New findings from a team of U.C. Davis ecologists show that sedimentary bedrock fertilizes plant growth in a northern Californian forest, filling an ecological role previously thought to be reserved for bacteria. Graduate student Scott Morford and colleagues have found that the rock provides ecologically available nitrogen (N), an essential nutrient for plant growth. Since plants cannot use N in its most abundant form as an atmospheric gas, they rely largely on bacteria to ‘fix’ atmospheric N into a usable form in soil. Now, Morford’s team shows that plants can also use ancient N stored in rocks, unveiling a hitherto unaccounted for reservoir of fertilizer that promotes plant growth and carbon sequestration in forests.
The sedimentary rocks fertilizing Morford’s study site formed on ocean and lake floors roughly 140 million years ago during the Cretaceous period. As dinosaurs frolicked and battled around these bodies of water, layers of N-rich organic material slowly accumulated within bottom sediments. After millions of years of compaction, these sediments were exhumed as solid rock to form the modern forest floor. Over time, the rock has broken down enough to slowly release nitrogen into the soil.
Not all bedrock has the same fertilizing effect as this Cretaceous sedimentary rock. Igneous rocks, for example, come directly from the Earth’s interior and contain only a small fraction of the N available in sedimentary rocks. Morford compared plant productivity in a forest underlain by sedimentary rock versus a forest underlain by igneous rock and found that the sedimentary forest was 50% more productive and stored more carbon than the igneous forest. Since sedimentary rocks are extremely prevalent, covering 75% of the Earth’s surface, these findings have global implications for the carbon cycle and the capacity for forests to slow climate change through carbon sequestration.
Morford has yet to identify the rate at which the bedrock releases nitrogen, which will be important in understanding the full potential of sedimentary forests to store carbon in the future. Regardless, he has discovered a new reservoir of N that fundamentally changes the way scientists understand the global N-cycle and the ecological role of rocks in forests.
Posted by Laura Poppick, Assistant Editor of Maine Climate News
Image Description: photograph of study area, south fork mountain, CA