Land use change in desert grassland in Central Mexico: A belowground perspective.
Huber-Sannwald, Elisabeth 1, Arredondo, Tulio1, 1 Instituto Potosino de Investigacion Cientifica y Tecnologica, San Luis Potosi, S.L.P., Mexcio
ABSTRACT- Desert grasslands are the second most abundant vegetation type in the southern most extension of the Chihuahuan Desert. Over the last four decades, natural grasslands declined by 60 percent as a consequence of land conversion to rain-fed agriculture and shrub encroachment. The remaining 40 percent have been affected by overgrazing, the introduction of African forage grass species or by moderate shrub encroachment. All forms of land use change have led to drastic changes in plant species composition, productivity and plant cover. Desert grasslands play an important role in belowground carbon storage, as belowground productivity accounts for up to 70% of total net primary productivity. We examined the effect of long-term land use change on root growth and accumulation of soil organic matter. Thus, we selected four grassland types differing in land use impact: pristine grassland (P) dominated by Bouteloua gracilis, heavily grazed grassland (H) dominated by B. gracilis, grassland dominated by the African species Eragrostis curvula (E), and a shrub encroached savanna-type grassland (S) and compared root biomass and soil organic matter (SOM). Soil samples were excavated at two depths (0-15 cm, 15-30 cm) at two microsites (beneath plants, interspace) in the four grassland types. Root biomass differed significantly between grassland types, this depended on soil depth and microsite (P<0.0001; site x microsite x depth interaction). Root biomass was significantly lower in site H than in all other sites (P<0.0001). Root biomass beneath plants was 3.5 - 4.5 times higher in the top 15 cm than at greater depth in all sites. In the top 15 cm, root biomass was significantly lower in interspaces compared to plant microsites in sites E and S. Soil organic matter differed between grassland types, it was higher in site P and H than in site S; however there was no detectable difference between microsites and soil depth. Land use change over the last 40 years has affected root production and vertical and horizontal root distribution in desert grasslands; however thus far the changes in root distribution have not led to similar changes in the distribution of SOM supporting the notion that desert grasslands provide important long-term storage for carbon.
Key words: land use change, soil organic matter, grassland, root distribution
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