Macronutrient cycling in mountain grasslands of Sierra de la Ventana, Argentina

Abstract

Macronutrient cycling was assessed in mountain grassland sites located on a NE slope at 550, 850 and 1025 m asl, in Sierra de la Ventana, Buenos Aires, Argentina. Previous studies showed ecosystem changes with elevation increase: 1) temperature decreased, 2) evaporation increased, 3) clay and base content of soils decreased, 4) net primary productivity (NPP) did not change significantly, 5) the below-above-ground biomass ratio increased and, 6) NPP allocation to aboveground tissues diminished and to belowground tissues increased. The objective was to establish if similar NPP along the environmental gradient is attained through changes in nutrient allocation and cycling pathways resulting in nutrient use efficiency (NUE) changes. Plant biomass and necromass were harvested during one year and separated in compartments. Plant and soil samples were analyzed for N, P, K, Ca and Mg. Mineral mass was calculated as the product of nutrient concentration per dry mass. Nutrient fluxes and NUE were calculated. Aboveground live tissues had the highest concentration, Ca excepted. With elevation increase we noted that: i) the concentration of P, K and Mg in green grasses decreased, ii) the mineral proportion stocked in roots incremented, iii) requirements decreased in the range (kg ha^-1 y^-1) 120 - 107 N, 9 - 6 P, 84 - 44 K, 41 - 34 Ca and 12 - 9 Mg; and iv) Nitrogen, P and K resorption decreased its absolute and percent contribution to requirements. Except for K, absorption was the main pathway contributing to N, P, Ca and Mg requirements. The OM and mineral return input to soil was due mainly to root senescence (55 to 92% of total return, depending on nutrient and site), except for K whose main return was via litter fall. At leaf level, there were not differences in nutrient resorption among sites; the resorption ranges were 70-80 % K, 57-68% P and 42-48% N. Nitrogen resorption proficiency was high and P resorption proficiency was complete. Foliar N:P ratio (=11 to 18) suggested P-limitation at intermediate and upper sites. The Gray’s relative turnover rate indicated P-conservative behavior suggesting P was limiting also at the lower site. Nutrients circulated slower than biomass. With the exception of N, NUE was higher at higher elevation. Apparently, N was not limiting in any site. A comparison of grasslands sites located at both environmental - elevation gradient extremes showed that: 1) the grassland in more favourable environment invested more in ANPP, resulting in higher macronutrient requirements; this grassland had high turnover rates (except for K), the NPP was less-dependent on external –absorption- supply and had a more conservative relative circulation of K and P; 2) the grassland in less favourable environment was more P, K, Ca and Mg nutrient- use efficient (higher dry matter production per absorbed nutrient unit), due to higher residence time and a higher biomass and productivity allocation to roots. We concluded that similar NPP in different environments was attained through combined changes in two strategic components involved in nutrient acquisition and use: 1) Carbon fractions allocated to above- and below- ground net productivity and biomass, and 2) Nutrient concentration, total-, above- and below- ground mineralmass, requirements and nutrient supply pathways, and mineral turnover rates, that change NUE. Changes are nutrient specific and may be evidence of soil nutrient availability and limitation with regard to demand.