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Colorimetric methods based on the Berthelot reaction are used widely for quantitative determination of NH4‐N in biological and environmental samples. Studies to evaluate phenol and salicylate, the most commonly used chromogenic substrates, revealed minor interferences by metallic cations, whereas up to a threefold shift in absorbance was observed with 38 diverse N‐containing organic compounds. Interferences differed markedly between phenol and salicylate. The possibility of a simple correction was precluded by the fact that interferences were both positive and negative, and depended on the temperature during color development and the concentration of NH4‐N. Fourteen compounds were evaluated as alternatives to phenol and salicylate, of which the Na salt of 2‐phenylphenol (PPS) proved to be the most promising. Using PPS, macro‐ and microscale batch methods and an automated flow‐injection method were developed. These methods are simple, convenient, and sensitive. Using the PPS microscale method, for which the limit of detection is 0.17 mg NH4‐N L−1, recovery of NH4‐N added to soil extracts ranged from 98 to 104%, with a coefficient of variation of 1.4 to 2.7%. As with phenol and salicylate, precipitation of metal hydroxides was observed. Precipitation was controlled by chelation with citrate rather than ethylenediaminetetraacetic acid (EDTA), which suppressed color development by preventing monochloramine formation. Compared with Berthelot methods that use phenol or salicylate, interference by amino acids was decreased by up to 10‐fold. Interference by other organic N compounds was virtually eliminated.

Reduction of seed-bank persistence is an important goal for weed management systems. Recent interest in more biological-based weed management strategies has led to closer examination of the role of soil microorganisms. Incidences of seed decay with certain weed species occur in the laboratory; however, their persistence in soil indicates the presence of yet-unknown factors in natural systems that regulate biological mechanisms of seed antagonism by soil microorganisms. A fundamental understanding of interactions between seeds and microorganisms will have important implications for future weed management systems targeting seed banks. Laboratory studies demonstrate susceptibility to seed decay among weed species, ranging from high (velvetleaf) to very low (giant ragweed). Microscopic examinations revealed dense microbial assemblages formed whenever seeds were exposed to soil microorganisms, regardless of whether the outcome was decay. Microbial communities associated with seeds of four weed species (woolly cupgrass, jimsonweed, Pennsylvania smartweed, and velvetleaf) were distinct from one another. The influence of seeds on microbial growth is hypothesized to be due to nutritional and surface-attachment opportunities. Data from velvetleaf seeds suggests that diverse assemblages of bacteria can mediate decay, whereas fungal associations may be more limited and specific to weed species. Though microbial decay of seeds presents clear opportunities for weed biocontrol, limited success is met when introducing exogenous microorganisms to natural systems. Alternatively, a conservation approach that promotes the function of indigenous natural enemies through habitat or cultural management may be more promising. A comprehensive ecological understanding of the system is needed to identify methods that enhance the activities of microorganisms. Herein, we provide a synthesis of the relevant literature available on seed microbiology; we describe some of the major challenges and opportunities encountered when studying the in situ relationships between seeds and microorganisms, and present examples from studies by the ARS Invasive Weed Management Unit.

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Several long‐term studies suggest that no‐till (NT) practices do not increase soil organic matter (SOM) sequestration in all situations. We evaluated the interaction of tillage and soil texture effects on SOM in Illinois Mollisols and Alfisols by characterizing particulate organic matter (POM), potentially mineralizable N (PMN), and soil microbial biomass (SMB). Thirty‐six fields were sampled during spring and summer of 1995 and 1996. Each field had been under either conventional tillage (CT) (disc, moldboard plow, and/or chisel plow) or NT management for at least 5 yr. No‐till fields contained 15% (3.0 g C kg−1 soil) more soil organic C (SOC) than CT fields in the 0‐ to 5‐cm depth; however, tillage did not affect SOC contents in the 5‐ to 15‐ or 15‐ to 30‐cm depths, or in the overall sampling depth (0–30 cm). Fields under NT contained 33% more POM (1.4 g C kg−1 soil) and 54% more PMN in the 0‐ to 5‐cm depth, but there was no tillage effect on POM (0–15 cm) or PMN (0–30 cm) contents overall. Average POM contents were 29% lower (0.73 g C kg−1 soil) in the 5‐ to 15‐cm depth of the NT than of the CT soils. At sand contents below ≈50 g kg−1 soil, NT fields contained greater SOC, total N, and POM contents in the 0‐ to 5‐cm depth and lower POM contents in the 5‐ to 15‐cm depth than CT fields. In soils with sand contents higher than ≈50 g kg−1 soil, tillage practices did not affect the vertical distribution of SOC, total N, or POM.

Isoxaflutole (5-cyclopropyl isoxazol-4-yl-2-mesyl-4trifluoromethylphenyl ketone) is a new herbicide marketed for broadleaf and grass weed control in corn, but little information has been published on the soil behavior and environmental fate of the compound. The herbicide exhibits an unusual behavior in which it is functionally reactivated by rainfall events, providing control of small weeds that have emerged. Isoxaflutole is extremely labile in aqueous solution, thus measuring equilibrium sorption is challenging. A qualitative kinetic evaluation was performed to characterize the sorption of isoxaflutole, during rapid hydrolysis to its bioactive product, a diketonitrile derivative (2-cyclopropyl-3-(2-mesyl-4-trifluoromethylphenyl)-3-oxopropanenitrile). The transformation was measured over time in a herbicidetreated aqueous solution with or without soil. At 25 °C, 83% of the parent compound remained in solution at 24 h in the soil free system, but only 15% remained in the solution in the presence of soil. The sorbed phase consisted mainly of isoxaflutole, although a small percentage of diketonitrile was also detected in increasing concentrations as the study progressed. Hydrolysis prevented the attainment of sorption equilibrium, thus the apparent K d of isoxaflutole increased over time, while that of diketonitrile remained close to zero at both 5 and 25 °C. Batch sorption isotherms were conducted with both isoxaflutole and diketonitrile using four Illinois soils of the Drummer, Flanagan, Catlin, and Cisne series ranging in organic carbon (OC) from 1.0 to 2.5%. Freundlich K d values were 6-12-fold greater for isoxaflutole than diketonitrile, with the greatest difference in the lower organic carbon soils. After removing the hydrolysis effect, sorption of the isoxaflutole and diketonitrile was independent of temperature, suggesting that it was an entropy-driven process. Based on soil OC content, K oc values of 134 and 17 mL g -1 were calculated for isoxaflutole and diketonitrile, respectively. Results suggest that desorption coupled to hydrolysis promotes reactivation of the herbicide's function after rainfall and contributes to the efficacy of the compound by resupplying the soil solution with a bioactive product.

The effects of several environmental factors on the dissipation, transformation, and mineralization of isoxaflutole were investigated in laboratory incubations. In the soil, isoxaflutole hydrolyzed to a diketonitrile derivative, which is the active form of the herbicide. The diketonitrile was then metabolized to an inactive benzoic acid derivative and later into two unknown products, which were found only in small quantities. Degradation of isoxaflutole was faster in soil maintained at -100 or -1500 kPa compared to that in air-dry soil. At 25 °C, the half-lives for isoxaflutole were 9.6, 2.4, and 1.5 days in air-dry, -1500 kPa, and -100 kPa moisture regimes, respectively. A simple Arrhenius expression described the response of isoxaflutole transformation (mineralization and transformation) to temperature in the range of 5 to 35 °C. An activation energy value (E a ) of 67 kJ/mol for isoxaflutole suggested the transformation of the herbicide to the diketonitrile derivative was primarily a chemical reaction. Moreover, biological activity had little effect on the hydrolysis of isoxaflutole, with half-lives of 1.8 and 1.4 days in sterile and nonsterile soil, respectively. However, the transformation of diketonitrile to benzoic acid and the production of the unknown products were greatly reduced in the sterile soil, suggesting one or more biologically mediated processes.