Alarmed by rapid deforestation in an ecologically sensitive swath of central Minnesota, state regulators have ordered a broad environmental review that will temporarily halt conversion of the region's jackpine stands to potato fields.

Natural Resources Commissioner Tom Landwehr said Thursday he's ordering the study of water and wildlife impacts because the bulldozing of trees and plowing of soil is happening over a permeable aquifer that could be polluted by fertilizers and depleted by crop irrigation.

Until the study's completion, which could take nine months to a year, the state won't consider dozens of permit applications for high-capacity groundwater wells submitted by potato processor R.D. Offutt Co. as it expands its already formidable footprint in the region.

Even then, the DNR could block further conversions if the review finds there are already severe threats to natural resources associated with the pristine Pineland Sands Aquifer and its overlying woods, Landwehr said. The area of concern touches Becker, Cass, Hubbard and Wadena counties.

"We simply have to get a better handle on what's happening with overall water use and water quality in this very, very, very important aquifer," Landwehr said.

Offutt declined to comment Thursday on Landwehr's announcement, but General Counsel Paul Noah said the company is acquiring land in an effort to reduce the environmental impact of potato cultivation.

Pine forests such as this one south of Park Rapids, Minn., have been cleared for potato fields. R.D. Offutt Co. has already bought about 12,000 acres of land.

"Our intention in acquiring land is to allow for increased crop rotation as part of our ongoing commitment to sustainable farming practices," Noah said. He said Offutt, the nation's largest potato grower, is committed to preserving groundwater and surface water quality in the communities where it farms. In Minnesota, Noah said, the company is participating in a Byron Township water quality study in partnership with the DNR and other agencies. The first-of-its-kind study was launched last fall and will benefit communities and farmers statewide and elsewhere, he said.

The Star Tribune reported Sunday that the pines-to-potatoes conversion is part of a bigger, mostly invisible transformation in the watershed that drains into the Upper Mississippi River, a basin that supplies drinking water for 1.7 million people in the Twin Cities. Since 2006, about 275 square miles of natural land in the Upper Mississippi watershed has been converted to row-crop agriculture, according to a University of Minnesota analysis — much of it sandy soils and forests where no one ever expected to see farming.

The DNR estimates that North Dakota-based Offutt already has purchased about 12,000 acres of pine forests in northern Minnesota for conversion to irrigated cropland. A third of that land already has been converted and won't be affected by the DNR review. Observers say the forest loss in the four counties could expand to include 42 square miles depending on how much more land Potlatch, the giant wood products manufacturer, sells to agriculture-minded buyers.

The DNR controls permitting of high-capacity groundwater wells that Offutt and other farmers need for irrigation of the sandy soils in the region. The agency notified Offutt by letter Thursday that the review, known as an Environmental Assessment Worksheet, will preclude construction of any projects that depend on groundwater well approvals.

To date, the DNR has issued 32 irrigation permits to Offutt, and the company has proposed an additional 54.

The DNR's letter defined "construction" as "any activity that directly alters the environment, including the preparation of land."

On Thursday, Landwehr applauded Offutt for adopting techniques that produced "amazing" decreases in nutrient and pesticide use. But he said the magnitude of the company's land conversions in Minnesota demanded an examination of the cumulative effects.

The DNR said nitrate contamination in water, from fertilizer, is difficult to avoid when growing potatoes in sandy soils and that contamination could hurt groundwater, connected surface waters, fish, and other aquatic species. The cumulative volume of water being consumed also is a concern, as some municipalities in the region have already had to invest in deeper wells.

Landwehr also said the environmental worksheet will consider impacts of deforestation on the area's wildlife. He said jackpine stands are a rare forest type in Minnesota, home to a number of unique species, including the goshawk and Blanding's turtle.

Tony Kennedy • 612-673-4213