Effects of different N-mineral fertilizations on photosynthetic activity and biological denitrification process at two different stage of maize growth in a Mediterranean cropland.

It's well known N-fertilization can greatly affect both plant photosynthesis and denitrifyingactivity of soil in agroecosystems. Moreover it's noteworthy soil biologicalprocesses such as denitrification can interfere on soil-plant relationship because ofcompetitive demand of microbial community for N-mineral source.The aim of this study was to investigate the possible interactions between photosyntheticactivity and biological denitrification of soil in a Mediterranean cropland underirrigated field conditions. This is of great concern since actually only few data areavailable for Mediterranean agroecosystems.The experimental site, also registered as contributing to the FLUXNET network, islocated near Eboli, in Campania region, South Italy. Even if the parent material ofthe soil at the site is carbonate, most of the material has an alluvial origin, with aclay texture in the top 0.2 m, then more sandy and silty-clay at 1.2 m, moreover it isvery rich in carbonates and is characterized by hydromorphic traits related to winterwaterlogging.After manuring and maize sowing, three plots were determined in the field with threedifferent mineral-N supply: 5q/ha control (C), 3q/ha low mineral-N fertilization (N-),high mineral-N fertilization 7q/ha (N+).Measurements of gas exchange (portable gas exchange system Li-Cor 6400, NE,USA), denitrification rate (Acetylene Inhibition Technique, on intact soil cores) andN2O fluxes from soil by closed chambers were performed in each different plot, attwo different stages of maize growth (35 days and 50 days). Analyses of nitrate concentration,pH, soil temperature, WFPS and organic matter were also carried out.On the first sampling date, no differences were detected in NO-3 concentration betweenC and N- plots, while significant higher NO-3 values were found at the N+ plot.In response to the NO-3 pattern, the highest denitrification rate and N2O fluxes weremeasured at N+ plot, moreover despite the similar NO-3 concentrations, an higher denitrificationactivity was found in C as compared to N-, according to the higher WFPSin this plot.Differently from denitrifying activity, comparable values of maize photosynthesiswere found at the three plots, suggesting that CO2 assimilation was not affected bythe different amount of NO-3 concentrations at this stage of maize growth.On the second sampling data, no decrease was detected in NO-3 pool concentration inall the plots, probably because of the slow release to the soil of mineral-N from themanure applied on the field.Also denitrification rates in N+ and N- did not show significant changes referring tothe first sampling, with the highest value being detected in N+. On the contrary verylow denitrifying activity and N2O fluxes from soil were observed in C, probably ascribedto the low WFPS at the sampling time. The measure of photosynthetic activityhas shown significant difference among plots, in particular the higher and the lowervalues were found in N- and N+ plots, respectively, as compared to control. It may behypothesized that different NO-3 concentrations affect CO2 fixation mainly in the latestage of maize growth, indicating that a reduction in nitrogen supply induces limitationin the photosynthetic performance during the full development of photosyntheticapparatus. Nevertheless it could be suggested that the higher photosynthetic rate in N+plot is due to a prolonged ontogenesis derived by richness of N supply.It may be that an interaction between plant and soil systems occurred. In fact despiteno significant changes between the two samplings in denitrifying activity in N+ andN-(at similar values of WFPS), a steep decrease in N2O emissions was found on thesecond date in both plots. It's well known plant-derived C in soil can affect denitrifierbacteria, so it might be argued that the roots of the growing maize plants could havereleased some metabolizable C compounds able to modify the N2O/(N2+N2O) ratio