Watershed and global-scale nitrogen (N) budgets indicate that the majority of the N surplus in anthropogenic landscapes does not reach the coastal oceans. While there is general consensus that this 'missing' N either exits the landscape via denitrification or is retained within watersheds as nitrate or organic N, the relative magnitudes of these pools and fluxes are subject to considerable uncertainty. Our study, for the first time, provides direct, large-scale evidence of N accumulation in the root zones of agricultural soils that may account for much of the 'missing N' identified in mass balance studies. We analyzed long-term soil data (1957-2010) from 2069 sites throughout the Mississippi River Basin (MRB) to reveal N accumulation in cropland of 25-70 kg ha −1 yr −1 , a total of 3.8±1.8 Mt yr −1 at the watershed scale. We then developed a simple modeling framework to capture N depletion and accumulation dynamics under intensive agriculture. Using the model, we show that the observed accumulation of soil organic N (SON) in the MRB over a 30 year period (142 Tg N) would lead to a biogeochemical lag time of 35 years for 99% of legacy SON, even with complete cessation of fertilizer application. By demonstrating that agricultural soils can act as a net N sink, the present work makes a critical contribution towards the closing of watershed N budgets.

Rising costs and air quality regulations have created interest in California's San Joaquin Valley (SJV) in production systems that reduce tillage operations and soil disturbance. From 1999 to 2009, we evaluated conventional (CT) and reduced tillage (RT) systems for a cotton (Gossypium hirsutum L.)/tomato (Solanum lycopersicon Mill.) rotation with (CC) and without (NO) cover crops in a Panoche clay loam soil (fine‐loamy, mixed, superactive, thermic Typic Haplocambid) in Five Points, CA, in terms of yield, soil C, and the NRCS soil conditioning index (SCI). The RT reduced tractor operations by 50% for tomato and 40% for cotton. Cover cropping produced 38.7 t ha−1 of biomass. Tomato yields were 9.5% higher in RT vs. CT systems and 5.7% higher in NO vs. CC systems. The CT cotton yields were 10.0% higher than RT yields and 4.8% higher in NO systems, but yield patterns were not consistent from 2005 to 2009. Soil C content was uniform (0–30‐cm depth) in 1999 (19.72 t ha−1) and increased in all systems in 2007 (t ha−1): RTCC 29.11, CTCC 26.36, RTNO, 24.09, and CTNO 22.65. Soil C content of RT and CT systems did not differ, but was greater in CC than in NO systems. In the 0‐ to 15‐cm depth, RT increased soil C, indicating stratification, and also increased C in the occluded light and mineral fractions. The SCI was positive for RT treatments, predicting a soil C increase, and negative for CT systems, predicting a soil C decline, but measured soil C content increased in all systems. Results show that RT maintains or increases yields relative to CT, and CC stores more soil C than NO.

Despite a large body of scientific research that shows that soils change on relatively short time scales under different management regimes, classical pedological theory states that we should expect these changes to occur only in the surface few centimeters and that they are not of adequate magnitude to suggest fundamental changes in pedon character over short periods of time. In fact, rarely, do the scientists that make these comparisons report on any properties deeper than 30 to 45 cm in the soil profile. With this study, we evaluate soil transformation to a depth of 150 cm after 50 yr of intensive row-crop agricultural land use in a temperate, humid, continental climate (Iowa, United States), by resampling sites that were initially described by the United States soil survey between 1943 and 1963. We find that, through agricultural land use, humans are accelerating soil formation and transformation to a depth of 100 cm or more by accelerating erosion, sedimentation, acidification, and mineral weathering, and degrading soil structure, while deepening dark-colored, organic-matter rich surface horizons, translocating and accumulating organic matter deeper in the soil profile and lowering the water table. Some of these changes can be considered positive improvements, but many of these changes may have negative effects on the soils' future productive capacity.Abbreviations: SOC, soil organic carbon.

T he soil C sequestration potential of reduced tillage or CT has been extensively studied in other parts of the country, but there has been very little CT research under arid, irrigated agriculture systems, like those in California. California cropping systems are intensively managed and require tillage operations to address furrow irrigation needs. Diverse crop rotations and crops untested for reduced tillage regimes have complicated the development of CT approaches in this highly productive agricultural region. Reducing tillage of agricultural soils may improve agricultural sustainability by reducing fossil fuel consumption, labor needs, equipment maintenance, and soil erosion, and increase soil water conservation and soil C sequestration (Unger et al., 1997; Lal, 2001). Soil C sequestration potential is dependent on a number of factors including climate and parent material. California agricultural soils often have high clay content (loam to loamy clay) because of the depositional environment of the San Joaquin Valley. As a result, these soils may have a greater potential to promote organo-mineral interactions and stabilize C through aggregation; however, irrigation combined with California's warm climate (near-surface soil temperatures often averaging between 25 and 35°C) may enhance decomposer activity and limit the potential for soil C sequestration.

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debated [11,12,14,15], but less research has been devoted to the effect of oyster reefs on the surrounding sediment. Reefs act to attenuate wave energy, possibly facilitating deposition of fine sediment [8]; this process may work in concert with oyster filtration to increase light penetration that may then shift ecosystems towards more benthic primary producers [6]. Finer particles and much higher organic matter (OM) content in oyster-associated sediments suggests a substantial role for carbon and nutrient removal by burial [8,17] and benthic algal uptake where light penetration is sufficient [6]. However, mesocosm experiments show that physical factors such as bottom shear can influence sediment resuspension and benthic micro-algal biomass, making the system more complex and the likelihood of OM burial versus remineralization more difficult to predict [7]. Several recent studies suggest that sediments associated with natural and restored oyster reefs have high rates of denitrification and may thus represent important sites for long term nitrogen removal [17][18][19][20]. Whole-creek studies and some mesocosm studies do not parse the contributions of the oysters themselves versus the associated sediments but rather consider the reef-sediment system as a whole [3,5,17]. Indeed, it is difficult to separate these effects because the presence of the reef will likely alter the depositional environment and ultimately the biogeochemistry of the surrounding sediment.

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