How is the water content of the soil and the health of the soil increased without irrigation? Better cover it with a straw layer, concludes a new study.

Farmers in the Loess Plateau in China have used plastic and straw for decades as a cover for soil, or mulch, between crops. Much of the agriculture in the region, of which corn is a large part, is dry land; Crops depend only on seasonal rainfall. Drought periods can damage growth and crop yield. So the two mulches, both economical and readily available, are used to stop the loss of water by evaporation and keep the earth warm.

Although padding has been used since the 1970s, "no studies were being done on the effect of padding on soil quality, soil health and carbon sequestration," said Upendra M. Sainju. Sainju is a soil scientist at the USDA-ARS Agricultural Research Laboratory of the Northern Plains in Montana.

To investigate this, as well as the effects of mulch on corn yield, the scientists designed a five-year experiment comparing straw mulch, mulch and non-mulch. In their test plots at the Changwu Agro-Ecological Station, they covered the soil in May and eliminated the cover when the corn was harvested in October. They took soil samples at the end of each season to analyze the carbon. The scientists also compared maize yields between the different plots.

On average, maize yield was highest in plastic mulch, by 21 to 25 percent. Straw mulch also increased yield compared to bare soil, but only by five percent. What could be responsible for this difference? As the straw is rich in carbon, the decomposer microbes feed on free nitrogen in the soil to balance their diet. This takes nitrogen, a key nutrient from the plant, away from corn, which affects its yield.

However, the organic carbon of the soil was higher in the straw cover plots. This is desirable because carbon-rich soils are better at removing greenhouse gases from the air, said Sainju. Soils with a higher carbon content also contain more nutrients and have a structure similar to that of a cookie. Both qualities benefit the crops.

In the plastic cover plots, the extra water increased the microbial activity. Hungry microbes consumed organic matter from the soil, turning it into carbon dioxide in the process. Using soil carbon quickly reduces soil capacity to act as a carbon sink.

"We think that plastic mulch would be better because it increases soil moisture," Sainju said. "The result was surprising in the sense that if you have too much water, microbial activity increases, which can break down organic carbon."

The creation of carbon in the soil is an important long-term goal throughout the world, especially in China. Farmers in China can earn money with carbon credits when they improve the soil's capacity to store carbon. This makes the straw the winner, according to the researchers. Although straw mulch increased microbial activity using nitrogen, farmers can adjust the use of nitrogen fertilizers to help balance processes. This will increase the soil carbon and generate high yields.

Next, Sainju goes to test the mulches in a farmer's field. He adds that he also plans to help write a policy directive for the Chinese government on the benefits of mulching.

The study was published in the journal of the American Soil Science Society. The National Natural Science Foundation of China and the International Project for Cooperation and Scientific and Technological Change of Shaanxi Province, China, funded the project.

Source: SSSA