DETERMINING NUTRIENT EXPORT COEFFICIENTS AND SOURCE LOADING UNCERTAINTY USING IN‐STREAM MONITORING DATA1

ABSTRACT: Over a three‐year period, flow and nutrients were monitored at 13 sites in the upper North Bosque River watershed in Texas. Drainage areas above sampling sites differed in percent of dairy waste application fields, forage fields, wood/range, and urban land area. A multiple regression approach was used to develop total phosphorus (TP) and total nitrogen (TN) export coefficients for the major land uses in these heterogeneous drainage areas. The largest export coefficients were associated with dairy waste application fields followed by urban, forage fields, and wood/range. An empirical model was then established to assess nutrient contribution by major sources using developed export coefficients and point source loadings from municipal wastewater treatment. This model was verified by comparison of estimated loadings to measured in‐stream data. Monte Carlo simulation techniques were applied to provide an uncertainty analysis for nutrient loads by source, based on the variance associated with each export coefficient. The largest sources of nutrients contributing to the upper North Bosque River were associated with dairy waste application fields and forage fields, while the greatest relative uncertainty in source contribution was associated with loadings from urban and wood/range land uses.     

The Role of Soil Surface Crusting in Desertification and Strategies to Reduce Crusting

Soil crusting decreases infiltration, increases erosion, and impedes vegetation establishment, so reducing the impact of crusting is of major importance in combating desertification. Although surface crusting has been the subject of considerable research over the past 50 years or more, the practical management of soil crusts remains a challenge for many dryland communities. Crusting occurs in two steps, an initial aggregate breakdown period that occurs under rainfall and a subsequent hardening phase during drying. Several factors influence crust development, but the single most important one is soil aggregate stability. Strategies to reduce crusting can be based either on protecting the surface from raindrop impact or improving aggregate stability, or a combination of both. However, crust control is labor and/or capital intensive and must be thought out clearly in terms of the benefits to be achieved.     

Characterisation of the abiotic degradation pathways of oxytetracyclines in soil interstitial water using LC-MS-MS

The fate of oxytetracyclines (OTCs) in soil interstitial water was investigated and the structure of a number of degradation products elucidated in a time-related experiment. A previously developed separation method for LC–MS–MS able to base separate and quantify OTC and three of its epimers and degradation products was applied. Compounds detected were 4-epi-oxytetracycline (EOTC) (t_R=3.0 min), OTC (t_R=4.4 min), -apo-oxytetracycline (-apo-OTC) (t_R=11.4 min) and β-apo-oxytetracycline (β-apo-OTC) (t_R=18.4 min). Furthermore, we tentatively identified 4-epi-N-desmethyl-oxytetracycline (E-N-DM-OTC) (t_R=3.0 min), N-desmethyl-oxytetracycline (N-DM-OTC) (t_R=3.5), N-didesmethyl-oxytetracycline (N-DDM-OTC), 4-epi-N-didesmethyl-oxytetracycline (E-N-DDM-OTC) (t_R=3.7 and 4.7 min) and 2-acetyl-2-decarboxamido-oxytetracycline (t_R=8.7) in all samples. Most compounds were only present in trace concentrations (less than 2%) relative to the parent OTC. EOTC was on the other hand formed up to a ratio of 0.6 relative to parent OTC concentration. Only EOTC, E-N-DM-OTC, N-DM-OTC, N-DDM-OTC and E-N-DDM-OTC were formed during the time-related experiment. All other compounds were probably only present as impurities in the spiked OTC formulation as they declined in concentration from the start of the experiment. Half-lives (T_1/2, days) of the OTCs in soil interstitial water were in the order of 2 days (EOTC) to 270 days (β-apo-OTC).     

Leachate chemistry of field-weathered spent mushroom substrate

Passive leaching by rainfall and snowmelt is a popular method to treat piles of spent mushroom substrate (SMS) before its reuse. During this field weathering process, leachate percolates into the underlying soils. A field study was conducted to examine the chemistry of SMS leachate and effects of infiltration. Two SMS piles were deposited (90 and 150 cm in height) over a Typic Hapludult and weathered for 24 mo. Leachate was collected biweekly using passive capillary samplers. The SMS leachate contained high concentrations of dissolved organic carbon (DOC; 0.8-11.0 g L(-1)), dissolved organic nitrogen (DON; 0.1-2 g L(-1)), and inorganic salts. The pH, electrical conductivity, and acid neutralizing capacity were 6.6 to 9.0, 21 to 66 ds m(-1), and 10 to 75 mmolc L(-1), respectively. Inorganic chemistry of the leachate was dominated by K+, Cl-, and SO24-. Leachate DOC was predominantly low molecular weight (<1000 Da) organic acids. During 2 yr of weathering, the 90-cm SMS pile released (per cubic meter of SMS) 3.0 kg of DOC, 1.6 kg of dissolved N, and 26.6 kg of inorganic salts. The 150-cm pile released (per cubic meter of SMS) 2.8 kg of DOC, 0.7 kg of dissolved N, and 13.6 kg of inorganic salts. The 150 cm pile retained more water and exhibited lower net nitrification compared with the 90-cm pile. The top 90 cm of soil retained 20 to 89% of the leachate solutes. Weathering of SMS in piles of 90 cm depth or greater may adversely affect ground water quality.