Impact of Fire on Soil Nitrogen Forms in Central Semiarid Argentina

Burning effects on soil N are particularly important in the Caldenal region of Argentina because N is likely to be limited and is easily lost by volatilization during fire. Burning may cause an initial increase in the mineral N content of the surface soil and, perhaps more important, can reduce the proportion of N in hydrolyzable (that is, more mineralizable) forms. The modification in soil N fractions by fire is directly related to the degree of soil heating and to the proportion and amount of N fractions remaining in the ash. This research was aimed at studying the effects of a controlled burn on total N, inorganic (NO3 and NH4+) and organic (amino acids, ammonium, hexosamines) N forms, as well as hydrolyzable and nonhydrolyzable N compounds of top soils of the southern Caldenal. Soil samples were taken from beneath the canopy of shrubs and from the areas with herbaceous cover, before and after burning. In order to show that a greater proportion of N added to regularly burned soils will result in lower levels of mineralizable N, differences in the changes of the various pools of plant tissue N upon burning at different ignition tem peratures are also reported. The initial unburned top soils had no significant differences in total, inorganic, and organic N form concentrations between shrub and herbaceous vegetation-covered soil. Burning did not affect the net content of total N in soil under woody vegetation, but in the grassland soil a decrease of total N concentration was observed. Nitrate concentration increased and NH4+ concentration decreased in both shrub and grassland soils after burning. Controlled burning caused losses of amino acid and ammonium organic N forms from the soil under grass plants and gains of hexosamine N in both shrub-and herbaceous-coveredsoils. There was an observed tendency to increase the relative proportions of non hydrolyzable N forms in both the shrub and grassland soils after burning. Inorganic and organic N form concentrations were similar in initial shrub litter and grass, but litter had higher concentrations of total and nonhydrolyzable N than those of grass. Shrub litter ash obtained after muffling at 300 C contained higher NH4+-N, aminoacid N, ammonium N, and total and nonhydrolyzable N concentrations than those from grass. An increase of 100 C in the ignition temperature produced clear ashes, rich in mineral N with little concentration of total N and C. The organic N forms affected most by the ignition temperature increase were amino acid N and ammonium N, respectively, particularly in woody litter. The most striking feature was the constituent increase in the percentages of nonhydrolyzable N forms of the vegetation covers on passing from initial to 300 C to 400 C, particularly for the litter and its ashes. These results suggest that the soil mineralizable N pool may be reduced by frequent burning or by more severe burning than observed presently in the Caldenal region.     

Herbaceous vegetation response to controlled fire and postfire defoliation in a semiarid rangeland of Argentina

Semiarid rangelands of central Argentina are shaped by fire and grazing. The aim of this study was to quantify the effect of controlled fire on aboveground biomass production of (1) herbaceous species as a whole group and (2) herbaceous species subclassified as desirable or undesirable perennial grasses and as herbaceous forbs at the end of two growing seasons after an experimental fire in a semiarid rangeland of central Argentina. The effect of the length of the resting period after fire on aboveground biomass production of desirable perennial grass species was also assessed; treatments consisted in defoliations at the end of the first and second postfire growing seasons, at the beginning and at the end of the second postfire growing season and only at the end of the second postfire growing season. Total biomass production of the burned treatments was higher than that of the control treatment, the difference being attributed to an increase in desirable perennial grass species biomass. This in turn was negatively affected by defoliation at the end of the first postfire growing season, suggesting that the beneficial effect of fire on biomass production might have been overridden by the stress imposed by an early defoliation. Overall, the results of the present study suggest that grazing should be held back for at least a whole year after the fire to achieve the pursued aim of increasing forage production by the use of controlled fire.