Biogeochemical consequences of livestock grazing in a juniper-oak savanna

Abstract

Native and domestic herbivores can alter the structure and function of ecosystems. Chronic livestock grazing can cause shifts in species composition and/or life-forms, which may subsequently alter the quality and quantity of organic matter inputs to the soil system. Despite its probable significance, little is known regarding the effects of livestock grazing on biogeochemical processes, I investigated the effects of grazing intensity (heavy, moderate, none) on soil carbon (C) and nitrogen (N) concentrations, microbial biomass C (MBC) and N (MBN), and potential C (Cmi,,) and N (Nmj mineralization rates under different plant communities (shortgrass, midgrass, juniper, oak) in a juniper-oak savanna in west-central Texas. Grazing treatments were maintained for 47 years. Soil texture and pH were similar across grazing treatments and vegetation types Grazing had no significant effect on any of the variables, perhaps due to the high clay and high CACO, content of these soils. There were no significant grazing x vegetation interactions; however, plant community type was significant (p<0.001) for all variables. Soil C and N concentrations (g kg-') was greater under oak (C = 96.7 ︢9.3; N = 6.1 ︢0.6) than all other communities (C = 54.4 ︢3.3; N = 3.5 ︢0. 1). Microbial biomass (mg kg-') was greater under oak (NMC = 2358 ︢495; MBN = 360 ︢127) than all other communities (NMC = 1799 ︢21 1-MBN = 259 ︢39). Similarly, potential mineralization (mg kg-' day-') was greater under oak (C.i. 49.9 ︢13.3; Nil = 7.8 ︢3.5) than all other communities (Cmi,, = 32.2 ︢6.8; Nmi,, = 3.4 0.7). Results indicate that vegetation type is more important than grazing as a determinant of soil nutrient concentrations and flux rates in this savanna. Although grazing did not have direct effects on soil organic C, total N, C and N mineralization, or microbial biomass, it may have indirect effects on these biogeochemical parameters by affecting plant community composition and structure. These conclusions apply only to concentrations and not absolute quantities of nutrients. Prior documentation of grazing-mediated soil erosion suggests that soil nutrient losses due to grazing have occurred in this region. Therefore, although the results of this study show no effect of livestock grazing on soil nutrient concentrations and dynamics, it should not be inferred that livestock grazing has no adverse effects on the soil resource.