Yet some experts say the partnership has devolved into a classic neo-colonial relationship in which China has the upper hand. Nearly 84 percent of Brazil’s exports to China last year were raw materials, up from 68 percent in 2000. But about 98 percent of China’s exports to Brazil are manufactured products — including the latest, low-priced cars for Brazil’s emerging middle class — that are beating down Brazil’s industrial sector.

“The relationship has been very unbalanced,” said Rubens Ricupero, a former Brazilian diplomat and finance minister. “There has been a clear lack of strategy on the Brazilian side.”

While visiting China last month, Brazil’s new president, Dilma Rousseff, emphasized the need to sell higher-value products to China, and she has edged closer to the United States. “It is not by accident that there is a sort of effort to revalue the relationship with the United States,” said Paulo Sotero, director of the Brazil Institute at the Woodrow Wilson International Center for Scholars. “China exposes Brazil’s vulnerabilities more than any other country in the world.”

China’s moves to buy land have made officials nervous. Last August, Luís Inácio Adams, Brazil’s attorney general, reinterpreted a 1971 law, making it significantly harder for foreigners to buy land in Brazil. Argentina’s president, Cristina Fernández de Kirchner, followed suit last month, sending a law to Congress limiting the size and concentration of rural land foreigners could own.

Mr. Adams said his decision was not a direct result of land-buying by China, but he noted that huge “land grabs” in Latin America and sub-Saharan Africa, including China’s attempt to lease about three million acres in the Philippines, had alarmed Brazilian officials.

“Nothing is preventing investment from happening, but it will be regulated,” Mr. Adams said.

A World Bank study last year said that volatile food prices had brought a “rising tide” of large-scale farmland purchases in developing nations, and that China was among a small group of countries making most of the purchases.