Biochar (BC) amendment to soils is a proposed strategy to improve soil fertility and mitigate climate change. However, before this can become a recommended management practice, a better understanding of the impacts of BC on the soil biota is needed. We determined the effect of addition rates (0, 1, 5, 10 and 20% by mass) of a fast-pyrolysis wood-derived BC on the extraction efficiency (EE), abundance and temporal dynamics of phospholipid fatty acids (PLFAs, microbial community biomarkers) in four temperate soils during a 1-year incubation. Additionally, the effects on microbial mineralization/incorporation of BC-C were determined by measuring CO 2 efflux and the BC contribution to CO 2 and PLFA-C using the natural 13 C abundance difference between BC and soils. Biochar addition proportionally increased microbial abundance in all soils and altered the community composition, particularly at the greatest addition rate, towards a more gram-negative bacteriadominated (relative to fungi and gram-positive) community. Though chemically recalcitrant, the BC served as a substrate for microbial activity, more so at large addition rates and in soil with little organic matter. Microbial utilization of BC-C for growth could only partially explain the observed increase in microbial biomass, suggesting that other, potentially abiotic, mechanisms were involved. The strong decrease in PLFA EE (−77%) in all soils with biochar addition emphasizes the need to measure and correct for EE when using PLFA biomarkers to estimate soil microbial responses to BC additions. Overall, our study provides support for BC use as a soil amendment that potentially stimulates microbial activity and growth.

[1] Hyporheic exchange plays a key role in the biogeochemical evolution of water and in ecosystem functioning at the local, reach, and watershed scales. Residence time is a fundamental metric to describe the possible transformation taking place in this exchange zone. With this in mind, we use a simple conceptual model to explore the residence time distributions (RTDs) of sinuosity-driven hyporheic zones (HZs) and to discriminate the individual effect of sinuosity (s), valley slope (J x ), hydraulic conductivity (K), aquifer dispersivity (a L ), and the biogeochemical timescales (BTSs) that characterize the degradation of dissolved organic carbon in these hydrologic systems. We find that RTDs are characterized by one early mode and a late time power law behavior. For a given aquifer dispersivity, the shape of these distributions is stretched or compressed by changes in J x , K, and s, having a strong influence on the net biogeochemical transformations within the HZ. Using BTSs proposed in previous studies and sensitivity analyses, we show the potential of s, J x , and K to classify meander HZs as net sinks of nitrates or only modulators of the residence times in the subsurface where nitrate reduction is negligible. These findings can be used as predictive tools to quantify the potential of meanders as biogeochemical reactors at the watershed scale with the aid of remote sensing data and GIS processing techniques. These tools can guide experimental design, suggesting important locations to visit, sample, and/or instrument. Also, hyporheic restoration projects can use them for initial site selection and design of channel modifications.

A dynamic model for uptake of pesticides in potatoes is presented and evaluated with measurements performed within a field trial in the region of Boyacá, Colombia. The model takes into account the time between pesticide applications and harvest, the time between harvest and consumption, the amount of spray deposition on soil surface, mobility and degradation of pesticide in soil, diffusive uptake and persistence due to crop growth and metabolism in plant material, and loss due to food processing. Food processing steps included were cleaning, washing, storing, and cooking. Pesticide concentrations were measured periodically in soil and potato samples from the beginning of tuber formation until harvest. The model was able to predict the magnitude and temporal profile of the experimentally derived pesticide concentrations well, with all measurements falling within the 90% confidence interval. The fraction of chlorpyrifos applied on the field during plant cultivation that eventually is ingested by the consumer is on average 10(-4)-10(-7), depending on the time between pesticide application and ingestion and the processing step considered.

Two field studies with six large aperture scintillometers (LASs) were performed using horizontal and slant paths. The accuracy of this novel and increasingly popular technique for measuring sensible heat fluxes was quantified by comparing measurements from different instruments over nearly identical transects. Random errors in LAS measurements were small, since correlation coefficients between adjacent measurements were greater than 0.995. However, for an ideal set-up differences in linear regression slopes of up to 21% were observed with typical inter-instrument differences of 6%. Differences of 10% are typical in more realistic measurement scenarios over homogeneous natural vegetation and different transect heights and locations. Inaccuracies in the optics, which affect the effective aperture diameter, are the most likely explanation for the observed differences.

[1] Natural systems are driven by dynamic forcings that change in time as well as space, behavior that is inherited by the system flow field and results in time-varying age distributions (ADs). This work presents a review of the mathematical tools and solution approaches used to model ADs in dynamic time-varying flow systems. A simple conceptual, numerical model is then used to explore the role of flow dynamics in ADs for topographydriven flow systems. This model is an analog for regional groundwater systems and hyporheic zones. This model demonstrates that relatively small fluctuations in the forcing, even though importantly affecting the flow in the system, can have minimal effects in ADs. However, as the intensity of fluctuation increases, still within the bounds observed in natural systems, ADs in shallow parts of the system become highly sensitive to dynamic flow conditions, leading to considerable changes in the moments and modality of the distributions with time. In particular, transient flow can lead to emergence of new modes in the AD, which would not be present under steady flow conditions. The discrepancy observed between ADs under steady and transient flow conditions is explained by enhancement of mixing due to temporal variations in the flow field. ADs in deeper parts of the system are characterized by multimodality and tend to be more stable over time even for large forcing fluctuations.

Saturation of large aperture scintillometer (LAS) signals can result in sensible heat flux measurements that are biased low. A field study with LASs of different aperture sizes and path lengths was performed to investigate the onset of, and corrections for, signal saturation. Saturation already occurs at C 2 n ≈ 0.074D 5/3 λ 1/3 L −8/3 , where C 2 n is the structure parameter of the refractive index, D is the aperture size, λ is the wavelength, L is the transect length, which is smaller than theoretically derived saturation limits. At a transect length of 1 km, a height of 2.5 m, and aperture ≈0.15 m the correction factor exceeds 5% already at C 2 n = 2 × 10 −12 m −2/3 , which will affect many practical applications of scintillometry. The Clifford correction method, which only depends on C 2 n and the transect geometry, provides good saturation corrections over the range of conditions observed in our study. The saturation correction proposed by Ochs and Hill results in correction factors that are too small in large saturation regimes. An inner length scale dependence of the saturation correction factor was not observed. Thus for practical applications the Clifford correction method should be applied.

RESUMENEn el ejido El Zapote de la Reserva de la Biosfera "Sierra de Huautla", estado de Morelos, México, se seleccionaron doce árboles de Quercus magnoliaefolia Née y se determinó su biomasa en la parte aérea. El modelo alométrico quedó expresado como: B = 0.0345 * DAP 2.9334 en donde B es la biomasa (kg) y DAP es el diámetro a la altura del pecho (cm), con un coeficiente de determinación (R 2 = 0.98; P<0.001). Adicionalmente, se estimó la biomasa mediante una regresión lineal múltiple que consideró el área basal (AB), la densidad específica de la madera (DEM) y la altura de los árboles (H). Una alta proporción de la varianza de la biomasa fue explicada solamente por el área basal. La DEM y H no incrementaron significativamente la precisión de los modelos. El porcentaje promedio de carbono en los árboles de Q. magnoliaefolia fue de 47.14 con valores de 46.29 % para el fuste, 46.83 % para las ramas y 48.31 % para el follaje. La proporción promedio de los componentes de la especie estudiada fue de 62.5 % para el fuste, 27.8 % para las ramas y 9.6 % para el follaje. ABSTRACTAt the El Zapote ejido in the "Sierra de Huautla" Biosphere Reserve in the state of Morelos, Mexico, twelve Quercus magnoliaefolia Née trees were selected and their above-ground biomass determined. The proposed allometric model was expressed as: B = 0.0345 * DBH 2.9334 , where B is biomass (kg) and DBH is diameter at breast height (cm), with a coefficient of determination (R 2 = 0.98; P<0.001). In addition, aboveground biomass was estimated by a multiple linear regression based on basal area (BA), specific gravity of wood (SGW) and tree height (H). A high proportion of the biomass variation was explained by basal area alone. SGW and H did not significantly increase the accuracy of the models. The average percentage of carbon in Q. magnoliaefolia trees was 47.14 with values of 46.29 % for the bole, 46.83 % for branches and 48.31 % for foliage. The average proportion of the components of the species studied was 62.5 % for the bole, 27.8 % for branches and 9.6 % for foliage. PALABRAS CLAVE:Ecuaciones alométricas, Quercus magnoliaefolia, biomasa aérea, densidad específica de la madera.

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