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Despite the wealth of information available on respiratory fluxes of CO2 from the soil surface, relatively few studies have examined the depth-related flux and production of CO2 within the soil. We developed a depth-integrated CO2 flux model based on soil profile CO2 measurements to explore both the temporal and spatial patterns of, and controls on, CO2 production in two contrasting California coastal ecosystems: a redwood [Sequoia sempervirens (D. Don) EndI.] forest and a seasonal grassland ecosystem. In the forest, CO2 production increased exponentially with temperature but the dominant control on CO2 efflux was due to changes in soil diffusivity with water content. Here, ∼67% of the total efflux was produced in the top 20 cm, with little seasonal variation. In the grassland, soil temperature and water content both exerted strong positive controls on CO2 production, with CO2 efflux highest in early spring (8 g Cm-2 d-1) and lowest in late summer (0.4 g Cm-2 d-1). Here, the relative proportion of total respiration in the top 20 cm varied from 5% at the end of the dry season to 70% during the wet winter months. Soil respiration in the redwood soil appeared to be well buffered from climate extremes due to the stable microclimate created by the forest canopy. The grassland soil, however, lacking such buffering, was subject to large and rapid changes in CO2 production throughout the year. These results indicate that despite some uncertainty in the calculation of diffusion rates, significantly more information on soil biological processes is gained from the CO2 production-diffusion approach than from surface flux measurements alone. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the effect of climate and saprolite weathering on lanscape evolution. The study focuses on the erosion rate and weathering in saprolite and soil in the Nevada Mountains in Western Sierra, California to determine the influence of climate in denudation. The result reveals that climate, weathering and erosion are closely related and suggests that climate controls chemical weathering through temperature and moisture control in chemical reaction rates.

In this study, we examined changes in isotopic (13C and 14C) and spectroscopic (UV and 13C NMR) properties of dissolved organic carbon (DOC) in relation to soil organic matter (SOM) to elucidate the sources and sinks of DOC as water percolates through the soils of two contrasting upland coastal California ecosystems—a redwood forest and a coastal prairie. Despite differences in the distribution of C stocks and litter chemistry at these two sites, we found similar shifts in DOC chemistry with soil depth. DOC concentrations dropped rapidly with increasing depth, with an accompanying decrease in the C:N ratio, an increase in the δ13C value and an decrease in specific UV adsorption. In the grassland soil, Δ14C values declined from current atmospheric values (+70‰) in the surface horizon to −75‰ at 100 cm. In the redwood soil, the Δ14C value of 111‰ in O horizon leachates was indicative of OM with a residence time of 8–10 yrs, with a decrease in Δ14C values to −80‰ at 100 cm. Solid-state CP/MAS 13C NMR spectra were generally most similar to highly humified OM, with a general decrease in the relative abundance of aromatic compounds and an increase in the alkyl C/O-alkyl C ratio with increasing depth. All of these trends are consistent with the shifts in SOM properties with increasing depth, which are interpreted to mean a shift from fresh plant material to older, highly altered OM. In this Mediterranean climate, we found distinct seasonal shifts in the quantity and composition of DOC found in soil solution during the winter rainy period that was also consistent with a shift from recent labile substrates to older, highly altered OM. These results fit in with a growing body of literature suggesting that the source of much of the DOC within mineral soils is the local soil OM, and the 14C data, in particular, indicate that DOC at depth is not simply the fraction of surficial leachates that have not been adsorbed or decomposed. Rather, exchange reactions with a portion of the more stabilized SOM pool exert the strongest control on both the concentration and composition of DOC found in these soils. [ABSTRACT FROM AUTHOR]

Little is known about the role of vegetated hillslope sediment transport in the soil C cycle and soil-atmosphere C exchange. We combined a hillslope sediment transport model with empirical soil C measurements to quantify the erosion and temporal storage of soil organic carbon (SOC) within two grasslands in central California. The sites have contrasting erosional mechanisms: biological perturbation (Tennessee Valley (TV)) versus clay-rich soil creep (Black Diamond (BD)). The average SOC erosion rates from convex slopes were 1.4-2.7 g C m-2 yr-1 at TV and 5-8 g C m-2 yr-1 at BD, values that are <10% of above ground net primary productivity (ANPP) at both sites. The eroded soil accumulates on depositional slopes. The long term Soc accumulation (or C sink) rates are ∼1.9 g C m-2 yr-1 in the TV hollow and 1.7-2.8 g C m-2 yr-1 in the BD footslope. We found that the hillslope C sink is driven primarily by the burial of in situ plant production rather than preservation of eroded SOC, a finding that differs from existing hypotheses. At TV, the net sequestration of atmospheric C by long-term hollow evacuation and refilling depends on the fate of the C exported from the zero order watershed. This study suggests that erosion and deposition are coupled processes that create a previously unrecognized C sink in undisturbed upland watersheds, with a potential to substantially affect the global C balance presently, and over geological timescales. [ABSTRACT FROM AUTHOR]

• On the California coast N is tightly cycled by biological activity whereas S appears to be in excess. • Depth profiles of C, N, and their isotopes provides insights into rates of C and N cycling. • Soil age in this setting seemed to have a limited effect on C, N, and S cycling. Carbon (C), nitrogen (N), and sulfur (S) are elements strongly influenced by biological cycling and redox reactions in soils, but few comparative analyses have investigated the behaviors of these elements with time. Thus, we examined changes in content and isotope composition of soil profiles along a chronosequence (58–212 kyr) of marine terraces on the central California coast, in an area with significant background geochemical research. Unlike in other chronosequences in more humid locations, the total C, N, and S in these soils did not vary strongly with age, possibly due to the retention of phosphorus (P). The total pools of soil N and S cannot be explained by wet deposition of NO 3 and SO 4 alone, suggesting other sources of atmospheric inputs such as NH 4 + and dimethyl sulfide. Total C and N declined in a characteristic logarithmic pattern with depth, while S did not. The ratio of extractable soil nitrate (NO 3) to total N declined with depth, suggesting strong biological demand via various avenues. In contrast, the ratio of extractable sulfate (SO 4) to total S increased with depth, suggesting that S was in biological excess. We used a simple reactive transport model to integrate the depth profiles of total C and N and their isotope values. The depth trends of total concentrations suggested one-pool residence times of approximately 500 to 1000 y, consistent with turnover times calculated by mass balance. Depth trends of stable isotope values indicated that N is isotopically fractionated at a magnitude twice that of C, consistent with observed 15N-depleted nitrous oxide (N 2 O) emissions during the dry summer months. The isotope composition of S (total and SO 4) suggests some isotope enrichment during biogeochemical cycling, but far less than observed for N. Thus, despite significant chemical weathering and elemental loss over time, the biogeochemical cycles of C, N, and S remain relatively unaffected by soil age in this climatic setting. [ABSTRACT FROM AUTHOR]