W. M. Jarrell

Direct measurement of denitrification in a Prosopis (Mesquite) dominated Sonoran Desert ecosystem

SummaryDenitrification was directly measured using the acetylene inhibition technique in a Sonoran Desert ecosystem dominated by Prosopis glandulosa. Soil under Prosopis and from the unvegetated area between Prosopis was wetted with 50 mm of water and denitrification measured for 48 hours. The mean denitrification rate under Prosopis was 11.6 g N ha-1h-1 compared to only 0.2 g N ha-1h-1 away from Prosopis. The denitrification response to wetting was rapid and rates peaked about 24 h after water application.The much higher denitrification under Prosopis probably results from high available organic C under Prosopis, but other soil chemical and physical changes effected by Prosopis may influence denitrification rates. About 0.5 kg N ha-1 of Prosopis cover may be lost from this ecosystem by denitrification after infrequent major rainfalls.

Seasonal dynamics of nitrogen cycling for a Prosopis woodland in the Sonoran Desert@@@Dinamica estacional del ciclo de nitrógeno de un bosque de Prosopis en el desierto Sonorense

Prosopis woodlands in the Sonoran Desert have levels of above-ground biomass and productivity much higher than those predicted for desert plant communities with such low levels of precipitation. A stand ofP. glandulosa near the Salton Sea, California, has 13,000 kg ha−1 aboveground biomass and a productivity of 3700 kg ha−1 yr−1. Such a high level of productivity is possible because Prosopis is decoupled from the normal limiting factors of water and nitrogen availability. Soil nitrogen contents for the upper 60 cm of soil beneath Prosopis canopies have 1020 g m−2 total nitrogen, 25 per cent of which is in the form of nitrate. Such accumulations of nitrogen may be the result of active symbiotic nitrogen fixation. Early estimates suggest that about 25–30 kg N ha−1 yr−1 is fixed in these stands. Since Prosopis covers only 34% of the ground surface and its water resources are not limiting, much higher levels of nitrogen fixation and productivity may be possible in managed stands at greater densities.ResumenLos bosques de Prosopis en el desierto Sonorense tienen niveles de producción de biomasa (parte aérea) y productividad mucho mayores que las predecibles para comunidades de plantas de desierto con muy bajos niveles de precipitación. Los bosques freatofiticos deP. glandulosa cerca del Mar de Salton, California, producen 13 000 kg ha−1 de biomasa aérea con una productividad 3700 kg ha−1 año−1. Tan alto nivel de productividad es posible porque Prosopis no es afectado por los factores que limitan la aprovechabilidad de agua y nitrógeno.Los primeros 60 cm del perfil del suelo bajo el dosel de Prosopis contienen 1020 g m−2 de nitrógeno total, el 25% existe en la forma de nitrato. Tales acumulaciones de nitrógeno pueden ser el resultado de la fijación simbiótica activa. Los primeros valores estimados sugieren que son fijados entre 25–30 kg N ha−1 año−1 en estos bosques. Puesto que Prosopis cubre solamente 34% de esta área y sus recursos de agua no son limitantes, puede ser posible la obtención de mayores niveles de fijación de nitrógeno y productividad de los cultivos si se manejan con mayores densidades.

Comparisons of ureide and acetylene reduction methods for estimating biological nitrogen fixation by glasshouse-grown cowpea

Abstract In dry field conditions, obtaining estimates of biological nitrogen fixation using either the acetylene reduction assay or measurements of ureide concentrations in xylem sap is limited by the difficulties involved in digging out root-nodule systems or obtaining xylem sap. This study was conducted to determine whether ureide concentrations in extracts from stem tissues are as reliable for estimating rates of biological nitrogen fixation by cowpea [ Vigna unguiculata (L.) Walp.] as acetylene reduction assays or ureide concentrations in xylem sap. Glasshouse-grown cowpeas were subjected to different levels of NO 3 − (0, 2, 10 mM) in a basal nutrient solution to achieve different levels of nitrogen fixation. Both levels of added nitrogen (2 and 10 mM) reduced nodulation and nitrogen fixation as measured by the ureide methods and acetylene reduction assay. All methods used to estimate rates of nitrogen fixation exhibited similar seasonal trends but some differences were apparent among treatments. Hot-water extraction of ureides from dried stem tissue appears promising as a rapid technique that could be effective under dry field conditions where the other techniques are not effective. Relative ureide abundance, expressed as 100 × [ureide (N)]/[ureide (N) + NO 3 − (N) + NH 4 + (N)], in either xylem sap or stem tissue, provided approximate estimates of the extent to which cowpeas rely on biological nitrogen fixation. Values of 85–95% were obtained for plants relying totally on nitrogen fixation, whereas non-nodulated plants grown on NO 3 − had values of 3–12%.

Extraction and reverse‐phase high‐pressure liquid chromatography of gibberellic acid in soil

Abstract Several aqueous extractants buffered above pH 6.5 were evaluated for their ability to extract gibberellic acid from a Xerollic Calciorthid. More than 90% of the GA3 added to soil was recovered with 0.01 M KH2PO4, pH 7.4. Gibberellic acid in soil extracts concentrated under vacuum was determined by reverse‐phase high‐pressure liquid chromatography (HPLC) by comparing the reproducible GA3 peak areas with those of standards prepared in the same background solution.

Fiber‐optic spectrophotometry of cowpea nodules treated with nitrate or ammonium

Abstract Cowpea nodules (Vigna unguiculata [L.] Walp. cv. CB5) were pierced with two opposing optical fibers (280 μm diameter), and absorbance spectra of 0.1 mm of nodule tissue were recorded from 415 to 600 nm using a modified spectrophotometer with a nodule sampling stage. The nodule spectra exhibited two absorbance bands, a major band in the near‐UV (415–450 nm) and a lesser one in the green‐yellow region (510–585 nm); the latter exhibited a prominent peak at 550 nm. Nodule spectra were consistent with the superposition of the spectra of ferroleghemoglobin (Lb2+), oxyleghemoglobin (Lb2+‐ O2), and cytochrome c (550 nm). The detection of leghemoglobin in vivo was confirmed by demonstrating the reversibility of binding of CO to Lb, and by comparing the spectra of live nodules with those obtained from anaerobic leghemoglobin preparations. The effects on the nodule spectra of two successive applications (36 and 39 days after planting) of 5 and 10 mM NO3 ‐ or NH4 + to the nutrient solution bathing the nodula...

Dwarf pea response to gibberellic acid applied to soil through a drip irrigation system, and gibberellic acid biodegradation in soil

Gibberellic acid (29 or 290 μM) injected into drip irrigation lines significantly stimulated internode elongation of dwarf peas, and the 290-μM soil treatment produced significantly taller plants than did the 29-μM treatment. GA3 uptake may limit GA-induced internode elongation when GA3 is applied to soil, in contrast to results obtained for hydroponically grown plants, where uptake initially appeared to exceed the rate of hormone metabolism (andersonet al.). It is likely that biodegradation or chemical inactivation limited the plant-availability of GA3 in the soil. Degradation of moderate GA3 concentrations in a moist, aerobic loamy fine sand was nearly complete within five days, indicating that the inefficiency of soil applications may outweight the benefits provided by reducing labor costs associated with foliar-spray applications.

Effect of two soil moisture levels and wetting‐drying cycles on manganese release in NaCl‐amended soils

Abstract Effect of two moisture levels (22.5 and 13.5%, w/w) and wetting‐drying cycles on manganese solubility was studied in NaCl‐amended soil. During 6 d incubation, higher moisture level released 40‐fold more water‐soluble Mn and 60‐fold more NH4OAc‐exchangeable‐Mn in non‐salinized soil. In NaCl‐treated soil, 50 to over 200% greater soluble and exchangeable Mn was recovered from samples incubated at 22.5% compared to 13.5% water levels. Wetting‐drying cycles significantly (P≤0.05) decreased water‐soluble Mn, which accounted for 50 to 60% increases in the exchange‐able Mn. Since other non‐oxidizing/reducing cations (Ca, Mg, Na, K) also demonstrated similar behavior, it is proposed that in addition to oxidation upon drying and reduction upon wetting, the increases in exchangeable Mn and simultaneous decreases in soluble Mn concentration are due to sorption processes. These results suggest that under field conditions, the insolubility of Mn due to continued wetting‐drying cycles may eventually lead to Mn ...