Relationship between plant nitrogen conservation and the dynamics of soil nitrogen in the arid patagonian Monte, Argentina

CARRERA, A.L.; BERTILLER, M.B; SAIN, C.L; MAZZARINO, M.J.

PLANT AND SOIL

SPRINGER

Lugar: Berlin; Año: 2003 vol. 255 p. 595 - 604

0032-079X

During three consecutive years with contrasting precipitation, we analysed the relationship between strategies of N conservationin the dominantplant functionalgroups(perennialgrasses and evergreenshrubs)of the Patagonian MonteandthemaincomponentsofN cyclinginsoil. We hypothesisedthat thedifferentpatternsofNconservation in perennial grasses and evergreen shrubs would have direct consequences for soil-N, inorganic-N release and microbial-N flush in soil. In autumn and late spring of 1999, 2000, and 2001, we assessed N and C concentration ingreenandsenescedleaves,N-resorptionefficiencyandC/N ratioinsenescedleavesofthreedominantspeciesof eachplantfunctionalgroup.Inthesoilassociatedwithspeciesofeachplantfunctionalgroup,wedeterminedNand C concentration,potential-N mineralisation,and the associated microbial-Nflush. Slow-growingevergreenshrubs exhibited low N-concentration in green leaves, high N-concentration in senesced leaves and low N-resorption from senescing leaves. In contrast, fast-growing perennial grasses showed high N-concentration in green leaves, low N-concentration in senesced leaves, and high N-resorption from senescing leaves. In evergreen shrubs, the maintenance of long-lasting green leaves with low N-concentration was the most important mechanism of N conservation. In contrast, perennial grasses conserved N through high N-resorption from senescing leaves. SoilN concentration, potential N-mineralisation, and microbial-N flush in the soil were higher underneath evergreen shrubs than beneath perennial grasses. Observed differences, however, were lower than expected considering the qualityofthe organicmatter suppliedbyeach plantfuctionalgroupto the soil. A possible reasonfor this relatively weak trend may be the capacity of evergreen shrubs to slow down N cycling through low leaf turnover and the presence of secondary compounds in leaves. Alternatively or simultaneously, the weak relationship between plant and soil N could result from shrubs being able to colonise N-poor soils while grasses may preferably occupy fertile microsites previously influenced by the decomposition pathway of evergreen shrubs. Differences between evergreenshrubsandperennialgrassesinthemechanismsofplantN-conservationandincomponentsofNcycling in the underlying soil were consistent over the three years of the study with differing precipitation. Inter-annual differences in N concentration in green leaves and in the microbial-N flush in soil indicate that during the wettest year fast-growing perennial grasses would outcompete slow-growing evergreen shrubs and microorganisms for N uptake.