Abstract
The soil carbon (C) saturation concept suggests an upper limit to the storage of soil organic carbon (SOC). It is set by the mechanisms that protect soil organic matter from mineralization. Biochar has the capacity to protect new C, including rhizodeposits and microbial necromass. However, the decadal-scale mechanisms by which biochar influences the molecular diversity, spatial heterogeneity, and temporal changes in SOC persistence, remain unresolved. Here we show that the soil C storage ceiling of a Ferralsol under subtropical pasture was raised by a second application of Eucalyptus saligna biochar 8.2 years after the first application—the first application raised the soil C storage ceiling by 9.3 Mg new C ha−1 and the second application raised this by another 2.3 Mg new C ha−1. Linking direct visual evidence from one-, two-, and three-dimensional analyses with SOC quantification, we found high spatial heterogeneity of C functional groups that resulted in the retention of rhizodeposits and microbial necromass in microaggregates (53–250 µm) and the mineral fraction (<53 µm). Microbial C-use efficiency was concomitantly increased by lowering specific enzyme activities, contributing to the decreased mineralization of native SOC by 18%. We suggest that the SOC ceiling can be lifted using biochar in (sub)tropical grasslands globally.
Introduction
Human activities risk releasing 260 Pg of ecosystem carbon (C) as carbon dioxide (CO2) globally that is irrecoverable on a timescale relevant to avoiding profound climate impacts1,2. Agriculture contributes a major part, releasing an average of 2 Mg C ha−1 y−1 from soil globally3,4,5. Plants release ~50% of photosynthetically fixed C into the soil, which supports microbial growth and metabolism, including respiration that produces CO2. It has been estimated that 122 Mg soil organic C (SOC) ha−1 to a depth of 1 m has been lost over 1 Mha of land converted to tropical grasslands6, with 40% of this area occurring on Ferralsols7. The grand challenge humanity now faces is to urgently reverse this loss of SOC and associated decline in soil health by increasing the amount of C retained in soil5,8,9.
The Intergovernmental Panel on Climate Change (IPCC) has identified that substantial CO2 removal will be required to limit global warming to 2 °C. To this end, the IPCC has identified soil C management4,5,6 and the application of biochar10 as carbon dioxide removal (CDR) methods11 with considerable potential, with corollary benefits of improving soil health, sustaining agricultural productivity12,13, and increasing the resilience of ecosystem services14,15. Protecting and rebuilding soil C could sequester 5.5 Pg CO2 y−1, representing 25% of the potential of natural climate solutions to deliver CDR through conservation, restoration, and improved land management practices6,16,17.
Application of biochar is a recognized CDR method because of its persistence9,11 in the environment. The pyrolysis of biomass can deliver bioenergy, as well as agronomic and non-CO2 greenhouse gas benefits through the use of biochar as a soil amendment18,19,20,21,22. Biochar systems generally show life-cycle climate change impacts of net emission reduction in the range of 0.4–1.2 Mg CO2 equivalent Mg−1 dry feedstock23, through C persistence and avoided non-CO2 emissions. The capacity for biochar to further contribute to mitigation by protecting and building SOC is often overlooked.