No abstract

Changes in land use across the semiarid grasslands of northern Mexico have driven a decline of plant cover and alteration of plant species composition. A number of different plant communities have resulted from these changes. Their implications, however, on the carbon (C) cycle and regional carbon balance are still poorly understood. Here, we examined the effects of plant cover loss and changes in species composition on net ecosystem CO₂ exchange (NEE) and their biotic and abiotic controls. NEE was measured in five representative plant community types within a semiarid grassland by temporarily enclosing the entire aboveground ecosystem using a chamber method (i.e., geodesic dome). Sites included an oat crop (crop), a moderately grazed grassland (moderate grazing), a 28 yr-old grazing exclosure (exclosure), an overgrazed site with low perennial grass cover (overgrazed), and an overgrazed site presenting shrub encroachment (shrub encroachment). For natural vegetation, rates of standardized daytime NEE for sites with a high plant cover (exclosure and moderate grazing) were similar (P > 0.05) as compared to sites with low plant cover (overgrazed and shrub encroachment). However, yearly total nighttime NEE (carbon loss) was more than double (P < 0.05) for sites with high plant cover compared to sites with low cover, resulting to slight C sinks for the low plant cover sites, and neutral or sources for the high plant cover sites as accounted by daytime and nighttime NEE annual balance. Differences in plant cover and its associated biomass defined the sensitivity to environmental controls. Thus, daytime NEE in low plant cover sites reached light compensation points at lower photosynthetic photon flux density than those from high plant cover sites. Differences in species composition did not influence NEE rates even though there were transient or permanent changes in C3 vs. C4 functional groups. Our results allowed the detection of the large variability and contribution of different plant communities to regional C balance in patchy landscapes. Identification of the role of landscape patches in the regional C balance as either sinks or sources may provide tools allowing land use management strategies that could favor C uptake in patchy landscapes

El objetivo fue revisar el estado actual de los pastizales semiáridos y matorrales áridos del norte y centro de México, así como analizar los retos y perspectivas del uso de estos ecosistemas. Desde la década de los años 1950, el INIFAP en colaboración con otras instituciones han generado y transferido conocimientos sobre manejo de pastizales, lo cual se ha reflejado en el uso de prácticas de manejo en los ranchos ganaderos del país. Los pastizales y matorrales han sufrido disturbios, principalmente la apertura de tierras para cultivos y se encuentran deteriorados principalmente por el sobrepastoreo. La utilización de los pastizales y matorrales a través del pastoreo debería incluir una carga animal adecuada, sistemas de pastoreo y prácticas estratégicas de distribución del pastoreo. A pesar del deterioro, existe una gran diversidad de recursos genéticos, principalmente de pastos, que pueden ser utilizados para conservación y producción de semilla para rehabilitación de los pastizales. La resiembra, aunque de alto costo y riesgo, es una opción para rehabilitar aquellos pastizales o matorrales deteriorados. Estos ecosistemas pueden proporcionar servicios ambientales, principalmente captura de carbono para mitigar el cambio climático. Los retos son generar, transferir y aplicar conocimientos e innovaciones tecnológicas para lograr un manejo sostenible de los pastizales y matorrales, a pesar de algunas amenazas como la baja inversión en ciencia y tecnología, el cambio climático y la avaricia humana. Para lograr esto, es imprescindible la participación conjunta y comprometida de todos los actores e instituciones involucrados en el uso de estos ecosistemas.

Changes in land use across the semiarid grasslands of Northern Mexico have driven a decline of plant cover and alteration of plant species composition. A number of different plant communities have resulted from these changes, however, their implications on the carbon cycle and regional carbon balance are still poorly understood. Here, we examined the effects of plant cover loss and changes in species composition on net ecosystem CO₂ exchange (NEE) and their biotic and abiotic controls. Five typical plant community types were examined in the semiarid grassland by encasing the entire above-ground ecosystem using the geodesic dome method. Sites included an oat crop (crop), a moderately grazed grassland (moderate grazing), a 28 yr-old grazing exclosure (exclosure), an overgrazed site with low perennial grass cover (overgrazed), and an overgrazed site presenting shrub encroachment (shrub encroachment). For natural vegetation, rates of daytime NEE for sites with a high plant cover (exclosure and moderate grazing) were similar (<i>P</i>>0.05) as compared to sites with low plant cover (overgrazed and shrub encroachment). However, night time NEE (carbon loss) was more than double (<i>P</i><0.05) for sites with high plant cover compared to sites with low cover, resulting into slight C sinks for the low plant cover sites and neutral or sources for the high plant cover sites on an annual basis. Differences in plant cover and its associated biomass defined the sensitivity to environmental controls. Thus, daytime NEE in low plant cover sites reached light compensation points at lower PPFD values than those from high plant cover sites. Differences in species composition did not influence NEE rates even though there were transient or permanent changes in C3 vs. C4 functional groups

Single application of biosolids increases forage production on semiarid grasslands. Residual effects of biosolids on forage production have been scarcely measured in semiarid grasslands. The objective was to evaluate the residual effects of biosolids application on forage production of blue grama (Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths) and other grasses at a semiarid grassland in Jalisco, Mexico. The study was performed at shortgrass prairie in northeast Jalisco. Field plots were selected to include blue grama plants before rainy season in 2002. Aerobic biosolids were applied at 0 (control), 15, 30, 45, 60, 75, or 90 dry Mg ha −1 under a completely random design with five replications. Forage production was estimated by clipping at the end of the growing season during five years. Data analysis was performed with linear mixed model and repeated measures. Forage production was influenced by a rate × year × species interaction ( = 0.0001). Blue grama forage production increased with increasing biosolids rates during all years, with the magnitude of this response varying among years. Forage production of other grass species slightly decreased with biosolids application. Single biosolids application had a residual effect on forage production throughout five years in semiarid grasslands.