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Uncertainty about the effects of climate change on terrestrial soil organic C stocks has generated interest in clarifying the processes that underlie soil C dynamics. We investigated the role of soil mineralogy and aggregate stability as key variables controlling soil C dynamics in a California conifer forest. We characterized soils derived from granite (GR) and mixed andesite‐granite (AN) parent materials from similar forest conditions. Granite and AN soils contained similar clay mineral assemblages as determined by x‐ray diffraction (XRD), dominated by vermiculite, hydroxy‐interlayered vermiculite (HIV), kaolinite, and gibbsite. However, AN soils contained significantly more Al in Al‐humus complexes (6.2 vs. 3.3 kg m−2) and more crystalline and short‐range order (SRO) Fe oxyhydroxides (30.6 vs. 16.8 kg m−2) than GR soils. Andesite‐granite pedons contained nearly 50% more C relative to GR soils (22.8 vs. 15.0 kg m−2). Distribution of C within density and aggregate fractions (free, occluded, and mineral associated C) varied significantly between AN and GR soils. In particular, AN soils had at least twice as much mineral associated C relative to GR soils in all horizons. Based on 14C measurements, occluded C mean residence time (MRT) > mineral C > free C in both soil types, suggesting a significant role for aggregate C protection in controlling soil C turnover. We found highly significant, positive correlations between Al‐humus complexes, SRO Al minerals, and total C content. We suggest that a combination of aggregate protection and organomineral association with Al‐humus complexes and SRO Al minerals control the variation in soil C dynamics in these systems.

Coupled climate-ecosystem models predict significant alteration of temperate forest biome distribution in response to climate warming. Temperate forest biomes contain approximately 10% of global soil carbon (C) stocks and therefore any change in their distribution may have significant impacts on terrestrial C budgets. Using the Sierra Nevada as a model system for temperate forest soils, we examined the effects of temperature and soil mineralogy on soil C mineralization. We incubated soils from three conifer biomes dominated by ponderosa pine (PP), white fir (WF), and red fir (RF) tree species, on granite (GR), basalt (BS), and andesite (AN) parent materials, at three temperatures (12.5 1C, 7.5 1C, 5.0 1C). AN soils were dominated by noncrystalline materials (allophane, Al-humus complexes), GR soils by crystalline minerals (kaolinite, vermiculite), and BS soils by a mix of crystalline and noncrystalline materials. Soil C mineralization (ranging from 1.9 to 34.6 [mg C (g soil C) À1 ] or 0.1 to 2.3 [mg C (g soil) À1 ]) differed significantly between parent materials in all biomes with a general pattern of ANoBSoGR. We found significant negative relationships between Fe-oxyhydroxides, Al-oxyhydroxides, and Al-humus complex content and soil C mineralization, suggesting mineral control of C mineralization. Modeled decomposition rates and mineralizable pool size increased with increasing temperature for all parent materials and biomes. Further, d 13 C values of respired CO 2 suggest greater decomposition of recalcitrant soil C compounds with increasing temperature, indicating a shift in primary C source utilization with temperature. Our results demonstrate that soil mineralogy moderates soil C mineralization and that soil C response to temperature includes shifts in decomposition rates, mineralizable pool size, and primary C source utilization.

We present a model for prediction of pedogenic environments and soil properties based on energy input to the soil system. The model estimates rates of precipitation and net primary production (NPP) energy input using the Parameter‐Regression Independent Slope Model (PRISM) climate data, and a parent material index (PMI). Soil order, soil C, and clay data from the State Soil Geographic (STATSGO) database were compared with rates of NPP and precipitation energy input for major geographic regions of the continental USA, including California, Oregon, Washington, Texas, North Dakota, Alabama, Pennsylvania, and New Hampshire. Soil orders in all states show differences in total energy input (Ein, kJ m−2yr−1) and the percentage of Ein from NPP (%Enpp) (e.g., Ultisols Ein = 29915, %Enpp = 49%; Mollisols Ein = 5880, %Enpp = 90%). Using linear regression models, rates of NPP estimated (R2 = 0.82***) trends in soil C content in western states, but failed to estimate soil C in other geographic areas. Parent material index adjusted energy flux estimated soil clay content for the majority (99.5%) of igneous parent materials in California and Oregon (R2 = 0.67**), the only states with digital geologic data. The model underestimated clay content in steeply sloping Inceptisols and Andisols (0.5% of igneous land area). Results suggest that rates of NPP may be used to estimate soil C for climate regimes with steep environmental gradients. Landscape age and stability components might improve clay prediction in young and erosive landscapes. Modeled energy input provides a tool for estimating pedogenic environments, soil order, and soil properties. Energy input parameters may aid efforts to pre‐map broad landscape units for soil survey.

4The impact on soil health of long-term no-tillage (NT) and cover cropping (CC) practices, alone 5 and in combination, was measured and compared with standard tillage (ST) with and without 6 cover crops (NO) in irrigated row crops after 15 years of management in the San Joaquin Valley 7 (SJV) CA, USA. Soil aggregation, rates of water infiltration, content of carbon, nitrogen, water 8 extractable organic carbon (WEOC) and organic nitrogen (WEON), residue cover, and 9 biological activity were all increased by NT and CC practices relative to STNO. However, 10 effects varied by depth with NT increasing soil bulk density by 12% in the 0 -15 cm depth and 11 10% in the 15 -30 cm depth. Higher levels of WEOC were found in the CC surface (0 -5cm) 12 depth in both spring and fall samplings in 2014. Surface layer (0 -15 cm) WEON was higher in 13 the CC systems for both samplings. Tillage did not affect WEON in the spring, but WEON was 14 increased in the NT surface soil layer in the fall. Sampling depth, CC, and tillage affected 1-15 day soil respiration and a soil health index assessment, however the effects were seasonal, with 16 higher levels found in the fall sampling than in the spring. Both respiration and the soil health 17 index were increased by CC with higher levels found in the 0 -5 cm depth than in the 5 -15 and 18 15 -30 cm depths. Results indicated that adoption of NT and CC in arid, irrigated cropping 19 systems could benefit soil health by improving chemical, physical, and biological indicators of 20 soil functions while maintaining similar crop yields as the ST system. 21 . 22 23 Keywords 24

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Abbreviations: AL-7, alpine biome; Al d , citrate-dithionite-extractable aluminum; Al o , oxalate-extractable aluminum; Al p , pyrophospate-extractable aluminum; BO-1, blue oak biome; Fe d , citrate-dithionite extractable iron; Fe o , oxalate-extractable iron; PO-2, pine-oak biome; PP-3, ponderosa pine biome; RF-5, red fi r biome; SA-6, subalpine biome; Si o , oxalate-extractable silicon; SRO, short-range ordered; WF-4, white fi r biome; XRD, x-ray diffraction.

The cycling of temperate forest soil C is likely to be altered with climate change. Climate change may induce changes in forest litter that promotes priming, or enhanced decomposition of extant soil C. The effects of environmental factors such as temperature, litter quality, and soil mineralogy on priming are not well understood. The objectives of this study were to determine the interaction of temperature and soil mineral assemblage on priming of temperate forest soil C. We incubated soils from three forest types (ponderosa pine, white fir, and red fir), on granite (GR), basalt (BS), and andesite (AN) parent materials at three temperatures (12.5, 7.5, and 5.0°C), with the addition of 13C‐labeled ponderosa pine litter. Soil C mineralized from each parent material differed in response to increasing temperature (i.e., relative increases of 38–70% from 5.0–12.5°C), following a pattern of GR > BS > AN. The percentage of C derived from litter and soil C pools varied significantly by parent material and forest type. Andesite soils, dominated by short‐range‐oder (SRO) aluminosilicates demonstrated decreased priming relative to BS and GR soils across all forest types. Soil C mineralization rate data indicated that the majority of priming effects were short term (within the first 20 d of a 90‐d incubation). Regression analysis indicated control of priming by soil C, C/N, and soil C 13C signature, SRO Fe oxyhydroxides, and Al–humus complexes. Variation in the soil mineral assemblage was the dominant control of both cumulative soil C mineralization and soil C priming.