Selective preservation of pyrogenic carbon across soil organic matter fractions and its influence on calculations of carbon mean residence times

The long-standing perspective that recalcitrance of soil organic carbon (SOC) controls its stability and persistence has shifted to one in which physical inaccessibility of SOC to microorganisms plays a predominant role. This paradigm shift has been facilitated by analytical techniques that isolate SOC into physical fractions protected from decomposers by different mechanisms. The correlation between these fractions and SOC age has reinforced the emphasis of SOC inaccessibility. Pyrogenic C (PyC; also called charcoal or black carbon), which has been thermally altered by fire, is known to contain highly recalcitrant components that decompose very slowly and could represent an exception to this paradigm shift. We employed hydrogen pyrolysis to quantify the contribution of PyC to total SOC across soil fractions from three long-term agricultural experiments with land use conversions that caused reductions in SOC. We show that all soil fractions contain PyC and up to one-fifth of SOC in soil fractions considered to have low accessibility is comprised of PyC. Regardless of the soil fraction in which it was located, PyC was relatively unaffected by land use conversion compared to biogenic C (organic C not altered by fire), which suggests that selective preservation, rather than physical protection, is the dominant mechanism limiting PyC decomposition in these sites. We accounted for PyC in calculations of C mean residence times (MRTs) using differences in stable C isotope ratios between PyC and SOC. Though results varied by site and soil fraction, MRTs for biogenic C were generally shorter than for total SOC. Based on these results, PyC decomposition is controlled by a different mechanism than biogenic C, and this should be considered in studies of soil C dynamics. In addition, methods based on physical fractionation alone may place too great an emphasis on the role of inaccessibility for long-term SOC persistence

Greenhouse gas mitigation in agriculture

Agricultural lands occupy 37% of the earth's land surface. Agriculture accounts for 52 and 84% of global anthropogenic methane and nitrous oxide emissions. Agricultural soils may also act as a sink or source for CO2, but the net flux is small. Many agricultural practices can potentially mitigate greenhouse gas (GHG) emissions, the most prominent of which are improved cropland and grazing land management and restoration of degraded lands and cultivated organic soils. Lower, but still significant mitigation potential is provided by water and rice management, set-aside, land use change and agroforestry, livestock management and manure management. The global technical mitigation potential from agriculture (excluding fossil fuel offsets from biomass) by 2030, considering all gases, is estimated to be approximately 5500–6000 Mt CO2-eq. yr−1, with economic potentials of approximately 1500–1600, 2500–2700 and 4000–4300 Mt CO2-eq. yr−1 at carbon prices of up to 20, up to 50 and up to 100 US$t CO2-eq.−1, respectively. In addition, GHG emissions could be reduced by substitution of fossil fuels for energy production by agricultural feedstocks (e.g. crop residues, dung and dedicated energy crops). The economic mitigation potential of biomass energy from agriculture is estimated to be 640, 2240 and 16 000 Mt CO2-eq. yr−1 at 0–20, 0–50 and 0–100 US$ t CO2-eq.−1, respectively