Biochar and Nitrogen Fertilizer in Paddy Soil: Study Results

What changes when biochar is added to a rice field already receiving nitrogen fertilizer? A two-year field study in China examined that question by measuring changes in soil, microbes, and rice yield.¹

The treated soil held more carbon and supported more microbial biomass. Several of the biochar treatments also produced more grain.¹

A field test of biochar and fertilizer

The researchers compared eight combinations of biochar and nitrogen fertilizer. They used four biochar rates, including a no-biochar comparison, and two nitrogen rates. Biochar went into the soil once at the start of the study. Fertilizer was added in both growing years.¹

The biochar was made from cassava straw. The team tested it in a rice field with slightly acidic soil.¹

More carbon and life in the soil

Compared with plots that received nitrogen alone, the biochar-treated plots had higher soil pH, soil carbon, and total nitrogen. The soil was also less dense. Together, these changes point to a more supportive growing environment for rice.¹

The researchers measured more microbial biomass in several biochar treatments. They also found changes in the mix of soil bacteria and fungi. Soil microbes help cycle nutrients and break down organic matter. Biochar’s effects on soil conditions can therefore matter to the living system around plant roots.¹

These findings add a field example to the broader picture in Benefits of Biochar for Agricultural Soils. That article explores biochar’s roles in soil structure, nutrient management, and soil life.

Higher rice yields in several treatments

In both years, several treatments combining biochar and nitrogen produced more rice than nitrogen alone. Three treatments using 20–30 tonnes of biochar per hectare produced about 32% more grain in 2019 and 24–28% more in 2020 than the lower-nitrogen treatment without biochar. Two of those three treatments also received more nitrogen than the comparison, so these percentages reflect differences in the treatment combinations, not biochar alone.¹

The study recorded changes in soil carbon, nutrients, and microbial life alongside higher yields, without establishing which changes caused the yield gains. Biochar did not take the place of fertilizer. It worked alongside it. That partnership is also the focus of another Learning Center article, “Is Biochar a Fertilizer?”