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  1. Abstract

    Soil nutrient distribution is heterogeneous in space and time, potentially altering nutrient acquisition by trees and microorganisms. Ecologists have distinguished “hot spots” (HSs) as areas with enhanced and sustained rates of nutrient fluxes relative to the surrounding soil matrix. We evaluated the spatial and temporal patterns in nutrient flux HSs in two mixed-conifer forest soils by repeatedly sampling the soil solution at the same spatial locations (horizontally and vertically) over multiple seasons and years using ion exchange resins incubated in situ. The climate of these forests is Mediterranean, with intense fall rains occurring following summers with little precipitation, and highly variable winter snowfall. Hot spots formed most often for NO3and Na+. Although nutrient HSs often occurred in the same spatial location multiple times, HSs persisted more often for PO43−NH4+, and NO3, and were more transient for Ca2+, Mg2+, and Na+. Sampling year (annual precipitation ranged from 558 to 1223 mm) impacted the occurrence of HSs for most nutrients, but season was only significant for PO43−, NH4+, NO3, and Na+, with HSs forming more often after fall rains than after spring snowmelt. The frequency of HSs significantly decreased with soil depth for all nutrients, forming most commonly immediately below the surficial organic horizon. Although HSs accounted for less than 17% of the sampling volume, they were responsible for 56–88% of PO43−, NH4+, and NO3resin fluxes. Our results suggest that macronutrient HSs have a disproportional contribution to soil biogeochemical structure, with implications for vegetation nutrient acquisition strategies and biogeochemical models.

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  2. Abstract

    The lateral destination and potential decomposition of soil organic matter mobilized by soil erosion depends on factors such as the amount and type of precipitation, topography, the nature of mobilized organic matter (OM), potential mixing with mineral particles, and the stabilization mechanisms of the soil OM. This study examined how the relative distribution of carbon (C) and nitrogen (N) in different OM fractions varied in soils from eroding slopes and in eroded sediments in a series of low‐order forested catchments in the western Sierra Nevada, California. We found that precipitation amount played a major role in mobilizing OM. More than 40% of the OM exported from these forested catchments was free particulate OM, or OM physically protected inside relatively less stable macroaggregates, compared to OM inside microaggregates or chemically associated with soil minerals. Years with high amounts of precipitation generally transported more mineral‐associated OM, with lower C and N concentrations, while sediment transported in drier years was more enriched in unprotected, coarse particulate OM derived from surficial soils. When incubated under the same conditions, sediment C (from material captured in settling basins) produced 72–97% more CO2during decomposition than soil C did. Our results suggest that without stabilization through burial or reconfigured organomineral associations, this sediment OM is prone to decomposition, which may contribute to little to no terrestrial CO2sink induced from erosion in these Mediterranean montane forest ecosystems.

     
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