114. Until the 1990s, the principal objectives of managing SOM in agricultural production systems were those related to improving soil fertility for increasing agronomic productivity. Therefore, SOM (SOC) concentrations and temporal changes were measured in the plow layer or the zone where roots of seasonal crops are concentrated. In this regard, SLM included cropping systems (rotations, sequences, combination, agroforestry measures), tillage methods (NT, reduced tillage, conservation tillage), surface cover (residue mulching, cover cropping), and nutrient management (compost, manure, biological N fixation, mycorrhizae, fertilizer) options which enhanced and sustained SOC concentration in the root zone. The latter has been measured in the units of percentage or weight basis (g/kg, mg/kg). With growing interest since 1990s in using soils as a sink for atmospheric CO2, SOM enhancement has acquired multi-functional considerations. As a C sink, soil depth of interest is 1-2 m rather that just the root zone. The labile SOC fractions and their dynamics (mineralization to release plant nutrients) are important to plant growth and agronomic productivity. In contrast, recalcitrant fractions and their permanence (long residence time without leakage) are important to addressing CC. Rather than the narrow interest in the plot-scale data to manage soil fertility using precision farming for crop production, it is the changes in the SOC pool at the watershed, regional or national scale which are relevant to off-setting anthropogenic emissions for mitigating CC. Thus, appropriate units of measurement are kg/ha, kg/ha/yr, Gt C/yr, etc. In addition to adoption of SLM options at farm scale, the goal is to accrue benefits by adoption at regional, watershed or national scale. It is important, therefore to scale up SLM options to regional and national scales for C sequestration. The need for upscaling of C sequestration activities, however, necessitates understanding and improving: (i) the process of SCS in relation to soil quality, (ii) methods for assessment of soil C at landscape scale in a cost-effective and credible manner, and (iii) predicting soil C pool at different scales. Indeed, improving soil quality through increase in SOC pool is an appropriate adaptation-mitigation win-win strategy, because it affects one of the largest terms of the global C balance through exchange of C between soils and the atmosphere (Gardi and Sconosciuto, 2007) while enhancing food/fiber production. Successful synergetic adaptation-mitigation SLM strategies are those which improve local livelihoods and strengthen resilience of target communities while making soils/ecosystem a net sink for atmospheric CO2 (Cerri et al., 2007a, b; Mauere et al., 2008).
