Tropical grasses: feed and plumber
The pastures that cattle graze also act as their “toilets”. This is because, as cattle eat grass, they periodically urinate and, therefore, randomly deposit urine on the soil surface. Once in the soil, the deposited urine results in the creation of patches that are generally characterized by high concentrations of nitrogen.
As the concentrations of nitrogen on these patches can be as high as 1 ton of nitrogen per hectare, these “toilets” can be considered to be “hotspots of nitrogen”. Nitrogen, an essential plant nutrient, is often first used by soil microbes and then taken up by plants. However, if concentrations of nitrogen are too high, as is the case when cattle urine is deposited on soil, the nitrogen transformed by soil microbes is lost to the environment (beyond the reach of plants), where it can cause serious damage (including climate change).
The roots of tropical grasses, such as the Brachiaria humidicola, produce exudates (chemical compounds) that can constrain the microbial transformation of soil nitrogen from a stable form (organic or ammonium) to a highly unstable form (nitrate) ─ a process known as biological nitrification inhibition (BNI). The latter (nitrate) is vulnerable to loss via leaching into ground water or as nitrous oxide (N2O, a potent greenhouse gas). Soil N2O losses are popularly described using a mechanistically-based conceptual model – the “hole-in-the-pipe” model, which suggests that nitrogen lost as N2O may be likened to a leaky “toilet”.
A recent study conducted on a long-term experiment at CIAT, under the LivestockPlus project funded through the CGIAR Research Programme on Climate Change, Agriculture and Food Security (CCAFS) (with additional support from the United States Agency for International Development (USAID), the University of California Davis Research for Innovation Fellowship in Agriculture, and the University of Hohenheim), tested the efficacy of root exudates from B. humidicola to reduce N2O emissions from cattle urine deposited in grazed pastures. Findings from this study show that B. humidicola indeed constrains the transformation of nitrogen on cattle urine patches and, consequently, reduces N2O emissions. This study, which was conducted during the dry season, showed a 60% reduction in cumulative N2O emissions from urine deposited on pastures with high BNI capacity compared to those with low BNI capacity over a one month period. The mechanisms driving these emissions reductions are described in detail in “Biological nitrification inhibition by Brachiaria grasses mitigates soil nitrous oxide emissions from bovine urine patches” (https://dx.doi.org/10.1016/j.soilbio.2016.12.029).
This work suggests that grasses such as B. humidicola can serve the dual role of providing nutritious animal feed and improving nitrogen-use efficiency on cattle “toilets” by mending the “holes in the pipe”. Moreover, the fact that having grasses such as B. humidicola means that the “toilet” and the “plumber” are located in the same place, which implies that the amounts of N2O produced in grasslands grazed by cattle can be reduced without farmers incurring additional costs. Therefore, in tropical countries, increasing the area under grasses such as B. humidicola will contribute towards improving farm productivity, employment creation along the cattle value chain (i.e., labour for grassland management, meat/milk processing and sales), and achieving emissions reduction targets set under the 2015 Paris Climate Agreement, among other benefits.
Acknowledgements: The work presented in this blog is based on research findings in an article (https://dx.doi.org/10.1016/j.soilbio.2016.12.029) co-authored by Ryan C. Byrnes, Jonathan Núñez, Laura Arenas, Idupulapati Rao, Catalina Trujillo, Carolina Álvarez, Jacobo Arango, Frank Rasche, and Ngonidzashe Chirinda.