The sustainability of grazing land depends on several factors, including nutrient cycling from short-term and long-term plant litter decomposition. The plant material that falls to the ground decomposes at different rates. Plant characteristics, such as the carbon (C) to nitrogen (N) ratio and lignin to nitrogen ratio, affect the process. So do the soil characteristics, like microbial biomass and diversity. UF/IFAS researchers at the Range Cattle REC in Ona, Fla., studied plant litter decomposition due to increasing levels of management intensification in grazing lands.

“More specifically, we wanted to investigate how plant litter quality and soil C and N stocks affected short-term decomposition,” said Marta Kohmann, postdoctoral associate at the REC.

“We used the litterbag technique to estimate the 168-day decomposition of saw-palmetto, which represents the majority of biomass in native rangelands, and of bahiagrass and rhizoma peanut, two important forage species in Florida” she explained. “We did that in two coarse-textured Coastal Plain soils. Those were similar in texture, but had contrasting C and N concentrations and microbial characteristics.”

Furthermore, they evaluated the 120-day soil C and N changes after the addition of plant litter and of N fertilizer.

Results

The study produced several key findings. One is that the soil characteristics had no effect on litter decomposition.

“In other words, litter chemical characteristics are the main drivers of plant decomposition in the systems we evaluated,” said Maria Silveira, professor of soil chemistry and nutrient management in the UF/IFAS Soil and Water Sciences Department. “And as native vegetation is substituted by forage species with more favorable chemical composition – particularly lower C/N and lignin/N ratios and lower proportion of insoluble in total N – the decomposition rate increases.”

Another finding from the 168-day evaluation was a net N immobilization in saw-palmetto. That means soil organisms take up nitrate and ammonium, making them unavailable to crops. Meanwhile, bahiagrass and rhizoma peanut showed N net mineralization, the process by which microbes decompose organic N to ammonium.

“We also found that the addition of plant litter did not change soil C or N concentrations,” said Joao Sanchez, postdoctoral associate at the REC. “This indicates that nutrient addition via plant litter was not sufficient to cause detectable changes in soil characteristics in our short-term study.”

Moreover, N addition did not change the total soil C concentration. However, it promoted an increase of C distribution towards more stable forms in the soil.

“For producers, this indicates that increase in management intensity through the adoption of forage species and N fertilizer promotes nutrient cycling, increasing plant litter decomposition and N mineralization in coarse-textured soils,” Kohmann added.

The research was recently published in the journal Agrosystems, Geosciences & Environment. You can find the full article here: https://doi.org/10.1002/agg2.20075

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