@article{glasener_wagger_mackown_volk_2002, title={Contributions of shoot and root nitrogen-15 labeled legume nitrogen sources to a sequence of three cereal crops}, volume={66}, ISSN={["0361-5995"]}, DOI={10.2136/sssaj2002.0523}, abstractNote={Legume mulches are important sources of N for cereal crop production, particularly for organic and resource-poor producers. A field study was conducted using a direct method to determine if the amount of N in cereal crops derived from either the shoots or roots of preceding tropical legume cover crops was affected by their chemical composition and mineralization potential. Desmodium ovalifolium Guill. & Perr. [= D adscendens (Sw.) DC. and Pueraria phaseoloides (Roxb.) Benth.], were grown in 6.0-m2 microplots and foliar-labeled with 99 atom % 15N urea. A cereal sequence of maize (Zea mays L.)–rice (Oryza sativa L.)–maize followed the legumes. Cereal accumulation of legume N from either the shoot (shoot + leaf litter) or the root-soil sources was evaluated by spatially separating the legume N sources. This was achieved by interchanging surface applications of nonlabeled and 15N-labeled legume shoots with in situ 15N-labeled and nonlabeled legume roots. Initially the Desmodium shoot N source contained 316 kg N ha−1 and roots contained 12.5 kg N ha−1 Pueraria shoots and root N sources initially contained 262 and 14.8 kg N ha−1, respectively. About 90 g kg−1 of the initial N of each legume shoot was recovered in the total aboveground tissues from the three cereal crops, while 490 g kg−1 of Desmodium and 280 g kg−1 of Pueraria root-soil N sources were recovered. Of the 181 kg N ha−1 accumulated aboveground by the cereal sequence, the contribution of shoot plus root-soil N sources was 200 g kg−1 from Desmodium and 150 g kg−1 from Pueraria Cereal N was derived primarily from mineralization of soil organic matter present before the legumes and possibly from N deposition (precipitation and dry) occurring during the cereal crop sequence. After harvest of the last cereal crop, 13 and 180 g kg−1 of the initial legume N was present as inorganic and organic N fractions, respectively, in the top 75 cm of soil. Even though Pueraria shoots had a lower C:N ratio and concentration of polyphenols than Desmodium shoots, the relative contributions of the shoot N source were similar for both legumes. Decomposition of legume residues, particularly legume shoots, make a meaningful contribution to the N economy of cereal crops grown in the tropics. The legume cover crops (root + shoot) contributed nearly 280 g kg−1 of the aboveground N in the first cereal crop and as much as 110 g kg−1 of the N in the third crop during the 15-mo sequence of cereals.}, number={2}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Glasener, KM and Wagger, MG and MacKown, CT and Volk, RJ}, year={2002}, pages={523–530} }
 @article{muriuki_king_volk_2001, title={Nitrogen-15 recovery in soil incubated with potassium nitrate and clover residues}, volume={65}, ISSN={["0361-5995"]}, DOI={10.2136/sssaj2001.6551430x}, abstractNote={In the southeastern USA, legumes are used as green manure to meet crop N requirements of a following crop, but recovery is usually lower than from conventional fertilizers. We conducted a laboratory study for 26 wk under aerobic conditions to monitor recovery of 15N‐labeled KNO3 (fertilizer) and crimson clover (Trifolium incarnatum L.) residues (clover) in organic, inorganic, and microbial biomass N pools. Volatilization of NH3 from decomposing clover residues was monitored for 12 wk. Three hundred–gram samples of a Typic Kanhapludult soil were amended with N (0.051 mg N kg−1 dry soil in fertilizer and 0.049 mg N kg−1 dry soil in clover). A control with no N was also included. Although inorganic N (NH4, NO2, and NO3) accumulated throughout (fertilizer > clover > control), the rate of accumulation did not differ among treatments. Organic and microbial biomass N concentration did not differ among treatments, but applied N recovery in microbial biomass was greater in clover than fertilizer (P < 0.05) throughout. Ammonia volatilized was negligible. After 26 wk, applied N recovered in soil inorganic N was 66% for fertilizer and 40% for clover; in soil organic N, 18% for fertilizer and 50% for clover; and in microbial biomass N, 0.75% for fertilizer and 1.5% for clover. Applied N presumed denitrified was 16% in fertilizer and 10% in clover. We concluded that clover green manure can meet the N requirements of a following crop from the time of emergence in the southeastern USA.}, number={5}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Muriuki, AW and King, LD and Volk, RJ}, year={2001}, pages={1430–1436} }
 @article{glasener_wagger_mackown_volk_1998, title={Nitrogen-15 labeling effectiveness of two tropical legumes}, volume={200}, ISSN={["1573-5036"]}, DOI={10.1023/A:1004330514833}, number={2}, journal={PLANT AND SOIL}, author={Glasener, KM and Wagger, MG and MacKown, CT and Volk, RJ}, year={1998}, month={Mar}, pages={149–156} }
 @article{feng_volk_jackson_1998, title={Source and magnitude of ammonium generation in maize roots}, volume={118}, ISSN={["0032-0889"]}, DOI={10.1104/pp.118.3.835}, abstractNote={Abstract}, number={3}, journal={PLANT PHYSIOLOGY}, author={Feng, JN and Volk, RJ and Jackson, WA}, year={1998}, month={Nov}, pages={835–841} }
 @article{crozier_king_volk_1998, title={Tracing nitrogen movement in corn production systems in the North Carolina Piedmont: A nitrogen-15 study}, volume={90}, ISSN={["0002-1962"]}, DOI={10.2134/agronj1998.00021962009000020009x}, abstractNote={Abstract}, number={2}, journal={AGRONOMY JOURNAL}, author={Crozier, CR and King, LD and Volk, RJ}, year={1998}, pages={171–177} }
 @article{volk_1997, title={Unidirectional fluxes of nitrate into and out of maize roots: measurement and regulation by prior nitrate nutrition}, volume={123}, DOI={10.1016/S0168-9452(96)04560-8}, abstractNote={A method for rapid assay of concurrent fluxes of nitrate into and out of intact plant roots has been developed. Maize (Zea mays L.) seedlings were grown on ‘normal’ 14N-nitrate prior to exposure for 5 min to 99 at.% 15N-nitrate. Influx and efflux rates were calculated from the depletion of nitrate and its 15N enrichment in the external solution. Once developed, the assay was used to examine the regulation of nitrate fluxes by prior nitrate nutrition. Maize seedlings were pretreated for 24 h with 0, 20 or 200 μM 14N-nitrate, providing root nitrate concentrations of 5.0, 8.2 and 30.7 μmol/g fw, respectively. Nitrate influx rates increased 55% when the root nitrate concentration rose from 5.0 to 8.2 μmol/g fw, but remained constant when root nitrate increased to 30.7 μmol/g fw. By contrast, efflux of nitrate continued to accelerate as root nitrate concentrations increased. As a consequence of these differential responses, net nitrate uptake was 26–30% higher at a root nitrate concentration of 8.2 μmol/g fw than at the lower and higher concentrations.}, number={1997}, journal={Plant Science}, author={Volk, R. J.}, year={1997}, pages={1–7} }
 @article{volk_chaillou_mariotti_morot-gaudry_1992, title={Beneficial effects of concurrent ammonium and nitrate nutrition on the growth of Phaseolus vulgaris: A 15N study}, volume={30}, number={4}, journal={Plant Physiology and Biochemistry}, author={Volk, R. and Chaillou, S. and Mariotti, A. and Morot-Gaudry, J. F.}, year={1992}, pages={487} }
 @article{volk_pearson_jackson_1979, title={REDUCTION OF PLANT-TISSUE NITRATE TO NITRIC-OXIDE FOR MASS-SPECTROMETRIC N-15 ANALYSIS}, volume={97}, ISSN={["0003-2697"]}, DOI={10.1016/0003-2697(79)90336-1}, abstractNote={Abstract A procedure based on that of Van Slyke and LoMonte ( Microchem. J. 14 , 608–626, 1969) is described for the mass spectrometric analysis of 15 N in nitrate extracted from plant tissue. The extract is evaporated to dryness in a disposable borosilicate test tube, which forms the basal part of a reaction flask. After evacuation of the flask, nitrate is reduced to nitric oxide by sonication with mercury in 18 n H 2 SO 4 . The atom percentage 15 N is calculated from the relative intensities of the 14 NO and 15 NO peaks of the mass spectrum. The method is unaffected by reduced nitrogen compounds which are present in plant tissue extracts.}, number={1}, journal={ANALYTICAL BIOCHEMISTRY}, author={VOLK, RJ and PEARSON, CJ and JACKSON, WA}, year={1979}, pages={131–135} }
 @article{volk_jackson_1972, title={Photorespira-tory phenomena in maize: Oxygen uptake, isotope discrimination and carbon dioxide efflux}, volume={49}, DOI={10.1104/pp.49.2.218}, abstractNote={Concurrent O(2) evolution, O(2) uptake, and CO(2) uptake by illuminated maize (Zea mays) leaves were measured using (13)CO(2) and (18)O(2). Considerable O(2) uptake occurred during active photosynthesis. At CO(2) compensation, O(2) uptake increased. Associated with this increase was a decrease in O(2) release such that a stoichiometric exchange of O(2) occurred. The rate of O(2) exchange at CO(2) compensation was directly related to O(2) concentration in the atmosphere at least up to 8% (v/v).When illuminated maize leaves were exposed to saturating CO(2) concentrations containing approximately equal amounts of (12)CO(2) and (13)CO(2), the latter was taken up more rapidly, thus depressing the atom% (13)C in the atmosphere. Moreover, upon exposure to CO(2) containing 96 atom% (13)C, there occurred a directly measurable efflux of (12)CO(2) from the leaves for at least 15 minutes. During this period an equimolar evolution of (16)O(2) and uptake of (13)CO(2) was observed. Thereafter, although the rate of (16)O(2) evolution remained unchanged, the rate of (13)CO(2) uptake declined markedly, suggesting continual (13)C enrichment of the photorespiratory substrate.It is concluded that a finite photorespiratory process occurs in maize and that the CO(2) generated thereby is efficiently recycled. Recycling maintains the internal CO(2) concentration at a level difficult to detect by most photorespiratory assays.}, number={2}, journal={Plant Physiology}, publisher={USA}, author={Volk, R. J. and Jackson, W. A.}, year={1972}, pages={218} }