Measurement of greenhouse gas (GHG) fluxes between the soil and the atmosphere, in both managed and unmanaged ecosystems, is critical to understanding the biogeochemical drivers of climate change and to the development and evaluation of GHG mitigation strategies based on modulation of landscape management practices. The static chamber-based method described here is based on trapping gases emitted from the soil surface within a chamber and collecting samples from the chamber headspace at regular intervals for analysis by gas chromatography. Change in gas concentration over time is used to calculate flux. This method can be utilized to measure landscape-based flux of carbon dioxide, nitrous oxide, and methane, and to estimate differences between treatments or explore system dynamics over seasons or years. Infrastructure requirements are modest, but a comprehensive experimental design is essential. This method is easily deployed in the field, conforms to established guidelines, and produces data suitable to large-scale GHG emissions studies.

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California rice (Oryza sativa L.) growers typically use two forms of preplant N fertilizer: aqua NH 3 applied 7 to 10 cm below the soil surface (subsurface N) and surface-applied N. Th e rational for applying about 25% of the total N rate to the surface is to provide a readily available N source for young rice seedlings; however no research has been done to verify this. On-farm fi eld studies were conducted over a 3-yr period (12 site-years) with the specifi c objectives of determining when rice begins to use subsurface N and to compare the effi ciency of surface and subsurface applied N. Rice seedlings began accumulating subsurface N within 2 wk aft er sowing at some sites. When a portion of the N rate was applied to the surface, early season plant biomass and N uptake was higher than when all of the fertilizer-N was applied subsurface. In contrast, grain yields were higher when all of the N fertilizer was applied subsurface. Averaged across all sites, the fertilizer-N recovery effi ciency of surface-applied N was 38% compared to 53% when only subsurface N was applied. As aqua NH 3 is less expensive than NH 4 + based fertilizers and the application of surface N requires an additional fi eld operation, there is no justifi cation to recommend the practice of applying surface N fertilizer in these rice systems. Instead, all of the preplant N should be applied subsurface as aqua NH 3 .

Intensive crop production on grassland-derived Mollisols has liberated massive amounts of carbon (C) to the atmosphere. Whether minimizing soil disturbance, diversifying crop rotations, or re-establishing perennial grasslands and integrating livestock can slow or reverse this trend remains highly uncertain. We investigated how these management practices affected soil organic carbon (SOC) accrual and distribution between particulate (POM) and mineral-associated (MAOM) organic matter in a 29-y-old field experiment in the North Central United States and assessed how soil microbial traits were related to these changes. Compared to conventional continuous maize monocropping with annual tillage, systems with reduced tillage, diversified crop rotations with cover crops and legumes, or manure addition did not increase total SOC storage or MAOM-C, whereas perennial pastures managed with rotational grazing accumulated more SOC and MAOM-C (18 to 29% higher) than all annual cropping systems after 29 y of management. These results align with a meta-analysis of data from published studies comparing the efficacy of soil health management practices in annual cropping systems on Mollisols worldwide. Incorporating legumes and manure into annual cropping systems enhanced POM-C, microbial biomass, and microbial C-use efficiency but did not significantly increase microbial necromass accumulation, MAOM-C, or total SOC storage. Diverse, rotationally grazed pasture management has the potential to increase persistent soil C on Mollisols, highlighting the key role of well-managed grasslands in climate-smart agriculture.