PS 33-139 - CANCELLED - Cover crops: How does mixing legume and grass cover crops influence nitrogen release in subtropical sandy soils?

Background/Question/Methods

Cover crops are often an integral component of tropical agroecosystems. Environmental conditions in Florida, namely high rainfall, warm temperatures and coarse-textured soils, can create cover crop decomposition and nitrogen (N) mineralization patterns different from the rest of the US. Quantifying the decomposition and nutrient mineralization rates and patterns in this particular environment is critical in predicting the nutrient credits usable by the subsequent crop. The litterbag technique was used to study the decomposition patterns of two grasses (sorghum sudangrass (Sorghum bicolor x S. bicolor var. Sudanese) and pearl millet (Pennisetum glaucum (L.) R. Br.) and two legumes (sunn hemp (Crotalaria juncea L.) and cowpea (Vigna unguiculata Walp.), and mixtures of 2 or 4 species. Litter mass loss was measured after accounting for ash content. This was complemented by an 8-week lab incubation conducted in the fall of 2020 after the termination of cover crops in summer. Incubation samples were terminated after 1, 2, 4 and 8 weeks, and net mineralization rate was measured as the buildup of inorganic organic N using colorimetry.

Results/Conclusions

Results indicate that litter mass loss in Florida was < 90% 6 months after litter incorporation. Mixtures enhanced decomposition and nitrogen release in mixtures, but the magnitude of this effect was timing and specie specific. Legumes had the highest N mineralization rate, followed by mixtures and lastly grasses. This pattern emerged as soon as cover crops were terminated and remained until the end of the incubations (8 weeks). Our results indicate that legumes in particular could provide N credits to farmers as soon as they are terminated, with N release continuing at a lower rate during later stages of decomposition. Cover crops can significantly provide nitrogen credits and contribute to the soil carbon pool in tropical agroecosystems.