Enhanced rock weathering application on sub-tropical agroecosystems for carbon sequestration and sustainable crop production

Achieving increased or sustained crop yields while minimizing environmental impact is a pressing challenge in agricultural science. Enhanced Rock Weathering (ERW) has emerged as a novel negative emission technology that accelerates natural geological processes of carbon (C) sequestration by applying crushed silicate rocks, specifically basalt, to croplands. However, field-scale evaluations to demonstrate carbon sequestration potential and agricultural co-benefits have been limited. This study quantitatively assessed the carbon and nitrogen (N) flow resulting from basaltic rock application in sugarcane and upland rice cultivation in a sub-tropical agroecosystem.

Two types of experiments were conducted on Ishigaki Island, located in the subtropical zone of Japan, where the annual rainfall and annual temperature were 2,500 mm and 24.0 ℃, respectively. Firstly, an outdoor field experiment on sugarcane cultivation was conducted. The following six treatments were applied with four replicates using a completely randomized block design: bare soil (no sugarcane), control without any amendment, lime application, manure application (10 Mg C ha⁻¹), basaltic rock application (10% by weight), and a combination of manure and basaltic rock. Soil water at the 0.6 m depth, which we defined as leaching water, was collected from all plots using the porous cup method and subjected to dissolved C and N analysis. Secondly, an indoor lysimeter experiment was conducted on upland rice cultivation to understand the comprehensive flow of C and N, including greenhouse gas emissions. The following four treatments were assigned with three replicates to 12 concrete lysimeters with 2 m² and 1 m depth: control without any amendment, torrefied plant residue (1% by weight), basaltic rock application (10% by weight), and a combination of torrefied plant residue and basaltic rock.

In the outdoor field experiment, continuous monitoring of soil volumetric water content and electric conductivity showed that the weathering of basaltic rock was enhanced by solid-liquid contact. Soil pH increased by the basaltic rock application the same as lime application. Similarly, the acidification of leaching water was buffered in the basaltic rock application treatments. Consequently, the inorganic carbon concentration in leaching water was higher in the basaltic rock application treatments than in the control and lime application treatments. Although there were no significant differences in the plant height of sugarcane, the number of leaves and Single Photon Avalanche Diode (SPAD) increased by basaltic rock application. Similarly, the indoor lysimeter experiment observed higher pH and dissolved inorganic C concentrations in leaching water. Although carbon dioxide flux increased by basaltic rock application in the first month, it was decreased in the crop growing period. These results indicated that the ERW application has a C sequestration potential and co-benefit on crop production. The annual C and N budget and further discussion will be presented in the session.