Overgrazing and land conversion to rainfed agriculture are the principal forms of land use driving grassland degradation in Central Mexico. Both forms of land use may affect physical and biological processes of soils and contribute to the enhancement of CO2 fluxes to the atmosphere. In the monospecific semiarid grassland of central Mexico selective herbivory may transform the functional attributes of grass populations. We examined possible sceneries of selective herbivory by manipulating phenotypic population composition of the key species (B. gracilis). Our interest was to study the impact of B. gracilis phenotypic populations that exhibited distinctive functional attributes which were defined by the lignin content in tissue and associated morphological traits. We established model populations that exhibited three phenotype signatures, low (3.1 - 3.7 % MS), high (4.3 - 5.5 % MS) and a combination of both tissue lignin contents. Monthly we measure in these plots, soil respiration, soil water content, meteorological variables and productivity. Except for the wettest month (September), soil respiration was poorly correlated (P<0.05) with soil temperature but highly correlated to soil water content. During summer and fall 2006 we observed slightly higher soil respiration rates in plots with low lignin content phenotypes during the driest months (May to July). This trend was reversed during the wettest months in fall (August to October). Our study provides evidences that genetic erosion or alteration of population genetic composition may alter processes at the ecosystem level.