million in 2005. A table below of some voluntary environmental or conservation programs and their estimated 2005 costs is provided below. Together these programs cost $3.5 billion in tax payer funds. Under a mandatory policy, these funds might be available for other uses, or could be returned to taxpayers in the form of lower tax rates.

References and Links:
Natural Resources Conservation Service Farm Bill 2002: Summary of NRCS Conservation Programs

Wildlife Habitat Incentives Program (WHIP) from the NRCS

Measuring the Success of Conservation Programs (USDA/ERS)

Effectiveness of Oregon's Voluntary Conservation Programs (Fish and Wildlife Service)

Voluntary Incentives for Reducing Agricultural Non-Point Source Water Pollution (USDA/ERS), Peter Feather and Joseph Cooper.