Creating Positive Soil and Ecosystem Carbon Budgets.

Soil carbon (C) pool, comprising of 1550 Pg of organic and 950 Pg of inorganic components, impacts global C cycle by either being a source or sink of atmospheric CO₂.  Most agricultural soils, with lower soil organic C (SOC) pool than those under natural ecosystems, can sequester atmospheric CO₂ as humus through adoption of those land use and soil management practices which create a positive C budget. Soil C sequestration occurs when the input of biomass-C exceeds the output, and depletion when output exceeds the input.  The input of C includes crop residues, root biomass, cover crop, compost, manure, sludge, deposition etc.  The output of C includes losses due to erosion, decomposition/mineralization and leaching.  The rate of mineralization is determined by mean annual temperature and precipitation, soil texture, clay mineralogy, landscape position, etc. Soil and crop management practices which impact C-input and C-output include tillage, residue management, cover cropping, crop rotation, nutrient management including manuring and fertilizers, soil amendments such as biochar etc.  Rather than linking soil C sequestration to soil-specific management (e.g., no-till, nitrogen fertilizer, biochar, crop rotations), it is important to assess soil C budget in relation to predominant processes.  Similar to the soil C budget, the ecosystem C budget is assessed through complete life cycle analysis of all input and output, including the hidden C costs of farm operations (e.g., tillage, fertilizers, pesticides, harvesting, drying, etc). Conducting life cycle analysis and evaluating soil C budgets are essential to determining whether soil is a source or sink of atmospheric CO₂.