The use of 13C natural abundance (δ13C) to follow C input to soil has gained widespread acceptance. However, inorganic C present in the soil as carbonates will interfere with the measurement of soil organic 13C unless removed or excluded from measurement. We report a simple and convenient HCl‐fumigation method to remove inorganic C from soil. Soil samples are weighed in Ag‐foil capsules, arranged on a microtiter plate, wetted with water to approximately field capacity, and placed in a desiccator containing a beaker with concentrated (12 M) HCl. The carbonates are released as CO2 by the acid treatment in 6 to 8 h. The soil samples are then dried at 60°C prior to isotope determination. The advantages of the HCl‐fumigation method to remove inorganic C from the soil are that: (i) no water soluble C will be lost from the soil; (ii) a large number of samples can be processed simultaneously; (iii) the removal of inorganic C is rapid and complete; and (iv) the method could also be used to determine both organic and inorganic C content in the soil. A potential disadvantage, however, is that the HCl fumigation changed the 15N natural abundance of soil N.

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The continuous increase of nitrous oxide (N 2 O) abundance in the atmosphere is a global concern. Multiple pathways of N 2 O production occur in soil, but their significance and dependence on oxygen (O 2 ) availability and nitrogen (N) fertilizer source are poorly understood. We examined N 2 O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O 2 concentrations following urea or ammonium sulfate [(NH 4 ) 2 SO 4 ] additions in loam, clay loam, and sandy loam soils that also contained ample nitrate. The contribution of the ammonia (NH 3 ) oxidation pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N 2 O production was determined in 36-h incubations in microcosms by 15 N-18 O isotope and NH 3 oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH 3 oxidation pathways increased as O 2 concentrations decreased from 21% to 0.5%. At low (0.5% and 3%) O 2 concentrations, nitrifier denitrification contributed between 34% and 66%, and HD between 34% and 50% of total N 2 O production. Heterotrophic denitrification was responsible for all N 2 O production at 0% O 2 . Nitrifier denitrification was the main source of N 2 O production from ammonical fertilizer under low O 2 concentrations with urea producing more N 2 O than (NH 4 ) 2 SO 4 additions. These findings challenge established thought attributing N 2 O emissions from soils with high water content to HD due to presumably low O 2 availability. Our results imply that management practices that increase soil aeration, e.g., reducing compaction and enhancing soil structure, together with careful selection of fertilizer sources and/or nitrification inhibitors, could decrease N 2 O production in agricultural soils.

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Soil chemical properties during the transition from conventional to organic and low‐input farming practices were studied over 8 yr in California's Sacramento Valley to document changes in soil fertility status and nutrient storage. Four farming systems differing in crop rotation and external inputs were established on land previously managed conventionally. Fertility in the organic system depended on animal manure applications and winter cover crops; the two conventional systems received synthetic fertilizer inputs; the low‐input system used cover crops and animal manure during the first 3 yr and cover crops and synthetic fertilizer for the remaining 5 yr. At 4 and 8 yr after establishment, most changes in soil chemical properties were consistent with predictions based on nutrient budgets. Inputs of C, P, K, Ca, and Mg were higher in the organic and low‐input systems as a result of manure applications and cover crop incorporations. After 4 yr, soils in the organic and low‐input systems had higher soil organic C, soluble P, exchangeable K, and pH. Ceasing manure applications in the low‐input system in Year 4 resulted in declining levels of organic C, soluble P, and exchangeable K. Crop rotation (the presence or absence of corn) also had a significant effect on organic C levels. Differences in total N appeared to be related in part to inputs, but perhaps also to differing efficiency of the farming systems at storing excess N inputs: the low‐input system appeared to be most efficient, and the conventional systems were least efficient. Electrical conductivity (EC), soluble Ca, and soluble Mg levels were tightly linked but not consistently different among treatments. Relatively stable EC levels in the organic system indicate that animal manures did not increase salinity. Overall, our findings indicate that organic and lowinput farming in the Sacramento Valley result in small but important increases in soil organic C and larger pools of stored nutrients, which are critical for long‐term fertility maintenance.