Predictions of the future dynamics of temperate savannah ecosystems require an improved understanding of their responses to simultaneous environmental changes.  For example, in the southwestern U.S., savannahs are subject to both the invasion of non-native grass species and altered precipitation regimes due to climate change.  Due to the differing physiological responses of native vs. non-native grasses to soil water availability, these simultaneous global change factors can have interactive effects on population dynamics, community composition, and ecosystem functioning.  The objective of this study was to determine the responses of soil carbon pools to the interactions of invasive plants and altered precipitation regimes. We utilized constructed stands of native and non-native grasses at the Santa Rita Experimental Range, AZ that had been exposed to several scenarios of seasonal precipitation under rain-out shelters between 2001 and 2006. Soils were sampled at the end of the precipitation experiments, following the final application of precipitation in June 2006.  We analyzed soils to assess the cumulative impact of precipitation scenarios, grass species, and soil texture on several relatively labile carbon pools and the abundance and diversity of soil organisms.

Results/Conclusions

Several soil properties responded to precipitation regime, vegetation type, and soil texture, as well as the interaction of these factors.  Bulk density was greater on the sandy soil than on the clay soil and was affected by precipitation regime.  Soil organic carbon on the clay soils was on average more than twice that of the sandy soils on an areal basis, and was affected by both precipitation regime and dominant vegetation.  In contrast, root biomass was greater on the sandy soil than the clayey soil, differed with dominant vegetation, but was not affected by precipitation.  Short-term C mineralization assays indicated that soil type, vegetation type, and precipitation regime all affected labile C availability for soil microbes.  Soils under the non-native grass had twice the potentially mineralizable C as those under the native grass, and both vegetation types elevated labile C in comparison with intercanopy soils. Precipitation also affected labile C and mineralizable C rates.  As an indicator of responses of soil biota, nematode abundances were influenced by soil texture and vegetation type, but did not respond to precipitation regimes.  These results indicate that the interaction of global change factors can have important implications for the short-term dynamics of soil C pools, which can alter regional C budgets and dynamics.