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The importance of building/maintaining soil carbon, for soil health and CO 2 mitigation, is of increasing interest to a wide audience, including policymakers, NGOs and land managers. Integral to any approaches to promote carbon sequestering practices in managed soils are reliable, accurate and cost-effective means to quantify soil C stock changes and forecast soil C responses to different management, climate and edaphic conditions. While technology to accurately measure soil C concentrations and stocks has been in use for decades, many challenges to routine, cost-effective soil C quantification remain, including large spatial variability, low signal-to-noise and often high cost and standardization issues for direct measurement with destructive sampling. Models, empirical and process-based, may provide a cost-effective and practical means for soil C quantification to support C sequestration policies. Examples are described of how soil science and soil C quantification methods are being used to support domestic climate change policies to promote soil C sequestration on agricultural lands (cropland and grazing land) at national and provincial levels in Australia and Canada. Finally, a quantification system is outlinedconsisting of well-integrated datamodel frameworks, supported by expanded measurement and monitoring networks, remote sensing and crowd-sourcing of management activity datathat could comprise the core of a new global soil information system. Take Home messages:Increasing soil organic carbon (SOC) stocks would improve the performance of working (managed) soils especially under drought or other stressors, increase agricultural resilience and fertility, and reduce net GHG emissions from soils.There are many improved management practices that can be and are currently being applied to cropland and grazing lands to increase SOC.Land managers are decision makers who operate in larger contexts that bound their agricultural and financial decisions (e.g. market forces, crop insurance, input subsidies, conservation mandates, etc.).Any effort to value improvements in the performance of agricultural soils through enhanced levels of SOC will require feasible, credible and creditable assessment of SOC stocks, which are governed by dynamic and complex soil processes and properties. This paper evaluates currently accepted methods of quantifying and forecasting SOC that, when augmented and pulled together, could provide the basis for a new global soil information system.

Soil, crop and fertilizer management practices may affect the amount and quality of organic C and N in soil. A long-term field experiment (growing barley, wheat, or canola) was conducted on a Black Chernozem (Albic Argicryoll) loam at Ellerslie, Alberta, Canada, to determine the influence of 19 (1980 to 1998) or 27 years (1980 to 2006) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [S Rem ]and straw retained [S Ret ]) and N fertilizer rate (0, 50 and 100 kg N ha -1 in S Ret and 0 kg N ha -1 in S Rem plots) on total organic C (TOC) and N (TON), and light fraction organic C (LFOC) and N (LFON) in the 0-7.5 and 7.5-15 cm or 0-5, 5-10 and 10-15 cm soil layers. The mass of TOC and TON in soil was usually higher in S Ret than in S Rem treatment (by 3.44 Mg C ha -1 for TOC and 0.248 Mg N ha -1 for TON after 27 years), but there was little effect of tillage and N fertilization on these parameters. The mass of LFOC and LFON in soil tended to increase with S Ret (by 285 kg C ha -1 for LFOC and 12.6 kg N ha -1 for LFON with annual rate of 100 kg N ha -1 for 27 years) , increased with N fertilizer application (by 517 kg C ha -1 for LFOC and 36.0 kg N ha -1 for LFON after 27 years), but was usually higher under CT than ZT (by 451 kg C ha -1 for LFOC and 25.3 kg N ha -1 for LFON after 27 years). Correlations between soil organic C or N fractions were highly significant in most cases. Linear regressions between crop residue C input and soil organic C or N were significant in most cases. The effects of tillage, straw management and N fertilizer on soil were more pronounced for LFOC and LFON than TOC and TON, and also in the surface layers than in the deeper layers. Tillage and straw management had little or no effect on C:N ratios, but the C:N ratios in light organic fractions significantly decreased with increasing N rate (from 20.06 at zero-N to 18.91 at 100 kg N ha -1 ). Compared to the 1979 results, in treatments that did not receive N fertilizer (CTS Rem 0, CTS Ret 0, ZTS Rem 0 and ZTS Ret 0), CTS Rem 0 resulted in a net decrease in TOC concentration (by 1.9 g C kg -1 ) in the 0-15 cm soil layer in 2007 (after 27 years), with little or no change in the CTS Ret 0 and ZTS Rem 0 treatments, while there was a net increase in TOC concentration (by 1.2 g C kg -1 ) in the ZTS Ret 0 treatment. Straw retention and N fertilizer application at 50 and 100 kg N ha -1 rates showed a net positive effect on TOC concentration under both ZT (ZTS Ret 50 by 2.3 g C kg -1 and ZTS Ret 100 by 3.1 g C kg -1 ) and CT (CTS Ret 50 by 3.5 g C kg -1 and CTS Ret 100 by 1.6 g C kg -1 ) treatments in 2007 compared to 1979 data. In conclusion, the findings suggest that retention of straw, application of N fertilizer and elimination of tillage would improve soil quality, and this might increase the potential for N supplying power of the soil and sustainability of crop productivity.

Long-term use of soil, crop and fertilizer management practices alters some soil properties, but the magnitude of change depends on soil type and climatic conditions. A field experiment with a barley (Hordeum vulgare L.)-wheat (Triticum aestivum L.)-canola (Brassica napus L.) rotation was conducted on a Gray Luvisol (Typic Cryoboralf) loam soil at Breton, Alberta, Canada. Effects of 19 or 27 years (from 1980 to 1998 or 2006 growing seasons) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [S Rem ] and straw retained [S Ret ]) and N fertilizer rate (0, 50 and 100 kg N ha -1 in S Ret , and 0 kg N ha -1 in S Rem plots) were determined on total organic C (TOC) and N (TON), light fraction organic C (LFOC) and N (LFON), macro organic matter C (MOM-C) and N (MOM-N), microbial biomass C (MB-C), and mineralizable C (C min ) and N (N min ) in the 0-7.5 and 7.5-15 cm or 0-5, 5-10 and 10-15 cm soil layers. Zero tillage and S Ret tended to have higher, and N fertilizer treatment usually had higher mass of TOC, TON, LFOC, LFON, C min and N min in soil compared to the corresponding CT, S Rem and zero-N control treatments, especially in the surface soil layers. Soil MB-C, MOM-C and MOM-N in soil generally tended to be higher with S Ret than S Rem , and also with N fertilizer than zero-N. There was no additional beneficial effect of ZT in increasing MB-C in soil. There were close and significant correlations among most soil organic C or N fractions, except for MB-C which did not correlate with MOM-N, and N min did not correlate with MOM-C. Linear regressions between crop residue C input and soil organic C or N were significant in most cases, except for MB-C and N min . Compared to the 1979 data, all treatments that did not receive N fertilizer (CTS Rem 0, CTS Ret 0, ZTS Rem 0 and ZTS Ret 0) showed a decrease in TOC concentration in the 0-15 cm soil layer over time, with the highest decrease in the CTS Rem 0 treatment. Straw retention and N fertilizer application at 50 and 100 kg N ha -1 under both ZT (ZTS Ret 50 and ZTS Ret 100) and CT (CTS Ret 50 and CTS Ret 100) resulted in a strongest increase in TOC during the first 11 years, and since then the TOC decreased under both N rates but 50 kg N ha -1 rate under CT (CTS Ret 50) showed the strongest negative effect on TOC in soil. In conclusion, elimination of tillage, straw retention and N application all improved organic C and N in soil, and generally differences were more pronounced for light fraction organic C and N, and between the most extreme treatments

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